Kelvin Probe Force Microscopy under Variable Illumination: A Novel Technique To Unveil Charge Carrier Dynamics in GaN

Abstract

GaN is a widely used material for optical and power devices. However, the performance of GaN-based devices is often reduced by charge trapping processes due to defect levels in the band gap. In this paper, we studied a GaN-on-silicon sample via Kelvin probe force microscopy under variable illumination. This novel methodology allows the surface photovoltage (SPV) to be followed on the GaN surface as a function of time, light intensity, and topography with nanometric resolution. The measured SPV decay after turning off the light is too slow (on the order of seconds to days) to correspond to electron–hole recombination, suggesting that it is due to charge trapping. Two processes of opposite sign contributing to SPV generation and decay were observed, indicating two different charge trapping mechanisms. In this study, we propose a model that accounts for the observed SPV behavior at different measurement temperatures. Furthermore, the contribution to the electronic behavior of possible contamination surface states is discussed.