Bulk wurtzite-GaN crystal was obtained with a size more than 3 mm along the length of the crystal and with a thickness 200 ∼ 300 µm at around 750°C and 100 bar for 24 hrs in the flux growth method. The structural and compositional property of the GaN bulk single crystal was also studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. Photoluminescence, cathodoluminescence and micro-Raman measurements at room temperature are investigated for bulk single crystals of wurtzite GaN. The cathodoluminescence peak of near band-to-band transition at 365.5 nm and the E2(high energy, 568 cm−1) and A1(LO, 737 cm−1) Raman phonon modes were obtained according to the different position of the (0001) surface of GaN grown by flux method. Sharp line shape for the strain-sensitive E2 (high) mode is considered to be due to the high crystalline quality of the crystal.

Scanning capacitance spectroscopy has been used to characterize, at the nanoscale, the frequency-dependent response of surface charge and of charge in the two-dimensional electron gas of an AlxGa1−xN/GaN heterostructure field-effect transistor structure. dC/dV spectra were measured in a scanning capacitance microscope with a voltage signal consisting of a triangle wave at frequencies of 1 – 50 Hz applied to the sample. The spectra were obtained in the dark (except for 600 nm laser light from the scanning capacitance apparatus) and under illumination. Measurements were performed in the vicinity of and away from charged threading dislocations visible in scanning capacitance images. In the absence of illumination, the dC/dV data indicate that electrons are trapped at or near the AlxGa1−xN surface, consistent with suggestions in the literature that a high density of surface states exists on the free AlxGa1−xN surface. Frequency-dependent measurements show that emission times for these traps can be as long as several hundred ms. In the presence of illumination, reduced electron trapping is observed. The nature and behavior of trap states in the vicinity of threading dislocations is found to differ significantly from that in regions between dislocations for measurements in the dark, and suggest that the electrostatic potential due to the charged threading dislocation is negligible at the surface.

The breakdown voltage of wurtzite and zinc-blende GaN rectifiers as function of a doping concentration and the width of epitaxial layer were successfully modeled in the reach-through case. The breakdown voltage was derived by the impact ionization integral employing the effective impact ionization coefficient and an accurate approximation. Our model shows that the breakdown voltage of wurtzite GaN rectifier was larger than those of zinc-blende GaN rectifier and SiC rectifiers including 4H-SiC and 6H-SiC in the condition that both the thickness and doping concentration of epitaxial layer are identical.

The influence of growth conditions and susceptor purity on the residual contamination in undoped 4H-SiC epitaxial layer grown at higher temperature such as 1600 °C is investigated. Residual N concentration is found to increase with growth temperature. Growth temperature dependence of residual N concentration is stronger than that of Al or B. The effect of degradation of SiC coating layer on the purity of the epitaxial layer is studied. SiC coating layer is degraded after very few repetition of growth-run. SIMS measurement reveals that the concentration of N, Al, B, Ti and V in epitaxial layer increases with the deterioration of SiC coating layer. Therefore, serious consideration on the effect of contamination from the susceptor graphite is required. By using various grades of graphite as susceptor, the dependence of the purity of epitaxial layer on the susceptor purity is studied. High concentration (1017 to 1018 cm−3) of N is found in graphite. Contamination of all types of impurities such as p-type impurities, transition metal impurities and N in graphite is found to affect the purity of epitaxial layers.

Silicon carbide (SiC) is a material with very attractive properties for high power/high temperature electronic devices. Its mechanical strength, high thermal conductivity ( κ), large bandgap, and extreme chemical inertness are a few of the characteristics making SiC interesting for semiconductor electronics. Due to the significant head load generated over large areas in high power devices, it is desirable forthe thermal properties of the substrate to be uniform and optimal. Scanning thermal microscopy (SThM), which provides nondestructive, absolute measurements of the thermal conductivity with a spatial/depth resolution in the 2-3 μm range, was used to examine the room temperature κ as a function of position of four 2” diameter SiC wafers. Wafers of 4H and 6H polytype were fabricated with carrier concentrations in the(1-3)×1018 cm−3 and (6-9)×1017 cm−3 ranges, respectively. A radial distribution of the thermal conductivity was determined for all the investigated samples. For a radius r < r1 (r1 ∼ 0.3”) and r > r2 (r2 ∼ 0.7”) highest thermal conductivity values were measured in the range of (3.8-3.9) W/cm-K, comparable to the highest κ reported for this material [D. Morelli et al., Inst. Phys. Conf. Ser. 137, 313 (1993); E.A. Burgemeister,.et al., J. Appl. Phys. 50, 5790 (1979)]. For r1 < r < r2 the thermal conductivity drops to about (2.85-3.25) W/cm-κinterval. Atomic force microscopy (AFM) investigation revealsthat the influence of surface roughness effects on κ is negligible. The κ dip may arise from a higher basal plane defect density in this region that could be associated with the presence of super screw dislocations, or “micropipes” [M. Dudley et al., J. Phys. D: Appl. Phys. 28, A63 (1995)]. The implications of these findings for device applications and design are considered.

