Chemical reactions and local charge redistribution at metal-CdS and CdSe interfaces

The surface electronic structure of CdS and CdSe has been studied during the initial stages of Schottky-barrier formation with metals. Surface photovoltage spectroscopy, low-energy-electron-loss spectroscopy, and ultraviolet photoemission measurements taken as a function of metal overlayer coverage show that interface chemical reactions and local charge redistribution dominate the Schottky-barrier formation for both of these semiconductors. Despite the reported contrast in Fermi-level pinning behavior of CdS and CdSe, intrinsic surface states play no role in determining their Schottky-barrier heights. The microscopic metal-semiconductor bonding causes work-function and band-bending changes which determine the macroscopic space-charge characteristics of the interface. A critical heat of reaction $\Delta H_R^C\sim 0.5$ eV/(metal atom) characterizes the compound formation inferred from alterations in the interface dielectric properties.