Electrical, optical, and structural properties of GaN films prepared by hydride vapor phase epitaxy

Two sets of undoped GaN films with the thickness of 10-20 μm were prepared by hydride vapor phase epitaxy (HVPE) and characterized by capacitance-voltage (C-V) profiling, microcathodoluminescence (MCL) spectra measurements, MCL imaging, electron beam induced current (EBIC) imaging, EBIC dependence on accelerating voltage, deep levels transient spectroscopy, high resolution X-ray diffraction measurements. The difference in growth conditions was mainly related to the lower (850 °C, group 1) or higher (950 °C, group 2) growth temperature. Both groups of samples showed similar crystalline quality with the dislocation density close to 108 cm−2, but very different electrical and optical properties. In group 1 samples the residual donors concentration was ∼1017 cm−3 or higher, the MCL spectra were dominated by the band-edge luminescence, and the diffusion length of charge carriers was close to 0.1 μm. Group 2 samples had a 2-4.5 μm thick highly resistive layer on top, for which MCL spectra were determined by green, yellow and red defect bands, and the diffusion length was 1.5 times higher than in group 1. We also present brief results of growth at the “standard” HVPE growth temperature of 1050 °C that show the presence of a minimum in the net donor concentration and deep traps density as a function of the growth temperature. Possible reasons for the observed results are discussed in terms of the electrical compensation of residual donors by deep traps.