ELECTRONIC-STRUCTURE OF SOME I-III-VI2 CHALCOPYRITE SEMICONDUCTORS STUDIED BY SYNCHROTRON RADIATION

Photoemission spectra were measured for the I-III-IV2 chalcopyrite semiconductors CuInSe2, CuInS2, CuInTe2, and CuGaS2 with various photon energies (32 to 140, and 1253 eV). The partial density-of-states (DOS) of Cu 3d orbital is selectively observed at the photon energies beyond 100 eV. The DOS of Cu 3d spreads from the top of valence band (VB) to 5 or 6 eV below. Three dominant peaks are labeled as A, B, and C, the structures of which are interpreted as antibonding, nonbonding, and bonding states. Reflection spectra were also measured for CuInSe2 from 2 to 100 eV at room temperature and 98.5 K. The In 4d core reflection is observed at 17.5, 18.9, 19.8, and 21.2 eV at 98.5 K; very similar characteristics are also observed at 5.28, 6.21, 7.35, and 8.67 eV. These contributions are due to the Cu 3d nonbonding electrons. Photoemission and reflection experiments thus confirm conclusively the existence of nonbonding Cu 3d states in I-II-VI2 chalcopyrites. This result proves the theoretical model that attributes the upper VB to the antibonding state, in contrast with the bonding state of sp3-hybridized semiconductors.