In an effort to investigate the stability of the surface and hetero-interface of AlGaN/GaN HEMTs during high temperature device processing steps, AlGaN/GaN HEMT samples were subjected to temperatures from 650°C to 1150°C for a period of 30 seconds prior to processing. Hall and photoluminescence measurements were performed on samples before and after temperature stressing. The samples annealed at 700°C and 1150°C were then processed, and electrical parametric data were collected during and after processing. Large increases in HEMT Schottky gate diode reverse leakage current are observed at higher pre-process annealing temperatures, while the low-field mobility decreases.

AlN epitaxial layers in a thickness range of 0.065 to 0.6 microns have been grown on double-side polished c-plane sapphire by molecular beam epitaxy (MBE) using either RF nitrogen or ammonia as the nitrogen source. The samples were characterized by XRD, room temperature (RT) and low temperature (2K) optical absorption (transmission) measurements. The XRD (0002) peak FWHM diminishes from 380 arcsec to 84 arcsec when the thickness of the AlN films is increased from 65 nm towards 0.6 μm. All the samples grown with ammonia (NH3), representing N-rich conditions, exhibited optical bandgap exceeding 6 eV at RT. For samples grown with RF nitrogen source, we find that higher nitrogen flow (partial pressure of 8.3-9.4 × 10-5 Torr) results in optical bandgap values larger than 6.0 eV, regardless of the XRD results. The bandgap is found to be smaller than 6.0 eV for the samples grown with lower nitrogen partial pressure (1.0-3.5 x 10-5 Torr), regardless of sample thickness and the XRD data.

Very high critical field, reasonable bulk electron mobility, and high thermal conductivity make 4H-Silicon carbide very attractive for high voltage power devices. These advantages make high performance unipolar switching devices with blocking voltages greater than 1 kV possible in 4H-SiC. Several exploratory devices, such as vertical MOSFETs and JFETs, have been reported in SiC. However, most of the previous works were focused on high voltage aspects of the devices, and the high speed switching aspects of the SiC unipolar devices were largely neglected. In this paper, we report on the static and dynamic characteristics of our 4H-SiC DMOSFETs. A simple model of the on-state characteristics of 4H-SiC DMOSFETs is also presented.

We investigated the recombination dynamics of the AlInGaN grown by a pulsed metal organic chemical vapor deposition (PMOCVD) by using the temperature dependent photoluminescence (PL) and time resolved photoluminescence (TRPL). The indium mole fractions of our samples are 0-3% and the PL measurement temperatures are 10-300K. The PL data show that AlInGaN layers with higher indium ratios exhibit significantly stronger PL intensities and less intensity reduction to the temperature increase. The TRPL data show that higher indium layers yield shorter lifetime in the low temperature range and longer lifetime in the high temperature range. These results indicate that the indium contents into the AlInGaN layers generate more localized states, which are likely to make the recombination processes in the AlInGaN layers less sensitive to the variation of the temperature.

ZnCdO thin films were deposited on (001) sapphire substrates by pulsed laser deposition. Modulation of the energy band gap of ZnCdO was induced by changing the processing parameters. The optical energy band gap of ZnCdO thin films, measured by photoluminescence and transmittance, changed from 3.289 eV to 3.311 eV due to the variation of annealing temperatures. The change of the optical properties was attributed to the change of the stoichiometry of ZnxCd1-xO as illustrated by Rutherford backscattering spectroscopy.

Photoconductive Semiconductor Switches (PCSS) were fabricated in planar structures on high resistivity 4H-SiC and conductive 6H-SiC and tested at DC Bias voltages up to 1000 V. The gap spacing between the electrodes is 1 mm. The average on-state resistance and the ratio of on-state to off-state currents were about 20 Ω and 3×1011 for 4H-SiC, and 60 Ω and 6.6×103 for 6H-SiC, respectively. The typical maximum switch current at 1000 V is about 49 A for 4H-SiC. Photoconductivity pulse widths for all applied voltages were 8-10 ns. The observed performance is due in part to the removal of the surface damage by high temperature H2 etching and surface preparation. Atomic Force Microscopy (AFM) images revealed that very good surface morphology, atomic layer flatness and large step widths were achieved with this surface treatment and these atomically smooth surfaces likely contributed to the excellent switching performance of these devices.

The low temperature growth of GaN crystal using epitaxy lateral overgrowth (ELO) on SiO2 dot pattern below 900°C by hydride vapor phase epitaxy (HVPE) have been studied. It is observed that the growth rate of GaN hexagonal pyramidal crystals along [1101] direction increases as growth temperature decreases. At low temperature of ∼ 850°C, hexagonal GaN columnar crystals with high index facet at the top can be observed. It is proposed that the surface diffusion length of precursors, such as NH3 and GaCl, decreases at lower temperature that reduces the probability of desorption and increase the lifetime. The condensation of Ga liquid droplets on the GaN surface will change the relative stability of {1101} facet. Therefore, the formation of high index planes such as {2122} facet on the top of hexagonal column along with the formation of stacking fault on the (0001) plane can be observed. A detailed study of the effect of growth temperature on the crystal growth mechanism will be presented.

