Pitfalls of using pressure to assign the luminescence of large-lattice-relaxation defects

Deep defects are often assumed to be insensitive to applied pressure because of their localized character. However, in recent photoluminescence (PL) experiments, several deep acceptor bands in ZnSe were found to shift with pressure substantially faster than the ZnSe bandgap. This shows that the optical (viz., PL) levels of these acceptors become more shallow under compression, a result that, if also true for the thermal defect levels, is important for p-type doping problems in II-VI semiconductors. We report investigations of the C_3v-relaxed isolated Zn-vacancy (V_Zn) in ZnSe that help to resolve these issues. High-pressure PL and PL-excitation (PLE) experiments and calculations are performed on this system. We find that the V_Zn-related PL and PLE bands have pressure coefficients that are, respectively, larger and smaller than that of the ZnSe bandgap. Hence, the Stokes-shift decreases with pressure. These results can not be understood without taking explicit account of lattice relaxation. We employ a defect-molecule model with atomic wave-functions to calculate semi-empirical configuration-coordinate diagrams for the V_Zn defect as a function of pressure. We find that compression increases the Jahn-Teller coupling, but not sufficiently to overcome lattice stiffening. Overall, the V_Zn thermal level deepens, inhibiting p-type doping.