bluuebunny/crossref_metadata_2025_split huggingface dataset:
Photoluminescence investigations on a native donor in ZnO
Authors: Meyer Bruno K. , Lautenschläger S. , Graubner S. , Neumann C. , Sann J. 
Published: April 2011

ABSTRACTThe shallow donor impurities in ZnO with binding energies between 46 and 56 meV have been studied in great detail in the recent years. They give rise to neutral donor bound exciton recombinations with the A- and B-valence bands, show rotator states and two-electron-satellite transitions. These properties allowed to establish the excited state splittings of the donors as well as confirming Hayne's rule in ZnO. So far they all seem to be of extrinsic origin, hydrogen, aluminum, gallium and indium in order of increasing binding energy. For many years it was common sense that intrinsic defects would dominate the n-type conductivity of ZnO. Interstitial zinc as well as oxygen vacancies should be double donors, and in order to contribute to the n-type-conduction they should have shallow levels, and low formation energies to be abundant. In PL-measurements at T∼100 K on various ZnO samples, single crystals as well as thin films, a luminescence around 3.31 eV was detected. Due to its line shape and temperature behaviour it is identified as bound-to-free recombination. If we assume that the 3.31 eV band with its level at EC ∼ 130 meV is the ++/+ level of the zinc interstitial we calculate for the binding energy of the +/0 level ∼ 130 meV, i.e. around 33 meV. Undoped Zn-rich epitaxial films grown by CVD show a dominant I3 recombination at 3.367 eV which according to Haynes rule is consistent with a shallow donor level at 33 meV. Moreover, they have free n-type carrier densities of 2×1019 cm−3 and as revealed by SIMS the common donor impurities (Al, Ga, In) cannot account for the high carrier densities.
Laser Assisted Molecular Beam Deposition of High Mobility Zinc Oxide
Authors: Li Meiya , Chokshi Nehal , DeLeon Robert L. , Tompa Gary , Anderson Wayne 
Published: April 2011

ABSTRACTWe have utilized laser assisted molecular beam deposition (LAMBD) to deposite zinc oxide (ZnO) at room temperature. The production of ZnO films via LAMBD utilizes an excimer laser induced ablation plume of zinc in combination with pulsed oxygen gases to create a molecular beam of ZnO clusters. The deposited films have been characterized structually and electrically in an as deposited state as well as after post deposition excimer laser annealing. The films undergo a clear structural change from a nanoparticle like film to either a microgranular film or a smooth continuous film depending upon the laser annealing power.Al ohmic contacts were made to both as-deposited and laser-annealed ZnO thin films. Thickness and refractive index, chemical composition, and surface morphology of the thin films were analyzed by using ellipsometry, electron spectroscopy for chemical analysic (ESCA), and feild emission scanning electron microscopy (FESEM), respectively. Dark current-voltage (I-V), DC photo I-V, Hall-effect, and photoluminescence measurements were employed for testing the device performance.In some cases, the ZnO thin film was changed from n-type into p-type after laser annealing, and photoconductive behavior was clearly seen on the laser-annealed samples, with values of 2.8 × 10−4 Ω−1. Also, both samples with and without laser annealing show near-band emission at ∼3.3 eV. Furthermore, the full width at half maximum (FWHW) values of the laser-annealed LAMB ZnO film was reduced from 20.02 nm or 21.68 nm to 18.2 nm when compared with that of non-laser annealed samples. This indicates that the laser annealing provided an improved stochiometric quality and crystallinity to the ZnO thin films.
Czochralski Crystal Growth of Zinc Oxide-Tellurium Oxide System
Authors: Nawash Jalal M. , Lynn Kelvin G. 
Published: April 2011

