Interface chemistry and Schottky-barrier formation of Y/GaAs interfaces are studied by high-resolution photoemission (PES) and inverse-photoemission (IPES) spectroscopies and are compared with results from other transition metals on cleaved GaAs surfaces. As with all other transition metals studied so far, the interface chemistry is characterized by a heavily reacted layer, which involves both Ga and As atoms. Ga diffuses into the overlayer, while As stays close to the interface, in contrast to the inverse behavior found in many other transition-metal GaAs systems. Contrary to, e.g., Ti or V, the PES and IPES spectra of Y overlayers show at low coverages no d-electron-related filled or empty states in the band gap of GaAs. The direct involvement of rehybridized d-electron states in the Fermi-level pinning, that has been proposed for several other transition metals, can be ruled out at least for the Y/p-type-GaAs interface. Alternative mechanisms responsible for the observed Schottky barrier are discussed. The variation of the barrier height with coverage indicates an influence of the metallic properties of the overlayer on the final Fermi-level pinning position. This applies especially for n-type GaAs, for which the final barrier height is not established before the overlayer reaches a thickness of 10 A. The variation of the band bending with coverage suggests a change in the energetic distribution of acceptor states with increasing Y coverage. © 1987 The American Physical Society.