Tingkai Li Hunan Gongchuang Photovoltaic Science & Technology Co., Ltd.
Michael Mastro U.S. Naval Research Laboratory
Armin Dadgar Otto-von-Guericke-Universitaet Magdeburg
Hongxing Jiang Texas Tech University
Jihyun Kim Korea University
O1: III-Nitride LEDs
Tuesday AM, November 29, 2011
Room 304 (Hynes)
9:00 AM - O1.1
Absence of Electron Accumulation at the InN(11-20) Cleavage Surfaces.
Holger Eisele 1 , Sarah Schaafhausen 2 , Andrea Lenz 1 , Aizhan Sabitova 2 , Lena Ivanova 1 , Mario Daehne 1 , Y. Hong 3 , Shangjr Gwo 3 , Philipp Ebert 2 Show Abstract
1 Institut für Festkörperphysik, Technische Universität Berlin, Berlin Germany, 2 Peter Grünberg Institut, Forschungszentrum Jülich, Jülich Germany, 3 Department of Physics, National Tsing Hua University, Hsinchu Taiwan
InN in principle opens up the possibility of using only one ternary III-V semiconductor alloy (InGaN) in optoelectronic devices to cover the whole visible spectral range. Despite this, key material properties of InN are still under debate. The intrinsic energetic position of the Fermi level is unclear, i.e., whether the Fermi level is located within the fundamental band gap or shifted slightly into the conduction band. The latter case induces electron accumulation at the surfaces of the crystal. This electron accumulation is typically observed at InN surfaces upon air contact, raising the question whether it is an intrinsic material property or not?In order to probe bulk properties by STM and not only contamination or surface effects, a clean and stoichiometric, cross-sectional surface is necessary. This can be achieved by cleaving InN along non-polar planes. To analyze the origin of the different electronic states in detail, we investigated the clean non-polar (11-20) cleavage surface using cross-sectional scanning tunneling microscopy (XSTM) and spectroscopy (XSTS). Using combined XSTM and XSTS we were able to locate an InN layer grown on top of a Si(111) substrate. XSTS spectroscopy on InN(11-20) cleavage surface yield normalized conductivity spectra, where three contributions to the tunneling current can be observed: (i) the contribution from the conduction band density of states for biases above the conduction band minimum at +0.3 V, (ii) a defect induced current, dominating the spectra between biases of 0 and -0.4 V, and (iii) a valence band related tunneling current rising at a bias of about -0.4 V and dominating the spectrum for biases below. The defect induced current arises from semi-filled defect states being present at the surface steps, and probably also from other (point) defects at the surface. Within the bulk band gap of Eg = 0.7 eV no intrinsic surface states could be observed. Furthermore, the Fermi level pinning at about 0.3 eV below the conduction band minimum indicates the absence of an electron accumulation layer. The results illustrate that electron accumulation at InN surfaces is not a universal property on InN. For clean stoichiometric cleavage surfaces no electron accumulation is observed. Thus, electron accumulation results primarily from the details of the surface structure and is not an intrinsic property of the material InN.
9:15 AM - O1.2
Visible-Color-Tunable Light-Emitting Diodes.
Young Joon Hong 2 3 1 , Chul-Ho Lee 2 3 , Gyu-Chul Yi 3 , Aram Yoon 4 , Miyoung Kim 4 , Han-Kyu Seong 5 , Hun Jae Chung 5 , Cheolsoo Sone 5 , Yong Jo Park 5 Show Abstract
2 Mateirals Science and Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of), 3 Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of), 1 Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo Japan, 4 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 5 , Samsung LED Co. Ltd., Suwon Korea (the Republic of)
We report on monolithically integrated full-color, tunable light-emitting diodes (LEDs) with electroluminescent color that changes continuously from red to blue by adjusting the external electric bias. High-efficiency full-color light sources are required for mobile device displays with high brightness and low power consumption. For full-color display applications, inorganic compound semiconductors have many advantages over organic materials, including their high carrier mobility and radiative recombination rate as well as their long-term stability and reliability. However, conventional inorganic thin-film LEDs emit only a single color that is determined by quantum-well layer thicknesses and compositions. To overcome this obstacle, we utilized multi-facetted GaN nanorod arrays with InGaN/GaN multiple quantum wells (MQWs) anisotropically formed on the nanorod tips and sidewalls. For various electroluminescence (EL) colors, current injection paths were controlled through a continuous p-GaN overlayer in the nanorod-embedded thin-film p−n junction structure by the applied current. The mechanism of the color tunability in the nanorod-embedded LED was explored by cross-sectional transmission electron microscopy, presenting the spatially graded distribution of the thickness and composition of 3-dimensional InGaN/GaN MQWs that can produce full-color ELs. By altering the applied voltage, the electric current was forced to travel through layers of different thickness and composition, thus changing the color of EL. Monolithically integrated red, green, and blue LEDs on a single substrate, operating at a fixed drive current, are also demonstrated for inorganic full-color LED display applications.
