Symposium Organizers
Shangjr (Felix) Gwo National Tsing-Hua University
Joel W. Ager Lawrence Berkeley National Laboratory
Fan Ren University of Florida
Oliver Ambacher Fraunhofer-Institut für Angewandte Festkörperphysik (IAF)
Leo Schowalter Crystal IS Inc.
I1: Growth and Doping
Session Chairs
Monday PM, November 30, 2009
Independence W (Sheraton)
9:30 AM - **I1.1
Heteroepitaxy of m-plane (1010) InN on (100)-LiAlO2 Substrates and its Anisotropic Optical Behaviors.
Kuei-Hsien Chen 1 2 , Jr-Tai Chen 1 , Ching-Lien Hsiao 2 , Ying-Chieh Liao 3 , Li-Wei Tu 4 , Mitch Chou 5
1 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan, 2 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, 3 Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei Taiwan, 4 Department of Physics, National Sun Yat-Sen University, Kaohsiung Taiwan, 5 Department of Materials Science & Opto-electronic Engineering, National Sun Yat-Sen University, Kaohsiung Taiwan
Show AbstractHeteroepitaxial growth of m-plane (1010) InN film on (100)-LiAlO2 (LAO) substrate has been realized by plasma-assisted molecular-beam epitaxy with pre-annealing treatment of substrate. X-ray diffraction, reflection high-energy electron diffraction, and electron back scatter diffraction patterns reveal pure m-plane InN film. On the contrast, with substrate nitridation, c-plane (0001) InN columnar structure is grown instead of m-plane InN film. Surface treatment of LAO substrate has been proven to play an important role in controlling the phase and purity of the m-plane InN. Angle-dependent polarized UV-Raman spectra show that all the InN phonon modes with pure structures follow Raman selection rule well. High polarization degree of 87% of the m-plane InN determined by polarized photoluminescence spectroscopy exhibits strong polarization anisotropy.Reference:1. J. T. Chen et al. J. Phys. Chem. A 111, 6755-6759 (2007).2. C. L. Hsiao et al. Appl. Phys. Lett. 92, 111914-(1-3) (2008).
10:00 AM - I1.2
Epitaxial Growth and Interface Study of GaN Films on MgAl2O4 Substrates.
Guoqiang Li 1 2 , Vlado Lazarov 1 , Shao-Ju Shih 1
1 Department of Materialsq, University of Oxford, Oxford United Kingdom, 2 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan China
Show AbstractGallium nitride (GaN) based materials/devices have shown great potential for electronic lighting, which is one of the solutions to the global warming and energy crisis facing to us nowadays. GaN films are usually grown on sapphire. However, the lattice mismatch between GaN and sapphire is as high as 16%, which makes the GaN films suffering from a high density of crystalline defects. The use of MgAl2O4 (111) as a substrate, which has a much smaller lattice mismatch with GaN is a promising way to solve this problem. However, the high Mg partial pressure of MgAl2O4 at high temperature makes the conventional MOCVD or MBE inappropriate in this case. In this work, GaN films were grown by pulsed laser deposition (PLD) with the pulse energy of about 3J/cm2. A metal Ga target and the rf nitrogen plasma were utilized as the Ga and N sources, respectively. The growth temperatures were varied from RT to 800 C. High resolution electron microscopy and electron diffraction were carried out to study the GaN/MgAl2O4 films. We found GaN grew epitaxially under 600 C with the epitaxial relationship of GaN (0002) // MgAl2O4 (111) and GaN [11-20] // MgAl2O4 [110]. The thickness of the interfacial layer decreased from 10 nm at 600 C to less than 1 nm at RT. The atomic interface structure of GaN/MgAl2O4 has been studied by high resolution TEM (HRTEM) and compared to model interface structures. The interface models comprising of O-Ga atomic structure is the best fit to experimental data. Convergent beam electron diffraction revealed the N film polarity which is in good agreement with the modeling results of the interface. In addition, using dark field imaging, we studied the dislocation density of GaN films as a function of temperature growth of GaN films.
10:15 AM - I1.3
Efficient InGaN Quantum Wells and Light Emitters Grown by MOVPE on Misoriented GaN Substrates.
Piotr Perlin 1 2 , Tadek Suski 1 , Martin Albrecht 3 , Szymon Grzanka 2 , Grzegorz Staszczak 1 , Robert Czernecki 1 2 , Grzegorz Targowski 2 , Marcin Krysko 1 , Grzegorz Nowak 1 , Grzegorz Kamler 1 , Przemek Wisniewski 1 2 , Mike Leszczynski 1 2 , Boleslaw Lucznik 1
1 , Institute of High Pressure Physics, Warsaw Poland, 2 , TopGaN Ltd. , Warsaw Poland, 3 , Leibniz-Instituts für Kristallzüchtung, Berlin, Germany, Berlin Germany
Show AbstractAs it is well known very important requirements for efficient light emitting diodes and laser diodes consist of i) a high internal efficiency of the quantum wells located in the active region of emitters and ii) a sufficient injection of electron and holes to this region. In case of devices employing InGaN/GaN material, achieving large concentration of holes by Mg doping, represents one of the crucial issues. We have demonstrated very recently that the use of GaN substrates misoriented with respect to c-polar direction (with misorientation angle α) acts as a factor increasing this concentration even by an order of magnitude. On the other hand, we have found also that In-concentration in InGaN layers or quantum wells decreases significantly with increasing GaN-substrate misorientation. In contrast to this observation the width of photoluminescence tends to increase with α, what is treated as an obstacle in achieving efficient lasing.The purpose of this work was to determine optimal growth conditions compromising these requirements, to study basic structural and optical properties of InGaN material and finally to demonstrate efficient laser diodes grown on misoriented GaN substrates (α up to 2 deg) by means of MOVPE technique. Following results have been obtained in this work:i)the most promising misorientation, α, of GaN substrate for the InGaN/GaN/ AlGaN laser fabrication is 0.5 deg with respect to c-plane of the wurtzite structure. We will demonstrate such laser diodes and compare them with devices grown on other orientations of GaN substrates;ii)In-concentration in InGaN layers and quantum wells decreases with increasing substrate misorientation, for all used growth conditions (the previous foundings were based on structures grown at the same growth conditions except the growth temperature);iii)width of the photoluminescence band changes with α in a step-like manner. It stays roughly constant for α up to about 0.5 deg, then increases drastically at α≈1 deg, and is similar for α between 1-2 deg.
10:30 AM - I1.4
Selective Area Epitaxy of InGaN/GaN Stripes, Hexagonal Rings, and Triangular Rings for Achieving Green Emission.
Wen Feng 3 2 , Vladimir Kuryatkov 3 2 , Dana Rosenbladt 4 2 , Nenad Stojanovic 4 2 , Mahesh Pandikunta 3 2 , Sergey Nikishin 4 2 , Mark Holtz 1 2
3 Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas, United States, 2 Nano Tech Center, Texas Tech University, Lubbock, Texas, United States, 4 Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas, United States, 1 Physics, Texas Tech University, Lubbock, Texas, United States
Show AbstractWe report experiments using selective area epitaxy to produce quantum wells (QWs) of InGaN on micron-scale stripes and rings. The objective of this study is to elevate indium incorporation for achieving blue and green emission on semi-polar crystal facets. Growth is carried out using selective metal-organic vapor phase epitaxy (MOVPE). In each case, GaN structures are first produced, and the InGaN QWs are subsequently grown and investigated using cathodoluminescence (CL) spectroscopy and imaging in a scanning electron microscope (SEM). InGaN QWs have been grown on GaN stripes oriented along the [11-20] direction. Several different window widths were designed in the SiO2 mask. Completed pyramidal InGaN stripes with flat and smooth {1-101} sidewall were produced on 2-µm windows while trapezoidal stripes with both {1-101} sidewall and (0001) top surface were obtained on the 5-µm windows. The former has uniform CL emissions at 500 nm on the {1-101} sidewall and at 550 nm on the narrow ridge. The latter exhibits similar CL emissions at 500 nm on the sidewall and at 570 nm on the top surface. These wavelength shifts relative to the CL spectrum peak (450 nm) from the reference region are attributed to thickness enhancement and indium enrichment in selective MOVPE InGaN QW hexagonal and triangular microrings were grown using selective area epitaxy on patterned (0001) AlN/sapphire. In the case of hexagonal microrings, the well defined shapes are comprised of {11-22} and {21-33} facets on inner sidewalls, and {1-101} facets on outer sidewalls, as well as (0001) top surfaces. The sidewall facets exhibit distinct emission spectra when investigated using CL, with peak wavelengths as long as 500 nm. Differences in emission spectra are attributed to variations in growth rate and indium incorporation on the facets. We observe a weak blueshift with increasing CL excitation current, verifying that the internal piezoelectric fields of the semi-polar sidewall facets are lower than reference (0001) InGaN QWs. In comparison, the triangular microrings reveal smooth inner and outer sidewalls falling into a single type of {1-101} planes. All these {1-101} sidewall facets demonstrate similar CL spectra with one InGaN-related peak at 460 nm. In addition, spatially matched striations are observed in the CL intensity images and surface morphologies of the {1-101} sidewall facets. The striations seen in the InGaN QW triangular microrings are found to be pertinent with surface morphologies of the underlying GaN layers.The growth factors leading to long-wavelength emission will be described along with variations in optical properties due to InGaN growth on different crystal facets.This was partly supported by the National Science Foundation (ECS-0609416), U.S. Army CERDEC (W15P7T-07-D-P040), and J. F Maddox Foundation.
10:45 AM - I1.5
Free-Standing Zinc-Blende (Cubic) GaN Layers and Substrates Grown by Molecular Beam Epitaxy.
Anthony Kent 1 , Sergei Novikov 1 , Norzaini Zainal 1 , Andrey Akimov 1 , Thomas Foxon 1
1 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom
Show AbstractThe group III-nitride semiconductors are being increasingly used for amber, green, blue and white light emitting diodes (LEDs), for blue/UV laser diodes (LDs) and for high-power, high-frequency and high temperature electronic devices. However, one of the most severe problems hindering progress in this field is the rarity of suitable lattice matched substrates. AlN, GaN, InN and their solid solutions are commonly grown on non-lattice matched substrates such as sapphire, GaAs or SiC, but bulk GaN substrates would be preferable for the highest-quality nitride-based devices. The group III-nitrides normally crystallise in the hexagonal (wurtzite) structure. The unique feature of wurtzite group III-nitrides, in comparison with conventional III-V semiconductors, is the existence of very strong electric fields inside the crystal structure. The charge separation within quantum wells can lead to a significant reduction in the efficiency of optoelectronic structures. As a result, the growth of non-polar group III-nitride structures has been the subject of considerable recent interest. The electric fields can be eliminated (reduced) in wurtzite material by growing in non-polar (semi-polar) directions. However, a direct way to eliminate electric fields would be to use non-polar (100) oriented zinc-blende (cubic) III-nitride layers.Recently, we have developed, for the first time, a process for MBE growth of free-standing cubic GaN layers with potential application as substrates. Undoped thick cubic GaN films were grown on semi-insulating GaAs (001) substrates by modified plasma-assisted molecular beam epitaxy (PA-MBE) using arsenic as a surfactant to initiate the growth of cubic phase material. At a thickness of ~30µm, free-standing GaN wafers can easily be handled without cracking. As a result, free-standing GaN wafers with thicknesses in the 30–100μm range may be used as substrates for further growth of cubic GaN-based structures and devices. We will present a detailed study of the initial stages of the growth of cubic GaN, which are crucial steps in the epitaxy of thick free-standing cubic GaN layers. A set of characterisation techniques including X-ray diffraction, reflection high energy electron diffraction (RHEED), secondary ion mass spectroscopy (SIMS) and photoluminescence (PL) were used to optimise the properties of free-standing cubic GaN and to measure for the first time the basic parameters of the bulk zinc-blende GaN.
11:30 AM - **I1.6
Doping of InN and AlN Bulk and Surfaces.
Chris Van de Walle 1
1 , University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractInN exhibits a high tendency for unintentional n-type conductivity, both in the bulk and on the surface. AlN, on the other hand, is difficult to dope either n-type or p-type, but surface states on AlGaN alloys may lead to doping of GaN/AlGaN heterostructures. We have used first-principles calculations based on density functional theory (DFT) to investigate various possible causes for the doping. Techniques to address the DFT band-gap error were employed, including hybrid functionals.For InN, our results indicate that native point defects are unlikely to be the source of conductivity. Instead, attention should be focused on unintentional incorporation of impurities. In particular, hydrogen has a low formation energy in InN and acts as a shallow donor [1]. Hydrogen can also substitute for nitrogen in InN, bonding equally to the four In nearest neighbors in a multicenter-bond configuration (a highly unusual type of chemical bond). Substitutional hydrogen, somewhat counterintuitively, is a double donor. In addition to the bulk conductivity, accumulation of electrons has been almost universally observed on InN surfaces. While donor impurities adsorbed on the surface could of course contribute to this conductivity, we have proposed that the accumulation layers are an intrinsic property of the material that can be attributed to the fact that on polar surfaces occupied surface states are located above the conduction-band minimum (CBM). Fermi-level pinning occurs due to occupied surface states above the CBM, for all In/N ratios, thus explaining the observed electron accumulation [2]. We have found an absence of electron accumulation on nonpolar surfaces of InN at moderate In/N ratios, a prediction that has been experimentally confirmed [3]. For AlN, we have studied the stability and the electronic structure of Al- and O-polar c-plane surfaces, as well as the effects of adsorption of oxygen atoms. Systematic trends and driving forces for oxidation are identified, and the relationship between atomic structure and the presence of surface donor states is clarified. Consequences for devices are discussed.This work was performed in collaboration with A. Janotti, J. L. Lyons, M. Miao, and D. Segev, and was supported in part by ONR and by NSF.[1] A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 92, 032104 (2008).[2] D. Segev and C. G. Van de Walle, EuroPhys. Lett. 76, 306 (2006).[3] C. L. Wu et al., Phys. Rev. Lett. 101, 106803 (2008).
12:00 PM - **I1.7
Two Mg Related Acceptors in GaN.
Bo Monemar 1 , Plamen Paskov 1 , Galia Pozina 1 , Carl Hemmingsson 1 , Peder Bergman 1 , Hiroshi Amano 2 , Isamu Akasaki 2 , Stephan Figge 3 , Detlef Hommel 3
1 , Linkoping University, Linkoping Sweden, 2 , Meijo University, Nagoya Japan, 3 , University of Bremen, Bremen Germany
Show Abstract Mg is the only acceptor that has been successfully applied for p-doping in GaN. While electrical properties (such as Hall data) of Mg-doped p-GaN can be reasonably understood using a model with a single acceptor level with an activation energy varying with doping between 170 and 130 meV clarified. Early studies of photoluminescence (PL) of p-GaN showed that the characteristic shallow donor-acceptor pair (DAP) or free-to bound (FB) emission peaking at 3.27 eV was unstable against annealing above 500 C, while the Mg acceptor responsible for the main electrical hole activation was known to be activated and stable in this temperature range. Several subsequent studies of the 3.27 eV PL concluded that the instability was caused by instable shallow donors, believed to be related to H or to N vacancies. In this work we present strong evidence that the instability instead is connected with the acceptor involved in the 3.27 eV PL, while the shallow donors involved at low temperatures are shown to be stable. It is found that Mg-doping introduces two acceptors in GaN, one of them is unstable in p-GaN.This work includes studies of donor and acceptor bound excitons (DBEs and ABEs) in Mg-doped GaN, and in order to avoid the large strain induced broadening and energy shifts of BE transitions, we have exclusively employed about 1-μm-thick Mg-doped layers grown by MOCVD on strain free thick (200-300 μm) bulk GaN templates. Samples were grown both at Meijo University and Bremen University. The ABE near bandgap PL spectra at lower energies show a dramatic sensitivity to the annealing. The ABE1 peak at 3.466 eV (related to acceptor A1) is observed for the unannealed condition, while the ABE2 peak (related to acceptor A2) at about 3.454 eV is dominant in the annealed condition. Similar observations vs annealing are made for the lower energy DAP spectra, where the 3.27 eV DAP relates to A1 and the broader 3.15 eV peak relates to A2 [1]. A2 appears to have the properties expected for the substitutional Mg acceptor, while A1 is unstable in p-GaN, and should be a complex acceptor, possibly involving Mg and H. The two acceptors have very similar binding energies for the hole. [1]. B Monemar et al, Phys Rev Lett 102, 235501 (2009)
12:30 PM - I1.8
Calculation of Defect Distribution at Interfaces from Ab-initio Based Thermodynamic Data.
Christoph Freysoldt 1 , Bjoern Lange 1 , Joerg Neugebauer 1
1 Computational Materials Design, MPI für Eisenforschung, Düsseldorf Germany
Show AbstractIII-nitride alloys are key materials for optoelectronic devices because the band gap and lattice constant can be tuned over a large range. While the electronic structure parameters of these alloys have been intensively studied, much less is known about point defect and impurity distributions in spatially inhomogeneous structures, for instance near surfaces around precipitates or at interfaces. Technologically important examples are the incorporation of H in p-type GaN, the activation of Mg acceptors due to thermal annealing, or the formation of parasitic phases. A challenge in describing defects in spatially inhomogeneous systems is the large range of relevant length scales, and the fact that concentrations may span several orders of magnitude. Combining thermodynamical concepts with ab-initio formation energies and diffusion barriers, it is possible to cope with this problem in principle. However, equilibrium is reached only after long simulation times, making such calculations exceedingly expensive. We have therefore developed an alternative approach to determine the thermodynamical equilibrium directly. This allows us to treat spatially inhomogeneous systems very efficiently.We apply the formalism to the hydrogen distribution in Mg-doped GaN. During growth, Mg is incorporated as passivated MgH complex. At very high Mg concentrations, Mg3N2 may form in addition as parasitic phase and induce an inversion of the growth polarity. To activate the Mg acceptors, the sample is annealed at elevated temperature, where H+ defects becomes mobile. On the other hand, Mg3N2 binds H+ much stronger than GaN. We demonstrate that the enhanced affinity for H+ produces space charge zones at the inversion domain boundaries and also estimate the amount of hydrogen stored in the boundaries. This provides direct insight into the mechanism preventing a complete activation of the Mg acceptors.
12:45 PM - I1.9
Surface Reconstructions and Magnesium Incorporation on Semipolar GaN(10-1-1) Surfaces.
Toru Akiyama 1 , Disuke Ammi 1 , Tomoki Yamashita 1 , Kohji Nakamura 1 , Tomonori Ito 1
1 , Mie Univsesity, Tsu-shi Japan
Show AbstractWurtzite nitride based optoelectronic devices such as light-emitting diodes and laser diodes have been conventionally fabricated by the crystal growth along the polar [0001] direction. However, this leads to large spontaneous and piezoelectric polarization fields inside multiquantum wells, reducing the radiative recombination rate within the quantum wells. In order to eliminate these polarization effects, the growth of devices on nonpolar and semipolar planes are currently being pursued extensively. Because of numerous nonradiative recombination centers contained in the layers on nonpolar orientations, there is now an increasing interest in semipolar planes. Recently, it has been shown that high hole concentrations are realized in Mg-doped semipolar (10-1-1) GaN in addition to eliminating the effects of polarization-induced electric fields. [1] In particular, Mg-doping in (10-1-1) orientated layers on vicinal (100) MgAl2O4 substrates miscut in the <011> direction is more efficient than that in (0001) oriented layers, although the solubility of Mg for (10-1-1) layers estimated to be lower than that for (0001) GaN. These experimental results can be attributed to the incorporation of a larger percentage of Mg onto electrically active substitutional lattice sites for (10-1-1) oriented GaN. In order to clarify the origin of high hole concentrations in Mg-doped semipolar (10-1-1) GaN, clarifying the stability of Mg-incorporated GaN(10-1-1) surfaces is a vital issue. In this work, we perform first-principles total-energy calculations that clarify the reconstructions of GaN(10-1-1) surfaces and the stability of Mg-incorporated surfaces. The calculated surface formation energy reveals characteristic features of surface reconstructions depending on growth conditions such as temperatures and pressures. [2] The formation energy of Mg-incorporated surfaces also clarified the stability of Mg-incorporated surface in which single Mg atom can substitutes topmost Ga atom of reconstructed 1×2 surface when the surface includes steps along the [10-1-2] direction. This implies that the Mg atom can be incorporated into electrically active substitutional lattice sites during the growth, qualitatively consistent with experimental results. [1] J.F. Kaeding et al.: Appl. Phys. Lett. 89, 202104 (2006). [2] T. Akiyama et al., submitted.
I2: LEDs and Optical Properties I
Session Chairs
Monday PM, November 30, 2009
Independence W (Sheraton)
2:30 PM - **I2.1
Challenges and Opportunities in Solid-state Lighting.
E. Schubert 1 , Jong Kyu Kim 1
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractThe field of photonics, that is, the science and technology of light interacting with semiconductors, is now becoming equally important as electronics [1, 2, 3]. Photonics starts with the efficient generation of light that can be controlled in terms of its optical properties. Light-emitting diodes (LEDs) allow for the generation of efficient and controllable light. The efficiency of white GaInN LEDs can be 20 times greater efficiency than that of incandescent light sources (conventional light bulbs). We will present some of the major challenges in solid-state lighting and new opportunities that are suited to overcome these challenges. The opportunities include new optical thin-film materials with unique properties and new polarization-matched active regions for enhanced carrier transport in GaInN LEDs. [1] E. F. Schubert and J. K. Kim “Solid-state light sources getting smart” Science 308, 1274 (May 2005)[2] E. F., Schubert, J. K. Kim, H. Luo, and J.-Q. Xi “Solid-state lighting – A benevolent technology” Reports on Progress in Physics 69, 3069 (November 2006)[3] Jong Kyu Kim and E. Fred Schubert “Transcending the replacement paradigm of solid-state lighting” Optics Express 16, 21835 (December 2008)
3:00 PM - I2.2
TiO2-based Transparent Conducting Oxide for GaN Light Emitting Diodes.
Taro Hitosugi 1 2 , Junpei Kasai 2 , Miki Moriyama 3 , Koichi Goshonoo 3 , N. Huong 4 , Shoichiro Nakao 2 , Naoomi Yamada 2 , Tetsuya Hasegawa 2 4
1 , Tohoku University, Sendai Japan, 2 , Kanagawa Academy of Science and Technology (KAST), Kawasaki Japan, 3 , Toyoda Gosei Co., LTD., Aichi Japan, 4 , University of Tokyo, Tokyo Japan
Show AbstractAnatase Nb-doped TiO2 transparent conducting oxide has been formed on GaN(0001) surfaces using a sputtering method. Amorphous films deposited at room temperature were annealed at a substrate temperature of 500 degrees C in vacuum to form single-phase anatase films. Films with a thickness of 170 nm exhibited a resistivity of 8x10^-4 Ohm cm with absorptance less than 5% at a wavelength of 460 nm. Furthermore, the refractive index of the Nb-doped TiO2 was well matched to that of GaN. These findings indicate that Nb-doped TiO2 is a promising material for use as transparent electrodes in GaN-based light emitting diodes (LEDs), particularly since reflection at the electrode/GaN boundary can be suppressed, enhancing the external quantum efficiency of blue-LEDs.
3:15 PM - I2.3
Graded-refractive-index Micro-patterns on GaInN Light-emitting Diodes for Enhanced Light Extraction and Control Over the Far Field Emission Pattern.
Ahmed Nayaz Noemaun 1 , Frank Mont 1 , David Poxson 1 , Jong Kyu Kim 1 , E. Fred Schubert 1
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractA significant portion of the light generated by the active region of light-emitting diodes (LEDs) remains confined inside the semiconductor material due to the high refractive index contrast between the semiconductor and the surrounding medium. Efforts to address this fundamental problem have persisted for several decades, and included wet-chemical texturing of the LED chip surface, using 2D photonic crystals, planar graded-refractive-index anti-reflection coatings, patterning of sapphire substrates, and the shaping of LED chips. Wet chemical texturing is known to be very efficient to enhance light extraction of GaInN LEDs, but can be applied only to thin-film LEDs with the N-face GaN surface being exposed. Furthermore, KOH based wet chemical etching needs additional processing steps such as deposition and removal of a protection layer covering the area of the ohmic contact. In comparison, non-random features that are designed and deliberately fabricated on the surface of an LED chip are believed to provide greater flexibility for simultaneously achieving high light-extraction as well as control over the far-field emission pattern.In this work, graded-refractive-index micro-patterns are modeled and demonstrated and are shown to completely eliminate total internal reflection. The thickness and refractive index of each layer in the multi-layer stack is optimized to enhance light extraction. GaInN LEDs with five-layer graded-refractive-index micro-patterns, fabricated by co-sputtering TiO2 and SiO2, show a light-output power enhanced by 73% and a strong side emission, consistent with analytical calculations and ray-tracing simulations. [1] In addition, the multi-layer stack can be further modified to control the far field pattern of the LED. By varying the refractive index and the thickness of each layer and the sidewall tilt angle of micro-patterns, the direction of the maximum emission intensity of the LED, can be controlled. This control over the far field emission has potential application for LEDs, in which the directionality of emission is particularly important, for example, LED backlighting for liquid crystal displays. [1] Jong-Kyu Kim et al., “Elimination of total internal reflection in GaInN light-emitting diodes by graded-refractive-index micropillars”, App. Phys. Lett. 93, 221111 (2008)
3:30 PM - I2.4
Nitride Based Light Emitting Diodes Embedded with a Wire-grid Polarizer.
David Meyaard 1 , Jaehee Cho 1 , Martin Schubert 1 , Sameer Chhajed 1 , Jong Kyu Kim 1 , E. Fred Schubert 1
1 Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractPolarized light is desirable for use in liquid crystal displays (LCDs) and projection display systems, which are heavily used in the consumer electronics industry. By using light emitting diodes (LEDs) that efficiently emit polarized light for the backlighting unit of the display, energy consumption of these displays may be decreased. It has been shown that unpackaged GaInN LEDs have side emission highly polarized in the plane of the multiple quantum wells [1]. By using innovative reflectors and encapsulation shapes, this polarization effect may be utilized to extract light that is mostly polarized in one direction [2, 3]. However, for use in LCDs, the polarization ratio should be as high as possible. One way to achieve a high polarization ratio is by using a wire-grid polarizer (WGP). A WGP consists of an array of conducting lines, all oriented in the same direction. An electric field component (TE) parallel to these lines is reflected, while an electric field component (TM) perpendicular to the wire grid is transmitted. In this work, fabrication of a WGP embedded on a GaN LED chip is described and demonstrated. The dimensions of the WGP designed for use on GaN LEDs emitting at 460 nm are optimized for maximizing polarization ratio between TE and TM modes of light as well as transmitted TM intensity. The LEDs are grown on double side polished sapphire by metal-organic chemical vapor deposition and fabricated with reflective p-contacts. The WGP itself is directly fabricated on the substrate side of the LED. Fabrication of the WGP is accomplished by utilizing electron-beam lithography to pattern an aluminum layer deposited by electron-beam evaporation. Highly polarized light emission can be observed from a light source with a small footprint. Finally, experimental results are compared to theoretical calculations of the intensities for transmitted TE and TM modes of light. [1] M. F. Schubert et al., Appl. Phys. Lett. 91, 051117 (2007). [2] M. F. Schubert et al., Opt. Express 15, 10453 (2007). [3] M. F. Schubert et al., Opt. Express 15, 11213 (2007).
3:45 PM - I2.5
Spatially and Spectrally Resolved Cathodoluminescence of Hexagonal GaN Pyramids Covered by InGaN Single Quantum Well.
Frank Bertram 1 , Sebastian Metzner 1 , Thomas Hempel 1 , Juergen Christen 1 , Michael Jetter 2 , Clemens Waechter 2 , Peter Michler 2
1 , Otto-von-Guericke-University Magdeburg, Magdeburg Germany, 2 , University of Stuttgart, Stuttgart Germany
Show AbstractOne principal problem in the nitrides is the quantum confined Stark effect (QCSE) as a consequence of the strong internal polarization fields in c-direction. The most common strategy to overcome the QCSE-problem is the growth of heterostructures in other directions than the c-axis. One approach to reduce the polarization fields is the growth in semi polar directions. However, the epitaxial growth on such planes is by far less developed than the growth on the commonly used c-plane. Moreover, in ternary and quaternary alloys and their heterostructures, nano scale fluctuations of stoichiometry as well as interfaces have strong impact on the radiative recombination in light emitters. In this study we correlate the optical properties of hexagonal GaN pyramids overgrown by a InGaN single quantum well with the crystalline real structure using highly spatially, spectrally, and time resolved cathodoluminescence microscopy (CL). The selective epitaxial growth of GaN was performed with MOVPE. First, a 1 µm GaN buffer layer was deposited on sapphire using an AlN nucleation layer. Subsequently, a 60 nm thick sputtered SiO2 mask was patterned by photolithography to form arrays of hexagonal windows. GaN was selectively grown on top of this mask creating periodic arrays of hexagonally shaped pyramids. The self assembled pyramids exhibit perfectly formed semi polar {1011} facets. No GaN was grown on the SiO2 mask. The pyramids were covered by a nominal 6 nm thick In0.18Ga0.82N quantum well followed by a 30 nm thick GaN cap layer.The luminescence shows a characteristic lateral distribution: While an intense GaN luminescence can be obtained from both, the pyramids as well as the buffer layer, the InGaN luminescence is exclusively emitted from the pyramids. The most intense InGaN SQW luminescence originates from the base and the upper part of the pyramids. A striation-like contrast is observed at the pyramids’ bases. Here, the stripes of high and low intensity directly correlate with the local emission wavelength: lower intensities are associated with shorter wavelengths, resulting in an average wavelength of 550 nm. In the upper part of the pyramids two distinctly different wavelengths are emitted: while 590 nm dominates at the edges and at the very top of the pyramids, the center of the facets is dominated by 530 nm emission. These results directly visualize a higher indium incorporation and/or a thicker quantum well at the edges and tops of the pyramids, i.e. the self organized formation of quantum wires at the edges and quantum dots at the tips of the pyramids.
