Physical and chemical effects at rare-earth-metal-SiO2-Si structures.

Abstract

Thin rare-earth-metal overlayers (Pr, Eu, Gd, and Yb) have been deposited in an ultrahigh-vacuum environment onto thin ${\mathrm{SiO}}_{2}$ layers on Si(111) substrates, and the resulting metal-insulator-semiconductor (MIS) structures have been investigated by photoelectron spectroscopy of core and valence states using synchrotron radiation, Auger electron spectroscopy, and by inverse-photoemission spectroscopy. The spectroscopic data, recorded as a function of metal coverage, clearly reveal the chemical reaction between the ${\mathrm{SiO}}_{2}$ and the rare-earth-metal atoms at room temperature for coverages >1 monolayer. The reaction yields metal silicide and metal oxide, thereby reducing the ${\mathrm{SiO}}_{2}$. At low metal coverages (1 monolayer) the Si 2p components of the Si substrate and of the ${\mathrm{SiO}}_{2}$ layer display different core-level shifts to higher binding energy, which are discussed in terms of changes of band bending in the Si as a result of charge injection, and in terms of changes of the band offset at the buried ${\mathrm{SiO}}_{2}$-Si interface; the latter is possibly mediated by rare-earth-atom diffusion through the insulating layer. Annealing of the reacted MIS structures to \ensuremath{\sim}500 \ifmmode^\circ\else\textdegree\fi{}C induces an additional solid-state reaction, and the results are consistent with the formation of silicate islands on the Si substrate.