Study of transient photoconductivity of GaN epilayer grown by metalorganic chemical vapor deposition

The transient photoconductivity (PC) properties of a GaN thin film on a (0001) sapphire substrate are investigated. The decay curves of PC obtained by a YAG:Nd pulsed laser are divided into two special regions, which are the beginning and tail decay regions, corresponding to two time constants, 0.1 ms and 1.0 ms, respectively. Keeping the same light intensity and bias voltage, when the sample is heated to 300°C, the time constants are both reduced. It shows that there are a large number of traps at the position of tens of meV below the conduction band edge. The PC ascends S-shaped when the data are collected during the first 200 ns. As the sample is not shined, the PC sharply reduces and oscillates, indicating that the carriers redistribute between the deep traps and the conduction band. As an approximation, a model involving two trapping levels, shallow and the deep trapping levels, is used to represent the structure of the electron trapping levels in the gap. The simulation based on this model is in good accord with the experimental results. Moreover, the mechanism of the PC curve variation with light intensity is explained by the model