ABSTRACTCzochralski technique was employed in an attempt to grow a single crystal of the system ZnO-TeO2. A good quality grown crystal is expected to be transparent with a very light yellow color. The crystals exhibit a high resistivity of the order of 1013 Ω-cm. Different mole percentages have been tested for growth. Several attempts were performed to pull a single crystal. It was found that the best mole percentage of ZnO-TeO2 is 35.5:64.5. The pulled material grows uniformly, such that necking and conning are noticed. The pulled material contained multiple single crystals which were isolated and studied. Each one of them was transparent. Some properties will be presented. The pulled material of the 40:60 ZnO-TeO2 mole percentage returned four phases. Two of these phases are very uncommon.
Manipulation of ZnO Nanowire by Low-Temperature Solution Approach
Authors: Lin Chia-Hsin , Cheng Syh-Yuh , Lin Ren-Jay , Wang Yi-Hui 
Published: April 2011

ABSTRACTA catalyst free, structure-induced heterogeneous nucleation and direct growth of ZnO nanowires on organic and inorganic substrates was prepared by low-temperature solution approach process. The experimental results showed that ZnO nanowires could be directly synthesized upon the concave of substrate without any pre-seeding. In this work ZnO nanowires were grown on both polystyrene bead layer and physical-grinded wafer substrate. ZnO nanowires with a broad aspect ratio of 10−2 ∼ 102 was controlled mainly by adjusting of reactant concentration and pH state of solution. A needle-like ZnO nanotip were also prepared by a two-step limited growth condition as a result that tip diameter is several nanometers only, which may be highly in favor of the field emission. Structure-induced heterogeneous nucleation and growth facilitates the fabrication of ZnO nanowires as the potential photoeletronic units in field-emission displays.
Sn-based Group-IV Semiconductors on Si: New Infrared Materials and New Templates for Mismatched Epitaxy
Authors: Tolle John , Roucka Radek , D'Costa Vijay , Menendez Jose , Chizmeshya Andrew , Kouvetakis John 
Published: April 2011

ABSTRACTWe report growth and properties of GeSn and SiGeSn alloys on Si (100). These materials are prepared using a novel CVD approach based on reactions of Si-Ge hydrides and SnD4. High quality GeSn films with Sn contents up to 20%, and strain free microstructures have been obtained. The lattice mismatch between the films and Si is relieved by Lomer edge dislocations located at the interface. This material is of interest due to the predicted cross-over to a direct gap semiconductor for moderate Sn concentrations. We find that the direct band gap, and, consequently, the main absorption edge, shifts monotonically to lower energies as the Sn concentration is increased. The compositional dependence of the direct band gap shows a strong bowing, such that the direct band gap is reduced to 0.4 eV (from 0.8 eV for pure Ge) for a concentration of 14% Sn. The ternary SiGeSn alloy has been grown for the first time on GeSn buffer layers. This material opens up entirely new opportunities for strain and band gap engineering using group-IV materials via decoupling of strain and composition. Our SiGeSn layers have lattice constants above and below that of pure Ge, and depending on the thickness and composition of the underlying buffer layer they can be grown relaxed, with compressive, or with tensile strain. In addition to acting as a buffer layer for the growth of SiGeSn, we have found that GeSn can act as a template for the subsequent growth of a variety of materials, including III-V semiconductors.
Deep Ultraviolet Light Emitting Diodes with Emission below 300 nm
Authors: Khan M. Asif 
Published: April 2011

AbstractIn this paper we will describe the problems in growth and fabrication of deep UV LED devices and the approaches that we have used to grow AlGaN-based multiple quantum well deep UV LED structures and to overcome issues of doping efficiency, cracking, and slow growth rates both for the n- and the p-type layers of the device structures. Several innovations in structure growth, device structure design and fabrication and packaging have led to the fabrication of devices with emission from 250-300 nm and cw-milliwatt powers at pump currents of only 20 mA (Vf ≤ 6 V). Record wall plug efficiencies above 1.5 % are now achievable for devices with emission at 280 nm. Thermal management and a proper device design are not only key factors in achieving these record performance numbers but are also crucial to device reliability. We will also discuss some of our initial research to clarify the factors influencing the lifetime of the deep UV LEDs. In addition to our own work, we will review the results from the excellent research carried out at several other laboratories worldwide.
Effects of the high-refractive index SiNx passivation on AlGaN/GaN HFETs with a very low gate-leakage current
Authors: Kambayashi Hiroshi , Wada Takahiro , Ikeda Nariaki , Yoshida Seikoh 
Published: April 2011

