Optical spectroscopy of Mg- and C-related donor and acceptor levels in GaNgrown by MBE

We present photoreflectance (PR) in combination with temperature and densit y dependent photoluminescence (PL) on undoped as well as on Mg- and C-doped GaN grown by molecular beam epitaxy (MBE). We determined the binding energ y of the free A-exciton, the residual donor-bound exciton and the Mg-accept or-bound exciton to (26 +/- 1), (5 +/- 1) and (18 +/- 1) meV, respectively. In n-type GaN a residual DAP band appears independent of the particular ty pe of dopant. Free-hole concentrations up to 10(18) cm(-3) in MBE grown p-G aN:Mg were achieved without generation of deep compensating donors. Moreove r, we found evidence for a second DAP series involving a shallow compensati ng donor level built in along with Mg doping. The correlation of different DAP bands to the respective type of conductivity opens an alternative way t o control the p-type doping in GaN:Mg without the need for electrical conta cting.