Transmission electron microscopy (TEM) has been applied to analyze the thickness and the In-concentration of InGaN layers in GaN/InGaN/GaN- and AlGaN/InGaN/AlGaN-quantum well (QW) structures. Two series of samples were grown by metal organic chemical vapor deposition varying either only the growth duration for the InGaN QW or by changing the Al- concentration in the buffer layers at unaltered InGaN growth conditions. A rising average In- concentration from 6.5 to 15.4 % and a decreasing growth rate are observed with increasing growth duration. The increase of the Al-concentration in the buffer layers from 0 to 36 % strongly affects the In-incorporation during the InGaN growth, which decreases from 17.5 to 2.5 %. All samples are characterized by an inhomogeneous In-distribution containing In-rich agglomerates with a size of only a few nanometers and less pronounced composition fluctuations on a scale of 100 nm.

We have investigated the nitrogen incorporation dependency on temperature, pressure, C/Si ratio and growth rate in a horizontal hot-wall CVD reactor. The incorporation mechanism for 4H- and 6H-SiC both for Si- and C-face material is presented. A comparison with previously published results in a cold-wall reactor is also made.

Photoionization spectroscopy (PS) measurements, previously carried out for the GaN MESFET, have been extended to the more complicated AlGaN/GaN HEMT structures. In all cases, the spectra revealed that the same two traps causing current collapse in the high resistivity (HR) GaN buffer layer of the MESFET were also responsible for current collapse in the HEMT structures. The HR buffer layers supporting the HEMT structures were prepared by MOVPE at varying growth pressures, in order to vary the incorporation of deep trapping centers. Lower growth pressures were observed to correlate with more severe current collapse and with an enhanced incorporation of carbon. Detailed analysis of the PS data suggests that one of the two responsible traps is related to carbon, while the other may be associated with structural defects in the material.

Copper-based ohmic contacts to n-type 6H-SiC have been investigated. In this study, ohmic contacts have been fabricated with pulsed excimer laser irradiation to Cu-deposited substrates at room temperature. It is shown that current-voltage characteristics depend on the laser energy density. Contacts formed by the laser irradiation at the energy density above 1.2 J/cm2 have shown the ohmic behavior. Cu atoms have slightly diffused into SiC by the laser irradiation at 1.4 J/cm2. As a result, a thin ohmic contact layer has been obtained by the laser processing. AES and XRD study have revealed that a Cu-SiC alloy containing Cu silicide (Cu3Si) is formed by the laser irradiation.

Infrared absorption and resistivity are studied in two different types of semi-insulating (SI) 4H SiC substrates. In type A the silicon vacancy dominates the infrared absorption spectra whereas in type B the UD-1 defect dominates. Both types of substrates are still SI (> 105 Ω cm at 200°C) after annealing at typical CVD growth temperatures (1600°C). The type A material shows a decreasing absorption of the silicon vacancy after 1600°C at the same time as the resistivity decreases from high 107 Ω cm to mid 106 Ω cm measured at 200°C. The thermal activation energy of the resistivity for the type A material is 0.85 eV for temperatures above 250°C both before and after annealing. The type B material does not change with annealing, besides that the absorption increases in intensity with a 1600°C anneal. The resistivity is > 1010 Ω cm at 200°C and the thermal activation energy is 1.4 eV.