AbstractWe have reported on the stress effects of the SiNx passivation film on AlGaN/GaN heterojunction field effect transistors (HFETs). The AlGaN/AlN/GaN heterostructure was grown using a metalorganic chemical vapor deposition (MOCVD). We first investigated the dependences between SiNx films and the refractive index of these. As a result, the stress type of the SiNx films was all Tensile. Moreover, the refractive index of SiNx increased, the stress was decreasing and almost invariable over refractive index 2.1. Furthermore, we fabricated some kinds of AlGaN/GaN HFETs and estimated the gate-leakage current and the current collapse of HFET. As a result, we confirmed the relationship between the stress of passivation film and gate-leakage current, and that the low stress SiNx film with a high-refractive index can suppress both a gate-leakage current and a current collapse.
Properties of InN grown by High-Pressure CVD
Authors: Alevli Mustafa , Durkaya Goksel , Woods Vincent , Habeck Ute , Kang Hun , Senawiratne Jayantha , Strassburg Martin , Ferguson Ian T. , Hoffmann Axel , Dietz Nikolaus 
Published: April 2011

AbstractGroup III-nitride compound semiconductors (e.g. AlN-GaN-InN) have generated considerable interest for use in advanced optoelectronic device structures. The fabrication of multi-tandem solar cells, high-speed optoelectronics and solid state lasers operating at higher energy wavelengths will be made possible using (Ga1-y-xAlyInx)N heterostructures due to their robustness against radiation and the wide spectral application range. To date, the growth of indium rich (In1-xGax)N films and heterostructures remains a challenge, primarily due to the large thermal decomposition pressures in indium rich group III-nitride alloys at the optimum growth temperatures. In order to control the partial pressures during the growth process of InN and related alloys, a unique high-pressure chemical vapor deposition (HPCVD) system with integrated real-time optical monitoring capabilities has been developed. We report initial results on InN layers grown at temperatures as high as ∼850°C with reactor pressures around 15 bar. Such process conditions are a major step towards the fabrication of indium rich group III-nitride heterostructures that are embedded in wide band gap group III-nitrides. Real-time optical characterization techniques are applied in order to study the gas phase kinetics and surface chemistry processes during the growth process.For an ammonia to TMI precursor flow ratio below 500, multiple phases with sharp XRD features are observed. Structural analysis perform by Raman scattering techniques indicates that the E2 high mode improves as NH3:TMI ratio is decreased to below 500. Optical characterization of these InN layers indicates that the absorption edge shifts from down from 1.85 eV to 0.7 eV. This shift seems to be caused by a series of localized absorption centers that appear as the indium to nitrogen stoichiometry varies. This contribution will correlate the process parameters to results obtained by XRD, Raman spectroscopy and optical spectroscopy, in order to assess the InN film properties.
Growth and characterization of semipolar InGaN/GaN multiple quantum wells and light-emitting diodes on (10 11) GaN templates
Authors: Chakraborty Arpan 
Published: April 2011

AbstractInGaN/GaN MQW samples were grown by metal organic chemical vapor deposition (MOCVD) on (10-1-1) oriented GaN templates. Effects of growth temperature and reactor pressure on the photoluminescence (PL) properties were investigated. The emission intensity improved significantly when the QWs were grown at 100 Torr, compared to higher pressure growths. The effect of well-width on the luminescence properties was investigated and an optimum well width of 40 Å was determined. Excitation dependent PL measurements revealed no shift in the PL emission wavelength suggesting the absence of electric field in the quantum wells. Furthermore, LEDs fabricated on (1011) GaN templates, emitting at 439 nm, showed no shift in the EL emission wavelength with the increase in drive current, reconfirming the absence of polarization.
Origins of Parasitic Emissions from 353 nm AlGaN-based UV LEDs over SiC Substrates
Authors: Park Ji-Soo , Fothergill Daryl W , Wellenius Patrick , Bishop Seann M. , Muth John F. , Davis Robert F. 
Published: April 2011

