Symposium Organizers
Ruediger Kniep Max-Planck-Institute for Chemical Physics of Solids
Francis J. DiSalvo Cornell University
Ralf Riedel Technische Universitaet Darmstadt
Zachary Fisk University of California
Yoshiyuki Sugahara Waseda University
Monday PM, November 26, 2007
Back Bay B (Sheraton)
9:30 AM - Q1
Opening Remarks by Symposium Organizers
Show Abstract9:45 AM - Q1.1
Raman Spectroscopy of Single Crystal ZnGeN2.
Timothy Peshek 1 , Kathleen Kash 1 , John Angus 2 , Tula Paudel 1 , Walter Lambrecht 1
1 Physics, Case Western Reserve University, Cleveland, Ohio, United States, 2 Chemical Engineering, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractA convenient picture of the ZnGeN2 lattice can be realized by replacement of the Ga atoms in the GaN lattice by alternating Zn and Ge atoms. This replacement results in a 2x2x1 orthorhombic lattice accompanied by a slight distortion of cell shape, and bond angles, and a bimodal distribution of bond lengths.1 GaN and ZnGeN2 have nearly identical lattice constants2 of the underlying wurtzite lattice, and nearly identical band gaps.3 However, we find that the Raman spectra for the two materials are substantially different. The period doubling in two directions in the c-plane compared to the wurtzite lattice results in 78 phonon modes for ZnGeN2, all of them Raman active,4 versus 6 Raman-active modes for GaN. Here, we present polarized Raman spectra on individual, oriented single crystal ZnGeN2 for several scattering geometries. The crystallites, grown by a vapor-liquid-solid method using NH3 and elemental Zn and Ge, are single crystal rods, as determined by electron diffraction, of lengths of the order of 100 μm and approximately hexagonal cross sections of up to 6 μm in width. A comparison of our results with a previously published unpolarized micro-Raman spectrum for polycrystalline material reveals major differences2. These include our observation of many individually resolved Raman peaks, and the absence of spectral weight for frequencies above 850 cm-1. The previously published spectrum shows a strong Raman signal in the entire region from 850-1300 cm-1. This portion of the spectrum, which, again, is absent in our spectrum, has been attributed tentatively to second harmonic overtones of the single phonon spectrum.4 A comparison with theory, including a discussion of the relevant selection rules for different scattering geometries, will be presented.This work was supported partially by grants from the Department of Education ( APR P200A030186), the National Science Foundation (DMR-0420765) and the Air Force Office of Scientific Research (F49620-03-1-0010).1 S. Limpijumnong, S.N. Rashkeev and W.R.L. Lambrecht, MRS Internet J. Nitride Semicond. Res. 4S1, G6.11 (1999).2 R. Viennois, T, Taliercio, V. Potin, A. Errebbahi, B. Gil, S. Charar, A. Haidoux, and J.-C. Tédenac, Mat. Sci. Eng. B82, 45 (2001).3 K. Du, C. Bekele, C.C. Hayman, J.C. Angus, P. Pirouz, and K. Kash, “Synthesis and Characterization of ZnGeN2 grown from Elemental Zn and Ge Sources”, submitted to J. Crystal Growth.4 Walter R.L. Lambrecht, Erik Alldredge and Kwiseon Kim, Phys. Rev. B72, 155202 (2005).
10:00 AM - Q1.2
Characteristic Features of PVT Growth of Bulk AlN and SiC Crystals: Modeling Analysis and Optimization.
Alexander Segal 1 , Denis Bazarevskiy 1 , Mark Ramm 1 , Yuri Makarov 2
1 , Soft-Impact, Ltd, St.Petersburg Russian Federation, 2 , STR, Inc, Richmond, Virginia, United States
Show AbstractBulk AlN and SiC crystals are currently grown using Physical Vapor Deposition (PVT) technique. This technique has some characteristic features as compared to various Chemical Vapor Deposition (CVD) methods, related to growth of the crystals in more or less tightly closed crucibles. In this case, total pressure and vapor composition in the crucible cannot be directly controlled but are spontaneously determined by the conditions of mass and species exchange between the crucible and the ambient. This effect essentially worsens controllability of the PVT technique and hampers its modeling analysis and optimization.We have developed original models of the AlN and SiC PVT growth that describe this effect and provide the correct prediction of the total pressure and vapor composition in both the hermetically closed and somewhat open crucibles. The models give also detailed description of the interplaying processes of heat exchange (conductive, convective, and radiant), gas flow dynamics, multi-component species diffusion, and surface chemical kinetics. It is essential that they allow modeling the process evolution during long growth of large crystals, including gradual movement of the evaporation/crystallization fronts and shift of the initial growth conditions. The models are validated using the available experimental data and embodied as the commercial computational codes Virtual Reactor AlN TM and Virtual Reactor SiC TM.The developed codes are applied to find the optimal technological conditions for growth of bulk AlN and SiC crystals. Computations with the codes revealed some unexpected effects, including possible high jumps of the species partial pressures at the Knudsen layers on the reactive surfaces, considerable increase of the total pressure in the crucible due to the diffusion of inert carrier gas from the ambient, complicated movement of the evaporation/crystallization fronts relative to the isotherms, disappearance of the species diffusion in the strictly stoichiometric vapor, and others. The results of computations were applied to study of particular machines for sublimation growth of bulk AlN and SiC crystals. Optimization of the processes allowed growing the crystals of the desired slightly convex shape that favors high quality of the crystals.
10:15 AM - **Q1.3
Solids with Mobile Nitrogen Ions.
Martin Lerch 1
1 Institut fuer Chemie, TU Berlin, Berlin Germany
Show AbstractSolid electrolytes are an important class of materials used in fuel cells, sensors, or batteries. In the case of anion conductors solids with high oxygen mobility are most prominent. Zirconia doped with aliovalent oxides such as yttria or scandia is commonly used in Solid Oxide Fuel Cells or automotive oxygen sensors. These zirconia-based materials can be described as anion-deficient fluorite-type structures. This structure type tolerates a large amount of oxygen vacancies and an activation energy of around 1 eV was found for the jump process of the oxygen ions. Thinking about solids with mobile nitrogen ions, fluorite-type based nitride oxides are promising candidates from a structural point of view. Nitriding zirconium dioxide doped with small amounts of yttria leads to nitride oxides with randomly distributed anion vacancies. For detailed investigations of these potential nitrogen conductors large single crystals are necessary. A special method for the growth of nitride oxide crystals was developed, the ‘reactive skull melting’. Impedance spectroscopy studies resulted in ionic conductivities of the Y-Zr-O-N samples comparable to commonly known YSZ ceramics (Y-Zr-O) but with increased activation energy for the conduction process. This was confirmed by tracer diffusion and neutron diffraction experiments. Whereas the activation energy for oxygen ions is nearly the same for nitride oxide as for oxide (YSZ) materials (1 eV), the barrier for nitrogen ions is 2 eV. The described nitride oxides are mixed oxygen/nitrogen ion conductors but limited to a nitrogen concentration of 15 anion-%. Consequently, another class of nitride oxides with a higher nitrogen amount was investigated. Beta tantalum nitride oxide shows the monoclinic baddeleyite-type structure at ambient temperature and can be considered as nitrogen-rich analogue of zirconium dioxide. Partial substitution of tantalum by yttrium ions leads to cubic anion-deficient fluorite-type phases with randomly distributed vacancies. The vacancy concentration can be optimized by varying the amount of yttrium. Nevertheless, compared to the above described zirconia-based materials, which are stable up to more than 2000 K in nitrogen, the thermal stability of the cubic tantalum nitride oxides is poor (decomposition at 1200 K). Consequently, no dense ceramics or single crystals can be prepared. In general, the thermal stability of the fluorite-type nitride oxides decreases with increasing nitrogen content.A nitrogen ion conductor without additional oxygen conductivity must base on a totally different concept. Mayenite (Ca12Al14O33), a good oxygen ion conductor, can be described as a framework structure in which 32 of the 33 oxygen anions are tightly bound, forming large cages, 1/6 of them being filled randomly by the remaining “free” and mobile oxygen. First results on the preparation and characterization of N-mayenite, where the “free” oxygen is substituted by nitrogen, are presented and discussed.
10:45 AM - Q1.4
MOCVD Growth of Hexagonal Nitride on Si(100).
Qian Sun 1 , Soon–Yong Kwon 1 , Jung Han 1
1 Electrical Engineering, Yale University, New Haven, Connecticut, United States
Show AbstractRecently two research groups have demonstrated hexagonal GaN-based LED and HEMT on offcut Si(100). The GaN material quality on Si(100) is at present much worse than that of GaN on sapphire or Si(111). In this paper we investigated the growth of AlN and Al0.13Ga0.87N on 4-deg offcut Si(100) to achieve single crystalline hexagonal phase. It is found that an optimum Al-pre-deposition and a high growth temperature play significant roles in the morphological and structural quality. V/III ratio during subsequent AlGaN growth is crucial in determining in-plane alignment. Al0.13Ga0.87N grown under a low V/III ratio shows a very rough surface with many misaligned grain boundaries prohibiting coalescence and x-ray phi-scan of the AlGaN (1011) shows two sets of 6-fold diffraction peaks with nearly equivalent intensity. As for the smooth AlGaN grown under high V/III, however, the (1011) x-ray diffraction intensity of the major type of AlGaN domains, whose [1010] is parallel to Si[110], is much stronger than that of the minor type of domains. Moreover, room temperature photoluminescence of smooth Al0.13Ga0.87N epilayer obtained under high V/III condition presents a strong near band-edge emission around 334 nm, while the rough AlGaN gives only a broad deep level emission. Evolution of hetero-nucleation and AlGaN heterostructures will be reported.
11:30 AM - Q1.5
Negative or Zero Thermal Expansion in Silicon Dicarbodiimide, Si(NCN)2.
Peter Kroll 1 , Emanuel Ionescu 2 , Ralf Riedel 2
1 Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas, United States, 2 Fachbereich Material- und Geowissenschaften, Technische Universität Darmstadt, Darmstadt Germany
Show AbstractFor many compounds the linear thermal expansion coefficient is not a constant of temperature, but becomes negative at some teperature. Thus, the crystal lattice parameter contracts as the temperature increases. Even diamond and other zinc-blende structures behave in such a way at low temperatures. While the technical importance is evident, usage of β-eucryptite in CERAN cooking tops is a major application, there are just a few compounds that exhibit an isotropic negative thermal expansion at ambient and elevated temperatures, resulting in a volume contraction upon heating. All of them are oxides of tungten, vanadium, and molybdynum; the standard reference being ZrW2O4 [1].Here we show computational results that indicate that silicon dicarbodiimide, Si(NCN)2, is the first non-oxide material that exhibits strong isotropic negative thermal expansion in a temperature range from 300-700 K. Our results are based on extensive ab initio molecular dynamics simulations. We investigated a wide field of parameters for temperature and volume to locate the points of lowest free energy. We find a negative thermal expansion coefficient of -1*10-5 K-1, comparable to that of ZrW2O4.Our subsequent experimental study using synchroton radiation shows that the thermal expansion of Si(NCN)2 is at least zero in a temperature range from 500-800 K. Further experimental studies with better quality crystals are currently under way. The property is mainly related to the combination of tetrahedral environment and the flexible carbodiimide, -NCN-, functional group present in Si(NCN)2. The discovered property, thus, is likely be a general phenomenon of all such carbodimide compounds.[1]M. P. Attfield and A. W. Sleight, Chem. Comm. 1998, 601.[2]R. Riedel, A. Greiner, G. Miehe, W. Dressler, H. Fuess, J. Bill, and F. Aldinger, Angew. Chem. Int. Ed. Engl. 1997, 36, 603.
11:45 AM - Q1.6
Analysis of Structural Defect Distributions in Aluminum Nitride (AlN) Bulk Crystals Grown by the Seeded Physical Vapor Transport (PVT) Technique.
Balaji Raghothamachar 1 , Michael Dudley 1 , Rafael Dalmau 2 , Ziad Herro 2 , Zlatko Sitar 2 , Raoul Schlesser 2
1 Materials Science & Engineering, Stony Brook University, Stony Brook, New York, United States, 2 Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractFor III-nitride device technology, aluminum nitride (AlN) substrates are a more attractive choice compared to sapphire substrates that are currently used. This is due to several favorable properties such as crystal structure and chemical compatibility of AlN with device epilayers of gallium nitride (GaN) and AlGaN alloys, close lattice match, negligible thermal expansion difference and high thermal conductivity. Several efforts are ongoing to grown bulk AlN crystals from which substrate wafers can be obtained. Using the seeded physical vapor transport (PVT) method, we have grown AlN single crystal boules in a RF reactor. Wafers sliced and polished from these boules have been systematically imaged by synchrotron white beam x-ray topography (SWBXT) to map the defect distribution and trace the defect evolution down the length of the boule. High resolution x-ray diffraction (HRXRD) measurements have also been carried out to quantify the variation in structural defect distribution. Results show a marked change in defect density across the length of the boule as well as considerable variation within each wafer. The effect of polarity as well as other growth conditions on defect generation and distribution is discussed.
12:00 PM - Q1.7
Free-Standing Zinc-Blende (Cubic) GaN Substrates Grown by a Modified Molecular Beam Epitaxy Process.
Anthony Kent 1 , Sergei Novikov 1 , Nicola Stanton 1 , Richard Campion 1 , Charles Foxon 1
1 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom
Show AbstractWe demonstrate bulk, free-standing, zinc-blende (cubic) GaN substrates grown by a modified molecular beam epitaxy process. We have grown free-standing cubic GaN layers up to 60 microns in thickness. Even though our growth rate is currently not particularly fast, it is already comparable with the growth rate for bulk wurtzite GaN crystals from the liquid Ga at high pressure. We present measurements, which confirm the cubic nature of the GaN wafers and show that the hexagonal content of the material is less than about 10%. Cubic (001) GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0001) c-axis wurtzite GaN, therefore, our free standing GaN wafers make ideal lattice-matched substrates for the growth of cubic GaN-based structures for blue and ultraviolet optoelectronic devices, and high power and high frequency electronic applications.
12:15 PM - Q1.8
Development of Homoepitaxially Grown GaN Thin Film Layers on Freestanding Bulk m-plane Substrates by Metalorganic Chemical Vapor Deposition (MOCVD).
Vibhu Jindal 1 , James Grandusky 1 , Mihir Tungare 1 , Neeraj Tripathi 1 , Fatemeh Shahedipour-Sandvik 1 , Peter Sandvik 2
1 , CNSE, Albany, New York, United States, 2 Global Research Centre, General Electric, Niskayuna, New York, United States
Show AbstractA design of experiment approach is used to investigate the growth space for optimization of homoepitaxial m-plane GaN films on freestanding HVPE m-plane substrates by metalorganic chemical vapor deposition. Under optimized c-plane GaN growth conditions, the homoepitaxy resulted in large areas without nucleation along with a high density of defects. These structural defects were mainly of arrow head shape caused by the difference in growth rates in a- and c- crystallographic directions. The growth conditions were optimized with respect to growth temperature, V/III ratios and reactor pressure to obtain smooth and coalesced epitaxial layers on bulk substrates. For example, growth at lower temperature resulted in increased nucleation, with a rough surface morphology. Higher growth temperatures led to smoother surfaces due to increased surface diffusion of adatoms. Overall, growth at higher temperature, lower V/III ratio and lower pressure decreased the surface roughness of GaN thin films with better optical properties, as measured by photoluminescence, on m-plane substrates as compared to standard c-plane growth conditions.
12:30 PM - Q1.9
Phonons in Zn-IV-N2 Semiconductors.
Tula Paudel 1 , Walter Lambrecht 1
1 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractA family of materials closely related to wurtzite GaN van be formed by substituting Ga by Zn and a group IV element: Si, Ge or Sn. Some of these materials, which all share the same ordering pattern of the cations, have recently been grown in thin film form and their properties start to be explored. Here we present a study of the lattice vibrations and structural properties using the linear response pseudopotential plane wave approach implemented in the ABINIT code using the local density approximation as well as the generalized gradient approxiation. Using the calculated Born effective charges and phonon eigenvectors, the oscillator strengths for infrared absorption have been calculated for all optically active modes for all three materials. The LO-TO splittings were also obtained. A detailed comparison with experiment is made for ZnSiN2 for the b1 modes, the only case for which experimental data are currently available. Good agreement (peak positions to within 5-8 %) is obtained although the interpretation is somewhat different from the experiment in the sense that some observed rather broad peaks are apparently superpositions of two modes. Interestingly, ZnSiN2 does not show a clear separation in frequency range of optic and acoustic type modes. A high oscillator strength mode occurs in the region where only low optical activity folded acoustic modes are expected. This mode, however, has not yet been observed, possibly because of a short lifetime due to mode coupling. High frequency and static dielectric constant tensors are also obtained and found to be in good agreement with experimental data for ZnSiN2. For ZnGeN2 and ZnSnN2 this low frequency mode is weaker and the spectrum splits into a region of 6 strongly active higher frequency modes and 5 weaker modes in the low frequency region.
12:45 PM - Q1.10
Electronic Properties of Mixed Conducting Solid Oxides Containing Nitride.
Hans Wiemhoefer 1 , Mustafa Dogan 1 , Vera Ruehrup 1 , Ilia Valov 2 , Juergen Janek 2 , Martin Lerch 3 , Eberhard Schweda 4
1 Institute of Inorganic & Analytical Chem., University of Muenster, Muenster Germany, 2 Institute of Physical Chemistry, University of Giessen, Giessen Germany, 3 Institute of Chemistry, Technical University of Berlin, Berlin Germany, 4 Institute of Inorganic Chemistry, University of Tuebingen, Tuebingen Germany
Show AbstractMonday PM, November 26, 2007
Back Bay B (Sheraton)
2:30 PM - **Q2.1
Ion-conducting Nitride Oxides: Transport, Reactions and Electrochemistry.
Juergen Janek 1 , Ilia Valov 1
1 Institute of Physical Chemistry, Justus-Liebig-University Giessen, Giessen Germany
Show Abstract3:00 PM - Q2.2
Seeded Growth of AlN on m-plane Seed.
Peng Lu 1 , Rafael Dalmau 1 , Zlatko Sitar 1
1 Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractSeeded growth of AlN was achieved on m-plane AlN seeds by physical vapor transport (PVT). The seeds with high crystalline perfection were cut from freestanding AlN single crystals obtained by self-seeded growth. The seeded growth was performed at temperatures above 2200°C in N2 atmosphere at 500 Torr total pressure. Crystals were grown at a growth rate of 200 μm/hr in the [10-10] direction while the in-plane growth rates were highly anisotropic. The growth surface showed macroscopic prismatic facets parallel to the c-axis and each of these facets consisted of micro-steps. X-ray diffraction analysis confirmed seeded growth and a high crystalline quality of grown boules. The defects formed in m-plane AlN growth were studied by aqueous solution and molten KOH etching.
3:15 PM - Q2.3
Growth and Texturing of Rare-earth Nitride Thin Films.
Jianping Zhong 1 , Andrew Preston 1 , B. Ruck 1 , H. Trodahl 1
1 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington New Zealand
Show AbstractThe rare-earth nitrides combine extended band electrons with highly localized 4f electrons that carry large magnetic moments. They lie on the boundary between metals and insulators, and advanced electronic structure calculations have predicted that their numbers include half metals with potential applications in the field of spintronics. Experimentally, tests of the theoretical predictions have been hampered by the absence of stoichiometric samples, and by the propensity of the rare-earth nitrides to react with atmosphere. Thus, there is presently an imperative to explore growth techniques that provide high quality samples for study. We have grown a range of rare-earth nitride thin films at room temperature on silicon and sapphire by evaporating the rare-earth element in the presence of a partial pressure of high purity nitrogen gas. Rutherford backscattering spectroscopy shows that the films are stoichiometric, and x-ray diffraction has shown that they all possess the rock-salt structure with the expected lattice constant. For SmN, DyN, ErN, and LuN the films consist of approximately 10 nm randomly oriented crystallites, while GdN shows a strong [111] orientation independent of substrate. We have probed the electronic states through the temperature dependent resistivity, and find semiconducting behaviour for all of the samples.
3:45 PM - Q2.5
Contact Formation on GaN Investigated with Electron and Soft X-ray Spectroscopies.
Sujitra Pookpanratana 1 , Marcus Baer 1 , Lothar Weinhardt 1 , Clemens Heske 1 , Ryan France 2 , Tao Xu 2 , Theodore Moustakas 2 , Oliver Fuchs 3 , Monika Blum 3 , Jonathan Denlinger 4
1 Dept. of Chemistry, University of Nevada, Las Vegas, Las Vegas, Nevada, United States, 2 Dept. of Electrical and Computer Engineering, Boston University, Boston, Massachusetts, United States, 3 Experimentelle Physik II, Universität Würzburg, Würzburg Germany, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract (Al, Ga, In)N-based semiconductors are of great interest for their applications in light emitting diodes (LEDs) and laser diodes. Traditionally, Ti-based metal contacts have been used for these types of materials. However, this becomes less than ideal for wide-gap nitrides with increasing AlN content [1]. In that case, it has been shown that there is a significant improvement in ohmic character in the contacts when V replaces Ti [1,2]. The metal contacts implemented on some n-type nitrides use a layered stack formation consisting of Au/V/Al/V and subsequent rapid thermal annealing (RTA) in N2 atmosphere. It is hypothesized that the RTA treatment forms VN at the nitride interface [3], and that the VN is responsible for the improvement of the electronic contact properties [2]. In order to understand the processes during contact formation between wide band gap nitrides and a V-based metal contact, we have used photoelectron spectroscopy (PES) and synchrotron-based x-ray emission spectroscopy (XES). These techniques give information about the chemical and electronic properties at or near a surface. Samples were measured before and after subsequent heat treatments using RTA as well as a conventional furnace at different temperatures. N-type GaN samples were grown by molecular beam epitaxy onto sapphire substrates and V-based contact stacks were deposited by e-beam evaporation with varying thickness. Our experiments allow us to paint a detailed picture of the contact formation, which indicates a rather complex behavior. While the PES spectra of Au/V/Al/V/GaN samples before annealing are dominated by Au photoemission lines, the PES signals of Al, V, Ga, and N can also be detected upon annealing. This indicates either pronounced intermixing processes or island formation induced by the heat treatment. The study of the respective morphology by atomic force microscopy, which is currently ongoing, will clarify these effects. N K XES spectra of RTA-annealed Au/V/Al/V/GaN samples show the formation of N-V bonds (in contrast to the samples annealed in the conventional furnace). At the same time, the emission feature indicating N-Ga bonds (i.e., of the GaN layer) disappears. Both findings support the previous hypothesis of the formation of VN [3]. The V L emission spectra, however, show that the situation is more complicated, because they consist of superimposed emission features of V, VN, and VxOy. In-situ PES experiments of V deposition and controlled VN formation on clean GaN, which will also allow us to measure the work functions and the electronic level alignment precisely, are currently being conducted to shed further light on the various observed V species.1.A. Sampath et al., Mater. Res. Soc. Symp. Proc. 482, 1095 (1998).2.R. France et al., Appl. Phys. Lett. 90, 062115 (2007).3.I. Galesic and B. O. Kolbesen, Thin Solid Films 349, 14 (1999).
4:30 PM - Q2.6
AlN Thermal Expansion Coefficients Determined from Bulk Crystals.
Stephan Figge 1 , Hanno Kroencke 1 , Boris Epelbaum 2 , Detlef Hommel 1
1 Department of Physics and Electrotechniques, University of Bremen, Bremen Germany, 2 Department of Materials Science, University of Erlangen, Erlangen Germany
Show Abstract4:45 PM - Q2.7
Enhancement of Light Extraction Efficiency in GaInN Blue Light-emitting Diodes by Graded-refractive-index Antireflection Coating of Co-sputtered Titanium Dioxide and Silicon Dioxide.
Frank Mont 1 2 , David Poxson 1 3 , Jong Kim 1 2 , E. Fred Schubert 1 2 3 , Arthur Fischer 4 , Mary Crawford 4
1 Future Chips Constellation, Rensselaer Polytechic Institute, Troy, New York, United States, 2 Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechic Institute, Troy, New York, United States, 3 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechic Institute, Troy, New York, United States, 4 Semiconductor Materials and Device Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractThe large refractive index (n) contrast between semiconductors (n = 2.5 to 3.5) and air (n = 1.0) results in low light extraction efficiencies in light-emitting diodes (LEDs) due to the total internal reflection and high Fresnel reflection losses. Single antireflection (AR) coatings are widely used to reduce reflections and thus to maximize transmitted light from the LED chip into the ambient. However, conventional AR coatings only function at a single wavelength and at normal incidence. In contrast, if the refractive index of an AR coating continuously varies from the substrate’s index to the ambient’s index, such graded-index optical coatings yield broad-band omni-directional AR characteristics with transmittance near 100% by complete elimination of Fresnel reflection. In this work, we have demonstrated the enhancement of light extraction efficiency in GaInN blue LEDs by using a graded-refractive-index AR coating, which is deposited by reactive RF magnetron co-sputtering of TiO2 and SiO2 targets. The refractive index of the coating varies linearly from the refractive index of TiO2, index matched to GaN, to that of SiO2, index-matched to an epoxy encapsulant. The linear index grading is achieved by adjusting the electrical power to the respective targets to their appropriate values in order to control the relative composition, and hence the refractive index of the mixture of TiO2 and SiO2. Normal-incidence and angle-dependent transmittance measurements are taken using a white light source and a He-Ne laser, respectively. A transmittance enhancement larger than 10% is shown for graded-refractive-index AR coating on GaN compared to conventional single-layer AR coating over a broad-band range of wavelengths. The graded-refractive-index AR coatings are incorporated into GaInN blue LEDs. The enhancement of light extraction efficiency and patterning of the graded-refractive-index AR coatings for further enhancement will be discussed in terms of theoretical and analytical considerations.
5:00 PM - Q2.8
HRTEM Observation of Dislocations in AlN Films.
Yuki Tokumoto 1 , Naoya Shibata 2 , Teruyasu Mizoguchi 2 , Masakazu Sugiyama 3 , Yukihiro Shimogaki 3 , Takahisa Yamamoto 1 , Yuichi Ikuhara 2
1 Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba, Japan, 2 Institute of Engineering Innovation, The University of Tokyo, Bunkyo, Tokyo, Japan, 3 Department of Materials Science, The University of Tokyo, Bunkyo, Tokyo, Japan
Show AbstractGroup III-nitride wide-gap semiconductors, including AlN, GaN, InN and their alloys, have been widely studied because of their potential applications for new, high performance optical and electronic devices. However, it has been known that the group III-nitride films epitaxially grown on the substrate normally contain a high density of threading dislocations (TDs), and these TDs have deleterious effects on the optical and electrical properties of the films. Therefore, reduction of the TDs is a key issue for the group III-nitride thin film growth. The generation mechanism of the TDs, especially edge dislocations, is expected to have relation with the mosaic growth mode of the film. In the mosaic growth mode, the nuclei formed on the substrate are slightly misoriented, and the films eventually consist of sub-grains slightly misoriented with respect to each other. Consequently, at the sub-grain boundaries, dislocations are introduced to compensate small angle deviations. Although the mosaic growth model has been supported by many studies, the atomic structures of threading dislocations have not well been characterized in connection with the mosaic growth model. In the present study, we characterize the structures of threading dislocations in AlN films mainly using high-resolution transmission electron microscopy (HRTEM), in order to understand the generation mechanism of the dislocations.AlN films were grown on (0001) sapphire by metal-organic chemical vapor deposition (MOCVD) using trimethylaluminum (TMAl) and ammonia (NH3) as precursors for Al and N, respectively. Plan-view specimens for TEM observations were prepared by conventional methods including mechanical polishing and argon-ion-beam thinning. Conventional and high-resolution TEM observations were performed using JEOL JEM-2010HC (200kV) and JEOL JEM-4010 (400kV), respectively. As a result of conventional TEM observations, the density of TDs in the AlN films fabricated in this study was estimated to be about 5×1010cm-2. HRTEM observations confirmed that the threading dislocations are mostly edge-type dislocations, which are perfect dislocations with the Burgers vector of b=⅓<11-20>. The edge dislocations were periodically introduced and formed sub-grain boundaries, indicating that these dislocations are introduced to compensate the small angle mismatch between the sub-grains. Thus, the arrays and directions of the threading dislocations strongly suggest the validity of the mosaic growth model. The detailed generation mechanism and atomic core structures of the threading edge dislocations will be discussed in the presentation.
5:15 PM - Q2.9
Epitaxial Lateral Overgrowth of Thick AlN Layers by Migration Enhanced Metalorganic Chemical Vapor Deposition.