4:30 PM - **I2.6
On the Light Emission in GaN Based Heterostructures at High Injection,
Hadis Morkoc 1 , Umit Ozgur 1 , Huiyong Liu 1 , Xing Li 1 , Xianfeng Ni 1
1 ECE and Physics, Virginia Commonwealth University, Richmond , Virginia, United States
Show AbstractRadiative recombination efficiency in nitride-based light emitting diodes (LEDs) degrades at high injection current levels beyond that which is expected from thermal effects, an undesirable attribute which has been dubbed the “efficiency droop”. It is imperative to overcome this shortcoming in order for LEDs to produce high luminous flux with reasonably high efficiencies at high current densities for use in lighting. Various models for this efficiency droop have been proposed, including carrier spillover, limited carrier injection efficiency, polarization field, Auger recombination, etc. Auger recombination is an intrinsic process and can only be reduced by increased active region thickness providing that the carrier density is kept sufficiently low. The absence of efficiency droop in photo excitation experiments where carriers are generated only in the quantum wells (QWs) with generation rates much higher than high electrical injection levels indicates that efficiency droop is most likely related to the skewed carrier injection, relatively large hole mass, transport, current crowding, and carrier spillover processes, etc. In this presentation arguments in favor and against various proposals backed with experiments will be provided. Furthermore, design philosophies along with experimental data for reduced efficiency degradation at high injection levels will be discussed.
5:00 PM - I2.7
The Origin of the High Diode-ideality Factors in GaInN/GaN Multiple Quantum Well Light-emitting Diodes.
Di Zhu 1 2 , Jiuru Xu 1 2 , Ahmed Noemaun 1 3 , Jong-Kyu Kim 1 3 , E. Fred Schubert 1 3 , Mary Crawford 4 , Daniel Koleske 4
1 Future Chips Constellation, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 4 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractIn recent years, great progress has been made in developing high-efficiency, high-power GaN-based light-emitting diodes (LEDs) for use in general illumination systems, display backlighting system, and headlamps for automobiles. One of the key parameters to characterize LEDs is the diode-ideality factor, which is directly related to carrier transport and carrier recombination. A high diode-ideality factor generally results in high device differential resistance and forward voltage, and thus limits the power efficiency. It should be noted, however, that III-N-based p-n junctions with multiple quantum wells (MQWs) show abnormally high diode-ideality factors (nideality ≈ 5~7). This phenomenon has been a scientific “puzzle” for more than a decade, and the physical origin of the high diode-ideality factor is still under discussion.In this work, four GaInN/GaN MQW LEDs, each comprising five GaInN QWs, are grown and studied. The four LED heterostructures are distinguished by the number of quantum barriers (QBs) that are intentionally Si-doped.[1] In four different samples, we employ doping in 1, 2, 3, and 4 of the QBs. We find a significant decrease in the diode-ideality factor of GaInN/GaN LEDs, from 5.5 to 2.4, as Si-doping is applied to an increasing number of QBs. The minimum ideality factor of 2.4 is obtained when all QBs are doped. It is also shown that the forward voltage at 20 mA decreases as the ideality factor decreases, indicating that lower ideality factor generally results in a reduced forward voltage.Due to the spontaneous and piezoelectric polarization effects in III-V nitrides, the QBs, for the undoped case, have a triangular shape that the electrons need to overcome. Each triangular-shaped barrier is reminiscent of a Schottky barrier. Here we propose that the undoped QBs indeed act similar to Schottky barriers. Since there are several undoped QBs in series, each having an associated ideality factor, the summation of ideality factors from each undoped QB contributes to the measured high ideality factor in GaInN/GaN LED samples with undoped QBs.Numerical simulations, using APSYS modeling software, are performed to obtain the band diagrams and current-voltage characteristics of the LED structures. The simulated diode-ideality factors of the LEDs are in excellent agreement with the measured ideality factors and their dependence on QB doping. The simulation results also verify that the band profiles of QBs in the active region have a quasi-triangular shape and have a significant impact on the carrier transport mechanism, showing that unipolar heterojunctions inside the active region play an important role in determining the diode-ideality factor.* Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.[1] Di Zhu et al., App. Phys. Lett. 94, 081113 (2009)
5:15 PM - I2.8
Reduced Efficiency Droop in High-power InGaN-based Vertical-structure LEDs using External Stress Modulation.
Jun Ho Son 1 , Jong-Lam Lee 1
1 Materials Science and Engineering, POSTECH, Pohang Korea (the Republic of)
Show AbstractAlthough significant progress in InGaN-based light-emitting diodes (LEDs) has been made, higher light output and higher efficiency are essential for the general illumination applications. The phenomenon known as ‘efficiency droop’ that the efficiency of LEDs decreases with increase of injection current is a major obstacle for high-power and high-efficiency LEDs. Several mechanisms have been suggested such as Auger recombination, electron leakage, carrier delocalization, and lack of hole injection, but the origins of efficiency droop remain controversial. Therefore, to overcome the efficiency droop is a key aspect for solid-state lightings. In this work, we significantly reduced the efficiency droop of vertical-structure LEDs (V-LEDs) using external stress modulation and the possible mechanism of efficiency droop is proposed based on experimental results. The InGaN/GaN multi-quantum-wells (MQWs) LED structures were grown by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrate. V-LEDs with 1 mm × 1 mm in size are fabricated using laser lift-off (LLO) technique. The thick Ni metal substrate for laser lift-off (LLO) process was formed by electroplating and LLO process was performed using a 248 nm KrF pulsed excimer laser (38 ns pulse width). After LLO process, the curvature of LED wafer was changed from convex to concave due to the external stress formed by the Ni metal substrate. Furthermore, the concave curvature of V-LEDs varies due to the thickness variation of Ni metal substrate, resulting in the changes of external stress. The light output increased and the efficiency droop is significantly reduced as the concave curvature increase. The peak wavelength of V-LEDs is blue-shifted as the concave curvature increase, indicating the changes in band bending of MQWs. Furthermore, as the injection current is increased, the shift of peak wavelength is not shown, because the Quantum Confined Stark Effects is reduced due to the band bending in MQWs by external stress. Synchrotron radiation photoemission spectroscopy (SRPES) was performed to investigate the change of binding energy in GaN surface of V-LEDs. Based on the experimental and simulation results, we propose the mechanism of reduced efficiency droop in stress-modulated V-LEDs.
5:30 PM - I2.9
High Performance of GaN Thin Films Deposited on Sapphire Substrates Coated with a Silica-submicron-sphere Monolayer.
Nobuhiro Hagura 1 , Ferry Iskandar 1 , Kikuo Okuyama 1
1 Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima Japan
Show Abstract Monolayers of submicron size silica (SiO2) particles (550 and 300 nm) were rapidly deposited on sapphire substrate (Ø 2 in.) by a spin coating method. The processing time to prepare the monolayer film was only 25 s, a very short time compared with previously reported methods used for assembling particles in monolayers. The concentration of Silica particles in the solution, the ambient humidity (relative humidity, RH) and the spin speed were all important parameters in achieving a large area monolayer film. A relatively high surface coverage and uniform monolayer film of Silica particles in the range of 60–81% from the center to the edge of the substrate (or the average is around 72%) was achieved by appropriate control of the above preparation parameters. We conclude that this method could be used in industrial applications, because of the speed and cost of the process. A high-performance, GaN-based light emitting diode (LED) was prepared using a metal organic chemical vapor deposition method on a silica-sphere, monolayer-coated sapphire substrate. Various surface coverage ratios of the silica submicron spheres with diameters ranging from 300 to 550 nm were deposited on the sapphire substrate using a spin-coating method. The LED output power was increased 2.5-fold compared with the LED constructed without silica spheres and uniform light distribution was achieved. In addition, LED output power was dependent on silica-sphere size and surface coverage of the substrates.
5:45 PM - I2.10
Self-segregated In Clusters Observed in High Efficient InGaN-based Blue LEDs by using Cs-corrected STEM and 3D Atom Probe Tomography.
Gil Ho Gu 1 , Chan Gyung Park 1 2
1 Materials Sciecne and Engineering, POSTECH, Pohang, Gyungbuk, Korea (the Republic of), 2 National Center for Nanomaterials Technology, POSTECH, Pohang, Gyungbuk, Korea (the Republic of)
Show AbstractThe InGaN-based multiple quantum well (MQW) structure is used as an active layer in commercial optoelectonic devices - blue and green light emitting diodes (LED) and blue-violet laser diodes. The high quantum efficiency observed in these devices can be related with the In(indium)-rich regions within the well, which can act as quantum dots. It has been reported that the In-rich region can be the major cause of the high efficiency even when large densities of threading dislocations propagate into active regions. In order to understand the exact role of In-rich quantum dots on the device performance, the precise information on the composition, morphology and size of these In-rich regions is prerequisite. High resolution electron microscopy (HREM) has been believed to be a direct tool for detecting In–rich clusters. However, the formation of In clusters within InGaN quantum wells was demonstrated under high voltage electron beam with long exposure times by Humphreys. This suggests that In clusters are artifacts resulted from electron beam induced damage. On the contrary to this argument, others have also proposed that artifacts can be negligible, and thus, the initial In distribution in the InGaN layers is very close to that determined from the HREM images, taken with fast exposure of electron beam. As an alternative method to determine the In composition in InGaN well layers without electron beam damage, three-dimensional atom probe tomography (3D-APT) is suggested. In addition, STEM-HAADF study can be better than HREM study in identifying the In clusters because of strong contrast dependence on atomic number. In the present study, thus, we have tried to investigate the inhomogeneity of InGaN-MQW by using STEM-HADADF and 3D-APT.The average mole-fraction of In within InGaN-MQW, by 3D-APT, was found to be about 18 %, which was quite consistent with data obtained by secondary ion mass spectrometry (SIMS). The In composition within InGaN-MQW layer, analyzed by using the iso-curve mapping techniques of 3D-APT, revealed discontinuous InGaN active layers. The In clusters or In-rich region was in the range of 2-3 nm in size along the active layer, which was quite consistent with the STEM results. The results clearly indicated less pronounced electron beam damage in InGaN layers as described previous reports. Our present results, thus, strongly support the intrinsic In-clustering in MQW, which can be essential for the high brightness InGaN based LEDs. We have also observed the discontinuous active layer of 50-100 nm in length as well as In clusters of 2-3 nm in size in InGaN active layers.
Symposium Organizers
Shangjr (Felix) Gwo National Tsing-Hua University
Joel W. Ager Lawrence Berkeley National Laboratory
Fan Ren University of Florida
Oliver Ambacher Fraunhofer-Institut für Angewandte Festkörperphysik (IAF)
Leo Schowalter Crystal IS Inc.
I3: Surface and Interface Properties I
Session Chairs
Tuesday AM, December 01, 2009
Independence W (Sheraton)
9:30 AM - **I3.1
Interface, Bulk and Surface Electronic Properties of InN.
Philip King 1 , Tim Veal 1 , Chris McConville 1
1 Department of Physics, University of Warwick, Coventry United Kingdom
Show AbstractA three-region model of the high n-type conductivity in InN will be presented, including contributions from the bulk, interface with the buffer layer and surface of the InN films. In particular, a parallel conduction analysis, incorporating dislocation and ionized impurity scattering and the differing surface and bulk mobilities, can account for the variation of both the Hall effect-measured electron concentration and the mobility with film thickness. For a set of In-polarity InN samples grown on GaN buffer layers under the same conditions (V/III ratio and temperature), as the film thickness is varied from 200 to 12000 nm, the electron density changes from 2 × 10E19 cm-3 to 3 × 10E17 cm-3 and the mobility from 300 cm2V-1s-1 to 2000 cm2V-1s-1 [1,2]. Similar results have also been observed for InN films grown on AlN buffer layers [3]. The findings of additional studies of the effects of the V/III ratio during growth on the interface-related electron density will also be presented [4]. Finally, our results will be placed in the context of recent data from other groups and the origins of the interface, surface and bulk conductivity will be discussed.[1] L. F. J. Piper, T. D. Veal, C. F. McConville, H. Lu, and W. J. Schaff, Appl. Phys. Lett. 88 (2006) 252109.[2] P. D. C. King, T. D. Veal, C. F. McConville, J. Phys.: Condens. Matter 21 (2009) 174201.[3] V. Cimalla, V. Lebedev, F. M. Morales, R. Goldhahn, and O. Ambacher, Appl. Phys. Lett. 89 (2006) 172109.[4] P. D. C. King, T. D. Veal, C. S. Gallinat, G. Koblmüller, L. R. Bailey, J. S. Speck, and C. F. McConville, J. Appl. Phys. 104 (2008) 103703.Corresponding author:
[email protected]10:00 AM - I3.2
In-situ Control of the Carrier Density Profile of MBE-Grown Hexagonal Indium Nitride.
Katharina Kloeckner 1 , Marcel Himmerlich 1 , Roland Koch 1 , Vladimir Polyakov 2 , Anja Eisenhardt 1 , Thomas Haensel 1 , S. Imad-Uddin Ahmed 1 , Stefan Krischok 1 , Juergen Schaefer 1 3
1 Institut für Physik and Institut für Mikro- und Nanotechnologien, TU-Ilmenau, Ilmenau Germany, 2 , Fraunhofer Institute for Applied Solid State Physics, Freiburg Germany, 3 Department of Physics, Montana State University, Bozeman, Montana, United States
Show AbstractIndium nitride is a promising candidate material for high speed and high frequency electronic devices due to its favorable electron transport properties and as small effective mass, high mobility and high drift velocity. In this respect phonon lifetime, electron-phonon interaction as well as phonon-plasmon coupling, hot carrier thermalization and carrier recombination are relevant issues that need to be thoroughly understood. The latter properties, in particular, strongly influence the static and dynamic transport properties of InN. One important aspect for understanding the performance of InN based electronic devices is the electron accumulation at the surface of the material. Up to now, the electron distribution as well as vibrational properties of InN, determined by inelastic electron scattering are reported for different InN samples grown either by molecular beam epitaxy (MBE) or by plasma enhanced chemical vapor deposition (PECVD).In this paper we report upon new experiments involving high resolution electron energy loss spectroscopy (HREELS) that are interpreted using dielectric theory in order to get new insights into the formation of the InN surface and interface grown by plasma assisted molecular beam epitaxy (PAMBE). We are able to model our HREELS spectra, where the carrier concentration goes far beyond the accumulation layer regime. Using a very wide primary beam energy regime from 1eV up to 200eV, the depth of information can be varied from extremely surface sensitive to bulk sensitive. Due to the phonon-plasmon coupling two modes ω- and ω+ appear besides the surface optical phonons, the so-called Fuchs Kliewer phonons, which are analyzed and discussed. We use a model which has been successfully applied to in-situ HREELS data for homogeneous and δ-doped GaAs(001) films grown by MBE. To simulate the measured HREELS-spectra, knowledge about the electron density distribution versus depth beneath the surface is required. For this purpose the Schrödinger and Poisson equations are solved self-consistently to obtain the profile of the electron density as a function of depth underneath the surface, which is treated as input data to further simulate the energy-loss spectra. Then, the depth profile of the electron density is discretized into a histogram of a finite number of sublayers in order to derive the effective dielectric function. Given the effective dielectric function, the energy loss probability is determined according to dipole scattering theory, which is directly compared with our measured HREELS-spectra.The advantage of our approach lies in circumventing any surface cleaning procedure, as our samples are measured directly after growth and transfer under vacuum conditions into the HREELS-spectrometer. There, only trace amounts of hydrocarbons and contaminations from dissociated water from the residual gas with HREELS-accumulation time are detected. Further details will be discussed at the conference.
10:15 AM - I3.3
Angle-resolved Photoemission Spectroscopy of Quantized Electron Accumulation at N-polar InN Surfaces: Effects of Surface Preparation and Alkali-metal Deposition.
Leyla Colakerol 1 , Louis Piper 1 , Alexei Fedorov 2 , Papo Chen 3 , Theodore Moustakas 3 , Kevin Smith 1
1 Department of Physics, Boston University, Boston , Massachusetts, United States, 2 Advanced Light Source, Lawrence Berkeley National Laboratory,, Berkeley, California, United States, 3 Department of Electrical and Computer Engineering, Boston University, Boston, New York, United States
Show AbstractWe present angle-resolved photoemission spectroscopy (ARPES) measurements of the intrinsic quantized electron accumulation layer subbands at both clean and potassium-deposited InN surfaces. We previously reported the ability of ARPES to investigate such states,[1] and their influence on the band structure.[2] Here we highlight the effects of electronic damage (in the form of surface preparation) and charge-transfer, and discuss their roles in the formation of quantized electron accumulation layers. We show how InN surfaces are highly susceptible to defect-induced near-surface carrier profiles even using mild ion bombardment and low temperature (~500 C) annealing cycles, in contrast to surfaces prepared by low temperature annealing alone. This is clearly shown by our observation of up to 5 subbands within the near-surface region of InN surfaces prepared by sputter and anneal cycles. This result is consistent with high-resolution electron energy loss spectra of clean InN surfaces prepared by N ion sputtering and annealing cycles.[3] We go further by reporting extreme downward band bending (few hundreds of meV) following thin (<~ 1 ML coverage) alkali-metal deposition due to charge-transfer between the highly electronegative K atoms and the semiconductor. These results are compared with alkali-deposited InAs,[4] in order to provide experimental insight into the microscopic origin of the surface states responsible for the electron accumulation at InN surfaces.[1] L. Colakerol, T. D. Veal, H.-K. Jeong, L. Plucinski, A. DeMasi, T. Learmonth, P. Glans, S. Wang, Y. Zhang, L. F. J. Piper, P. H. Jefferson, A. Fedorov, T.-C. Chen, T. D. Moustakas, C. F. McConville, and K. E. Smith Phys. Rev. Lett. 97, 237601 (2006).[2] L. Colakerol, L. F. J. Piper, T.-C. Chen, T. D. Moustakas and K. E. Smith. Europhys. Lett. 83, 47003 (2008).[3] L. F. J. Piper, T. D. Veal, C. F. McConville, Phys. Stat. Sol. (c), 3, 1841 (2006).[4] M. G. Betti, V. Corradini, G. Bertoni, P. Casarini, C. Mariani, and A. Abrama Phys. Rev. B 63, 155315 (2001)
10:30 AM - I3.4
Influence of Defects, Dopants, and Surface Orientation on Free Carrier Properties of InN.
V. Darakchieva 1 2 , M. Schubert 3 , E. Alves 1 , K. Lorenz 1 , M. Xie 2 , Tino Hofmann 3 , W. Schaff 4 , L. Chen 5 , L. Tu 6 , Y. Nanishi 7
1 , Instituto Tecnológico e Nuclear, Sacavém Portugal, 2 , Linköping University, Linköping Sweden, 3 , University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 4 , Cornell University, Ithaca, New York, United States, 5 , National Taiwan University, Taipei Taiwan, 6 , National San Yat-Sen University, Kaohsiung Taiwan, 7 , Ritsumeikan University, Shiga Japan
Show AbstractWe provide new insight into the free charge carrier properties and doping mechanisms in InN using novel non invasive optical Hall effect method [1] and generalized infrared spectroscopic ellispometry (GIRSE) [2] in combination with ion beam analysis techniques. We studied a large number of state-of-the-art InN films with different surface orientations and different bulk free carrier concentrations grown by molecular beam epitaxy (MBE) at four growth laboratories. The optical Hall effect and GIRSE allows decoupling of interface from bulk free charge carrier properties. We thereby obtain independently information from the bulk and from the surface free carrier densities in the InN films [3]. We find a distinct electron accumulation to occur at the polar, nonpolar and semipolar surfaces of n-type conductive InN films. We obtain indications for the variability of the surface Fermi level to a certain extent, and we reveal the existence of a new doping mechanism [3]. Comparison with structural analysis allows for new conclusions about the important role of impurities for the unintentional bulk conductivity in InN. We will discuss in detail the impurity distributions and behaviors with respect to surface orientation, defect structure and surface morphology of the films and in relation to their free charge carrier properties. Furthermore, the surface sheet charge densities and surface electron mobilities of InN films with different surface orientations and different dopant, and impurity levels will be discussed. Finally, future research strategies and pathways to control the doping mechanisms in InN and related alloys will be suggested.[1] T. Hofmann et al., J. Electron. Mater. 37, 611 (2008).[2] V. Darakchieva et al., Phys. Status Solidi (a) 205, 905 (2008).[3] V. Darakchieva et al., Appl. Phys. Lett. 94, 022109 (2009).
10:45 AM - I3.5
Raman Scattering by LO-phonon-plasmon Coupled Modes in InN Epilayers: Dependence on the Excitation Laser Intensity and Wavelength.
Ramon Cusco 1 , Jordi Ibanez 1 , Esther Alarcon-Llado 1 , Tomohiro Yamaguchi 2 , Yasushi Nanishi 2 , Luis Artus 1
1 , Inst. Jaume Almera (C.S.I.C.), Barcelona Spain, 2 , Faculty of Science and Engineering,Ritsumeikan University, Shiga Japan
Show AbstractInN is attracting much interest because of its remarkable material properties: narrow band gap, small effective mass, high electron mobility and high drift velocity. InN is also unique among III-V semiconductors in that it exhibits an extreme accumulation layer at the surface. The presence of a conducting sheet of electrons at the surface complicates the analysis of transport measurements and the evaluation of the bulk electron density. Raman scattering by long-wavelength LO-phonon-plasmon coupled modes (LOPCMs) can be used to probe optically the bulk electron density in the layers. However, the LOPCM spectrum in InN is not as obvious as in other well-known III-V semiconductors. Wave-vector nonconserving mechanisms have been proposed to explain some features of the Raman spectra of n-type InN. Such mechanisms yield coupled mode peaks whose frequencies are rather insensitive to changes in electron density or excitation wave vector. Here we present a Raman scattering study on a set of high-quality plasma-assisted MBE grown InN epilayers spanning one order of magnitude in background electron density, from ≈2E18 up to ≈2E19 cm-3. We observe an L- coupled mode peak whose frequency is below that of the A1(TO) mode. The L- mode shifts to higher frequencies and sharpens up as the electron density in the layers increases. The observed L- behavior is consistent with scattering by long-wavelength LOPCMs. Raman spectra obtained with near-IR excitation display the expected downward frequency shift of the L- peak corresponding to the lower LOPCM wave vector. Raman line shape calculations were performed by using a dielectric model based on the Lindhard-Mermin model assuming wave vector conservation. Bulk electron densities were extracted from fits to the Raman spectra. A reasonably good agreement with Hall data is found. An independent means of modulating the electron density in the InN layers is given by above-band-gap photoexcitation at high photon flux densities. Provided that the surface recombination velocity is not too high, a spatially homogeneous population of electron-hole pairs can be sustained by continuous wave laser excitation. To achieve sufficiently high photoexcitation levels, we carried out micro-Raman experiments in which the photon flux density was varied over one order of magnitude. The L- peak shifts to higher frequencies with increasing excitation power. The density of photoexcited carriers was extracted from line shape fits to the Raman spectra, and it shows a roughly linear increase with excitation power. All the reported measurements confirm the assignment of the observed L- peak to long-wavelength LOPCM scattering. The frequency of the L- peak is sensitive enough to variations of electron density to allow us to extract the bulk electron density of the InN layers from line-shape fits to the Raman spectra.
11:30 AM - **I3.6
InN and its Native Oxide In2O3: Electronic and Optical Properties from First Principles.
Friedhelm Bechstedt 1
1 IFTO , Friedrich-Schiller-University Jena, Jena Germany
Show AbstractWhile the indium oxide In2O3 hase been used for decades as transparent conducting electrode in optoelectronic and photovoltaic devices, indium nitride InN has recently been rediscovered as an material for optoelectronics and electronic applications after the observation of the fundamental gap of about 0.7 eV [1]. The possible lattice-match of the two indium compounds would also open the way to metal-oxide-semiconductor hetero-structures. Therefore, electronic single- and two-particle excitations and their influence on electron, optical and X-ray spectra are studied and discussed in the light of recent experiments for InN polytypes and pure or doped In2O3 polymorphs. The studies are based on parameter-free, i.e. ab-initio calculations. They start from the iterative solution of the quasiparticle equation with an exchange-correlation (XC) potential resembling the self-energy in Hedins GW approximation. More precisely, the starting point is a spatially non-local hybrid XC-potential which accounts for screened exchange [2]. The final quasiparticle band-structures are obtained by G0W0 corrections of the starting-point band-structures, using a fully frequency-dependent screening in the calculation of the screened Coulomb potential W. The In4d electrons are taken into account as valence-states. The excitonic effects in optical spectra are described by the statically screened electron-hole attraction and the unscreened electron-hole exchange. A novel computational method [3] posses an accuracy in order to predict the binding energies of Wannier-Mott-like excitons. Results are presented for the fundamental energy gaps. The so-called indirect gap of In2O3 is traced back to inter-conduction band transitions in the presence of free carriers. The density of states and their site- and orbital-pro jections allow to describe the lineshape of photoelectron and X-ray absorption or emission spectra [4, 5]. Extremely small exciton binding energies are predicted for InN. Band discontinuities between the two materials are derived from alignments of the vacuum levels as well as the branch-point [6]. [1] V. Davydov, et al., phys. stat. sol. (b) 229(3), R1 (2002). [2] F. Bechstedt, F. Fuchs, and G. Kresse, phys. stat. sol. (b) (2009) (to be published). [3] F. Fuchs, C. Rödl, A. Schleife, and F. Bechstedt, Phys. Rev. B 78(8), 085103, (2008). [4] L. F. J. Piper, et al., Phys. Rev. B 76(24), 245204, (2007).
12:00 PM - **I3.7
Origin of High Indium Incorporation on the Semipolar InGaN(11-22) Surface.
John Northrup 1
1 , Palo Alto Research Center, Palo Alto, California, United States
Show AbstractSemipolar surfaces such as the (11-22) surface are currently under investigation as substrates for growth of InGaN optoelectronic devices emitting in long wavelength regions of the visible spectrum (λ > 500 nm.). Fabrication of such devices requires controlled incorporation of high concentrations of indium. I will present arguments explaining why indium incorporation on semipolar surfaces such as (11-22) is expected to be greater than on c-plane and m-plane surfaces. The arguments are based on an extensive set of first-principles calculations. These calculations show that there is a repulsive interaction between incorporated indium atoms that is significantly reduced on the (11-22) surface in comparison to the m-plane and c-plane surfaces, and that it is relatively easier for indium to win the competition with Ga to occupy surface sites on the (11-22) surface.
12:30 PM - I3.8
First-principles Study of Bare and Oxidized AlN and GaN Polar and Non-polar Surfaces.
Maosheng Miao 1 , Anderson Janotti 1 , Chris Van de Walle 1
1 Materials Department, University of California, Santa Barbara, California, United States
Show AbstractGaN, AlN and their alloys have attracted great interest for more than a decade due to their promising applications in light emitting diodes and high-power and high-frequency electronic devices. Future improvements in the performance of AlGaN/GaN devices, including the pursuit of normally-off HEMTs, are hindered by a lack of understanding of the structural and electronic properties of the surface. This is in part due to the fact that the nitride surfaces can exist in various reconstructions under different physical and chemical conditions. Furthermore, when exposed to air, nitride surfaces will not preserve the surface reconstructions corresponding to bare surfaces, but will tend to oxidize. First-principles calculations can shed light on these issues, but the band-gap underestimation inherent in density functional theory leads to large uncertainties in the position of surface states. In this work, we apply the Perdew-Burke-Ernzerhof (PBE) functional to optimize the surface structures, combined with the screened hybrid functional approach of Heyd-Scuseria-Ernzerhof (HSE) [1] to investigate the nature and position of the surface states. We find that for AlN clean surfaces, Al dangling-bond (DB) states tend to be close to the conduction-band minimum (CBM) and N DB states close to the valence-band maximum (VBM). Al-N bonding states also occur near the VBM, while Al-Al bonding states occur in the middle of the gap. The Al (Ga) DB states are high in energy and tend to be unoccupied. The lack of a surface donor states in the upper part of the gap suggests that the surface states on clean AlGaN surfaces are unlikely to be the source of carriers in the two-dimensional electron gas (2DEG) in AlGaN/GaN HEMTs. On the other hand, we demonstrate that oxidized AlGaN (0001) surfaces result in surface donor states in the upper part of the gap, consisting of partially filled Al or Ga DB states. We find that the density and energy distribution of the surface donor state depends on the composition of the oxidized surface layer, explaining many of the experimental observations [2,3]. Acknowledgement: This work was supported by ONR under award number N00014-08-1-0095. It made use of NSF-funded TeraGrid resources under grant No. DMR070072N. We thank G. Kresse for providing an unreleased version of the VASP code. [1] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003). [2] J. P. Ibbetson, P. T. Fini, K. D. Ness, S. P. DenBaars, J. S. Speck, and U. K. Mishra, Appl. Phys. Lett. 77, 250 (2000). [3] G. Koley and M. G. Spencer, Appl. Phys. Lett. 86, 042107 (2005).
12:45 PM - I3.9
Defect Levels from Partial Dislocations and Stacking Faults in GaN.
Iskander Batyrev 1 , T. Zheleva 1 , K. Jones 1
1 , US Army Rsearch Laboratory, Adelphi, Maryland, United States
Show AbstractWe present results of first principles calculations of partial dislocation and stacking faults in GaN. The density functional theory and supercell-cluster hybrid approach were used to model the dislocations’ core structure and intrinsic stacking faults. We considered dissociation of a perfect 60° dislocation in wurtzite into a pair of Shockley partials (SP): 30° glide and 90° glide (single and double period) with both Ga and N cores. Formation of the partials results in a variety of defect levels in a band gap of GaN. The deepest defect levels (~1.1 eV from CBM) are related to dangling bonds in the dislocation core. We also calculated the segregation of the n- dopant, Si or O, and the p- dopant, Mg, to the dislocation core. All of the dopants have an energy gain (0.2-0.4 eV) after segregation to the dislocation core of a SP 30° glide. The intrinsic stacking fault (…AaBbCcAaCc…) with two violations of the wurtzite stacking sequence (…AaBbAaBb…) was considered between the Shockley partial dislocations. In the system of SP 30° glide – intrinsic stacking fault - SP 90° glide (double period) we found a slight narrowing of the band gap (0.03 eV) in addition to the formation of defect levels in the band gap. The results of the calculations are compared with the experimental evidence of stacking faults in GaN obtained from high resolution transmission electron microscopy.
I4: Surface and Interface Properties II
Session Chairs
Tuesday PM, December 01, 2009
Independence W (Sheraton)
2:30 PM - I4.1
Manipulation of Surface Charge on n-type GaN.
