Empty semiconductor surface states: Core‐level photoyield studies

Abstract

Using partial yield spectroscopy, we have investigated the variation in the excitation of core electrons to the empty surface states on II–VI, III–V, and group IV semiconductors. From measurements of cleaved surfaces of ZnS, ZnSe, ZnTe, CdTe, GaAs, Si, and Ge, strong transitions to empty surface states from p‐core electron initial states are observed from the anion in ZnS (110) and the cation in ZnSe (110). Neither Ga (3p) nor As (3d) core electrons were photoexcited to the GaAs (110) unfilled surface states. Further, p‐core electrons do not make optical transitions to Si (111) or Ge (111) unoccupied surface states. We propose that the empty orbitals for covalent surfaces are mainly p‐character, which for compound semiconductors are localized on the cation; if there is anion state density, it is mainly s‐like. In contrast, for ionic surfaces, s‐like anion and cation wave functions form the unoccupied surface states. The II–VI core‐to‐surface transition is ∠2.5 eV below the photoemission‐deduced conduction band edge; this final‐state effect is much larger than for either bulk transitions in the same solid or surface transitions in less ionic crystals (by an amount much greater than the ratio of the bandgaps).