bluuebunny/crossref_metadata_2025_split huggingface dataset:
Growth of a-plane ZnO Thin Films on r-plane Sapphire by Plasma-assisted MBE
Authors: Xie Junqing Q. , Dong J. W. , Osinsky A. , Chow P. P. , Heo Y. W. , Norton D. P. , Pearton S. J. , Dong X. Y. , Adelmann C. , Palmstrøm C. J. 
Published: April 2011

ABSTRACTZnO thin films have been epitaxially grown on r-plane sapphire by RF-plasma-assisted molecular beam epitaxy. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies indicate that the epitaxial relationship between ZnO and r-plane sapphire is (1120)ZnO // (1102)sapphire and [0001]ZnO // [1101]sapphire. Atomic force microscopy measurements reveal islands extended along the sapphire [1101] direction. XRD omega rocking curves for the ZnO (1120) reflection measured either parallel or perpendicular to the island direction suggest the defect density anisotropy along these directions. Due to the small lattice mismatch along the ZnO [0001] direction, few misfit dislocations were observed at the ZnO/Al2O3 interface in the high-resolution cross-sectional TEM image with the zone axis along the ZnO [1100] direction.
Structural and optical properties of Zn1−xCoxO and ZnCo2O4 thin films
Authors: Samanta Kousik , Bhattacharya Pijush , Katiyar Ram S. , Iwamoto W. , Urbano R. R. , Pagliuso P. G. , Rettori C. 
Published: April 2011

ABSTRACTThin films of Co substituted ZnO and ZnCo2O4 were deposited on c-axis (0001) oriented Al2O3 substrates using pulsed laser deposition. The XRD results showed all the films were highly (002) oriented with a less intense peak of (311) for ZnCo2O4 thin film. Micro-Raman spectra of ceramic targets showed the modes related to wurtzite ZnO and spinel ZnCo2O4 structures. In thin films of Zn1−xCoxO no modes corresponding to ZnCo2O4 were detected. The intensity of E1(LO) and multiphonon peak at 584 and 540 cm−1 respectively, increased with increase in Co substitution. The optical absorption of the films showed that the band gap decreased with increase of Co concentrations at room temperature along with the sub-bandgap absorptions due to d-d transitions of Co2+. Similar sub-bandgap d-d transition was also observed in the absorption spectra ZnCo2O4 thin films. The highest saturated magnetization (0.2μB/Co) was obtained for 5%Co substituted ZnO.
Concentration and Defect Dependent Ferromagnetism above Room Temperature in Co Doped ZnO Films Prepared by Metalorganic Decomposition
Authors: Kharel P. , Sudakar C. , Lawes G. , Suryanarayanan R. , Naik R. , Naik V. M. 
Published: April 2011

ABSTRACTZn1−xCoxO (x = 0.0 – 0.047) thin films (thickness ∼0.5 – 1 μm) have been prepared on sapphire substrates using metalorganic decomposition (MOD) method. The X-ray diffraction and Raman scattering studies indicate films to be polycrystalline ZnO with wurtzite structure. The optical absorption spectra show an expected bandgap of ∼3.2 eV. The magnetization studies show that the as prepared films lack the room temperature ferromagnetic order, whereas the films when vacuum annealed at a temperature 500 – 600 °C acquire ferromagnetic ordering at room temperature. Further, the observed ferromagnetism (FM) appears only for a limited range of Co concentration, 0.03 < x < 0.10 (after heat treating in vacuum at 550 °C), and it reversibly disappears upon re-annealing in air. The data presented here seem to suggest that the appearance of ferromagnetic order is dictated by both the oxygen defects and the critical concentration of Co, and thus may lend support to a recent model proposed by Coey et al. [Nature Materials4, 173 (2005)].
Electrical Properties of Diamond MISFETs with Submicron-Sized Gate on Boron-Doped (111) Surface
Authors: Saito Takeyasu , Park Kyung-ho , Hirama Kazuyuki , Umezawa Hitoshi , Satoh Mitsuya , Kawarada Hiroshi , Liu Zhi-Quan , Mitsuishi Kazutaka , Okushi Hideyo 
Published: April 2011

