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

Deep defects are often assumed to be insensitive to applied pressure becaus e of their localized character. However, in recent photoluminescence (PL) e xperiments, several deep acceptor bands in ZnSe were found to shift with pr essure substantially faster than the ZnSe bandgap. This shows that the opti cal (viz., PL) levels of these accepters become more shallow under compress ion, a result that, if also true for the thermal defect levels, is importan t for p-type doping problems in II-VI semiconductors. We report investigati ons of the C-3v-relaxed isolated Zn-vacancy (V-Zn) in ZnSe that help to res olve these issues. High-pressure PL and PL-excitation (PLE) experiments and calculations are performed on this system. We find that the V-Zn-related P L and PLE bands have pressure coefficients that are, respectively, larger a nd 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 atom ic wavefunctions to calculate semi-empirical configuration-coordinate diagr ams for the V-Zn defect as a function of pressure. We find that compression increases the Jahn-Teller coupling, but not sufficiently to overcome latti ce stiffening. Overall, the V-Zn thermal level deepens, inhibiting p-type d oping.