AbstractThe effects of p-GaN capping layers and p-type carrier-blocking layers on the occurrence of parasitic emissions from 353 nm AlGaN-based LEDs have been investigated. LEDs without a p-type Al0.25Ga0.75N carrier-blocking layer showed a shoulder peak at ∼370 nm due to electron overflow into the p-Al0.10Ga0.90N cladding layer and subsequent electron-hole recombination in the acceptor levels. Broad emission between 380 and 450 nm from LEDs having a p-GaN capping layer was caused by 420 nm luminescence from the p-GaN capping layer, which was optically pumped by 353 nm UV emission from the quantum wells. Broad, defect-related luminescence at ∼520 nm was emitted from the AlGaN layers within the quantum wells.
Comparison of the Electroluminescence of Blue and Deep-UV Light-Emitting Diodes at Elevated Temperatures
Authors: Cao Xian-An , Stecher T. , LeBoeuf S. 
Published: April 2011

AbstractThe performance of InGaN and AlGaN-based blue (465nm) and deep ultraviolet (UV) (280 nm) light-emitting diodes (LEDs) at elevated temperatures (25-175 °C) were investigated. As a result of uniform high-Al content AlGaN alloys yielded by migration-enhanced metalorganic chemical vapor deposition, the deep-UV LED showed dominant band-edge emission, much smaller alloy broadening and weaker localization effects as compared to the InGaN LED. Strong carrier localization was retained in the blue LED up to 175 °C, leading to temperature-independent emission intensity at low-energy tails. The UV LED, however, showed a much more rapid decrease in light output with increasing temperature. The characteristic temperature was 37 K, compared to 270 K for the blue LED. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the deep UV LED and suggest that increasing carrier confining potentials will provide a critical means to improve its thermal stability.
Epitaxal growth of InGaN quantum dots grown by MOVPE: Effect of capping process on the structural and optical properties
Authors: Yamaguchi Tomohiro , Sebald Kathrin , Gutowski Juergen , Figge Stephan , Hommel Detlef 
Published: April 2011

AbstractThe surface morphology of thin InxGa1-xN layers in uncapped structures and the overgrowth using GaN or InxGa1-xN with a lower In content than the InxGa1-xN layer as the protection layer of the InxGa1-xN layer were investigated in addition to their dependence on growth temperature during the growth by metalorganic vapor phase epitaxy (MOVPE). It was necessary to decrease the growth temperature of the InxGa1-xN layer in order to realize homogeneous dots. In order to obtain a high luminescence efficiency, on the other hand, it was essential to increase the growth temperature of the protection layer. Sharp lines related to the localized electronic states, which are attributed to InGaN quantum dots, were observed by micro-photoluminescence (μ-PL) for the samples with the InxGa1-xN layers protected by the InxGa1-xN layers grown at 700∼740°C.
Raman scattering of self-assembled gallium nitride nanorods synthesized by plasma-assisted molecular beam epitaxy
Authors: Wang D. , Tin C.-C. , Williams J. R. , Park M. , Park Y. S. , Park C. M. , Kang T. W. , Yang W.-C. 
Published: April 2011

AbstractRaman scattering experiment was carried out to study material and electronic properties of the aligned GaN nanorods grown by plasma-assisted molecular beam epitaxy. Due to the small size of the nanorods and the relatively high power density of the laser illumination used in this investigation, local heating of the sample cannot be neglected. Special care has been taken to identify the frequency shift in the optical phonon peaks induced by local heating prior to any spectral analysis. The residual stress in the GaN nanorods was estimated by analyzing the frequency shift of its E2 Raman mode. It was found that the frequency of E2 mode is shifted by only 0.1 cm-1 comparing with the stress-free frequency, indicating a negligible residual stress in nanorods. Owing to aligned geometry of the nanorods and the back-scattering scattering geometry used in the experiment, A1(LO) Raman mode was collected. The free carrier concentration as well as electron mobility of the GaN nanorods were obtained by the line shape analysis of the coupled A1(LO) phonon-plasmon mode. The electron concentration and mobility of electron obtained from line shape analysis are 3.3×1017 cm-3 and 140 cm2/Vs, respectively. We presented a general method of better determining the electronic parameters of the GaN nanostructures via Raman scattering.
Visible Light-Emitting Diodes Grown by Plasma Assisted Molecular Beam Epitaxy on Hydride Vapor-Phase Epitaxy GaN Templates and the Development of Dichromatic (Phosphorless) White LEDs
Authors: Cabalu Jasper S. , Williams Adrian , Chen Tai-Chou P. , France Ryan , Moustakas Theodore D. 
Published: April 2011