R. Jain 1 , J. Zhang 1 , W. Sun 1 , X. Hu 1 , M. Shatalov 1 , J. Deng 1 , I. Shtrum 1 , A. Lunev 1 , Y. Bilenko 1 , J. Yang 1 , R. Gaska 1
1 , Sensor Electronic Technology, Inc., Columbia, South Carolina, United States
Show AbstractIt has been shown that internal quantum efficiency and reliability of III-nitride visible and ultraviolet (UV) light emitting diodes (LEDs) is strongly affected by the material quality of buffers and subsequent layers of device structure. Therefore, for deep UV LED we developed epitaxial lateral overgrowth (ELOG) of thick AlN layers over sapphire substrates by using a combination of conventional metal-organic chemical vapor deposition (MOCVD) and our proprietary migration-enhanced MOCVD (MEMOCVD) processes. In this paper we will present our recent results on growth of high quality thick AlN layers over sapphire substrates that were used as templates for deep UV LED structure growth. We also will report on fabrication and characterization of deep UV LED structures grown over these high quality thick AlN layers.For this study, high-quality thick c-plane AlN layers were grown over sapphire substrates by a combination of conventional MOCVD and proprietary MEMOCVD processes. Initially, 2 μm to 5 μm thick AlxGa1-xN (x=0.9-1) films were heteroepitaxially deposited on sapphire substrates by MOCVD. Deep grooves for lateral overgrowth were formed in the AlGaN layer along <1-100> direction by using standard photolithography and reactive ion etching in BCl3/Cl2/Ar plasma. Fully coalesced 15 - 20 μm thick AlN films were laterally overgrown on the grooved templates at growth rates ranging from 1 to 5 μm/hr. Cracking, typical for thick AlN films due to the differences in lattice constants and thermal expansion coefficients of epilayer and of substrate, was eliminated by accommodating the strain in grooved AlGaN templates resulting in completely crack-free layers. V/III ratio was found to be the key growth parameter in improving layer coalescence and surface morphology. Preliminary structural and optical characterization was carried out confirming high quality of AlN layer. Thick low-defect density AlN layers were used as templates for subsequent growth of AlInGN based deep UV LED device structures. Standard UV LEDs were fabricated using reactive ion etching to access n-AlGaN layers combined with electron beam deposition for n- and p-contact metallization. Packaged devices with peak emission at 310 nm exhibited CW output power of 0.6 mW at 20 mA. Reliability tests of packaged devices show variation of the output power of about 10 % of initial value after 400 hours of operation at 20 mA CW. Based on the results of ongoing reliability measurements, the device operation lifetime of the order of 10,000 h at 20 mA CW can be estimated.
5:30 PM - Q2.10
Large Area Aluminum Nitride Substrates for UV Optoelectronics.
Robert Bondokov 1 , Kenneth Morgan 1 , Stephan Mueller 1 , Sandra Schujman 1 , Glen Slack 1 , Leo Schowalter 1
1 , Crystal IS, Inc. , Green Island, New York, United States
Show AbstractFree-standing, nitride-based substrates with dislocation densities of less than 1000 per sq. cm, high crystallinity, and uniformity are a highly desirable platform for the epitaxial growth of the next generation of III-nitride devices. These devices meet the increasing market demand for deep ultra-violet (UV) optoelectronics as well as high frequency applications. The choice of Aluminum Nitride (AlN) as substrate to fill this need offers the advantage of lower lattice and thermal expansion mismatch to Gallium Nitride (GaN) and AlGaN alloys than e.g. sapphire or silicon carbide. Additionally, AlN substrates exhibit high radiation hardness and a thermal conductivity superior to GaN. In this study we discuss the development of large area AlN substrates cut from bulk crystals, grown from the vapor phase by the sublimation-recondensation technique. Parameters influencing the growth rate and crystal quality include the thermal gradient across the growth cell, the nucleation conditions and the seed quality. By optimizing these parameters we have been able to grow large AlN boules for commercial production of AlN wafers up to 2” diameter. A clear advantage of cutting AlN substrates from a bulk crystal is the possibility to prepare wafers with different orientations, i.e. non-polar or quasi-polar orientations, which are not readily obtainable by epitaxial growth on foreign substrates. The lack of both piezoelectric and spontaneous polarization fields, for nitride heterostructures grown in non-polar directions, has recently created significant interest in high quality, non-polar nitride substrates to boost the performance of light emitting diodes (LEDs) and laser diodes.AlN boules and wafers have been characterized by X-ray Laue backscattering, diffraction and rocking curves measured in double axis configuration. We demonstrate AlN substrates with a high crystallinity, showing a full width at half maximum (FWHM) of 28 arcsec and 32 arcsec for the symmetric and asymmetric rocking curves, respectively. The high material quality was also confirmed by measurements of the etch pit densities (EPD) for AlN substrates oriented parallel to the c-, m-, and a-planes. The etch pits were revealed by a preferential chemical etching technique using molten Potassium and Sodium hydroxides in a previously reported setup. The EPD for all AlN substrate orientations varied in the range 100 - 10E4 per sq. cm. The impurity levels were measured by a glow discharge mass-spectrometry (GDMS) and secondary ion mass spectroscopy (SIMS). The oxygen content as measured by SIMS showed values of < 10E18 per cc. The room temperature UV transparency indicates absorption coefficients ≤ 50 cm-1 @ 280 nm and the high thermal conductivity measured by the laser flash method of ~ 270 W/mK correlates well with the low oxygen content and high crystallinity of the AlN substrates.
5:45 PM - Q2.11
Sapphire Nano-Patterning and GaN Nano-heteroepitaxy.
Hongwei Li 1 , Jason Perkins 1 , Sreya Dutta 1 , Yik Khoon Ee 2 , Ronald Arif 2 , Nelson Tansu 2 , Richard Vinci 1 , Helen Chan 1 , Pavel Capek 3 , Naveen Jha 3 , Volkmar Dierolf 3
1 Center for Advanced Materials and Nanotechnology, Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania, United States, 2 Center for Optical Technologies, Department of Electrical and Computer Engineering, Lehigh University, Bethlehem, Pennsylvania, United States, 3 Center for Optical Technologies, Department of Physics, Lehigh University, Bethlehem, Pennsylvania, United States
Show AbstractGaN grown epitaxially on single crystal sapphire suffers both high density of misfit dislocations and internal stress due to lattice and thermal mismatch. Nano-heteroepitaxy has been shown as a promising means to reduce dislocations as well as stress in large lattice-mismatched epitaxial systems, but it is difficult to apply to the GaN/sapphire system due to challenges associated with sapphire patterning. Here a novel process of sapphire nano-patterning by Aluminum deposition, Growth of Oxide and Grain growth (AGOG) is demonstrated. Nanoscale aluminum patterns were first fabricated on sapphire substrates through E-beam lithography and lift-off processes, then the aluminum was oxidized in air and fully converted to single crystal sapphire through grain growth under high temperature. Successful conversion is evident in cross section TEM images and by SEM-based electron backscattered diffraction (EBSD). Heteroepitaxy of GaN on the conventional and patterned sapphire substrates was performed using metalorganic chemical vapor deposition (MOCVD) with a low-pressure vertical-type MOCVD system. The growth of the GaN template consists of a ~30 nm nucleation layer of low-temperature GaN layer at 535oC and a 2.8 μm GaN at 1080oC under H2 ambient gas. During the growth of the high-temperature GaN template layer, the flow rate of trimethylgallium (TMGa) and NH3 were 10 μmol/min and 2500 sccm, respectively, corresponding to a V/III ratio of about 1800. Photoluminescence (PL) structure comprising of 4 period In0.18Ga0.82N / GaN quantum wells were subsequently grown on the GaN virtual template. The peak and integrated room temperature (RT) photoluminescence of the quantum wells grown on nano-patterned sapphire shows ~2 times intensity improvement compared to the quantum wells grown on plain sapphire. The dislocation density in the nano-heteroepitaxially-grown GaN is on the order of 108/cm2, as determined from cross sectional TEM characterization. This is on the low end of the typical range for planar substrates, demonstrating that the patterned substrate may be positively influencing the GaN growth even though the processing has not yet been fully optimized. Further optimization on the growth and reducing the AGOG pattern dimensions will also be conducted.
Symposium Organizers
Ruediger Kniep Max-Planck-Institute for Chemical Physics of Solids
Francis J. DiSalvo Cornell University
Ralf Riedel Technische Universitaet Darmstadt
Zachary Fisk University of California
Yoshiyuki Sugahara Waseda University
Tuesday AM, November 27, 2007
Back Bay B (Sheraton)
9:30 AM - Q3.1
Sublimation Growth and Defect Characterization of AlN Single Crystals.
Shaoping Wang 1 , Balaji Raghothamachar 2 , Michael Dudley 2 , Zaiyuan Ren 3 , Jung Han 3 , Andrew Timmerman 1
1 , Fairfield Crystal Technology, LLC, New Milford, Connecticut, United States, 2 Dept. of Materials Science & Engineering, State University of New York at Stony Brook, Stony Brook, New York, United States, 3 Dept. of Electrical Engineering, Yale University , New Haven, Connecticut, United States
Show AbstractAlN single crystal is a promising substrate material for high quality III-V nitride epitaxy, especially for fabrication of short wavelength (UV and blue) III-nitride-based devices employing high Al concentrations (e.g. 30% Al). This is because AlN has the same crystal structure as GaN, AlxGa1-xN epitaxial layers lattice match all crystal planes of AlN; that cannot be easily done with either sapphire or SiC. AlN also has a high thermal conductivity, III-nitride-based devices fabricated on AlN substrates will be better able to dissipate heat, which will contribute significantly to better efficiency, longer lifetime and lower degradation. In this paper, we report results from AlN single crystal growth experiments carried out using a sublimation physical vapor transport (PVT) technique at high temperatures. Stand-alone AlN single crystal boules up to 7mm in diameter were produced. Surface morphologies and crystal defects in these AlN single crystals were studied using optical microscopy and an etching technique. Selected AlN single crystals were studied using a high-resolution X-ray diffraction technique and a Synchrotron White Beam X-ray Diffraction Topography technique. Polished surfaces of AlN single crystal wafers were studied using an AFM and a hydrogen etching technique. Major structural defects identified in AlN single crystals include dislocations, grain boundaries and cracks.
9:45 AM - Q3.2
Low-V-defect Blue and Green GaInN/GaN Light Emitting Diodes.
Mingwei Zhu 1 2 , Yong Xia 1 2 , Wei Zhao 1 2 , Yufeng Li 1 2 , Jayantha Senawiratne 1 2 , Shi You 1 2 , Theeradetch Detchprohm 1 2 , Christian Wetzel 1 2
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
Show AbstractGaInN/GaN material system has made rapid progress in recent years for their realization of blue and green light emitting diodes (LEDs) and laser diodes. In order to achieve longer emission wavelength in green and deep green range, higher indium composition is essential in the active region. However, as more indium is incorporated, more defects, including V-defects may be generated. It has been reported that V-defects interrupt the homogeneity of the QWs and act as the non-radiative recombination centers. Therefore, it is necessary to understand and suppress the generation of V-defects in LEDs.In this study, we characterized the structural defects in blue and green GaInN/GaN LEDs by cross-sectional and plan-view transmission electron microscopy (TEM). The samples were grown on bulk GaN or sapphire substrates by metal organic vapor phase epitaxy. LEDs grown on bulk GaN show low defect density in the active region. High resolution TEM measurements further suggest highly homogenous quantum wells and barriers with sharp edges.We observe two types of V-defects in blue and green LEDs. Both types were initiated by edge-type threading dislocations (TDs). Large V-defects with diameters around 500 nm were found in the blue LEDs on bulk GaN. They are initiated around the epitaxial growth boundary. The V-defects density is as low as 2E5cm-2. The optical output power of blue LEDs on bulk GaN is five times better than those on sapphire, which has one order of magnitude higher V-defect density. On the other hand, high density (2E9cm-2) of smaller V-defects with {1-101} facets was observed in the active region of green LEDs. The diameters of these smaller V-defects are approximately 150 to 200 nm. A second set of narrower QWs and barriers were grown on the sidewalls of these V-defects. The TDs that lead to the generation of such V-defects were mostly initiated during the growth of active region. After successfully suppressing the generation of these TDs and smaller V-defects, the optical output power is improved by one order of magnitude.This work was supported by a DOE/NETL Solid-State Lighting Contract of Directed Research under DE-FC26-06NT42860.
10:00 AM - Q3.3
Growth and Characterization of High-Performance GaN and AlxGa1-xN Ultraviolet Avalanche Photodiodes Grown on GaN Substrates.
Russell Dupuis 1 , Dongwon Yoo 1 , Jae Hyun Ryou 1 , Yun Zhang 1 , Shyh-Chiang Shen 1 , Jae Boum Limb 1 , Drew Hanser 2 , Edward Preble 2 , Keith Evans 2
1 School of ECE, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 , Kyma Technologies, Raliegh, North Carolina, United States
Show AbstractWide-bandgap GaN-based avalanche photodetectors (APDs), such as AlGaN p-i-n diodes, are excellent candidates for short-wavelength photodetection due to the capability of operating in the solar-blind UV spectral region, λ < 290 nm. For the growth of GaN-based heteroepitaxial layers on lattice-mismatched substrates such as sapphire and SiC, a high density of defects is introduced, thereby causing device failure by premature microplasma breakdown before the electric field reaches the level of the bulk avalanche breakdown field, which has hampered the development of Group III-nitride based APDs. We have demonstrated GaN p-i-n APDs with record-high gains grown on free-standing GaN substrates. In order to achieve intrinsically solar-blind APDs, the use of wider-bandgap material than GaN is required; but the growth of AlGaN APD structures on GaN substrates introduces technological challenges such as less perfect materials quality due to dislocations and strain and even strain-induced cracking, as well as a limitation in doping, etc.. In this study, we investigate on the growth and characterization of the AlGaN-based APDs on GaN substrates.Epitaxial layers of GaN and AlGaN p-i-n ultraviolet avalanche photodiodes on GaN substrates were grown using a Thomas Swan CCS 7x2 close-coupled showerhead MOCVD reactor. Improved crystalline and structural quality for GaN and AlxGa1-xN epitaxial layers were achieved by employing optimum growth parameters on low-dislocation-density bulk GaN substrates in order to minimize the defect density in epitaxially grown materials. In this work, GaN APDs with avalanche gains >10^5 have been obtained. For Al0.05Ga0.95N APDs, a compositionally graded layer from unintentionally doped GaN to Al0.05Ga0.95N was inserted as a strain-management layer for the crack-free growth. The epitaxial layer structure consists of an n-type Al0.05Ga0.95N:Si layer, followed by an unintentionally doped Al0.05Ga0.95N drift region (0.25 μm, n < 5×1016 cm-3), a p-type AlGaN:Mg+ layer, and topped with p-type GaN:Mg++ (heavily doped) contact layer. The devices were fabricated into 30μm- and 50μm-diameter circular mesas. The forward I-V characteristics and low reverse-bias voltage (up to -100V) I-V characteristics were measured. No microplasmas or side-wall breakdown luminescence was visually observed. The photocurrent was obtained using a UV lamp-monochromator operating at a peak emission wavelength of ~250 nm. The avalanche gain reaches a maximum value of ~50 at a voltage of 86.75V. For AlxGa1-xN APDs with higher Al-content, the crack-free growth of thick AlxGa1-xN on a bulk GaN substrate has been investigated by employing various strain management layers such as AlxGa1-xN (x > 0.5) interlayers and/or AlN/GaN multiple short-period superlattice structures. Growth of AlxGa1-xN (x>0.1) PIN structures with higher Al-content and APD device performance of will be reported.
10:15 AM - **Q3.4
Synthesis and Physical Properties of LixZrNCl Superconductors.
Yasujiro Taguchi 1
1 , Institute for Materials Research, Tohoku Universiy, Sendai Japan
Show Abstractβ-ZrNCl and β-HfNCl have been known to become superconductors when doped with electrons by means of alkali-metal intercalation. These superconductors can be thought of as two-dimensional analogue of well-known superconductors, ZrN and HfN with NaCl-type structure. Doped electrons are accommodated into ZrN or HfN double-honeycomb layers, and form a rather simple, two-dimensional electronic state according to recent band calculations. Upon the change in crystal structure and electronic state from three-dimensional one to two-dimensional one, the Tc increases from 10.7 K to 15.2 K, and from 8.8 K to 25.5 K in the case of Zr- and Hf-based materials, respectively. It should be noted that the Tc value of 25.5 K is even higher than those of the materials that are currently used for practical applications, such as NbTi and Nb3Sn. Despite the high Tc value, the difficulties in synthesizing single phase materials and in treating the samples with extreme sensitivity to the air have thus far prevented the systematic investigations on the fundamental properties of these materials. Recently, we have successfully developed a method to obtain single phase samples of LixZrNCl with controlled doping levels, and clarified the electronic phase diagram as well as doping-evolution of physical properties of the materials[1].The as-intercalated samples of LixZrNCl in the lightly doped region of x<0.1 tend to be easily phase-separated into Li-doped and pristine phases, but we found that single phase samples can be obtained by high temperature annealing at 873 K. For thus obtained samples, we carefully confirmed the formation of solid solution, which is a very rare case from a view point of materials chemistry of intercalation compounds. As the doping concentration is reduced from x=0.4, the Tc value remains almost constant, but rapidly increases below x=0.12, taking the maximum value of 15.2 K at x=0.06. At x=0.05, the Tc suddenly disappears and the material turns to an Anderson insulator. Another remarkable feature of the present material is that the electronic specific heat coefficient is much smaller than other superconductors having the similar value of Tc[2]. In the presentation, I would like to discuss about the doping-evolution of the fundamental properties of the present superconductors.This work was done in collaboration with T. Takano, T. Kishiume, A. Kitora, T. Kawabata, M. Hisakabe and Y. Iwasa. [1] Y. Taguchi, A. Kitora, and Y. Iwasa, Phys. Rev. Lett. 97, 107001(2006).[2] Y. Taguchi, M. Hisakabe, and Y. Iwasa, Phys. Rev. Lett. 94, 217002 (2005).
10:45 AM - Q3.5
Investigation on Origin of Efficiency Droops in InGaN-based High-Power Blue Light Emitting Diodes.
Min-Ho Kim 1 2 3 , Martin Schubert 1 2 , Jong-Kyu Kim 1 2 , E. Schubert 1 2 , Hee Seok Park 3 , Yong Jo Park 3 , Joachim Piprek 4
1 Future Chips Constellation, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Electrical, Computer, & Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Central R&D Institute , Samsung Electro-Mechanics, Su-Won, Gyunggi-Do, Korea (the Republic of), 4 , NUSOD Institute LLC, Newark, Delaware, United States
Show AbstractRecently, excellent progress in the III-nitride semiconductor light-emitting devices (LEDs) has paved the way for the use of III-nitride LEDs as a light source suitable for headlamps in automobiles and lighting systems. Such applications generally demand a high optical flux as well as a high luminous efficiency, which mandates a great effort to realize LEDs operating at higher forward currents. It should be noted, however, that most high-power III-nitride LEDs show a severe external quantum efficiency (EQE) droop of about 30~40%, as the forward current increases to 350 mA. The physical origin of the efficiency droop is not yet clearly understood. In this paper, we present simulations on band diagrams, carrier distribution, and light output-current-voltage (L-I-V) characteristics of InGaN/GaN multiple quantum well (MQW) blue LEDs with various parameters including hole mobility, band offset, and polarization effect in order to identify the origin of EQE droop. The LED structure employed in the simulation consists of 5 period MQWs of 3 nm-thick In0.2Ga0.8N wells and 18 nm-thick GaN:Si barriers and a p-type Al0.13Ga0.87N electron blocking layer, followed by a p-type GaN layer. These reference high-power blue LED chips show an optical output power of about 250 mW at a forward current of 350 mA (J = 35 A/cm2). The simulated L-I-V characteristics of the reference LED structure are in excellent agreement with experimental results.Our simulations have shown that the electron overflow over the EBL indeed occurs even at a low forward current, and increases linearly with increasing forward current, resulting in the external quantum efficiency (EQE) droop of ~ 25%. It is also shown that when the hole mobility decreases from 10 to 1 cm2/Vs, the efficiency droop further increases from 25% to 37% due to a significant increase in electron overflow. Furthermore, our simulations reveal that piezoelectric polarization fields introduced in the EBL and the MQW region have significant role in the EQE droop. Based on our findings, a promising LED structure with minimized efficiency droop will be presented and discussed in detail.
11:45 AM - Q3.7
Growth and Characterization of Non-polar GaN Multi-Quantum-Well Structures on LiAlO2.
H. Behmenburg 1 , A. Alam 1 , Y. Dikme 1 , B. Dlugosch 2 , C. Sommerhalter 2 , N. Rzheutski 3 , R. Schreiner 1 , E. Lutsenko 3 , A. Gurskii 3 , G. Yablonskii 3 , M. Heuken 1
1 , AIXTRON AG, Aachen Germany, 2 , AIXTRON Inc., Sunnyvale, California, United States, 3 , National Academy of Sciences of Belarus, Minsk Belarus
Show Abstract12:00 PM - Q3.8
Green Light Emitting Diodes under Photon and Electron Beam Modulation.
Yufeng Li 1 2 , Jayantha Senawiratne 1 2 , Yong Xia 1 2 , Mingwei Zhu 1 2 , Wei Zhao 1 2 , Theeradetch Detchprohm 1 2 , Christian Wetzel 1 2
1 Future Chips Constellation, Rensselaer Polytechinic Institute, Troy, New York, United States, 2 Physics, Applied Physics and Astronomy, Rensselaer Polytechinic Institute, Troy, New York, United States
Show AbstractEfficiency is a subject of continued debate in green and deep light emitting diodes (LEDs) that employs active regions of GaInN/GaN multiple quantum wells (MQWs). It is now widely recognized that piezoelectric (PZ) fields play a dominant role in the spectral emission properties of such strained MWQs structures. It therefore must be expected, that piezoelectric properties should also control their power performance and efficiency. Here we therefore present a study of the efficiency of green (520-545 nm) GaInN/GaN MQWs LEDs under external modulation of bias illumination and electron beam excitation. In this way we explore the role of the electrical field inside the quantum well and its impact on the optical power of the device. We modulate the electroluminescence performance in operational devices under a bias of laser and electron beam excitation and use lock-in technique for sensitive detection. As a function of drive current, we find a modulation of the electroluminescence (EL) power that well supersedes photoluminescence (PL) and cathodoluminescence (CL) by itself. At low diode current (1 mA), resonant laser excitation (488 nm) is found to increase LEDs efficiency by 19% in higher light-output dies (A) and 7% in lower light-output dies (B). The enhancement increases with the driving current up to 4-5 mA. The enhanced intensity at 5 mA is 4 times larger than that of PL. We attribute the extra luminescence to an improved efficiency of the device. The same behavior is observed in a commercial green LED. Under electron beam modulation, the same phenomenon occurs but with a different magnitude. The resulting effect is not a simple modulation of the electrical transport properties as concluded from the current-voltage characteristics. Upon further investigation we expect clues as to the origin and mechanisms of green LED performance sloop. This work was supported by a DOE/NETL Solid-State Lighting Contract of Directed Research under DE-FC26-06NT42860.
12:15 PM - Q3.9
Nanoporous GaN p-n Junctions Fabricated by a Simple Chemical Vapor Deposition Approach.
Dominique Drouin 1 , Juan Carvajal 2 , M. Aguilo 2 , Arnaud Beaumont 1 , F. Diaz 2 , J. Rojo 3
1 Nanofabrication and nanocharacterization research center, Electrical and computer engineering, Universite de Sherbrooke, Sherbrooke, Quebec, Canada, 2 Física i Cristalolografia de Materials, Universitat Rovira i Virgili, Tarragona Spain, 3 , GE Global Research, Niscayuna, New York, United States
Show AbstractThe unique properties that porous semiconductor materials exhibit compared to their bulk counterparts have propelled the utilization of these materials in the fabrication of devices with enhanced functionality for advanced microelectronics, sensors, interfacial structures and catalysis. Among these materials, wide bandgap semiconductors, and specially GaN are expected to potentially contribute to the advancement of novel technologies in magnetism, catalysis, and biotechnology. The actual application of these materials does, however, critically hinge on the development of processing methods able to precisely control the optical and electrical properties of the resulting porous materials. A major difficulty for the application of these materials also appears with the lack of simple routes for their integration on the most common semiconductor technologies, based on silicon.Using a simple chemical vapor deposition approach based on the direct reaction of gallium and ammonia, we have been able to grow n-type nanoporous GaN particles with pore sizes below 100 nm on Si (111) and Si (100) substrates. SEM analysis of the nanoporous GaN crystalline micron-size particles reveals an almost regular array of nanopores closely aligned along the [0001] crystallographic direction. By introducing Mg3N2 during the growth process we were able to dope GaN particles with Mg while still maintaining the nanoporosity, which allowed producing p-type GaN in a simple and costless synthetic route. Furthermore, in a two step growth process we were able to produce nanoporous GaN p-n junctions, by producing first n-type GaN nanoporous particles, and then, in a second step, growing Mg-doped GaN on these nanoporous particles. The resulting particles with a p-n junction in them and with diameters around 1.5 um still exhibited porosity in one of their surfaces.Cathodoluminescence studies of these structures revealed a sharp and well defined band-gap emission in pure GaN porous particles that shifted to longer wavelengths and broadened, while the intensity of the peak decreased. The p-n junctions fabricated following this simple approach showed emission at around 395 nm. In all cases, the porous faces of the samples showed stronger emission that the rest of their flat faces, allowing a better efficiency of light extraction when compared with conventional flat GaN p-n junctions, with benefits for applications in high bright white LEDs that require increased external quantum efficiency.Compared to other reported approaches, this process is unique in that it results in the formation of nanoporous p-n junctions already integrated on Si substrates during the growth process without requiring any post-growth treatment.
12:30 PM - Q3.10
Photoluminescence of Gallium Nitride in Air with Acidic and Basic Vapors.
Vidhya Chakrapani 1 , John Angus 1 , Kathleen Kash 1 , Chandrashekar Pendyala 2 , Mahendra Sunkara 2
1 , Case Western Reserve University, Cleveland, Ohio, United States, 2 , University of Louisville, Louisville, Kentucky, United States
Show AbstractIn humid air the yellow band luminescence increases in intensity and is blue-shifted in the presence of HCl vapors; in the presence of NH3 the intensity decreases and is red-shifted. The intensity of the near-band-edge luminescence decreases in HCl vapors and increases in NH3 vapors. These observations are interpreted as arising from electron transfer between the GaN and an electrochemical redox couple in an adsorbed water film on the GaN surface. In equilibrium the surface Fermi level of the GaN is equal to the electron chemical potential in the water film, which in turn is fixed by the oxygen redox couple,O2 + 4H+ + 4e− = 2H2O. The electron chemical potential of the redox couple is a function of pH and spans the energy range of the midgap states. At low pH the chemical potential is low in the gap and midgap states are emptied; at high pH the electron chemical potential is higher in the gap, filling midgap states. This process directly mediates the intensity and average photon energy of the yellow band emission. This interpretation of the effect of ambient on the luminescence from GaN is consistent with prior observations [1-4] and may explain disparate results obtained from experiments done in air. Electrochemically mediated charge transfer is not unique to GaN and is believed to be responsible for the unusual p-type surface conductivity observed in hydrogen-terminated diamond [5-7]. In humid air, adsorbed water films are ubiquitous [8] and related effects may occur in other semiconductors as well.[1] M. A. Reshchikov and H. Morkoc, J. Appl. Phys. 97, 061301 (2005).[2] M. A. Reshchikov, M. Zafar Iqbal, D. Huang, L. He, and H. Morkoc, Mat. Res. Soc. Symp. Proc. 743, L11.2.1 (2003).[3] M. Z. Iqbal, M. A. Reshchikov, L. He, and H. Morkoc, J. Electron. Mater. 32, 346 (2003).[4] U. Behn, A. Thamm, O. Brandt, and H. T. Grahn, J. Appl. Phys. 87, 4315 (2000). [5] M. I. Landstrass, and K. V. Ravi, Appl. Phys. Lett., 55, 975 (1989).[6] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472-3475.[7] V. Chakrapani, S. C. Eaton, A. B. Anderson, M. Tabib-Azar, and J. C. Angus, Electrochem. and Solid State Lett. 8 (2005) E4-E8[8] A.W. Adamson, "Physical Chemistry of Surfaces," John Wiley, NY, 4th edition, 1982.
Tuesday PM, November 27, 2007
Back Bay B (Sheraton)
3:00 PM - Q4.2
Molecular Beam Epitaxy of Nonpolar Cubic AlxGa1-xN/GaN Epilayers.