9:30 AM - O1.3
First Demonstration of InGaN/GaN Based Blue Light Emitting Diodes Grown on 8-Inch Diameter Si (111) Substrates.
Jun-Youn Kim 1 , Hyun-Gi Hong 1 , Yeonhee Kim 1 , Suhee Chae 1 , Youngjo Tak 1 , Jae Kyun Kim 1 , Jae Won Lee 1 , Hyoji Choi 1 , Junghun Park 1 , Bokki Min 1 , Bokki Min 1 , Youngsoo Park 1 , U-In Chung 1 Show Abstract
1 , Samsung electronic company, Yongin Korea (the Republic of)
We have grown LED structures on top of a robust n-type GaN template on 8-inch diameter silicon substrates achieving both a low dislocation density and a thick crack-free thickness even at a sufficient Si doping condition. The n-type GaN template consisted of AlN layer, which acts as a nucleation layer and barrier layer of Ga-Si eutectic reaction. The transition layer which consists of AlGaN layers and the unique epitaxial structure which consists of the dislocation reduction layer and stress compensation layers were then grown to control the stress and reduce the dislocation, simultaneously. After that, over 3 μm-thick Si doped GaN layer with 4.5×1018 cm-3 doping concentration has been grown successfully without any cracking. The crystalline quality of n-type GaN templates on Si substrates was evaluated by high resolution x-ray diffraction (HR-XRD) rocking curve. The full width at half maximum (FWHM) of a symmetric (0002) and an asymmetric (10-12) ω-scan were 280 arcsec and 380 arcsec, respectively. On top of n-GaN layer, 20 pairs of InGaN(2nm)/GaN(2nm) layers for the effective current spreading and stress releasing, a five-period multi-quantum-well (MQW) active region consisting of 3 nm-thick InGaN wells and 4.5 nm-thick GaN barriers, AlGaN layer for blocking electron overflow and p-GaN cap layers were grown. The emission wavelengths were 436 +- 4 nm from the mapping of LED wafer by Photoluminescence (PL). The convex wafer bow during MQWs growth steps results in shorter emission wavelength around a hotter wafer edge. The internal quantum efficiency (IQE) of the InGaN/GaN MQWs grown on Si wafers was measured by temperature- and power-dependent PL method. We used a 405 nm-wavelength laser as an excitation source for in-well pumped PL measurement. The IQE of our samples, which was evaluated from the ratio of PL efficiency obtained at 10 K and 300 K, was over 65%. After the deposition of Ag based p-metal on the front side of LED structure, it was transferred from the mother Si substrate to a new Si substrate as a submount by eutectic wafer-to-wafer bonding. Then, the mother Si substrate was removed by lapping and successive etching. Comparing with the costly laser-lift-off (LLO) process which is typically applied to manufacture vertical LEDs (V-LEDs) on sapphire, this process can provide easier and more reliable result to fabricate the high-power V-LED chips on Si. Next, the transferred LED structure was etched until the Si-doped GaN was exposed. An n-contact was formed on the exposed N-face n-GaN and each device was isolated from its neighbors by reactive ion etching. Finally, in order to increase light extraction efficiency, the top of surface was etched for 20 min with KOH solution kept 50 °C. Optical output power measurements of various sized V-LED chips have been performed in the integrating sphere at different injection currents under un-encapsulated condition. World first results of InGaN/GaN LED on 8-inch Si substrates will be presented.
9:45 AM - O1.4
High-Performance Semipolar (20-2-1) InGaN/GaN Light-Emitting Diodes.
