A combined structural and optical assessment of cubic (3C-) SiC thin films grown on Si (100) substrates by chemical vapor epitaxy (CVD) is presented. The CVD growth was performed at both atmospheric and low (100 Torr) pressure, using a vertical reactor. The CVD-grown 3C-SiC films with different growth time were characterized by X-ray diffraction, Raman scattering and Fourier transform infrared (FTIR) spectroscopy to be single crystalline with a high degree of crystal perfection. The film thickness was determined from FTIR spectra. Variations of X-ray, FTIR and Raman spectra with different growth conditions and film thicknesses are studied comparatively. Related problems are discussed.

Silicon carbide films have been grown on 6H-SiC (0001) and Si (001) wafers by laser ablation using an excimer laser. The films were deposited at heater plate temperatures between 970° C to 1270° C. Film composition, morphology and polytypism were determined by Auger electron spectroscopy, atomic force microscopy and high resolution transmission electron microscopy (TEM). In the course of these experiments growth of 2H-SiC on 6H-SiC was observed at the highest heater plate temperatures. Cross-sectional TEM images clearly show the symmetry of a film grown at 1270° C as c-axis oriented 2H-SiC containing columnar grains with average diameter of 20 nm and length of 100 nm.

Silicon carbide has excellent physical and electronic properties for use in devices when higher temperatures or higher power densities are required. We have investigated a direct laser conversion technique to create electrical conductors on the high band-gap silicon carbide. Thin films of silicon carbide (SiC) were sputter deposited on AI2O3, SiO2, and Si substrates using a SiC target with an RF planar magnetron. These films were irradiated at 308 nm with multiple 15 ns excimer laser pulses creating 0.5 to 2 mm wide electrically conducting paths. Both the irradiated and unirradiated films were evaluated as a function of substrate type, deposition temperature, finish, stoichiomelry, annealing temperature, sputter gas, film thickness, and laser processing conditions. The lowest resistivity films, originally 10 ohm-m, were calculated to be 160 μohm-m obtained after irradiation, which compares to a value of 50 μohm-m obtained after irradiating bulk SiC. The films were characterized using XPS, SIMS, AES, SEM, and Raman spectroscopy. We were able to characterize the composition of the films and conducting traces, the surface oxide, the critical binding energies, the lattice structure, and the morphology of the microstructure. Models for the phase transformations and conductivity have been formulated.

Double-modulation mass spectrometry is used to analyze the identity and energy distribution of the particle flux during low-temperature growth of Si1−xCx by reactive magnetron sputtering of a silicon target in 3 mTorr argon and 0 – 0.35 mTorr methane. The ion fluxes, methane consumption, film deposition rate, and film composition are determined as a function of the methane partial pressure at fixed target current. Argon, silicon, carbon and hydrocarbon ions are detected at the substrate position. The argon and hydrocarbon ions have a narrow energy distribution consistent with the plasma potential. The silicon and carbon ions have a broad energy spread, consistent with that of sputtered neutrals which are post-ionized by the plasma. The film composition (C/Si) varies in the same manner as the arrival ratio of (C+/Si+) ions, but does not correlate well with the arrival ratio of (ΣCHX+/Si+) ions or the methane consumption rate. We conclude that the main source of carbon in Si1−xCx films is the sputtering of C atoms from the Si target surface.

High-quality AlGaN/GaN and AlGaN/GalnN DHs were fabricated by MOVPE using low-temperature deposited AlN buffer layer. We applied the newly designed dual-flow-channel reactor, by which high-quality and well-controlled AlGaN and GalnN alloys and their heterostructures have been grown. AlGaN/GaN-DH shows low-threshold stimulated emission by optical pumping at room temperature for both edge and surface modes. The peak wavelength of stimulated emission for edge mode was 369.5nm. The peak wavelength of stimulated emission was affected by the strain due to heterostructure as well as the many body effect under high excitation. The wavelength for stimulated emission can be widely changed by using GalnN as the active layer. AlGaN/GalnN DH with InN molar fraction of the active layer of 0.09 shows room temperature low-threshold stimulated emission for edge mode by optical pumping with peak wavelength of 402.5nm. A few mW-class symmetrical AlGaN/GaN DH LED and anti symmetrical AlGaN/GalnN/GaN DH LED using low energy electron beam irradiation (LEEB1) treated Mg-doped P-AlGaN cladding layer were fabricated. These results show that column-Ill nitrides are promising for the realization of practical short wavelength LED and LD.

