Dual nature of the BeGa acceptor in GaN: Evidence from photoluminescence

Experimental studies of Be-doped GaN by photoluminescence (PL) confirmed theoretical predictions that the ${\mathrm{Be}}_{\mathrm{Ga}}$ acceptor in GaN has dual nature, namely a coexistence of a deep state with a localized hole and a shallow state with a delocalized hole. Electron transitions via the deep polaronic states of this defect cause the broad yellow luminescence band with a maximum at 2.1--2.2 eV. Analysis of the PL dependence on temperature reveals two polaronic configurations (labeled Be1 and Be2) with the \ensuremath{-}/0 transition levels at 0.3--0.4 eV above the valence band. The Be1-related luminescence band transforms into the Be2-related luminescence band at a critical temperature of about 100 K. In addition, the shallow state of the ${\mathrm{Be}}_{\mathrm{Ga}}$ (Be3) is found at 0.2 eV above the valence band. The associated ultraviolet PL band with the main peak at 3.26 eV can be activated by transitions of holes from state Be2 to state Be3 over a potential barrier at $T>100$ K. The origin of another Be-related acceptor with a level at 0.113 eV above the valence band, which is responsible for the ultraviolet PL band with a maximum at 3.38 eV, remains unknown. We do not find experimental evidence for the dual nature of other acceptors, such as ${\mathrm{Mg}}_{\mathrm{Ga}}$ in GaN and ${\mathrm{Li}}_{\mathrm{Zn}}$ in ZnO.