We report on high quality GaN layers grown with the use of one intermediate layer. The defect analysis shows that the density of dislocation is only 8×107/cm2 in these layers, compared to over 1010/cm2 for layers grown without the intermediate layer (IL). Electron microscopy on cross-section samples shows that deposition under certain specific conditions of a low- temperature IL directly benefits the quality of the subsequently deposited GaN layer. The growth of the GaN top layer appears to be similar to growth observed for lateral epitaxial overgrowth layers. This first time observation opens the possibility for using standard growth methods of GaN compounds to achieve a dislocation density comparable to that achieved with lateral overgrowth epitaxy.

The reactive ion etching of diamond in O2, CF4/O2, CHF3/O2, O2/Ar) discharges has been examined as a function of bias voltage, flow rate and composition of the gas mixtures. Etching in O2 and O2/Ar plasmas (with flow ratio of O2/Ar >25% ) was characterised by a high etch rate (∼35 nm/min) and an increase in surface roughness with rising bias voltage. The CF4/O2 plasmas also produced a high etch rate (∼50 nm/min) but with only minor dependence of roughness on bias voltage. In comparison, the O2/Ar (with O2/Ar flow ratio <25%) and CHF3/O2 plasmas resulted in a low etch rate (7-10 nm/min). The high and low rate regimes were identified as ion- enhanced chemical etching and physical sputtering respectively. Etching in the O2/Ar plasmas has been attributed to a combination of the two processes dependent on the O2 content.

We report preliminary electrical and diffusion barrier characteristics of Ti (100nm)/TaSi2 (200nm)/Pt (300nm) thermally stable ohmic contact metallization on n-type 6H-SiC epilayers. These contacts exhibited linear ohmic characteristics with contact resistance in the range of (0.3×10−4−8×10−4) Δcm2 on n-type epilayer doped between 0.6 to 2×1019cm−3. The I-V characteristics and contact resistance remained stable after heat treatments at 500°C and 600°C in air for over 600 hours and 150 hours, respectively. Auger Electron Spectroscopy (AES) was used to analyze the metal and semiconductor interfaces to understand the prevailing reactions. The thermal stability of the ohmic contact between 500°C and 600°C in air is believed to be due to the formation of silicides and carbides of titanium after being annealed at 600°C in H2 (5%)/N2 forming gas for 30 minutes. The oxidation of silicon species that migrated after TaSi2 decomposition is proposed as the diffusion barrier mechanism.

A drift-diffusion model has been used to explore the performance capabilities of Npn AlGaN/GaN heterojunction bipolar transistors. Numerical results have been employed to study the effect of the p-type Mg doping and its incomplete ionization on device performance. The high base resistance induced by the deep acceptor level is found to be the cause of limited current gain values for Npn devices. Several computation approaches have been considered to improve their performance. Reasonable improvement of the DC current gain β is observed by realistically reducing the base thickness in accordance with processing limitations. Base transport enhancement is also predicted by the introduction of a quasi-electric field in the base.

We report on our progress on the fabrication of AlGaN/GaN HEMTs with extremely short gate length. AlGaN/GaN HEMTs with different gate length from 6 νm down to 60nm were fabricated to investigate DC- and high frequency behavior as well as short channel effects. We have found that the transistors with gates in the 100 nm range can be improved in the device performance with respect to transconductance and high frequency but shows also short channel effects as the loss of saturation in the output characteristics and a strong dependency of the threshold voltage on the gate length.

MOCVD growth of As-doped GaN using dimethylhydrazine, triethylgallium and tertiarybutylarsenic has been investigated. A maximum doping concentration of 4.0 × 1019cm−3 at growth temperatures between 600°C and 800°C was obtained. At 1000°C the As doping level dropped below the SIMS detection limit of ∼1.0 × 1017cm−3. The As incorporation depended only weakly on variations of the V/III molar flow ratio between 11 and 61. Raising the As/V molar flow ratio from 0.01 to 0.06 increased the As concentration which then decreased by further increase to 0.11. Different morphologies of the layers were found depending on the growth conditions. A surfactant-like behavior of As was observed leading to smooth GaN films grown on top of the As-doped GaN layer. Two characteristic luminescence peaks at 3.31 eV and 3.425 eV were found for samples doped with As below 900°C. These spectral features are believed to originate at extended lateral defects - presumably stacking faults.

The contribution of spontaneous and piezoelectric polarization to the formation of a 2DEG in AlGaN/GaN heterostructures was investigated using undoped AlGaN/GaN structures. Hall measurements of 2DEG density on such structures with varying Al percentage (8%-27%) and varying thickness of the AlGaN layer (30-500Å) indicated that donor-like surface states at an energy of 1.42eV below the conduction band were the source of electrons in the 2DEG. Field effect transistors were fabricated on such undoped heterostructures. For an AlGaN/GaN structure with 0.27 Al mole fraction, power density in excess of 3.5 W/mm at 6 GHz with corresponding maximum PAE of 33.5% was obtained. These results exceed the best reported power performance of MBE grown GaN HFETs on sapphire, thus demonstrating the excellent capability of MBE grown GaN heterostructures for microwave power applications.

