Yellow luminescence and persistent photoconductivity of undoped n-type GaN

Deep-level defect-related optical properties of undoped n-type GaN grown by metalorganic chemical vapor deposition are investigated using photolumines cence (PL), optical absorption (OA), photoconductivity (PC), and persistent photoconductivity (PPC) measurements. From the temperature dependence of t he PL and OA, we find that the yellow luminescence (YL) is due to shallow-t o-deep donor recombination. PL, PC, and PPC results manifest a strong corre lation in properties related to deep levels. Samples which emit YL exhibit a PC peak at 1.9 eV due to the photoionization of deep levels as well as to the persistent photoconductivity effect, whereas samples with no YL have n o PC peak in the forbidden gap and no PPC at any photon energy, suggesting a common origin. Furthermore, two types of PPC behavior were observed depen ding on the sample quality: typical stretched exponential decay in relative ly thick samples and photocurrent quenching and a subsequent reduction of t he dark current in thin samples. An explanation of the latter phenomenon ba sed on photoinduced metastable electron traps in a highly defective layer n ear the interface is suggested from the temporal behavior of the PC. These traps seem to disappear slowly after the illuminating light is turned off. (C) 2001 American Institute of Physics.