Photoemission Studies of Cesium Telluride

Photoemission measurements have been made at photon energies from 3 to 12 eV on Cs₂Te films at pressures less than 5 10^-10 Torr. Inelastic electron-electron scattering and electron-phonon scattering have a dramatic effect on the photoemission data. By using the three-step model of photoexcitation, hot-electron transport, and escape to describe photoemission in Cs₂Te, both the quantum yield above the main threshold and the energy distributions of photoemitted electrons (EDC's) can be qualitatively understood. Important features in the band structure Cs₂Te have also been deduced from the behavior of structure in the EDC's. Three maxima in the conduction-band density of states are located at 4. 05 0. 1, 4. 9 0. 1, and 5. 4 0. 1 eV above the top of the valence band. Two peaks observed in the valence-band density of states at 0. 7 0. 1 and 1. 4 0. 1 eV below the top of the valence band have been assigned to the spin-orbit-split 5p orbitals of Te. The value for the spin-orbit splitting (0. 65 0. 1 eV) is in excellent agreement with the theoretical free-atom value. In addition, an upper bound of 2 eV was set for the over-all width of the valence band in Cs₂Te. Cs₂Te films overcoated with 5% additional Te were also studied. Both these films and the Cs₂Te films prepared without additional Te display a low yield (10^-5 electrons/incident photon) below the main threshold of the quantum yield. It was found that this low yield cannot be explained in terms of a Cs or Te in an otherwise stoichiometric single-phase compound.