Study on characteristics of GaN epilayers grown on Al2O3 by using surface photovoltage

Three types of gallium nitride (GaN) materials grown on sapphire (001) by metal chemical vapor deposition (MOCVD) were design for the study of different doping types on the characteristics of GaN epilayers. Surface photovoltage (SPV) spectroscopy of samples was measured in the photo energy range 2.9≤hv≤4.5eV with different chopping frequency. The similar surface photovoltage signals were obtained under sub-band-gap illumination in the photo energy range 2.9≤hv≤3.4 eV and the origins about signals were determined by comparing the surface photovoltage magnitude and phase spectra. By changing the incident light frequency, it can be found that the surface photovoltage magnitude shows a downward trend in the entire photon energy range, and the surface photovoltage magnitude has a more significant decrease in the sub-band gap region. In additional, a laser with 3.06eV photo energy was used to aid measured the surface photovoltage signals of sample with Si doped in the photo energy range 2.9≤hv≤4.5eV with chopping frequency at 172 Hz, 440 Hz, 1k Hz and 3k Hz. The defect states at the interface of the undoped GaN layer and Al2O3 were proved to be the major contribution to the SPV signals in the sub band gap region and were "slow processes" during the formation of SPV of the GaN samples.