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

We present photoreflectance (PR) in combination with temperature and density dependent photoluminescence (PL) on undoped as well as on Mg- and C-doped GaN grown by molecular beam epitaxy (MBE). We determined the binding energy of the free A-exciton, the residual donor-bound exciton and the Mg-acceptor-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 type of dopant. Free-hole concentrations up to 10¹⁸ cm^-3 in MBE grown p-GaN:Mg were achieved without generation of deep compensating donors. Moreover, we found evidence for a second DAP series involving a shallow compensating donor level built in along with Mg doping. The correlation of different DAP bands to the respective type of conductivity opens an alternative way to control the p-type doping in GaN:Mg without the need for electrical contacting.