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

GaNis a widely used material for optical and power devices. However,the performance of GaN-based devices is often reduced by charge trappingprocesses due to defect levels in the band gap. In this paper, westudied a GaN-on-silicon sample via Kelvin probe force microscopyunder variable illumination. This novel methodology allows the surfacephotovoltage (SPV) to be followed on the GaN surface as a functionof time, light intensity, and topography with nanometric resolution.The measured SPV decay after turning off the light is too slow (onthe order of seconds to days) to correspond to electron-holerecombination, suggesting that it is due to charge trapping. Two processesof 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 atdifferent measurement temperatures. Furthermore, the contributionto the electronic behavior of possible contamination surface statesis discussed.