Microstructure and thermal stability of ZrN/ZrB2 bilayer deposited on GaN have been studied using transmission electron microscopy methods (TEM) and secondary ion mass spectrometry (SIMS). It has been demonstrated that annealing of the contact structure at 1100°C in N2 atmosphere does not lead to any observable metal/semiconductor interaction. In contrast, a failure of the integrity of ZrN/ZrB2 metallization at 800°C, when the heat treatment is performed in O2 ambient has been observed.

Thermal expansion and lattice parameters are useful thermophysical properties of materials. A knowledge of their temperature dependence is essential for optimizing device design and crystal growth conditions while minimizing the residual stress in epitaxial films and electronic devices. Currently first principles theory can predict lattice parameters to 0.01%. Information required for device design and thin film crystal growth requires improvements two to three orders of magnitude better than this. Available experimental results from our work and the literature will be reviewed in terms of a high temperature predictive model. Tables of thermal expansion and lattice parameters for AlN, GaN, 6H-SiC, β-SiC, MgO, Al2O3, ZnO and GaAs are provided.

A simple ion-implanted bipolar transistor technology in 4H-SiC is presented. Suitable for both high-voltage vertical devices and lateral high-temperature transistors (for circuit applications), the technology is based on an implanted boron p-well with nitrogen and boron (or aluminium) implanted n+ and p+ regions respectively. The effects of base doping and carrier lifetime on device performance have been studied using TCAD techniques. It is shown that understanding the strong variation of carrier concentration with temperature (due to deep activation levels) and applied field (so-called field ionization) is critical in device design optimisation. The effects of post-implant anneal conditions on the physical and electrical characteristics of the junctions are investigated. It is shown that annealing can remove much of the damage induced by high dose nitrogen implantation but that residual damage is still present. The electrical characteristics of simple BJT transistors with breakdown voltages in excess of 1000V and common-emitter gains of ∼2 is related to the level of such residual damage.

n-type 3C-SiC was heteroepitaxially grown on n-type Si(100) substrates using HMDS (hexamethyldisilane) and characterized by DLTS (deep level transient spectroscopy) measurements. In order to investigate relationship of defect density with epilayer thickness, epilayers with various thicknesses were grown. Relatively thin (<1.0μm thick) epilayers were found to contain defects with energy levels distributed in a wide energy range, while relatively thick (>2.2μm thick) epilayers contain a defect with an activation energy of 0.25eV. This defect level is slightly shallower than that in 3C-SiC grown by SiH4 and C3H8 (∼0.3eV).

GaN dots have been grown on c-plane sapphire and (111) Si substrates by reactive molecular beam epitaxy. A new method involving two-dimensional growth followed by a controlled annealing during which dots are formed was employed. Due the dot nature and large dot density, relatively high luminescence efficiencies were obtained on both substrates. Single layer dots were used for AFM analysis whereas 30 layer dots were used for photoluminescence experiments. AlN barrier layers, some too thick for mechanical interaction, some thin enough for vertical coupling were used. Strong polarization effects lead to a sizeable red shift, which depends on the size of the dots.

We have investigated surface treatment effect on the interfacial reaction of Pd/p-GaN interface and also room temperature ohmic contact formation mechanism of Pd-based ohmic contact. In order to examine room temperature ohmic behavior, various metal contact systems were deposited and current-voltage measurements were carried out. In spite of large theoretical Schottky barrier height between Pd and p-GaN, Pd-based contact showed perfect ohmic characteristic even before annealing. According to the results of synchrotron X-ray radiation, the closed-packed atomic planes (111) of the Pd film were quite well ordered in surface normal direction as well as in the in-plane direction. The effective Schottky barrier height of Au/Pd/Mg/Pd/p-GaN was 0.47eV, which was estimated by Norde method. This discrepancy between theoretical barrier height and the measured one might be due to the epitaxial growth of Pd contact metal and so the room-temperature ohmic characteristic of Pd-based ohmic contact was related strongly to the in-plane epitaxial quality of metal on p-GaN.

High-dose titanium implantations have been performed into ion beam synthesized heteroepitaxial layer systems of Si/3C-SiC/Si(100) in order to study the formation of titanium silicide layers in the silicon top layer. The structure and composition of layers was analysed using RBS, XRD, XTEM and EFTEM. The sputtering rates of 180 keV Ti ions were determined using the lower SiC/Si interface as a marker. A homogeneous surface layer with the stoichiometry of TiSi2 was formed by a nearly stoichiometric implantation and subsequent annealing. The formation of more metal-rich silicides was observed at doses where the peak Ti concentration largely exceeds the TiSi2 stoichiometry and where the total amount of Ti atoms in the top layer is greater than the amount needed to convert the entire Si top layer into TiSi2. Under these conditions, strong solid state reactions of the implanted Ti atoms with the buried SiC layer and the silicon substrate are observed.