Michael Foussekis 1 , Josephus Ferguson 1 , James C. Moore 2 , Michael A. Reshchikov 1 , Alison Baski 1
1 Physics, Virginia Commonwealth Univ., Richmond, Virginia, United States, 2 Chemistry and Physics, Longwood University, Farmville, Virginia, United States
Show AbstractThe presence of surface charge can be an important factor for the proper functioning of GaN devices. Negative charge on n-type GaN is known to cause an upward band bending of up to 1.5 eV. This band bending can be reduced by 0.3 to 0.9 eV via the surface photovoltage (SPV) effect, where UV illumination causes photogenerated holes to reach the surface. We have determined the SPV values for several n-type GaN samples by measuring the surface contact potential using a Kelvin probe mounted in an optical cryostat, as well as an ambient scanning Kelvin probe microscope (SKPM). Specifically, we explore how the SPV behavior can be influenced by the effect of prolonged UV exposure in air ambient, as well as local charge injection. The SPV signal for a new sample is typically 0.5 to 0.6 eV after initial UV illumination (365nm, 0.03 W/cm2), and can decrease by up to 0.3 eV over an hour exposure. After ceasing illumination, the measured surface contact potential then gradually restores to its baseline over hours. This behavior indicates the presence of extra accumulated negative surface charge during illumination, which we have determined arises from the photo-induced adsorption of negatively charged oxygen species. We have recently observed an "aging" effect on samples exposed to UV illumination for hours under air ambient conditions. In this case, the initial SPV value upon illumination is slightly higher (~0.1 eV) than for the new sample and remains relatively constant during illumination. This behavior is consistent with the presence of a thicker surface oxide (presumably grown during prior UV exposure) that prevents bulk electrons from reaching the surface. Experiments are in progress to determine an efficient method for restoring the initial surface conditions. In addition to prolonged UV exposure, the surface contact potential and SPV behavior can also be changed using local charge injection. In this procedure, we inject electrons onto the surface using a grounded, metallized atomic force microscope tip scanned in contact mode with the sample held at +10V. Subsequent SKPM measurements indicate a decrease in the surface contact potential of 0.5 to 3 eV for the charged region. When the sample is then illuminated with UV light, the surface potentials in both the charged and un-charged regions equalize and increase to a value ~0.5 eV above the dark value. The SPV value for the charged region, which initially had a lower potential, is therefore correspondingly higher. This technique can be utilized as a method to quickly restore negative surface charge after UV illumination, which typically takes hours under dark conditions.
2:45 PM - I4.2
Photocatalytic Cleavage of Self-assembled Organic Monolayers on GaN Surfaces.
J. Howgate 1 , S. Schoell 1 , Ian Sharp 1 , M. Hoeb 1 , W. Steins 1 , B. Baur 1 , M. Stutzmann 1 , M. Eickhoff 2
1 , Walter Schottky Institut, Technische Universität München, Garching Germany, 2 , I. Physikalisches Institut, Justus-Liebig-Universität, Giessen Germany
Show AbstractWide-bandgap semiconductors such as gallium nitride (GaN) and silicon carbide (SiC) exhibit excellent characteristics for bio-electronic applications; they are naturally biocompatible, allow band-gap engineering, and already have been employed for a variety of chemical and biological sensors. Thus far, these semiconductors have largely been used as passive bio-electronic elements. However, many organic systems possess energetic levels whose occupations can be altered by direct electronic charge transfer to or from a semiconductor substrate and may be exploited for use in the emerging field of molecular and bio-molecular electronics. Here, we study the impact of illumination on n- and p-type GaN and SiC with covalently bound self-assembled monolayers formed from octadecyltrimethoxysilane (ODTMS) and 1-octadecene. We demonstrate the occurrence of low energy photo-induced charge transfer ionization of the alkyl chains well below the energy normally required for molecular cleavage. By exploiting the large energetic window spanned by the valence and conduction bands of n- and p-type GaN and SiC, we determine the alignment required to accommodate such a transfer and assess the stability of SAMs in the presence and absence of such photocatalytic processes. Using a range of chemical, structural, and electrical characterization techniques, we show that photocatalytic cleavage of surface-bound aliphatic chains occurs for the case of n-type GaN, but not for p-type GaN or for n- or p-type SiC. An interesting application of such a photo-induced charge transfer process leading to the cleavage of organic molecules at the semiconductor surface is the programmable release of covalently grafted enzymes from GaN surfaces under illumination.
3:00 PM - I4.3
Photoelectron Spectroscopy and X-ray Diffraction Investigation of Ultrathin InN/AlN Heterojunction.
Cheng-Tai Kuo 1 , Hong-Mao Li 1 , Shangjr Gwo 1
1 Physics, National Tsing-Hua University, Hsinchu Taiwan
Show AbstractInN is a very promising semiconductor material for sensing applications due to its unique surface electron accumulation phenomenon. Recently, ultrathin InN/AlN heterojunction ion selective field effect transistors (ISFETs) have been fabricated with high sensitivity and fast response time [1]. For the ultrathin layer case, the spontaneous polarization difference could play an important role for InN/AlN heterojunction ISFETs. Grown in the polar orientation (-c or (000-1)-direction), a positive bound sheet charge is induced at the interface owing to discontinuities of spontaneous polarizations and would cause a two-dimensional electron gas (2DEG) at the heterointerface [2]. In contrast, a negative bound sheet charge is induced at the InN/AlN(0001) heterointerface and would cause depletion of electrons. The phenomena mentioned above have been confirmed by photoelectron spectroscopy (PES). Therefore, the polarity-tunable InN/AlN heterostructures can have strong implications for ISFFT applications. Compared with conventional band gap engineering, spontaneous polarization effects in polar semiconductors add a new degree of freedom in the design of group-III nitride heterojunction devices. Moreover, structural properties of InN/AlN with both polarities have been studied by in-situ reflection high-energy electron diffraction (RHEED) and ex-situ x-ray diffraction (XRD) measurements.[1] Y.-S. Lu, C.-L. Ho, J. A. Yeh, H.-W. Lin, and S. Gwo, Appl. Phys. Lett. 92, 212102 (2008).[2] C.-L. Wu, H.-M. Lee, C.-T. Kuo, S. Gwo, and C.-H. Hsu, Appl. Phys. Lett. 91, 042112 (2007).
3:15 PM - I4.4
Spontaneous and Piezoelectric Polarization Effects in Wurtzite Group III Nitride Heterojunction Varactor Characteristics.
Choudhury Praharaj 1
1 Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, United States
Show AbstractWe present theoretical calculations for the effect of spontaneous and piezoelectric polarization on wurtzite group III nitride heterojunction varactors, a class of devices of potential interest for realizing high-frequency power sources using frequency multiplication, possibly in the terahertz regime. Spontaneous and piezoelectric polarizations in the 1013 electrons per cm2 range exist in group III nitride semiconductors of wurtzite crystal symmetry. Our calculations show that the polarization-induced bound charges produced at hetero-interfaces have a significant impact on the magnitude as well as the amount of asymmetry in the non-linear capacitance characteristics in varactors. Since asymmetric characteristics will give rise to odd as well as even harmonics of the input excitation frequency, the amount of power realizable at any particular harmonic will be affected by this asymmetry. We calculate the bias-dependent elastance of varactor diodes for a wide range of barrier material compositions, and we discuss in detail the effect of polarization on these characteristics, for single and double barriers. We also calculate the current-voltage characteristics of different varactors from the quantum mechanical tunneling flux of particles across the devices. Since the current conduction is a parasitic element for the varactor mode of operation for high-frequency generation, we use our calculations to discuss trade-offs between elastance characteristics and current conduction across the devices. While Aluminium Gallium Nitride is a common material of choice for realizing varactor-like barriers in group III nitrides, we also extend the analysis to other barriers like Aluminium Indium Nitride and to barriers made from adjacent Aluminium Gallium Nitride and Indium Gallium Nitride layers. Aluminium Indium Nitride provides the possibility of using compensating spontaneous and piezoelectric polarization components to obtain more symmetric varactor characteristics. We discuss in detail the effect of barrier thickness and material composition on varactor capacitance characteristics with Aluminium Indium Nitride barriers. Our calculations lead to an explanation of how compensating compressive and tensile biaxial strain in the basal hexagonal plane in Aluminium Gallium Nitride and Indium Gallium Nitride layers affect the overall device characteristics for heterojunction varactors. Our calculations show that polarization effects are crucial to the design of varactors in group III nitride semiconductors, and that a wide range of device characteristics can be obtained by using polarization effects to engineer the bias-dependent band profiles and charge distributions in devices.
3:30 PM - I4.5
AlGaN/GaN Two-dimensional Electron Gas (2DEG) Structures on Low-defect Density Quasi-bulk GaN Substrates as Lateral Devices for Use in Emerging Electric Power Distribution Electric Switch Applications.
Mark Johnson 1 , Judith Grenko 1 , Matt Veety 1 , Mike Morgensen 1 , Doug Barlage 1 , Tanya Paskova 2
1 , nc state university, Raleigh, North Carolina, United States, 2 , Kyma Technologies, Raleigh, North Carolina, United States
Show AbstractIII-Nitride semiconductors have had tremendous commercial impact over the past decade through the introduction of solid-state lighting and high-frequency power electronics. An emerging area of application is power electronics for switching and inverters in electric power distribution. For example, the reduction in switching and resistive losses in solar photovoltaic systems through the use of III-N based wide bandgap semiconductor based inverter has great potential to improve the overall system efficiency. Minimization of loss in inversion of DC to AC can have an overall impact on the viability of renewable energy as significant as materials related improvements in quantum efficiency for solar cells. As such, III-N semiconductor materials have the potential to impact both the generation and distribution of renewable electric energy. Normally-off field effect transistors with low switching and low resistive losses are essential components in solid-state inverters. The high electron mobility of coherency strain engineered AlGaN/GaN two-dimensional electron gas heterostructures are candidate materials for high efficiency power inverter applications. However, most AlGaN/GaN structures are fabricated on highly lattice mismatched substrates such as Si of SiC, resulting in a high density of defects. Such defects have potential to increase both switching and conductive losses. In order to improve understanding of AlGaN/GaN 2DEG structures and the influence of defects on their electrical properties, a series of test structures were grown on low defect, quasi-bulk GaN substrates. These substrates were synthesized by a modified Halide Vapor Phase Epitaxy (HVPE) process as boules and formed into substrates. The typical defect density of the resulting substrates was less than 1e8cm-2, with samples 2-orders of magnitude lower. Further, the polarity of the GaN substrate could be controlled by the orientation of the boule cut. This provided a framework for investigating the influence of substrate defect density and polarity on 2DEG carrier concentration and mobility. 20nm thick AlGaN/GaN test structures were grown by MOCVD with no mobility enhancement layers. Carrier concentration up to 2e13 cm-3 and mobilities up to 1800 cm2/Vs were measured by van-der-Pauw Hall. SIMS and TEM studies were performed to investigate the influence of MOCVD grown GaN buffer layer thickness as on 2DEG properties related to transport of unwanted substrate interface impurities. Inclusion of a lightly doped p-type compensation GaN buffer layer was also investigated. A comparison was made of 2DEG properties with electronic transport requirements for electric power distribution electronics.
4:15 PM - I4.6
Atomic and Electronic Structure of the Non-polar GaN(1-100) Cleaved Surface.
Marco Bertelli 1 , Peter Loeptien 1 , Martin Wenderoth 1 , Angela Rizzi 1 , Rainer Ulbrich 1 , Maria Clelia Righi 2 , Andrea Ferretti 2 , Layla Samos 2 , Carlo Bertoni 3 , Alessandra Catellani 4
1 IV. Physikalisches Institut, Georg-August University Goettingen, Goettingen Germany, 2 CNR-INFM S3 and Physics Dept. , Università di Modena e Reggio Emilia, Modena Italy, 3 S3 and CNISM, Università di Modena e Reggio Emilia, Modena Italy, 4 , CNR-IMEM, Parma Italy
Show AbstractWurtzite InGaN-based alloys and heterostructures grown along non-polar or semi-polar directions are the best candidates for efficient solid-state light sources in the visible, including the green wavelength range where up to now no other material system has achieved satisfactory quantum efficiencies. Basic knowledge of the atomic structure and the energetics of the growing surface is invaluable for controlled growth of device quality heterostructures. Existing band structure and total energy calculations of the non-polar GaN(1-100) stoichiometric surface lead to contradictory results. [1,2] The question about the existence of intrinsic surface states within the bandgap at the center of the BZ is still open due to the lack of experimental studies on clean and atomically flat GaN(1-100) stoichiometric surfaces with low defect concentrations.In this work we report a comprehensive study of the cleaved atomically flat non-polar (1-100) surface (m-plane) of n-type HVPE free-standing GaN quasisubstrates following our previous work on cleaved non-polar 6H-SiC(11-20) surfaces. [3] Cross-section scanning tunneling microscopy-spectroscopy (X-STM/STS) and ab initio density functional theory (DFT) simulations are combined and provide strong evidence for the lack of intrinsic Ga-like surface states within the band gap at the center of the BZ.Empty and filled states on the clean cleaved GaN(1-100) surface (defect concentration ≤ 2×1012 cm−2) were imaged by STM with atomic resolution. The STS measurements show that the Ga-like surface state band at the center of the BZ lies outside the fundamental energy bandgap, as predicted by the DFT calculations. The Ga-like electronic states dominate the tunneling experiment which is explained in a consistent way with tip induced band bending. The experimental measurements confirm the results of simulated IU characteristics obtained from a self-consistent one-dimensional Poisson solver. They show that the contribution to the tunneling current of N-like states is negligible compared to the one of Ga-like states. Tunneling through filled N-like states can be measured on cleaved GaN(1-100) surfaces where the Fermi level is pinned at midgap energy by defect derived surface states (defect concentration ≥ 3×1013 cm−2).The DFT-based simulations show that, with respect to the ideal truncated surface, the length of the Ga-N bond at the surface is reduced and a bond buckling is observed with an inward displacement of the Ga-atom with respect to the N-atom. According to simulated STM topographies, filled and empty states are concentrated on N- and Ga-atoms, respectively.[1] J. E. Northrup and J. Neugebauer, Phys. Rev. B 53, R10477 (1996).[2] D. Segev and C. G. V. d.Walle, Europhys. Lett. 76, 305 (2006).[3] M. Bertelli, J. Homoth, M. Wenderoth, A. Rizzi, R. G.Ulbrich, M. C. Righi, C. M. Bertoni, and A. Catellani,Phys. Rev. B 75, 165312 (2007).
4:30 PM - I4.7
Low Contact Resistance Metal Semiconductor Field Effect Transistors with N-polar GaN Source/Drain by MOCVD.
Jinqiao Xie 1 , Seiji Mita 1 , Anthony Rice 2 , James Tweedie 2 , Ramon Collazo 2 , Zlatko Sitar 2
1 , Hexatech Inc, Morrisville , North Carolina, United States, 2 Materials Science and Engineering , North Carolina State University, Raleigh, North Carolina, United States
Show AbstractLow Ohmic contact resistance is critical for the development of semiconductor optoelectronic and electronic devices. However, it is a challenge to fabricate Ohmic contacts with low contact resistance in GaN based device because of its relatively wide bandgap. Recently, Si ion implantation [Electron Lett 43,1466 (2007)] and regrown n+ GaN source/drain [Appl. Phys. Lett. 93, 102102 (2008) ] have been reported to significantly improve the source/drain contact resistance, owing to the high carrier density in source/drain regions. Similarly, newly developed selective growth of N- /Ga-polar GaN on sapphire substrates as a lateral polar junction (LPJ) structure could be used to design new device structures by taking advantage of the high conductivity of N-polar GaN. [J. Cryst. Growth 287, 586 (2006)] Because of the large difference in the adsorption energy (~1.3 eV/atom) for oxygen between N- and Ga-polar surfaces, [Appl. Phys. Lett., 74, 1695 (1999)]. N-polar GaN usually has several orders of magnitude higher oxygen concentration than Ga-polar GaN grown within the same MOCVD reactor. Oxygen, as a shallow donor, can contribute more than 5x1018cm-3 free carries in N-polar domains, while Ga-polar domains remain highly resistive. The difference in the electronic properties of the two types of domains allows us to fabricate metal semiconductor field effect transistor (MESFET) devices having N-polar source/drain and Ga-polar channel to improve contact resistance. The polarity control is achieved by the choice of the buffer layer. When GaN is directly grown on sapphire substrates at high temperature without any buffer layer, the film is N-polar. If low temperature (LT) AlN buffer is used, the GaN film is Ga-polar. Therefore, selective growth is realized by selective etching of the LT-AlN buffer in the source/drain regions as patterned by photolithography. After the deposition of a 30 nm LT AlN buffer layer, the wafer was unloaded from the reactor for source/drain and TLM patterning followed by RIE etching of the AlN buffer layer. Then the wafer is reloaded for growth of a ~1.5 μm unintentionally doped GaN film followed by a 400 nm n-channel (n=3.6x1017cm-3, μ=340 cm 2/Vs). A Ti/Al/Ni/Au (30/130/50/50 nm) metal stack annealed at 800 °C serves as the Ohmic contact to the N-polar domain source/drain. TLM measurements reveal a contact resistance (Rc) of 0.25 Ωmm and specific contact resistance (ρc) 5.7x10-7Ω cm2 for the MESFET having N-polar source/drain, compared to Rc of 0.84 Ωmm and ρc of 6.3x10-6 Ω cm2 for the conventional structure. The fabrication process and device characteristics with a recessed gate having a N-polar source/drain will be presented.
4:45 PM - I4.8
Cubic AlGaN/GaN Hetero-Field Effect Transistors with Normally On and Normally Off Operation.
Donat As 1 , Elena Tschumak 1 , Florentina Niebelschuetz 2 , W. Jatal 2 , Joerg Pezoldt 2 , Ralf Granzner 3 , Frank Schwierz 3 , Klaus Lischka 1
1 Department of Physics, University of Paderborn, Paderborn Germany, 2 FG Nanotechnologie, Institut für Mikro- und Nanotechnologien, TU Ilmenau, Ilmenau Germany, 3 FG Festkörperelektronik, TU Ilmenau, Ilmenau Germany
Show AbstractAlGaN/GaN heterojunction field-effect transistors (HFETs) are presently of outstanding interest for electronic devices, in particular, for high-power and high-frequency amplifiers. This is motivated by the potential commercial and defense applications, e.g., in the area of communication systems, radar, base stations, high-temperature electronics and high-power solid-state switching. Currently, state of the art AlGaN/GaN HFETs are fabricated of c-plane surface material of the stabile wurzite crystal structure with inherent spontaneous and piezoelectric polarization fields which produce extraordinary high sheet carrier concentration at the heterointerface. Therefore, usually these devices are of the normally-on type. However, for power and consumer applications, normally-off operation is required to simplify the design of driving circuits and for the safety of the products. A direct way to fabricate HFETs without undesirable parasitic polarization effects is the use of cubic group III-nitrides. In this work, non-polar cubic AlGaN/GaN HFETs were grown by plasma assisted MBE on 3C-SiC substrates. Both normally-on and normally-off HFETs were fabricated using contact lithography. Our devices have a gate length of 2 µm, a gate width of 25 µm, and source-to-drain spacings of 8 µm. For the source and drain contacts the Al0.36Ga0.64N top layer was removed by reactive ion etching (RIE) with SiCl4 and Ti/Al/Ni/Au ohmic contacts were thermally evaporated. The gate metal was Pd/Ni/Au. At room temperature the DC-characteristics clearly demonstrate enhancement and depletion mode operation with threshold voltages of +0.7 V and –8.0 V, respectively. A transconductance of about 5 mS/mm was measured at a drain source voltage of 10 V for our cubic AlGaN/GaN HFETs, which is comparable to that observed in non-polar a-plane devices. From capacity voltage measurements a 2D carrier concentration of about 2x1012 cm-2 is estimated. The influence of source and drain contact resistance, leakage current through the gate contact and parallel conductivity in the underlaying GaN buffer are discussed and the different effects are simulated using a two dimensional device simulation program (ATLAS).
5:00 PM - I4.9
Double Heterostructure AlGaN/GaN/AlGaN HEMT Based on Grading AlGaN/AlN Buffer Layer.
Hongbo Yu 1 , Sefer Lisesivdin 1 , Basar Bolukbas 1 , Ozgur Kelekci 1 , Mustafa Ozturk 1 , Huseyin Cakmak 1 , Pakize Demirel 1 , Ekmel Ozbay 1 2 3
1 , Nanotechnology Research Center, Bilkent University, Ankara Turkey, 2 Department of Physics, Bilkent University, Ankara Turkey, 3 Department of Electrical and Electronics Engineering, Bilkent University, Ankara Turkey
Show AbstractWide-bandgap AlGaN/GaN based high electron mobility transistor (HEMT) is emerging as an excellent candidate for high power, high voltage operations at microwave frequencies. Enhancing the breakdown voltage is an effective approach to increase the power density of such devices. W. S. Tan et al reported that buffer layer leakage currents are much more significant than surface leakages in high-voltage operation [1]. So the double heterostructure HEMT utilizing advantages of the wide bandgap from AlN or AlGaN buffer and high mobility from GaN channel was demonstrated in recent years [2-4]. However, since the two-dimensional hole gas (2DHG) caused by negative polarization at the bottom GaN/AlN interface could possibly counteract the two-dimensional electron gas (2DEG), which is enhanced by this same polarization effect at the top AlGaN/GaN interface [2]. To avoiding the negative effect of 2DHG, a grading AlGaN was introduced between GaN channel and AlN buffer in this study. The band simulation results show that there is no 2DHG formation at the interface of GaN and grading AlGaN. Furthermore, the sheet carrier density of 2DEG can be calibrated from -8.12E12 cm-2 up to -1.15E13 cm-2 by variation of the GaN channel thickness from 100 to 50 nm. For comparison, undoped HEMT structures were grown on 200 nm AlN/Sapphire templates by using both normal GaN and grading AlGaN as buffer layer. For DH-HEMT, the thickness of GaN channel is 50 nm, and Al content is graded from 90% to 5% in 250 nm thick grading AlGaN layer. Sample HEMT devices with LSD/LSG/WG/LG = 3/1/250/1 µm were fabricated from the epilayer structure. Ti/ Al/ Ni/Au (200/2000/400/500Å) was used for source and drain ohmic contacts annealed at 850 deg for 30 s in forming gas ambient. Ni/Au (400/1000Å) Schottky gates were then metalized. A reactive ion-etched mesa was used for the device isolation. Good pinch-off DC IDS–VDS characteristics for both of two HEMT devices without passivation. It is obvious that the DH-HEMT has a lower output conductance and better pinch-off. The breakdown voltage of the GaN/AlGaN normal HEMT device is 34 V, while it is higher than 80 V for the DH-HEMT. This result is very promising for the further higher power operation of high-frequency HEMTs.[1] W. S. Tan, et. al, IEEE Electron Device Lett., Vol. 27, No. 1, pp. 1–3, (2006).[2] Z. Y. Fan, et. al, Appl. Phys. Lett. 88 073513 (2006).[3] Z. Chen, et. al, Appl. Phys. Lett. 94 171117 (2009).[4] Takashi Inoue, et. al, IEEE Transactions on Electron Devices, VOL. 55, NO. 2, (2008)*Corresponding author email:
[email protected]I5: Poster Session: III-Nitride Growth, Doping, and Device Processing
Session Chairs
Wednesday AM, December 02, 2009
Exhibit Hall D (Hynes)
9:00 PM - I5.1
TEM Analysis of the Microstructures of Undoped and Doped AlN/sapphire Grown by MOCVD.
Bo Cai 1 2 , Mim Nakarmi 1 2
1 Physics, Brooklyn College of the CUNY, Brooklyn, New York, United States, 2 Graduate Center, the City University of New York, New York, New York, United States
Show AbstractAluminum nitride (AlN) has emerged as a promising deep ultraviolet (UV) material for the development of deep ultraviolet optoelectronic devices such as light emitting diodes and detectors in the spectral range down to 200 nm. High quality AlN/sapphire can also be used as templates to grow nitride based ultraviolet and deep ultraviolet photonic devices due to high thermal conductivity and transparency of the light. The performance of the devices depends on the density of dislocations and the microstructures of the templates. We report on the microstructure analysis of AlN epilayers grown on sapphire. Both plane and cross section views are investigated by high resolution transmission electron microscopy (TEM). It has been revealed that the dislocations are greatly reduced by using high temperature buffers. Edge dislocations dominate the total density of dislocations. The microanalysis of doped AlN epilayers grown on AlN/sapphire templates is also performed. Comparative TEM analysis of the microstructures due to different dopants will also be presented. Implications of our finding for the applications in deep UV optoelectronic devices will be discussed.
9:00 PM - I5.10
Kinetics and Mechanism of Formation of GaN from β-Ga2O3 by NH3.
Toshiki Sakai 1 , Hajime Kiyono 1 , Shiro Shimada 1
1 , Hokkaido University, Sapporo Japan
Show AbstractNitridation of β-Ga2O3 to GaN in an atmosphere of NH3/Ar was investigated by thermogravimetric analysis (TGA) and microstructural observation. As-received and sintered dense Ga2O3 rod-like samples with an average particle size of 5 μm were used as the starting sample, the latter sample having a flat surface without pores. Non-isothermal TGA of the as-received sample showed that the weight loss begins at about 700 °C, continuing to 850 °C and ceases in a little while at 850 – 900 °C, after which temperature the weight loss begins again to 1200 °C. Since XRD showed that the transformation to wurtzite GaN was completed in the sample obtained by TGA at 870 °C, the weight loss at 700 – 850 °C is due to nitridation of Ga2O3 and that above 900 °C due to decomposition of GaN formed. Isothermal TGA at 800 – 1000 °C showed that the weight loss due to nitridation proceeds linearly with time, suggesting that nitridation is controlled by an interfacial reaction with an activation energy of 110 kJ mol-1. Microstructure of the as-received and coarsen samples nitrided at 800 °C was observed by SEM, showing that fine round and pyramidal shaped particles of GaN deposit on surfaces of Ga2O3 particles with the size and number of the deposits increasing with the progress of nitridation. These results suggest that the formation and transportation of gaseous species such as Ga2O or GaH participate the nitridation of Ga2O3. At a high degree of nitration, dense particles of Ga2O3 change to tube-like forms. A nitridation mechanism will be discussed on the basis of TGA results and microstructural observation. The cathode luminescence (CL) measurement of partially and fully nitrided Ga2O3 is in progress to indicate the dependence of the degree of nitridation on the CL properties.
9:00 PM - I5.11
Characteristics of p-type Mg Dopants Activation in Polar and Non-polar GaN Grown by Metalorganic Chemical Vapor Deposition.
Jisu Son 1 2 , Kwang Hyeon Baik 1 , SungHo Lee 1 , Yong Gon Seo 1 , Sung-Min Hwang 1 , Tae Geun Kim 2
1 Green-energy Research Center, Korea Electronics Technology Institute, Seongnam Korea (the Republic of), 2 Electronic Engineering, Korea University, Seoul Korea (the Republic of)
Show AbstractThe samples were grown on polar c-plane (0001) sapphire as well as non-polar a-plane (11-20) GaN on r-plane (1-102) sapphire by metalorganic chemical-vapor deposition (MOCVD). The major challenges related to p-type doping in GaN include high activation energies of Mg dopants, as large as c-plane (~170 meV) and a-plane (~118 meV); solubility limits for Mg in the range of low 1020 cm-3 and the hydrogen passivation effect, in which Mg forms Mg–H complex defects in GaN. One of the efficient ways of improving Mg dopant activation is to perform thermal annealing at optimum temperature and ambient. Mg doping concentration from secondary ion mass spectrometry (SIMS) measurements in GaN shows constant values of 6.58x1019 cm-3 by bis-magnesium (Cp2Mg) gas during epitaxial growth. The samples were activated at various temperatures for different periods of time in air, O2 and N2 gas ambient by conventional furnace annealing (CFA) and rapid thermal annealing (RTA), respectively. After annealing, hole concentration, mobility and resistivity were characterized by room-temperature Hall-effect measurements by Accent HL5500IU Hall system using the van der Pauw method. It was measured that the highest hole concentration of c-plane GaN was limited to 4.068x1017 cm-3 with mobility μh of 7.18 cm2/V-s and specific resistance of 2.136 ohm-cm in N2 ambient at 685°C for 40 min by CFA. In contrast, the highest hole concentration of 2.504x1018 cm-3 with mobility µh of 0.339 cm2/V-s and specific resistance of 7.35 ohm-cm was obtained in a-plane GaN, when annealed in air ambient at 700°C for 28 min by CFA. SIMS depth profile analysis was performed to confirm the dissociation of hydrogen. The hydrogen concentration was greater than 4.56x1018 cm-3 in as-deposited sample, while it decreased to 2.18x1018 cm-3 in activated sample, showing p-type conductivity. Thus, it can be said that Mg dopants activation in nonpolar a-plane GaN is also dominated by the dissociation of hydrogen. Low-temperature photoluminescence (PL) spectra (13K) measurements for annealed samples show the same trends as electrical characteristics by Hall measurements, confirming that optimum activation can enhance the electrical and optical properties of Mg-doped GaN layers.
9:00 PM - I5.12
Control of Facet Structures in Selective Area Growth (SAG) of a-plane GaN by MOVPE.
Bei Ma 1 , Reina Miyagawa 1 , Hideto Miyake 1 , Kazumasa Hiramatsu 1
1 , Department of Electrical and Electronic Engineering, Mie University, Mie University, Tsu Japan
Show AbstractNonpolar nitrides are a promising approach since it overcomes quantum-confined Stark effect (QCSE) to improve quantum efficiency. Unfortunately, due to the lack of adapted substrate, nonpolar nitrides still suffer from high densities of thread dislocations (TDs) and basal stacking faults (BSFs). In c-plane nitrides, selective area growth (SAG) has been proved as a mature technique to improve crystal quality and fabricate novel devices [1], such as filed emitters, waveguides, facet laser and nanostructures. In the present study, we have investigated facet structure of a-plane GaN during SAG and achieved facet control by MOVPE growth condition.Prior to the growth, ~6 µm thick a-plane GaN templates were grown on r-plane sapphire substrates by MOVPE [2]. Then, 5 µm × 5 µm square patterns with 5µm spacing of a 100nm thick SiO2 mask were fabricated on the a-GaN using conventional sputtering, lithography and reactive ion etching techniques. The patterned templates were reloaded into the MOVPE for SAG. The reactor pressures and growth temperature were 100~500 Torr, and 1000~1100oC, respectively.The sample grown under 500 Torr exhibited rectangular crystallites consisting of (0001), (000-1), {1-100} and {1-101} faces. With decreasing the pressure to 300 Torr, the growth rate along <11-20> prominently decreased; however, none of novel facets emerged. Further decreasing the pressure to 100 Torr, the growth rate along <11-20> correspondingly decreased to form a novel facet, (11-20), at the top face. Thus, our result proved that adjusting reactor pressure can achieve facet control in SAG of a-plane GaN, and the mechanism should be attributed to the stability of different facets which mainly depended on the “the surface energy” and “the stability of surface atoms”.Acknowledgements: This work was partly supported by Ministry of Education, Culture, Sports, Science and Technology, the Knowledge Cluster Initiative (the Second Stage) and grants-in-aids for Scientific Research (Nos.21360007, 21560014, 18069006).[1] M. Funato, T. Kondou, K. Hayashi, S. Nishimura, M. Ueda, Y. Kawakami, Y. Narukawa, T. Mukai, Appl. Phys. Express 1 (2008) 011106.[2] B. Ma, R. Miyagawa, W. G. Hu, D. B. Li, H. Miyake and K. Hiramatsu, J. Cryst. Growth 311 (2009) 2899
9:00 PM - I5.13
Influence of Gallium Supersaturation on the Growth of N-polar GaN Films by Metalorganic Chemical Vapor Deposition.