Donat As 1 , Stefan Potthast 1 , Joerg Schoermann 1 , Elena Tschumak 1 , Marcio de Godoy 1 , Klaus Lischka 1
1 Department of Physics, University of Paderborn, Paderborn Germany
Show AbstractGroup III-nitrides crystallize in the stable wurtzite structure or in the metastable zincblende structure. An important difference between these material modifications is the presence of strong internal electric fields in hexagonal (wurtzite) III-nitrides grown along the polar c-axis, while these “build-in” fields are absent in cubic (zincblende) III-nitrides. Since polarization fields can limit the performance of devices some attention has been focused recently on the growth of wurtzite structures with nonpolar orientations e.g., growth along a, m or R directions and also on cubic nitrides. The cubic III-nitride polytype is metastable and can only be grown successfully in a narrow window of process conditions. For the fabrication of electronic devices it is essential to realize AlxGa1-xN epilayers with a well defined Al mole fraction x which determines e.g. the barrier height of heterojunctions.Nonpolar cubic AlxGa1-xN films were grown by molecular beam epitaxy on freestanding 3C-SiC (001) substrates with an Al mole fraction of x = 0 to 1. Using the intensity of a reflected high energy electron beam as a probe we find optimum growth conditions of c-AlGaN when a one-monolayer gallium coverage is formed at the growing surface. Clear reflection high energy electron diffraction oscillations during the initial growth of AlxGa1-xN/GaN layers were observed. The growth rate was about 177 nm/h. We find that the aluminium mole fraction is only determined by the aluminium flux, and that the AlxGa1-xN growth rate is independent on the aluminium content. Atomic force microscopy exhibits smooth surfaces with a RMS roughness of about 5 nm on 5x5 µm2 areas. Cathodoluminescence spectroscopy revealed clear band edge emission up to an aluminium mole fraction of x = 0.52, showing a linear relation between the band gap energy and the Al composition.
3:15 PM - Q4.3
Two- and Three-Dimensional Design of InGaN White Light Emitting Diodes Nanostructures.
Zhiwen Liang 1 , Edwin Garcia 1
1 Materials Engineering, Purdue University, West Lafayette, Indiana, United States
Show Abstract3:45 PM - Q4.5
Multifunctional Ultracomposites: Piezoelectric Materials Grown on Binary Metallic Glasses.
Michael Brougham 1 2 , Colin Ophus 1 2 , Steven Melenchuk 1 2 , Jia Luo 1 2 , Erik Luber 1 2 , Mohsen Danaie 1 2 , Fraser Forbes 1 , Velimir Radmilovic 3 , Zonghoon Lee 3 , David Mitlin 1 2
1 Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada, 2 , National Institute for Nanotechnology, Edmonton, Alberta, Canada, 3 NCEM, Lawrence Berkeley National Laboratory, University of California, Berkeley, California, United States
Show AbstractAs MEMS devices enter the regime of the nanoscale, it becomes increasingly important to effectively scale each component. The present study explores the feasibility and advantages of a new class of "multifunctional ultracomposites" by examining the characteristics of a key piezoelectric, AlN, grown on a novel metastable alloy, Al-32 at.% Mo. Using a parameter optimization procedure and advanced tools such as XRD and TEM, we have demonstrated the following: the ultrasmooth surface of AlMo combined with its incredibly high nucleation site density together enable a competing growth pattern whereby the desired piezoelectric (0002) orientation dominates from the very outset of growth. Our initial results also suggest a third contributing factor and new physical phenomenon namely local epitaxial growth on nanocrystallites through the enhanced mechanical compliancy of the amorphous matrix. Taken together, this novel approach opens new possibilities in the NEMS-scale miniaturization of both actuation and detection capabilities with applications including self-sensing nanorobotics and scanning probe microscopy.
4:30 PM - Q4.6
GaN Nanowalls Grown by RF-plasma Assisted Molecular Beam Epitaxy.
Akihiko Kikuchi 1 2 , Takayuki Hoshino 1 , Shunsuke Ishizawa 1 2 , Hiroto Sekiguchi 1 2 , Katsumi Kishino 1 2
1 Engineering of Electrical and Electronics, Sophia University, Tokyo Japan, 2 , CREST, JST Japan
Show AbstractSemiconductor nanocrystals are promising candidates for future nano-photonics and nano-electronics applications. Recently, there are many repots on growth and device application of semiconductor nanocrystals. We also have been reported on GaN nanocolumns, that is self-assembled one-dimensional columnar nanocrystals [1] with superior optical characteristics [2] and their LED applications [3-4].
In this study, we demonstrated growth of two-dimensional GaN nanocrystals that is nanowalls. GaN nanowalls with the width of 300 nm, height of 1000 nm and length of over 100 um were successfully grown by rf-plasma assisted molecular beam epitaxy (RF-MBE) on GaN template substrate. The GaN nanowalls were grown c-axis (0001) perpendicular to the substrate surface and having very smooth side planes. The width of the nanowalls was same from the bottom to the top. It is considered that for the GaN nanowalls along the [1_100] direction have a-plane (11_20) as side-planes and for the nanowalls along [11_20] direction have m-plane (1_100) as side-planes.
The GaN nanowalls were grown on GaN template. The GaN template was grown by a metal-organic vapor deposition (MOCVD) on a (0001) sapphire substrate. Prior to the growth, titanium metal was deposited on the MOCVD-GaN template with electron beam deposition and stripe windows with ~300 nm width was open by electron beam lithography and dry-etching technique to appear the GaN surface.
In generally, it was known that the selective growth of III-V semiconductors by MBE is quite difficult compare to the MOCVD method. But as shown in the our previous study on GaN nanocolumn [1], RF-MBE can grow high aspect nanocrystals under a optimum growth conditions. By use of nanocolumn growth condition for the metal masked GaN template with stripe windows, we could achieve a complete selective growth of GaN nanowalls by RF-MBE.
From the SEM observation, it was confirmed that the GaN nanowalls were quite uniform and aligned very well. For example, a hundred nanowalls with a 300 nm width, 1000 nm height and 150um length were ordered with 100nm spacing. We also demonstrated some other structures with GaN nanowalls such as single stripe, hexagonal ring cavity, and mesh pattern.
The well controlled GaN nanowalls wil be used for many kinds of nano-electronic and -photonic devices such as laser diodes, LEDs, optical circuits, FETs, and so on.
Acknowledgements:
This study was supported by a Grant-in-Aid for Scientific Research #18069010 and #18310079 from the MEXT, and NEDO Industrial Technology Research Grant #02A23041d.
References:
[1] M. Yoshizawa, A.Kikuchi, M.Mori, N.Fujita and K.Kishino, Jpn. J. Appl. Phys 36, L459 (1997).
[2] A. Kikuchi, K. Yamano, M. Tada and K. Kishino, phys. stat. sol. (b), 241, 2754 (2004).
[3] A. Kikuchi, M. Kawai, M. Tada and K. Kishino, Jpn. J. Appl. Phys. 43, L1524 (2004).
[4] A. Kikuchi, M. Tada, K. Miwa and K. Kishino, Proc. SPIE 6129, 6129-05 (2006).
4:45 PM - **Q4.7
Semiconducting and Metallic Perovskite Nitrides: Structures and Properties.
Rainer Niewa 1
1 Chemistry, TU Munich, Garching Germany
Show AbstractInverse Perovskite chemistry emerges to display all basic crystallographic features known from normal Perovskites. However, chemistry and physical properties differ substantially. Perovskite nitrides of the general composition (Ca3N)E with E = P, As, Sb, Bi, Ge, Sn Pb, Tl are known for some years. All these compounds crystallize as cubic Perovskites or distortion variants thereof [1, 2]. The compounds of group 15 elements obey the (8 - N) rule and accordingly show properties compatible with semiconductors, while the compounds with E = Tl and E = Ge, Sn, Pb were described as electron-deficient metals. Compounds (A3N)E with A = Sr, Ba and E = Sb, Bi crystallize as cubic (Sr) and hexagonal 2H (Ba) Perovskites and represent diamagnetic semiconductors [3]. On gradual substitution of Sr by Ba (i. e. the quaternary system (Sr3-xBaxN)Bi) partial order Sr-Ba leads to phases with 4H and 9R Perovskite structures. The intriguing order scheme of the alkaline-earth metal ions presents the possibility of stacking engineering of such hexagonal Perovskites [4]. On insertion of oxygen recently even the n = 1, 3 members of a Ruddlesden-Popper series with the general composition ”n (A3Z)Bi ● ABi” (Z = N, O) and Sr–Ba site preference were obtained [5]. By exchange of E = group 15 elements by the E = group 14 elements Sn, Pb exclusively cubic Perovskites (A3Nx)E with x in the range of 0.60 < x < 0.85 and intrinsic defects in the nitride substructure are obtained [6]. Electrical resistivity and magnetic susceptibility studies indicate these phases to exhibit metallic properties. The composition with x = 2/3 would obey the (8 – N) rule, consequently one might expect semiconducting properties. However, electronic band structure calculations on ordered superstructure models reveal metallic behavior and indicate the tendency to higher nitrogen site occupancy as was observed in experiments. This chemistry is prolonged on turning to rare-earth metal systems of nitrogen, carbon and oxygen as compared to the alkaline-earth metal systems with nitrogen. Here, typically metallic properties of Perovskite compounds containing rare-earth metal species in the electronic R3+ state are obtained. Eu and Yb present exceptions. The introduction of rare-earth metals in such systems leads to extensive magnetic order phenomena [7]. [1] M. Y. Chern, D. A. Vennos, F. J. DiSalvo, J. Solid State Chem. 1992, 96, 415.[2] R. Niewa, W. Schnelle, F. R. Wagner, Z. Anorg. Allg. Chem. 2001, 627, 365.[3] F. Gäbler, M. Kirchner, W. Schnelle, U. Schwarz, M. Schmitt, H. Rosner, R. Niewa, Z. Anorg. Allg. Chem. 2004, 630, 2292.[4] F. Gäbler, R. Niewa, Inorg. Chem. 2007, 46, 859. [5] F. Gäbler, Yu. Prots, R. Niewa, Z. Anorg. Allg. Chem. 2007, 633, 93.[6] F. Gäbler, M. Kirchner, W. Schnelle, M. Schmitt, H. Rosner, R. Niewa, Z. Anorg. Allg. Chem. 2005, 631, 397.[7] M. Kirchner, W. Schnelle, R. Niewa, Z. Naturforsch. 2006, 61b, 813 and references therein.
5:15 PM - Q4.8
LED it be.
Andries Meijerink 1 , Volker Bachmann 1 2 , Cees Ronda 1 2
1 Chemistry, Debye Institute, Utrecht Netherlands, 2 , Philips Research Labortaories, Aachen Germany
Show AbstractThe discovery of blue emitting GaN LEDs had an enourmous impact on the lighting industry because it makes it possible to generate white light with LEDs. The market for white light emitting LEDs is now expanding rapidly. From niche applications, like flashlights and traffic lights, white light LEDs are finding their way into general lighting applications. This has an impact on phosphor research. New phosphors are needed efficiently absorb in the near UV to blue spectral range and emit in the visible. The energy difference between excitation and emission wavelength is small which is good for the energy efficiency, but it lowers the choice of activator ions that can be used. Research is mainly focused on Eu2+ and Ce3+. For an LED phosphor to be applied in commercial products several criteria have to be met such as: high quantum efficiency, high thermal quenching temperature, and the possibility to adjust the color. In this contribution the luminescence properties of various LED phosphors will be discussed to understand the mechanism for thermal quenching and energy transfer processes that determine the luminescence characteristics. First, the widely applied phosphor YAG:Ce will be revisited. It is shown that the intrinsic luminescence quenching temperature is very high (above 700 K). The variation in the (lower) quenching temperatures reported in the literature is explained on the basis of thermally activated concentration quenching and the temperature dependence of the oscillator strength of the 450 nm absorption band.Next, a new class of LED-phosphors will be discussed: oxynitrides doped with Eu2+. The luminescence and luminescence quenching mechanism for the Eu2+ and Yb2+ luminescence in SrSi2O2N2 will be presented. The Eu2+ doped compound is a very promising material for application in white light LEDs due to the luminescence quantum efficiency (>90%), the stability and the high luminescence quenching temperature. Only above 600 K the luminescence quenches. Comparison with the (anomalous) luminescence of Yb2+ in the same host show that the quenching mechanism is thermally activated photoionization from the f-d excited state. Finally, color tuning will be discussed. It is important to shift the emission to longer wavelengths (orange or red) to change the cool white light from LEDs with YAG:Ce to a warmer white for home lighting. For SrSi2O2N2:Eu and YAG:Ce it will be shown how the emission color can be tuned by changing the chemical composition, while retaining good thermal quenching behavior. Finally an outlook will be given on the future of solid state lighting based on GaN LEDs based on the rapid developments in this field.
5:30 PM - Q4.9
Polarization Anisotropy in the Light Emission of Blue GaInN/GaN Light-emitting Diodes Grown on (0001) Oriented Sapphire Substrates.
Martin Schubert 1 , Sameer Chhajed 1 , Jong Kim 1 , E. Fred Schubert 1 2 , Jaehee Cho 3
1 Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Physics, Applied Physics, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Opto System Laboratory, Corporate R&D Institute, Samsung Electro-Mechanics, Suwoon Korea (the Republic of)
Show AbstractPolarized light-emitting diodes (LEDs) would be highly advantageous for applications such as liquid crystal display backlighting, sensing, imaging, and free space optical communications. Previously, partially polarized light emission has been reported for GaInN/GaN LEDs grown on non-polar or semi-polar substrates. However, the polarization of light emitted by LEDs grown on conventional polar sapphire substrates has not been thoroughly examined. Here we report measurements of the polarization of light emission by blue 460 nm GaInN/GaN LEDs with multi-quantum-well (MQW) active regions grown on (0001) oriented sapphire substrates. Light emitted from conventionally packaged devices and bare unpackaged chips is measured over a wide range of emission angles from vertically above the LED surface (φ = 0°) to nearly directly below it (φ = 160°). For the unpackaged chips, ray-tracing simulations are used to calculate the angular distribution for top emitted light in order to distinguish top-emitted light from light emitted through the chip facets. It is found that unpackaged chips emit a majority of light from the facets, and that this side emission is highly polarized with the electric field in the plane of the MQW. The ratio of intensity of in-plane polarized light to normal-to-plane polarized light reaches values as high as 7:1. When integrated over all emission angles, the total power of in-plane polarized light is found to be more than double the power of normal-to-plane polarized light. The packaged devices do not show any polarization anisotropy, and emit each polarization with equal intensity due to the design of the LED chip packaging.
Symposium Organizers
Ruediger Kniep Max-Planck-Institute for Chemical Physics of Solids
Francis J. DiSalvo Cornell University
Ralf Riedel Technische Universitaet Darmstadt
Zachary Fisk University of California
Yoshiyuki Sugahara Waseda University
Wednesday AM, November 28, 2007
Back Bay B (Sheraton)
9:30 AM - Q5.1
Host Dependence of Photoluminescence from Eu-implanted AlGaN Alloys.
Kevin O'Donnell 1 , Ke Wang 1 , Benjamin Hourahine 1 , Robert Martin 1 , Katharina Lorenz 2 , Eduardo Alves 2 , Ian Watson 3
1 Physics, University of Strathclyde, Glasgow United Kingdom, 2 , ITN, Sacavem Portugal, 3 Photonics, University of Strathclyde, Glasgow United Kingdom
Show AbstractThe photoluminescence (PL) and PL excitation (PLE) spectra of Eu-implanted and annealed AlxGa1-xN alloy are obtained over the whole composition range for the first time. A single Eu centre, which can be photoexcited both above and below the bandgap, dominates the PL of the 5D0-> 7F2 band, which is found near 622 nm in pure GaN. The emission band broadens and then narrows, similar to an alloy exciton, but downshifts by only 7 meV (~2.2 nm) as x increases from 0 to 1. The trans-gap PLE spectrum of Eu emission measures the bandgap energy of AlGaN from 3.5 towards 6.2 eV. Below gap, we identify 2 related excitation bands: X1 has previously been observed in GaN at 380 nm (3.25 eV); it upshifts linearly by 0.23 eV as x increases to 1. A second band, X2, with a very similar energy shift with composition, emerges when x increases above 0.5. In AlN, the energy of X2 is 4.59 eV. We identify X1,2 as bound-exciton like complexes of Eu emitting centres, formally equivalent to charge transfer states. Hence the emission mechanism of Eu doped III-nitride luminescence is clarified and the need for “RE-related deep defects” to “sensitise” or “activate” that luminescence is eliminated by these studies.
9:45 AM - Q5.2
Electron Band Structure of MnGaN.
Dimiter Alexandrov 1 , Nikolaus Dietz 2 , Ian Ferguson 3 , Hang Yu 1
1 Electrical Engineering, Lakehead University, Thunder Bay, Ontario, Canada, 2 Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia, United States, 3 School of Elect. & Comp. Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractTheoretical determination of the electron band structure of MnGaN semiconductor compound alloy is performed in this paper. The authors model the MnGaN alloy on Mn content. Segregated Mn species in wurtzite GaN containing Mn are not considered, only alloyed species with Mn substituting on the Ga site. For this arrangement a new ternary semiconductor MnGaN is identified and it has two binary constituents – GaN (the existing binary crystal) and MnN that exists in the structure of MnGaN. It allows that MnN primitive cell satisfying the following conditions to be introduced: a) Mn atom substitutes for a Ga atom saving the tetrahedral shape of the crystal cell; b) Mn atom has valence equal to 2 and the N atom has valence equal to 5; c) Mn-N bond is ion-covalent. Using the conditions a), b) and c) the authors have found that the 2p orbital of N atom attracts an electron from the 3d orbital of Mn atom and this electron replaces on the 4p orbital of Mn atom. In this way Mn-N bond becomes of sp3 type. The following parameters of MnN binary constituent are found: 1) The energy band gap of tetrahedral MnN is 6.10 eV. 2) Charge transfer between 3d-orbital of Mn atom and 4p-orbital of the same atom is one electron and as result the valence electron cloud of Mn atom is replaced in direction to nitrogen atom, i.e. donor behavior can be expected from the alloyed Mn atoms. 3) Using the Hartree-Fock method it is found that the electron energy of the 4p orbital of Mn atom, which forms MnN tetrahedral cell, is -3.41 eV. 4) Using the valence orbital radii of both atoms Mn and N and considering the tetrahedral shape of MnN cell, the distance between Mn and N atoms is found to be 1.56 Angstrom. LCAO electron band structure of wurtzite MnGaN for points Γ is calculated in term of optical properties and in consideration of the following ratio Mn:Ga forming quasi-elementary cells: 1) GaN quasi-elementary cell. 2) mixed Mn-GaN quasi-elementary cell containing 1.5 atoms of Ga and 0.5 atoms of Mn; 3) mixed Mn-GaN quasi-elementary cell containing 1 atom of Ga and 1 atom of Mn; 4) mixed Mn-GaN quasi-elementary cell containing 0.5 atoms of Ga and 1.5 atoms of Mn; 5) pure MnN quasi-elementary cell. This electron band structure has the following features: a) Inter-band optical absorption can be observed for photon energy ~1.60 eV, which is close to the experimentally observed absorption band ~1.50 eV in the samples of MnGaN without silicon impurities; b) Tunnel optical absorption can be observed for photon energies ~1.59 eV and greater than 1.86 eV. c) Peaks of the PL spectra can be observed for energies 1.60 eV, 2.05 eV that is close to the experimental value of 2.20 eV, and ~2.67 eV, which is close to the experimental value of 2.70 eV. The exact value of the PL peak depends on the technological circumstances of the ratio Mn:Ga forming mixed quasi-elementary cells in the structure of MnGaN. The same is valid for domination of either inter-band or tunnel optical absorption.
10:00 AM - Q5.3
Deep Ultraviolet Photoluminescence Studies of AlN Epilayers Grown on Different Substrates.
Neeraj Nepal 2 , B. Pantha 2 , T. Tahtamouni 2 , J. Li 2 , M. Nakarmi 2 , J. Lin 2 , H. Jiang 2 , J. Zavada 3
2 Department of Physics, Kansas State University, Manhattan, Kansas, United States, 3 , U. S. Army Research Office, Durham, North Carolina, United States
Show AbstractAlN is an ideal material for applications in ultraviolet (UV) emitters and detectors active in the spectral wavelength region down to 200 nm. The increased importance of AlN for various applications and the diverse choices of substrates call for a systematic comparative study concerning the basic structural and optical properties of AlN epilayers grown on different substrates, which however has been highly challenging because the growth conditions of AlN epilayers on different substrates have to be independently optimized. We report on the epitaxial growth, and DUV photoluminescence (PL) and x-ray diffraction (XRD) studies of AlN epilayers grown on sapphire, SiC, Si, and AlN bulk single crystal substrates. The variations of the free exciton emission peak position and lattice constant with respect to the growth substrate have been systematically probed and correlated. It was confirmed by PL and XRD results that AlN epilayers grown on AlN bulk substrates (or homo-epilayers) have the same lattice parameters as AlN bulk crystals and strain-free, while AlN hetero-epilayers grown on sapphire substrates experience a compressive strain and those grown on SiC and Si substrates undergo a tensile strain. Based on the variations of the FX emission peak position and lattice constant with respect to the growth substrate, a linear relationship between the FX transition and in-plane stress was obtained and a value of 45 meV/GPa for the linear coefficient of the stress-induced bandgap shift in AlN epilayers was deduced, which is about 88% larger than that in GaN. The work here establishes PL as an alternative simple and effective method for monitoring the substrate induced biaxial stress in AlN epilayers.* Presently at Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC.
10:15 AM - **Q5.4
Shock Wave Synthesis and Exploration of High-pressure Nitrides and Related Materials.
Toshimori Sekine 1
1 , National Institute for Materials Science, Tsukuba Japan
Show AbstractShock wave achieves high pressure and high temperature states that give us unique conditions for material synthesis. We have carried out shock wave synthesis and exploration of high-pressure nitrides and related systems recently. We investigated the high-pressure phases by shock recovery experiments and in situ measurements of shocked materials. The samples obtained after shock-loadings were characterized by the techniques of XRD, TEM, NMR, DTA and the Hugoniot data were determined by the inclined mirror method to record wave velocities. These results will be reviewed. We have successfully developed a method to manufacture spinel-type Si3N4, which can be synthesized only above ~15 GPa, and also a chemical treatment method to separate the spinel-type phase from the low-pressure phase. Similar methods could be applied for the SiAlON systems. In order to explore high-pressure nitrides and oxynitrides, we extended toward the system Si3N4-AlN-Al2O3. According to the results of in situ measurements of the high-pressure behavior up to pressures of 200 GPa, there appears to be post-spinel phase in the system. This is consistent with the results from the first principles calculations. However we could not obtain the post-spinel phase by the shock recovery experiments at present. We also carried out shock recovery experiments on carbon nitrides. We employed several reactions including the followings: (1) X CBr4 + 4X NaN(CN)2 = [C3N4]3X + 4X NaBr , (2) 3 C2N4H4 = 2 C3N4 + 4 NH3 , (3) C3N6H6 = C3N4 + 2 NH3 .Nitrogen-rich starting materials, included a C-N-O amorphous precursor, dicyandiamide, melamine, and a mixture of carbon tetrahalide and sodium dicyanoamide, were used and the recovered samples were investigated by X-ray diffraction technique, elemental analysis, transmission electron microscopy and so on. Experimental results showed formation of a new carbon nitride, high stability of melamine up to a shock pressure of 37 GPa, and production of amorphous C-N materials with a highest N/C ration of 1.26 from the reaction between carbon tetrahalide and sodium dicyanoamide. We extended toward the system C3N4-Si3N4 after taking into account the results on shock synthesis of spinel-type nitrides.
10:45 AM - Q5.5
Superfluorescence in Green Emission GaInN/GaN Quantum Well Structures under Pulsed Laser Excitation.
Jayantha Senawiratne 1 2 , Stephanie Tomasulo 2 , Theeradetch Detchprohm 1 2 , Mingwei Zhu 1 2 , Yufeng Li 1 2 , Wei Zhao 1 2 , Yong Xia 1 2 , Peter Persans 2 , Christian Wetzel 1 2
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
Show AbstractGaInN/GaN quantum well (QW) structures are the system of choice for UV and blue laser diodes and also hold the biggest promise for green laser diodes. We report optical investigation of green emission GaInN/GaN multi-QW (MQW) structures near lasing conditions under pulsed and continuum (cw) laser excitation. We compare the optical properties of c-plane grown MQW with m-plane MQW structures under strong laser field.GaInN/GaN MQW structures have been grown by metal organic vapor phase epitaxy on c-plane sapphire and m-plane on GaN substrates. The MQW consists of five GaInN/GaN quantum wells of nominal well width of 3 nm separated by barriers of nominal width 11 nm. High pump intensity measurements were carried out with a nitrogen laser (λ = 337.1 nm) with 10 ns pulse width and maximum pulse energy of 0.1 mJ. Low pump intensity measurements were carried out with a HeCd cw laser of wavelength 325 nm. Photoluminescence (PL) was collected in surface and edge emission configurations under variable excitation power. Edge emission PL of a sample grown on the c-plane of sapphire substrates blue shifts by 200 meV as the pump intensity is increased. In contrast, the PL blue shift measured under the same conditions is less than 10 meV for a sample grown on the nonpolar m-plane of the bulk GaN substrate. A large PL blue shift is explained by considering the screening of large piezoelectric polarization in the c-plane grown sample. Under intense pulse excitation, the full width half maximum of PL narrows to ~4 nm and couples out of the sample in two distinct modes. Variation of the PL intensity with excitation stripe length is consistent with superfluorescence.This work was supported by a DOE/NETL Solid-State Lighting Contract ofDirected Research under DE-FC26-06NT42860
11:30 AM - Q5.6
Magnetic Exchange Interactions in Mn-doped ScN.
Aditi Herwadkar 1 , Walter Lambrecht 2 , Mark Schilfgaarde 3
1 Department of Physics and Astronomy, University of Nebraska Lincoln, Lincoln, Nebraska, United States, 2 Department of Physics, Case Western Reserve University , Cleveland , Ohio, United States, 3 Department of Chemical and Materials Engineering, Arizona State University , Tempe, Arizona, United States
Show AbstractScN is a relatively little studied semiconductor with rocksalt structure. Here we study the possibility of using ScN as a diluted ferromagnetic semiconductor host using Mn doping. We present a study of the exchange interactions in Mn-doped ScN using the linear response approach implemented in the linear muffin-tin orbital Green's function method. In this approach, exchange interactions are calculated directly from the Green's function and the change in scattering t-matrix due to a rotation of a spin within the rigid spin approximation (maintaining constant spin density direction within each atomic sphere). The validity of the approach is tested by comparison with direct total energy calculations in small cells of 64 atoms with different collinear and non-collinear magnetic configurations. Special quasirandom structure cells of 256 and 432 atoms are used with different concentrations of Mn. We find that the exchange interactions are long-range and strongly influenced by the disorder in the system. It is found crucial to include a gap correction beyond the local density approximation. Without it ScN is a zero gap semiconductor and the exchange interactions turn out to become dominated by long-range antiferromagnetic interactions leading to a spin-glass state rather than a ferromagnet. This is confirmed by the fact that the lowest spin-wave excitation has negative energy. Including a gap correction, however, we find ferromagnetic interactions to be dominating up to a distance of about 3 cubic lattice constants. The concentration dependence is studied and the critical temperatures are estimated using a recently introduced extension of the cluster variation method for the Heisenberg Hamiltonian. We also study the effect of adding nitrogen vacancies which appears to further increase the critical temperature. Above room temperature Tc's appear feasible with modest Mn concentrations.
11:45 AM - Q5.7
Blue Light Emitting Diodes Based on ZnO/GaN Wafer Bonding.
Akihiko Murai 1 , Daniel Thompson 1 , Natalie Fellows 1 , Hitoshi Sato 1 , Umesh Mishra 1 , Shuji Nakamura 1 , Steven DenBaars 1
1 , University of California Santa Barbara, Santa Barbara, California, United States
Show AbstractWe have investigated blue light emitting diodes (LEDs) based on a wafer bonding of a ZnO wafer to a III-nitride LED wafer for the purpose of obtaining high light extraction efficiency from a LED chip. The light extraction efficiency is critically important for improving external quantum efficiency of LEDs. A single crystal n-type ZnO wafer has the characteristic advantages of high transparency, electrical conductivity, and shape processing for using a transparent electrode to p-type GaN. For this study, we used a III-nitride LED layer grown on a (0001) sapphire wafer by metal-organic chemical vapor deposition and a commercially available n-type ZnO wafer. After surface cleaning, the two wafers were joined together and uniaxial pressure of 2 MPa was applied and thermal process (800 °C for 1 h in nitrogen) was performed. After this process, the two wafers were successfully wafer bonded. Then, the sapphire substrate was detached from the III-nitride LED film by a method of laser lift-off. An n-contact was formed on the exposed N-face GaN. A p-contact was formed on a ZnO. An anisotropic chemical etching was done on the O-face ZnO wafer to generate a hexagonal ZnO pyramid shape. This pyramidal shape is considered to have high light extraction efficiency. Fabricated samples were packaged. In this presentation, we will discuss the optical and electrical characteristics of these types of LED structures in detail.