In this study, the near-surface regions of air-exposed thin GaN layers deposited by hydride vapour phase (HVPE) epitaxy on 6H-SiC substrates have been examined. Chemical-state identification and in-depth elemental distribution profiles are evaluated using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and secondary ion mass spectroscopy (SIMS). The epilayers show a high degree of chemical purity as determined by XPS and SIMS. Low temperature photoluminescence (PL) were performed and is dominated by donor-acceptor pairs (DAP) emission. Layer thickness was measured to be ∼ 600-700 nm and an abrupt, well-defined heterointerface is observed using scanning electron microscopy (SEM) and SIMS.

We demonstrate that the use of pure gallium (Ga) as a buffer layer results in improved crystal quality of GaN epilayers grown by plasma-assisted molecular beam epitaxy on c-plane sapphire. The resulting epilayers show electron Hall mobilities as high as 400 cm2/Vs at a background carrier concentration of 4 × 1017 cm−3, an outstanding value for an MBE-grown GaN layer on sapphire. Structural properties are also improved; the asymmetric (101) X-ray rocking curve width is drastically reduced with respect to that of the reference GaN epilayer grown on a low-temperature GaN buffer layer. Nitrided Ga metal layers were investigated for different Ga deposition time. These layers can be regarded as templates for the subsequent Ga main layer growth. It was found that there is an optimum Ga metal layer deposition time for improving the electron mobility in the epilayer. Heating of the Ga metal layer to the epilayer growth temperature under nitrogen plasma is found to be sufficient to produce highly oriented GaN crystals. However, nonuniform surface morphology and incomplete surface coverage were observed after nitridation of comparatively thick Ga metal layers. This is shown to be the reason for the decreasing electron mobility of the epilayers as the Ga metal layer thickness exceeds the optimum value.

Carrier decay transients in 4H-SiC n-type and p-type epilayers have been charac-terized using a non-destructive, non-contact microwave photoconductivity technique. Decay transients show a two-stage exponential decay with first decay constants as high as 400 ns in 10 νm p-type epilayers. The second decay constant increases with temperature and is dominated by interface recombination.

The present status of development of SiC and GaN devices for high-voltage power electronics applications is reviewed. Device structures that are particularly applicable to these two wide bandgap semiconductors are considered and compared to those commonly used in silicon. The simulated and experimental performance of two-terminal rectifiers and three- terminal transistors and thyristors are compared. The effects of material parameters (mobility, ionization coefficients, lifetimes) and defects on device characteristics are pointed out. Similarities and differences between electronic and photonic device development in these semiconductors are discussed.

We present an optically-detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in a p-i-n diode, and show how it may be used to determine the electron transit time and velocity-field characteristic in GaN at room temperature. The transit time drops with increasing electric field E in the intermediate field regime (50-100kV/cm), and the electron velocity possesses a weak, quasi-linear dependence on E attributed to polar optical phonon scattering. In the high field regime the transit time and the electron velocity gradually become independent of E. The peak electron velocity of 1.9×107 cm/sec, corresponding to a transit time of ∼2.5 ps across the 0.53 μm depletion region, is attained at ∼ 225 kV/cm. The experimental results are in qualitative agreement with theoretical steady-state velocity-field characteristics found in the literature.

Low-frequency noise is investigated in n-type GaN film grown by rf-plasma assisted molecular beam epitaxy. The temperature dependence of the voltage noise power spectra, SV(f), was examined from 400K to 80K in the frequency range between 30Hz and 100KHz, which can be modeled as the superposition of 1/f (flicker) noise G-R noise. At f > 500 Hz the noise is dominated by G-R noise with activation energies of 360meV and 65meV from the conduct band. The results clearly demonstrate the trap origin for both the 1/f noise and G-R noise. At the low-frequency range the fluctuation was dominated by 1/f noise. To determine the origin of the noise we considered both the bulk mobility fluctuation and the trap fluctuation models. Our experimental results showed that rapid thermal annealing (RTA) at 800°C resulted in over one order of magnitude decrease in the Hooge parameter. Annealing at temperatures in excess of 1000°C resulted in significant increase in the noise. Photoluminescence and x-ray diffraction measurements also showed that the crystallinity of the films improved with RTA at 800°C with an accompanying reduction in deep levels. Annealing at 900°C and 1000°C resulted in an increase in the FWHM of the x-ray diffraction indicative of thermal decomposition of the materials. The results are in excellent agreement with the trend of Hooge parameters as a function of annealing temperature, strongly indicating trap origin of the observed 1/f noise.