Seiji Mita 1 , Ramon Collazo 2 , Anthony Rice 2 , James Tweedie 2 , Jinqiao Xie 1 , Rafael Dalmau 1 , Zlatko Sitar 2
1 , Hexa Tech Inc., Morrisville, North Carolina, United States, 2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractThe growth of N-polar GaN enables the fabrication of a variety of novel GaN based devices, due to the opposite direction of the spontaneous polarization to that of Ga-polar GaN and its highly chemically reactive surface suitable for sensor applications. A thermodynamic supersaturation model for gallium (Ga) was applied to the growth characteristics of N-polar GaN in low-pressure metalorganic chemical vapor deposition (MOCVD). Supersaturation is a measure of the driving force for deposition as obtained from the deviation from thermodynamic equilibrium. The model takes into account the simplified GaN chemical reaction that occurs at the growth interface. This involves many variable MOCVD growth parameters. The concept of Ga supersaturation can provide useful information for predicting N-polar GaN growth characteristics in terms of the behavior of growth morphologies, film nucleation, and impurity incorporation. All 10-20 nm-thick nucleation layers (NLs) and 1.3 µm-thick main layers of GaN were grown on nitrided sapphire under the mass-transport limited growth regime. (0001) sapphire with misorientation angle of 2° toward the m-plane was used as substrate. After sapphire nitridation, GaN NLs were deposited at 900°C by using a V/III ratio of 2000. N-polar GaN layers were grown on these NLs by our standard growth conditions: growth temperature of 1150°C, V/III ratio of 100, input Ga partial pressure of 8 mTorr at a total reactor pressure of 20 Torr. Under this growth condition, the use of H2 as a diluent gas during the growth led to N-polar films with a smooth surface morphology with less than 1 nm RMS roughness, while the hexagonally facetted surface morphology were obtained in the case of simply replacing N2 for H2 as the diluent gas. The choice of H2 diluent gas played a role in significantly decreasing the degree of supersaturation, since H2 is the product in the chemical reaction of GaN formation. An equivalent growth condition with respect to Ga supersaturation was found at 1050°C that led to the same surface smooth morphology. Smooth N-polar films were always obtained at lower Ga supersaturation conditions than those needed for smooth Ga-polar films (typical growth conditions). In addition to growth morphology, the degree of Ga supersaturation also influenced the oxygen incorporation into the N-polar films. 0.7 µm thick GaN layers were grown on 1.3 µm thick smooth N-polar GaN templates by drastically increasing the Ga supersaturation. Growth conditions for these films were as follows: V/III ratio of 2000, growth temperature of 950°C, and input Ga partial pressure of 40 mTorr at a total reactor pressure of 100 Torr. Secondary ion mass spectrometry studies identified that the oxygen incorporation levels were 1x1019 and 1x1018 cm-3 in smooth N-polar GaN templates and the top layers, respectively. Details of the supersaturation calculation procedure and N-polar growth conditions will be presented.
9:00 PM - I5.14
High-quality Ti-based Schottky Contacts to p-type GaN.
Ja-soon Jang 1 , Sun-Ho Jang 1
1 Department of ECE, Yeungnam University, Gyeongsan-si, Gyeongbuk, Korea (the Republic of)
Show AbstractWe report on high-quality Schottky contacts to p-type GaN using a Ti single layer. Electrical measurements showed that the as-deposited contacts become rectifying, indicating the formation of Schottky contact. As considering the fact that the formation of Schottky contact is very difficult (because there are high density of defects in the p-GaN), our finding can be of technological importance. Based on the current-voltage-temperature (I-V-T) data, Schottky barrier height and ideality factor were calculated to be 1.1 eV and 2.2, respectivley. Glancing angle X-ray diffraction (GXRD) showed that TiN-based peaks are detected on the as-deposited sample, indicating that the generation of nitrogen vacancy even at room temperature can contribute to the formation of Schottky contacts. Possible formation and hole transport mechanisms will be described in terms of surface reaction between Ti and GaN, the reduction of surface defects, and the surface energy band-bending effects.
9:00 PM - I5.15
Quantitative Description of GaN Photoelectrochemical Oxidation and Etching Using an Equivalent Circuit Model.
Shao-Ning Pei 1 , Kieren Chen 2 , Daniel Porto 1 , Luke Lee 1 , Pei-Cheng Ku 1
1 EECS, University of Michigan, Ann Arbor, Michigan, United States, 2 MSE, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe success of silicon technologies is largely attributed to the ability of forming high quality silicon/SiO2 interfaces. Similarly, high quality oxide/GaN interfaces can greatly enhance the performance of GaN MOSFETs (metal-oxide-semiconductor field-effect transistors), improve surface passivation, and enable device isolation, all of which are critical for power electronics and optoelectronics applications. Due to the stability of GaN, it is difficult to thermally oxidize and chemically etch GaN. Alternatively, deposition of oxide layers on GaN can generate a high density of interface traps at the interface while dry etching often results in surface damaging. Recently, photoelectrochemical (PEC) method has been successfully applied to GaN oxidation and etching. In PEC oxidation, photogenerated holes in GaN lead to the oxidation of GaN near the GaN/aquatic solution interface. In PEC etching, the oxidized GaN (Ga2O3) then dissolves in the acid or base. Although partial and qualitative understanding of the PEC process in various acids, bases, and neutral solutions have been reported, a unified and quantitative model is still lacking. Such a model is crucial for one to accurately and repeatedly control the oxide growth rate in device processing. In this work, we have developed a unified PEC oxidation/etching model using a simple equivalent circuit that consists of a photovoltaic cell, a few resistors, and a switch to model the electrochemical process of GaN oxidation. Our model can quantitatively determine the oxide growth rate under several external parameters including solution type, solution pH value, optical intensity, external bias, and electrical impedence (e.g. GaN doping level). Importantly, two limiting regimes for the PEC process have been predicted and experimentally verified using this model: (1) surface gallium atom ionization under low optical intensity; (2) ion transport in the solution under high optical intensity.
9:00 PM - I5.16
Low-resistance Ohmic Contacts to N-face p-GaN for the Fabrication of Functional Devices.
Seung Cheol Han 1 , Jae-Kwan Kim 1 , Jun Young Kim 1 , Joon Seop Kwak 1 , Kangho Kim 2 , Jong-Kyu Kim 3 , E.Fred Schubert 3 , Kyoung-Kook Kim 4 , Ji-Myon Lee 1
1 Department of Materials Science and Metallurgical Engineering, Sunchon National University, Sunchon, Chonnam, Korea (the Republic of), 2 , Korea Photonics Technology Institute, Gwangju Korea (the Republic of), 3 Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 4 Department of Nano-Optical Engineering, Korea Polytechnic University, Siheung, Kyonggi, Korea (the Republic of)
Show AbstractIt is well known that GaN exhibit high spontaneous polarization fields due to the partial ionic bonding nature of GaN. By this polarization effects, the interface property and electrostatics of structures grown along the N-polar, or (0001) direction, are very different from electrostatics along the Ga-polar, or (0001) direction. Most of the researches in III-nitride optoelectronics in the past have focused on materials and devices based on Ga-polar GaN. However, the N-polar based structures can be used in various device structures, such as thin vertical LEDs. Recently, some researchers have reported p-side down light emitting diode that can suppress the efficiency drooping effectively [1]. In contrast to the many studies of ohmic contact on Ga-face p-type GaN, fewer detailed investigations of ohmic contact on N-polar p-type GaN are reported. In this study, we will investigate the low resistance ohmic contacts to N-face p-GaN.
Ga-polar GaN films were grown by metal organic chemical vapor deposition system and N-polar GaN films were produced by laser lift-off (LLO) of sapphire substrate after conventional growth of Ga-polar GaN films. After LLO processing, the films were dipped in HCl solution in order to etch off the Ga droplet. The undoped GaN layers were then etched by inductively coupled plasmas to expose the p-GaN layer using Cl2/BCl3. Subsequently, Ni (20 nm)/Au (40 nm) films were then deposited by electron-beam evaporator. Samples were rapid-thermal annealed at various temperatures for 1 min in N2 ambient. Current-voltage (I-V) data were measured at room temperature using a parameter analyzer.
Electrical properties of Ni/Au contact on N-face p-GaN before and after annealing at 100, 200, and 300 °C in N2 ambient was investigated. Annealed sample exhibit better electrical properties than as-deposited sample. For the sample annealed at room temperature, the I-V curve showed a Schottky contact characteristics. However, the slope of I-V curve was increased as the annealing temperature was increased, indicating that the reaction of metal and surface of GaN was occurred. The specific contact resistance was determined from the plots of the measured resistance versus the spacing between the circular transfer length method pads. Measurement showed that the specific contact resistances were 1.01, 4.04×10-1, 6.47×10-2, and 9.05×10-3 Ω cm2 for the as-deposited and the annealed Ni/Au contacts, respectively. It is of noteworthy that the specific contact resistance of N-face p-GaN exhibits a liner decrease with increasing annealing temperature, which is different from those of Ga-polar GaN. In this presentation, we will also report the ohmic contact mechanism as well as the electrical property with various annealing temperature and thickness of metals.
[1] M. L. Reed et al. Appl. Phys. Lett. 93, 133505 (2008).
9:00 PM - I5.17
Low Temperature Growth of c-polar and a-nonpolar GaN on Sapphire Substrates using UHV PLD.
Pranav Gupta 1 , Thomas Rawdanowicz 1 , Titas Dutta 1 , Alok Gupta 1 , Jagdish Narayan 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractDevelopment of growth techniques for GaN thin films has been of immense scientific interest owing to their applications in UV-visible LEDs, photodetectors, and high temperature-high power devices. Device quality GaN thin films are generally grown IN TWO STEPS using conventional MBE and OMCVD; first low-temperature (500-600oC) growth, followed by high-temperature (over 1000oC) growth. The first step leads to the formation of smooth (but defective) GaN layer of cubic Zince Blende structure on which hexagonal (Wurtzite) GaN is grown. This hexagonal device layer is often quite defective containing high-density stacking faults and Shockley partials. Recent efforts to grow wurtzitic GaN thin films using conventional PLD systems have still hovered in 850oC temperature range. Using UHV-Pulsed Laser Deposition process we have been able to grow hexagonal GaN at significantly low temperatures (600-650oC), which is considerably less defective, and this can provide a much better template to grow device-quality GaN (for both polar and nonpolar orientations). Epitaxial nature of (0001) GaN thin films deposited on c-Al2O3 was determined using X-ray diffractometry and φ-scans. Further characterization of the interface quality and epitaxy was performed using the high resolution Transmission Electron Microscopy (HRTEM) which showed clean interface with large grain size ~250 nm and low angle intermediate grain boundaries. In-situ surface characterization using RHEED during and after the growth showed distinct spots pointing towards well constructed and flat grain surfaces. The Atomic Force microscopy studies revealed the large island like features having the Z variation of 5-10 nm near boundaries and an RMS roughness of 3-4 nm. Growth in non-polar directions (a-plane and m-plane) is especially desired for the III-nitrides as it gets rid of unwanted interface charge which develops across the device interfaces in polar directions. Such non-polar growth will enable faster switching speeds for GaN based electronic devices. We also report the growth of non-polar a-plane (2-1-10) GaN on the r-plane Al2O3 using the same deposition process and 600oC temperature range. The direct a-plane GaN growth on r-plane Al2O3 surface is difficult because of large anisotropic lattice mismatch between r-plane Al2O3 and a-plane GaN and the tendency of GaN to grow in 3-D island like growth mode. We have explained the epitaxial growth using domain matching epitaxy paradigm. We also report the results from X-ray diffractometry and φ-scan studies which confirm the epitaxial growth of our non-polar GaN thin films on r- Al2O3 substrates.
9:00 PM - I5.19
Effects of Controlling the Indium to Nitrogen Growth Ratio on the Structural and Optical Properties of InN Grown by Molecular-Beam Epitaxy.
Paul Minor 1 , Michael Sattler 1 , Morgan Ware 1 , Eric Decuir 1 , Omar Manasreh 1 , Martin Schmidbauer 2 , Yuriy Mazur 1 , Gregory Salamo 1
1 , University of Arkansas, Fayetteville, Arkansas, United States, 2 , Leibniz-Institut für Kristallzüchtung, Berlin Germany
Show AbstractRecently, InN has become very interesting as several groups have established its high quality growth epitaxially [1,2]. This heightened interest is due in large part to the extension, with InN, of the direct bandgap III-N material system throughout the visible and into the NIR region of the electromagnetic spectrum. However, the interplay between structural quality and optical quality is still not well understood. It has been observed in this study that InN growths with high measures of crystallinity from narrow x-ray diffraction rocking curves lack completely or have very poor photoluminescence (PL). At the same time, growths that exhibit very broad rocking curves, and are assumed to have poor crystallinity, are found to have a considerably stronger PL response. It has also been demonstrated in this study that the growth of InN in RF assisted molecular beam epitaxy may occur in two very different regimes. 2-D planar growth and 1-D columnar growth of InN can be achieved by controlling the ratio of the In to N in the growth flux. In addition to control of surface morphology, this study also reveals interplay between the dimensionality or growth regime of the crystal and the strength of its PL. This study discusses the observed trends in the surface morphology, crystalline quality, and optical properties and presents a physical model explaining these observed trends. [1] K. M. Yu, Z. Liliental-Weber, W. Walukiewicz, W. Shan, J. W. Ager, III, S. X. Li, R. E. Jones, E. E. Haller, H. Lu, and W. J. Schaff, “On the crystalline structure, stoichiometry and band gap of InN thin films” Appl. Phys. Lett. vol. 86, Feb. 2005. [2] I. Gherasoiu, M. O'Steen, T. Bird, D. Gotthold, A. Chandolu, D. Y. Song, S. X. Xu, M. Holtz, S. A. Nikishin, and W. J. Schaff, “Characterization of high quality InN grown on production-style plasma assisted molecular beam epitaxy system,” J. Vac. Sci. Technol. A, vol. 26, no. 8, pp. 399-405, May 2008.
9:00 PM - I5.2
Growth of High Quality c-plane AlN on a-plane Sapphire.
Reina Miyagawa 1 , Jiejun Wu 1 , Hideto Miyake 1 , Kazumasa Hiramatsu 1
1 , Mie Univ., Tsu Japan
Show AbstractStrong demands for high efficiency deep UV-LED and sensors have led to a renewed interest in the growth of high-quality AlN. Although normally c-plane sapphire is used as substrate, a-plane sapphire is actually preferable for some applications like edge emitting lasers [1] due to the easy cleavage along r-plane. Despite the improved GaN were grown on a-plane sapphire [2], it is seldom reported the growth of AlN on a-plane sapphire by MOVPE or HVPE. In our knowledge, one group tried to grow AlN on a-plane sapphire by MBE, but crystal quality is low (FWHM=0.8 deg.) [3]. In this paper, MOVPE and HVPE are used to grow AlN (0001) on sapphire (11-20). The optimal growth conditions are investigated.AlN (0001) layers were grown on sapphire (11-20) and (0001) substrates by MOVPE and HVPE, respectively. The growth temperatures were adjusted from 1200-1500 oC and the reactor pressure was kept constant at 30 Torr.Mirror and flat c-plane AlN were obtained grown on both a-plane and c-plane sapphire. In-plane epitaxial relationship of c-plane AlN on a-plane sapphire, such as AlN[11-20]//sapphire[0001] and AlN[1-100]//sapphire[1-100], was decided by x-ray phi–scans. Crystalline quality and surface roughness are improved with increasing growth temperature, detected by high resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM). FWHM values of (10-12) diffraction are 519 and 1219 arcsec for c-plane AlN grown on a-plane sapphire and c-plane sapphire, respectively. It indicates that a-plane sapphire substrate benefits to decrease dislocations density. AFM image show the micro-morphology was characterized as flow-steps with RMS roughness of 0.33 nm. AFM RMS values are also reduced by increasing temperature. When thickness is larger than 3.0 um, compared with the cracking nets (three <11-20> directions) in AlN on the c-plane sapphire, only crack along one <11-20> direction is observed in AlN layer on the a-plane sapphire with the dramatically reduced cracking density. Moreover, although the in-plane anisotropic lattice constants and thermal expansion coefficients for a-plane sapphire substrate, in-plane anisotropic FWHM values of AlN along c-axis and m-axis of substrate aren’t observed. Acknowledgments This work was partly supported by Ministry of Education, Culture, Sports, Science and Technology, the Knowledge Cluster Initiative (the Second Stage) and grants-in-aids for Scientific Research (Nos.21360007, 21560014, 18069006).Reference[1] S. Nakamura, M. Senoh, S. Nagahama et.al., Jpn. J. Appl. Phys. 35 (1996) L217.[2] H. K. Chauveau, P. D. Mierry, H. Cabane, and D. Gindhart, J. Appl. Phys. 104 (2008) 113516.[3] J. Xu, J. S. Thakur, G. Hu, and Q. Wang, et al., Appl. Phys. A 83 (2006) 411.
9:00 PM - I5.20
Growth and X-ray Diffraction Structure Study of InGaN/GaN Quantum Wells Implanted with Eu3+ Ions.
Mohammad Ebdah 1 , Wojciech Jadwisienczak 2 , Martin Kordesch 1 , Andre Anders 3
1 Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States, 2 School of EECS, Ohio University, Athens, Ohio, United States, 3 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractGrowth and structure characterization of rare earth (RE) ion doped low dimensional III-nitride structures such as quantum wells (QW) have received recently increasing interest due to the technological applications of such nanostructures. In this work, InGaN/GaN QWs of 25 periods with a fixed well/barrier thickness ratio were grown by metal-organic chemical-vapor deposition (MOCVD) technique on GaN/(0001) sapphire substrate with a capping layer of GaN on top of the superstructure. Doping was done by implanting Eu3+ ions in QWs at energy of 150 keV with a dose of up to 5.5x1015 cm-2 at room temperature. The as-grown and Eu3+ ion implanted InGaN/GaN QWs were subjected to thermal annealing in quartz tube furnace at different temperatures ranging from 600°C to 950°C in nitrogen ambient for 30 minutes. The quality of as-grown and as-implanted QW interfaces, as well after annealing of each has been investigated by X-ray diffraction (XRD) and the characteristic satellite peaks of the QWs were measured for the (0002) reflection up to the second order in the symmetric Bragg reflections. The simulation of the XRD spectrum of the as-grown QWs indicates a high quality of modulated composition of wells/barriers, and sharp interfaces between the InGaN and GaN sub-layers. Annealing the as-grown QWs does not show to affect the interfacial quality of the superstructure. The simulation results show 7% indium percentage in the InGaN well sub-layers, with thicknesses of 2.5 and 3 nm for a single InGaN well and GaN barrier, respectively. It has been found that the Eu3+ ion implanted InGaN/GaN layered quantum structure undergo a partial induced degradation. Annealing the implanted QWs shows a gradual improvement of the multilayer periodicity and a reduction of the induced degradation with increasing the annealing temperature as indicated by the XRD experimental spectra.
9:00 PM - I5.21
The Characterization of Indium-rich InGaN Alloys Grown by High-pressure CVD.
Nikolaus Dietz 1 , Mustafa Alevli 1 , Ramazan Atalay 1 , Buegler Max 1 , Goksel Durkaya 1 , Ronny Kirste 2 , Jan-Hindrik Schulze 2 , Axel Hoffmann 2
1 Physics & Astronomy, Georgia State University, Atlanta, Georgia, United States, 2 Technische Universität Berlin, Institut für Festkörperphysik, Berlin, Berlin, Germany
Show AbstractThe In1-xGaxN ternary alloy system has a great potential for the development of high efficiency solar energy conversion and advanced optoelectronic device applications. Ga1-yInyN/In1-xGaxN based heterostructures of various compositions can be engineered to the responsive from UV to IR wavelength regime, so that devices based on such heterostructures can cover the whole visible spectrum. At present however, the growth of such heterostructures is limited by incompatible processing windows, as well as high lattice mismatch, interfacial fields, relaxation and phase segregation, which affects the film quality and thus efficiency of thought device applications. This contribution focuses on the structural and optical characterization of In1-xGaxN layers and heterostructures grown by ‘high-pressure chemical vapor deposition (HPCVD), a growth technique explored to stabilize indium-rich In1-xGaxN alloys at elevated temperatures using 15 to 20 bar nitrogen overpressure in order to suppress the thermal disassociation of In1-xGaxN layers. We will present the structural and optical analysis of In1-xGaxN layers studied by Raman spectroscopy, X-Ray Diffraction, transmission spectroscopy and atomic force microscopy. The composition dependent behavior of observed phonon features in Raman is provided. The compositional information obtained from Raman and the XRD Bragg peaks are correlated to results obtained by transmission spectroscopy and linked to the band-gap shift in In1-xGaxN. The effects of composition and growth conditions on the layer surface topography and growth modes are studied by AFM.
9:00 PM - I5.22
Selective Growth of InGaN/GaN Quantum Dot by Nonlithographic Nanopatterning through Anodic Aluminum Oxide.
Joonmo Park 1 , Hyung-A Do 1 , Se-Hoon Moon 1 , Sang-Wang Ryu 1
1 Physics, Chonnam National Unoversity, Gwangju Korea (the Republic of)
Show AbstractWe fabricated high density InGaN quantum dot (QD) embedded in GaN hexagonal pyramids. This approach was based on the selective epitaxial growth of GaN hexagonal pyramids and shape control of InGaN QD in nano-sized SiO2 pores formed onto GaN/sapphire. Photoluminescence obtained at room temperature from the capped nanodots indicated confinement of the excitons in the nanostructures. This approach enables fabrication of dense and uniform arrays of epitaxial nanostructures and is potentially applicable to a variety of materials systems.
9:00 PM - I5.24
Materials Study of the Competing Group-V Element Incorporation Process in Dilute Nitride Films.
Wendy Sarney 1 , Stefan Svensson 1
1 Sensors & Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland, United States
Show AbstractIn this presentation we detail a materials study of InAsSbN films grown for infrared (IR) detector applications. Common III-V semiconductors, such as alloys based on GaAs, GaSb, or InAs, do not have sufficiently small bandgaps to be used for mid to long-wave infrared detector applications. The energy bandgap for GaAs, for instance, is 1.52 eV, which is in the near-infrared (NIR) range. For this reason, quantum structures such as quantum well infrared photodetectors (QWIPs) and type-II strained-layer superlattices are grown to take advantage of confinement effects and induce an effective bandgap in the desired range. The disadvantage of such structures is that they require many interfaces which must be atomically precise, and the quality of such films may be fundamentally limited.A small proportion of N substituted for the group V element in a III-V alloy dramatically decreases both the bandgap and lattice parameter while maintaining high quality crystalline film characteristics. Dilute nitride semiconductors based on III-V semiconductors allow a direct-gap, single-film material with a sufficiently small bandgap for mid to very-long-wavelength applications.Quaternary films such as InAsSbN can be lattice matched in InAs substrates. It has been reported that the presence of Sb enhances N incorporation. The nature of the interaction and incorporation of three group V elements is not exactly known and the determination of the mole fractions of these elements is non-trivial. In this presentation, we will discuss a methodology for determining the group-V mole fractions and to confirm the enhanced incorporation of nitrogen in the presence of Sb.A series of four-layer test structures consisting of InAs/InAsSb/InAsN/InAsSbN on InAs substrates were grown by molecular beam epitaxy (MBE). Since the lattice constant is affected by both N and Sb, x-ray diffraction (XRD) data alone cannot be used for unambiguous determination of the composition. At least one more chemical characterization method must be used. Transmission electron microscopy (TEM) with energy-dispersive x-ray spectroscopy (EDS) allows the capability to do localized composition analysis on very thin strained layers. Because EDS cannot detect small amounts of N with high enough precision, we restricted the analysis to provide concentration ratios of As/Sb in the InAsSb and the InAsSbN layers. This information in combination with the XRD peak positions allows us to uniquely determine the mole fractions of the three group V elements. We have obtained high material quality films with N incorporation levels that well exceed the values needed for long-wavelength infrared (LWIR) absorption.
9:00 PM - I5.25
Structural Characterization of ScxGa(1-x)N(001)/MgO(001) Films Grown by Molecular Beam Epitaxy.
Costel Constantin 1 , Kangkang Wang 2 , Abhijit Chinchore 2 , Jeonhim Pak 2 , Arthur Smith 2 , Kai Sun 3
1 Physics, Seton Hall University, South Orange, New Jersey, United States, 2 Physics and Astronomy, Ohio University, Athens, Ohio, United States, 3 , University of Michigan, Ann Arbor, Michigan, United States
Show AbstractGallium nitride is one of the most promising materials for making blue-light-emitting devices due to its wide band gap (i.e. EG ~ 3.4 eV). Alloying gallium nitride with aluminum nitride and indium nitride constitute another motivation for studying gallium nitride. However, indium nitride has a low melting temperature, therefore, precluding its alloying with gallium nitride at high temperatures. A very good substitute for indium nitride is scandium nitride which has a lattice match < 2% with the zincblende gallium nitride. In this study, ScxGa(1-x)N (x = 0 – 1) films were grown on MgO(001) substrates by radio frequency plasma assisted molecular beam epitaxy. The preliminary data obtained from reflection high energy electron diffraction, x-ray diffraction, and atomic force microscopy suggests that the Sc0.25Ga0.75N, and Sc0.5Ga0.5N films have a zincblende crystal structure. Above x = 0.5, there is a transition towards the rocksalt crystal structure in which scandium nitride is most stable. Our results agree very well with theoretical results of Zerrough1 et al. who predicted that zincblende crystal structure is more favorable than rocksalt in films with Sc concentrations of 0%, 25%, and 50%; whereas for Sc concentrations of 75% and 100% the rocksalt structure is the most stable configuration with respect to zincblende. This collaborative work is supported by: Seton Hall: University Research Council. Ohio University: DOE-BES Grant No. DE-FG02-06ER46317 and NSF Grant No. 0730257.1S. Zerroug, F. Ali Sahraoui, and N. Bouarissa, J. App. Phys. 103, 063510 (2008)
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AlN Periodic Multiple-layer Structures Grown by MOVPE for High Quality Buffer Layer.
V. Kuryatkov 1 2 , W. Feng 1 2 , M. Pandikunta 1 2 , D. Rosenbladt 1 , B. Borisov 1 2 , S. Nikishin 1 2 , Mark Holtz 1 3
1 Nano Tech Center, Texas Tech University, Lubbock, Texas, United States, 2 Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas, United States, 3 Physics, Texas Tech University, Lubbock, Texas, United States
Show AbstractObtaining high quality AlN is a critical factor for devices based on wide-bandgap III-Nitride semiconductors. AlN plays an essential role in functioning as a buffer layer which initiates epitaxy and is transparent throughout the deep UV spectral region 210-300 nm. Our experiments combine two recent approaches: the ammonia pulsed multi-layer AlN (ML-AlN) method and control of the growth mode by varying the V/III ratio. We report growth of high-quality AlN buffer appropriate for deep-UV devices via the pulse-flow ML-AlN growth method. The growth was performed on sapphire (0001) substrates by low-pressure metalorganic vapor phase epitaxy (MOVPE). Trimethylaluminum (TMAl) precursor was used with H2 as carrier gas and NH3 as nitrogen source. Growth temperature was varied from 550 to 1200 oC for LT and HT AlN layers, respectively denoting low and high temperature. The growth pressure was controlled between 20 and 200 Torr. AlN epilayers were grown under various V/III ratios (80 to 10,000). This was accomplished by varying the NH3 flow rate and TMAl flow. The total flow was kept constant. The amount of TMAl flow during the growth was varied for LT and HT AlN. The structural quality of the epilayers was characterized by X-ray rocking curve (XRC) analysis. The surface morphology of the epilayers was evaluated by atomic force microscopy (AFM) along with scanning electron microscopy (SEM). The transition of the growth mode by using the periodical AlN ML can reduce the number of dislocations at the interface between growth regions produced by pulsed and continuous modes. MOVPE growth of AlN on c-plane sapphire substrates was investigated as a function of the ML period. This method leads to a reduction in the screw-type dislocation density of AlN. The screw-type dislocation densities of AlN layer was 6.2 × l05 cm-2, as determined from the X-ray data for layers with thickness up to 300 nm. The RMS surface roughness obtained from atomic force microscopy was 0.2 nm. The FWHM of the X-ray (002) ω-scan rocking curve of the AlN buffer layer was reduced to 11.3 arcsec by introducing the periodic ML-AlN, as compared with 86.8 arcsec for layers grown without the ML-AlN approach. AlN layers with an atomically flat surface are obtained without cracks. Mechanisms responsible for the annihilation of dislocations at ML-AlN layer are under investigation. Detailed aspects of MOCVD AlN layer growth, employing the periodic multiple-layer approach, will be described. Dependence on sapphire preparation and growth conditions will be presented, including the influence of the pre-growth annealing, the nitridation conditions, and both LT and HT growth conditions will be discussed. This work at TTU was partly supported by the National Science Foundation (ECS-0609416), U.S. Army CERDEC (W15P7T-07-D-P040), and J. F Maddox Foundation.
9:00 PM - I5.5
Properties of Digital Aluminum Gallium Nitride Alloys Grown via Metal Organic Vapor Phase Epitaxy.
