The N levels in 4H-type material were determined by means of thermal admittance spectroscopy. The values of Ec - 0.053eV, for N at the hexagonal site, and Ec - 0.10eV, for N at the quasi-cubic site, agreed with previous data that had been obtained using other techniques. The deep levels in 4H-type material were studied by using optical admittance spectroscopy. As well as the conductance peak that corresponded to band-to-band transitions, these spectra revealed 4 other conductance peaks. These peaks corresponded to the photo-excitation of carriers from the defect levels and into the conduction band. It was deduced, from a comparison with 6H-type material, that one conductance peak (b4) was due to the excitation of electrons from the V donor at Ec - 1.73eV. Photo-conductance build-up transients of the Ec - 1.73eV level could be fully described by a single exponential term. This suggested that only 1 center contributed to the observed conductance. The decay kinetics of a persistent photo-conductance that was due to the Ec - 1.73eV level obeyed a stretched exponential law. The potential barrier to the recapture of photo-excited carriers was estimated to be equal to 0.018eV for the V donor level in 4H-type material.

A.O.Evwaraye, S.R.Smith, W.C.Mitchel: Journal of Applied Physics, 1996, 79[10], 7726-30