Epitaxially-grown layers of C-doped material, which exhibited n-type conductivity, were investigated by means of direct-current photoconductivity measurements and optical admittance spectroscopy. It was found that the doping led to the formation of a defect level, at a photon energy of 0.04 to 0.07eV below the band-gap, which became more pronounced with increasing C concentration. At the same time, the responsivity at 300nm decreased with increasing power of the C filament. This was attributed to the incorporation of deep defects, with an energy position of between 0.35 and 0.5eV below the band-gap, that were introduced by doping. The time-constant of the persistent photoconductivity decreased markedly with increasing doping level, and was related to a decrease in the defect concentration 2.2 and 2.4eV (yellow band); as observed by means of optical admittance spectroscopy.

Influence of Carbon Doping on Photoconductivity in GaN Layers M.Lisker, A.Krtschil, H.Witte, J.Christen, A.Krost, U.Birkle, S.Einfeldt, D.Hommel: Physica Status Solidi B, 1999, 216[1], 593-7