L. Rodak 1 , D. Korakakis 1 2
1 Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, West Virginia, United States, 2 , National Energy Technology Laboratory, Morgantown, West Virginia, United States
Show AbstractDeep Ultra Violet (UV) emitters are of particular interest for applications including, but not limited to, biological detection and sterilization. Within the III-Nitride material system, Aluminum Gallium Nitride (AlxGa1-xN) alloys are the most promising for UV device fabrication due to the wide, direct band gap. The growth of high quality AlxGa1-xN alloys via Metal Organic Vapor Phase Epitaxy (MOVPE) is challenging due to large sticking coefficient of the Al species compared to that of Ga and also the high reactivity of Al precursors. As a result, films are often characterized by large dislocation densities, cracks, and poor conductivity. Digital alloy growth, or Short Period Superlattices (SPS), consisting of layers of binary or ternary alloys with a period thickness of a few monolayers has been shown to be a viable means of growing high quality ternary alloys via Metal Organic Vapor Phase Epitaxy (MOVPE) [1]. In certain materials, such as AlGaInP, the electronic properties of digitally grown alloys differ considerably from the equivalent random alloy [2]. Specifically, the bandgap has been shown to differ significantly from the equivalent random alloy. As a result, digital alloy growth presents the potential to further engineer material properties. However, the influence of digital growth on the electronic properties of III-Nitride alloys has not been extensively characterized. This study focuses on Aluminum Gallium Nitride (AlxGa1-xN) alloys grown using a digital technique via MOVPE. The differences in material and electronic properties between the digital and random growth techniques will be discussed. The influence of the growth technique over a wide range of compositions will also be reported. [1] M. E. Hawkridge, Z. Liliental-Weber, H. Jin Kim, S. Choi, D. Yoo, J. Ryou, and R. Dupuis. Appl. Phys. Lett. 94, 071905 (2009).[2] O. Kwon, Y. Lin, J. Doeckl, and S. Ringel. J. Electronic Materials, 34, 1301 (2005).
9:00 PM - I5.6
Studies of Ni and Co Doped Amorphous AlN for Magneto-optical Applications.
Wojciech Jadwisienczak 1 , Hiroki Tanaka 1 , Marty Kordesch 2 , Aurangzeb Khan 3 , Savas Kaya 1 , Ravikiran Vuppuluri 1
1 School of EECS, Ohio University, Athens, Ohio, United States, 2 Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States, 3 Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio, United States
Show AbstractTransition metals (TM) doped group-III nitride semiconductors are well known for their potential in spintronics [1]. The room-temperature magneto-optic Kerr effect (MOKE) in the magnetic semiconductor materials combines the magnetic and optical properties of the materials together, making them attractive for the development of magnetic recording media, as well as for multifunctional photo-magneto-electronics applications including sensors where the strong light-matter coupling is essential [2]. In this paper we report on magneto-optical investigations of Ni and Co doped amorphous AlN thin films. The a-AlN was grown by rf-sputtering on Si (0001) substrates at low temperature and doped with Ni and Co at various concentrations. As-grown materials were annealed up to 700 C in nitrogen ambient and analyzed with XRD and EDX spectroscopy. No evidence of the metal clusters or phase segregations was observed in as-grown and annealed samples implying that the material retained its amorphous nature after the thermal treatment. The morphological changes of surface during high temperature annealing have been investigated by AFM microscopy in terms of the RMS and surface roughness. Significant enhancement of the magnetization in the Ni and Co-doped a-AlN was observed in annealed materials as indicated by the polar MOKE measurements. The largest remnant magnetization and coercivity field were observed for Ni- and Co- doped a-AlN materials annealed at 650 C and 400 C samples, respectively. The studied materials have shown strong magnetic isotropy in polar geometry whereas the MOKE measurement in longitudinal geometry did not show a measurable signal. This indicates that the magnetization in the active Ni and Co-doped a-AlN layers is out of plane. Furthermore, we will present and discuss the Kerr rotation spectra measured for studied TM-doped a-AlN materials focusing on the relation between the Kerr rotation and TM concentrations. Finally, we will compare the obtained results with other available magnetic semiconductor materials in terms of their potential magneto-optical applications. [1] S. Wolf, D. Awschalom, R. Buhrman, J. Daughton, S. Von Molnar, M. Roukes, A. Chatchelkanova, and D. Treger, Science, 294, 1488 (2002). [2] H. Song, L. Mei, Y. Ahang, S. Yan, X. Ma, Y. Wang, Z. Zhang, L. Chen., Physica B, .388, 130 (2007).
9:00 PM - I5.7
Homoepitaxial Deposition of AlN on (0001)-oriented AlN Substrates by MOCVD.
Anthony Rice 1 , Ramon Collazo 1 , Seiji Mita 2 , James Tweedie 1 , Jinqiao Xie 2 , Rafael Dalmau 2 , Zlatko Sitar 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 , HexaTech, Inc., Morrisville, North Carolina, United States
Show AbstractSingle crystalline AlN is a promising substrate material for AlGaN based UV laser diodes, UV photodetectors, and UV light-emitting diodes owing to similar thermal expansion coefficients and the small lattice mismatch. AlN thin films were deposited on single crystalline (0001)-oriented, Al-polar AlN substrates by metalorganic chemical vapor deposition (MOCVD). All AlN substrates used in this study were mechanically polished (MP) for planarization, and a number of the substrates were subsequently chemo-mechanically polished (CMP). Substrates were cleaned with solvents and acids prior to loading in the growth reactor. The growth reactor was pumped to high vacuum before being back-filled with NH3 and increasing the reactor temperature to 1000–1250 °C. Following this NH3 treatment, ~1 µm thick AlN films were deposited at 1100–1250 °C under 20 Torr total pressure with a V/III ratio of 250–500 in either N2 or H2 diluent. Decoration of the substrate microstructural defects, such as scratches, voids, and low-angle grain boundaries, by the epilayer suggests nucleation of AlN grains at these sites. Atomic force microscopy (AFM) characterization of regions away from such macroscopic defects showed uniform terraces with step heights of ~2 nm. Step widths of the terraces varied with wafer miscut away from AlN (0001), ranging from ~100 nm to ~350 nm for wafers with ~13° and <1° miscut, respectively. Hexagonal pits of 300–500 nm width were observed by AFM in the as-grown epitaxial films with site density ~107 cm-2. The pits exhibited a hexagonal pyramid morphology with sidewalls inclined ~30° or ~60° relative to the sample normal. The pitting is thought to be unrelated to threading dislocations as AlN steps were not pinned at such pits and threading dislocation densities of the AlN wafers were below 105 cm-2. Triple axis high-resolution x-ray diffraction measurements of the (0002) Bragg peaks (i.e., 2θ-ω scans which are highly sensitive to lattice dilations) of AlN thin films deposited on CMP wafers indicated that the films were epitaxial and strain-free. On the other hand, AlN films deposited on wafers that were not CMP exhibited shoulders on the (0002) 2θ-ω scan peaks, suggesting that the AlN epitaxial layers were strained, likely due to re-nucleation on mechanically damaged surfaces. Calibrated secondary ion mass spectrometry analysis of homoepitaxial films indicated a significant reduction in the incorporation of unintentional impurities relative to the substrates: the concentrations of carbon, oxygen, and silicon were 3×1018, 5×1018, and less than 1×1018 cm-3, respectively; while the concentrations of these impurities in the bulk substrates were ~1×1019 cm-3. Results of epilayer growth on MP and CMP substrates will be presented, and implications for AlN homoepitaxy will be discussed.
9:00 PM - I5.8
Thermodynamic Analysis and Purification for Source Materials in Sublimation Growth of Aluminum Nitride.
Li Du 1 , James Edgar 1
1 , Kansas State Univeristy, Manhattan, Kansas, United States
Show AbstractSource material purification according to the thermodynamic analysis is reported for sublimation growth of aluminum nitride. The study is based on available thermodynamic data and sublimation growth experiment. The Al-O-H-N system was studied to determine the species most responsible for the vapor transports of oxygen impurities from the growth environment and AlN source. OAlOH is strongly favored over all other possible oxygen containing compounds, while Al2O, proved to be the most favorable oxygen containing gas species for Al-O-N system in previous study, become secondary favorable gas species. The low-temperature source treatment (950 ~ 1000°C) is effective in eliminating oxygen and hydrogen from the source powder as aluminum hydroxide (AlOOH or Al(OH)3) is not stable at this temperature. Carbon monoxide was shown to be another important oxygen containing gas species in Al-O-C-N and Al-O-H-C-N system, and is favored over Al2O at lower temperature and higher pressure. Carbothermic reduction can further minimize the oxygen concentration after the low-temperature source treatment, and does not consume any AlN source. High-temperature sintering continues minimize oxygen concentration and reduces the specific surface area of the source. The combination of heat treatments produced a source with oxygen and hydrogen concentrations as low as 0.015 wt% O (1.9E19 atoms O cm-3) and 1.7ppm H (3.4E18 atoms H cm-3).
9:00 PM - I5.9
Ion Induced SiN Substrate and GaN Growth at Room Temperature on Si(111) Surface.
Praveen Kumar 1 , Mahesh Kumar 1 , Govind Gupta 1 , Sonanda Shivaprasad 2
1 Surface Physics and Nanostructures Group, National Physical Laboratory New Delhi, New Delhi, Delhi, India, 2 , Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore India
Show Abstract GaN and related nitride semiconductors have attracted great attention in view of their wide applications in light emitting diodes, full spectrum solar cells, laser diodes and high temperature & high power electronic devices. Various substrates such as c-plane sapphire, 6H-SiC (0001), GaAs (111) and Si (111) have been used for Wurtzite GaN growth due to the lack of availability of large sized single crystal GaN. However, Si substrate for the GaN epitaxial growth is very attractive because of its large-scale industrial production due to a mature technology and low cost and large diameter wafers. However lattice mismatch and thermal expansion coefficient differences between GaN and Si, affect GaN growth and induce defect formation. Thus several Si modifications are being attempted to reduce defect density. In the present study ion-induced conversion of Si (111) surface into silicon nitride at room temperature is optimized and used as substrate for the growth of Ga films. These Ga films are again nitrided by optimal N+ ion bombardment. Experiments have been performed in-situ in an ultra high vacuum chamber equipped with a Ga source and X-ray photoelectron spectrometer (XPS) at base pressure of 2x10-10 torr. The energy dependence of the nitridation is carefully performed at constant flux. The results clearly demonstrate the Si-N bond formation after threshold energy of 2 keV and the formation of GaN layer after 800eV of N2+ ion bombardment on Si (111) 7x7 surface and Ga adsorbed silicon nitride surface respectively. The FWHM and chemical shifts in the core-level spectra of Si(2p), Ga(3d), Ga(2p) and N(1s) have been deconvoluted in to the components to see the interface reactions. The valence-band spectra show the modification of the electronic structure during the formation of silicon nitride and GaN. The secondary electron emission measurement during the nitridation process shows the dependence of work function on the chemical composition at the interface. The room temperature results demonstrate a possible approach to integrate Si and group III-nitride technologies. References:1.P. Kumar, S. Bhattacharya, Govind, B. R. Mehta and S. M. Shivaprasad, J. Nano Sci. Nanotech. 9, (2009) 5659.2.P. Kumar, L. Nair, S. Bera, B. R. Mehta and S. M. Shivaprasad, Appl. Surf. Sci. 255 (2009) 6802.
Symposium Organizers
Shangjr (Felix) Gwo National Tsing-Hua University
Joel W. Ager Lawrence Berkeley National Laboratory
Fan Ren University of Florida
Oliver Ambacher Fraunhofer-Institut für Angewandte Festkörperphysik (IAF)
Leo Schowalter Crystal IS Inc.
I6: Sensors and Related Devices
Session Chairs
Wednesday AM, December 02, 2009
Independence W (Sheraton)
10:00 AM - **I6.1
Recent Advances in Wide Bandgap Semiconductor Biological and Gas Sensors.
Stephen Pearton 1 , Fan Ren 2 , Yu-Lin Wang 2 , B. Chu 2 , K. Chen 2 , C. Chang 2 , Wantae Lim 1 , Jenshan Lin 3 , D. Norton 1
1 Materials Science and Engineering, University of Florida, Gainesville , Florida, United States, 2 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 3 Electrical Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractThere has been significant recent interest in the use of surface-functionalized thin film and nanowire wide bandgap semiconductors, principally GaN, InN, ZnO and SiC, for sensing of gases, heavy metals, UV photons and biological molecules. For the detection of gases such as hydrogen, the semiconductors are typically coated with a catalyst metal such as Pd or Pt to increase the detection sensitivity at room temperature. Functionalizing the surface with oxides, polymers and nitrides is also useful in enhancing the detection sensitivity for gases and ionic solutions. The wide bandgaps of these materials make them ideal for solar-blind UV detection, which can be use of use for detecting fluorescence from biotoxins. The use of enzymes or adsorbed antibody layers on the semiconductor surface leads to highly specific detection of a broad range of antigens of interest in the medical and homeland security fields. We give examples of recent work showing sensitive detection of glucose, lactic acid, prostate cancer and breast cancer markers and the integration of the sensors with wireless data transmission systems to achieve robust, portable sensors.
10:30 AM - I6.2
A Novel Bio-functionalization of AlGaN/GaN-pH-Sensors for AlGaN/GaN-DNA-sensors.
Stefanie Linkohr 1 , Stefan Schwarz 1 , Pierre Lorenz 2 , Stefan Krischok 2 , Volker Cimalla 1 , Christoph Nebel 1 , Oliver Ambacher 1
1 , Fraunhofer IAF, Freiburg Germany, 2 , Technical University Ilmenau, Ilmenau Germany
Show AbstractDuring the past years, there has been an increasing interest in chip-based sensor devices for medical and pharmaceutical screening as well as for monitoring of environmental properties. Due to its high chemical stability in physiological fluid and the large spontaneous polarization AlGaN/GaN-sensors are of particular importance for pharmaceutical and medical sensors. These characteristics are advantageous for the fabrication of very sensitive and robust biosensors to detect ions, bio-molecules and the bioactivity of cells in solutions, gases and polar liquids. AlGaN/GaN-sensors afford the investigation of DNA molecules and proteins after suitable preparation of GaN surface. Group III-nitrides are promising bio-sensors because of the particularly good bonding stability of bio-molecules to GaN.The AlGaN/GaN-fluid-sensors are based on the sensing principles of a GaN/AlGaN/GaN heterostructure. Such an ion-sensitive field effect transistor (ISFET) is realized by the open gate surface placed in an appropriate measuring solution and controlled by a reference electrode. Ions cause a change of the two-dimensional electron gas (2DEG) underlying the surface. Negative charge causes depletion and positive charge causes an accumulation of the 2DEG. With an achievable sensitivity (S) of 59 mV/pH these sensors are able to measure the ph value at Nernst limit. The sensitivity of the sensor was determined from the measured transfer characteristics. The key issue to get a stable pH-sensor is an appropriate passivation of the contact and inactive regions which is found in a SiO2-SiNx-multilayer.These AlGaN/GaN-pH-sensors are the basis of the AlGaN/GaN DNA biosensors. To bio-functionalize GaN with DNA, the open gate with an active area of 0,04 – 3 mm2 was terminated with hydrogen by hydrogen plasma or in a hydrofluoric acid dip. To functionalize with 10-amino-dec-1-ene molecules, which are passivated at one side with Trifluoroacetic Acid (TFAAD) and obtain covalent bonding of Olefin molecules to GaN, a photochemical method is used. Therefore the GaN surface was exposed to UV light (240 nm, 5,3 mW/cm) for 30 min – 3 h after wetting with TFAAD. The functionalization experiments showed an insular growth of the Olefin molecules at low exposure times. Olefin monolayers arise after a 3 h exposure which is tested with AFM and XPS-measurements. The sensor characteristics displayed the dependency with an increasing current and decreasing sensitivity. These functionalized devices based on the pH-sensitive AlGaN/GaN ISFET will establish a new family of adaptive, selective sensors for biomolecular detection with various potential biosensing applications.
10:45 AM - I6.3
Pressure Sensing with PVDF Gated AlGaN/GaN High Electron Mobility Transistor.
Sheng-Chun Hung 1 , Byung Hwan Chu 1 , Chih Yang Chang 2 , Chien-Fong Lo 1 , Ke-Hung Chen 1 , Yulin Wang 2 , S Pearton 2 , Amir Dabiran 3 , P. Chow 3 , G. Chi 4 , Fan Ren 1
1 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Material Science and Engineering, University of Florida, Gainesville, Florida, United States, 3 , SVT Associates, Eden Prairie, Minnesota, United States, 4 Department of Physics, National Central University, Jhong-Li Taiwan
Show AbstractWe have demonstrated a mini-pressure sensor with polyvinylidene difluoride (PVDF) gated AlGaN/GaN high electron mobility transistors (HEMTs). Piezoelectric materials are used widely as sensitive pressure sensors and piezoelectric gauges are typically fabricated with materials such as PZT, lithium niobate and quartz. In 1969, Kawai found a very high piezoelectric effect in polarized polyvinylidene fluoride (PVDF). Since then, polarized PVDF has also become an important piezoelectric material due to its flexibility, low density, low mechanical impedance and easy fabrication as a ferroelectric. In this work, we integrated the PVDF thin film with HEMTs by depositing PVDF on the gate region (2 X 50 μm2) of the HEMT with an inkjet plotter. In order to display the piezoelectric characteristics, the PVDF was needed to be polarized, which was realized by applying 10 kV across the PVDF film at 70 oC. The polarized PVDF gated HEMT exhibited significant changes in channel conductance upon exposure to different ambient pressures. Variations in ambient pressure induced changes in the charge in the polarized PVDF, leading to a change of surface charges on the gate region of the HEMT. Changes in the gate charge were then amplified through the modulation of the drain current in the HEMT. By reversing the polarity of the polarized PVDF film, the drain current dependence on the pressure could be reversed. In this talk, we will present the effect of ambient pressure on the drain current of GaN/AlGaN HEMT sensors with a polarized PVDF thin film coated on the gate area. The sensitivity, the temporal resolution, the detection limit, and the influence of the bias voltage polarity on the drain current will also be discussed.
11:30 AM - **I6.4
pH Responses of Ultrathin (10nm) InN based ISFETs.
Yen-Sheng Lu 1 , Yuh-Hwa Chang 2 , J. Yeh 2 , Yu-Liang Hong 3 , Shangjr Gwo 3
1 Institute of Electronics Engineering, National Tsing Hua University, Hsinchu Taiwan, 2 Institute of NanoEngineering and MicroSystems, National Tsing Hua University, Hsinchu Taiwan, 3 Department of Physics, National Tsing Hua University, Hsinchu Taiwan
Show AbstractUltrathin (~10 nm) InN ion selective field effect transistors (ISFETs) show a current variation ratio of 3% per pH decade with a response time of less than 10 s. When the ISFET is employed as an electrolyte FET, the current variation of 18% was measured as the gate bias changes from zero to 0.3 V given a drain-source voltage of 0.1 V. The high current (resistance) variation ratio enables a chemical sensor with high sensitivity and resolution, which permits detection of a slight concentration variation of the electrolyte.[1] Such a sensor is appealing in the regime of chemical and biological sensing applications. For ultrathin InN ISFETs, the high current variation ratio is attributed to the ultrathin epilayer and an unusual phenomenon of intrinsic strong electron accumulation on InN surface.[2]The pH response measurement in ultrathin (~10 nm) InN ISFETs investigated were performed in an aqueous solution titrated with diluted NaOH and HCl. The Helmholtz potential built at the electrolyte-InN interface is governed by direct adsorption of ions at the given sites of the InN surface, modulating the channel current of the InN ISFETs. The channel current monotonically decreased as the pH value of an aqueous solution increases from 2 to 10. The sensitivity and resolution were found to be 58.3 mV per decade and 0.02 pH change, respectively.[1] Y. Cui, Q. Wei, H. Park and C. M. Lieber, Science 293, 1289 (2001).[2] Y.-S. Lu, C.-L. Ho, J. A. Yeh, H.-W. Lin, and S. Gwo, Appl. Phys. Lett. 92, 212102 (2008).
12:00 PM - I6.5
Chloride Ion Detection with InN Gated AlGaN/GaN High Electron Mobility Transistor.
Byung Hwan Chu 1 , Hon-Way Lin 2 , Shangjr Gwo 2 , Yu-Lin Wang 3 , S. Pearton 3 , J. Johnson 4 , P. Rajagopal 4 , J. Roberts 4 , E. Piner 4 , K. Linthicum 4 , Fan Ren 1
1 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Physics, National Tsing-Hua University, Hsinchu Taiwan, 3 Department of Material Science and Engineering, University of Florida, Gainesville, Florida, United States, 4 , Nitronex Corporation, Durham, North Carolina, United States
Show AbstractWe have demonstrated a chloride ion (Cl-) sensor with InN gated AlGaN/GaN high electron mobility transistors (HEMTs). InN is a small bandgap semiconductor material which has potential application in solar cells and high speed electronics. It exhibits an electron accumulation phenomenon which is very unusual in III-V semiconductors. Positively charged surface donor states on InN function as fixed surface sites for reversible anion coordination. Because of this property, InN has been proposed as a useful material for sensing applications. In this work, we deposited a 10nm-thick InN film on the gate region (2 X 50 μm2) of the HEMT by molecular-beam epitaxy. The InN gated HEMT exhibited significant changes in channel conductance upon exposure to different concentrations of chloride solutions. Variations in chloride concentration induced change of surface charges on the gate region of the HEMT. Changes in the gate charge were then amplified through the modulation of the drain current in the HEMT. In this talk, we will present the effect of chloride ions on the drain current of AlGaN/GaN HEMT sensors with an InN thin film coated on the gate area. The sensitivity and durability of the sensor and the influence of pH on the drain current will also be discussed.
12:15 PM - I6.6
High Sensitivity of Hydrogen Sensing Through N-polar GaN Schottky Diodes.
Yu-Lin Wang 1 , Byung Hwan Chu 2 , Chihyang Chang 2 , Ke-Hung Chen 2 , Yu Zhang 3 , Qian Sun 3 , Jung Han 3 , Steve Pearton 1 , Fan Ren 2
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 3 Electrical Engineering, Yale University , New Haven, Connecticut, United States
Show AbstractN-polar and Ga-polar GaN grown on c-plane sapphire by a metal-organic chemical vapor deposition (MOCVD) system were used to fabricate platinum deposited Schottky contacts for hydrogen sensing at room temperature. Wurtzite GaN is a polar material. Along the c-axis, there are N-face (N-polar) or Ga-face (Ga-polar) orientations on the GaN surface. The Ohmic contacts were formed by lift-off of e-beam deposited Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (1200 Å). The contacts were annealed at 850°C for 45 s under a flowing N2 ambient. Isolation was achieved with 2000 Å plasma enhanced chemical vapor deposited SiNx formed at 300°C. A 100 Å of Pt was deposited by e-beam evaporation to form Schottky contacts. After exposure to hydrogen, Ga-polar GaN Schottky showed 10% of current change, while the N-polar GaN Schottky contacts became fully Ohmic. The N-polar GaN Schottky diodes showed stronger and faster response to 4% hydrogen than that of Ga-polar GaN Schottky diodes. The abrupt current increase from N-polar GaN Schottky exposure to hydrogen was attributed to the high reactivity of the N-face surface termination. The surface termination dominates the sensitivity and response time of the hydrogen sensors made of GaN Schottky diodes. Current-voltage characteristics and the real-time detection of the sensor for hydrogen were investigated. These results demonstrate that the surface termination is crucial in the performance of hydrogen sensors made of GaN Schottky diodes.
12:30 PM - I6.7
Surface-charge Lithography for the Fabrication of Gallium Nitride Based Gas Sensors.
Tim Schuller 1 , Peter Heard 2 , Veaceslav Popa 3 , Olesea Volciuc 3 , Ion Tiginyanu 3 4 , Jo Das 5 , Stefan Degroot 5 , Marianne Germain 5 , Andrei Sarua 1 , Martin Kuball 1
1 H.H. Wills Physics Laboratory, University of Bristol, Bristol United Kingdom, 2 Interface Analysis Centre, University of Bristol, Bristol United Kingdom, 3 National Center for Materials Study and Testing, Technical University of Moldova, Chishinau Moldova (the Republic of), 4 Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau Moldova (the Republic of), 5 , Interuniversity Microelectronics Center, Leuven Belgium
Show AbstractThe development of electronic gas sensors capable of operation in extreme environmental conditions (notably high temperature and chemically aggressive ambients) is an area of considerable interest. The outstanding stability of Gallium Nitride (GaN) together with a remarkable progress in GaN device technologies in recent years makes this material an ideal candidate for these applications. Surface-adsorption-induced conductivity change has been previously proposed to be responsible for the transducing mechanism of GaN resistive gas sensors. Therefore, to improve the sensitivity of these devices an enhanced surface to volume ratio has to be achieved compared to a bulk or thin film topology. This is possible by integration of micro- or nanostructure into a GaN sensor. Here we demonstrate the use of novel surface-charge lithography (SCL) for the controlled fabrication of nanostructures from n-type GaN, for gas sensing applications.Surface-charge lithography combines focused Ga ion-beam (FIB) treatment with photoelectrochemical (PEC) etching. This quick and maskless approach directly writes patterns of negative surface charge, which are shielded from etching when the sample is placed in KOH (0.01-0.1M) solution and exposed to UV optical excitation from a mercury lamp. Regions which have not been exposed to the ion beam are quickly etched due to the oxidising action of the photogenerated holes and electrolyte. Thanks the FIB instrument’s superior resolution it is possible to etch structures with lateral dimensions as small as ~100nm using this process, limited by the surface depletion region width in our samples.Resistive hydrogen gas sensors are fabricated by depositing a pair of Ti/Al/Ti/Au ohmic contacts about 200 µm apart onto an n-type ~4 µm thick epitaxial GaN layer (Si doped; 1.2x1018 cm-3) grown by molecular organic chemical vapour deposition on a sapphire substrate. We report on resistive gas sensing devices consisting of various types of electrically connected GaN nano-wires, fabricated in a controlled manner using the SCL method. These devices do not rely on the thermally unstable catalytic Schottky gate contacts, therefore have a potential to be operated at relatively high temperatures (up to 500-700°C).
I7: Energy Conversion
Session Chairs
Wednesday PM, December 02, 2009
Independence W (Sheraton)
2:30 PM - **I7.1
Applications of Group III-Nitride Alloys for Solar Power Conversion.
Wladek Walukiewicz 1
1 Solar Energy Materials Research Group, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractDiscovery of the low energy gap of InN greatly expanded the range of direct band gaps of group III-nitride alloys offering an unprecedented potential for using these materials in a variety of optoelectronic and photovoltaic devices. I will review recent progress in applications of group III-nitride alloys for solar power conversion devices. Most of the research in this area focuses on single junction cells with the energy gap of about 1.7 to 2.4 eV which requires controlled and reproducible growth of uniform n- and p-type doped In1-xGaxN films with 0.55
3:00 PM - I7.2
Plasmonic Nanoparticle Enhanced Light Absorption in InGaN Quantum Well Solar Cells.
Imogen Pryce 1 , Daniel Koleske 2 , Arthur Fischer 2 , Harry Atwater 1
1 Thomas J Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, United States, 2 Semiconductor Materials and Device Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractThe design of high efficiency low cost thin film solar cells is inherently limited by the absorption properties of the absorbing layer. Increasing cell efficiency and reducing the amount of material used necessitates exploring new ways of increasing the absorption in photovoltaic materials. Recently it has been shown that metallic nanoparticles can be used to enhance absorption in thin film solar cells. We have previously demonstrated enhanced solar cell absorption and photocurrent due to scattering by nanoparticles in a single InGaN quantum well photovoltaic device based on a standard LED design. Nanoparticles scatter light diffusely into the device leading to enhanced absorption and effective path length increase for the incident light. In optically thin films, nanoparticles scatter light into guided wave modes thereby enhancing absorption. Here we extend this work in order to design an optimum cell based on plasmonic nanoparticle enhancement. Solar cell structures were grown via metallorganic chemical vapor deposition with an active layer consisting of a 2.5 nm InGaN quantum well with a band gap of 2.67 eV. The p- and n-type layers are GaN with the n-type layer serving as a 2-3 µm buffer layer for growth. The spectrally-resolved photocurrent is measured both before and after nanoparticle deposition under the AM 1.5 solar spectrum at 1 Sun illumination. Cells decorated with 100 nm nanoparticles show a 15% increase in total photocurrent over control cells. The current-voltage and spectra response characteristics were modeled using the AFORS Het device physics transport model. Both the experimental optical absorption data and experimental spectral response are reproduced in the AFORS Het model, suggesting that this is a useful platform for optimization of the device design. Preliminary modeling of the p-type GaN shows that thinning the layer from 200 nm to 20 nm will increase external quantum efficiency of the cell at the band edge by up to 20% and result in an overall photocurrent enhancement of up to 23%. We will discuss fabrication of nanoparticle arrays using a mechanically masked nanolithography process. We fabricate anodic aluminum oxide templates with controllable inter-pore spacing ranging from 100 nm to 300 nm, which are then used as mechanical masks for thermal evaporation of Ag through the mask to form ordered arrays of Ag nanoparticles on the surface of the InGaN quantum well solar cells. Given that both particle size and spacing can be varied, the resonance of the nanoparticles can be tailored to the optimized cell. Finite-difference time-domain full field electromagnetic simulation is used to guide the optimization of particle array design for scattering into the substrate.
3:15 PM - I7.3
Improvement of Photoelectrochemical Reaction for Hydrogen Generation from Water using N-face GaN.