12:00 PM - Q5.8
Alkaline-earth Nitrodocobaltates(I) Containing [CoIN2]5- Complexes.
Joanna Bendyna 1 , Peter Hoehn 1 , Walter Schnelle 1 , Ruediger Kniep 1
1 chemistry, Max-Planck-InstituteCPFS, Dresden Germany
Show Abstract12:15 PM - Q5.9
Comparison of UV, Blue- and Yellow-Band Micro-Photoluminescence Maps Near Defects in Semi-Insulating GaN.
Bruce Claflin 1 2 , David Look 1 2
1 Materials and Manufacturing Directorate, AFRL/MLPS, WPAFB, Ohio, United States, 2 Semiconductor Research Center, Wright State University, Dayton, Ohio, United States
Show AbstractGrowth of semi-insulating (SI) GaN, either by Fe- or C-doping, has received considerable attention over the past few years in an effort to develop heteroepitaxial substrates with improved isolation for GaN-based electronic device applications. However, these SI materials have not been extensively characterized. Photoluminescence (PL) has been widely used to characterize unintentionally doped and n-type GaN to study the behavior of free and bound excitons, as well as to investigate the properties of extended- and point-defects. In the present work, room temperature micro-PL mapping of the near-band-edge (UV), blue- and yellow-band spectral regions is used to characterize structural defects in SI GaN. Intensity variations observed in the PL maps are correlated with structural features observed using plan-view scanning electron microscopy (SEM) and atomic force microscopy (AFM). The PL spectra from these SI GaN samples are dominated by near-band-edge emission throughout. The yellow-band luminescence is more than 20x lower than the UV emission. Two SI GaN:Fe samples, one bulk and one epitaxial layer, contain low densities (< 5 cm-2) of macro-defects, 100-250 μm in diameter. The cores of these macro defects exhibit intense UV-PL, as much as 30x higher than the surrounding bulk material, suggesting that these sites getter radiative recombination centers. In the bulk SI GaN:Fe sample, these macro-defects are also correlated with an increase in yellow-band luminescence, although still well below the level of the UV-PL. In addition, for the bulk SI GaN:Fe sample, six-pointed-star patterns about 100 μm in diameter are observed that exhibit increased UV-PL along the crystallographic axes. It is possible that these star-patterns emanate from a dislocation at their center, however these patterns do not correlate with any structural features observed with SEM or AFM.
12:30 PM - Q5.10
Electronic and Magnetic Properties of Mixed Valence Monoclinic SrN.
Piotr Boguslawski 1 , Oksana Volnianska 1
1 , Institute of Physics PAN, Warsaw Poland
Show AbstractNitrides span a wide variety of compounds, beginning with the well-established semiconducting GaN and its alloys that currently are of fundamental importance in the microelectronics and blue photonics, and include also the recently investigated transition metal nitrides such as IrN2 and PtN2 [1] with exceptional material properties. Finally, first principles calculations predict that II-N nitrides with the rock salt structure, such as CaN and SrN, are ferromagnetic half-metals [2] due to the non-vanishing spin polarization of the p-shell of N ions, given by the Hund's rule, which persists in solids after formation of bonds. Recent synthesis of Sr nitrides [3], namely SrN2, SrN, and Sr2N, opens a question about their properties. They all share the monoclinic (m) crystal structure, with nitrogen anions occupying octahedral sites formed by Sr cations. Of particular interest is m-SrN, in which there are two non-equivalent types of nitrogen ions that occupy two non-equivalent lattice sites. The first type consists in isolated N ions, while in the second case two neighboring N form a N2 quasi-molecule, characteristic also for the transition metal nitrides [1] and SrN2 [3]. Consequently, one may expect that charge states of the two kinds of N ions are different. This conjecture is supported by calculations of the electronic and magnetic structure based on the density functional theory. We find that SrN is indeed a mixed valence compound. A comprehensive analysis of the results shows that (i) m-SrN is a metal with a pronounced, almost half-metallic spin polarization at the Fermi level, (ii) isolated N ions are in the 3- charge state with the closed-shell configurations, and their magnetic moment vanishes, and (iii) the N2 molecules are in the 2- charge state, and have a magnetic moment close to 1 μB, because of the partial population of the N2 antibonding molecular orbitals. The obtained results point out to the existence of the antiferromagnetic ordering of m-SrN at low temperatures. Presence of stable magnetic moments and their ordering in m-SrN are due to its mixed valence character. Differences between the properties of the monoclinic and rock salt phases of SrN are discussed. [1] J. C. Crowhurst et al., Science 311, 1275 (2006); A. F. Young et al., Phys. Rev. Lett. 96, 155501 (2006). [2] O. Volnianska and P. Boguslawski, Phys. Rev. B 75, 224418 (2007). [3] G. Auffermann, Yu. Prots, and R. Kniep, Angew. Chem. Int. Ed. 40, 547 (2001); G. Auffermann et al., Anal. Bioanal. Chem. 373, 880 (2002).
Wednesday PM, November 28, 2007
Back Bay B (Sheraton)
2:30 PM - **Q6.1
Thermodynamic and Kinetic Investigations in the System Ga-O-N.
Manfred Martin 1
1 Institute of Physical Chemistry, RWTH Aachen University, Aachen Germany
Show AbstractThe wide band gap semi-conductors Ga2O3 and GaN are of interest to industrial applications, ranging from opto-electronic components to sensor materials. While there is high solubility of oxygen in gallium nitride only little is known on the solubility of nitrogen in gallium oxide and on the existence of gallium oxynitrides. Here we report on our thermodynamic and kinetic investigations in the system Ga-O-N. Samples were prepared by classical solid state reactions, by ammonolyis of gallium oxide, and by pulsed laser deposition (PLD). Structural and analytical characterization was performed by X-ray and neutron diffraction, X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), and secondary ion mass spectromery (SIMS). The kinetics of the ammonolyis of gallium oxide was followed by means of in situ XAS at elevated temperatures. The optical band gap of the materials was investigated by ex situ and in situ optical absorption spectroscopy, and the electrical conductivity was measured using the van der Pauw technique. Density functional theory (DFT) was used to calculate electronic structures. Using our experimental and theoretical results we will discuss implications for the stability, the defect structure and the transport properties of some materials in the system Ga-O-N.
3:00 PM - Q6.2
Density Functional Theory Calculations for the Binding Energies and Adatom Diffusion on Strained AlN (0001) and GaN (0001) Surfaces.
Vibhu Jindal 1 , James Grandusky 1 , Neeraj Tripathi 1 , Mihir Tungare 1 , James Raynolds 1 , Fatemeh Shahedipour Sandvik 1
1 , CNSE, Albany, New York, United States
Show AbstractDensity functional theory calculations were carried out to study the binding energies and diffusion barriers for adatoms on AlN and GaN surfaces. The binding energies and diffusion barriers were calculated for Al, Ga and N adatoms on both Al (Ga) terminated and N terminated (0001) surfaces of AlN (GaN). Calculation of the diffusion paths and diffusion energy barriers for Al, Ga and N adatoms on AlN and GaN was made. Further the surfaces were subjected to a hydrostatic compressive and tensile strain in the range of 0 to 5% to investigate the effect of strain on diffusion barriers. Deeper understanding of the effect of strain on these properties may help explain differences observed in growth conditions for homoepitaxy and heteroepitaxy on bulk III-Nitride substrates as opposed to III-Nitride template layers on foreign substrates.
3:15 PM - Q6.3
Light Emitting Diode and Lasers Beyond 1.55 μm with GaAsSbN/GaAs Single Quantum Wells
Kalyan Nunna 1 , Shanthi Iyer 1 , Jia Li 1 , Ward Collis 1
1 Electrical and Computer Engineering, North Carolina A&T State University, Greensboro, North Carolina, United States
Show AbstractIn this report, GaAsSbN/GaAs single quantum well (SQW), separate confinement heterostructure has been successfully used as the emission region for a room temperature (RT) light emitting diode and laser diode operation beyond 1.55 μm. These diodes were grown by solid source molecular beam epitaxial (MBE) technique with an RF assisted plasma source for N. The emission wavelength of 1.61 μm was achieved for ~1.4% N in the quaternary active layer. The light emitting diodes exhibited a RT electroluminescence (EL) peak at 1.61 μm for 30 mA forward current, with a low turn-on voltage of ~500 mV and an abrupt reverse break-down voltage of ~7.5 V. A 1.61 μm pulsed mode laser operation was also achieved at RT with a threshold current density of ~0.94 kA/cm2 for a diode area of 1250 μm x 100 μm. Variation of the laser output power with the drive current as a function of stripe width and temperature will be presented.This work was supported by Army Research Office under the Grant No. W911NF-04-1-0025 and Contract No. W911NF-06-1-0396(Dr.Michael Gerhold, Technical Monitor)
3:30 PM - Q6.4
GaN Nanowires for Label-free Bio-sensing Applications.
Li-Chyong Chen 1 , Chin-Pei Chen 1 , Abhijit Ganguly 1 , Chen-Hao Wang 1 , Chih-Wei Hsu 2 , Yu-Kuei Hsu 2 , Kuei-Hsien Chen 1 2
1 Center for Condensed Matter Sciences, National Taiwan University , Taipei Taiwan, 2 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show AbstractA novel bio-sensing system based on GaN nanowires (NWs) is presented coupled with their electrochemical impedance and photoluminescence measurements. GaN is well established now for a variety of optoelectronic applications. However, while its inherent bio-compatibility has also been recognized, its application as bio-sensors has been surprisingly lacking till date. Meanwhile, one-dimensional nanostructures have attracted huge interest as potential building blocks for the future nanoelectronic devices. In this report, GaN NWs are used as a transducer for DNA-sensors, by immobilizing single-strand DNA (ssDNA) molecules through covalent binding using organosilane linker (MPTS). The MPTS-modified GaN NWs exhibited an electrochemical window remarkably wider than those of boron-doped diamond or carbon nanotubes reported to date. The immobilization of ssDNA and the subsequent hybridization to double-strand DNA (dsDNA) were confirmed using confocal microscope. Electrochemical impedance measurement showed that interfacial electron-transfer resistance (Ret), from solution to transducer surface, increased significantly when pristine GaN NWs were immobilized with ssDNA, along with a formation of additional semicircle region at lower frequency in Nyquist plot. The unique appearance of double-semicircle region for ssDNA-immobilized NWs, compared to single-semicircle region for pristine GaN NWs, leads to the idea of formation of double-capacitance layer in series. The phenomenon is more prominent by the appearance of double peaks in the plot of phase angle vs. frequency (Bode plot), the second peak, formed after ssDNA-immobilization, showed further increase under the hybridization to dsDNA, and consequently reduces the overall impedance. In contrast, characteristic impedance spectra remain the same when complimentary target DNA is replaced by mismatched DNA. Moreover, quenching behavior in photoluminescence of the GaN NWs was distinguishable for the ones immobilized with ssDNA and the same hybridized to dsDNA. Both the technique implies the ability of oligonucleotides, immobilized on the NW-surface, to interact with other biomolecules. The dual and label-free sensing capability in impedance and photoluminescence of GaN NWs makes them effective DNA transducers.
3:45 PM - Q6.5
Spinel-type Structured Gallium Oxonitride with Composition Ga3O3N.
Isabel Kinski 1 , Stefanie Hering 2 , Carmen Zvoriste 1 , Ralf Riedel 1 , Hubert Huppertz 2
1 Institute of Materials Science, Darmstadt University of Technology, Darmstadt Germany, 2 , Ludwig-Maximilians-Univeristaet Munich, Munich Germany
Show Abstract4:30 PM - Q6.6
Growth and Characterization of Zinc-blende InN Thin Films on R-plane Sapphire Substrates by Molecular Beam Epitaxy.
Ching-Lien Hsiao 1 , Ting-Wei Liu 2 , Hsu-Cheng Hsu 1 , Wen-Yu Hsiao 3 , Chih-Chung Yang 3 , Chia-Chun Chen 2 , Li-Chyong Chen 1 , Kuei-Hsien Chen 4 1
1 Center for Condensed Matter Sciences, National Taiwan University , Taipei Taiwan, 2 Department of Chemistry, National Taiwan Normal University, Taipei Taiwan, 3 Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei Taiwan, 4 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show AbstractIn the past decade, nonpolar III-nitrides have attracted much attention due to the absence of the spontaneous and piezoelectric polarizations that can limit the device performance. Cubic system with Zinc-blend (ZB) InN is expected to have high electron transport properties and low bandgap energy about 0.58eV. However, the growth window for ZB-InN is narrower than that of wurtzite (WZ) type because of its metastable phase and the lack of suitable substrate. Hence, the investigation on the electric and optoelectronic properties of ZB-InN is important for future device application. Here, we demonstrate a simple method for growing ZB-InN in MBE system by substrate nitridation. The InN thin films were grown on r-plane sapphire(0112) substrate by plasma-assisted molecular beam epitaxy (PAMBE). Substrate nitridation was performed before InN growth at 200oC for 30 to 120 min. Following the nitridation, the growth of InN films was carried out at 320oC for 3 hours. The structural evloution of the InN films was analyzed by the θ/2θ-scan x-ray diffraction (XRD) patterns. By increasing the initial nitridation time, the phase of the InN films changes from pure WZ-InN(1011) to mixture of WZ-InN and Zb-InN, to pure ZB-InN(002). However, the reciprocal space maps of the XRD analysis shows that a certain content of WZ-InN is still embedded in the pure phase InN film. Nevertheless, this result indicates that the interlayer produced by substrate nitridation plays an important role in controlling the InN phase. Optical characterization of InN films was analyzed by temperature-dependent photoluminescence (PL) spectroscopy using Nd+ YAG laser (532nm) as the excitation source. At room temperature, the PL peak emission at 0.670eV with a full width at half maximum (FWHM) of 130 meV is obtained from the pure zb-InN. At 22k, PL consists of an emission band at about 0.680 eV with a FWHM of 93 meV. With increasing temperature, the emission peak slightly broadened and shifted to lower energy. These PL results indicate that the optical quality of the ZB InN films is comparable to that of the high quality ZB InN films from other literatures. Furthermore, a S-shape shift of peak energy from temperature-dependent PL measurement is believed to be related to a type-II semiconductor behavior which was ever observed in type-II semiconductor with mixture phases of WZ and ZB-GaN films.
4:45 PM - Q6.7
High Stress PECVD Silicon Nitride Films For 65nm SOI Technology and Beyond.
Hartmut Ruelke 1 , Volker Jaschke 1 , Kai Frohberg 1 , Mihaela Balseanu 2 , Tsutomu Kiyohara 2 , Li-Qun Xia 2 , Derek Witty 2 , Hichem M'Saad 2 , Olaf Hiller 3 , Wolfgang Senninger 3
1 , Advanced Micro Devices, AMD Fab36 LLC& Co. KG, , Dresden Germany, 2 , Applied Materials, Sunnyvale, California, United States, 3 , Applied Materials, Dresden Germany
Show AbstractHigh stress nitride films have been used at 90nm node and beyond to increase transistor drive current performance for both NMOS (tensile stress) and PMOS devices(compressive stress). In this paper we discuss the mechanisms controlling the stress of the PECVD silicon nitride (SiN) films in the GPa range and present the drive current improvement induced by those films. Silicon Nitride films with tensile stress up to 1.7GPa can be produced at 400°C using PECVD deposition followed by UV cure in a separate chamber. The removal of hydrogen from the film by the UV treatment leads to an increase in the cross-linking of the nitride network, a three-dimensional shrinkage of the film and an increase in the tensile stress beyond 1.3GPa. Ab initio calculations are used to study the bond dissociation in the excited states and to determine the absorption peaks of SiN in the 200-300nm wavelength range. The tensile nitride deposition process and hardware have been optimized based on the ab initio calculations and experimental results of hydrogen evolution during UV cure. To minimize the hydrogen content next to the poly gate, which is believed to impact device reliability, a multi-layer deposition/cure process has been implemented allowing further stress improvement and hydrogen reduction. 4% drive current improvement was demonstrated on AMD’s 65nm SOI Technology NMOS transistor using 1.7GPa tensile nitride with UV cure. The stress transfer into the channel is enhanced by the use of integrated multi-layer deposition / UV cure scheme. PECVD can also be used to deposit silicon nitride with large compressive residual stress. SiN films with compressive stress greater than 3.0GPa have been achieved by optimizing the ion bombardment during the deposition. Dilution gases and RF plasma power were the main knobs to control the type, energy and concentration of the film-forming radicals. The intrinsic compressive stress is build-up by modulating the ratio of the bombarding species to film forming radicals: increasing ion bombardment can lead to “stress relaxation” due to SiN bond breakage and/or sputtering of H atoms. This phenomenon is best described by the variation of the compressive stress with RF power which shows an optimum for different deposition chemistries. The effect of the compressive stress liner on hole mobility is proportional to the stress and thickness of the dielectric layer and shows no saturation up to 3.5GPa. Device modeling shows that PMOS device performance is expected to further improve with higher compressive strain induced in the channel. Drive current improvement of >16% was achieved for AMD's SOI PMOS devices using the high compressive stress SiN film.SiN films with tunable stress from 1.7GPa tensile to greater than 3.5GPa compressive have been demonstrated using PECVD and UV cure methods below 500°C. The use of both tensile and compressive films directly impact transistor performance by increasing electron and hole mobility respectively.
5:00 PM - Q6.8
(11-22)-oriented GaN/AlN Quantum Wells Grown on m-sapphire by PAMBE.
Lahourcade Lise 1 , Renard Julien 1 , Bellet-Amalric Edith 1 , Chauvat Marie Pierre 2 , Ruterana Pierre 2 , Monroy Eva 1
1 DRFMC/SP2M/NPSC, CEA-Grenoble, Grenoble France, 2 SIFCOM UMR 6176, CNRS ENSICAEN, Caen France
Show Abstract5:15 PM - Q6.9
Optical and Magnetic Properties of Fe and Mn Doped GaN for Spintronic Applications.
Enno Malguth 1 2 , Axel Hoffmann 1 , Wolfgang Gehlhoff 1
1 Institut für Festkörperphysik, TU Berlin, Berlin Germany, 2 Microstructural Analysis Unit, University of Technology Sydney, Sydney, New South Wales, Australia
Show AbstractIn the context of spintronic applications we studied Mn and Fe doped GaN. Samples with different transition metal concentrations and n or p co-doping were investigated by means of optical and magnetic experiments. The results allow us to elucidate the following issues that are of crucial significance on the way to eventually realize a ferromagnetic coupling at room temperature: (1) Which charge states the transition metals are found in, (2) whether and where they form levels within the band gap, (3) the formation of bound states consisting of a hole localized at a transition metal ion in the charge state 2+. We also assess the influence of the transition metal concentration on the structural, electronic and optical properties of GaN. Additionally, the effects of n and p co-doping on these properties are investigated.
5:30 PM - Q6.10
Non-cesiated GaN Photocathodes Using Surface Bandstructure Engineering by Molecular Beam Epitaxy.
Shouleh Nikzad 1 , L. Bell 1 , Amir Dabiran 2
1 , Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, United States, 2 , SVT Associates, Eden Prairie, Minnesota, United States
Show AbstractGallium nitride has attracted attention as a promising candidate material for solar-blind ultraviolet (UV) photocathodes, due to its large band gap, direct bandgap, chemical inertness, low electron affinity, and the possibility of achieving a negative electron affinity (NEA) surface. Most surface preparations to date on GaN have involved conventional use of low work function metal and treatments with Cs to achieve NEA. However, due to high reactivity of Cs and as in all Cs-based NEA surfaces, all fabrication and incorporation processes of cesiated photocathodes have to be performed a vacuum enclosure.We report promising results using delta-doping to approach NEA surface conditions. This technique is reminiscent of delta doping for back-illuminated silicon imagers to achieve breakthrough performance with short wavelength photons. The success of the delta doped imagers suggests that using a near-surface delta-layer in GaN can create strong band-bending in GaN to approach NEA without the use of Cs. Due to the relatively unreactive surface of GaN, this method also offers the possibility of fabricating photocathode devices that do not require evacuated enclosures, thus reducing size and increasing robustness of the devices.Photocathode structures grown with near-surface Si delta layers will be described. A thin, high-density Si layer is deposited on a GaN template, followed by a thin GaN cap. Dopant density in excess of 2 x 1014 Si/cm-2 has been verified. A strong reduction in emission threshold has been achieved in these devices relative to bare GaN. Emission characteristics as a function of doping density, GaN template doping, and GaN cap thickness will be presented. Prospects for achieving NEA with this method will be discussed.
5:45 PM - Q6.11
Tunable Hyperspectral Imaging Detector based on III-Nitride Dielectric Heterostructures.
Douglas Bell 1 , Neeraj Tripathi 2 , James Grandusky 2 , Fatemeh Shahedipour-Sandvik 2
1 , Jet Propulsion Laboratory, Pasadena, California, United States, 2 College of Nanoscale Science and Engineering, University at Albany, Albany, New York, United States
Show AbstractHyperspectral imaging, or imaging spectrometry, has important applications to the remote investigation of the Earth's surface via chemical composition mapping and analysis. The technique provides chemical information on vegetation as well as inorganic materials; thus it is valuable in environmental evaluation and resource identification. We describe a new type of wavelength-tunable pixel element, based on layered dielectric materials. This detector will have intrinsically hyperspectral pixels, each pixel being tunable in real time through a range of wavelengths determined by pixel design. This eliminates the need for external gratings and filters, substantially decreasing weight, size, and complexity and increasing robustness. Since the detector requires no wavelength dispersion along a spatial dimension, the detector array dimension can be decreased by one. Expected range of wavelength tunability for this detector will be as much as a factor of three (e.g. 400-1200 nm), with a wavelength resolution Δλ/λ of about 0.005. Unlike conventional hyperspectral detectors, the number and wavelength spacing of spectral channels is not fixed, but can be adjusted dynamically.This new type of hyperspectral imager is based on voltage-tunable electron tunnel barriers fabricated from layered dielectrics. Shape-engineering of the barrier potential profile allows the barrier height to be tuned by application of a voltage, enabling energy selectivity of photoexcited electrons and thus a spectroscopy of photon wavelength. Of particular interest is the GaN/AlN materials family, which offers a large range of achievable band gaps and band offsets and thus a wide range of wavelength sensitivity.We will describe growth of these detector structures and will present the results of structural, electrical, and optical characterization. Internal photoemission has been used to assess the performance of these barrier structures as tunable detectors. Measurements relating to selection of wavelength sensitivity, quantum efficiency, and tunability range will be discussed. Growth of the tunable barrier structures was primarily on sapphire substrates, but Si substrates were also used with no loss of performance. Preliminary results indicate tunability energy ranges as large as half of the applied voltage across the structures, and external quantum efficiencies greater than 10%. Materials and growth challenges, and methods of improving performance, will be described.
Symposium Organizers
Ruediger Kniep Max-Planck-Institute for Chemical Physics of Solids
Francis J. DiSalvo Cornell University
Ralf Riedel Technische Universitaet Darmstadt
Zachary Fisk University of California
Yoshiyuki Sugahara Waseda University
Thursday AM, November 29, 2007
Back Bay B (Sheraton)
9:30 AM - Q7.1
Two-dimensional Growth Mode and Reduction of Dislocations in Nitride Layers.
Krzysztof Pakula 1 , Jacek Baranowski 1 , Jolanta Borysiuk 2
1 Institute of Experimental Physics, Warsaw University, Warsaw Poland, 2 , Institute of Electronic Materials Technology, Warsaw Poland
Show AbstractHeteroepitaxial two-dimensional (2D) and three-dimensional (3D) MOVPE growth of nitride layers on sapphire substrates using low temperature (LT) AlN nucleation layer are discussed. It is shown by Atomic Force Microscopy and Transmission Electron Microscopy that the 2D or 3D growth mode of successive AlGaN layers depends predominantly on the growth conditions of the underneath AlN nucleation layer.Modifications of growth conditions of LT AlN nucleation layer in respect of the growth in the conventional way, leads to a drastic change in properties of the successively grown at high temperature AlGaN layer. Main modification is connected with a drastic reduction of the growth rate of LT AlN nucleation layer. In this case the growth at high temperature starts evenly on the whole surface, retaining atomic flatness, and occurs in the 2D mode. Therefore, it is possible to grow even a very thin, continuous AlGaN layers directly on top of LT nucleation layer. The 2D-growth mode leads to significant change in extended defect distribution. Edge dislocation density being close to 1e10 cm-2, result in the effective relaxation of the internal stress, which is confirmed by photoluminescence measurements. On the other hand mix dislocations density are close to 1e8 cm-2. All observed mix dislocations occur in pairs with dislocations with opposite sign. This suggests that all mix dislocations originate from dislocation half loops. Both types of dislocations, edge and half loops can be effectively reduce in-situ, during the growth using annealing in the presence of SiH4 (SiN interlayer). This process and precise control of the growth conditions leads to decreasing of mix dislocations density below 2e6 cm-2 and edge dislocations density to 1e7 cm-2. This mode of growth is in contrary to commonly describe in literature 3D growth mode of LT AlN or GaN nucleation layer. Successive growth of secondary layer grown at high temperature begins in this case from separated sites, where individual 3D crystallites are formed. Threading dislocations present in crystallites are bending on their facets, which reduces the quantity of dislocations. From the other hand, slight crystallographic misorientations between crystallites lead to creation of new dislocations during coalescence of the crystallites. As a result, edge and mix dislocations appear at similar densities of about 1e9 cm-2.Thus, the newly developed two-dimensional growth mode of nitride layer opens up a new way to reduce strain, decrease density of dislocations and improvement of crystallographic properties.
9:45 AM - Q7.2
Atomic Scale Z-contrast Imaging Study of N Distribution in GaAsN Quantum Wells.
Miriam Herrera Collado 1 , Quentin Ramasse 2 , David Morgan 1 , Nigel Browning 1 3 , David Gonzalez 4 , Rafael Garcia 4 , Mark Hopkinson 5
1 Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California, United States, 2 Lawrence Berkeley National Laboratory, National Centre for Electron Microscopy, Berkeley, California, United States, 3 Chemistry, Materials and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, United States, 4 Departamento de Ciencia de los Materiales e I.M. y Q.I, Universidad de Cádiz, Puerto Real, Cadiz, Spain, 5 Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield United Kingdom
Show Abstract10:00 AM - Q7.3
Toward Tantalum Nitride Nanostructures.
Andrew Hector 1 , Baishakhi Mazumder 1 , Paul O'Brien 2 , James Tabernor 2
1 School of Chemistry, University of Southampton, Southampton United Kingdom, 2 School of Chemistry, University of Manchester, Manchester United Kingdom
Show AbstractColloidal Ta3N5 nanoparticles could find applications as non-toxic alternatives to cadmium chalcogenides in photocatalytic applications and for use in optoelectronic devices. Its 2.08 eV band gap of places Ta3N5 midway between CdS (2.53 eV) and CdSe (1.74 eV). TaN is a good conductor and finds application in diffusion barriers, wear and corrosion resistant materials etc. Soluble nanoparticles of this material would be potentially useful for the production of barrier layers under mild conditions. The solution phase synthesis of early transition metal nitride nanoparticles generally is uncharted and thus we have been working on solvothermal and injection growth of tantalum nitride as an initial study.Solvothermal reactions of tantalum chloride with Mg3N2 or LiNH2 in mesitylene lead to TaN and Ta3N5 respectively. These are amorphous and we have not been able to crystallise them in the solution phase reactions, post-annealing at similar temperatures to the solution phase growth was necessary to obtain crystalline compounds. TaN particles were found to be spheres of 5-7 nm in diameter. The Ta3N5 particles are spheres (10-20 nm in diameter) and rods (100-200 nm in length and 20-40 nm in diameter). The rod morphology is closely linked to lithium impurities. Thorough lithium extraction prior to annealing leads to tantalum carbonitride materials with a Ti3O5 type structure.The difficulty with obtaining crystalline early transition metal nitrides from solution is also found with injection reactions, in which metal nitride precursors are injected into hot solvents. This presentation will discuss our synthetic work in both these areas and examine the parallel issues of solvent temperature and nucleation in these systems.
10:15 AM - **Q7.4
New High-Pressure Nitrides: Synthesis and Properties.