ABSTRACTAn H-terminated-surface conductive layer of B-doped diamond on a (111) surface was used to fabricate a metal insulator semiconductor field effect transistor (MISFET) using CaF2, SiO2 or Al2O3 gate insulators and a Cu-metal stacked gate. For a CaF2 gate, the maximum measured drain current (Idmax) was 240 mA/mm and the maximum transconductance (gm) was 70 mS/mm, and the cut-off frequency of 4 GHz was obtained. For a SiO2 gate, Idmax and gm were 75 mA/mm and 24 mS/mm, respectively, and for an Al2O3 gate, these characteristics were 86 mA/mm and 15 mS/mm, respectively. These values are among the highest reported DC and RF characteristics for a diamond homoepitaxial (111) MISFET.
Optimization of Ga(In)NAs thin film growth by atomic hydrogen-assisted molecular beam epitaxy
Authors: Shimizu Yukiko , Miyashita Naoya , Okada Yoshitaka 
Published: April 2011

ABSTRACTThe effect of growth temperature on the crystal quality and optical properties of Ga(In)NAs films was investigated over a range of 340 ∼ 520 °C. We found that Ga(In)NAs films fabricated at lower growth temperatures generally result in an improved crystal quality. An XRD linewidth of as low as 45 arcsec was obtained for a 1 µm-thick Ga0.94In0.06N0.01As0.99 thin film grown at 380 °C. This is ∼1/2 of that grown at the conventionally-adopted growth temperature of 480 °C. After annealing, an improved optical property represented by a higher PL intensity compared to the conventional growth method (annealed, Tgrowth = 480 °C) was also obtained in the 1 µm-thick Ga0.94In0.06N0.01As0.99 thin film grown at low temperature of 380 °C.
Potentially Modulated GaAs/GaNAs/InGaAs Quantum Wells for Solar Cell Applications
Authors: Kobayashi Naoto , Sasaki Naoyuki , Okada Yoshitaka 
Published: April 2011

ABSTRACTWe have fabricated GaAs/GaNAs and GaAs/GaNAs/InGaAs MQWs introduced into the intrinsic region of GaAs p-i-n solar cells by atomic H-assisted RF-molecular beam epitaxy. Compared to the more conventional GaAs/InGaAs MQW cells, GaAs/GaNAs/InGaAs MQW cells exhibited improved external quantum efficiencies in the low energy region. This is due to the extension of band edge into longer wavelengths as achieved by use of GaNAs materials.
Chemical Vapor Deposition and Defect Characterization of Silicon Carbide Epitaxial Films
Authors: Chen Yi , Dhanaraj Govindhan , Chen Hui , Vetter William , Dudley Michael , Zhang Hui 
Published: April 2011

ABSTRACTSiC homo-epitaxial layers have been grown using a halide chemical vapor deposition (HCVD) process. The thermodynamic process of SiC CVD in SiCl4-C3H8-H2 gas system was studied using equilibrium model. The predicted growth rate decreases gradually with the increase in growth temperature, and this trend is consistent with our experimental results. Good quality epitaxial layers with low density of basal plane dislocations could be grown. Some elementary screw dislocations present in the substrate do not seem to be propagating into the epitaxial layer.
X-ray standing wave investigations of Si dopant incorporation in GaN
Authors: Siebert Michael , Schmidt Th. , Flege J. I. , Zegenhagen J. , Lee T.-L. , Figge S. , Hommel D. , Falta J. 
Published: April 2011

AbstractThe synchrotron radiation technique of x-ray standing waves (XSW), which allows to directly obtain structural and element-specific data, was successfully used for the investigation of the dopant site distribution in Si doped GaN films grown on (0001) sapphire substrates for the first time. The Si dopant concentration was chosen to 5×1018 cm-3 and 5×1019 cm-3. The measurements were performed on 300 nm thin doped films deposited on high-quality thick undoped GaN films. With this sample structure, influences of the Si dopant induced decrease of the crystalline quality on the XSW signal are suppressed. The XSW data are compared to those obtained from thick homogeneously doped GaN films. All XSW measurements were performed in (002) backscattering geometry. Independent of the dopant concentration, the results indicate that Si atoms are solely incorporated on substitutional Ga sites.
The Use of Cathodoluminescence in Gallium Nitride During Growth to Determine Substrate Temperature
Authors: Lee Kyoungnae , Schires Eric D , Myers Thomas H 
Published: April 2011