Recent progress in the growth of high quality 6H-SiC single crystals has led to an ideal substrate material for GaN epitaxial films. Nearly matching lattice constants of wurzite GaN to 6H-SiC in the hexagonal plane can reduce strain effects at the interface. We employed the sublimation sandwich method to grow single crystal layers at reasonable growth rates with free carrier concentrations of 2×1017 cm-3. Very sharp x-ray diffraction peaks of the GaN (0002) plane are obtained indicating the high quality of this system (Δ(2θ) < 0.1 degrees). These findings are directly reflected in the optical properties. The photoluminescence is dominated by a single sharp exciton line, impurity related donor acceptor transitions are seen with very weak intensities. However, at lower energies the internal luminescence transitions of the 3d transition metal ions Fe and V are observable. The incorporation of Fe is confirmed by electron paramagnetic resonance.

The acceptor binding energy is calculated to find out the best acceptor impurity in InN, GaN and AlN. Be is predicted to be the shallowest acceptor and the next are Mg and Zn. Group IV elements such as C or Si are very deep. Band lineup is calculated to be ΔEc : ΔEv = 2.1 eV : 0.76 eV = 0.73 : 0.27 = 2.8 : 1 for GaN/AlN and ΔEc : ΔEv = 0.88 eV : 0.66 eV = 0.57 : 0.43 = 1.3 : 1 for GaN/InN. GaN is grown on GaAs and GaP-coated Si substrate by MOCVD. GaAs intermediate layer gives better GaN compared to GaP intermediate layer. It is suggested that the lower bulk modulus of GaAs than that of GaP gives this difference.

The effect of epitaxial growth temperature in the range 985 – 1050°C on the preparation of device quality Si-doped GaN layers on self-nucleated (00.1) sapphire has been explored. Not unexpectedly, several device-related properties monotonically improve with increasing growth temperature, including: (a) carrier density; and, (b) volume fraction of heteroepitaxial domains. However, a number of equally important device-related properties show a local maximum or minimum and include: (a) optical second-harmonic generation intensity; (b) structural coherence; and, particularly (c) surface morphology. The antecedents of the first class is found in an increase in surface and bulk diffusion and a reduction in film defect incorporation and stress at the GaN/GaN (nucleation layer)/α-Al2O3 heterointerface (even for overlays with thicknesses near 1 μm). The second class apparently stems from the limited range over which the thermally annealed GaN nucleation layer stimulates pseudo two-dimensional growth of the overlayer.

Wurtzite GaN films on AlN buffer layers were grown on Si (111) by electron cyclotron resonance microwave plasma assisted MBE (ECR-MBE). High resolution x-ray diffraction studies indicate that the mosaic disorder decreases with increasing growth temperature. The grain size is related to the growth temperature. The best (0002) diffraction peak full width at half maximum was 22 min. for a film 1.7 μm thick. Prominent exciton luminescence is observed at 3.46 eV at 10 K.

The plasma I-V characteristics were measured with a Langmuir probe near the growth position. The nitrogen ion density has been extracted from the data, and is a strong function of the N2 flow rate, the microwave power and the aperture size of the ECR source. The crystal quality of AlN is strongly affected by the plasma conditions.