Andreas Zerr 1
1 , LPMTM-CNRS, Universite Paris Nord, Villetaneuse France
Show AbstractProgress in high-pressure synthesis of new binary nitrides of groups IVA-, IVB-, and VB-elements and in experimental determination of their properties is the subject of this presentation. For nitrides of the group IVA elements having cubic spinel structures, γ-M3N4 (M = Si, Ge, Sn), the emphasis will be made on a completion of experimental information on their elastic moduli as well as estimation and comparison of their thermal shock resistances. The information on elastic moduli was obtained from high-pressure measurements of equation of states of the spinel nitrides combined with the results of nanoindentation experiments. The thermal shock resistance estimations were derived from pressure dependences of Raman band frequencies of the spinel nitrides measured on hydrostatic compression in a diamond anvil cell. The obtained experimental data were used to examine if there is a systematics between hardness and bulk or shear moduli at least for this family of compounds. From the consideration followed that the hardness of a hypothetical γ-C3N4 will not exceed 100 GPa thus similar to that of diamond. Further, results of high pressure - high temperature experiments on a post-spinel phase transition in this family of compounds will be presented. Similarly, recent experiments on synthesis of large amounts of the group IVB nitrides having cubic Th3P4-type structures, c-A3N4 (A = Zr or Hf) and measurements of their elastic moduli and hardness will be discussed. The obtained data have also been used to estimate thermal shock resistances of these compounds from high-pressure Raman spectroscopic measurements. Finally, recent results on high pressure – high temperature treatment of the group VB nitrides having stoichiometry X3N5 will be presented. Possible structures of the obtained product, derived from powder XRD measurements, will be discussed. Examination of the samples texture using scanning electron microscopy revealed a possibility for industrial applications of the obtained product as a structural material.This work was partially supported by the Agence Nationale de la Recherche/France (grant NT05-3_42601); the European Synchrotron Radiation Facility; Materials and Structures Laboratory of the Tokyo Institute of Technology; the Deutsche Forschungsgemeinschaft and the Adolf-Messer-Foundation/Germany.
10:45 AM - Q7.5
In-situ Stress Measurements on GaN Epitaxial Layers Grown on Low Stress AlN Buffer Layers on Semi-Insulating 6H and 4H SiC Substrates.
Volker Heydemann 1 , David Rearick 1 , Joshua Robinson 1 , Xiaojun Weng 2 , Joan Redwing 2 , David Snyder 1
1 Electro-Optics Center, Pennsylvania State University, Freeport, Pennsylvania, United States, 2 Department of Materials Science and Engineering, Pennsylvania State University, State College, Pennsylvania, United States
Show AbstractGallium nitride (GaN) epitaxial layers without intentional doping are used as the drift layer in high electron mobility RF transistors (RF HEMTs) for high power, high frequency applications. To achieve the desired electrical properties of advanced HEMT devices, the GaN layers are commonly deposited onto a thin aluminum nitride (AlN) buffer layer on semi-insulating silicon carbide (SiC) substrates. The lattice mismatch, thermal expansion coefficient mismatch between the SiC substrate, the AlN buffer layer and the GaN epitaxial layer, as well as the epitaxial process of dissimilar materials introduce stress in the AlN buffer layer and the GaN epitaxial layer that impacts the electrical properties of the SiC / AlN / GaN epitaxial system and the properties of AlxGa1-xN epitaxial layers subsequently deposited upon the GaN layer. Epitaxial gallium nitride layers ranging in thickness from 200nm to 1500nm have been deposited on stress-optimized AlN buffer layers on (0001) on-axis 6H and 4H semi-insulating silicon carbide (SI-SiC) substrates by molecular beam epitaxy (MBE) utilizing a nitrogen plasma source. The stress introduced by thermal cycling and the hetero-epitaxial deposition was monitored in-situ during the growth runs utilizing a multiple beam optical stress sensor (MOSS). The MOSS system continuously tracks position and intensities of a matrix of laser dots projected onto the growth surface, permitting the deriviation of the bow of the substrate / epitaxial layer stack. The bow data is converted into relative stress information using Stoney’s equation. A matrix of experiments was conducted to correlate the stress evolution from compressive to tensile conditions during the deposition of GaN epitaxial layers on AlN buffer layers on SI-SiC substrates, as well as the final cumulative stress observed in the GaN / AlN / SI-SiC layer system with growth parameters such as film thickness, substrate temperature, aluminum and nitrogen flux and the surface and stress properties of the AlN buffer layers. Morphology, crystal quality and interface properties of the GaN epitaxial layers as investigated by white light interferometry, atomic force microscopy (AFM), x-ray diffraction (XRD), and cross-sectional transmission electron microscopy (X-TEM) were correlated with the stress data and the growth parameters. The stress evolution of MBE-grown epitaxial structures is compared to the stress evolution observed in equivalent MOCVD-grown layers.
11:30 AM - Q7.6
Investigation of Vertical Transport in GaN-based Heterostructures: Tunneling Trapping and Bi-stability.
Sylvain Leconte 1 , Sebastian Golka 2 , Gianmauro Pozzovivo 2 , Gottfried Strasser 2 , Thilo Remmele 3 , Martin Albrecht 3 , Eva Monroy 1
1 DRFMC / SP2M / PSC, CEA-Grenoble, Grenoble France, 2 Zentrum für Mikro- und Nanostrukturen, Technische Universität Wien, Vienna Austria, 3 , Institut für Kristallzüchtung, Berlin Germany
Show AbstractIII-nitride semiconductors are promising candidates for various quantum effect device applications, such as intersubband (ISB) optoelectronic devices or resonant tunneling diodes. Although a lot of progress has been done to validate the optical properties of nitride heterostructures, little is known about vertical electron transport mechanisms, whose control is necessary in order to optimize the design of ISB devices. Indeed, photovoltaic GaN/AlN quantum well infrared photodetectors with promising performance have been reported. However, to enhance the responsivity of these detectors, it is necessary to evolve towards a photoconductive configuration, where vertical resonant tunneling transport plays a dominant role.In order to investigate the vertical electron transport in III-nitride heterostructures, we have first studied the behavior of single AlGaN barriers in a GaN matrix, with the Al mole fraction varying from 0.25 to 1, and the barrier thickness varying from 0.5 nm to 5 nm. The structures are grown by plasma-assisted MBE on 10-μm-thick GaN templates, with a dislocation density of ~8e7 cm-2. We first investigate the charge distribution by CV measurements, which reveal the presence of a 2DEG at the bottom interface of the barriers, even for 0.5-nm-thick AlN barriers. The variation of the 2DEG and the band diagram as a function of barrier thickness and Al mole fraction is analyzed and compared with self-consistent Schrödinger-Poisson simulations. The effective blockage of current by the barrier is verified by conductive AFM, which shows a leakage current path density of in the 1e7 cm-2 range.In a second stage, AlN/GaN/AlN double barrier RTD structures with an AlN barrier thickness of 0.5 nm and a GaN well thickness varying from 0.5 nm to 2 nm have been synthesized. High-resolution TEM images confirm the thickness and homogeneity of the barriers. The electrical characterization of the RTD structures, patterned into 28 μm2 mesas, display a reproducible negative differential resistance (NDR) effect around 4V. This behavior appears only when scanning from negative (<-4V) to positive bias. These results suggest that the NDR should be interpreted as a bi-stable behavior, with a low and a high current state. It is possible to switch from one state to the other by applying an absolute voltage larger than about 3V, depending on the well thickness. For bias lower than this threshold, the system remains stable in its previous state. For higher bias, time resolved measurements reveal a random transition in time between both states. This bi-stability is attributed to an interaction between the dislocations and the quantum well.
12:00 PM - Q7.8
Measuring the Enthalpy and Free Energy of Formation of GaN.
Timothy Peshek 1 , Kathleen Kash 1 , John Angus 2
1 Physics, Case Western Reserve University, Cleveland, Ohio, United States, 2 Chemical Engineering, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractDirect measurement of the thermochemical potentials of GaN has proven difficult, and little data exist in spite of their potential importance to areas such as GaN wafer processing, thermal annealing and bulk growth. We present the results of experiments conducted to measure the free energy of formation of GaN, ΔG0f,GaN(T). The stability of GaN was determined as a function of temperature by measuring N2 pressures where the GaN decomposed into N2 gas and molten Ga or was stable. In this manner an estimate of the free energy of formation was found that rules out, with high confidence, a value widely quoted in the literature. We also observed the formation and decomposition of GaN for various concentrations of ammonia in hydrogen as a function of temperature. The values of ΔG0f,GaN(T) found from the stability of GaN in N2 and NH3 in H2 are in agreement and are consistent with ΔG0f,GaN(T) determined by high N2 pressure studies on GaN 1,2. In brief, we find that ΔG0f,GaN(T)=0 at T=1215 ± 35 K, and the enthalpy of formation, ΔH0f,GaN(298 K) is approximately -160 kJ/mol.This work supported by U.S. Department of Education grant APR P200A030186 and NSF grant DMR-04207651. R. Madar, G. Jacob, J. Hallais, R. Fruchart, J. Crystal Growth 31, 197 (1975)2. J. Karpinski, S. Porowski, J. Crystal Growth 66, 11 (1984)
12:15 PM - **Q7.9
Subnitrides with Group 1 and 2 Metals.
A. Simon 1 , G. Vajenine 1 , V. Smetana 1 , V. Babizhetskyy 1
1 , Max-Planck-Institut fur Festkorperforschung, Stuttgart Germany
Show Abstract12:45 PM - Q7.10
A Role of the Built-in Piezoelectric Field in InGaN/AlGaN/GaN Multiple Quantum Wells in the Electroferlectance Experiments.
Pavel Bokov 1 , Lev Avakyants 1 , Mansur Badgutdinov 1 , Anatoly Chervyakov 1 , Stas Shirokov 1 , Alexander Yunovich 1 , Elena Vasileva 2 , Feodor Snegov 2 , Dmitry Bauman 2 , Boris Yavich 2
1 Physics, M.V. Lomonosov Moscow State University, Moscow Russian Federation, 2 , JSC “Svetlana-Optoelectronica”, Saint-Petersburg Russian Federation
Show AbstractThe InGaN/AlGaN/GaN multiplies quantum wells (MQW) p-n- heterostructures have been studied by means of electroreflectance (ER) in the spectral range from 380 to 1000 nm. The heterostructures under investigation are grown for light emitting diodes (LED) by MOCVD technology on the sapphire substrate and “flip-chip” mounted.In the ER spectra of each sample observed two spectral peculiarities: third derivative like-line [1] corresponds to interband transition in the LEDs’ InGaN/GaN active layer and series of interference bands. The energy of the third derivative like-line changes from 2.70 eV for royal blue LED to 2.30 eV for green LED, which corresponds to the data observed from the electroluminescence experiments [2]. The two interference bands with two different period have been observed. It was established that the amplitude and the period of interference depends on doping level of barriers in the MQW.The dependence of ER spectra from applied voltage in the range of biases from +2 to -8 V has been studied. It is found that the energy of the third derivative like-line, changes from 2.70 to 2.77 eV with the changing of bias voltage from +2 to -8 V. This phenomenon is found to originate in the fact that the applied reverse voltage cancels the internal piezoelectric field, which points from the growth surface to the substrate [3]. The cancellation of internal piezoelectric field is appeared at reverse voltages, which value is higher than -2 V. The discussion of the role of piezoelectric field in such devices will be presented.1.D.E. Aspnes. Surf. Sci. 1973, Vol. 37 p. 4182.A.E. Yunovich, L. Avakyants, M. Badgutdinov, P. Bokov, A. Chervyakov, S. Shirokov, E. Vasileva, A. Feopentov, F. Snegov, D. Bauman, B. Yavich. Mater. Res. Soc. Symp. Proc. 2007, Vol. 955, p. 0955-I15-363.T. Takeuchi, C. Wetzel, S. Yamaguchi, H. Sakai, H. Amano, I. Akasaki, Y. Kaneko, S. Nakagawa, N. Yamada. Appl. Phys. Lett. 1998, Vol. 27(3), p. 1691
Thursday PM, November 29, 2007
Back Bay B (Sheraton)
2:30 PM - **Q8.1
N-containing Solid-state Materials by Quantum-chemical and Synthetic Approaches.
Richard Dronskowski 1
1 Institute of Inorganic Chemistry, RWTH Aachen University, Aachen, NRW, Germany
Show AbstractModern solid-state electronic-structure calculations of density-functional type have become effective tools for the synthetic solid-state chemist and materials scientist striving to design and prepare novel extended compounds. The use of these computational strategies is justified in three ways, by, 1.) the existence of sufficiently reliable exchange-correlation functionals and pseudopotentials, 2.) the ease with which the electronic structure may now be analyzed in depth using short-range basis sets and energy-partitioning schemes and, 3.) not to forget, relatively simple techniques for converting atomistic energetics into macroscopic thermodynamics. The results of a cooperative synthetic effort between theory and experiment are demonstrated using a number of characteristic examples, such as the understanding, subjection to proof, prediction and synthesis of advanced nitrides (simple 3d nitrides, PtN, RhFe3N etc.), the subjection to proof, prediction and characterization of transition-metal oxynitrides (TaON, VON and related phases), and also the prediction and synthesis of novel carbodiimides (MnNCN and other correlated nitrogen-based solids) of the magnetic transition metals.
3:00 PM - Q8.2
Characterization of Spectral Emissions from Laser Irradiated Titanium.
Ravindra Kumar Akarapu 1 , Puneit Dua 1 , Alan Campbell 1 , Dana Scott 1 , Abdalla Nassar 1 , Judith Todd 1 , Steve Copley 1
1 Engineering Science and Mechanics, Penn State University, State College, Pennsylvania, United States
Show AbstractTitanium nitride (TiN) is a candidate material for hard and wear resistant coatings on metallic substrates such as Ti, stainless steel and aluminum. Coating processes may include chemical vapor deposition, ion implantation, plasma and thermal nitriding under vacuum and in controlled environmental conditions. The present research aims to develop a laser plasma process for depositing TiN coatings on Ti substrate under atmospheric conditions. Laser induced plasma was generated by a pulsed CO2 laser on a titanium substrate. Spatial and temporal distribution of the species in the vapor plume was conducted by optical emission spectroscopy as a function of laser power, position of substrate relative to focal plane, pulse parameters, and shielding gases. Spectral emissions combined with microstructural characterization of surface deposits identified potential windows of optimal process parameters for depositing nitride and oxynitride coatings at ambient pressures.
3:15 PM - Q8.3
Electrical Properties of GaAs/GaN pn Heterojunction Diodes Formed by Wafer Fusion.
Chuanxin Lian 1 , Huili Xing 1
1 , University of Notre Dame, Notre Dame, Indiana, United States
Show AbstractCombining the unity emitter injection efficiency and large base transport factor of AlGaAs(emitter)/GaAs(base) structures and the high breakdown field of GaN (collector), AlGaAs/GaAs/GaN HBTs can be promising candidates for high-speed high-power applications. Wafer fusion, instead of epitaxial growth, has been exploited to form such heterostructures due to the large lattice constant mismatch between GaAs and GaN. Recently, we have reported AlGaAs/GaAs/GaN HBTs formed by wafer fusion exhibiting current gain as high as ~ 7-9. Since electrons need to transport through the fused GaAs/GaN interface to reach the GaN collector, detailed knowledge of the electrical properties of fused GaAs/GaN heterojunction interfaces is essential to device design and fusion process optimization. In this work, GaAs/GaN heterojunction pn diodes, fused at 550C for 1 hr, have been studied by I-V, C-V and temperature dependent I-V measurements. Room temperature I-V of the fused pn diode shows good rectifying effect and the ideality factor is about 1.8. But the turn-on voltage (~0.3 V) at forward bias is much smaller than the theoretical built-in voltage (~1.7 V) obtained by considering the doping concentrations at both sides and assuming the conduction band offset is 0.4 eV. The room temperature 1/C^2-V plot also suggests a small diffusion potential barrier ~ 0.3 eV. Since GaAs is degenerately doped with carbon and GaN lightly doped with Si, the heterojunction can be treated, at the first glance, as a Schottky barrier. The Schottky barrier height has also been extracted by fitting Js/T^2 ~ 1/T and was found to be ~ 0.3 eV as well. The small diffusion potential barrier consistently extracted from all the techniques mentioned above suggests that interface charges play a significant role in electron transport. We propose that the interface states assisted tunneling attributes to a large proportion of the current at forward bias. Electrons tunnel from the conduction band of GaN into empty interface states and then recombine with holes in the valence band of GaAs. This was testified by inspecting the slopes of semilog plots of I-Vs at different temperatures. If tunneling current plays an important role in electron transport, d(lnI)/dV is independent (or weakly dependent) on temperature. It was observed that I-Vs measured at different temperatures were parallel to each other, supporting the hypothesis that interface states assisted tunneling was predominant.
3:30 PM - Q8.4
Metastability of Platinum-metal Nitrides.
Daniel Aberg 1 , Babak Sadigh 1 , Jonathan Crowhurst 1 , Alexander Goncharov 1 2
1 , Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Geophysical Laboratory, Carnegie Institution of Washington, Washington, District of Columbia, United States
Show AbstractUsing density functional theory methods we have performed a comprehensive study of all platinum-metal (Pt, Ir, Os, Ru, Rh, Pd) nitrides for a subclass of crystal structures with nitrogen dimers centered at the octahedral interstitials of a fcc metal lattice. By treating all compounds on the same footing we are able for the first time to study their relative and absolute thermodynamic stability at various pressures. We address the issue of metastability of these compounds at ambient conditions.
3:45 PM - Q8.5
The Realization of Vertical Light Emitting Diodes (V-LEDs) Using Chemical Lift-off (CLO) Processes.
Seogwoo Lee 1 , Junseok Ha 1 , Hyunjae Lee 1 , Hyojong Lee 1 , Hiroki Goto 1 , Sanghyun Lee 1 , Takenari Goto 1 , Takashi Hanada 2 , Katsushi Fujii 2 , Meoungwhan Cho 2 , Takafumi Yao 1 2
1 , Center for Interdisciplinary Research, Tohoku University, Sendai Japan, 2 , Institute for Materials Research, Tohoku University, Sendai Japan
Show AbstractThe development and research of GaN-based light emitting diodes (LEDs) have been focused on improvement of luminous efficiency. However, heat dissipation from an active layer in LEDs has been a primary limitation on the luminous efficiency of the LEDs. Therefore, research in LEDs field is concentrated to improvement of luminous efficiency and heat dissipation simultaneously. Though sapphire substrate is a suitable material for growth of GaN-based epilayer, it has a poor thermal- and electrical-conductivity. Thus, removing the LED structure from the sapphire substrate, and re-bonding with a better thermal-conductive substrate is the most desirable method for high efficiency device. Therefore, debonding techniques such as laser lift-off (LLO), i.e., detaching method from a substrate, were applied to develop highly efficient vertical light emitting diodes (V-LEDs). The GaN-based V-LEDs was proved to be better suited for high power and reliable operation. However, LLO technique have some problems 1) active layer damage by laser, 2) complicated process, 3) not suitable for mass production, 4) expensive laser system, 5) formation of micro cracks on debonding and 6) lift-off side damage by laser. In order to prevent those problems, a detaching technique, i.e., a simple and damage-free process such as selective chemical etching, is strongly desirable.This work purposes to introduce CrN as a lattice matched and chemically etchable buffer layer for V-LEDs. We propose CrN as a novel buffer for GaN growth. Rock-salt structured CrN (111) has an intermediate bond distance (a=2.927Å) between sapphire (0001) (a=2.747Å) and GaN (0001) (a=3.188Å). Moreover the thermal expansion coefficients of CrN (α=6.00×10-6/K) lies in between those of sapphire (α=6.66×10-6/K) and GaN (α=5.59×10-6/K). Hence, high quality GaN films and LED structures were successfully grown on sapphire substrate with novel CrN buffer layer. The V-LED structure was realized by selective chemical etching processes of CrN layer, i.e. CLO method. GaN growth on CrN buffer layer and fabrication processes of V-LEDs using CLO technique will be reported for the first time. The CLO technique has advantages of damage-free and simple process. Hence, the V-LEDs manufactured by CLO processes have a high-efficient and reliable characteristic. Furthermore, this technique has a high reproducibility and is suitable for mass production. The impact of the V-LED using CLO processes is that we can easily produce effective and reliable V-LEDs, and use these to high-brightness applications, such as backlights for flat panel display in television, automotive headlights and general room lighting. Therefore, this new V-LED using CLO processes will attract people who are interested in GaN-based LEDs as well as lighting applications.
4:30 PM - Q8.6
Synthesis and Characterization of New Nanostructured, Anti-adhesive and Wear Resistant SiOx/[Si(NCN)2]n Sol-gel Films.
Emanuel Ionescu 1 , Ralf Riedel 1 , Jens Harenburg 2
1 Institute for Materials Science, TU Darmstadt, Darmstadt Germany, 2 , FEW Chemicals GmbH, Wolfen Germany
Show AbstractIn the poster presentation, studies on the synthesis and characterization of new nanostructured SiOx/[Si(NCN)2]n sol-gel films will be presented. For the preparation of the films, silicon oxide sols (particle size ca. 15 nm) were mixed either with [Si(NCN)2]n-based sols or with pyrolyzed [Si(NCN)2]n nanoparticles (particle size ca. 60 nm). Whereas the silicon oxide sols were prepared by hydrolysis-supported polycondensation techniques, the [Si(NCN)2]n sols and nanoparticles were synthesized by a non-oxidic Stöber process. [1] The films, which were prepared via doctor blade technique followed by cross-linking at 130 °C, were investigated concerning their chemical composition (elemental analysis, IR and Raman spectroscopy, and micro probe analysis), their microstructure (SEM) as well as their anti-adhesive properties, and hydrolysis and wear resistance. The synthesized sol-gel films were found to have antiadhesive properties (hydrophobic and oleophobic) and hydrolysis resistance, due to the fluorine-modified silicon oxide sols used. Furthermore it could be shown that the use of pyrolyzed [Si(NCN)2]n nanoparticles for the preparation of the films remarkably increase their wear resistance in comparison with the analogous silicon oxide films, due to the hard [Si(NCN)2]n nanoparticles incorporated into the silicon oxide matrix. References[1]Y.-L. Li, E. Kroke, A. Klonczynski, R. Riedel, Adv. Mater. 2000, 12, 956.
4:45 PM - Q8.7
Nanoscale Characterization of Nucleation Layer for GaN based LEDs.
Punam Pant 1 , Jagdish Narayan 1 , Wei Wei 1 , Roger Narayan 2 , John Budai 3
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Department of Biomedical Engineering, University of North Carolina, Chapell Hill, North Carolina, United States, 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe growth of a low-temperature (LT) nucleation layer (NL) on sapphire substrate enabled the growth of device quality GaN thin films (h-GaN), which in turn has led to rapid development of GaN based light-emitting-diodes. The deposition of a low-temperature nucleation layer prior to film growth substantially improves the surface morphology and reduces defects in h-GaN layers for LEDs and lasers. The NL which is grown at a LT~500-600°C is single crystal and has been reported to have a cubic zinc-blende structure. In this work we present systematic studies on the NL grown on c-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). A detailed characterization of the NL was done by AFM, High-Resolution TEM, x-ray diffraction and Raman spectroscopy techniques. Detailed microstructural characterization by TEM shows that the NL consists of nanostructured islands or grains which are separated by small-angle grain boundaries. The islands grow epitaxially on sapphire substrate. Qualitatively, TEM analysis also shows that the islands have a predominantly faulted cubic structure and a small fraction of unfaulted c-GaN. It was interesting to note that the faults in the islands were confined to one of the four sets of {111} planes of c-GaN. The x-ray analysis of NL which included low and high-resolution θ-2θ scans, Φ scans, and low resolution L scans (10L scans in hexagonal coordinates) gave quantitative information about the NL. Using high-resolution θ -2θ scans the average misorientation between the islands was found to be ~1°. X-ray scans also revealed the presence of an in-plane tensile strain in the NL contrary to the expected compressive strain due to thermal strain. In addition the L-scans provided quantitative information on the percentage of h-GaN and c-GaN in the NL. Using the Hendricks-Teller model the x-ray diffraction intensity was calculated as a function of L and was fitted to the experimental data to give the fraction of c-GaN and h-GaN in the NL. Unpolarized backscattered Raman study of the NL showed the presence of Raman modes belonging to c-GaN and h-GaN phases, thus confirming the TEM and x-ray analysis that the NL consists of c-GaN and faulted c-GaN (h-GaN). Extra Raman modes that belong neither to c-GaN nor to h-GaN were also detected for the first time, and their presence is attributed to micro structural defects in the NL and to the tensile strain in the islands.
5:00 PM - Q8.8
Effect of B2O3 Additive on Structural Ordering, Consolidation and Orientation of Stacking Disordered BN.
Naoki Toyofuku 1 , Yuri Kaneda 1 , Natsuki Yamasaki 1 , Hiroyasu Yamasaki 1 , Manshi Ohyanagi 1 , Zuhair Munir 2
1 Materials Chemistry, Ryukoku University, Ohtsu, Shiga, Japan, 2 Chemical Engineering and Materials Science, University of California, Davis, Davis, California, United States
Show Abstract Much attention has been paid on the consolidation of hexagonal boron nitride (h-BN) as high temperature material with high heat resistance, electric insulation and so on. For the high melting point of h-BN and the difficulties in consolidation, it has been generally preformed by hot pressing with sintering additives such as boric oxide, calcium boroaluminate glasses. In the cases, the basal plane (002) of h-BN was preferably aligned with the facet perpendicular to the pressure direction for the grain orientation in the BN oriented from crystalline h-BN powder. Copper-activated hot pressing of mixture of micro h-BN and crystalline h-BN was also performed to improve the sinterability and the mechanical properties. The h-BN compacts were prepared by hot pressing of mixtures of the boron nitrides with copper. A positive effect of copper on boron nitride crystallization and densification was reported. However, the preferred orientation of boron nitride grains revealed that the basal plane of h-BN crystallites was arranged preferably within a plane parallel to the direction of uniaxial pressure. Such an arrangement was interpreted as a result of the uniaxial compression of the boron nitride grains extensively growing within the c crystallographic axis.On the other hand, recently we have shown that structural changes occurring during sintering can aid the densification process, making it possible to sinter such materials as nanostructured SiC to relatively high densities (98%) without the use of additives or exceptionally high pressures. A related study on the synthesis and densification of B4C from the elements showed a direct correspondence of the rate of consolidation with the annealing out of twins. In this work, Boron nitride (BN) with stacking disordered structure was formed from crystalline hexagonal BN by high-energy mechanical grinding under an inert gas atmosphere and consolidated to a relative density of 95% by spark plasma sintering above at 1600oC and 40-100MPa with and without an additive of B2O3. A sharp increase in density in the temperature region 1500 -1600oC was associated with a sharp decrease in the disorder with the additive. During the densification with ordering, the basal plane (002) of h-BN was preferably aligned with the facet parallel to the pressure direction which stood in contrast to the grain orientation in the BN oriented from crystalline h-BN powder. BN with stacking disordered structure was also fully consolidated without the additive, but still keeping the disordered and turbostratic structure. We describe, herein, the effect of B2O3 additive on the ordering to h-BN, the consolidation, the orientation, and also their mechanical properties.
5:15 PM - Q8.9
Formation of Aluminum Nitrides by Precipitate-Accommodated Plasma Nitriding.
Tatsuhiko Aizawa 1 2 , Patama Vissutipitukul 3
1 R & D Division, AsiaSEED, tokyo Japan, 2 Materials Science and Engineering, Osaka Prefecture University, Osaka Japan, 3 Metallurgical Engineering, Chularongkong, Bangkok Thailand
Show AbstractAluminum nitride has become one of the most useful engineering ceramics for improvement of wear toughness and heat-sink. Thin film of AlN up to 1 micro-meter is easily formed as a protective layer or a surface heat sink by CVD or PVD methods or conventional nitriding. Owing to much low diffusion coefficient of nitrogen in the formed AlN, thick film formation is still difficult to make for tailored coating of aluminum alloy parts and components. Authors have been concerned with development of new type of plasma nitriding both for steel and aluminum alloy parts. In the precipitate accommodated plasma nitriding, a precipitate of Al2Cu embedded in the aluminum alloy parts works as a catalysis to drive the solid state synthesis of AlN from the nitrogen supplied from plasma source and the aluminum as a constituent of parts. Owing to this solid state synthesis mechanism, the formation rate of AlN is accelerated by 80 times than the conventional nitriding or other PVD processing. In the present paper, this solid state synthesis mechanism is discussed with consideration on the role of Al2Cu to drive the aluminum nitride formation. Structural aluminum alloys are employed as a target material to demonstrate the effectiveness of formed aluminum nitride coating in improvement of wear toughness and reduction of friction coefficient.
5:30 PM - Q8.10
Non-oxide Sol Gel Preparation of Silicon Nitride Thin Films and Aerogels.
