Band structure of CdS and CdSe at high pressure

The indirect energy gaps of NaCl-structured CdS and CdSe are measured using absorption spectroscopy to approximately 55 and 42 GPa, respectively. The gaps shift linearly with pressure and their pressure dependence generally is in good agreement with previous measurements to 16 GPa. Also, band calculations using pseudopotentials and density-functional theory are conducted on CdS as a function of pressure. The shift in the energy gap of CdS with pressure (dEgap/dP) is experimentally determined to be -5.7(6)x10-3 eV/GPa, in good agreement with the theoretical calculation of -5.6x10-3 eV/GPa. The pressure dependence of the gap of CdSe is measured to be -4.0(2)x10-3 eV/GPa, while previous theoretical work yielded -6.0x10-3 eV/GPa. Relative to a previous experimental result on CdSe, our measured shift is in considerably closer agreement with theory. The energy gap in both materials is indirect throughout the pressure range of these measurements. There is also no compression-induced change in the location in k space of the valence-band maxima for either the experimental or theoretical results. In contrast to previous inferences for CdS, the gap appears to remain from the L point to the X point up to our maximum pressure. Based on a simple linear extrapolation of our data, we estimate the metallization pressures of NaCl-structured CdS and CdSe to be above 250 and 150 GPa, respectively. Because phase transitions occur in both materials below these pressures, it is unlikely that either of the NaCl-structured phases will undergo metallization. copyright 1996 The American Physical Society