Gallium nitride layers were grown by organometallic vapor phase epitaxy on AlN buffer layers deposited in the range of 450–650°C. The GaN growth conditions were kept constant so that changes in film properties were due only to changes in the buffer layer growth temperature. A monotonie improvement in relative crystallinity as measured by double-crystal X-ray diffraction corresponded with a decrease in buffer layer growth temperature. Improvements in GaN electron transport at 300 and 77 K were also observed with decreasing AlN buffer layer temperature. Photoluminescence spectra for the lowest temperatures studied were dominated by sharp excitonic emission, with some broadening of the exciton linewidth observed as the buffer layer growth temperature was increased. The full width at half maximum of the excitonic emission was 2.7 meV for GaN grown on a 450°C buffer layer. These results indicate that minimizing AlN buffer layer temperature results in improvements in GaN film quality.

Gallium nitride is one of the most promising materials for ultraviolet and blue light-emitting diodes and lasers. Both Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) have recently made strong progress in fabricating high-quality epitaxial GaN thin films. In this paper, we review materials-related issues involved in MBE growth. We show that a strong understanding of the unique meta-stable growth process allows us to correctly predict the optimum conditions for epitaxial GaN growth. The resulting structural, electronic and optical properties of the GaN films are described in detail.

Electron cyclotron resonance-metalorganic molecular beam epitaxy (ECR-MOMBE) has been used to deposit cubic and hexagonal gallium nitride (GaN) on various substrates, namely GaAs, ZnO and Al2O3. This paper will report on the effect of the growth rate of the GaN layer on the surface morphology, as analyzed using scanning electron microscopy (SEM). Structural characterization of this material was conducted using cross-sectional transmission electron microscopy (XTEM) and x-ray diffraction. Conditions such as pre-deposition annealing, growth rate and growth temperature are critical in determining the phase and crystallinity of the deposited layers. These parameters were optimized to obtain the cubic GaN phase on GaAs substrates and single crystal wurtzitic GaN on ZnO and Al2O3 substrates.

The growth of high-quality thin films of the Group IIIA nitrides is exceedingly difficult given their propensity for nonstoichiometry and the lack of suitable substrates for either homoepitaxial or heteroepitaxial growth. A novel deposition technique, ultrahigh vacuum electron cyclotron resonance-assisted reactive magnetron sputtering, has been developed for the preparation of Group IIIA nitride thin films. Thus far, thin films of the semiconductor InN have been deposited on AlN-seeded (00.1) sapphire substrates, and the properties (structural, morphology, and electrical transport) of these films studied as a function of growth temperature. Comparison to InN thin films grown by conventional reactive magnetron sputtering shows enhanced Hall mobilities (from about 50 to over 100 cm2/V-sec), a decreased carrier concentration (by about a factor of 2–3), an increased optical bandgap, and an apparent reduction in homogeneous strain that is in part to be due to film relaxation induced by the ECR beam and in part to enhanced nitrogen content and more nearly stoichiometric films.

Raman spectroscopy and x-ray diffraction are used to characterize Gallium Nitride (GaN) films grown on (100) Gallium Arsenide (GaAs) substrates. Reflection X-ray diffraction data from (200) planes of GaAs and cubic GaN are presented. The linewidth of the cubic GaN diffraction peak is shown to be a strong function of the growth temperature. Raman spectra are presented for a series of samples grown at different temperatures. Raman scattering is characterized by strong peaks at 560 cm-1 and at 736 cm-1, corresponding to TO and LO phonon modes of cubic GaN, respectively. An additional, unexplained feature at 768 cm-1 is clearly observed in Raman spectra of c-GaN samples grown at lower temperatures. The polarization dependence of the intensity of the GaN LO phonon mode is presented and compared with the GaAs LO phonon mode to establish the relative orientation of the c-GaN epitaxy on GaAs.

