Metal-derived band gap states: Ti on GaAs(110)

The emergence of filled and empty Ti-derived interface states in the band gap of GaAs has been observed for coverages as low as 0.03 monolayer with synchrotron radiation excited photoemission (XPS) and inverse photoemission (IPS) spectroscopies. The onset of the photoelectron threshold for filled states in the valence band spectra and of the IPE photon threshold for the empty states follow directly the position of the surface Fermi level in the band gap. The origin of these states is associated with the rehybridization of the Ti-3d electrons with the valence electrons of the Ga and As atoms of the interface. This notion is supported by XPS core level studies, which show the occurrence of exchange reactions between the Ti and Ga, and the out-diffusion of As and its reaction with Ti. Chemical rebonding at the interface, which produces bonding (filled) and nonbonding (empty) states, is thus the first demonstrable mechanism for Fermi level pinning and Schottky barrier formation