The absorption measurements on the silicon vacancy also show that it is still present in the as-grown material after a 1600°C anneal. We interpret this as follows: The silicon vacancy is stable even at 1600°C and does not get mobile at 750°C as earlier suggested.

Stable and low-resistance Ohmic contacts are especially important for laser diodes where high current levels are required. Good contacts are especially difficult on p-type GaN which was the motivation for this study. The GaN was epitaxially grown on (0001) sapphire substrates by MOCVD. Resistivity of this layer was 3.5 Ohm-cm and thickness was 2 microns. After conventional cleaning followed by treatment in boiling HNO3: HCl (1:3), metallization was by thermally evaporating 40 nm Au / 60 nm Ni or 70 nm Au / 55 nm Pd. Heat treatment in O2 + N2 at various temperatures followed, with best results at 600 °C or 700 °C, respectively. Best values of the contact resistance were 1.8×10−4 Ohm-cm2 for Pd/Au and 2.65×10−4 Ohm-cm2 for Ni/Au contacts. After repetitive cycling from room temperature to 600 °C, the Ni contacts were very stable and more stable than the Pd contacts. X-ray photoelectron spectroscopy depth profiling showed the Ni contacts to be NiO followed by Au at the interface for the Ni/Au contacts whereas the Pd/Au contacts exhibited a Pd: Au solid solution. Some contacts were quenched in liquid nitrogen following sintering. These contacts were much more uniform under atomic force microscopy examination and gave a 3 times lower contact resistance with the Ni/Au design. Current-voltage-temperature analysis revealed that conduction was predominantly by thermionic field emission.

Aluminum nitride (2H-AlN) films were grown by plasma-assisted molecular beam epitaxy (MBE) on Si(001). By conventional (CTEM) and high resolution transmission electron microscopy (HRTEM) investigations the influence of the off-axis angle of the substrate surface on the film structure was studied. Three types of Si(001) substrates were used: on-axis, ∼1°, and ∼5° off-axis. The 2H-AlN layer on an exact oriented Si(001) substrates consists of 3 AlN film domains: two main film domains, AlNI and AlNII, and a small domain AlNIII at substrate surface defects. Their c-axis orientations are parallel to the c-axis of the substrate: [0001]AlNI,II,III ∥ [001]Si. The a-axes of AlNI and AlNII rotated by 30° to each other: [11 20]AlNII∥[01 10]AlNII ∥ [1 1 0]Si. The orientation of AlNIII is [01 10]AlNIII ∥ [100]Si. In 2H-AlN films grown on off-axis Si(001) substrates (∼1° and ∼5°) the ratio between the AlNI and AlNII film domains changes dramatically as far as a single domain film structure consisting of mainly AlNI is reached. The AlN c-axes of all domains on the off-axis substrates are not parallel to the Si c-axis but tilted by the off-axis angle of the Si(001) substrate (∼1° respectively ∼5°), i.e. [0001]AlN is parallel to the Si(001) substrate surface orientation. Determination of the AlNI domain / Si(001) interface structure by HRTEM illuminates the origin of the preference of this domain in the 2H-AlN film by using off-axis Si(001) substrates. On the on-axis substrate a regular array of misfit dislocations causes a 5:4 fit between the (1 1 00)AlN and ( 1 1 0)Si lattice planes. The off-axis Si(001) leads to a rotation of the AlN lattice in respect of the Si lattice. An array of misfit dislocations with a 4:3 fit between (1 1 01)AlN and ( 1 1 1)Si lattice planes decreases the residual lattice misfit from -1.6% to -0.8%.

We investigate the adsorption of a Ni monolayer on the β-SiC(001) surface by means of highly precise first-principles all-electron FLAPW calculations. Total energy calculations for the Si- and C-terminated surfaces reveal high Ni adsorption energies, with respect to other metals, confirming the strong reactivity and the stability of the transition metal/SiC interface. These high binding energies, about 7.3-7.4 eV, are shown to be related to strong p-d hybridization, common to both surface terminations and different adsorption sites, which, despite the large mismatch, may stabilize overlayer growth. A detailed analysis of the bonding mechanism, in terms of density of states and hybridization of the surface states, reveals the strong covalent character of the bonding. We also calculate and discuss the Schottky barrier heights at the Ni/SiC junction for both terminations.