Introduction of a buffer layer to facilitate heteroepitaxy in thin films of the Group IIIA nitrides has had a tremendous impact on growth morphology and electrical transport. While AIN- and self-seeded growth of GaN has captured the majority of attention, the use of AIN-buffered substrates for InN thin films has also had considerable success. Herein, the properties of InN thin films grown by reactive magnetron sputtering on AIN-buffered (00.1) sapphire and (111) silicon are presented and, in particular, the evolution of the structural and electrical transport properties as a function of buffer layer sputter time (corresponding to thicknesses from ∼50Å to ∼0.64 μm) described. Pertinent results include: (a) for the InN overlayer, structural coherence and homogeneous strain normal to the (00.1) growth plane are highly dependent on the thickness of the AIN-buffer layer; (b) the homogeneous strain in the AIN-buffer layer is virtually nonexistent from a thickness of 200Å (where a significant X-ray intensity for (00.2)AIN is observed); and (c) the n-type electrical mobility for films on AIN-nucleated (00.1) sapphire is independent of AIN-buffer layer thickness, owing to divergent variations in carrier concentration and film resistivity. These effects are in the main interpreted as arising from a competition between the lattice mismatch of the InN overlayer with the substrate and with the AIN-buffer layer.

CNx films with x around 1.0 have been made by inverted cylindrical DC magnetron sputtering. RBS, XPS, IR spectroscopy, ERD and SEM were used to characterize the composition and bonding properties of the films, while X-ray diffraction was used for crystal structure determination. XPS data indicated the existence of the tetrahedral C3N4 phase in the CNx films, which was consistent with the C-N single bond suggested by IR spectra. The annealing effect on CNx films will also be discussed.

Nuclear magnetic resonance (NMR) spin lattice relaxation is highly sensitive to the nature, amount, and homogeneity of unpaired-electron-containing impurities in silicon carbide and similar solids, and is a promising tool for the study of dopants and impurities. In nitrogen-doped 6H silicon carbide, 13C and 29Si NMR spin lattice relaxation is highly site-dependent. Not only do carbon sites relax much more rapidly than the corresponding silicon sites, but also there are unprecedented differences in relaxation efficiency among the different carbon (and silicon) sites, consistent with much higher unpaired electron density at the higher-symmetry (Types A and B) sites than at the lowest-symmetry (Type C) site. In contrast, all sites relax at equivalent rates in undoped samples and in commercial abrasive grade material with high levels of impurities, although there are large differences in relaxation efficiency among samples. The change in the nature of the relaxation process, from exponential in high-purity to stretched exponential in lower-purity samples, is apparently related to changes in unpaired-electron mobility.

We performed infrared absorption and reflectivity measurements on serverai 6H-SiC samples at variable temperatures between T=5K and T=300K. From the temperature dependence of the observed absorption lines we separated electronic from vibronic transitions. The electronic transitions are assumed to be due to transitions from the neutral nitrogen donor occupying the three different carbon lattice sites in 6H-SiC into excited states. We determined polarization and the oscillator strengths of these transitions. From the temperature dependence of the electronic transitions we determined the valley-orbit-splitting energy and we demonstrated the influence of compensation. For the different vibronic transitions we determined the Grüneisen constants.

We have investigated the electron spin resonance of donors in bulk and thin film β-SiC at T = 10K to 40K. In both, two donor spectra are resolved; a three line spectrum associated with nitrogen on the carbon site and a broader line of unidentified origin. The observable hyperfine splitting decreases with increasing temperature due to a small valley-orbit splitting, in qualitative agreement with photoluminescence results. The lineshape has a significant lorentzian character even at the lowest temperatures, indicating exchange interactions with residual conduction electrons, possibly from a shallower donor. We find a lower concentration of both donors in the bulk material and a different temperature dependence of the lineshape for the two samples.

We have measured the photoluminescence decay time of the bound excitons at the neutral nitrogen donors in the 6H and 3C polytypes of SiC. At 2K the decay times are 8.0 ns, 1.8 ns and 1.5 ns, for the P, R and S bound excitons in 6H SiC. For the nitrogen exciton in 3C, we find a decay time of 160 ns. These values are faster than previously reported for shallow donors in other indirect bandgap materials such as Si or GaP. Each of the observed decay times is found to be independent of the doping level in the sample, is temperature independent at low temperatures but decrease when the bound excitons are thermally ionised. The decay time related to different donor levels in 6H exhibits a strong dependence on the donor binding energy. We suggest that the dominant mechanism responsible for the observed decay time is a phonon-less Auger process. In high-purity 6H samples we have also measured the free exciton decay time at low temperatures to be 12 ns.