Silicon carbide has been proposed as an excellent material for high-frequency, high-power and high-temperature electronics. High power and high frequency applications have been pursued for quite some time in SiC with a great deal of success in terms of demonstrated devices. However, self-heating problems due to the much higher power densities that result when ten times higher electrical fields are used inside the devices needs to be addressed. High-temperature electronics has not yet experienced as much attention and success, possibly because there is no immediate market. This paper will review some of the advances that have been made in high-temperature electronics using silicon carbide, starting from process technology, continuing with device design, and finishing with circuit examples. For process technology, one of the biggest obstacles is long-term stable contacts. Several device structures have been electrically characterized at high temperature (BJTs and FETs) and will be compared to surface temperature measurements and physical device simulation. Finally some proposed circuit topologies as well as novel solutions will be presented.

The structural and optical properties of InGaN/GaN multiple quantum wells (MQWs) grown on sapphire by MOCVD have been investigated using high-resolution XRD, PL and TEM. The samples consisted of 10 periods of InGaN wells with 6.5nm thickness. The designed indium compositions were 15, 20, 25 and 30% (samples C15, C20, C25, C30, respectively). The thickness of GaN barrier was 7.5nm. The MQW in sample C15 maintained lattice coherency with the GaN epilayer underneath, the MQWs in the other samples, however, experienced lattice relaxation. The crystallinity of the samples decreased considerably with In concentration. As In composition increased, PL peak energy showed a red-shift, and the FWHM of the peaks increased. The increase in the FWHM is attributed to the defects due to the lattice relaxation. For C25 the PL peak intensity increased sharply in spite of the defects due to the lattice relaxation of the sample. It is concluded that the results are related to the In-rich region due to indium phase separation which was observed by TEM image.

The density and distribution of micropipes (MPD's), a screw dislocation with a hollow core, are significant criteria for SiC wafer selection in device fabrication. For this reason the measurement of micropipes in SiC is an important problem. The micropipe distributions of 2’’ diameter 6H SiC wafers from three different PVT furnace designs in three different laboratories were statistically characterized. The micropipe distributions were measured in two ways: by KOH etching the wafer and then counting the hexagonal pits formed on the wafer surface using a microscope and imaging software; and by manually counting with an optical transmission microscope the micropipes on a un-etched wafer within a grid of 0.5mm squares superimposed over the wafer. None of the wafers had a random distribution of micropipes i.e. they did not follow a Poisson distribution. The micropipes were found to associate with each other i.e. there is a higher probability of there being other micropipes near a location where a micropipe has been found. In the eight samples characterized, 1.9% of the wafer area contained 32% of the total micropipes. We have analyzed the repeatability of the optical transmission microscopy method of micropipe characterization and found that it is dependent on the micropipe density; the standard deviation of the measurement increases with increasing MPD. The number of micropipes counted increases by about 10% with increasing magnification and 15% when the wafer is double side polished.

High quality undoped AlGaN/GaN high electron mobility transistors(HEMTs) structures have been gorwn by Hydride Vapor Phase Epitaxy (HVPE). The morphology of the films grown on Al2O3 substrates is excellent with root-mean-square roughness of ∼0.2nm over 10×10μm2 measurement area. Capacitance-voltage measurements show formation of dense sheet of charge at the AlGaN/GaN interface. HEMTs with 1μm gate length fabricated on these structures show transconductances in excess of 110 mS/mm and drain-source current above 0.6A/mm. Gate lag measurements show similar current collapse characteristics to HEMTs fabricated in MBE- or MOCVD grown material.

Bragg mirrors are highly interesting structures for a large set of applications including vertical cavity lasers and the upcoming range of devices based on microcavities. Although the nitride semiconductors are performing fairly well in optoelectronic applications, it is not straightforward to realize Bragg mirrors based on this material system, due to the low optical index differences between GaN and AlN. Moreover, the lattice parameter difference between these materials will generate crystal defects, which prevent the stacking of a large number of periods, adding to the difficulty. In this work, we have grown high reflectivity Bragg mirrors, with a band centered in the visible blue range. The structures were first modelled, then grown by low pressure MOCVD, and were optimised using an in-situ reflectivity system.

This in-situ reflectivity measurement was compared to a calculated profile, to enable real-time control of the structures. The samples were characterized by transmission electron microscopy and reflectivity. It was possible to realize samples with 90% reflectivity near 400 nm.

The deep levels in both undoped and Si-doped GaN layer grown by metalorganic chemical vapor deposition have been characterized by photocapacitance and transient capacitance spectroscopy. The increase in the photocapacitance was observed in both GaN samples in the range of 1.8 to 2.2 eV. This is due to the photoionization of carriers from the deep levels associated with the yellow luminescence (YL). In addition, the transient capacitance measurements after the photoionization were also performed in the range of 1.8 to 3.4 eV. The notable transient of capacitance was observed at the photon energies of about 2.1 and 3.4 eV, the former could be associated with the change in the charge state of the YL center and latter might stem from some other defects capturing photogenerated carriers. By using the isothermal capacitance transient spectroscopy (ICTS) analysis, the ICTS peaks due to the deep levels associated with YL were detected at about t = 150 s in both GaN samples. In addition, another ICTS peak was detected only in the Si-doped GaN samples. It is considered that this peak is associated with the deep levels deeper than YL levels and the deeper levels originate from defects induced by Si doping.