Shereen Hassan 1 , Ali Kalaji 1 , Andrew Hector 1 , Jason Hyde 2 , David Smith 2
1 School of Chemistry, University of Southampton, Southampton United Kingdom, 2 School of Physics and Astronomy, University of Southampton, Southampton United Kingdom
Show AbstractSol-gel processes allow formation of solid materials through gelation of precursor solutions and can be used to produce a large number of useful morphologies. Historically this is mainly a method used for oxides, but sol gel processes for non-oxide materials are of growing interest [1]. Often the non-oxide work has focused on producing powders for applications such as catalysis, where controlled porosity and basic catalytic sites are the point of interest. However, some groups have begun to produce other forms of material such as mesoporous silicon nitride membranes [2] and semiconductor aerogels [3]. This paper will describe the formation of thin films of amorphous silicon nitride on alumina tiles prepared using a non-oxide sol-gel technique. Tetrakis(methylamino)silane (Si(NHMe)4) solutions in THF were reacted with ammonia in the presence of a triflic acid catalyst. The effect of using different ammonia concentrations has been shown using thermogravimetric analysis (TGA). A simple dip coating technique yielded films of reasonable quality. This is an important step because a straightforward film formation method will facilitate the possibility of making templated structures. A small change in the conditions also leads to the formation of monolithic gels and preliminary results on supercritical drying of these to yield aerogel monoliths will also be presented.[1] A. L. Hector, Chem. Soc. Rev., in press (DOI:10.1039/b608838b).[2] F. Cheng, S. M. Kelly, S. Clark, J. S. Bradley, M. Baumbach and A. Schültze, J. Membr. Sci., 2006, 280, 530.[3] J. L. Mohanan, I. U. Arachige and S. L. Brock, Science, 2005, 307, 397.
Q9: Poster Session
Session Chairs
Friday AM, November 30, 2007
Exhibition Hall D (Hynes)
9:00 PM - Q9.1
The Effect of Composition on the Properties of Semiconducting Transition Metal Nitrides.
Maria Moreno-Armenta 1 , Armando Reyes-Serrato 1 , Gerardo Soto 1
1 Centro de Ciencias de la Materia Condensada, Universidad Nacional Autonoma de Mexico, Ensenada, B.C., Baja California, Mexico
Show AbstractThe transition metal nitrides (TMN) constitute an interesting class of materials with many technological applications. Most of the TMN tend to form ordinary structures: face centered cubic (fcc), hexagonal closed packed (hcp), simple hexagonal (hex) or simple cubic, with nonmetal atoms located in interstitial sites. In most cases the number of interstitial vacancies is considerable; consequently their representation as simple fcc, hcp, etc., is merely an idealization. Moreover, the stoichiometry is an important factor to think about when computing the properties of TMN. By controlling the nonmetal atom content in the alloy is possible to fine tune the hardness, brittleness, conductivity, etc. There has been a longstanding debate in the literature as to whether some nitrides are semiconductor or conductor, for example: scandium nitride and copper nitride. In this work we are guessing that the experimental discrepancy is caused by a variation in the electronic structure due to nitrogen (or metal) deficiencies in the TMN lattice. With that idea in mind, the aim of this work is two fold. In one hand, we want to present our methodology to study the composition variations in interstitial alloys. In the other hand, we aspire to reveal the electronic structure of TMN as a solid solution with large non-stoichiometric variants. Using the full potential linearized augmented plane wave (FP-LAPW) method we investigate the structural and electronic properties of bulk of scandium-, yttrium-, and copper-nitride over a wide range of nitrogen concentrations. The N atom was gradually incorporated into metal matrix with and without metal vacancies. The ground state properties like equilibrium lattice constants, bulk moduli, densities of states and formation energies are determined for each of the calculated alloys. We have found that the semi-conducting state in copper nitride have a tinny compositional margin. Any deviation of the ideal stoichiometry will produce a metallic character. What is more, the stabilities of the conductive phases are very close to the stability of the semi conducting phase, with a little margin favorable to the conducting phases. One interesting result is that copper vacancies are needed to obtain the semi conducting state. For Yttrium nitride study the calculations show that, for very low nitrogen incorporations the hexagonal and fcc phases may coexist. However, for high nitrogen concentration the cubic phases are favored by a 30 kJ mol-1 margin. Our calculations revel that the density of states for scandium- and yttrium nitrides undergoes a gradual modification as nitrogen is incorporated into the metal lattice. For non-stoichiometric nitrogen content the materials behave as metal, whereas at stoichiometric composition the DOS becomes zero at Fermi level, confirming in this way the semiconductor character of these nitrides.
9:00 PM - Q9.10
High-pressure Crystal Structure and Properties of Cu3N.
Ulrich Schwarz 2 , Lev Akselrud 2 , Aron Wosylus 2 , Matt Tucker 3 , Michael Hanfland 4 , Komalavalli Thirunavukkuarasu 5 , Christine Kuntscher 5 , Jörg von Appen 6 , Richard Dronskowski 6 , Dieter Rau 1 , Rainer Niewa 1
2 , MPI CPfS, Dresden Germany, 3 ISIS Facility, Rutherford Appleton Laboratory, Oxon United Kingdom, 4 , ESRF, Grenoble France, 5 Experimentalphysik II, University of Augsburg, Augsburg Germany, 6 Institute of Inorganic Chemistry, RWTH, Aachen Germany, 1 Chemistry, TU Munich, Garching Germany
Show AbstractThe ambient-pressure modification of the semiconducting nitride Cu3N crystallizes in an inverse ReO3 type, an exclusive situation for binary transition-metal nitrides [1, 2]. Moreover, a pressure-induced phase transition of ReO3-type Cu3N is predicted to occur below about 25 – 35 GPa on the basis of electronic-structure calculations [3]. However, pressure-dependent electrical resistivity measurements indicate a semiconductor–metal transition at the much lower pressure of 5 GPa [4, 5].Recent in-situ X-ray investigations, carried out in a DAC with argon as pressure-transmitting medium (undulator beamline ID 9A, ESRF, Grenoble), yield a high-pressure phase at 7.8(5) GPa. The tetragonal cell parameters reveal a discontinuous volume decrease of about 20 %. The phase transition is completely reversible on decreasing pressure, with a hysteresis of about 4 GPa. Subsequent ex-situ investigations in a multi-anvil press evidence a completely reversible re-formation of LP-Cu3N from identical XRD patterns. For the full structure analysis, high-pressure neutron-diffraction experiments were carried out in a Paris-Edinburgh cell (Pearl beamline, ISIS pulsed-neutron source, Rutherford Appleton Laboratory). The development of the unit-cell parameters corresponds closely to the X-ray data. Initially increasing volumes with increasing pressure of the high-pressure phase seems to be associated with a thermodynamic barrier for the phase formation. The structural transformation is indicated by disappearing reflections of LP-Cu3N. While the reflections of HP-Cu3N grow slowly from background on increasing pressure and stay broad and weak, the tetragonal substructure can clearly be confirmed. No superstructure reflections indicating any nitrogen order are visible. The structure refinement with N atoms disordered in distorted octahedral voids of the tetragonal body-centred Cu-substructure leads to an occupation of about 1/3 and thus to a composition of Cu3N1.1(1). Density-functional theory supports the existence of tetragonal HP-Cu3N but also indicates an ordered arrangement of the N atoms, necessary for quantitative agreement in terms of the c/a ratio and the transition pressure. The compressibility of B0 = 146(24) GPa for LP-Cu3N is significantly higher than the value for the isotypic ReO3 phase (100 GPa [6]). Optical absorption measurements (IR-VIS) at various pressures indicate a semiconductor–metal transition concurrent with the structural transition.[1] R. Juza, H. Hahn, Z. Anorg. Allg. Chem. 239 (1938) 282.[2] U. Zachwieja, H. Jacobs, J. Less-Common Met. 161 (1990) 175.[3] Z. Cancarevic, J. C. Schön, M. Jansen, Z. Anorg. Allg. Chem. 631 (2005) 1167.[4] L. X. Yang et al., Chin. Phys. Lett. 23 (2006) 426.[5] J. G. Zhao et al., phys. stat. sol (b) 243 (2006) 573.[6] J.-E. Jørgensen, J. Staun Olsen and L. Gerward, J. Appl. Cryst. 33 (2000) 279.
9:00 PM - Q9.11
Carbometalates and Their Structural Relationship to Nitrido-Compounds.
Enkhtsetseg Dashjav 1 , Guido Kreiner 1 , Walter Schnelle 1 , Frank R Wagner 1 , Ruediger Kniep 1
1 , MPI-CPfS Dresden, Dresden Germany
Show AbstractTernary and higher carbides are classified into two main groups, carbometalates and metal-rich carbides, as was introduced only recently, for compounds AxTyCz (A = rare-earth metals and actinoids; T = transition metals) with monoatomic species C4− [1]. Carbometalates are compounds containing complex anions [TyCz]n- as structural entities in analogy to fluoro-, oxo- and nitridometalates. They exhibit strong covalent T–C interactions, charge transfer from the rare-earth metal onto the complex anions and secondary metal–metal interactions. They are clearly distinguished from the metal–rich carbides, which may be viewed as interstitial carbides, with broad homogeneity ranges and chemical bonding situations determined predominantly by metal–metal interactions. The polyanions in carbometalates occur as discrete entities, chain-, layer- or framework-substructures. Structurally, they are closely related to the nitrido-compounds. For example, typical building units are anionic dumb-bells as present in the crystal structures of Li4[FeN2] and Th2[NiC2], linear chains in Ca[NiN] and Y[CoC], and zigzag chains Y2[ReC2] as well as in LiSr2[Fe2N3]. Within the last few years we explored the synthesis of new ternary carbides, and bonding analyses already revealed that the concept of carbometalates holds for a large number of novel compounds [2-7]. Here, we present recent developments in syntheses, crystal and electronic structures, as well as physical properties of carbometalates. References[1]E. Dashjav, G. Kreiner, W. Schnelle, F. R. Wagner, R. Kniep W. Jeitschko, J. Solid State Chem. 2007, 180, 636-653.[2]E. Dashjav, G. Kreiner, W. Schnelle, F. R. Wagner, R. Kniep, Z. Anorg. Allg. Chem. 2004, 630, 689-696.[3]E. Dashjav, G. Kreiner, W. Schnelle, F. R. Wagner, R. Kniep, Z. Anorg. Allg. Chem. 2004, 630, 2277-2286.[4] E. Dashjav, W. Schnelle, G. Kreiner, R. Kniep, Z. Kristallogr. NCS. 2005, 220, 129-130.[5]E. Dashjav, W. Schnelle, F.R. Wagner, G. Kreiner, R. Kniep, Z. Anorg. Allg. Chem. 2006, 632, 2094. [6]E. Dashjav, G. Kreiner, F.R. Wagner, W. Schnelle, R. Kniep, Z. Anorg. Allg. Chem. 2007, in press [7]E. Dashjav, Yu. Prots, G. Kreiner, F.R. Wagner, W. Schnelle, R. Kniep, STAM. 2007, in press
9:00 PM - Q9.12
On the AE3[MIIIN3] and (Ca3N)2[MIIIN3] Compound Series (AE: Sr, Ba; M: Fe, Mn, Cr).
Joanna Bendyna 1 , Peter Hoehn 1 , Walter Schnelle 1 , Ruediger Kniep 1
1 chemistry, Max-Planck-InstituteCPFS, Dresden Germany
Show AbstractNitridometalates of the AE3[MN3] series contain the planar complex [MIIIN3]6- and crystallize in the space group P63/m (AE: Sr, Ba; M: Cr, Ga, Mn, Fe)[1-5] and Cmcm (Ca3[MN3] M: Cr, Mn, V)[6-8], respectively. With the exception of Ba3[FeN3][9] (Mössbauer spectroscopy) the physical properties of compounds containing barium and strontium were not investigated up to now. Nitridometalates of the (AE3N)2[MN3] series also contain the (trigonal) planar complex [MIIIN3]6- and crystallize in P63/mcm (M: Ga, Mn, Fe)[5,10-11], physical properties data are available for the Mn phase only [11]. Recently, we were able to prepare Sr3[FeN3] (P63/m, a = 7.6296(11) Å, c = 5.3268(16) Å)[1] and single phase samples of the isotypic compounds Sr3[CrN3][2], Sr3[MnN3][4], and Ba3[FeN3][2], as well as (Ca3N)2[MnN3][11] and (Ca3N)2[FeN3][10]. The isolated [MIIIN3]6- ions in the AE3[MN3] series are located within planar layers stacked in the sequence …ABAB… along [001]. In the crystal structure of (Ca3N)2[MN3] the planar [MIIIN3]6- complexes are placed between layers of edge sharing, N-centred Ca6- polyhedra, stacked in the sequence …ABAC… along [001]. The compounds of the AE3[MN3] series display different electrical and magnetic properties. Sr3[CrN3] is an insulating Van-Vleck paramagnet while the Mn-homologue is an insulating Curie-paramagnet with μeff = 2.4μB, Θ ≈ -63 K. Ba3[FeN3] shows a temperature-independent resistivity (≈1*10-4 Ω m) and displays a magnetic susceptibility with a broad maximum at 150 K and magnetic order at 68 K. The compounds of the (Ca3N)2[MN3] series revealed Curie-Weiss behaviour with effective moments at high temperature μeff = 2.82μB ((Ca3N)2[MnN3]) and μeff = 4.03μB ((Ca3N)2[FeN3]), compatible with a intermediate state (IS) state of the [MnIIIN3]6- units. X-ray absorption spectroscopy (XAS) was used to investigate the Fe, Mn, and Cr K-thresholds of the nitridometalates and selected oxides together with nitrides for comparison.References:[1] J.K. Bendyna, P. Höhn, R. Kniep, Z. Kristallogr. NCS (2007) submitted.[2] P. Höhn, R. Kniep, A. Rabenau, Z. Kristallogr. 196 (1991) 153.[3] M. G. Barker, M. J. Begley, P. P. Edwards, D. H. Gregory, S. E. Smith, J. Chem. Coc., Dalton Trans. (1996) 1.[4] A. Tennstedt, C. Röhr, R. Kniep, Z. Naturforsch. 48b (1993) 794.[5] D. G. Park, Z. A. Gal, F. J. DiSalvo, Inorg. Chem. 42 (2003) 1779.[6] D. A. Vennos, F. J. DiSalvo, J. Solid State Chem. 98 (1992) 318.[7] A. Tennstedt, C. Röhr, R. Kniep, Z. Naturforsch. 48b (1993) 1831.[8]D.A. Vennos, M.E. Badding, F.J. DiSalvo, Inorg. Chem. 29 (1990) 4059.[9] N. Jansen, H. Spiering, P. Gütlich, D. Stahl, R. Kniep, V. Eyert, J. Kübler, P. C. Schmidt, Angew. Chem. Int. Ed. 31 (1992) 1624.[10] G. Cordier, P. Höhn, R. Kniep, A. Rabenau, Z. Anorg. Allg. Chem. 591 (1990) 58.[11] D. H. Gregory, M. G. Barker, P. P. Edwards, D. J. Siddons, Inorg. Chem. 34 (1995) 5195.
9:00 PM - Q9.13
On the Mixed Valency Compounds Sr8M3N8 (M = Fe, Fe + Mn) and Sr8Mn3N9.
Joanna Bendyna 1 , Peter Hoehn 1 , Walter Schnelle 1 , Ruediger Kniep 1
1 chemistry, Max-Planck-InstituteCPFS, Dresden Germany
Show AbstractRecently, we were able to prepare the isostructural phases Sr8[MnN3]2[FeN2] (a = 18.8315(23) Å, b = 5.3206(7) Å, c = 7.5506(8) Å, β = 106.272(4)°, V = 726.23 Å3) and Sr8[FeN3]2[FeN2] (a = 18.80571(16) Å, b = 5.31658(4) Å, c = 7.50698(6) Å, β = 106.2449(5)°, V = 720.6 Å3) crystallize monoclinic (C2/m, No. 12). The related compound Sr8[MnN3]3 crystallizes also monoclinic in a superstructure variant (P21/c, No. 14, a = 7.5978(17) Å, b = 10.6686(17) Å, c = 18.943(3) Å, β = 106.2163(9)°, V = 1474.79 Å3).Sr8[FeIIIN3]2[FeIIN2] is the first mixed-valency nitridoferrate and combines the main motifs of both, the Sr3[FeIIIN3] [1] and Sr2[FeIIN2] [2] crystal structures. The isotypic Sr8[MnIIIN3]2[FeIIN2] is the first example for a nitridometalate containing two different transition metals with different oxidation states. Both, Sr8[MIIIN3]2[FeIIN2] (M = Fe, Mn) contain isolated planar [MIIIN3]6- units and [FeIIN2]4- dumbbells. The crystal structure of Sr8[MnN3]3 is closely related to Sr8[MIIIN3]2[FeIIN2], by replacing the [MIIN2]4- group by a [MIIIN3]6- unit and thereby doubling the unit cell.X-ray absorption spectroscopy (XAS) was used to investigate the K-threshold of Mn and Fe species. The comparison of the energies Sr8[FeIIIN3]2[FeIIN2] is exactly placed between Sr2[FeIIN2] and Sr3[FeIIIN3], respectively. The location of Sr8[MnIIIN3]2[FeIIN2] is directly within the range of Sr3[MnIIIN3], whereas the Fe energy is close to that of Sr2[FeIIN2]. The energy for Sr8[MnN3]3 was shifted to higher values compared to Sr3[MnIIIN3] and Sr8[MnIIIN3]2[FeIIN2], ergo indicating a higher oxidation state for Mn.Magnetic susceptibility data revealed Curie-Weiss behaviour with effective moments (above 150 K) of μeff = 6.51μB (Sr8[FeIIIN3]2[FeIIN2]), μeff = 2.33μB (Sr8[MnN3]3), and μeff = 7.07μB (Sr8[MnIIIN3]2[FeIIN2]) per formula units, respectively. The values for Sr8[MIIIN3]2[FeIIN2] are compatible with a LS state for the [MIIIN3]6- units, and a HS state for the [FeIIN2]4- complexes. The values for Sr8Mn3N9 are compatible with a LS for the two planar [MnIVN3]5- ions and one non-planar [MnIIIN3]6- ion.References[1] J. K. Bendyna, P. Höhn, R. Kniep Z. Kristallogr. submitted.[2] P. Höhn, R. Kniep, Z. Naturforsch. 47b (1992) 477.[3] A. Tennstedt, C. Roehr, R. Kniep, Z. Naturforsch. 48b (1993) 794.
9:00 PM - Q9.14
II-VI Material Development for Production Scale MOCVD Reactors.
Daniel Byrnes 1 , Matthew Youngers 1 , Dong Lee 1 , Sherman Li 1 , Paul Ahn 1 , William Quinn 1
1 R&D, Veeco Turbodisc, Somerset, New Jersey, United States
Show AbstractII-VI semiconductors are interesting materials for the development of integrated RGB and white LEDs for displays and general lighting, infra-red optics, and solar cells. We present ZnSe thin films grown epitaxially by MOCVD on 0,10, and 15 degree offcut GaAs substrates with varying growth conditions. The ZnSe samples were characterized using AFM, X-ray diffraction, and temperature dependent Photoluminescence. We show a band edge emission of around 460nm for a film thickness of around 100nm at a temperature of 300K which is suitable for blue LEDs. We show the correlation between surface morphology and crystal quality with enhanced emission spectra. Our measurements show the development of a high quality thin film by optimizing reactor growth conditions. We have demonstrated the capability to grow high quality II-VI materials using a vertical flow, high speed rotation MOCVD reactors. The reactors used in this experiment were the Veeco D180 research system with a 6, 2-inch wafer capacity and a Veeco E450 production system reactor with a 50, 2-inch wafer capacity.
9:00 PM - Q9.15
Structure of Isolated Oxygen Impurity States in InN.
Dimiter Alexandrov 1 , Scott Butcher 2 , Nikolaus Dietz 3
1 Electrical Engineering, Lakehead University, Thunder Bay, Ontario, Canada, 2 Physics, Macquarie University, Sydney, New South Wales, Australia, 3 Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia, United States
Show AbstractThe electron state structure of isolated interstitial O atoms in real InN (containing clusters of InN, of InON and of non-stoichiometric InN:In) is subject of investigation in this paper. There is different behavior of O atoms that depends on their positions due to different symmetry of the electrical charge of the valence electron cloud. It is shown that for the alloyed O atoms the corresponding symmetry gives interaction of these atoms only with atoms belonging to the crystal lattice, and for the interstitial O atoms the corresponding symmetry is equivalent to this of O atom in the vacuum. It allows that the method of the hydrogen like impurity atom to be applied for isolated interstitial O atom hosted in real InN. The authors have used this method and the following results have been obtained (it has been considered that the vacuum level has zero energy): i) If the O atom is interstitially incorporated in cluster of pure InN the impurity state has energy -5.11 eV, which acts as donor level with ionization energy +0.06 eV. Also this state is donor level with ionization energy -0.02 eV for cluster of InON if this cluster occurs on a distance less than 30 Angstroms from O atom, however if there is cluster of InN:In in the same range this level goes into the valence band of this cluster; ii) The impurity state has energy -5.15 eV if the O atom is interstitially incorporated in cluster of InON, which acts as donor level with ionization energy +0.02 eV. Also this state is donor level with ionization energy +0.10 eV for cluster of InN if this cluster occurs on a distance less than 60 Angstroms from O atom. This level has the same behavior as this in i) about cluster of InN:In only here the range is 60 Angstroms. iii) If the O atom is interstitially incorporated in cluster of non-stoichiometric InN:In the impurity state has energy -5.38 eV, which goes in the valence band of this cluster. However this state acts as donor level for both cluster of InN and cluster of InON if they are on distance less that 59 Angstroms from the O atom. The donor ionization energy for the first cluster is +0.33 eV, and for the second cluster it is +0.25 eV. Two conclusions can be made from the obtained results: 1) The energy of the electron state depends on location of the O atom in the real InN and it determines variation of donor ionization energy in the range -0.02 – +0.33 eV. 2) The levels created by the isolated interstitial O atoms in non-stoichiometric InN:In go into the valence band of this cluster. If due to presence of near InN or InON clusters these levels become donor ionized further they can act as acceptor levels of negative ionization energy. In this way these levels can be considered to be “bridge levels” between both the valence band and the conduction band of real InN, and they can occur in certain technological circumstances. Both conclusions can be considered to form one of the reasons for the electron conductivity in the real InN.
9:00 PM - Q9.17
Optimization of Functionally Graded Material(FGM) Layers on The Residual Stress of Polytypoidally Joined SI3N4-AL2O3 Using Three-Dimensional Analysis.
Caroline Sunyong Lee 1 , SungHoon Ahn 3 , JaeChul Lee 3 , Jong-ha Park 1 , SaeHee Ryu 1 , Hyun Jung Hong 1 , Gyu-Bong Jung 1 , Dong-joo Hong 1 , Gang-Hwan Jung 1 , Doh-Hyung Riu 2
1 Materials and Chemical Engineering, Hanyang university, Gyunggi-do, Gyunggi-do, Korea (the Republic of), 3 Department of Mechanical Engineering and Aerospace Engineering, Seoul National University, Shin-Rim Dong, Seoul, Korea (the Republic of), 2 Ceramic institute of ceramic engineering and Tech., Ceramic Engineering and Tech., Guemcheon-Gu, Seoul, Korea (the Republic of)
Show AbstractThe functionally graded material (FGM) is one of the promising candidates to obtain special properties unavailable from any one homogeneous monolithic material. The design of FGM is intended to take advantage of certain desirable features of each of the constituent phases. Due to the existence of physical and economic limitations for the manufacture of large parts, joining is essential. Furthermore, having different materials at different regions of the same component is desirable for several applications. Joining to form these components is often the only way for fabrication. Silicon nitride has been considered as one of the most promising structural materials for high temperature applications for the following unique properties - high strength, oxidation and corrosion resistance, thermal stability and resistance to thermal shock. Alumina has been chosen for the joining material since it has an intermediate coefficient of thermal expansion between Si3N4 and metals. Alumina can be used as a buffer layer for joining Si3N4 and metal. Moreover, it has been widely used in high temperature structural components due to its chemical stability. In order to join Si3N4 to Al2O3, a sialon polytypoid has been used because the structure of this polytypoid compound is determined by stacking-fault spacing (metal/non metal ratio), no undesirable reaction product is formed between Si3N4 and Al2O3, and glass-free interface can be obtained for high temperature application.A unique approach introducing sialon polytypoids as a Functionally Graded Material (FGM) bonding has been used to join Silicon Nitride and Alumina. Multilayered FGM samples were optimized from 20 layers to 15 layers using three dimensional analysis tool, ALGOR to fabricate a crack-free joining of heterogeneous ceramics. Previously, an FGM joint which consists of 20 graded layers, was introduced by a powder stacking method. 20 layers were added to minimize the CTE mismatch stresses as much as possible. In this study, these 20 layers were further reduced to 15 layers for optimization. The thickness and compositions of these joints were newly designed, based on the residual stress calculation using ALGOR and these FGM joints were fabricated for verification. According to the analysis, a layer for polytypoid 100% showed a highly stressed zone of the FGMs and this analysis actually matched with the result from fabricated sample. The individual stress plots for axial, radial, and hoop stresses showed highly stressed zone of the FGMs to indicate that such analyses can be especially useful for graded FGM samples where the residual stresses are very difficult to measure experimentally. Based on analysis, crack-free 15 layer FGM joint between Si3N4 and Al2O3 were obtained. After these joints were fabricated, strength of the joint and various interfaces were qualitatively characterized using oriented Vickers Indentation test.
9:00 PM - Q9.18
Mn- and Fe- doped GaN for Spintronic Applications.
Axel Hoffmann 1 , Enno Malguth 1 , Wofgang Gehlhoff 1
1 Inst. f. Fstkoerperphysik, TU Berlin, Berlin Germany
Show AbstractIn the context of spintronic applications we studied Mn and Fe doped GaN. Samples with different transition metal concentrations and n or p co-doping were investigated by means of optical and magnetic experiments. The results allow us to elucidate the following issues that are of crucial significance on the way to eventually realize a ferromagnetic coupling at room temperature: (1) Which charge states the transition metals are found in, (2) whether and where they from levels within the band gap, (3) the formation of bound states consisting of a hole localized at a transition metal ion in the charge state 2+. We also assess the influence of the transition metal concentration on the structural, electronic and optical properties of GaN. Additionally, the effects of n and p co-doping on these properties are investigated.
9:00 PM - Q9.19
Preparation of Si-B-O-N-C Ceramics from Precursors Prepared from a Borazine-derivative and Hydrorganosiloxanes via Hydrosilylation.
Ken-ichi Fuchigami 1 , Yoriyoshi Yoneyama 1 , Yuko Uchimaru 2 , Yoshiyuki Sugahara 1
1 Department of Applied Chemistry, School of Science and Engineering, Waseda University, Tokyo Japan, 2 , National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki Japan
Show AbstractPrecursors were prepared from a borazine-derivative and hydoorganosiloxanes via hydrosilylation and were pyrolyzed under the Ar atmosphere to yield Si-B-O-N-C ceramics. The B-triallylborazine (TAB) was used as a borazine-derivative, while hydride-terminated poly(dimethylsiloxane) (H-PDMS) or 1,3,5,7-tetramethyl-cyclotetrasiloxane (cycloS) was employed as a hydoorganosiloxane. Spectroscopic characterization indicated that C=C bonds in TAB were reacted with Si-H groups in the hydoorganosiloxane to yield co-polymers with three-dimensional networks. TG analyses revealed that the ceramic yields for the pyrolysis of co-polymers under the Ar atmosphere depended on the type of hydoorganosiloxanes; the ceramic yield of cycloS/TAB (1:1) co-polymer was 75 mass%, while that of PDMS/TAB (1:1) co-polymer was 20 mass%. The compositions of the ceramic residues pyrolyzed at 1000 °C under the Ar atmosphere were Si1.0O1.1C0.89B0.22N0.089H0.25 for cycloS/TAB (1:1) precursor and Si1.0O1.1C1.4B1.0N0.90H0.45 for PDMS/TAB (1:1). The ceramic yields of cycloS/TAB co-polymer also depended on the molar ratio of cycloS/TAB, and the cycloS/TAB (4:1) precursor exhibited the highest ceramic yield (85 mass%) among the precursors employed.
9:00 PM - Q9.2
Superplastic Deformation of Nano-structured Monolithic Silicon Nitride.
