Optical admittance spectroscopy studies near the band edge of gallium nitride

Nominally undoped n-type GaN layers grown by metalorganic chemical vapor deposition on silicon substrates were investigated using Thermal Admittance Spectroscopy and Optical Admittance Spectroscopy (OAS). A defect level was observed at E-c - 0.051 eV, and it is correlated with the nitrogen vacancy (N-V) which is a donor in GaN. Illuminating the samples with a monochromatic light with wavelengths ranging from 200 nm to 450 nm, the OAS spectrum was measured at different temperatures and with different excitation light intensities. A dominant peak was observed in the OAS spectrum at lambda = 365 nm (3.40 eV); this is attributed to transitions from the valence band to the donor level. Our results show that the saturation level, Gm, of the photoconductance is a function of both light intensity and temperature. The photoconductance decay, after the illumination has been terminated, is non-exponential but it is fully described by the stretched exponential function. The value of beta ranges from 0.78 to 0.86. The analysis suggests that the observed photoconductance decay is due to thermal emission of photo-excited carriers from the donor level. (C) 2014 AIP Publishing LLC.