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 Ec – 0.051 eV, and it is correlated with the nitrogen vacancy (NV) 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 λ = 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 β 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.