Temperature stability of high-resistivity GaN buffer layers grown by metalorganic chemical vapor deposition

Abstract

Nominally undoped GaN films were grown by metalorganic chemical vapor deposition under three different conditions, namely (1) “standard” growth conditions with growth temperature of 1000 °C and growth rate of 1 μm/h, (2) slightly reduced growth temperature of 975 °C, and (3) standard temperature, but higher growth rate of 2.5 μm/h. The standard sample had a net donor density <1015 cm−3, while the two other samples were semi-insulating, with sheet resistivity ∼1014 Ω/square and the Fermi level pinned at Ec−0.8 eV for the low temperature growth and at Ec−0.9 eV for the high growth rate conditions. The photoconductivity spectra of both of these latter samples show the presence of centers with optical threshold near 1.35 eV commonly attributed to C interstitials and centers with optical threshold near 2.7–2.8 eV and 3 eV often associated with C-related defects. However, no signals that could be attributed to substitutional C acceptors and C donors were detected. Current relaxation spectroscopy revealed deep traps with activation energies 0.2, 0.25, 045, and 0.8 eV. Annealing at 800 °C increased the concentration of these traps. The changes in resistivity induced by annealing in the high-growth rate sample were much stronger than for the low-temperature sample. The authors also observed a strong suppression of the yellow luminescence band intensity in the “standard” sample after annealing, as opposed to a slight increase of this band intensity in the two semi-insulating samples. The role of compensation by native defects and by deep levels related to carbon in the observed changes is discussed.