AbstractThe Use of Cathodoluminescence in Gallium Nitride During Growth to Determine Substrate Temperature Accurate measurement of the substrate temperature during growth of gallium nitride by molecular beam epitaxy is crucial. Typically, thermocouples are usually used to measure the temperature of the back side of block which is holding the substrate. Alternatively, pyrometers are often used. However, there is a big range of an error. In-situ cathodoluminescence (CL) occurring during reflection high energy electron diffraction is a strong candidate to determine the growth temperature. The electron beam supplied by our RHEED gun has an energy of 13keV which was used for each measurement. CL was easily detected up to and beyond typical growth temperatures. The CL was directed into a monochromator using fiber optics. The final signal was detected with a photomultiplier tube. This technique appears quite useful to accurately and reproducably determine substrate temperature during growth.The CL could also be observed using a ccd camera. Thus, we investigated using the CL to image the sample during growth. This could be used to see temperature inhomogenaities, and potentially to map alloy composition fluctuations. We calibrated the wavelength vs. growth temperature by using narrow band-pass interference filters. Background subtraction with blanking the e-beam could be used to remove black body radiation and other undesired sources of light. For gallium nitride, the photon energy at the growth temperature of 750°C is about 3.0eV. Using different filters, we can take a picture of e-beam on the surface of substrate for each filter and analyze the peak intensity using the line profile. We will present CL images of various samples at differing temperatures.This work was supported by the AFOSR MURI F49620-03-1-0330 monitored by Todd Steiner and Gerald Witt and by ONR Grants N00014-02-1-0974 and N00014-01-1-0571, both monitored by Colin E. C. Wood.
III-N Epitaxial Growth for Nitride Devices
Authors: Dupuis Russell , Chung Theodore , Lee Wonseok , Li Peng , Limb Jae , Ryou Jae-Hyun , Yoo Dongwon 
Published: April 2011

AbstractVarious GaN-based device structures were grown on (0001) sapphire and 6H-SiC substrates by metalorganic chemical vapor deposition. The device structures of this study include a variety of p-n junction-based devices, such as InGaN/GaN multiple-quantum-well green light emitting diodes, GaN p-i-n vertical rectifiers, and GaN/InGaN heterojunction bipolar transistors. This paper describes the epitaxial growth and device performance characteristics of these device structures. We have developed state-of-of-the-art growth techniques for the materials that are critical for high-performance electronic and optoelectronic devices. High-performance InGaN HBTs, high-voltage GaN rectifiers and long-wavelength green LEDs have been epitaxially grown, fabricated, and characterized. The details of the material growth, device fabrication, and device characterization will be presented.
Al mole fraction dependence of deep levels in AlGaN/GaN-HEMT structures estimated by CV profiling
Authors: Kikawa Junjiroh , Katsuhiro Imada , Tomoyuki Yamada , Tadayoshi Tsuchiya , Yuichi Hiroyama , Masayuki Iwami , Tsutomu Araki , Akira Suzuki , Yasushi Nanishi 
Published: April 2011

AbstractCurrent-voltage (IV) measurements and capacitance-voltage (CV) measurements have been carried out to investigate electrical properties of AlGaN/GaN-HEMT structures. By CV measurements of Schottky barrier diodes (SBDs) with large leak currents, we observed a distinct peak in CV profiling at low frequencies. The integral of this peak was found to have a correlation with a leak current. The behavior of this peak might be described by the Shockley-Read-Hall (SRH) model if we assume this peak is due to a phenomenon of an electron emission and capture by deep levels. Then Quasi-Fermi Level (Imref) at the bias point where this peak appears in CV profiling corresponds to energy depth of deep levels. That energy level can be approximated by Imref of two-dimensional (2D) electron gas. The result of our samples showed that the energy depth of deep levels from the conduction band is distributed from 320meV to 470meV for Al mole fraction from 0.19 to 0.30, respectively.
3D Simulations on Realistic GaN-Based Light-Emitting Diodes
Authors: Li Simon , Li Z.Q. , Shmatov O. , Xia C.S. , Lu W. 
Published: April 2011

