Photoemission Studies of Cesium Telluride

Abstract

Photoemission measurements have been made at photon energies from 3 to 12 eV on ${\mathrm{Cs}}_{2}$Te films at pressures less than 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ 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 ${\mathrm{Cs}}_{2}$Te, 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 ${\mathrm{Cs}}_{2}$Te 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 \ifmmode\pm\else\textpm\fi{} 0.1, 4.9 \ifmmode\pm\else\textpm\fi{} 0.1, and 5.4 \ifmmode\pm\else\textpm\fi{} 0.1 eV above the top of the valence band. Two peaks observed in the valence-band density of states at 0.7 \ifmmode\pm\else\textpm\fi{} 0.1 and 1.4 \ifmmode\pm\else\textpm\fi{} 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 \ifmmode\pm\else\textpm\fi{} 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 ${\mathrm{Cs}}_{2}$Te. ${\mathrm{Cs}}_{2}$Te films overcoated with 5% additional Te were also studied. Both these films and the ${\mathrm{Cs}}_{2}$Te films prepared without additional Te display a low yield (\ensuremath{\le} ${10}^{\ensuremath{-}5}$ 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.