Yuji Zhao 1 , Shinichi Tanaka 2 , Chih-Chien Pan 2 , Chia-Yen Huang 2 , Kenji Fujito 3 , Daniel Feezell 2 , James Speck 2 , Steven DenBaars 1 2 , Shuji Nakamura 1 2 Show Abstract
1 ECE, University of California, Santa Barbara, Santa Barbara, California, United States, 2 Materials, University of California, Santa Barbara, Santa Barbara, California, United States, 3 Optoelectronics Laboratory, Mitsubishi Chemical Corporation, Ushiku, Ibaraki, Japan
Semipolar and nonpolar orientations of group III-Nitrides have attracted considerable attention for realizing high-efficiency light-emitting diodes (LEDs) and laser diodes (LDs). Several advantages over commercially available c-plane structures have been reported, including reduced polarization-induced electric fields in the quantum wells (QWs), increased indium uptake, and polarized light emission. The former characteristics are promising for achieving high-performance green emitters, while the latter characteristic contributes to anisotropic optical gain in LDs fabricated on these planes. The relative magnitude of the intensity parallel to and perpendicular to the c-axis is described by the polarization ratio and high values are preferred for improved LD performance. In this work, we report high optical polarization ratios for LED devices on the semipolar (20-2-1) plane, which is inclined at 15° and 30° toward the [000-1] direction from the m-plane and (20-21) plane, respectively. Using integrated electroluminescence measurements, the polarization ratio was 0.67 at 519 nm and 20 mA for (20-2-1) devices with an active area of 0.1 mm2. Comparable devices of a similar wavelength on (20-21) showed a polarization ratio of 0.34. To further examine the performance of LEDs on the (20-2-1) plane, a series of samples with 15 pair InGaN/GaN superlattice structures were grown on (20-2-1), (20-21) and m-plane substrates in a “co-loaded” experiment and characterized by XRD analysis. The indium composition of devices on the (20-2-1) plane (6.5 %) was nearly twice that of those on the (20-21) plane (3.3 %), and also higher than those on the m-plane (2.7 %). This is advantageous for improved crystal quality for long-wavelength structures on the (20-2-1) plane. A high-performance blue-violet LED operating with low droop up to 200 A/2 was also fabricated on the (20-2-1) plane. At a forward current of 20 mA, the LED showed a peak external quantum efficiency of 52% and an output power of 30.6 mW, which are comparable to the best values ever reported for semipolar and nonpolar LEDs. At higher current densities, the LED also showed outstanding performance, with droop ratios of 0.7% at 35 A/cm2, 4.3% at 50 A/cm2, 8.5% at 100 A/cm2, and 14.3% at 200 A/cm2. To the author’s knowledge, such a low droop has not been reported at current densities as high as 200 A/cm2. In summary, LEDs fabricated on the (20-2-1) plane have demonstrated higher optical polarization ratio, higher indium composition than LEDs fabricated on the (20-21) plane. We have also fabricated a high-power blue-violet semipolar (20-2-1) LED operating up to 200 A/cm2 with an EQE above 50% and remarkably low droop. These results suggest that the (20-2-1) orientation may provide benefits for high-performance LEDs in blue and green spectral region.
10:00 AM - O1.5
Incorporation of Colloidal Metallic Nanocrystals into InGaN/GaN MQWs: Bringing Together Top-down and Bottom-up Approaches in Order to Enhance Light Emission.
Sergio Pereira 1 , M. Martins 1 , T. Trindade 1 , A. Llopis 2 , Arup Neogi 2 , A. Krokhin 2 , Ian Watson 3 Show Abstract
1 Physics and Chemistry/CICECO, University of Aveiro, Aveiro Portugal, 2 Physics, University of North Texas, Denton, Texas, United States, 3 Institute of Photonics, University of Strathclyde, Glasgow United Kingdom
During the last few years, worldwide research has focused on various approaches to produce nanostructured semiconductors. The different approaches used to fabricate such nanostructures usually emerge from distinct scientific backgrounds, for example physicists mainly favour top-down approaches while chemists prefer bottom-up methods. This dichotomy of approach results in a technology gap between the different methods used to produce nanostructured materials, namely physical deposition, such as metal-organic vapor-phase epitaxy (MOVPE), and wet chemical synthesis. By way of example, tuneable photonic structures may be produced by using either nanofabrication or wet chemistry. However combinations of such approaches, as complementary realms of activity, might contribute to achieve novel functional nanomaterials and also give new scientific insights of a more fundamental kind; bridging the gap will open the way to the engineering of material systems that can offer radically new properties in particular through the exploitation of cross-coupling effects. In order to reach such synergetic integration we take advantage of specific surface features that arise in technologically relevant semiconductors for optoelectronics, namely group-III nitride and oxide semiconductors. In this contribution we report on the effects of incorporation of colloidal Au nanocrystals (NCs) at the surface of light emitting InGaN/GaN Multiple Quantum Well (MQW) heterostructures on its optical properties. For this purpose we exploit spontaneously formed Inverted hexagonal pits (IHPs) at the surface of InGaN/GaN MQWs to create well-defined assemblies of gold NCs on an optically and electrically active substrate [S. Pereira et al, Advanced Materials 20 (5), 1038 (2008) ]. Such incorporation of metallic NCs into InGaN/GaN IHPs results into a remarkable enhancemet of light emission which reaches about 60% enhancement on the photoluminescence emission and whose origin is not due to (but can be combined with) plasmonic coupling. We will briefly discuss the electrostatic mechanism behind this effect, which provides a new and exciting perspective for improving the efficiency of broad-band light emitters and controlling carrier concentration on the nanoscale.