AbstractComprehensive multiscale models have been employed to simulate the realistic GaN based light-emitting diodes (LED) using 3D finite-element analysis. The advanced features include drift-diffusion model for carrier transport, self-consistent Poisson-Shrodinger and K·P models for multi-quantum well band structure, quantum tunneling model for heterojunction, spontaneous and piezoelectric polarization models for built-in electric field, heat flow model for self-heating and ray-tracing model for photon extraction. All the advanced capabilities have been integrated into our software APSYS[1]. In this paper, we present the 3D simulations on the InGaN/InGaN LEDs. Based on the detailed simulation results, we were able to analyze the impact of micro- and nano-scale physical effects such as current crowding, carrier leakage, built-in interface charge and self-heating on the internal efficiency of the device. The macro-scale effect of the geometry on photon extraction was analyzed using 3D ray-tracing technique. Results of different structures will be given to demonstrate the power of the software in handling complicated realistic LED geometries. The simulation results can be used to optimize the design of quantum well layers, blocking layer materials and electrode geometries etc.
III-Nitride Epitaxial Material on Large-Diameter Semi-Insulating SiC Substrates for High Power RF Transistors
Authors: Saxler Adam , Hutchins Edward L. , Jenny Jason , Blew Austin 
Published: April 2011

AbstractMetalorganic chemical vapor deposition was employed to deposit high quality, highly uniform III-Nitride transistor structures on 100 mm diameter semi-insulating 4H-SiC substrates. Electron mobility was over 2000 cm2/Vs at room temperature. Sheet resistivity uniformity was as low as 0.75%. Typical standard deviations were about 1% in most properties including sheet resistivity, carrier concentration, mobility, and AlGaN composition. Additionally, wafers maintained their flat shape after deposition of these structures. Wafer bow and warp were typically less than 20 μm for optimized structures and <5 μm for the best wafers.
Metal Organic Chemical Vapor Deposition of Zinc Oxide
Authors: Fenwick William E. , Woods Vincent T. , Pan Ming , Li Nola , Kane Matthew H. , Gupta Shalini , Rengarajan Varatharajan , Nause Jeff , Ferguson Ian T. 
Published: April 2011

AbstractThin films of ZnO were grown by metal organic chemical vapor deposition (MOCVD) in a vertical injection rotating disk reactor (RDR) system on sapphire substrates. Kinetics of ZnO growth by MOCVD were studied and an optimal growth window for a RDR tool was determined. Experimental growth conditions were chosen based on calculations of Reynolds Number (Re) and mixed convection parameter in order to select a growth window with stable gas flow and uniform heat transfer. Growth parameters were systemically varied within this window to determine the optimal growth conditions for this MOCVD tool and to study how these parameters affect film growth and quality. Properties of ZnNiO films grown by MOCVD were also studied to determine the effects of Ni incorporation on structural, optical, and magnetic properties.
A semipolar (10-1-3) InGaN/GaN green light emitting diode
Authors: Sharma Rajat , Pattison P Morgan , Baker Troy J , Haskell Benjamin A , Farrell Robert M , Masui Hisashi , Wu Feng , DenBaars Steven P , Speck James S , Nakamura Shuji 
Published: April 2011

AbstractThe performance of conventional c-plane GaN-based optoelectronic devices suffers from the effects of strong polarization-induced electric fields along the conduction direction, which result in a reduced overlap between electron and hole wavefunctions. These devices consequently demonstrate low radiative recombination rates, and a blue-shift in peak emission wavelength with increasing bias. There have been several recent demonstrations of light emitting diodes (LEDs) fabricated on non-polar a- and m- plane GaN that show greatly reduced to zero blue-shift of peak emission wavelength, and other recent work on non-polar GaN has yielded hole concentrations that are almost an order of magnitude higher than for c-plane GaN.The effects of the strong polarization-induced electric fields may, conceivably, also be mitigated or potentially eliminated by growing films on so-called ‘semipolar’ planes. A semipolar plane is any plane that may not be classified as a c-, a- or m- plane, and has at least two non-zero h, i, or k Miller indices and a nonzero l Miller index (, and planes, for example). It is expected that devices grown on these semipolar planes should also demonstrate a reduced blue-shift in peak emission wavelength and higher hole concentrations. Further, recent work also suggests that the indium incorporation efficiency for growth on semipolar planes is comparable to that for growth on the c-plane.We demonstrate the first green InGaN/GaN LED grown on a planar semipolar GaN template. The LED structure is grown by metalorganic chemical vapor deposition (MOCVD), and the 20 μm-thick, specular and optically transparent template is grown by hydride vapor phase epitaxy (HVPE). The fabricated devices have a peak emission wavelength of ∼525 nm and demonstrate rectifying behavior, with a low turn-on voltage of 3.2 V. We observe a small ∼7 nm blue-shift in the peak emission wavelength during electroluminescence measurements, over the range 20 to 250 mA. We also see an almost linear increase in the output power from 5 mA to 200 mA, with no appreciable decrease in the external quantum efficiency over the same range. We also observe evidence of polarization anisotropy in the emission from the semipolar green LEDs.
Oxidation of Aluminum Nitride for Defect Characterization
Authors: Edgar James , Gu Z. , Taggart K. , Chaudhuri J. , Nyakiti L. , Lee R.G. , Witt R. 
Published: April 2011

