Chemical bonding and reactions at the Pd/Si interface

The electronic structure of the clean Pd/Si(111) and Pd/Si(100) interfaces has been investigated using angle-integrated and angle-resolved ultraviolet photoemission (UPS) and Auger electron spectroscopies (AES), in conjunction with transmission electron microscopy (TEM), work-function, and low-energy electron diffraction measurements. Since the interface is highly reactive, studies were made by processing the reaction in two ways: (i) sequential annealing steps of thick metal overlayers, and (ii) sequential deposition steps of very thin metal overlayers, considerably less than a monolayer per step. The UPS and AES spectra of the bulk reaction product (${\mathrm{Pd}}_2$Si) were first determined in films sufficiently thick for TEM phase analysis (≥5 Å), taking care to be sure that surface segregation effects (due to thin-film reaction kinetics) in these cases were negligible. On this basis, lower coverage (submonolayer) spectra could then be interpreted. Formation of the ${\mathrm{Pd}}_2$Si compound is interface driven and extremely rapid at the clean Pd/Si interface. This compound formation dominates the microscopic chemistry and electronic properties of the interface. Chemical shifts observed in the electron spectroscopy measurements can be interpreted in terms of real or effective stoichiometry variations in the ${\mathrm{Pd}}_2$Si compound (i.e., retaining its lattice structure); these appear as Si-rich ${\mathrm{Pd}}_2$Si near the interface, with the Si excess decreasing with distance from the interface. Although silicide formation accounts for most aspects of the electronic structure, other effects such as localized interface bonds, defect states, etc., may make additional contributions at low metal coverage.