Layers of material doped with Mg, Ca and C were grown onto sapphire by means of molecular beam epitaxy, and were investigated by using optical admittance spectroscopy. It was noted that, whereas p-type conductivity was achieved by Mg-doping, Ca and C doping did not over-compensate the background n-type conductivity. This was explained by the generation of compensating defects; mainly electron traps and donors. A correlation was observed between the creation of compensating centres and the flux of the doping element. It was concluded that the new donors due to C doping were N vacancies which generated a level at Ec – 0.053eV.

Incorporation of Deep Defects in GaN Induced by Doping and Implantation Processes A.Krtschil, H.Witte, M.Lisker, J.Christen, A.Krost, U.Birkle, S.Einfeldt, D.Hommel, A.Wenzel, B.Rauschenbach: Physica Status Solidi B, 1999, 216[1], 587-91