Kentarou Chihara 1 , Yutaka Shinoda 1 , Takashi Akatsu 1 , Fumihiro Wakai 1
1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan
Show AbstractSuperplasticity, which is the ability of polycrystalline solid to exhibit large plastic deformation at elevated temperatures, is favorable for shaping, e.g. superplastic forming and joining, for rigid materials. Liquid phase sintered silicon nitride (Si3N4) polycrystals can be deformed superplastically by viscous flow of intergranular glass phase or solution-precipitation in conjunction with grain boundary sliding. The strong point of the superplasticity in Si3N4 consists in the improvement of the mechanical properties owing to the microstructual evolution, growth of rod-shaped β-Si3N4 grain, occurs during the deformation. However, the strain rates of conventional superplastic-Si3N4 with a grain size of sub-micro meter lower than 10-4~10-3 s-1. Hence, further improvement of superplasticity in Si3N4 was demanded to materialize metal-like commercial superplastic forming. The semi-empirical equations for high temperature deformation gives a strategy for achieving superplasticity at high strain rates, i.e. grain refinement and reduction of the grain boundary viscosity. In this study, we developed a monolithic β-Si3N4 that was composed of nano-sized grains for the purpose of improving the superplasticity. The nano-structured monolithic β-Si3N4 ceramics, the average grain size was about 100 nm, was fabricated by liquid phase sintering a nano-Si3N4 powder. The nano-powder was conveniently produced by annealing of a commercially available amorphous Si3N4 powder containing 6 mass% Y2O3, 3 mass% Al2O3 and 2 mass% MgO, without using any high-energy ball milling. The sintered body reached almost full density (3.25 g/cm3) during Spark Plasma Sintering (SPS) at 1873 K. The deformation was investigated by uni-axial compression tests in nitrogen atmosphere at temperature ranging from 1773 K to 1923 K. The nano-structured monolithic Si3N4 could be deformed several times faster than conventional superplastic Si3N4-based materials. A strain rate of 10-2 s-1, which was criterion for high-strain rate superplasticity, was successfully achieved for the first time in the monolithic Si3N4 materials in 1923 K. The deformation was characterized by stress exponent n <1 in the higher stress region, n ~1 (Newtonian flow) in the intermediate stress region and n >1 in the lower stress region. This result suggests that the rate-controlling mechanism for deformation change depending on the applied stress. The transitions of stress exponents from 1 (Diffusion controlled-solution precipitation creep) to 0.5 (Shear-thickening creep) and from 2 (Interface reaction controlled-solution precipitation creep) to 1 with increasing applied stress were already observed in conventional superplastic Si3N4-based materials. It is reasonable deduction that the nano-structure allows the three dominant mechanisms to coexist in the same temperature. Researches on the nano-structured dense polycrystals are expected to develop the frontiers of the advanced rigid materials.
9:00 PM - Q9.20
Synthesis of Molecular Precursors to GaN QDs.
Pietro Chirico 1 , Andrew Hector 1 , David Smith 2
1 School of Chemistry, University of Southampton, Southampton United Kingdom, 2 School of Physics and Astronomy, University of Southampton, Southampton United Kingdom
Show AbstractGallium nitride (GaN) has recently attracted extensive experimental and theoretical interest due to its physical properties, such as a wide and direct band gap [1], low compressibility, and high thermal conductivity, which makes it strong candidate for short-wavelength electroluminescent device and high-temperature/high power diode and transistor [2,3]. Colloidal nanocrystals, or quantum dots (QDs) made from this material, are expected to combine good thermal, chemical, and radiation stability with excellent optical properties.The precursors studied include three amidogallium precursors: [Ga(NMe2)3]2, [Ga(NEt2)3]2, [Ga(NiPr2)3], these can be converted directly to GaN through pyrolysis under N2 atmosphere or NH3 in a mixture of coordinating solvents. These precursors also offer an opportunity to tune the particle size by controlling reaction conditions such as precursor concentration, pyrolysis temperature, reaction time, and identity and concentration of the capping ligands.[1] H.P. Maruska and J.J. Tietjen, Appl. Phys. Lett. 1969, 15, 327.[2] S. Strite and H. Morchoc, J. Vac. Sci. Technol. 1992, B 10, 1237.[3] S. Strite, M. E. Lin, and H. Morchoc, Thin Solid Films 1993, 231, 197.
9:00 PM - Q9.21
Synthesis and Sintering of Aluminium Nitride Nanoparticles Prepared in Liquid Ammonia.
Zhao Han 1 , Hongmin Zhu 1
1 , University of Science and Technology Beijing, Beijing China
Show AbstractAluminum nitride (AlN) nanoparticles were synthesized from aluminum chloride by sodium reduction in liquid ammonia. A liquid solution of sodium dissolved in ammonia was employed as a reduction-nitridation agent, which enabled direct nitridation of aluminum chloride at 228K. The synthesized particles were heat-treated at 1273K in vacuum, and were characterized by X-ray diffraction (XRD), scan electron microscopy (SEM), transmission electron microscopy (TEM) and BET surface area measurement. The results indicated that the product were hexagonal wurtzite AlN particles, with a specific surface area of 262m2/g. Spark plasma sintering(SPS) process was used to consolidate the as-prepared AlN nanoparticles, and a dense AlN ceramic (>98.5%) with average grain size of 200-400nm was obtained at 1873K without the use of sintering additives.
9:00 PM - Q9.22
Luminescence of Non-oxide Sol-gel Derived Lanthanide Doped Silicon Nitrides.
Shereen Hassan 1 , Andrew Hector 1
1 School of Chemistry, University of Southampton, Southampton United Kingdom
Show AbstractThe fluorescence of lanthanide doped silicon nitride materials prepared by the sol-gel method was studied. Due to the unique optical and magnetic properties of luminescent lanthanide ions which are used in diverse potential application in optical technologies [1], combining the low reactivity of silicon nitride materials with other functional properties via doping in different lanthanide is a compelling goal. Sol-gel methods offer an effective means of adding elements into the Si-N matrix. Strategies have included matching ammonolysis rates between amides of silicon and another element [2], or developing single source precursors with the second element already incorporated [3]. In this work we have evaluated the doping of lanthanide ions into silicon nitride with the aim of producing new photoluminescent materials.[1] e.g. see R. Reisfeld, G. Panczer, A. Patra and M. Gaft, Mater. Lett.,1999, 38, 413. [2] J. Löffelholz, J. Engering and M. Jansen, Z. Anorg. Allg. Chem., 2000, 626, 963.[3] F. Cheng, S. M. Kelly, S. Clark, N. A. Young, S. J. Archibald and J. S. Bradley, Chem. Mater., 2005, 17, 5594.
9:00 PM - Q9.23
New Metal Nitride Compounds: Can they be Synthesized at High-Pressures?
Peter Kroll 1
1 Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas, United States
Show Abstract9:00 PM - Q9.24
Non-planar Corrugated Layered Heptazine-based Carbon Nitride: The Lowest Energy Modifications of C3N4.
Jose Gracia 1 , Peter Kroll 1
1 Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas, United States
Show AbstractA theoretical revision of possible C3N4 structures is performed to explain latest experimental results. In our study we combine computational and experimental evidence [1,2,3] that heptazine-based motifs are energetically preferred and, therefore, ubiquitious in synthesized carbon nitride materials. We, furthermore, corroborate some recent computational results that established a trend towards non-planar wave-like structures in favor over planar sheet-like structures [4].We propose non-planar s-heptazine based two-dimensional corrugated sheet-like and three-dimensional extended models that constitute lowest energy structures of C3N4. Our investigations were carried out with the support of density functional calculations, including extensive ab-initio molecular dynamic simulations, and post-Hartree-Fock methods. Sheet-like structures comprising the heptazine motif are corrugated and become the most favorable in energy. Corrugation may be driven by repulsion of nitrogen lone-pairs as well as by a band-gap opening process. Extended models comprising the heptazine motif appear mechanically unstable and are very sensitive to shear distortions.The trend to form non-planar corrugated structures may also favor the possibility of tubular carbon nitride based on the heptazine motif. We investigated several different candidate models and found only a marginal energy difference to the ground state structure. [1]E. Kroke, M. Schwarz, E. Bordon, P. Kroll, B. Noll, and Arlan D. Normand, New. J. Chem. 2002, 26, 508; [2]B. Jürgens, E. Irran, J. Senker, P. Kroll, H. Müller, and W. Schnick, J. Am. Chem. Soc. 2003, 125, 10288.[3]B.V. Lotsch, M. Döblinger. J. Sehnert, L. Seyfarth, J. Senker, O. Oeckler, and W. SchnickChem. Eur. J. 2007, 13, 4969.[4]D. T. Vodak, K. Kim, L. Iordanidis, P. G. Rasmussen, A. J. Matzger, and O. M. Yaghi, Chem. Eur. J. 2003, 9, 4197.
9:00 PM - Q9.25
The Phase Boundary Between b-Si3N4 and g-Si3N4 at Elevated Temperatures and Pressures.
Atsuchi Togo 2 , Peter Kroll 1
2 Inorganic Chemistry, RWTH Aachen, Aachen Germany, 1 Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas, United States
Show AbstractThe discovery of spinel-type silicon nitride, g-Si3N4, by experimental and computational methods [1] motivated a number of further studies [2]. Meanwhile, it has been proposed that the phase diagram of Si3N4 contains two more phases, a post-spinell d-Si3N4 [3] and another high-pressure modification with unknown structure and density [4].To further complement the picture of silicon nitride at high-pressure/high-temperature conditions we set out to investigate the phase boundary between the ambient pressure modification b-Si3N4 and the high-pressure polymorph g-Si3N4. Effects of temperature are allocated through the differences of vibrational entropy of both structures, which we calculate using the harmonic approximation. All computations are made within density functional theory.In a further step we apply the same formalism to investigate the phase boundary between the monoclinic ambient pressure modification of Ca2Si5N8 and its orthorhombic high-pressure modification that has recently been synthesized. [1]A. Zerr, G. Miehe, G. Serghiou, M. Schwarz, E. Kroke, R. Riedel, H. Fuess , P. Kroll, R. Böhler, Nature 1999, 400, 340.[2]E. Horvath-Bordon, R. Riedel, A. Zerr, P. F. McMillan, G Auffermann, Y. Prots, W. Bronger, R. Kniep, and P Kroll, Chemical Society Reviews 2006, 35, 987.[3]Peter Kroll and Jörg von Appen, phys. stat. sol. (b) 2001, 226, R6.[4]A. Zerr, phys. stat. sol. (b) 2001, 227, R4.
9:00 PM - Q9.26
Phase Transitions in Silicon-Carbon-Nitride Compounds.
Peter Kroll 1 , Jose Gracia 1 , Aleksander Gurlo 2 , Ralf Riedel 2
1 Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas, United States, 2 Fachbereich Material- und Geowissenschaften, Technische Universität Darmstadt, Darmstadt Germany
Show Abstract9:00 PM - Q9.27
Nitride Based Schottky-Barrier Solar Cells.
Balakrishnam Jampana 1 , Omkar Jani 3 , Brian McCandless 2 , Steven Hegedus 2 , Christiana Honsberg 1
1 School of Electrical and Computer Engineering, University of Delaware, Newark, Delaware, United States, 3 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States
Show AbstractIn the past decade III-nitride semiconductors evolved as a promising material for optical systems and power devices. In recent years, the potential of III-nitrides is examined towards several applications including solar energy conversion. The most attractive property of the nitride system is the ability to vary the InGaN bandgap from 0.65 eV to 3.4 eV. A major challenge in forming a pn junction is doping the semiconductor, particularly at lower bandgaps. Alternatively, Schottky-barrier solar cells are demonstrated on other semiconductors and this paper utilizes those concepts to develop a nitride based Schottky-barrier solar cell.This paper presents the fabrication and characterization of p-GaN Schottky-barrier solar cell. The p-GaN (Sapphire/LT Buffer/u-GaN(0.5um)/p-GaN(1um), hole Con: mid 10^17cm-3 to mid 10^18 cm-3, mobility: 10-20 cm^2 V^-1 s^-1) is grown by MOCVD. The Schottky contact was Ti/Au and the Ohmic contact was a Ni/Au tunneling contact. The device was fabricated with a Schottky dot at the center and Ohmic contact surrounding the dot separated by few mm. It is observed that the forward turn-on voltage on this device is close to 2.5 V which is higher than the previous reported value [1], and attributed to lower Ohmic contact resistance. The turn-on voltage is affected by many factors, and turn-on voltages of the same magnitude are demonstrated in GaN Schottky-barrier power devices. The currents are observed to be in the order of nanoAmps. As the mobility of carriers is small the current density is observed to falls off exponentially to small values at large separation. The effect of separation between Schottky contact and Ohmic contact on current density is studied with interdigitated pattern and reported in this paper.The device with a single Schottky dot surrounded by Ohmic contact gave an open-circuit voltage of 0.257 V under AM 1.5 condition. This low value despite large turn-on voltage is attributed to low UV content in the spectrum. Higher Voc (~0.7-0.8 V) was observed with UV source.This paper enhances the approach to Schottky devices demonstrated earlier [1] and studies its application as a solar cell. Another important need for Schottky solar cells is in the field of bio-inspired nitride materials. Obtaining both types of doping during growth to form a pn junction is not demonstrated. To make solar cells out of these materials alternative techniques for depletion region formation have to be developed, like the Schottky-barrier solar cells. The Schottky-barrier devices form the basis for the evolution of nitride based-MIS solar cells. They also aid in determining the electrical quality of the semiconductor that affect the solar cell performance and hence can be used as test structures.[1] Khan, M. Asif et al., Appl. Phys. Lett., Vol. 63, (18), 1993, pp.2455-2456
9:00 PM - Q9.28
Aluminum Nitride Micro-Channels Grown via Metal Organic Vapor Phase Epitaxy for MEMs Applications.
L. Rodak 1 , Sridhar Kuchibhatla 1 , K. Kasarla 1 , P. Famouri 1 , D. Korakakis 1
1 Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, West Virginia, United States
Show AbstractAluminum nitride (AlN) possesses several material properties, including temperature and chemical stability, which make it promising for use in harsh environments. Furthermore, AlN maintains its piezoelectric properties at higher temperatures than more commonly used materials, such as Lead Zirconate Titanate (PZT) [1,2], making AlN attractive for high temperature MEMs and NEMs applications including, but not limited to, high temperature sensors and actuators, micro and nano channels for fuel cell applications, and micromechanical resonators. This work presents a novel AlN micro-channel fabrication technique using Metal Organic Vapor Phase Epitaxy (MOVPE). AlN easily nucleates on dielectric surfaces due to the large sticking coefficient and short diffusion length of the aluminum species resulting in a high quality polycrystalline growth on typical mask materials, such as silicon dioxide and silicon nitride [3,4]. The fabrication process introduced involves partially masking a substrate with a silicon dioxide striped pattern and then growing AlN via MOVPE simultaneously on the dielectric mask and exposed substrate. A buffered oxide etch is then used to remove the underlying silicon dioxide and leave a free standing AlN micro-channel. The width of the channel can be varied from 5 μm to 110 μm and the height of the air gap from 130 nm to 800 nm indicating the stability of the structure. Furthermore, this versatile process has been performed on (111) silicon, c-plane sapphire, and gallium nitride epilayers on sapphire substrates. The channels grown on GaN epilayers exhibit higher crystalline quality as confirmed by Reflection High Energy Electron Diffraction (RHEED) measurements, but are under more stress, as expected, than those on the sapphire or silicon substrates as there is observable curvature in the channel when viewed via Scanning Electron Microscopy (SEM). The effect of stress on the AlN micro-channels’ crystallinity and corresponding piezoelectric response will be discussed by investigating various substrates and structures engineered to further improve the material quality on the surface of the channel. Substrates include AlN, GaN, and AlxGa1-xN alloys with varying Al concentrations, x. [1] R. E. Eaton, C. A. Randall, T. R. Shrout, and S. E. Park. Jpn. J. Appl. Phys. 41, 2099-2104, 2002.[2] D. A. Stubbs and R. E. Dutton. JOM. 48, 29-31, 1996.[3] T. Katona, P. Cantu, S. Keller, Y. Wu, J. Speck, S. DenBaars. Appl. Phys. Lett. 84, 5025-5027, 2004.[4] Y. Kwaguchi, G. Sugahara, A. Mochida, T. Shimamoto, A. Ishibashi, Y. Yokogawa. Phys. Stat. Sol. (C) 0, 2107-2110, 2003.
9:00 PM - Q9.29
Use of Gallium Nitride for a Photonic Crystal-based, Enhanced Fluorescence Biomolecule Detection System.
J. Nightingale 1 , R. Tompkins 2 , B. Farmer 1 , O. Myers 2 , X. Cao 1 , T. Myers 2 , J. Dawson 1 , D. Korakakis 1 , L. Hornak 1
1 Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, West Virginia, United States, 2 Department of Physics, West Virginia University, Morgantown, West Virginia, United States
Show AbstractWe propose the use of gallium nitride (GaN) for the realization of an integrated optofluidic biosensor that employs a photonic crystal slab system for resonance-enhanced fluorescence emission detection of biomolecules. The sensor design utilizes an enlarged air hole defect microcavity tuned for resonance with a fluorescing biomolecule to enhance fluorescence and detection. While other optical sensing platforms often rely on the weak, decaying evanescent tail of a totally internally reflected electromagnetic wave to probe an analyte, photonic crystal biosensor designs instead offer attractive sensing possibilities through the use of spatially localized resonant cavity modes. A common biosensor design based on photonic crystal defect cavities focuses on the small spectral shifts seen in the transmission spectrum of the structure when analyte molecules bind to the photonic crystal’s air hole pores. While these designs offer high sensitivity, their selectivity requires complex internal surface modification and introduction of probe molecules that allow only for specific binding of desired target molecules.
To circumvent the need for surface modification chemistry, we propose the concept of using precisely tuned photonic crystal defect cavities to enable fluorescence enhancement and detection of fluorescing biomolecules. Enlarged air hole, acceptor state defects are introduced in a GaN photonic crystal slab by creating larger openings in which to pass fluorescing biomolecules of interest and creating high quality factor (Q) microcavities with discrete resonant frequencies. By engineering the size of the defect cavity, the allowable frequency of the cavity can be tuned in resonance with the fluorescence resonant energy transfer (FRET) event in target molecules. As the target molecule passes through the pumped defect cavity, its fluorescence at the resonant cavity frequency within the photonic band gap will create enhanced, spatially localized fluorescence that will allow for enhanced biomolecule detection. The biomolecule can be excited at its FRET pump frequency using frequencies within the allowable air or dielectric frequency bands of the photonic crystal. The use of a GaN photonic crystal core allows for the sensor to be used at visible wavelengths. Simulation results indicate that fluorescence intensity enhancement values of over 400 are achievable using high-Q defect cavities in GaN. This fluorescence enhancement potentially enables the detection of single molecule events.
In this paper, the design and fabrication considerations of this device are investigated and theoretical device characteristics are determined using frequency and time domain simulations. Additionally, the growth and etching of polarity-inverted GaN structures for obtaining nanopatterns with high aspect ratios is discussed.
9:00 PM - Q9.3
Brazing of B4C to B4C Using Ag-Based Filler Metal.
Ehsan Rezabeigi 1 , Ali Hadian 1
1 material & metallurgy, University of Tehran, Tehran, Tehran, Iran (the Islamic Republic of)
Show Abstract9:00 PM - Q9.30
Preparation of AlN Films by Means of CVD Using Iodide Source Under Atmospheric Pressure.
Hiroki Iwane 1 , Naoki Wakiya 1
1 , Shizuoka University, Hamamatu Japan
Show AbstractAlN thin films were prepared on (0001) sapphire substrate by chemical vapour deposition using auminum iodide (AlI3) and ammonia (NH3) resources under atmospheric pressure. Crystal orientations, surface microstructure, composition of the films and optical properties were investigated. Under optimized conditions, epitaxial AlN films were deposited and AlN film thickness was 1μm.
9:00 PM - Q9.31
Energy Absorbing Materials.
Ben Coapes 1
1 , AWE Plc, Reading United Kingdom
Show AbstractEnergy absorbing materials have many commercial applications, most notably in the automotive industry where they are used for both occupant and pedestrian protection. In general such materials are required to absorb relatively low quantities of kinetic energy. We are investigating the possibility of using materials to absorb a combination of heat and kinetic energy. For heat absorption, the materials chosen need to have a high melting point as well as appropriate heat capacity and solid density; for kinetic energy absorbing, the materials need to be made porous to dissipate energy through 'crush' strength. We present here the initial results on the use of boron nitride and silicon carbide to absorb energy. Both BN and SiC have been made into porous spheres (~25-35% porosity) using a novel and relatively simple sol-gel process. Rapid energy absorbing properties have been measured in gas-gun impact experiments and compared to quasi-static compression, both of which are presented here. ©British Crown Copyright 2007/MOD.
9:00 PM - Q9.32
Preparation of InN by Means of AP-HCVD Using In Buffer Layers.
Hiroaki Yokoo 1 , Naoki Wakiya 1
1 , Shizuoka University, Hamamatu Japan
Show AbstractUsing atmospheric pressure halide chemical vapor deposition (AP-HCVD), we have investigated the effect of In buffer layer on the crystal quality and electrical property of InN layer.It is found that the In buffer layer have a dramatic effect on the crystal quality and morphology.The Hall mobility and carrier concentration in the film was 178cm2/V and 1.0×1020cm-3, respectively.
9:00 PM - Q9.33
Analysis of Leakage Current Origins in Blue Light-Emitting Diodes.
Seong-Eun Park 1 , Jaewoong Han 1 , Hun Jae Chung 1 , Jungja Yang 1 , Ki-Ho Park 1 , Grigory Onushkin 1 , Hyunjung Lee 1 , Bae Kyun Kim 1 , Heeseok Park 1 , Cheolsoo Sone 1 , Yongjo Park 1
1 Corporate R&D Institute, Samsung Electro-Mechanics Co., Ltd., Suwon, Gyunggi-Do, Korea (the Republic of)
Show Abstract9:00 PM - Q9.4
Contact Characterizations of ZrN Thin Films Obtained by Reactive Sputtering.
Joshua Pelleg 1 , Assaf Bibi 1 , Michael Sinder 1
1 Materials Engineering, Ben Gurion University of the Negev, Beer Sheva Israel
Show AbstractThe contact properties of ZrNx on p-type Si obtained by magnetron reactive sputtering were investigated. Schottky diode characteristics were observed as determined by forward current-voltage (I-V), and capacitance voltage (C-V) measurements. The zero-bias barrier heights evaluated by I-V were in the range of 0.55-0.63V, which is higher than the value of 0.53 V of as deposited amorphous TiN1. The I-V curve is not ideal due to the presence of interfacial layer and interface states which are located at the ZrN-semiconductor interface. It is likely that n is controlled by the interface state densityFor each diode investigated in the ZrN/Si system, the barrier heights measured by the C-V method are considerably higher than those evaluated by I-V technique. At this stage we cannot present a theoretical explanation for the difference between the values of The contact properties of ZrNx on p-type Si obtained by magnetron reactive sputtering were investigated. Schottky diode characteristics were observed as determined by forward current-voltage (I-V), and capacitance voltage (C-V) measurements. The zero-bias barrier heights evaluated by I-V were in the range of 0.55-0.63V, which is higher than the value of 0.53 V of as deposited amorphous TiN1. The I-V curve is not ideal due to the presence of interfacial layer and interface states which are located at the ZrN-semiconductor interface. It is likely that n is controlled by the interface state densityFor each diode investigated in the ZrN/Si system, the barrier heights measured by the C-V method are considerably higher than those evaluated by I-V technique. At this stage we cannot present a theoretical explanation for the difference between the values of the narrier heights beyond the speculations that the presence of interface states might be responsible for the difference. Since I-V measurements tend to emphasize the lower value of the barrier height while C-V measurements would give a value of barrier height averaged over the entire interface the more probable magnitude of the barrier height can be assumed to be those measured by I-V technique.1Joshua Pelleg, A. Douhin, J. Vac. Sci. Technol. B23 (2005) 178.
9:00 PM - Q9.5
Magnetocaloric Effects of Binary Rare Earth Nitrides.
Yusuke Hirayama 1 , Naofumi Kusunose 2 , Takashi Nakagawa 3 , Koji Kamiya 4 , Takenori Numazawa 4 , Takao Yamamoto 1
1 graduate school of engineering , Osaka university , Suista, Osaka, Japan, 2 Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, Japan, 3 Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguro, Tokyo, Japan, 4 Tsukuba Magnet Laboratory, National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractMononitirides of some rare-earth elements, Gd, Tb, Dy, Ho and Er, possess the NaCl type crystal structure and exhibit ferromagnetism with a Curie’s temperature Θ varying with atomic number in a range ca. 70 – 10 K. Binary compounds consisting of neighboring or next-neighboring these elements LnxLm1-xN (Ln, Lm : rare-earth) form solid solution without miscibility gap and have an intermediate lattice constant and also an intermediate Curie’s temperature Θ. The packing density of rare-earth atoms is significantly higher than those of metal and alloy of hcp structure, which leads to a high packing density of magnetic moments undergoing the ferro-para magnetic transition. This high density and this Θs range are very beneficial to magnetic refrigerants and/or regenerators for the general cryogenic cooling systems working around this temperature region. The magnetic refrigeration based on the magnetocaloric effect, MCE, is a promising cooling technology at cryogenic temperatures. Therefore, these nitrides are materials that would be applicable (i) to magnetic cooling of hydrogen from liquid nitrogen temperature to and below 20 K at which liquefaction occurs, and (ii) to a regenerator making use of the large magnetic specific heat accompanied with the transition. We have already synthesized mononitrides of the five elements and a few binary solid solutions thereof, Gd-Dy, Gd-Tb, Tb-Ho, and pointed out the potential as materials for magnetic refrigerant and regenerator. In this paper, we report on ΔS of HoxEr1-xN (x =0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) synthesized by the carbothermic reduction method. The magnetic entropy change, ΔS , which is an indicator of performance of MCE, was obtained from data sets of magnetization M (H, T) measured at various temperature, T, and magnetic field, H, through equations, ΔS=∫H0 (∂M/∂T)HdHand(∂S/∂H)T=(∂M/∂T)H. Thus obtained ΔS vs. T curves have peaks at temperatures depending on x in a range of 8–18 K. These peak plots are along a convex curve, which may indicate an interaction between the two components. These results are going to be discussed by comparing them with previously reported data including our results on specific heat measurements with and without magnetic field.
9:00 PM - Q9.6
Synthesis and Characterization of Oxynitride Perovskites LaAON2 (A=Nb,Ta) from Metal Precursors in Supercritical Ammonia.
Keisuke Tajima 1 , Jinwang Li 1 , Tomoaki Watanabe 2 , Nobuhiro Matsushita 1 , Masahiro Yoshimura 1
1 , Tokyo Institute of Technology, Yokohama Japan, 2 , Meiji, Kawasaki Japan
Show AbstractTransition metal oxynitrides have received much attention due to various possible applications especially in electronic devices for their unusual dielectric and optical properties, ionic conductivity, as well as their catalytic activity. In spite of the great potential in various applications, those oxynitrides have much less been investigated than the corresponding oxides in their syntheses, properties and applications. One major reason is that those oxynitrides are more difficult to be synthesized. Therefore, it is important to develop their synthetic techniques. In the present study, we have developed the synthesis of perovskite LaAON2 (A=Ta, Nb), potential photocatalysts for water splitting, by reacting metal precursors in ammonia under super critical conditions. Previously, these compounds were synthesized from oxide precursors in an ammonia flow, but the products rarely showed the expected properties due to their low crystallinity. In our present study, we confirmed the phase purity of the oxynitrides by X-Ray diffraction (XRD). The morphology of the obtained products was investigated by scanning electron microscopy (SEM). The thermal stability was investigated by thermgravimetric analysis (TG). The dependence of crystallinity, crystallite size, photocatalytic activity, etc., of the products on the experimental parameters (temperature, reaction time, additives, etc.) has been studied. The results will be discussed in comparison with the previous ones in the literature.
9:00 PM - Q9.7
Mechanical Properties of Rhenium Diboride, an Ultra-Incompressible, Superhard Material.
Jonathan Levine 1 , Hsiu-Ying Chung 1 2 , Michelle Weinberger 1 , Robert Cumberland 1 , Abby Kavner 3 , Jenn-Ming Yang 2 , Sarah Tolbert 1 , Richard Kaner 1 2
1 Chemistry and Biochemistry, UCLA, Los Angeles, California, United States, 2 Materials Science and Engineering, UCLA, Los Angeles, California, United States, 3 Earth and Space Sciences, UCLA, Los Angeles, California, United States
Show AbstractNew superhard materials are rarely created without the use of high-pressure synthetic methods that require gigapascals of applied pressure. Here, we report the mechanical properties of superhard rhenium diboride synthesized via arc-melting under ambient pressure. The microhardness of crystalline ReB2 was determined using a Vickers indenter, resulting in an average hardness of 48 gigapascals under an applied load of 0.49 N. A bulk modulus of 360 gigapascals was calculated from in situ high-pressure x-ray diffraction measurements and radial diffraction experiments indicate that ReB2 is able to support a remarkably high differential stress. These results are discussed with respect to the structure and bonding of ReB2. Potential mechanisms for increasing the hardness of ReB2 will also be covered.
9:00 PM - Q9.8
Carrier Rremoval Rate in Electron Irradiated AlxGa1-xN Grown by Molecular Beam Epitaxy.