AbstractMuch of the work on III-Nitride-based LEDs that has been published and applied commercially has been done using metal-organic chemical vapor deposition (MOCVD) as a method of film growth. We report on the growth and fabrication of visible light emitting diodes, by combining hydride vapor-phase epitaxy (HVPE) and rf plasma-assisted MBE (PAMBE) methods. Thick (∼7 μm to 10 μm) HVPE n+-GaN smooth and textured templates, were used as substrates for the growth of LED structures by rf-PAMBE. The active regions of the LED structures, which consist of InGaN/GaN MQWs, were grown using the pulsed nitrogen plasma technique leading to abrupt well and barrier interfaces as confirmed by x-ray diffraction (XRD) measurements. Using this method, we obtained InGaN/GaN MQWs whose room temperature photoluminescence (PL) spectra have a full width at half maximum (FWHM) of 12 nm (105 meV). Visible LEDs on smooth GaN templates emitting in the blue to green were produced with EL spectrum FWHM as narrow as 27 nm. On the other hand, white LEDs without the use of phosphor have been produced utilizing textured MQWs as the active region, a phenomenon we initially attribute to different incorporation of In on the different QW-planes. The growth and fabrication of these devices was preceded by detailed growth and doping studies of the various layers of the LED structure. These include detailed nucleation studies on (0001) sapphire substrates as well as identification of kinetic factors which lead to good crystalline-quality InGaN alloys and InGaN/GaN MQWs.
Normally Off-Mode AlGaN/GaN HFET with p-Type Gate Contact
Authors: Tsuyukuchi Norio , Nagamatsu Kentaro , Hirose Yoshikazu , Iwaya Motoaki , Kamiyama Satoshi , Amano Hiroshi , Akasaki Isamu 
Published: April 2011

AbstractA normally off-mode AlGaN/GaN heterostructure field effect transistor (HFET) using a p-type GaN gate was fabricated and their static properties were compared with those of HFET having a Schottky gate. HFET having a p-GaN gate contact shows a very low leakage current density of 18.2 μA/mm at VGS and VDS of 0 V and 20 V, respectively.
Flip Chip Mounting for Improved Thermal Management of AlGaN/GaN HFETs
Authors: Ji Hangfeng , Sarua Andrei , Kuball Martin , Das Jo , Ruythooren Wouter , Germain Marianne , Borghs Gustaaf 
Published: April 2011

AbstractWe report on a temperature study of flip-chip mounted multi-finger AlGaN/GaN heterostructure field effect transistors (HFETs) using micro-Raman spectroscopy and infrared (IR) thermography. Flip-chip mounting can be used to improve thermal device management, in particular, for devices grown on low thermal conductivity substrates such as sapphire. In this study, we compare two flip-chip mounted HFETs of different flip-chip contact bump layout designs and a non flip-chip mounted HFET. Both temperature measurements and 3D temperature simulations are presented. The results show that minimizing the distance between the bumps and the active area of the HFET is essential for obtaining a low device operating temperature.
Perturbation of charges in AlGaN/GaN heterostructures studied by nanoscale capacitance-voltage technique
Authors: Koley Goutam , Lakshmanan Lakshminarayanan 
Published: April 2011

AbstractPerturbation of charges at the surface and interface of AlGaN/GaN heterostructures has been studied by quantitative nanoscale capacitance-voltage (C-V) measurements. The nanoscale C-V curves were found to have different slopes in the forward and reverse directions. These measurements indicate a change in confinement of the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface depending on the direction of the dc voltage sweep during C-V measurements, which can be explained by surface state charging and discharging during the bias sweep. Under UV illumination, the density of the 2DEG increased significantly as inferred from the increase in threshold voltage of the nanoscale C-V scans, and no change in 2DEG confinement, depending on the direction of the bias sweep, was observed.