Kinetics of optically excited charge carriers at the GaN surface

In this work, we combine conductance and contact potential difference measurements in a consistent and systematic way, in steady-state and transient modes, both in the dark and under illumination. With this we obtain valuable information about the kinetics of charges at and close to the surface of GaN. We compare the processes involved in the accumulation and the decay of charge carriers generated via excitation with above and below band-gap light with varying light intensity. In particular, we probed the role played by localized defect states in the kinetics of photogenerated charges. These states are responsible for the trapping of photogenerated electrons in the space-charge region close to the surface, which explains the slow response of the photocurrent to illumination. These states are also involved in the transfer of electrons back to the surface after illumination, which results in the slow recovery of the photocurrent and the contact potential difference in the dark.