Katsushi Fujii 1 , Keiichi Sato 1 , Takashi Kato 1 , Tsutomu Minegishi 1 2 , Takafumi Yao 1
1 Center for Interdisciplinary Research, Tohoku University, Sendai, Miyagi, Japan, 2 School of Engineering, University of Tokyo, Bunkyo-ku, Tokyo, Japan
Show AbstractPhotoelectrochemical water splitting is one of expecting renewable energy techniques. Nitride semiconductors are good candidates for the photo-illuminated working electrodes considered from the band edge energy and the stability in solutions. The energy conversion efficiency improvement is, however, one of the key issues to realize the photoelectrochemical hydrogen generation. Not only the band gap shrinkage but also the reduction of generated carrier loss improves the efficiency.The surface polarity could also affect the efficiency because of the difference in surface chemistry. In this report, we investigated the effects of the Ga- and N-face of GaN. We used both Ga- and N-face of 1.5 mm thick free-standing n-type GaN grown by hydrogen vapor phase epitaxy (HVPE). The carrier concentration and mobility were 1.8 x 1018 cm-3 and 280 cm2/Vs, respectively. The contact electrodes were the backsides of the photoelectrochemical reaction faces. The GaN samples grown by metal-organic vapor phase epitaxy (MOVPE) (n-type, C.C. = 2.2 ~ 10.0 x 1018 cm-3, 4.2 ~ 4.4 mm thickness) were also used as references. The contact electrodes were placed at the front surface edges because the insulating sapphire substrates were used. Thus, the series resistances for the reference samples were higher than those for the GaN grown by HVPE. The electrolytes were 1.0 mol/L HCl and NaOH aqueous solutions. The counterelectrode was Pt. The Ag/AgCl/NaCl reference electrode was used when it was needed.The flatband potentials of N-face GaN in both solutions obtained from the Mott-Schotky plot were about 0.2 V negative to the Ga-face GaN. Interestingly, even the flatband potentials of N-face GaN were about 0.3 V positive to those of the Ga-face GaN grown by MOVPE. The order of the turn on voltage versus reference electrode was Ga-face MOVPE GaN < N-face GaN < Ga-face GaN as expected from the flatband potentials. The order of the saturated photocurrent density at the positive biased region was Ga-face GaN > Ga-face GaN grown by MOVPE >= N-face GaN. The lower density of Ga-face GaN grown by MOVPE can be explained by the higher series resistance.The order of the turn on voltage for photocurrent density without reference electrode changed to be N-face GaN < Ga-face GaN < Ga-face GaN grown by MOVPE. The positive shift of Ga-face GaN grown by MOVPE can be explained by the high series resistance of the sample. The photocurrent density of N-face GaN at zero bias was the highest of the three as expected from the turn on voltage. The density was over 3 times higher than that of Ga-face GaN and was almost stable over 600 min.These results would be explained that slightly positive charged N-face GaN by spontaneous polarization. Positive charged surface can be enhanced the physical absorption of OH--ion at the surface and can help hole diffusion from semiconductor to electrolyte.
3:30 PM - I7.4
Au Nanoparticles Modified GaN Photoelectrodes for H2 Gas Generation.
Wen-Hsun Tu 1 2 , Yu-Kuei Hsu 1 , Li-Chyong Chen 3 , Kuei-Hsien Chen 1 , Chih-I Wu 2 , Cheng-Hsiung Yen 4
1 Institude of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan, 2 Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei Taiwan, 3 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, 4 Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung Taiwan
Show AbstractFor GaN, n-GaN nanowire shows very high photocurrent gain[1][2] and high sensibility to DNA[3], which worth to apply to the energy application. However, the growth of p-GaN nanowire is much more hard than n-GaN. Therefore, in this study, we compare the photo-electrochemical (PEC) properties of n- and p-GaN film modified with gold nano-particles for water splitting. To prevent uniformity variations within the wafer, we measured the same sample prior to and after Au nano-particles deposition. The Au nano-particles was deposited on GaN by sputtering system. The PEC property was measured in 1M HCl solution with Ag/AgCl electrode as reference electrode, Pt as counter electrode. As for the light source, we use xenon lamp with filters to make the spectrum similar to the sunlight, the light power was fixed at 100mW/cm2. The PEC current-voltage curve shows very different properties between n- and p-GaN with and without Au nano-particle. For p-GaN, Au nano-particles deposition enhanced the photocurrent, whereas n-GaN showed suppressed result. Although the saturated photocurrent is similar to the original p-GaN, the zero-bias versus Pt counter electrode measurement show significant advantage with Au nano-particles addition. The IPCE test demonstrates the enhanced efficiency, the highest IPCE increased from 3% to 24% at 300nm. The photocurrent increased from -0.016 to -0.418 mA/cm2 upon Au nano-particles addition. That means we have H2 generation directly without any external bias. The highest IPCE under zero bias is about 24% at 300nm. From the shifted on-set potential, the mechanism was explained by the difference of surface band bending between GaN/ solution and GaN/ Au NP/ solution interface.[1] “Ultrahigh photocurrent gain in m-axial GaN nanowires,” R. S. Chen, H. Y. Chen, C. Y. Lu, C. P. Chen, L. C. Chen, Y. J. Yang, and K. H. Chen*, Appl. Phys. Lett. 91, 223106-(1-3) (2007).[2] “On-chip fabrication of well aligned and contact barrier-free GaN nanobridge devices with ultrahigh photocurrent responsivity,” R. S. Chen, S. W. Wang, Z. H. Lan, J. T. H. Tsai, C. T. Wu, L. C. Chen, K. H. Chen*, Y. S. Huang, C. C. Chen, ACS Small 4, 925-929 (2008).[3] “Label-free dual sensing of DNA molecules using GaN nanowires,” C. P. Chen, A. Ganguly, C. H. Wang, C. W. Hsu, Y. K. Hsu, Y. C. Chang, K. H. Chen, and L. C. Chen, Anal. Chem. 81, 36-42 (2009).
3:45 PM - I7: Energy
BREAK
I8: Nanostructures
Session Chairs
Wednesday PM, December 02, 2009
Independence W (Sheraton)
4:15 PM - **I8.1
Nanostructured Nitrides in Augmenting Light Emission and Detection.
Arto Nurmikko 1 , Jung Hanin 1
1 Engineering, Brown University, Providence , Rhode Island, United States
Show Abstract4:45 PM - I8.2
Carrier Confinement in GaN/AlGaN Nanowire Heterostructures.
Florian Furtmayr 2 , Joerg Teubert 1 , Pascal Becker 1 , Jordi Arbiol 3 , Sonia Conesa-Boj 3 , Sonia Estrade 3 , Peiro Francesco 3 , Juan-Ramon Morante 3 4 , Martin Stutzmann 2 , Martin Eickhoff 1 2
2 Walter Schottky Institut, Technische Universitaet Muenchen, Muenchen Germany, 1 I. Physikalisches Institut, Justus-Liebig-Universitaet Giessen, Giessen Germany, 3 Departament d’Electrònica, Universitat de Barcelona, Barcelona, CAT Spain, 4 IREC, Catalonia Institute for Energy Research, Barcelona, CAT Spain
Show AbstractDue to the low density of structural defects and the absence of strain, group III-nitride nanowires are an ideal model system for the analysis of basic material properties. In addition, nanowire heterostructures present a promising approach for the realization of improved optoelectronic or nanoelectronic devices. We report on the optical properties and carrier confinement in AlGaN/GaN nanowire heterostructures grown by plasma assisted molecular beam epitaxy on Si(111) substrates. Stacks of 9 GaN nanodiscs (NDs) embedded in GaN/AlxGa1-xN nanowires with Al content x between 9% and 100% were realized. The ND height along the (0001) growth direction was varied between 1.5 nm and 4 nm. Photoluminescence measurements showed an increase of the emission energy with increasing Al content up to Al concentrations of approximately 40%, indicating the dominant influence of confinement effects in this regime. For higher Al concentrations, the emission energy decreases due to the increasing influence of the polarization-induced quantum confined Stark effect. This is further confirmed by the decrease of the emission energy in AlN/GaN ND stacks from 3.58 eV to 2.71 eV when the ND height is increased from 2 nm to 4 nm. In comparison, AlxGa1-xN/GaN stacks with x = 19 % only show a decrease from 3.58 eV to 3.51 eV for the same ND heights. The strong confinement in AlxGa1-xN/GaN MQWs leads to a weak thermal quenching of the emission intensity by a factor of 3 between 5K and room temperature for an Al content of approximately 35%, whereas for x = 19% the luminescence intensity is quenched by more than a factor of 30. The influence of the Al content in the barriers and the structural properties of the heterostructures on the electric fields in the nanodiscs, the thermal stability of the photoluminescence, and the emission broadening will be discussed.
5:00 PM - I8.3
Catalyst-free Selective Area Growth of InN Nanocolumns by MBE and Electrical Conductivity of Single InN Nanowires in Four-point Probe Measurements.
Christian Denker 1 , Boris Landgraf 1 , Florian Werner 1 , Friedrich Limbach 1 2 , Michael Carsten 1 , Joerg Malindretos 1 , Angela Rizzi 1
1 IV. Physikalisches Institut, Georg-August University, Goettingen Germany, 2 IBN-1, Forschungszentrum Jülich, Jülich Germany
Show AbstractIn the search of new concepts for the reduction of power consumption in large scale integrated circuits, nanowire transistors are intensively studied. Moreover, due to their high surface to volume ratio, nanowires are potential candidates for sensor applications and they are also discussed in connection with next generation solar cells. To fully exploit the potential of InN nanowires, their physical properties have to be understood in depth. The self-organized formation of InN nanocolumns in molecular beam epitaxy (MBE) is quite straightforward achieved under N-rich conditions on Si(111). A drawback for the characterization of the intrinsic physical properties of these new structures though is represented by the distribution of sizes, shapes and orientations in the ensemble. One approach is to separate the nanowires from the ensemble and perform the characterization of single objects. In this way we have investigated the electrical properties of InN nanowires in four-point probe measurements. Typical wires used for our measurements were between 800 nm and 1.5 μm long with radii in the range between 10 and 75 nm. The dependence of the conductance on the wire diameter allows distinguishing between ″core″ bulk (quadratic) and ″shell″ sheet (linear) contributions. Evidence of the formation of a thin In2O3 layer at the surface of the nanowires is provided by X-ray core level photoemission spectroscopy. The shell conductivity is therefore ascribed to an electron accumulation layer forming at the radial InN/In2O3 interface. Although conductance through the accumulation layer dominates for nanowires below a critical diameter of about 55 nm, the core channel cannot be neglected, even for small nanowires.A second approach that would allow for the growth of single, well separated columns with a controlled size, position and shape is selective area growth (SAG). Selective area growth of InN in MBE was only shown on a hole-patterned GaN template by using a focused ion beam to etch pitches into the substrate [1]. On Si(111) substrates, SAG has been demonstrated for MBE grown GaN nanocolums by Kishino et al [2]. In that work the authors used a titanium mask, but high substrate temperatures of at least 900°C are necessary for the occurrence of SAG. However, the much higher N2 equilibrium pressure for InN requires much lower growth temperatures of TS≈475°C. In this work we demonstrate SAG of InN nanocolumns by plasma assisted molecular beam epitaxy. The growth of arrays of single nanorods on a Si(111) substrate has been achieved with a thin molybdenum mask lithographically patterned with holes smaller than 60 nm. This is an important step towards the preparation of InN nanocolumns of controlled shape and size, providing exciting building blocks for nanoscale assemblies, structures, and devices.[1] S. Harui et al. J. J. Appl. Phys., 47, 5330, 2008.[2] K. Kishino et al. Electronics Letters, 44, 819, 2008.
5:15 PM - I8.4
AlxGa1-xN Epitaxial Columnar Nanostructures.
Ratnakar Palai 1 , J. Wu 1 , M. Rodriguez 1
1 Department of Physics, University of Puerto Rico, San Juan, Puerto Rico, United States
Show AbstractHeterostructures with multiple or tunable bandgaps holds promise for light emission and absorption over a wide spectral range. The most appealing application is the fabrication of multijunction solar cells for enhancing efficiency by absorbing the whole solar spectrum. In the present work, nanostructures of AlxGa1-xN(x = 0.1, 0.2, and 0.4) have been fabricated on Al2O3 and Si substrates with AlN buffer layer using plasma assisted molecular beam epitaxy (MBE). An attempt has been made to change the growth mechanism from 2D layer (film)-like to 3D columnar nanostructures. 3D-Columnar structures are very useful for device fabrication because of minimal lattice mismatch. Structural, microstructural, optoelectronic (photoluminescence, cathode luminescence, and x-ray photoelectron spectroscopy), and magnetotransport properties of the nanostructures were studied using different analytical techniques in order to understand the physical properties. Cross-sectional scanning electron microscopy of the nanostructures shows three well distinct layers showing 3D-like columnar structure. No defects and imperfections have been found at the interface. X-ray diffraction shows AlxGa1-xN nanostructures are highly epitaxial epitaxial with c-axis orientation. Cathode luminescence measurement of AlxGa1-xN(x = 0.1, 0.2, and 0.4) nanostructures shows blue emission at room temperature.
5:30 PM - I8.5
Controlled Epitaxy and Characteristics of InN Nanopyramids by MOCVD.
Muhammad Jamil 1 , Tianming Xu 1 , Tahir Zaidi 1 , Andrew Melton 1 , Ian Ferguson 1 , Chee-Loon Tan 2 , Boon-Siew Ooi 2
1 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Center for Optical Technologies, Lehigh University, Bethlehem, Pennsylvania, United States
Show AbstractIn this work, we report on the growth of unidirectional InN nanopyramids by using a simple pattern-free epitaxial technique. These nanopyramids were grown on GaN layers deposited on sapphire via metalorganic chemical vapor deposition (MOCVD). The successful growth of high density self-assembled InN nanopyramids offers a simple technique to design cold electron emitters. In addition, high quality nanopyramids which allow excitonic non-linearities could potentially be used in heterostructures for optical communications. The growth of InN films was conducted in a vertical reactor with TMIn and NH3 as the In- and N-precursors, respectively. InN films free of metallic droplets were achieved using a pulsed MOCVD technique. In this mode of deposition, NH3 was constantly flowing while the TMIn was delivered to the reactor chamber for a 30s pulse then routed away from the reactor chamber for 18s, giving a total cycle time of 48s. From our studies, we found that at a growth pressure of 100 Torr, the optimum growth temperature and V/III ratio of the InN film are 525-550°C and ~3×104, respectively. It was found that the epitaxy of InN nanopyramids is particularly sensitive to the growth pressure and temperature for a given V/III ratio of input precursors. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the surface morphology of the films. X-ray diffraction (XRD) measurement revealed the (0002) texture of the InN nanopyramids. The XRD full width at half maximum (FWHM), as low as 0.129° for the InN (0002) ω rocking curve, indicates very good crystal quality of InN nanopyramids. PL analysis of the films has been performed both at room temperature and at 77K. The bandgap of the InN nanopyramids as measured by photoluminescence is 0.78eV. Control of the nanopyramid density using growth conditions will be discussed, as well as the superior optical and crystal properties achieved.
I9: Poster Session: III-Nitride Characterization and Applications
Session Chairs
Thursday AM, December 03, 2009
Exhibit Hall D (Hynes)
9:00 PM - I9.1
Direct Microscopic Correlation of Real Structure and Recombination Kinetics in Semipolar grown InGaN Quantum Well.
Sebastian Metzner 1 , Frank Bertram 1 , Juergen Christen 1 , Thomas Wunderer 2 , Frank Lipski 2 , Stephan Schwaiger 2 , Ferdiand Scholz 2
1 , Otto-von-Guericke-University Magdeburg, Magdeburg Germany, 2 , University of Ulm, Ulm Germany
Show AbstractThe development of efficient InGaN/GaN based light emitting devices is hindered by built-in electric fields. These fields causing the quantum confined Stark effect arise from piezoelectric and spontaneous polarization. One approach for beating the negative impact of internal polarization fields of group-III nitrides is the growth on semi- or nonpolar crystal planes. The sample under investigation was grown by MOVPE. On top of a 2μm thick GaN buffer a 200nm thick SiO2 mask was patterned into hexagons. Subsequent MOVPE overgrowth results in the formation of 3D inverse pyramids due to selective area growth. The resulting surface planes of the inverted pyramids are formed by {10-11} and {11-22} facets. An InGaN single quantum well covered by a thin GaN cap was subsequently grown on top of the facets.We present results from ps-time+nm-spatially resolved cathodoluminescence (CL) microscopy (δt<35ps, δx<40nm). The electron beam is extremely fast switched on, stays “on” for a selected time, and is then extremely fast switched off by an electrostatic beam-blanking. This allows the analysis of excitation from thermal equilibrium into true steady state condition and the relaxation back into thermal equilibrium. The light is detected by a cooled micro-channel plate photomultiplier by means of single photon-counting in delayed coincidence. The focused electron beam is digitally scanned over the surface (typically 256×200 pixels) while CL transients are recorded at each pixel. The resulting data set consisting of 51,200 local CL transients i.e. CL(x,y,t), is stored and evaluated ex situ to produce: local transients, sets of time delayed CL intensity mappings, transient line scans, etc.. Using a digital box-car method, a microscopic lifetime mapping can be generated, directly revealing the nano scale kinetics in correlation with the morphology.Microscopic local transients taken at the center of the inverted pyramid exhibit a very short lifetime down to τ_initial=200ps. Upon going up to the ridge, the lifetime dramatically increases reaching τ_initial>10ns. Time delayed CL maps directly reveal the strong variation of the local lifetime. In particular at the center the CL intensity vanishes very fast in less than a few ns after switching off the excitation, whereas at the ridges the CL persists even after 55ns. The microscopic distribution of the initial lifetime is visible in lifetime mappings. It directly correlates with the micro-morphology as well as local emission wavelength of the InGaN QW. At the center we find very short emission wavelength (370nm) and here, the lifetime is very short (<0.4ns). Upon going up along the facet the emission wavelength exceeds up to 500nm where the initial lifetime becomes >13ns. Here, the persistent decay time exceeds 55ns. Normalizing the total lifetime map to the local quantum efficiency yields a microscopic map of the radiative lifetime, which directly correlates the local oscillator strength to the local SQW properties.
9:00 PM - I9.10
Fast Detection of Perkinsus Marinus, a Prevalent Pathogen of Oysters and Clams from Sea Waters.
Yu-Lin Wang 1 , Byung Hwan Chu 2 , Ke-Hung Chen 2 , Chihyang Chang 2 , Tanmay Lele 2 , Geoege Papadi 3 , James Coleman 3 , Barbara Sheppard 3 , Christopher Dungan 4 , Steve Pearton 1 , Wayne Johnson 5 , K. Linthicum 5 , P. Rajagopal 5 , J. Roberts 5 , E. Piner 5 , Fan Ren 2
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 3 Infectious Diseases & Pathology, University of Florida, Gainesville, Florida, United States, 4 Cooperative Oxford Laboratory, Maryland Department of Natural Resources, Oxford, Maryland, United States, 5 , Nitronex Corporation, Raleigh, North Carolina, United States
Show AbstractAntibody-functionalized, Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect Perkinsus marinus. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN/GaN HEMT were grown by a molecular beam epitaxy system (MBE) on sapphire substrates. Perkinsus marinus (P. marinus), a protozoan pathogen of oysters, is highly prevalent along the east coast of the United States. Perkinsus species infections cause widespread mortality in both natural and farm-raised oyster populations, resulting in severe economic losses for the shellfish industry and detrimental effects on the environment. The water samples were taken from the tanks in which Tridacna crocea infected with P. marinus were living and dead. The AlGaN/GaN HEMT drain-source current showed a rapid response of less than 5 seconds when the infected sea waters were added to the antibody-immobilized surface. The recyclability of the sensors with wash buffers between measurements was also explored. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN/GaN HEMTs for Perkinsus marinus detection.
9:00 PM - I9.12
Size Reduction and Rare Earth Doping of GaN Powders through Ball-milling.
Xiaomei Guo 1 , Tiju Thomas 2 , Kewen Li 1 , Yanyun Wang 1 , Xuesheng Chen 3 , Michael Spencer 2 , Hua Zhao 4 , Baldassare Bartolo 4
1 , Boston Applied Technologies, Inc., Woburn, Massachusetts, United States, 2 School of Electrical and Computer Engineering, Cornell University, Ithaca, New York, United States, 3 Department of Physics and Astronomy, Wheaton College, Norton, Massachusetts, United States, 4 Department of Physics, Boston College, Chestnut Hill, Massachusetts, United States
Show AbstractProper particle size and good size uniformity of starting powders are key factors to achieve desirable optical transparency for GaN ceramics since its crystallographic anisotropy effect can be reduced as the crystallite size falls into the submiron range.[1] GaN powders synthesized through a high-temperature ammonothermal process may possess better crystallinity and purity compared to other GaN powder synthesis methods. However, this process produces mostly micron-sized particles with an apparent broad size distribution.[2] Here, we report the successful size reduction and rare earth elements doping of GaN powders through a simple ball-milling process.Ball-milling of ammonothermally synthesized Eu:GaN powder with a Eu content of 0.5 at% was performed in ethanol solution for a variety of durations, resulting in average particle sizes of nanometer. Yttria-stabilized zirconia balls with mixed sizes were used with a total ball-to-powder weight ratio of around 20:1. The ball-milled powders showed noticeable brightened color and improved dispersability, indicating reduced levels of aggregation. X-ray diffraction (XRD) peaks of the ball-milled GaN powders were broadened compared to the as-synthesized powders. The broadening of the XRD peaks was partially attributed to the reduction in the average particle size, which was confirmed through SEM analyses. Rare earth doping of commercial GaN powders also were achieved through a ball-mill assisted solid-state-reaction process. Rare earth salts were mixed with GaN powder for ball-milling. The as-milled powders were heat treated at different conditions to help the dopants diffusing. Luminescence properties of the rare earth doped GaN powders at infrared and visible wavelengths were investigated and the results will be discussed. This work is supported in part by U.S. Army Research Office through contract No. W911NF-09-C-0012.[1] N. Kuramoto and H. Taniguchi, “Transparent AlN ceramics,” J. Mater. Sci. Lett. 3 471 (1984).[2] H. Wu, C. B. Poitras, M. Lipson, M. G. Spencer, J. Hunting and F. J. Di Salvo, Appl. Phys. Lett. 88 011921 (2006).
9:00 PM - I9.15
Room Temperature Ferromagnetic behavior in Yb-doped GaN Semiconductor.
J. Wu 1 , M. Rodriguez 1 , A. Rivera 1 , Ratnakar Palai 1 , H. Huhtinen 2 , K. Liu 3 , M. Shur 3
1 Department of Physics, University of Puerto Rico, San Juan, Puerto Rico, United States, 2 Department of Physics, University of Turku, Turku Finland, 3 Department of Physics, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractSince last four decades the information and communication technologies are relying on the semiconductor materials. Currently a great deal of attention is being focused on adding spin degree-of-freedom into semiconductor to create a new area of solid-state electronics, called spintronics. In spintronics not only the current but also its spin state is controlled. Such materials need to be good semiconductors for easy integration in typical integrated circuits with high sensitivity to the spin orientation, especially room temperature ferromagnetism being an important desirable property. GaN-based materials have unique properties and are extremely promising for high power and high temperature sensor applications. Development of GaN-based RT ferromagnetic semiconductors is one of the major worldwide research and development programs. In the present work Yb-doped (~1.5-2%) GaN thin films have been grown on Si (111) and Al2O3 substrates at different growth conditions using a plasma assisted molecular beam epitaxy (MBE). X-ray diffraction patterns of GaYbN thin films show films are single phase and highly epitaxial with c-axis orientation. The surface morphology of the films studied by atomic force microscopy (AFM) and electric force microscopy (EFM) shows uniform distributions grains and smooth surface. The magnetic properties of GaYbN thin films measured using a superconducting quantum interference device (SQUID) shows room temperature ferromagnetism. The magnetization vs. temperature shows no paramagnetic contribution at very low temperature as observed Mn and Eu-doped GaN thin films. The magnetic force microscopy (MFM) images show uniform distribution magnetic domains, which rules out the present of magnetic cluster or precipitation of magnetic ions. Phtotoluminescence spectra, Hall effect, and resistivity measurement of GaYbN thin films show typical semiconductor behavior.
9:00 PM - I9.16
Synthesis of Rare-Earth-Doped Metastable III-Nitride Nanopowders for Photonic Applications.
Geliang Sun 1 , Jonathan Doyle 1 , Stephen Tse 1 , Uwe Hommerich 2 , John Zavada 3 , Sudhir Trivedi 4
1 Mechanical and Aerospace Engineering, Rutgers University, Piscataway, New Jersey, United States, 2 Physics, Hampton University, Hampton, Virginia, United States, 3 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States, 4 , Brimrose Corporation, Baltimore, Maryland, United States
Show AbstractAmong III-nitride compound semiconductors, Er-doped hexagonal gallium nitride (h-GaN) and cubic boron nitride (c-BN) have gained attention for their potential application as near infrared light sources. It is well known that Er3+ ions in host crystalline materials emit temperature stable luminescence at 1.54um corresponding to an intra-4f shell transition. That transition originates at the first excited state (4I13/2) and terminates at the ground state (4I15/2). Works have reported that the Er3+ photoluminescence (PL) intensity depends strongly on both the band gap energy of the semiconductor and the host temperature. The band gaps of h-GaN and c-BN are 3.4 and 6.4 eV, respectively. Large band gaps not only offer wide wavelength range but also lower thermal quenching of the Er3+ luminescence, which is inversely proportional to the band gap of the host semiconductor. For RE doping, ion-implantation, in situ doping, MOCVD, ammonothermal route, and hydride vapor phase epitaxy (HVPE) are traditional processes. The common disadvantage of these processes is the limitation of dopant concentration in the final product due to thermal stresses or crystal mismatches between interphases. In this work, we investigate the synthesis and consolidation of c-GaN, c-AlN, and c-BN nanopowders, along with their doping using Nd, Er, and Yb. The powders are synthesized using a novel, aerodynamically-enhanced plasma process, with in-situ laser-based diagnosis of the gas-phase flow field and as-synthesized nanoparticles. In the plasma region, RE ions are mixed with other ions uniformly, and then these ions recombine as compounds, which homogeneously nucleate into nanoparticles due to supersaturation in a strong temperature gradient. As such, we can obtain metastable III-nitride nanopowders with high RE dopant concentration. Cases where RE is doped directly during the plasma synthesis process and where undoped powders are post-treated with dopant exposure and nitridization are investigated and compared. Pellets are fabricated from the RE-doped III-Nitride nanopowders, and the technical pathway for producing such bulk ceramics is examined. Finally, both powders and ceramic pellets are characterized spectroscopically. Photoluminescence of the powders and absorption/emission characteristics and emission life times of the pellets are measured.
9:00 PM - I9.18
Significant Configurational Dependence of the Electro-mechanical Coupling Constant of B0,125Al0,875N.
Igor Abrikosov 1 , Ferenc Tasnadi 1 , Ilia Katardjiev 2
1 Department of Physics and Measurement Technology (IFM), Linkoping University, Linkoping Sweden, 2 , Uppsala University, Uppsala Sweden
Show AbstractThe thin film electroacoustic technology has established itself as a viable technology for the fabrication of high frequency bandpass filters and duplexers for wireless communications.In addition to being integrated circuit compatible and low cost, it offers the unique possibility for tailoring the properties of thin piezoelectric films to suit the particular needs of the application in mind. Currently, AlN is the preferred material in electroacoustic applications but modern applications necessitate the synthesis of materials with a range of electroacoustic properties. A possible route to synthesize piezoelectric materials providing at the same time the flexibility for property engineering is alloying of two or more piezoelectric materials, as for instance group-III nitrides. Among the promising candidates are the wurtzite Boron-containing AlN alloys. In here we carry out ab initio study of the material properties of wurtzite B0.125Al0.875N [1]. The calculations performed in this study are based on the special quasirandom structure (SQS) method, where a random alloy is modeled with a cleverly designed ordered supercell. Although the size of SQS supercells should be commensurate with the composition, one can always generate several supercells, which are easily treatable on the level of ab initio calculations. This is shown in our study for the composition of x=0.125, where we minimized the Warren–Cowley pair short-range orders parameters up to the seventh coordination shell to generate the SQS supercells. All the structural optimizations and polarization calculations were done in the density-functional theory framework, and the proper piezoelectric tensor elements e33 and e31 were calculated with the modern Berry-phase approach. The results are compared with those of the Vegard’s rule for the lattice parameters, the stiffness and piezoelectric constants. During this analysis we further examine the configurational dependence of these quantities. A major finding is that neither the lattice parameters nor the stiffness constants show significant dependence on a particular atomic configuration in solid solution, whereas the calculated piezoelectric constants and equivalently the corresponding electromechanical couplings constant do so in a significant manner.[1] F. Tasnadi, I. A. Abrikosov, and I. Katardjiev, Appl. Phys. Lett. 94, 151911 (2009).
9:00 PM - I9.19
Electronic, Nonlinear Optical and Piezoelectric Properties of Zn-IV-N2 Semiconductors.
Tula Paudel 1 , Walter Lambrecht 1 , Mark van Schilfgaarde 2
1 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States, 2 School of Materials, Arizona State University, Tempe, Arizona, United States
Show AbstractThe Zn-IV-N2 semiconductors with the group IV element, Si, Ge or Sn form an attractive alternative to III-N semiconductors. We present quasiparticle self-consistent GW and hybrid functional calculations of their electronic band structure using a full-potential linearized muffin-tin orbital method. Like the III-N semiconductors, their band gaps span the infrared to ultraviolet region. Their orthorhombic structure leads to additional polarization dependencies of the near gap optical transitions. The spontaneous polarizations and piezoelectric coefficients were calculated using the Berry phase and density functional perturbation theory approaches by means of the ABINIT code. We find only small differences in spontaneous polarization between the members of the family. Thus, electric fields in quantum wells, which are mostly deleterious for optical applications, are avoided. The piezoelectric coefficients are similar in magnitude to those in the III-N semiconductors. The nonlinear optical coefficients and linear electro-optic effects in the clamped and unclamped conditions were calculated and compared to available data in III-N semiconductors.
9:00 PM - I9.2
Traps and Defects in AlGaN/GaN High Electron Mobility Transistors on Semi-Insulating SiC Substrates.
Yongkun Sin 1 , Erica DeIonno 1 , Brendan Foran 1 , Nathan Presser 1
1 Electronics and Photonics Laboratory, The Aerospace Corporation, El Segundo, California, United States
Show AbstractHigh electron mobility transistors (HEMTs) based on AlGaN/GaN hetero-structures are promising for high power, high speed, and high temperature operation. Especially, AlGaN/GaN HEMTs grown on semi-insulating (SI) SiC substrates are the most promising for both military and commercial applications. High performance characteristics from these devices are possible in part due to presence of high two-dimensional electron gas charge sheet density maintaining a high Hall mobility at the AlGaN barrier/GaN buffer hetero-interface and in part due to high thermal conductivity of the SiC substrates. However, long-term reliability of these devices still remains a major concern because of the large number of traps and defects present both in the bulk as well as at the surface leading to undesirable characteristics including current collapse.We report on the study of traps and defects in two MOCVD-grown structures: Al0.25Ga0.75N HEMTs on SI SiC substrates and Al0.25Ga0.75N Schottky diodes on conducting SiC substrates. Our HEMT structures consisting of undoped AlGaN barrier and GaN buffer layers grown on an AlN nucleation layer show a charge sheet density of ~1013/cm2 and a Hall mobility of ~1500cm2/V×sec. Deep level transient spectroscopy (DLTS) is employed to study deep traps in large area AlGaN HEMTs and Schottky diodes fabricated with different Schottky contacts -Pt/Au and Ni/Au. Focused ion beam is employed to prepare both cross-sectional and plan view TEM samples for defect analysis using a high resolution TEM. Lastly, we will report DC and RF characterization results of AlGaN RF HEMTs fabricated with passivation layers and gate recesses prepared under different deposition and etching conditions.