Mo Ahoujja 1 , S. Elhamri 1 , Y. Yeo 2 , M. Hogsed 2 , R. Henegehold 2
1 Physics, University of Dayton, Dayton, Ohio, United States, 2 ENP, AFIT, WPAF-B, Ohio, United States
Show AbstractThe effects of electron irradiation on the carrier concentration in Si doped AlxGa1-xN samples with Al mole fraction in the range of x = 0 to 0.30 grown by radio-frequency plasma activated molecular beam epitaxy on sapphire substrates were investigated by variable temperature dependent Hall-effect. Both the carrier concentration and mobility of electrons in the conduction band were found to decrease significantly in both GaN and AlGaN following 1.0 MeV electron irradiation at a fluence of 1x10^17 cm-2. A measure of the effect of the electron irradiation is typically expressed in terms of the dose carrier removal rate, defined as η = dn/dΦ, where dn is the change in room temperature carrier concentration after an electron fluence of Φ. The dose-averaged carrier removal rate was found to depend foremost on the initial carrier concentration, no, regardless of the aluminum mole fraction. For 6.5x10^16≤ n_o ≤ 8.2x10^17 cm-3, the carrier removal rate shows a linear dependence on no given by η = (3.96x10^-18 n_o – 0.15) cm-1.This relationship is attributed to a process whereby nitrogen interstitials passivate shallow silicon donors by forming Si-Ni complexes.
9:00 PM - Q9.9
Effect of Structural Properties of AlN Templates on the Optical, Electrical, and Structural Properties of n-type AlGaN.
Wonseok Lee 1 2 , Kaixuan Chen 1 3 , Qi Dai 1 3 , Sameer Chhajed 1 , Martin F. Schubert 1 4 , Frank W. Mont 1 4 , Jong Kyu Kim 1 4 , Christian Wetzel 1 3 , E. Fred Schubert 1 3 4
1 Future Chips Constellation, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States, 4 Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show Abstract
Symposium Organizers
Ruediger Kniep Max-Planck-Institute for Chemical Physics of Solids
Francis J. DiSalvo Cornell University
Ralf Riedel Technische Universitaet Darmstadt
Zachary Fisk University of California
Yoshiyuki Sugahara Waseda University
Friday AM, November 30, 2007
Back Bay B (Sheraton)
9:30 AM - Q10.1
Molecular Design of Carbon-Rich Silicon Carbonitrides.
Gabriela Mera 1 , Ralf Riedel 1
1 Materials Science, Dispersive Solids, TU Darmstadt, Darmstadt, Hessen, Germany
Show AbstractPolymer-derived ternary SiCN ceramics are a new class of materials possessing oxidation and creep resistance up to exceptionally high temperatures,[1] properties which are increased when the ceramics are fabricated with a high content of excess carbon, as published in the case of SiCO materials.[2] The high-temperature resistance is also discussed in terms of the presence of nanodomains with 1-3 nm in size in the microstructure as shown by SAXS measurements.[3]In this work, carbon-rich silicon carbonitride ceramics were obtained by the pyrolysis of novel polysilylcarbodiimides. The preceramic polymers –[SiPhR-NCN]n- (R = -Ph, -Me, -H, -Vinyl) were synthesized by the reaction of bis(trimethylsilylcarbodiimide) with phenyl-substituted dichlorosilanes.[4] The polymers were insensitive against air and moisture and soluble in all common organic solvents. Raman investigations proved the presence of π-π van der Waals interactions of the phenyl-groups resulting in self-assembly of the polymer chains. This observation was underlined also by rheological measurements.The pyrolysis of the polymers was done up to 1600°C. The ceramics are amorphous up to 1500°C. The microstructure of the ceramics, the mechanism of pyrolysis and the ceramic yields depend strongly on the molecular structure of the polymers. The presence of phenyl-substituents yields an increased formation of free-carbon in the ceramics. The nanocluster-size of the free-carbon was determined by means of Raman spectroscopy. The presence of the significant amount of free-carbon leads to different properties for these SiCN ceramics. The enthalpy of formation obtained from calorimetric measurements was used to quantify the thermodynamic stability of C-rich SiCN PDCs.[5]The characterization of the carbon-rich polymers and their transformation to ceramics was done by means of STA (TG-DTA/MS), FT-IR, Raman, XRD, MAS NMR, SEM, EA and rheology and will be discussed in the presentation. The electronic and optical properties of the precursors annealed at different temperatures will be also reported in terms of the nanostructure and composition.References:[1] R. Riedel, G. Mera, R. Hauser, A. Klonczynski, J. Ceram. Soc. Japan 2006, 114 [6], 425-444.[2] H.-J. Kleebe, G. Gregori, F. Babonneau, Y. D. Blum, D. B. MacQueen, S. Masse, Int. J. Mat. Res. 2006, 97, 6, 699-709; G. Gregori, H.-J. Kleebe, Y. D. Blum, F. Babonneau, Int. J. Mat. Res. 2006, 97, 6, 710-720.[3] A. Saha, R. Raj, D. L. Williamson, H.-J. Kleebe, J. Am. Ceram. Soc. 2005, 88 [1], 232.[4] G. Mera, R. Riedel, F. Poli, K. Müller, P. Kroll, paper in preparation.[5] R. M. Morcos, A. Navrotsky, D. Ahn, F. Poli, K. Müller, G. Mera, R. Riedel, R. Raj, paper in preparation.
9:45 AM - Q10.2
Rare-earth Chloride Seeded Growth of GaN nano- and micro-Crystals.
Jaehui Ahn 1 , H. Kim 1 , M. Mastro 2 , J. Freitas 2 , R. Holm 2 , R. Henry 2 , C. Eddy Jr. 2 , J. Kim 1
1 Chemical & Biological Eng., Korea Univ., Seoul Korea (the Republic of), 2 Electronic Science and Technology Division, US Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractThe market for III-nitride optoelectronics will continue to expand well beyond the current two billion (US) dollar valuation. The III-nitride material system has proven its suitability for large-scale production of light emitted diodes, lasers and sensors. A promising yet still developing technology is nanoscale optoelectronics based on the III-nitride material system. In recent years, several issues have hampered the transition of nanoscale technologies into volume production. A major hindrance has been the difficulty in position control of nanorods and nanowires. A more specific requirement, is the necessity to develop a nanoscale fabrication technology that is compatible with existing Si industrial facilities. The problems are addressed by growing GaN crystals via an erbium chloride seed. This vapor-liquid-solid (VLS) growth technique produces GaN crystals with improved crystallinity as well as the ability to control the orientation of the GaN crystal. Additionally, the placement of the ErCl3 seed selectively determined the location of the GaN nanocrystal. Employing a chloride compound is necessary to lower the melting temperature of erbium below the growth process temperature as required for the VLS growth mechanism. In contrast to a traditional VLS mechanism, the seed material was selected for its high solubility in the host GaN matrix. This incorporation resulted in a GaN crystal with strong optical transitions based on the erbium ion. Erbium seeded GaN-crystals were grown for 1, 5, 10, 20 and 30 min on c-,a- and r-plane sapphire substrate. The growth temperature of samples was 900 oC. GaN-crystal controllably produced with distinct orientation based on the underlying substrate. A growth time of 1 min produced GaN-crystals that were tens of nanometers in length. The crystal length was proportional to growth time. Analysis by XRD revealed that this technique produced a-plane GaN-crystals on r-plane sapphire, mixed m- and c-plane GaN-crystals on a-plane sapphire, and c-plane GaN-crystal on c-plane sapphire. In photoluminescence, a dominant green emission was generated by the trivalent Erbium ion. The high crystal quality of GaN:Er material was confirmed by micro-Raman spectroscopy. This rare-earth chloride seeded growth process is a promising technique for creating nano-scale in-lane LED devices, particularly for integration with Si microelectronics.
10:00 AM - Q10.3
The Pressure-induced Phase Transition from B4 to B1 Structure Type in Pure, Defect GaN and InGaN/AlGaN Solid Solutions.
Salah Eddine Boulfelfel 1 , Dirk Zahn 1 , Yuri Grin 1 , Stefano Leoni 1
1 Chemistry, MPI CPfS Dresden, Dresden Germany
Show AbstractExploring high pressure polymorphism gives access to dense compounds with novel physical and chemical properties. To fully unfold the potential of high-pressure compounds a precise atomistic elucidation of the reasons governing phase stability is required. For this, an essential step is the determination of the true transition path. This quest is especially important for compound classes linking the same structure types (e. g. B1-B2 for alkali-metal halides or B4-B1 for group-III nitrides), as the transformation mechanism depends on the chemical nature of species involved. Motivated by the recent debate on the pressure-induced wurtzite-to-rocksalt (B4-B1) phase transition in semiconducting group-III nitrides (AlN, GaN and InN), we have undertaken a mechanistic investigation within a recent computational scheme that combines molecular dynamics and topological modeling [1-2]. Irreversible in AlN only, the B4-B1 transition may exhibit different pattern of phase growth at higher pressure in mixed Al-Ga-In-N systems. Our simulations provide a clear picture of the B4-B1 phase transition in pure and deficient GaN in terms of nucleation and growth. The mechanism is identified as a two-step transformation through a metastable intermediate (wurtzite to tetragonal and tetragonal to rocksalt) [3]. The transition is initiated by the formation of nucleation centers that quickly develop into a 2D interfacial slab. Subsequent growth completely transforms the wurtzite structure into an intermediate tetragonal one. Afterwards, quasi-collective atomic displacements lead to the rocksalt type structure. Cation substitution with Al or In differently affects the mechanism. While a preference to nucleate the tetragonal phase at In-substituted sites appears, Al sites are more reluctant to forming the tetragonal intermediate and damp the reactivity of the interfacial slab. On substituting In for Ga the mechanistic details are preserved with a clear tendency to form islands in the B4 structure. Stepwise substitution of Ga by Al up to 5% significantly lowers the stability of the tetragonal intermediate up to its complete disappearance and establishment of a distinct transformation route. Combining Al, Ga and In in one nitride opens thus new perspectives to explore the stability of high-pressure polymorphs. GaN is an example of a trend of reactivity with respect to mechanisms, metastable intermediates and nucleation patterns that is clearly emerging for binary semiconducting materials [4]. 1. D. Zahn, S. Leoni, Phys.Rev.Lett., 2004, 92, 250201.2. S. E. Boulfelfel, D. Zahn, O. Hochrein, Yu. Grin and S. Leoni, Phys. Rev. B, 2006, 74, 094106.3. S. E. Boulfelfel, D. Zahn, Yu. Grin and S. Leoni, submitted4. S. E. Boulfelfel, Yu. Grin, S. Leoni, in preparation.
10:15 AM - **Q10.4
Rare-earth Activated Nitride-based Luminescent Materials:Interesting from a Scientific Point of View, Relevant from an Application Point of View.
H.T. Bert Hintzen 1
1 Dept. of Chemical Eng. and Chemistry, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractDuring the last decade several novel rare-earth activated nitride-based luminescent materials have been discovered, with improved or unprecedented properties as compared to traditional phosphors. An overview will be given of nitride-based phosphor materials, their crystal chemistry, synthesis methods, characterization and luminescence properties. Composition-structure-property relationships will be dealt with for amorphous (glasses) as well as crystalline (powders, ceramics) materials. Examples will be given of new applications (e.g. as conversion phosphors in white LEDs) or potential applications (e.g. as spectral conversion materials in solar cells), illustrating that the future of this promising class of materials is bright.
10:45 AM - Q10.5
Structure, Composition, and Defect Analysis of AlN on 6H-SiC Seeds.
Jharna Chaudhuri 1 , Luke Nyakiti 1 , Peng Lu 2 , James Edgar 2 , Peng Li 3
1 Mechanical Engineering, Texas Tech University, Lubbock, Texas, United States, 2 Chemical Engineering, Kansas State University, Manhattan, Kansas, United States, 3 Earth and Planetary Science, University of New Mexico, Albuquerque, New Mexico, United States
Show AbstractTransmission electron microscopy (TEM) study was performed to investigate the interface region of AlN/6H-SiC. Thick AlN layers were grown on a 3.5° off-axis 6H-SiC seeds at a temperature of 1790 °C for 100 hours by sublimation-recondensation method. Lattice images of cross-sectional TEM samples show a transition region exists at the interface. The EDAX measurement indicated considerable amount of aluminum and nitrogen present in the transition layer. Detailed investigation is underway to determine nature of the transition layer. The interface between AlN and the transition layer is abrupt with dislocations of Burgers vector running parallel to the interface (i. e. (0001) plane).ACKNOWLEDGMENTThis work was supported by the NSF grant #DMR 0408774 and # DMR0515858.
11:30 AM - Q10.6
Magnetic Properties of Rare Earth Nitrides Thin Films.
Claire Meyer 1 3 , Andrew Preston 1 , J. Zhong 1 , B. Ruck 1 , S. Granville 1 , G. Williams 2 , H. Trodahl 1
1 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington New Zealand, 3 Institut Néel, CNRS and Université Joseph Fourier, Grenoble France, 2 , Industrial Research Ltd., Lower Hutt New Zealand
Show AbstractThe active research on highly spin-polarised materials for spintronic integration has recently caused a revival of interest in rare earth nitrides (RN). Despite the appearance of new experimental and theoretical results about their electronic structure, controversies still remain. As an example, band structure calculation predicted half-metallic ground state for the ferromagnetic GdN [1]. However, our work on a highly stoichiometric GdN thin film has shown it to be a ferromagnetic semiconductor, in agreement with a revised band calculation [2,3]. The nitrogen concentration and the lattice volume seem to be key parameters that need to be well controlled in order to understand the band structure and the magnetic interactions within the whole RN series. The present work is focused on the magnetic behaviour of some of them (R=Sm, Dy, Er). The thin films were prepared by vacuum evaporation of the metal in a N2 atmosphere [2,4]. The typical thickness is 200-400 nm. The magnetic measurements were performed on GaN capped samples, with a SQUID magnetometer. All compounds are magnetically ordered at low temperature. DyN exhibits a ferromagnetic behaviour below Tc=25K, with a coercive field of 0.2T at 5K. For ErN, a small hysteresis loop is found below 6K. A non-reversible thermal variation of the magnetisation after cooling in zero field allows the determination of the Curie temperature at Tc=7K±0.5K. The temperature dependence of the susceptibility is dominated by the cubic crystal field effects, which lift the degeneracy of the Er3+ J=15/2 multiplet. The whole behaviour is in qualitative agreement with a previous calculation performed on ErN, suggesting a Gamma 8 quadruplet ground state and an overall splitting larger than 300K. The magnetisation does not saturate under 6T as expected for a polycrystal with strong magnetic anisotropy. For SmN, our experimental results are in agreement with a previously suggested antiferromagnetic behaviour: a sharp maximum is obtained in the low field temperature variation of the magnetisation at TN=30K±1K. In the magnetically ordered phase, the magnetisation process in a 6T applied field is complex, suggesting a metamagnetic behaviour.[1] C.M. Aerts et al, Phys. Rev. B 69, 045115 (2004)[2] S. Granville et al, Phys. Rev. B73, 235335 (2006)[3] P. Larson et al, Phys. Rev. B75, 045114 (2007)[4] J. Zhong et al, this conference
11:45 AM - Q10.7
Hard Chemistry Going Soft: A Metastable Modification of Ca3N2.
Peter Hoehn 1 , Stefan Hoffmann 1 , Jens Hunger 1 , Stefano Leoni 1 , Fabian Nitsche 1 , Rüdiger Kniep 1
1 , MPI-CPfS, Dresden Germany
Show AbstractThe structural chemistry of alkaline earth nitrides is dominated by just two structure types: insulating cubic AE3N2 (AE = Be [1], Mg [2], Ca [3]) and metallic AE2N (AE = Ca[4], Sr[5], Ba [6]). Other phases (Ca11N8 [7], γ-Ca3N2 [8]) have been proven to be cyanamide-nitrides, Ca11N6[CN2]2 [9] and Ca4N2[CN2] [9,10], respectively.Whereas alkaline earth azides are best prepared by reaction from sodium azide and alkaline earth halides, both, nitrides and diazenides are typically synthesized from the elements under various reaction conditions and sometimes at elevated pressures.To prepare metastable (m-)Ca3N2, freshly distilled Ca is heated (50 K/h) in a closed system (p0(N2) = 1200 mbar) while monitoring the N2 pressure. Heating is stopped at the onset of the reaction (typically around 700 K) indicated by a decrease of pressure, resulting in samples containing up to 95 % m-Ca3N2 together with cubic Ca3N2 making up the balance. Annealing for an extended time, heating to temperatures above 900 K or using “old” Ca metal (surface often passivated by oxidation products) prevents the presence of m-Ca3N2 in the reaction products.The crystal structure of black, brittle m-Ca3N2 (trigonal P -3c1, # 165, a = 618.94(1) pm, c = 1661.15(2) pm) is comprised of a network of NCa6/4 octahedra sharing common edges and vertices. It crystallizes in a subgroup variant of the corundum structure (R -3c) and is best described as a filled BiI3 type structure with the layers stacked in the sequence ABCA’B’C’ along [001].DTA/TG investigations under both, N2 and Ar, show that m-Ca3N2 transforms monotropically without any detectable mass change to cubic Ca3N2 (heating rate 10 Kmin-1, Ttrans. ~ 810 K). Chemical analysis confirms the composition (Ntheo/obs 18.89 % / 18.3(3) %); impurities (C, O) were not detected. IR and Raman investigations feature no bands above 600 cm-1, thereby indicating the absence of species with N-N or N-H bonds as well.The transformation mechanism of m-Ca3N2 to cubic Ca3N2 is analyzed by molecular dynamics investigations.References:[1]O. Reckeweg, C. Lind, A. Simon, F. J. DiSalvo, Z. Naturforsch. 58B (2003) 159.[2] J. David, Y. Laurent, J. Lang, Bull. Soc. Fr. Mineral. Cristallogr. 94 (1971) 340.[3] Y. Laurent, J. Lang, M. T. le Bihan, Acta Crystallogr. B24 (1968) 494.[4]J.-F. Brice, J.-P. Motte, J. Aubry, Rev. Chim. Miner. 12 (1975) 105.[5] N. E. Brese, M. O'Keeffe, J. Solid State Chem. 87 (1990) 134.[6] O. Reckeweg, F. J. DiSalvo, Z. Kristallogr. NCS 220 (2005) 519.[7]Y. Laurent, J. Lang, M. Th. Le Bihan, Acta Cryst. B25 (1969) 199.[8]Y. Laurent, Rev. Chim. Miner. 5 (1968) 1019.[9]O. Reckeweg, F. J. DiSalvo, Angew. Chem. Int. Ed. 39 (2000) 412.[10]P. Hoehn, R. Niewa, R. Kniep, Z. Kristallogr. NCS 215 (2000) 323.
12:00 PM - Q10.8
On Reactions between Alkali Metals and Active Nitrogen.
Grigori Vajenine 1
1 , Max-Planck-Institut, Stuttgart Germany
Show Abstract12:15 PM - Q10.9
Nitridoaluminosilicate CaAlSiN3 and its Derivatives - Theory and Experiment.
Masayoshi Mikami 1 , Hiromu Watanabe 1 , Kyota Uheda 1 , Naoto Kijima 1
1 Fundamental Technology Laboratory, Research and Development Division, Mitsubishi Chemical Group Science and Technology Research Center, Inc., Yokohama Japan
Show AbstractNitridoaluminosilicate MAlSiN3(M: alkaline-earth element) and its derivatives have attracted more and more attention owing to the fact that the material doped with rare-earth element has intense body color and exhibit efficient luminescence under InGaN diode irradiation. In particular, red phosphor, Eu-doped CaAlSiN3 (CASN), has good temperature dependence of luminescence and sufficient chemical durability for white LED use. [1-2] Still, for the lineup of various kinds of white color, it is more desirable to tune the wavelength of the red luminescence with other physical/chemical properties kept as possible. Thus the derivatives with different chemical compositions have been intensively explored so far. For the feasibility of such chemical composition change, it is necessary to understand its atomic/electronic structure of the unique crystal, which is a distorted AlN-based wurtzite superstructure (Cmc21, No.36) with Al and Si disordered on 8b site and Ca occupying 4a site. Recently, we have performed first-principles band calculation of CASN and clarified the origin of the Al/Si disorder configuration as well as the feasibility of the virtual crystal approximation (VCA) of heterovalent cations (Al and Si) for the reproducibility of atomic/electronic structure of CASN. [3]As a natural extension of the previous study, we have investigated some CASN-derivatives to confirm/predict the crystal structures. The VCA allows us to model the superstructure with various chemical compositions quite easily. In this work, we will present two examples of solid-solution, (Ca,Sr)AlSiN3 and CaAlSiN3-Si2N2O. The agreement between experiment and theory appears quite satisfactory. It is emphasized that the crystal structure of SrAlSiN3 has been successfully predicted by first-principles calculation prior to experimental result. The collaboration of experiment and theory promises us “crystal-engineering” to develop new nitrides/oxynitrides effectively and efficiently. [1] K.Uheda et al., Electrochem. Solid-State Lett. 9, H22 (2006).[2] K.Uheda et al., Phys. Stat. Sol. (a) 203, 2712 (2006).[3] M.Mikami, K.Uheda and N.Kijima, Physica Status Solidi (a) 203, 2705 (2006).
12:30 PM - **Q10.10
Ternary Nitrides with Thermo Electrical and Optical Properties and Related Silicon Nitride-based Composites.
Z. Lences 1 , P. Sajgalik 1 , T. Plachky 1 , L. Kipsova 1 , Y. Zhou 2 , K. Hirao 2 , R. Riedel 3
1 Institute of Organic Chemistry, Slovak Academy of Sciences, Bratislava Slovakia, 2 , National Institute of Advanced Industrial Science and Technology, Nagoya Japan, 3 Institute of Materials Science, Darmstadt University, Darmstadt Germany
Show AbstractAlmost single-phase MgSiN2 powder has been obtained from Si/Mg2Si/Si3N4 or LaSi/Si/Si3N4 mixtures by direct nitridation. The TG-DTA analysis indicated slightly different reaction paths for the two systems. Consequently, a modified stepwise heat treatment schedule including a nucleation period C has been adopted for both systems. The content of single metallic substances in the starting powder should not exceed the following ratios to obtain single-phase MgSiN2 in an unsealed reaction system: Mg2Si/Mg ≥ 3 and Si3N4/Sitot ≥ 0.5. Thermal conductivities of dense ternary nitrides were from 28.1 and 4.3 WM.m-1K-1 for MgSiN2 and LaSi3N5, respectively. The dielectric properties of MgSiN2 and electric properties LaSi3N5 (band gap) will be discussed in detail. Both ternary nitrides have luminescent character.Reaction bonded Si3N4 materials with the addition of ternary nitrides and polymer derived SiOC/SiNC ceramics have thermal conductivities in the range of 110 - 138 and 50-72 W.m-1K-1. Samples with SiOC/SiNC additives had also good creep resistance. The reaction bonding process and post-sintering heat treatment conditions had to be further optimised.
Friday PM, November 30, 2007
Back Bay B (Sheraton)
2:30 PM - Q11.1
Defect Formation and Anisotropy in Non-polar GaN.
Roland Kroeger 1 , Tanya Paskova 1
1 University of Bremen, Insitute of Solid State Physics, Bremen Germany
Show AbstractBulk GaN layers grown with a non-polar growth surface are desirable as substrates for nitride based light emitting devices such as laser diodes. A non-polar growth surface significantly improves the device efficiency, which is hampered by the built-in dipole field if a polar growth plane is used. As it turns out the growth of layers and bulk material along a non-polar axis by metal-organic vapor phase (MOVPE) epitaxy or hydride vapor phase epitaxy (HVPE) on heterosubstrates such as SiC or sapphire is accompanied by the formation of dislocations, stacking faults and voids. This observation hampers the application of these materials and is therefore motivating a detailed investigation of the character and origin of the observed defects. Commonly observed defect patterns in a-plane or m-plane nitride films differ significantly from those found in films with c-plane growth orientation. For the non-polar growth surfaces a strong impact of the anisotropic elasticity properties on the growth is found with respect to the lattice mismatch accomodation by the introduction of misfit dislocations at the film/substrate interface. In order to study the impact of anisotropy on the defect formation in a-plane GaN films deposited on r-plane sapphire transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies were performed. The interfacial structure was investigated by high resolution TEM revealing the anisotropic introduction of dislocations with Burgers vector components in the {0002} and the {1-100} planes almost completely relaxing the 1.2% misfit in the [1-100] direction and the 16 % misfit in the [0001] direction with respect to the GaN.Experimental data for the defect characterization were compared with numerical solutions obtained for dislocation line energies, stresses and strains based on the anisotropic linear elasticity theory. The stress fields of Frank-Shockley partial dislocations, bounding the frequently observed basal plane stacking faults, are found to be significantly stronger and highly anisotropic compared to the stress fields calculated based on the isotropic elasticity theory. It is suggested that these findings are connected to the commonly found anistropic strain in such layers as observed by X-ray diffraction.Furthermore, the experimentally observed average basal plane stacking fault length of several hundred nanometers was compared with values obtained based on the assumption of a dislocation split reaction as source for the stacking fault formation, resulting in a value of about 30 nm. The findings reveal that the stacking fault formation can not be explained by a dislocation split reaction and the anisotropic elastic properties of non-polar GaN significantly affect the defect formation process and hence essentially affect the quality of the layers.
2:45 PM - Q11.2
Evolution of Surface Morphology of GaN Thin Films During Photoelectrochemical Etching.
Jacob Leach 1 , Umit Ozgur 1 , Hadis Morkoc 1
1 Electrical Engineering, Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractThe evolution of the surface morphology of unintentionally doped and Si-doped GaN samples subjected to photoelectrochemical (PEC) etching in the carrier-limited regime in aqueous KOH is reported. It was found that a nanoporous structure precedes whisker formation in samples in which high densities of whiskers ultimately form. Increasing the light intensity accelerated the rate of change of the surface morphology, and increasing the molarity of the KOH had no effect on the etching. Thus, the surface morphology tends to only depend on parameters of the starting layers, as well as how much etching in total has occurred. The identification of variations in surface morphology at different times during PEC etching of GaN may have utility in that assorted nanopatterns of GaN surfaces can be intentionally achieved in a controllable, large-scale, and inexpensive manner.
3:00 PM - Q11.3
Defect Selective Etching of Thick AlN layers Grown on 6H-SiC Seeds – a Transmission Electron Microscopy Study.
Luke Nyakiti 1 , Jharna Chaudhuri 1 , Ed Kenik 2 , Peng Lu 3 , James Edgar 3
1 Mechanical Engineering, Texas Tech University, Lubbock, Texas, United States, 2 Oak Ridge National Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 Chemical Engineering, Kansas State University, Manhattan, Kansas, United States
Show Abstract3:15 PM - Q11.4
Phase Transformation of Sputtered TaxNy Thin Films as a Function of Nitrogen and Doping Element Concentrations.
Brian Coss 1 , F. Aguirre-Tostado 1 , R. Wallace 1 , J. Kim 1
1 , University of Texas at Dallas, Richardson, Texas, United States
Show AbstractTraditionally, tantalum nitride (TaN) has been used in semiconductor interconnects, memory applications, diffusion barrier layers, and in a variety of other Si device applications. Recently TaN has been extensively evaluated as a metal gate electrode due to its mid-gap properties on CMOS structures comprised of a high-k hafnium based dielectrics. Additionally, recent studies have reported that the work function of a TaN gate can be tuned closer to the conduction band edge of silicon by selectively doping the metal gate/high-k interface with rare-earth metal elements, which is necessary for 35 nm technology node and beyond. The aim of this work was to create a suitable TaN thin film for evaluation of its properties as a metal gate candidate for NMOS. A suitable TaN gate is one with low sheet resistance (~5 Ω/sqr), mostly stoichiometric, thermally stable, and is consistently reproducible. Thin TaN films (~50 – 200 nm) were deposited using reactive ion sputtering in an UHV cluster tool with an analysis chamber and sputter chamber connected in situ. The deposition was carried out at 5 mTorr and 4.4 W/cm2, with varying concentrations of nitrogen gas. Results of the depositions show a clear trend of as-deposited TaN phases as a function of N2 flow rate which is characterized by x-ray diffraction (XRD), x-ray photoelectron spectra (XPS) data as well as four-point probe measured sheet resistance. As shown is Fig. 1, above ~3% N2 flow rate, the diffraction pattern is the FCC structure of TaN with peak indices of (111), (200), and (220) (JCPDS Card #00-049-1283, see Fig. 1). As nitrogen content is reduced the (220) peak disappears and the (200) and (111) features merge and broaden (see Fig. 3). This is the onset of hexagonal TaN (JCPDS Card#00-039-1485, see Fig. 2). Further reduction in N2 flow rate results in several co-existing as-deposited phases, hexagonal TaN, hexagonal Ta2N (JCPDS Card# 00-026-0985) and BCC Ta[N] (JCPDS Card #00-004-0788, see Fig. 2) . The stoichiometry for the various N concentrations was obtained from XPS analysis (see Fig. 4 and 5). Fully crystallized patterns of each are presented along with XPS and resistivity data (see Table 1) that supports the XRD conclusions. In addition, we will discuss effects of doping elements, such as La and Al, on bulk properties and related electrical properties.