Poly-crystal ZnO films with c-axis (002) orientation have been successfully grown on the lead-based ceramic substrates by r.f. magnetron sputtering technique. The deposited films were characterized as a function of deposition time and argon-oxygen gas flow ratio. Crystalline structures of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Highly oriented films with c-axis normal to the substrates can be obtained by depositing under a total pressure of 10mTorr containing 50% argon and 50% oxygen and r.f. power of 70W for 3 hours. The phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency of SAW device with ZnO/IDT/PT-ceramic structure were investigated. It shows that the preferred oriented ZnO film is beneficial for improving the electromechanical coupling coefficient of SAW device.

A multi–layer gadolinium–doped yttrium oxide thin film was deposited in a combinatorial fashion on a Si (001) substrate using radio–frequency magnetron sputtering. Alternating layers of Y2O3 and Gd were deposited for a total of 9 layers. The film was homogenized in composition by an 850°C 12 hour thermal treatment producing a range of composition from Y1.96Gd0.04O3-Y1.54Gd0.46O3 in a single film. Ultraviolet emission with a peak wavelength at 314-315nm was observed from the gadolinium 8S7/2 - 6P7/2 transition via cathodoluminescence (CL) excitation. The CL–induced intensity was found to be maximum at 9 at% Gd (Y1.82Gd0.18O3) and an optimized electron probe of Vacc = 13 keV was determined for the 1 μm thick thin film. Gadolinium activator saturation was observed for sample current density greater than 0.03 μA/cm2. Non- radiative decay via thermal pathways is suspected for the observed activator saturation.

In this paper, we investigate the effect of post-implant annealing on the optical and electrical properties of Si-implanted GaN films. Results from several measurement techniques including room temperature photoluminescence (PL), micro-Raman scattering, high resolution X-ray diffraction (HRXRD) and Hall measurement are correlated to study the behavior of damage removal, dopant activation, crystalline quality and residual stress, etc. The Hall measurement demonstrates that reasonable activation percentage is achieved though there is only partial recovery of the PL intensity. Raman scattering shows the decrease of stress within the implanted films after thermal annealing. The carrier concentration increases monotonically with increasing annealing temperature up to 1100°C, which is in agreement with linewidth broadening of near band edge in PL spectrum. Moreover, systematic measurements implies that the implantation induced defects, especially point defects, which could play significant role in either the optical or electrical properties of films, cannot be completely annealed out at 1100°C.

Gallium Phosphide (GaP) Metal-Insulator-Semiconductor (MIS) capacitors were fabricated with synthesized SiN as the gate dielectric. The interface property and the SiN bulk quality were studied with capacitance-voltage (C-V) measurements and current-voltage (I-V) measurements. The total interface state density of 3×1012 cm-2 and the fixed charge density in SiN of 7×1012cm-2 were estimated from the C-V measurements. The leakage current density was as low as 15nA/cm2 at an effective electric field of 3MV/cm. The effective electric field in SiN at breakdown was as high as 10MV/cm. Constant current stress measurements showed bulk trap density of 2×1011 cm-2 in SiN, which is much lower than that for CVD SiN or SiO2.

The processing of Solid Grätzel Solar Cells (SGSCs) offers advantages of cost effective fabrication, possible flexible products, and high incident photon-to-electron conversion efficiencies (IPCE). In this paper, InP quantum dots (QDs) were synthesized and used as the photosensitizer to design new generation SGSCs. An organic p-type charge transport material (spiro-OMeTAD) was used as the hole-transport material (HTM). The InP QD has an average diameter of approximately 3nm. Preliminary research indicates that InP QD is a promising photosensitizer, which allows the sensitized SGSC TiO2 nanocrystalline cell to achieve over 400mV open circuit photovoltage, and a short-circuit current greater than 0.035mA/cm2 under the illumination of a solar simulator with an integrated light power of 58mW/cm2.

High reflectivity AlN/GaN DBR structures were grown by MOCVD under three ambient gas conditions during the AlN layer growth. Highest peak reflectivity of about 94.5% with a stopband width of 18 nm at a center wavelength of 442nm was obtained under pure N2 gas ambient growth condition. The center wavelength of the DBR structures blue-shifted to 418 nm and 371 nm and the peak reflectivity also showed a reduction to 92 % and 79 % under the ambient gas conditions of a N2/H2 mixture and pure H2 respectively. Surface roughness showed slight increase from 8nm to 12nm, and the groove depth shows an increase from 25nm to 60nm with increasing the H2 ambient gas ratio. TEM results showed that AlN grown rate was reduction with the increasing H2 content in the ambient gas. The simulation result indicated that optical loss of AlN layers was increased from 450 cm-1 to 1850 cm-1 with the increasing H2 content in the ambient gas.