9:00 PM - I9.20
Experimental Observation of Sequential Tunneling Transport in GaN/AlGaN Coupled Quantum Wells Grown on a Free-Standing GaN Substrate.
Faisal Sudradjat 1 , Kristina Driscoll 1 , Yitao Liao 1 , Anirban Bhattacharyya 1 , Christos Thomidis 1 , Lin Zhou 2 , David Smith 2 , Theodore Moustakas 1 , Roberto Paiella 1
1 Electrical Engineering, Boston University, Boston, Massachusetts, United States, 2 Physics, Arizona State University, Tempe, Arizona, United States
Show AbstractAs the materials and device technology of nitride semiconductors continues to make progress, new applications are constantly emerging in electronics and optoelectronics. As an important example, in the past few years substantial research efforts have been devoted to the study of novel device functionalities based on intersubband transitions in these materials. Recent milestones in this area include the demonstration of optically pumped light emission, all-optical switching, electro-optic modulation, and photodetection. On the other hand, the study of tunneling transport in nitride quantum wells (QWs), which is a key feature of more complex intersubband devices including quantum cascade lasers, has so far been limited. Here we report the measurement of highly nonlinear vertical transport characteristics in GaN/AlGaN coupled QWs, providing unambiguous evidence of sequential tunneling. The material used in this work was grown by rf-plasma-assisted molecular beam epitaxy on a free-standing GaN substrate, and consists of 20 repetitions of three weakly coupled GaN/Al(0.15)Ga(0.85)N QWs. Its high structural quality was confirmed via transmission electron microscopy. All layers are either undoped or n-type doped with silicon. According to simulations, near zero applied bias all electrons reside in the ground-state subbands of the center wells of the 20 repeat units. These subbands are energetically separated from the ground states of the neighboring wells by relatively large amounts (about 66 and 37 meV for the well immediately below and immediately above, respectively); as a result, the entire structure is in a relatively high-resistance state. As the bias is increased in either direction, the populated subbands become energetically aligned with the ground states of the adjacent wells downstream, so that vertical transport by sequential tunneling becomes allowed and the resistance is lowered. To experimentally verify this behavior, mesa-structure devices were fabricated by reactive ion etching and metallization, and their electrical characteristics were then recorded at various heat-sink temperatures from 20 to 300 K. As expected, the measured current-voltage characteristics are highly nonlinear, with a sharp low-temperature turn-on near an applied bias of about 2.3 and 3.5 V for positive (i.e., top to bottom) and negative polarity, respectively. It should be noted that this asymmetry is also expected, given the aforementioned asymmetric subband structure of the three-well repeat units. Furthermore, as the temperature is increased the current turn-on becomes more gradual and shifts to lower values of the applied bias, consistent with the thermal broadening of the electronic distribution in the initially occupied subbands. These results therefore indicate that sequential tunneling transport is possible in high quality GaN/AlGaN QWs, as required for the future development of advanced devices such as quantum cascade lasers.
9:00 PM - I9.3
Deep-Level Optical Spectroscopy Study of Interface States in AlGaN/GaN Hetero-Structure.
Yoshitaka Nakano 1 , Keiji Nakamura 1 , Yoshihiro Irokawa 2 , Masaki Takeguchi 2
1 , Chubu University, Kasugai, Aichi, Japan, 2 , National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractAlGaN/GaN-based electronic devices are of great current interest because of their capability of operating at high temperature, high power, and high frequency environments. The AlGaN/GaN hetero-structure has a narrow, high-mobility channel due to a 2DEG produced at the hetero-interface. Excellent device characteristics have been reported for high electron mobility transistors, employing these structures. However, these characteristics are not always reproducible, because the device performance at high frequencies can be limited by the presence of deep-level defects in the AlGaN/GaN hetero-structure. To date, a number of research approaches have been employed to detect and identify these defects in AlGaN/GaN hetero-structures, using various electrical characterization techniques. However, the electrical and physical properties of various deep-level defects existing in the AlGaN/GaN hetero-structures still remain uncertain. In particular, the lack of understanding, regarding in which region of the device structure they are located. Notably, few investigations of deep levels, particularly at the AlGaN/GaN hetero-interface, have been reported. In this study, the interface states have been revealed by capacitance-voltage (C-V) and capacitance deep-level optical spectroscopy (DLOS) techniques.The Al0.3GaN/GaN hetero-structure used in this study exhibited a sheet carrier concentration of 9.98x1012cm-2 and a mobilitity of 1456cm2/Vs, as determined by room-temperature Hall-effect measurements. Planar Pt/AlGaN/GaN Schottky barrier diodes (SBDs) were fabricated on a c-plane sapphire substrate, using Pt and Ti/Al/Pt/Au as Schottky and ohmic contacts, respectively. As a reference, planar Pt/GaN:Si (Si: ~4.5x1016cm-3) SBDs on a c-plane sapphire substrate were also prepared. Photo C-V measurements were conducted under white light illumination (λ > 390nm) from the back side with a 150W halogen lamp. Capacitance DLOS measurements were performed, measuring photo-capacitance transients as a function of incident photon energy, from 0.78eV (1600nm) up to 4.0eV (300nm).Compared the photo C-V characteristics to the dark C-V ones, an increased concentration Δns of the 2DEG on illumination is estimated to be at least ~1.8x1011cm-2 and is considered to be optically excited from deep-level defects to the 2DEG at the AlGaN/GaN hetero-interface. From DLOS measurements, in the partial pinch-off mode, two deep levels, G1 and G2, characteristic of AlGaN/GaN hetero-structures, are clearly observed and are respectively located at ~1.70 and ~2.08eV below the conduction band, being clearly different from the deep-level defects observed in GaN. Both deep levels show a significant increase in their corresponding steady-state photo-capacitance, ΔCss/C, in partial pinch-off mode. Therefore, these levels probably stem from the 2DEG region at the AlGaN/GaN hetero-interface. In particular, the 1.70eV level is likely to act as an efficient trap center for 2DEG carriers.
9:00 PM - I9.4
High-efficiency Schottky Diode-integrated GaN-based Light-emtting Diodes.
Ja-soon Jang 1
1 Department of ECE, Yeungnam University, Gyeongsan-si, Gyeongbuk, Korea (the Republic of)
Show AbstractWe have investigated high-efficiency GaN-based light-emtting diodes by using a reversly-connected Cr-based Schottky diode (SD). In addition, we have also studied the LED chip design factors such as the effective area of the SD and shape of the SD so as to find the optimal conditions for enhancing light-extraction from the LED. Measurement showed that the output power can be strongly dependent upon the area ratio (between whole transparent conducting electrode and the SD) and configuration of dispatch of the SD electrode. It was also shown that the turn-on voltage (at 20 mA) and series resistance are determined to be 3.2 V and 3.8 ohm respectively, while those of normal LEDs (without SDs) are 3.5 V and 9.2 ohm. It were found that there are also sensitively dependent on the SD-integrated chip design factors. The detailed chip design analysis and the corresponding results for the SD-integrated LED will be described by means of electro-optical data, device simulation results and reliability data.
9:00 PM - I9.5
Normally-off GaN MOSFETs on Silicon Substrates with High-temperature Operation.
Hiroshi Kambayashi 1 , Yuki Niiyama 1 , Takehiko Nomura 1 , Masayuki Iwami 1 , Yushihiro Satoh 1 , Sadahiro Kato 1
1 , The Furukawa Electric Co., Ltd., Yokohama Japan
Show AbstractWe report the normally-off GaN MOSFETs fabricated on silicon substrates in the temperature range of from room temperature to 300oC. For GaN-based transistors, normally-on AlGaN/GaN HFETs have been intensively reported so far. However, GaN-based normally-off devices are expected for high-power switching systems. GaN MOSFET is one of the candidates for normally-off operation and several groups have recently reported on their works. GaN MOSFETs fabricated on silicon substrates are attractive on the cost side. Therefore, we fabricated the FETs on this substrates. A heterostructure of Mg-doped (1×1017 cm-3) GaN/buffer layer was grown on a silicon (111) substrate using a metal organic chemical vapor deposition method. The n+ source and drain regions was selectively ion implanted of silicon and rapid thermal annealing at 1200oC for 10 sec in Ar ambient was performed to activate the implanted silicon. The sheet resistances of the n+ regions were 125 ohm/square. A 60-nm-thick silicon dioxide was deposited by a plasma-enhanced chemical vapor deposition as the gate insulator. We believe that silicon dioxide is a good gate insulator for GaN MOSFET since silicon dioxide has a direct wide bandgap of 9.0 eV and a large conduction-band offset on GaN of 2.3 eV. Then the silicon dioxide was annealed at 800oC for 30 min in N2 ambient to decrease the silicon dioxide-GaN interface-state density near the conduction band of GaN. The gate length was 4 μm and the gate width was 1.1 mm. From the transfer characteristics applied to drain voltage of 0.1 V, the maximum field-effect mobility was 108 cm2/Vs at room temperature, decreasing slightly to 84 cm2/Vs at 300oC. The threshold voltage was +2.7 V at room temperature and it slightly shifted to minus as the temperature was higher. The leakage currents at below the pinch-off voltage from 100oC to 300oC were about two orders of magnitude higher than that at room temperature. However, the leakage current at 300oC was less than 100 pA/mm. The specific on-state resistance at gate voltage of 20 V was 16.5 mΩ-cm2 at room temperature and 25.0 mΩ-cm2 at 300oC, respectively. So, the on-state resistance at 300oC was about 1.5 times as high as that at room temperature. On the other hand, in the case of AlGaN/GaN HFET, the on-state resistance at 227oC (500K) increased about 2.3 times as high as that at room temperature. Furthermore, Si-based FETs can not operate in such a high temperature. These results suggest that GaN MOSFET is suitable for high-temperature operation.
9:00 PM - I9.6
GaN Nanowires Formation via Thermal Reconstruction for Water Splitting.
Geng-Ming Hsu 1 , Antonio Basilio 2 3 , Yu-Kuei Hsu 4 , Kuei-Hsien Chen 1 4 , Li-Chyong Chen 1
1 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, 2 Department of Chemistry, National Taiwan University, Taipei Taiwan, 3 Taiwan International Graduate Program, Academia Sinica, Taipei Taiwan, 4 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show AbstractNanostructures offer interesting and useful properties over their bulk counterpart such as greater surface area, low reflectivity, and the possibility of good conduction pathways for carriers. Higher photocurrent gain in GaN nanowires over that of the thin films has been observed in our earlier reports. Up to three orders of magnitude higher photogain has been demonstrated in GaN nanowires[1,2]. Further investigations on the origin of the enhanced photogain suggested that the behavior was caused by its band bending rather than surface trapping effects, which can be beneficial for water splitting applications. GaN nanowires were successfully synthesized through Au-catalyzed thermal reconstruction method on a GaN thin film. Photoelectrochemical measurements in 1 M HCl solution under 100 mW/cm2 of Xe light illumination of the nanowire system has shown markedly improvement on the performance of the GaN nanowires compared to these synthesized by other methods such as thermal CVD and MOCVD. The improved photoelectrochemical performance can be attributed to the improved interfacial contact between the nanowires and the GaN thin film substrate and higher density of nanowires under the reconstruction method. Different nanowire samples were tested and the photoelectrochemical profile shows that the sample with longer and denser nanowires demonstrated improved photoelectrochemical properties. References[1] R. S. Chen, H. Y. Chen, C. Y. Lu, K. H. Chen, C. P. Chen, L. C. Chen, Y. J. Yang, Appl.Phys. Lett. Vol. 91 (2007), p. 223106.[2] R. S. Chen, S. W. Wang, Z. H. Lan, J. T. H. Tasi, C. T. Wu, L. C. Chen, K. H. Chen, Y. S.Huang, C. C. Chen, Small, Vol. 4 (2008), p. 925.
9:00 PM - I9.7
Site-Selective Biofunctionalization of Aluminum Nitride Surface Using Patterned Organosilane Self-Assembled Monolayer.
Chi-Shun Chiu 1 , Hong-Mao Lee 1 , Shangjr Gwo 1
1 Department of Physics, National Tsing-Hua University, Hsinchu Taiwan
Show AbstractSurface biochemical functionalization of III-nitride semiconductors has recently attracted much interest due to their biocompatibility, nontoxicity, and long-term chemical stability under physiochemical conditions for chemical and biological sensing. Among III-nitrides, aluminum nitride (AlN) and Aluminum gallium nitride (AlGaN) are particularly important since they are used as the sensing surfaces for sensors based on transistor or surface acoustic wave (SAW) resonator structures. To demonstrate the possibility of site-selective biofunctionalization on aluminum nitride (AlN) surfaces, we have fabricated two-dimensional antibody micropatterns on AlN surfaces by using patterned self-assembled monolayer (SAM) templates. Patterned SAM templates are composed of two types of organosilane molecules terminated with different functional groups (amino and methyl) which were fabricated on AlN/sapphire substrates by combining photolithography, lift-off process, and self-assembly technique. These patterned SAMs were used for selective immobilization of proteins. Because that organosilane SAMs have different surface properties on the same surface, clear imaging contrast of SAM micropatterns can be observed by field emission scanning electron microscopy (FE-SEM) operating at low accelerating voltage of 0.5-1.5 kV. Furthermore, the contrast in surface potential of the binary microstructures was confirmed by selective adsorption of negatively charged colloidal gold nanoparticles. The immobilization of gold nanoparticles was limited onto positively charged amino-terminated regions, while they were scarcely found on the surface regions terminated by methyl. Selective immobilization of green fluorescent protein (GFP) antibodies was demonstrated by the specific protein binding of enhanced GFP (EGFP) labeling. The observed strong fluorescent signal from antibody functionalized regions on the SAM-patterned AlN surface indicates the retained biological activity of specific molecular recognition due to the antibody–EGFP interaction. The results reported here show that micropatterning of organosilane SAMs by the combination of photolithographic process and lift-off technique is a practical approach for the fabrication of reaction regions on AlN-based bioanalytical microdevices.
9:00 PM - I9.8
Surface Acoustic Wave Sensors Deposited on AlN Thin Films.
Joshua Justice 1 , V. Pagan 1 , O. Mukdadi 2 , D. Korakakis 1 3
1 Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, West Virginia, United States, 2 Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia, United States, 3 National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, West Virginia, United States
Show AbstractOver the past few decades, there has been considerable research and advancement in surface acoustic wave (SAW) technology. At present, SAW devices have been highly successful as frequency band pass filters for the mobile telecommunications and electronics industries [2]. In addition to frequency selectivity, SAW devices are also highly sensitive to changes in their environment; including mass, density, and viscosity [1]. This sensitivity along with a relative ease of manufacture, makes SAW devices ideally suited for many sensing applications; most notably mass, pressure, temperature, and biosensors. In the area of biosensing, other technologies such as surface plasmon resonance (SPR) and quartz crystal microbalances (QCM) are still in the forefront of research and commercial production [1], but advancement in SAW sensors could prove to have significant advantages in this area. Lead zirconium titanate (PZT) is the most popular piezoelectric material for the use in SAW devices today, but its relatively low curie point limits its ability to operate in high temperatures. This study investigates the advantages of using Aluminum Nitride (AlN) as a material for SAW devices. AlN retains its piezoelectric properties at relatively high temperatures as compared to more common piezoelectric materials such as PZT and zinc oxide (ZnO). AlN is a very robust material making it suitable for biosensing applications where the sensing target is selectively absorbed by an active layer on the device [3]. AlN thin films have been deposited on silicon substrates by RF reactive sputtering. The quality and the properties of the AlN thin film as a function of film thickness are also investigated. Rayleigh wave SAW devices have been fabricated by the deposition of platinum contacts and interdigital transducers (IDT’s) onto the AlN thin films using image reversal and liftoff photo lithography processes. Experiments have been conducted to measure dispersion curves, Rayleigh velocities, resonant frequencies, and insertion loss. Experimental results are compared to theoretical calculations. Initial tests have found the group delay between IDT’s spaced 250 microns to be ~10 ns. The design and fabrication of Lamb wave devices for use in liquid environments is discussed.
References:
[1] T.M.A. Gronewold. Anal. Chim. Acta 603 (2007) 119
[2] F.S. Hickernell. Surface Acoustic Wave Devices: A Rewarding Past, a Signifigant Present, and a Promising Future. IEEE. 159
[3] A. Kabulski, V.R. Pagan, D. Cortes, R. Burda, O.M. Mukdadi, D. Korakakis. Mater. Res. Soc. Symp. Proc 1139 (2009) 1139-GG03-47
9:00 PM - I9.9
Detection of DNA Hybridization using Functionalized InN ISFETs.
Cheng-Yi Lin 1 , Yen-Sheng Lu 1 , Shih-Kang Peng 2 , J. Yeh 3 , Shangjr Gwo 4
1 Institute of Electronics Engineering, National Tsing-Hua University, Hsinchu Taiwan, 2 Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu Taiwan, 3 Institute of Nano Engineering and MicroSystem, National Tsing Hua University, Hsinchu Taiwan, 4 Department of Physics, National Tsing Hua University, Hsinchu Taiwan
Show AbstractUltrathin (~10 nm) InN ion selective field effect transistors (ISFETs) with gate region modified with 3-mercaptopropyltrimethoxysilane (MPTMS) by molecular vapor deposition (MVD) are used to detect hybridization of deoxyribonucleic acid (DNA). The ultrathin InN ISFETs have a high sensitivity and short response time for anion detection, showing a great potential for chemical and biological sensing applications.[1] Vapor-phase silanization of MPTMS using MVD substantially shortens the response time for surface modification compared to the conventional self-assembled monolayer (SAM) techniques.[2] The change of contact angle of water on InN surface was observed from 0°, indicating the O2 plasma cleaning, to 68° after 1.5 h vapor deposition. MPTMS with -SH terminal functional groups was used to immobilize probe DNA with acrylic phosphoramidite modification at 5'-end.[3] After immersed in 10 µM DNA probes solution for 12 h, the functionalized InN ISFET was used to perform the hybridization with complementary single stranded (ss) DNA 5'-ATTGTTATTAGG-3'. A drain-source current decrease (~2.9 µA) was observed when a complementary DNA was introduced to the gate region of ISFETs. The current decrease is attributed to the attachment of negatively charged DNA. For a 12-mer oligonucleotide probe, the detection of 143 nM target DNA was accomplished, while the noncomplementary DNA with one base mismatch did not show any obvious current variation.[1] Y.-S. Lu, C.-L. Ho, J. A. Yeh, H.-W. Lin, and S. Gwo, Appl. Phys. Lett. 92, 212102 (2008).[2] M. Hu, S. Noda, T. Okubo, Y. Yamaguchi, and H. Komiyama, Appl. Surf. Sci. 181, 307 (2001)[3] Z. Li, Y. Chen, X. Li, T. I. Kamins, K. Nauka, and R. S. Willaims, Nano Lett. 4, 245 (2004)
Symposium Organizers
Shangjr (Felix) Gwo National Tsing-Hua University
Joel W. Ager Lawrence Berkeley National Laboratory
Fan Ren University of Florida
Oliver Ambacher Fraunhofer-Institut für Angewandte Festkörperphysik (IAF)
Leo Schowalter Crystal IS Inc.
I10: LEDs and Optical Properties II
Session Chairs
Thursday AM, December 03, 2009
Independence W (Sheraton)
9:30 AM - **I10.1
AlGaN Quantum Wells Emitting Below 250 nm with Internal Quantum Efficiency as High as 50%.
Theodore Moustakas 1 , Anirban Bhattacharyya 1 , Lin Zhou 2 , David Smith 2 , William Hug 3
1 Department of Electrical and Computer Engineering, Boston University, Boston, Massachusetts, United States, 2 Department of Physics, Arizona State University, Tempe, Arizona, United States, 3 , Photon System Inc, Covina, California, United States
Show AbstractThe literature reports that AlGaN alloys are inferior to those of InGaN for emitter applications [1]. In this paper, we report the growth and characterization of number of identical Al0.7Ga0.3N /AlN MQWs on (0001) sapphire substrates by plasma-assisted MBE with internal quantum efficiency (IQE) as high as that of InGaN MQWs [2]. The group III to V flux ratio during growth of the AlN barriers was the same for all samples, while that for the AlGaN wells was varied from close to unity to significantly greater than unity. This was done by keeping the Al flux during growth of the wells constant to a value required for 70% AlN mole fraction and varying the Ga flux in the different samples. The microstructure of these MQWs was investigated by TEM and the results show that the QWs have sharp interfaces. These nominally identical MQWs were found to have room temperature luminescence spectra consisting of only a sharp near band-edge emission whose peak was found to vary from 220 to 250 nm as the Ga flux increased from III/V close to one to much greater than one. Furthermore, the IQE of these MQWs, evaluated by taking the ratio of the integrated photoluminescence intensity at room temperature to that at 10 K, was found to vary from 5% for emission at 220 nm to 50% for emission at 250 nm. To understand the variation of the emission spectra as well as the very high IQE of these identical MQWs we deposited a number of thick Al0.7Ga0.3N films using identical III/V flux conditions as the wells of the MQWs. We found that the Al0.7Ga0.3N films grown under Ga-rich conditions exhibit ordered superlattice structures that are incommensurate with the wurtzite crystal lattice. To account for these results we propose that the growth mechanism changes from vapor phase epitaxy in the absence of excess Ga to liquid phase epitaxy in the presence of excess Ga in the surface of the growing film. In the later case, the alloy growth proceeds by dissolving the arriving Al and active N species in the metallic Ga film and the deposition of the alloy proceeds from the liquid phase. In this growth mode, deposition in the growing front occurs when the Ga solution is saturated with N and Al and this unavoidably leads to vertical and lateral compositional inhomogeneities in the films. Thus, the observed red shift of the emission spectra of the identical Al0.7Ga0.3N /AlN MQWs with corresponding increase in IQE is due to compositional inhomogeneities in the AlGaN wells introduced by the liquid phase epitaxy growth mode. These compositional inhomogeneities give rise to a Stoke shift in the emission spectra and increase in IQE due to the localization of the excitons in the induced potential fluctuations. These results demonstrate that the AlGaN alloys hold the same promise as InGaN alloys for the development of deep-UV optoelectronic devices.[1] A. Khan et al., Nature Photonics, 2, 77 (2008)[2] A. Bhattacharyya et al., Appl. Phys. Lett. 94, 181907 (2009)
10:00 AM - I10.2
Performance of Ultraviolet-C Pseudomorphic LEDs on Bulk AlN Substrates.
Shawn Gibb 1 , James Grandusky 1 , Yongjie Cui 1 , Mark Mendrick 1 , Leo Schowalter 1
1 , Crystal IS, INC., Green Island, New York, United States
Show AbstractLow dislocation density epitaxial layers of AlxGa1-xN can be grown pseudomorphically on c-face AlN substrates prepared from high quality, bulk crystals. We will report on initial characterization results from deep ultraviolet (UV) light emitting diodes (LEDs) which have been fabricated and packaged from these structures. As reported previously, pseudomorphic growth and atomically smooth surfaces can be achieved for a full LED device structure with an emission wavelength between 250 nm and 280 nm. Additionally, temperature-dependant and time-resolved photoluminescence measurements and X-ray diffraction have demonstrated the high quality of these structures. Here, we report on fully fabricated and packaged LEDs.A benefit of pseudomorphic growth is the ability to run the devices at high input powers and current densities. The high aluminum content AlxGa1-xN (x~70%) epitaxial layer can be doped n-type to obtain sheet resistances < 200 Ohms/sq/μm due to the low dislocation density. Bulk crystal growth allows for the ability to fabricate substrates of both polar and non-polar orientations. Non-polar substrates are of particular interest for nitride growth because they eliminate electric field due to spontaneous polarization and piezoelectric effects which limit device performance. Initial studies of epitaxial growth on non-polar substrates will also be presented.
10:15 AM - I10.3
Short Period AlN/GaN and AlN/AlGaN Superlattices for Deep UV Light Emitters.
Sergey Nikishin 1 , Boris Borisov 1 , Vladimir Mansurov 1 , Mahesh Pandikunta 1 , Indra Chary 1 , Gautam Rajanna 1 , Ayrton Bernussi 1 , Yuriy Kudryavtsev 2 , Rene Asomoza 2 , Sergey Karpov 3 , Sandeep Sohal 1 , Mark Holtz 1
1 Nano Tech Center, Texas Tech University, Lubbock, Texas, United States, 2 SIMS Laboratory of SEES, CINVESTAV, Mexico D.F. Mexico, 3 STR Group, Soft-Impact, Ltd., St. Petersburg Russian Federation
Show AbstractAlN/GaN and AlN/AlGaN short period superlattices (SPSLs) have been shown to have optical bandgaps in the deep UV suitable for light emitting diodes (LEDs) operating down to ~ 240 nm. SPSLs, with respective well and barrier thickness from 0.5 to 1 nm and from 0.75 to 1.5 nm, have been grown using gas source molecular beam epitaxy (GSMBE) with ammonia on (0001) sapphire substrates. N-type and p-type doping is achieved with Si from silane and Mg from an effusion cell. LED devices are limited by factors including efficiency of radiative recombination in the active region and electrical resistivity (including contact resistance) of p-type wide bandgap emitter. We describe growth of SPSLs and extensive studies of their properties relevant to doping and contact formation.Growth modes of the SPSL layers are studied in situ using reflection high energy electron diffraction (RHEED). The structural, optical, and electrical properties are examined ex situ using X-ray diffraction (XRD), scanning electron and atomic force microscopy (SEM and AFM), optical reflectance (OR), cathodoluminescence (CL), time resolved photoluminescence (TRPL), secondary ion mass spectroscopy (SIMS), and temperature varied Hall measurements (TVHM). In addition, temperature varied transmission line measurements (TVTLM) are used to investigate the mechanism of current injection in Au/Ni/(AlGaN/AlN):Mg low resistive Ohmic contacts. Using RHEED, CL, and TRPL we found that efficiency of radiative recombination in active region of LEDs can be significantly increased when the well material is grown in three (3D) instead two dimensional (2D) mode. The transition time from 2D to 3D growth mode can be readily controlled by varying the ammonia flux. The longest carrier life time (highest radiative recombination efficiency) is observed when 3D islands are formed in all wells of active region. RHEED, XRD, SEM, SIMS, and wet etching in KOH are used to investigate the influence of Mg incorporation on growth rate and polarity of the well and barrier materials. Surface structure of the Mg-doped layers remains (1x1) at room temperature although undoped layers show (2x2) reconstruction. Growth rates of GaN, AlN, and AlGaN significantly decrease and vary when Mg flux reach and exceed ~ 5x1011 at/s cm-2, respectively. Using new processing approach we obtain very low specific contact resistance of ~ 5x10-5 Ω cm2 at room temperature for p-type SPSLs with average AlN content equal 65%. The TVTLM show that current injection mechanism in temperature range from 300 to 450 K is thermionic. Based on our results we propose a model describing the process of Mg activation in GaN and AlGaN (Al content is ~ 5%) well materials. The new approach takes into account complexities of the GaN valence band structure and its importance in analysis of TVHM. This work at TTU was partly supported by the National Science Foundation (ECS-0609416) and U.S. Army CERDEC (W15P7T-07-D-P040).
10:30 AM - I10.4
Surface Plasmon Polariton Enhanced Emission from AlGaN/GaN Quantum Wells.
A. Neogi 1 , Hadis Morkoc 2 , Amir Mohammadizia 1 , Jie Lin 1
1 , University of North Texas, Denton, Texas, United States, 2 , Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractWe examined a hybrid metal-semiconductor nanostructure and observed enhancement of photoluminescence (PL) in silver coated AlGaN/GaN quantum well (QW) due to surface plasmon (SP) coupling to two-dimensional confined excitons. Gold and Aluminum induced plasmons does not show any change in the electron-hole recombination efficiency of the lower dimensional system. Temperature time resolved photoluminescence (TRPL) measurement reveals the reduction of excitonic lifetime by a factor of two for silver coated sample comparing with reference sample. The enhancement of light emission from UV emitting quantum well and quantum dots is observed due to resonant plasmon exciton interaction. The plasmon-exciton coupling is also emission from AlGaN quantum well is also influenced by the wavelength of the excitation light. The role of nonequlibrium recombination carrier dynamics is also investigated.
10:45 AM - I10.5
Theoretical Investigation on Polarization Control of Deep-Ultraviolet AlGaN Quantum-Well Emission.
Atsushi Yamaguchi 1
1 , Kanazawa Institute of Technology, Tokyo Japan
Show AbstractIt has been pointed out that the topmost valence band in deep-ultraviolet (UV) AlGaN material has a Z-like character and that this causes the c-axis polarized emission unfavorable for light extraction from c-plane based LEDs. We previously proposed that this unfavorable polarization can be switched into favorable in-plane polarization by enhancing quantum confinement and/or compressive strain effects in c-oriented AlGaN-QWs [1]. After our proposal, Hirayama et al. achieved a remarkable enhancement in deep-UV LED output power by using this technique [2], and Banal et al. observed the predicted polarization switch experimentally [3]. The switching, however, can be realized only when the Al composition in QW layers is less than ~0.8. Therefore, additional methods still need to be explored to enhance the emission efficiency in deep-UV LEDs emitting further short-wavelength ( < ~220nm) light. Utilization of nonpolar or semipolar oriented substrates is one of the promising candidates for this purpose. However, the crystal growth technique on these substrates has not been established yet, and it is desired to find a solution on nearly c-oriented substrates at present stage. In this work, we investigated the polarization properties in AlGaN QW layers on vicinal c-face substrates, and found that even small inclination of c-plane substrate makes a drastic change in the characteristics. The calculations were performed for coherently-grown 1.5-nm AlN/Al(x)Ga(1-x)N-QWs on AlN substrates with various substrate inclination angles, using 6x6 k.p Hamiltonian. The calculated results show that the polarization switching occurs rather gradually in QWs on vicinal substrates, compared with those on exact c-face substrates. These phenomena can be explained by the valence band mixing effect caused by the structural symmetry breaking in quantum-well planes on vicinal substrates. As a result, the matrix elements for in-plane polarization are enhanced even for high Al composition beyond the critical Al composition of polarization switching, and thus surface emission from high Al-content AlGaN-QW LED could be enhanced by using vicinal substrates. It is numerically estimated that the surface emission intensity can be enhanced by one order of magnitude in Al(0.9)Ga(0.1)N-QWs on 10-degree misoriented AlN substrates compared with those on exact c-face substrates. This work was supported by Grant-in Aid for Scientific Research on Priority Areas (#21016006). [1] A. A. Yamaguchi, Phys. Stat. Sol. (c) 5 (2008) 2364. [2] H. Hirayama, N. Noguchi, T. Yatabe, and N. Kamata, Appl. Phys. Express 1 (2008) 051101. [3] R. G. Banal, M. Funato, and Y. Kawakami, Phys. Rev. B 79 (2009) 121308(R).
