Comparative TEM studies have been carried out on GaN/AlN epifilms grown on both on-axis and off-cut 6H-SiC substrates to study the defects formed in the GaN/AlN films and the state of strain relaxation at the interface. Prismatic Stacking Faults (PSFs) are observed to form at I1 type substrate steps in both the on-axis and vicinal samples. In the vicinal samples, the PSFs expand into GaN/AlN film forming intersecting stacking fault configurations comprising faults that fold back and forth from the basal plane (I1 Basal-Plane Stacking Faults; BSFs) to the prismatic plane (PSFs). On the other hand, in the on-axis sample the PSFs are observed to mostly annihilate each other to form enclosed domains confined to the near-interface region. In addition, HRTEM studies suggest the existence of Geometric Partial Misfit Dislocations (GPMDs) at the SiC/AlN interface of the vicinal sample, which form at I2 type substrate steps. These GPMDs simultaneously accommodate the lattice mismatch and stacking sequence mismatch present at the SiC/AlN interface. This provides explanation of the improved strain relaxation observed in the vicinal versus the on-axis sample.

Active nitrogen species produced by an Oxford Applied Research HD-25 plasma source have been monitored by optical emission spectroscopy and quadrapole mass spectroscopy. Both techniques confirmed that at higher RF powers and lower flow rates the efficiency of atomic nitrogen production increased; emission spectroscopy confirmed that this was at the expense of active molecular nitrogen (N2*). InN films grown on (0001) sapphire/GaN with higher relative molecular content were found to have lower carrier concentrations than the corresponding films grown with higher atomic content. However, electrical properties of films grown on (111) YSZ showed insensitivity to the active nitrogen content. Etching experiments revealed that films grown on sapphire/GaN were nitrogen-polar, while films grown on YSZ were In-polar, suggesting that film polarity can greatly influence the effect active species have on growth. Lattice relaxation, as measured by reflection high-energy electron diffraction, revealed that the N-polar films grown under high relative molecular flux relaxed fully after ∼60 nm of growth, while the corresponding In-polar film relaxed fully within the first several nm of growth.

The performance of III-Nitride high power, high frequency transistors and laser diodes is intimately connected with the ability to dissipate heat from the junction to the substrate. The thermal conductivity was characterized by the three omega method for undoped and doped gallium nitride bulk substrates grown by HVPE from room temperature to 450 K. The thickness of the samples varied from thin film epilayers on sapphire to 2 millimeter thick free standing samples Dislocation density of the substrates was measured by imaging cathodoluminescence, SIMS was used to measure impurity levels of oxygen, hydrogen, silicon, and iron, while carrier concentrations and resistivity were determined from electrical measurements and EPR. A semi-insulating, 2 mm thick iron doped sample had the highest thermal conductivity of 230W/K-m at room temperature. Undoped samples had comparable, but lower thermal conductivities throughout the temperature range from 300-450 K. By comparing these results with previously reported experimental results including those on MOCVD grown GaN free of grain boundaries, we establish an empirical relationship in a compact formula that relates the thermal conductivity of GaN and the dislocation density with three different regimes of low, intermediate, and high dislocation densities. In the high dislocation regime, the thermal conductivity improves significantly with reduction of dislocation densities. As material quality continues to improve it remains to be seen if in the low dislocation density regime, thermal conductivities will approach 300 W/K-m or plateau out near 250 W/K-m. As point defects start to limit the thermal conductivity when dislocation density becomes very low, gallium vacancies are expected to play an increasing role. Iron is postulated to substitute on the gallium site. The indication from this study is that iron doping at concentration of 1018 cm-3 is not limiting the thermal conductivity in the 300-450 K range.

By compositionally grading AlGaN layers over different thicknesses, high mobility electron gases are produced by polarization-induced doping. Temperature dependent Hall and capacitance-voltage measurements were performed on these AlGaN layers, and two degrees of freedom are found for choosing the carrier concentration of these slabs. Carrier mobilities determined from Hall measurements are observed to be much higher than impurity doped structures of similar carrier densities. Alloy and phonon scattering are determined to be the major contributors limiting the mobility of the electron in the graded layers. This form of polarization-induced doping offers an attractive alternative to the traditional doping techniques, and may be used for highly conductive AlGaN layers with high Al composition, both for lateral and vertical transport.

The interaction of InN epitaxial films grown by r.f. plasma assisted molecular beam epitaxy with atomic hydrogen and nitrogen, produced by remote r.f. H2 and N2 plasmas, is investigated. InN strongly reacts with both atomic hydrogen and nitrogen yielding depletion of nitrogen and concurrent formation of In clusters. The impact of hydrogen treatments on the optical properties of InN is assessed using photoluminescence (PL). It is found that hydrogen suppresses the intense PL band peaked at approximately 0.7eV for the as-grown InN epitaxial layers, and results in the appearance of a new PL band whose peak energy and intensity increase with H-dose. The effect of exposure to atomic hydrogen and nitrogen on electrical properties of InN is investigated using Hall effect measurements. Atomic force microscopy is also used for studying the morphological changes of InN upon interaction with atomic hydrogen and nitrogen.