Optical absorption and anomalous photoconductivity in undoped n-type GaN

Photoconductivity and optical absorption measurements were employed to anal yze deep levels in undoped n-type GaN films grown on sapphire substrate by metalorganic chemical vapor deposition. At room temperature, the photocondu ctivity measurement exhibits a broad level at around 1.90 eV. Similarly, th e optical absorption spectrum shows a deep level located at 1.87 eV within the band gap, which is best described by a transition from a donor charge-t ransfer level to the conduction band, according to Lucovsky theory. A persi stent photoconductivity whose behavior is distinctive from that of previous ly reported work for n- or p-type GaN epitaxial films was observed. The pho tocurrent quenching and decreased dark current in the persistent photocondu ctivity effect suggest that metastable electron states are formed in the ba nd gap to trap electrons which tunnel out the potential barrier with long r ecovery time. (C) 2000 American Institute of Physics. [S0003-6951(00)01508- 4].