Photocurrent spectroscopy of GaN and AlGaN epilayers grown on 6H (0001) silicon carbide

We have investigated the photocurrent of GaN and AlGaN epilayers grown on SiC substrates. The peculiar temperature and intensity dependence of the exciton photocurrent reveal the existence of two different carrier localization mechanisms, one due to shallow point defects, which is thermally activated above 150 K and the other due to extended defects, which causes the sublinear intensity dependence of the photocurrent in the whole temperature range comprised between 10 and 300 K. Comparison with temperature dependent photoluminescence experiments suggests that Ga-vacancies can be responsible for the observed localization at point defects, whereas the high-density of native extended defects (as inferred by Rutherford back-scattering experiments) is responsible for the intensity dependence of the photocurrent. However they are localized close to the interface and do not propagate across the bulk of the layer resulting in a good quality of the sample. (C) 2000 Elsevier Science Ltd. All rights reserved.