11:30 AM - **I10.6
Characteristics of Blue InGaN-Based Laser Diodes with InGaN Waveguides.
Russell Dupuis 1 , Jianping Liu 1 , Yun Zhang 1 , Jae-Hyun Ryou 1 , Seong-Soo Kim 1 , Zachary Lochner 1 , Shyh-Chiang Shen 1 , Douglas Yoder 1 , Qiyuan Wei 2 , Kewei Sun 2 , Yu Huang 2 , Ti Li 2 , Alec Fischer 2 , Fernando Ponce 2
1 School of ECE, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Department of Physics, Arizona State University, Phoenix, Arizona, United States
Show AbstractIII-nitride-based violet and blue laser diodes (LDs) have been rapidly developed for the applications of optical storage systems and display systems, resulting in the commercialization of high-density optical storage digital versatile disk systems such as the BluRay DiscTM. In spite of the impressive development of the devices in terms of optical output power, the reliability of the LDs, and their performance characteristics, details in the epitaxial structures and microstructural and optical characteristics have rarely been reported. In this study, we report on the characterization of the important layers composing blue laser diode epitaxial structures such as the p-type AlGaN/GaN short-period superlattice cladding layers, p-type GaN layers, InGaN/GaN multiple quantum well (MQW) active regions, AlGaN electron-blocking layers, and InGaN waveguides. The microstructural and optical characterization of the layers will be discussed, with an emphasis of the roles of InGaN waveguides in blue laser diodes. For blue and longer-wavelength InGaN based LDs, the refractive index difference between the GaN and AlGaN layers becomes smaller compared to violet LDs. To compensate for this effect, one option is to use InGaN as waveguides because of the larger refractive index of InGaN compared to GaN. Moreover, another advantage of InGaN waveguides is that they can improve the material quality of active region. Time-resolved cathodoluminescence (CL) data for blue LD structures without and with In0.03Ga0.97N waveguides show improved carrier lifetimes for the LDs with InGaN waveguides. At 4K, The CL transients of both LD structures show one-exponential decay that represents radiative recombination. Above 200K, the CL data in (a) have double-exponential decay behavior, but the CL data in (b) still have only one-exponential decay time constant, indicating the suppression of non-radiative recombination by using InGaN waveguides. A comparison of the spontaneous EL intensity and FWHMs of LD structures with and without In0.03Ga0.97N waveguides as a function of injection current also shows improved performance for LDs with InGaN waveguides. The EL intensity at 50 mA current of LD structure with In0.03Ga0.97N waveguides is 80% higher than that of LD structure without In0.03Ga0.97N waveguides. Both LD structures have been fabricated to laser diodes with ridge geometry. LDs without In0.03Ga0.97N waveguides can not lase, while LDs with In0.03Ga0.97N waveguides lase at λ=455 nm with FWHM~0.5nm at room-temperature. Figure 3 shows the light-current curve of LDs with In0.03Ga0.97N waveguides under pulse mode operation, demonstrating Jth = 3.3 kA/cm2. The devices with non-optimized heat sinking also lase CW at 300K at Jth ~11 kA/cm2. In this paper, we will report further characteristics of blue LDs with InGaN waveguides and improved active region designs.
12:00 PM - I10.7
Alloy Composition-dependent Biexciton Luminescence Dynamics in AlxGa1−xN Mixed Crystals.
Daisuke Hirano 1 , Takeshi Tayagaki 1 , Yoichi Yamada 2 , Yoshihiko Kanemitsu 1
1 , Institute for Chemical Research, Kyoto University., Uji, Kyoto, Japan, 2 Department of Electrical and Electronic Engineering, Yamaguchi University, Yamaguchi, Yamaguchi, Japan
Show AbstractThere have been extensive studies on optical properties in GaN-based mixed crystals because of their potential applications in highly efficient light-emitting devices in the UV spectral range. AlxGa1−xN mixed crystals are one of the key materials for applications, operating between the deep-UV and UV spectral range, because their band-gap energies can be tuned with varying their compositional fraction x. In such GaN-based semiconductors, the excitons and biexcitons (exciton molecules) play an essential role in optical responses due to their large binding energy. Moreover, in mixed crystals with alloy disorders, the optical properties in the wide-gap semiconductors are very sensitive to spatial potential fluctuations due to their very small exciton Bohr radii [1]. Therefore, the dynamical behaviors of excitons and biexcitons, such as their formation and localization to the spatial potentials, are essential for understanding the light emission processes. In this work, we study the localization dynamics of biexciton in AlxGa1–xN mixed crystals under exciton resonant excitation at low temperatures. The samples were AlxGa1−xN epitaxial films with different compositions x. The sample temperature was kept at 7 K. Femtosecond time-resolved photoluminescence (PL) spectra were measured by an optical Kerr-gate method. After exciton resonant excitation, the PL spectra show asymmetrical shapes with lower energy tails. The PL spectral shape can be fitted well by an inverse Boltzmann distribution, indicating the appearance of free biexcitons [2]. The PL spectral shape is determined by the effective biexciton temperatures and the edge energy E0, where the E0 corresponds to the lowest energy of biexcitons. We clarified that with an increase of the delay time, the E0 shifts to lower energies in AlxGa1–xN mixed crystals. This behavior is completely different from that in GaN crystals, reflecting that the appearance of localized biexcitons. We clarified that the edge energy E0 is the most reliable parameter for judging the localization of biexcitons. Based on the PL spectral analysis, we compared the samples with different compositions x, and clarified that the localization times of biexcitons become shorter with increasing alloy disorders [3]. Moreover, the biexciton binding energy increases with the inhomogeneous broadening due to alloy disorders. Our method for the PL spectral analysis provides useful information on the localization-dependent dynamics of biexcitons in AlxGa1–xN mixed crystals.[1] D. Hirano, T. Tayagaki, and Y. Kanemitsu, Phys. Rev. B 77, 073201 (2008).[2] D. Hirano, T. Tayagaki, Y. Yamada, and Y. Kanemitsu, Phys. Rev. B 77, 193203 (2008).[3] D. Hirano, T. Tayagaki, Y. Yamada, and Y. Kanemitsu, submitted for publication. (2009).
12:15 PM - I10.8
Short-Wavelength Intersubband Light Emission from Optically Pumped GaN/AlN Quantum Wells.
Roberto Paiella 1 , Kristina Driscoll 1 , Yitao Liao 1 , Anirban Bhattacharyya 1 , Lin Zhou 2 , David Smith 2 , Theodore Moustakas 1
1 Electrical Engineering, Boston University, Boston, Massachusetts, United States, 2 Physics, Arizona State University, Tempe, Arizona, United States
Show AbstractDue to their large conduction-band offsets, GaN/Al(Ga)N quantum wells (QWs) can accommodate intersubband (ISB) transitions at record short wavelengths, well into the near-infrared spectral region. As a result, they are currently the subject of extensive research efforts aimed at the demonstration of novel ISB device applications. Here we report our recent measurement of optically pumped ISB light emission from GaN/AlN QWs. The output optical spectra are peaked near 2 microns, which represents a new record for the shortest ISB emission wavelength from any QW materials system. The material used in this work was grown by rf-plasma assisted molecular beam epitaxy on a c-plane sapphire substrate, and consists of 200 repetitions of identical n-type-doped GaN/AlN QWs. To promote two-dimensional growth, all layers were formed under group-III-rich conditions; furthermore, a small flux of Ga was used during deposition of the AlN barriers to act as a surfactant. The high structural quality of the resulting QWs was confirmed via transmission electron microscopy. Optically pumped ISB light emission was demonstrated at room temperature, using an optical parametric oscillator producing nanosecond-scale pulses at tunable wavelengths. These pulses were used to pump electrons from the ground state to the second excited subband of each QW, followed by radiative relaxation into the first excited subband. It should be noted that these pumping transitions are allowed (albeit weakly) only due to the parity-breaking action of the internal pyro- and piezo-electric fields of nitride QWs. The emitted light is transmitted through a monochromator and then measured with an InGaAs photodetector. To increase the measurement sensitivity, gated detection was performed using a box-car integrator. The measured luminescence spectra are peaked near 2 microns, in good agreement with calculations of the QW subband structure and with results of ISB absorption measurements performed with the same samples. The ISB origin of the emitted light was also confirmed via an extensive study of its polarization properties and pump wavelength dependence. The peak integrated power is on the order of a few hundred nanowatts, and the emission linewidth is about 100 meV. The observation of such well resolved emission peaks represents an important milestone towards the demonstration of short-wavelength nitride-based ISB devices, and it is promising for further development in this field.
12:30 PM - I10.9
Extreme Ultraviolet/Vacuum Ultraviolet/ultraviolet Detector Based on AlGaN.
Fatemeh Shahedipour-Sandvik 1 , Neeraj Tripathi 1 , Blaze Messer 1 , Mihir Tungare 1 , Gregory Denbeaux 1
1 , College of Nanoscale Science and Engineering, UAlbany-SUNY, Albany, New York, United States
Show AbstractDespite the success of the AlGaN based detector and tremendous progress already made in the last two decades for their development and application in the solar blind region not much attention is paid to the tremendous potential of AlGaN based material for their application in Extreme ultraviolet (EUV), vacuum ultraviolet (VUV) and x-ray wavelength ranges. There are a few reports that demonstrate response of GaN based MSM detectors to wavelength range between 160 to 360nm. Most if not all of these devices have been tested using synchrotron sources. Here we report on the successful demonstration of in-band (EUV: 13nm) and out of band (13nm-360nm) radiation using a pulsed plasma source, Energetiq EQ-10 source using a GaN based MSM detector. These measurements are relevant as the plasma source used in this study is similar to the plasma sources that may be used in EUV lithography systems. The recently fabricated detectors have a dark current of less of 10nA and are expected to show superior detection characteristics. Material growth and characterization as well as device characteristics will be presented.
12:45 PM - I10.10
AlGaN Quadruple-band Ultraviolet Photodetectors.
Serkan Butun 1 2 , Mutlu Gokkavas 1 , Piotr Caban 4 , Vlodek Strupinski 4 , Ekmel Ozbay 1 2 3
1 Nanotechnology Research Center, Bilkent University, Ankara Turkey, 2 Physics, Bilkent University, Ankara Turkey, 4 , Electronic Materials Technology, Warsaw Poland, 3 Electrical Engineering, Bilkent University, Ankara Turkey
Show AbstractUltraviolet detectors have a wide range of applications in flame, fire and missile detection, chemical and biological analysis, short distance non-line-of-sight optical communications, as well as emitter calibration. The existing fire warning systems utilize infrared (IR)/IR, UV/IR, or UV/visible/IR channels. Multiband narrow-spectrum UV detectors increase the fire source and range recognition capabilities of warning systems and help to eliminate false alarms. We demonstrated quadruple back-illuminated ultraviolet metal-semiconductor-metal photodetectors with four different spectral responsivity bands on each of two different AlxGa1-xN heterostructures grown on sapphire by metalorganic chemical vapor deposition. Spectral quantum efficiency and current-voltage measurements were carried out. The quantum efficiency peak values were at 267, 286, 309, and 328 nm wavelength with 18.15 nm average of the full-width at half-maximum (FWHM) of the quantum efficiency peaks for sample A, which incorporated five 1000 nm thick epitaxial layers. In comparison, the quantum efficiency peak values were at 268, 295, 316, and 336 nm wavelength with 9.98 nm average FWHM for sample B, which incorporated nine 500 nm thick epitaxial layers.
I11: Terahertz Emission and Detection
Session Chairs
Thursday PM, December 03, 2009
Independence W (Sheraton)
2:30 PM - **I11.1
Terahertz Electronics Detectors.
Michael Shur 1
1 CIE, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractTerahertz electronics holds promise of greatly expanding numerous applications of terahertz technology that now mostly relies on bulky and expensive THz photonics setups. These applications include detection of biological and chemical hazardous agents, explosive detection, building and airport security, and applications in radio astronomy, space research, biology and medicine. Conventional THz electronics uses two-terminal devices, such as Gunn diodes and III-V Schottky diodes. However, THz transistor technology is now emerging. Short channel Si CMOS, InGaAs-based Heterostructure Bipolar Transistors (HBTs) and High Electron Mobility Transistors (HEMTs) have already reached cutoff frequencies and/or maximum frequencies of oscillations in the THz range. Si Schottky diodes fabricated using a standard CMOS-compatible process demonstrated millimeter wave detection. The device feature sizes have shrunk to the point, where ballistic mode of electron transport becomes important or even dominant. At THz and sub-THz frequencies, the ballistic transport (which is the manifestation of the electron inertia) also affects devices with relatively large feature sizes. THz radiation excites the oscillations of the electron density (called plasma waves) in transistor channels. Plasma waves propagate with velocities much larger that electron drift velocities and have characteristic frequencies in the THz range even for devices with feature sizes exceeding a few hundred nanometers The rectification of plasma waves by the device nonlinearity can be used for detecting THz radiation and for imaging and in-situ testing of transistor structures. In very short devices, plasma waves become unstable and cause THz emission. Plasma wave electronics detectors and sources are tunable by applied bias voltage and can be modulated at very high frequencies, approaching or even exceeding transistor cutoff frequencies. Using synchronized THz transistor arrays instead of individual devices is expected to yield dramatic performance improvements of plasmonic THz electronic detectors and sources and rejuvenate THz electronics.
3:00 PM - **I11.2
THz Emission from InN.
Hyeyoung Ahn 1 , Yi-Jou Yeh 1 , Yu-Liang Hong 2 , Shangjr Gwo 2
1 Department of Phtonics, National Chiao Tung University, Hsinchu Taiwan, 2 Department of Physics, National Tsing Hua University, Hsinchu Taiwan
Show AbstractRecently, THz science emerges as the new attractive field which offers the noninvasive tool for imaging and spectroscopy. Despite several technical advances, the output power of THz wave is yet low for many applications. Thus the development of higher power semiconductor THz emitters is of considerable practical significance. Due to its narrow band-gap and the superior electron transport properties, InN has emerged as a potentially important THz emitter. Several efforts have been done for the THz power enhancement from InN, such as the increases of effective surface area and the geometrical coupling through nanostructured InN. For a-plane InN, the intrinsic electric field is along the surface and thus higher THz extraction is expected. Time-domain terahertz waveforms measured from the c- and a-plane InN films indeed shows that THz emission from a-plane InN is at least one order of magnitude (one hundred times in intensity) larger than that from c-plane InN and it is even comparable to that of n-InAs. For a-plane InN, the stacking sequence of In and N atoms is alternatively lined up along the wurtzite c-axis direction so that the surface layers of c-plane InN have either an In- or a N-terminated polar surfaces. Therefore, the electric field generated by these In-N pairs directs perpendicular to the surface and the resultant out-of-surface radiation can be significantly limited by the geometrical reason mentioned above. On the other hand, the layers of a-plane InN have the same number of In and N atoms in a plane and these in-plane In-N pairs form in-plane intrinsic electric field along the surface. The photoexcited carriers can then be efficiently coupled to the in-plane electric field such that more fraction of emission can escape through the emission cone. Meanwhile, to realize the InN-based optoelectronic devices, it is essential to have the ability to fabricate both n- and p-type InN. But, due to its high electron affinity p-type doping of InN is known to be difficult. Currently, it is accepted that p-type InN can be realized by using Mg dopant. THz emission due to the photo-Dember effect and surface field acceleration depends on carrier concentration so that the amplitude and the polarity of THz waves can be determined by the competition between these emission mechanisms at a carrier concentration. From the carrier density dependence of THz emission from InN:Mg films, we found that the multiple emission mechanisms compete to dominate the emission mechanism in InN:Mg. For InN films with the carrier concentration larger than a critical value, the THz radiation intensity increases with the decrease of the carrier concentration. However for InN films with the carrier concentration smaller than nc, the polarity of THz radiation changes the sign and the emitted intensity is also decreased. The maximum THz radiation from Mg-doped InN then can be obtained near nc, whose amplitude is at least 20 times stronger than that of undoped InN film.
I12: Photonic Crystals and Microcavities
Session Chairs
Thursday PM, December 03, 2009
Independence W (Sheraton)
3:30 PM - I12.1
GaN/In1-xGaxN/GaN/ZnO Nanoarchitecture Light Emitting Diode Microarrays.
Chul-Ho Lee 1 , Jinkyoung Yoo 1 , Young Joon Hong 1 , Jeonghui Cho 1 , Yong-Jin Kim 1 , Seong-Ran Jeon 3 , Jong Hyeob Baek 3 , Gyu-Chul Yi 2
1 National Creative Research Initiative Center for Semiconductor Nanorods, Department of Materials Science and Engineering, POSTECH, Pohang Korea (the Republic of), 3 , Korea Photonics Technology Institute, Gwangju Korea (the Republic of), 2 National Creative Research Initiative Center for Semiconductor Nanorods, Department of Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of)
Show AbstractBottom-up approaches based on a nanometer-scale epitaxy have enabled the fabrication of high-quality materials without problems in material compatibility between the film and the substrate for the top-down approach. In particular, one-dimensional nanomaterial heterostructures can provide the versatility and power of designing numerous quantum structures for optoelectronic nanodevice applications such as light emitting diodes (LEDs). Despite the successful demonstration of nanometer scale LEDs based on the bottom-up method, the practical use of individual nanowire LEDs has still remained out of reach because of difficulties in manipulating and positioning individual nanostructures. To address this problem, a demand has arisen for precise controls of positions and dimensions during nanostructure growth. Particularly, position-controlled nanoarchitecture arrays with numerous quantum structures can be very useful for many device applications. Here, we report the fabrication and describe electroluminescent (EL) characteristics of GaN/In1-xGaxN/GaN/ZnO nanoarchitecture LED microarrays consisting of position-controlled GaN/ZnO coaxial nanotube heterostructures. The strong emissions from nanoarchitecture LED microarrays originated from individual nanoarchitectures in the green and blue visible range when a forward bias voltage was applied. Furthermore, the origins of dominant EL peaks are also discussed.
3:45 PM - I12.2
Directional Nanoscale White-Light Generation by Efficient Multiphoton Excitation in Gallium Nitride Nanowires.
Adam Schwartzberg 1 , Shaul Aloni 1 , Tevye Kuykendall 1 , James Schuck 1 , Jeffery Urban 1
1 The Molecular Foundry, Lawrence Berkeley National Labs, Berkeley, California, United States
Show AbstractDirectional white light generation is of great spectroscopic importance. On the millimeter or greater scale, it is relatively easy to attain and use using standard optical equipment. However, no nanoscale directional white light source currently exists at the nanoscale. We demonstrate that it is possible to generate white light in sub-wavelength diameter Gallium Nitride (GaN) wires by multi-photon absorption. In addition to spectroscopic applications, the generation of broad-band white light within the cavity of the GaN wire results in Fabry-Perot etalons, which have been used to perform interferometry on these nanowires. Interestingly, we have observed significant variation in refractive indices from wire to wire, with many deviating significantly from bulk values. We ascribe this to variations in the laser heating of the sample, which depends on the details of wire size and substrate coupling. This has been substantiated via power dependence studies which show threshold behavior at low laser power (<3 mW) at which the observed index drops rapidly. This thermal effect further explains the large 2-photon absorption cross section which is responsible for the sub band gap generation of white and band edge emission.
4:30 PM - **I12.3
GaN Photonic-Crystal Surface-Emitting Laser.
Susumu Yoshimoto 1 , Hideki Matsubara 1 , Kyosuke Sakai 1 , Susumu Noda 1
1 , Kyoto University, Kyoto Japan
Show AbstractRecently, there has been growing interest in photonic-crystal surface-emitting lasers (PC-SELs) (1–4). The lasing principle exploited by the lasers is based on the band-edge effect in a two-dimensional (2D) PC, where the group velocity of light becomes zero and a 2D cavity mode is formed. The output power is coupled to the vertical direction by the PC itself, which gives rise to the surface-emitting function. PC-SELs have the following features: first, perfect, single longitudinal, and lateral mode oscillation can be achieved even when the lasing area becomes very large (for example, devices >300 μm in diameter) (1, 2, 4); and second, the polarization mode (2) and the beam pattern (4) can be controlled by appropriate design of the unit cell and/or lattice phase in the 2D PC. For example, unique beam patterns including doughnut shapes with radial or tangential polarizations have been successfully generated, which leads to a realization of super-high-resolution light sources that could be focused to a spot smaller than wavelengths (4, 5). Recently, GaN PC-SELs have been realized in blue-violet wavelengths (6) by developing a unique method, named “air-holes-retained over-growth (AROG)”, in order to construct a 2D GaN/air photonic crystal structure. The device has successfully oscillated with a current injection at room temperature. In this presentation, the lasing principle, the device structures, a generation of unique beam patterns, and the recent results on current-driven blue-violet GaN PC-SELs will be described. (1)M. Imada, S. Noda, et al., Appl. Phys. Lett. 75, 316 (1999). (2)S. Noda, M. Yokoyama, et al, Science 293, 1123 (2001).(3)R. Colombelli et al., Science 302, 1347 (2003). (4)E. Miyai, S. Noda et al., Nature 441, 946 (2006). (5)R. Dorn, S. Quabis, G. Leuchs, Phys. Rev. Lett. 91, 233901 (2003). (6)H. Matsubara, S. Noda, et al, Science 319, 445 (2008).
5:00 PM - I12.4
Gallium Nitride Logpile Photonic Crystals for Visible Lighting.
Ganapathi Subramania 1 , Qi Ming Li 1 , George Wang 1 , Yun-Ju Lee 1 , Arthur Fischer 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractPhotonic crystals (PC) can fundamentally alter the emission behavior of light sources by suitably modifying the electromagnetic environment around them. Strong modulation of the photonic density of states especially by full 3D bandgap PCs, enables one to completely suppress emission in undesired wavelengths and directions while enhancing desired emission. This property makes them highly attractive for creating advanced light sources such as ultra-low threshold lasers, single photon sources and energy efficient and high brightness visible lighting. However, creating 3D gap PCs composed entirely from direct band gap semiconductor used in visible lighting, such as crystalline GaN, poses fabrication challenges due to their resistance to etching. An approach based on growth through a three dimensional nanostructured template offers a solution to this problem. Here, we describe the fabrication of a GaN logpile PC with visible photonic bandgap via MOCVD growth of GaN through an ‘inverse’ SiO2 logpile PC on GaN seed layer coated sapphire substrate. The SiO2 template with lattice constants as small as 300nm is fabricated using a multilayer e-beam direct write technique developed at Sandia[1, 2]. Optimization of GaN deposition conditions result in vertical growth of GaN from the bottom layer and complete filling of the void regions of the SiO2 template, as shown by cross-section SEM. After removing the SiO2 template, optical reflectance spectroscopy reveals a broad feature centered near 600nm corresponding to the photonic bandgap. We also examine modification of the yellow-green GaN defect mode emission by the logpile PC via spectrally and temporally resolved photoluminescence measurements. Finally, we discuss the potential for doping the GaN logpile PC to obtain p-n junction in order to achieve electrically pumped luminescence.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US DOE’s NNSA under Contract DE-AC04-94AL85000.[1]G. Subramania et al., Optics Express 15, 13049 (2007).[2]G. Subramania, and S. Y. Lin, Applied Physics Letters 85, 5037 (2004).
5:15 PM - I12.5
Electrically Injected GaN/InGaN Microdisks.
Adele Tamboli 1 , Michael Iza 1 , Steven DenBaars 1 , Shuji Nakamura 1 , Evelyn Hu 1
1 Materials, University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractMicrodisks, which consist of a disk-shaped resonant cavity supported in the center by a post, have been studied in a variety of materials systems because they support low loss, high quality whispering gallery modes and low threshold lasing. These whispering gallery modes propagate around the periphery of the circular resonant cavity, confined by total internal reflection to the periphery of the optically isolated, undercut region. In GaN especially, microdisks are a promising geometry because GaN microdisks emit in the blue, can be operated at room temperature, and are relatively insensitive to surface recombination. Unlike most other GaN laser geometries, microdisks can have very low loss and do not require mirror fabrication, which can be challenging. We have previously demonstrated optically pumped GaN microdisks with room temperature, continuous-wave operation and a record low lasing threshold. Electrically injected GaN microdisks offer increased potential for laser device applications, but because of challenges in the fabrication of GaN microdisks, there have so far been no reports of electrically driven GaN microdisks. We have recently fabricated electrically injected GaN microdisks for the first time. In these devices, optical isolation of the disk region is achieved by using bandgap-selective photoelectrochemical etching to remove a sacrificial InGaN layer. Electron injection occurs through the n-type post, while hole injection occurs through a metal contact placed directly on top of the microdisk cavity. This 6 μm contact is smaller than the 8 μm cavity and recessed so that the metal does not overlap the whispering gallery modes, which are confined at the edge of the disk.In our electrically injected microdisks, we observe first order whispering gallery modes under both optical and electrical excitation. The modes observed under optical pumping are much stronger compared to the background quantum well emission than the modes observed under electrical injection. Because the metal contact and n-type post only overlap the center of the microdisk, there is a significant amount of direct emission from the quantum wells in the center of the microdisk, where it does not contribute to the modes. In contrast, under optical pumping, the opaque metal contact obscures incident light from coupling into the center of the microdisk, leading to more efficient excitation into the modes. We have previously seen that decreasing microdisk size leads to increased coupling of light emitted in the cavity into whispering gallery modes. At smaller microdisk sizes, more of the quantum well emission couples into fewer modes, leading to increased relative mode intensity and decreased lasing threshold. Reducing the size of our electrically injected microdisks would likely lead to stronger modes and possibly electrically driven lasing.
5:30 PM - I12.6
Growth of III-Nitride Quantum Dots with Precise Position and Dimensional Control for Strong Light-Matter Interaction.
Luke Lee 1 , Pei-Cheng Ku 1
1 EECS, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractOwing to large exciton binding energy and oscillator strength, III-nitride materials are a promising candidate to observe strong light-matter interaction. Therefore, there have been considerable interests in recent years to study the coupling of a single III-nitride quantum dot (QD) to a high-Q optical cavity. The strong coupling of exciton and photon in such a system can potentially allow us to realize semiconductor quantum light sources beyond cryogenic temperature. In addition, the ability to make pn junctions in III-nitride materials and integrate them to optical cavities makes them particularly attractive for device applications compared to other material systems such as colloidal QDs. There are two criteria to achieve the strong light-matter interaction: (1) the QD must be precisely placed at the antinode of the optical mode in the cavity; (2) the QD resonance must be precisely matched to the cavity resonance. Due to small Bohr radius (~3nm) and large piezoelectric field in III-nitride QDs, the second criterion requires a very small QD (<10nm) for strong quantum confinement and precise control of QD dimensions. In this paper, we demonstrated the feasibility of growing high-quality optically active (In)GaN QDs that satisfy both criteria. The position and lateral dimensions of the QD are lithographically defined using a combination of nanoscale lithography (e.g. electron beam lithography) and a novel pattern shrinking technique using atomic layer deposition (ALD). The transverse dimension and shape of the QD are controlled by selective area epitaxy (SAE) and a model that we have developed. The fabrication procedure is as follows. First a SiO2 mask was deposited on a GaN template that was grown by MOCVD. Electron beam lithography was used to pattern an array of holes with diameters of 30 nm. These holes were shrunk by conformally depositing another dielectric layer using ALD followed by anisotropic dry etching. The amount of shrinkage only depends on the original slope of the hole and the thickness of ALD layer. Because ALD can control the film thickness with a resolution better than 1 nm over a large area, the dimensions of the shrunk holes can be precisely controlled. After the patterning, (In)GaN QDs were grown by MOCVD on the patterned GaN template using SAE. Because the QDs are not strain induced, the control of transverse dimension of the QD can be excellent. Indeed, an SAE model has been developed with results agreeing well with the experiment. In addition, the lack of wetting layer as in conventional Stranski-Krastanow growth can significantly improve the confinement of holes and the crystal quality of the QDs.
5:45 PM - I12.7
Optimisation of a GaN/AlN Quantum Dot Single Photon Source.
Stanko Tomic 1 , Nenad Vukmirovic 2
1 Computational Science and Engineering Department, STFC Daresbury Laboratory, Warrington, Cheshire, United Kingdom, 2 Computational Research Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractExcitons and biexcitons in GaN/AlN quantum dots (QD) were investigated with emphasis on the use of these QDs for single photon source applications. The theoretical methodology for the calculation of single-particle states was based on 8-band strain-dependent envelope function Hamiltonian, with the effects of spin-orbit interaction, crystal-field splitting, piezoelectric and spontaneous polarization taken into account. Exciton and biexciton states were found using the configuration interaction method [1]. The effect of the mirror charges due to periodic boundary conditions were eliminated by Makov-Payne correction adapted to a hexagonal lattice. Within the fixed value of base to height ratio bigger dots tend to have larger values of the biexciton shift due to reduction of the attractive part of the Coulomb interaction. On the other hand it is also important to have large values of optical dipole matrix elements on the exciton transition that can be obtained on smaller QD’s. Optimal QD heights for their use in single-photon emitters were determined for various base to height (D/h) ratios based on the optimization of the target function that depends on the biexcitonic shift and optical dipole matrix element of the excitonic transition. Our predictions suggest that for the QD’s diameter to height ratios in the range 6 - 10 the dots should be as small as possible, while for smaller ratios there exist an optimal quantum dot height. For D/h=4 and D/h=5 the optimization function is nonmonotonous with a maximum at h=2.5 nm and h=2.0 nm, respectively. For larger D/h the largest value of optimization function is reached for the smallest dots among those investigated, with the height of h=1.5nm. Our predictions that the optimal dots emit in the range 3.2 - 3.8 eV are in very good agreement with the experimental results on existing single GaN quantum dot sources [2], underpinning the validity of the model described here and QD morphology extracted from it. The competition between strong confinement in GaN QDs and internal electric field, generally reported in wurtzite GaN, was also discussed, as well as its effect on appearance of bound biexcitons.[1] S. Tomic and N. Vukmirovic, Phys. Rev. B 79 (2009), accepted for publication[2] S. Kako, C. Santori, K. Hoshino at al, Nature Materials 5, 887 (2006).