AbstractThe thermal oxidation of aluminum nitride was developed as a means to study defects in bulk aluminum nitride crystals. The oxidation kinetics was established for the dry oxidation of highly textured AlN polycrystals produced by sublimation-recombination crystal growth in a tungsten furnace. Despite seeding on polycrystalline tungsten, the grains were predominantly [0001] oriented as verified by electron backscattering diffraction (EBSD). The oxidation rate is dependent on the crystal’s orientation, polarity, stress, and surface condition, thus oxidation decorates grain boundaries, polishing scratches, and inversion domains by producing oxide layers of different thicknesses. The initial oxidation rate of nitrogen polar (0001) AlN is approximately 25% faster than on aluminum polar crystals. Low temperature (800 °C) dry oxidation produced an amorphous oxide layer and generated a high density of defects (vacancies, stacking faults, and dislocations) in the nitride near the oxide/nitride interface, as observed by cross-sectional transmission electron microscopy. In contrast, high temperature oxidation (1000 °C) produced a crystalline oxide layer, and left the nitride free of observable defects.
Effects of GaN passivation with SiO2 and SiNx studied by photoluminescence and surface potential electric force microscopy
Authors: Chevtchenko Serguei , Reshchikov M A , Zhu K , Moon Y-T , Baski A A , Morkoç H 
Published: April 2011

AbstractThe influence of passivation with SiO2 and SiNx on optical properties and surface band bending in unintentionally doped GaN has been studied by steady-state photoluminescence (PL) and surface potential electric force microscopy (SP-EFM). For both types of passivation we observed a significant increase of PL intensity in air ambient at room temperature. The measured surface potential was the same for control and passivated samples within the experimental error. The value of the surface band-bending was determined as 1.0±0.2 eV in all cases. We suggest that the strong enhancement of PL is caused by reduction of contribution of the surface states to recombination of photogenerated carriers after passivation.
LCA-based evaluation of ecological impacts and external costs of current and new electricity and heating systems
Authors: Dones Roberto , Heck Thomas 
Published: April 2011

AbstractA systematic study of current European electricity and heat systems performed in the frame of the Swiss LCA project ecoinvent was extended to a few new technologies and used as a basis for comparison and ranking using External Costs Assessment and one selected Life Cycle Impact Assessment (LCIA) method. The energy systems include full process chains from extraction of resources through waste disposal. The external costs from airborne emissions were estimated using the most recent findings of the ExternE series on the average damage factors for Europe.Current fossil electricity systems exhibit the highest LCIA scores as well as the highest external costs, unless greenhouse gas emissions (GHG) are valued very low (sensitivity) and advanced technologies are applied. Alpine hydropower always exhibits the lowest score. Environmental performance of current renewables is generally better than fossil but LCIA ranking for wind and PV may worsen when increased importance is attributed to abiotic resource depletion. Wood cogeneration has a relatively poor score compared to other renewables. Nuclear shows generally good environmental performance, unless the high radioactive wastes are given subjectively high negative value. For heating systems, oil has higher external costs than natural gas, with conventional wood in between. External costs of heat pumps strongly depend on the origin of the electricity supplied.Sensitivity analyses were performed for external costs to reflect uncertainties of impacts and variations in monetary valuation. Fossils remain worst performers. External costs of nuclear remain low. Using allocation by exergy, electricity by diesel and natural gas cogeneration ranks worse than oil and natural gas combined cycle, respectively, and never better than renewables or nuclear.