10:15 AM - **O1.6
Reducing the Cost of Ownership: MOCVD Advances for GaN LED’s and AsP CPV Technologies.
Eric Armour 1 Show Abstract
1 Turbodisc Division, Veeco Instruments, Somerset, New Jersey, United States
As compound semiconductors make inroads into common electronic devices, it remains important to continue to lower the cost of the primary MOCVD epitaxial deposition, which creates the foundation for the devices. Both GaN-based LED and AsP-based CPV markets have been focused on simultaneous cost-reduction, cycle time reductions, and device efficiency improvements, which can be achieved utilizing high growth rates and higher operating pressures. To achieve these goals, it has become increasingly important to understand the underlying growth mechanisms that drive the chemistry within the MOCVD process.In this presentation, I will discuss our experiments on understanding the optical, electrical and physical material properties associated with high growth rate GaN and GaAs, along with high pressure InGaN growth regimes. In all cases, there are tradeoffs that need to be made to achieve good crystal quality with abrupt interfaces, smooth surface morphology, and good minority carrier properties. While exceptional device performance has been achieved for both LED’s and CPV cells, it is primarily cost that is limiting full-scale adoption of compound semiconductors into these potentially enormous markets.
11:15 AM - O1.7
A Defect-Based Mechanism for Efficiency Droop in Nitride Light Emitting Diodes.
N. Modine 1 , A. Armstrong 1 , M. Crawford 1 , W. Chow 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Efficiency droop is a serious concern in InGaN/GaN light emitting diodes (LEDs) in which the radiative efficiency decreases as the current through the device increases. Droop is widely believed to be associated with a non-radiative recombination mechanism that increases with carrier concentration faster than the approximately quadratic dependence of radiative recombination. In modeling nitride LEDs, defect-induced recombination is often assumed to depend linearly on the carrier concentration. However, this is not generally true. Many defects in semiconductors have multiple charge states and therefore multiple defect levels. Any given defect can only be in one of its charge states at a given time, and changes in charge state are associated with the capture or emission of carriers. As the carrier concentration increases, the predominant charge state of the defect can shift, opening up new defect levels for recombination. We will show that such multilevel defects can induce recombination that has a highly non-linear dependence on carrier concentration. Furthermore, using a microscopic InGaN/GaN LED model, we will show that a multilevel defect with plausible properties (concentration, defect levels, and capture cross-sections) can reproduce the essential features of the experimentally observed droop phenomenon for InGaN/GaN LEDs in the absence of Auger recombination.This work was supported by Sandia’s Solid-State Lighting Science Energy Frontier Research Center, sponsored by the Department of Energy Office of Basic Energy Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.
11:30 AM - O1.8
Optimization of the Optical and Electrical Properties of GaN Vertical Light Emitting Diode with Current Block Layer.
Na Lu 1 , Zhiqiang Liu 3 2 , Enqing Guo 2 , Liancheng Wang 2 , Andrew Melton 3 , Ian Ferguson 3 Show Abstract
1 Engineering Technology , University of North Carolina at Charlotte, Charlotte , North Carolina, United States, 3 , Chinese Academy of Science , Beijing China, 2 Electrical and Computing Engineering , University of North Carolina at Charlotte , Charlotte , North Carolina, United States
The light emitted from the MQW area under the Metal electrode in light emitting diode (LED) cannot escape to free space. To save this part of energy, a structure called current block layer (CBL) is used in LEDs. Current cannot pass through CBL, so there is no light emitted in this area, and more currents will flow to other area to produce light. In this study, current block layer is used in GaN vertical light emitting diodes, and a new method to fabricate schoottly CBL was illustrated. Optical and electrical tests of the LEDs were carried out. The results show that vertical LEDs with CBL and LEDs without CBL not only have different light output power at the same operation currents, but also show different efficiency droop curve. Key words: current block layer; efficiency droop; vertical LED;
11:45 AM - O1.9
Fabrication of Large-Area Graphene-Based Transparent Conductive Electrode for UV LEDs.
Byung-Jae Kim 1 , Chongmin Lee 1 , Younghun Jung 1 , Jihyun Kim 1 Show Abstract
1 Deprtment of Chemical and Biological Engineering, Korea University, Seoul Korea (the Republic of)
Graphene has been recently highlighted for the applications in optoelectronics, sensors and nanoelectronics due to its superior properties, which include excellent thermal and electric conductivity and high transmittance from UV to NIR. Indium tin oxide (ITO) has been widely used as the transparent conductive electrode in LEDs. However, ITO has some issues such as high cost, instability in base or acid and poor transmittance in UV region. Therefore, graphene can be an attractive alternative to ITO for the application of transparent conduct