Metal overlayers on the MBE-grown ZnSe(001) surface

Abstract

N-type ZnSe thin films have been grown by MBE on GaAs (001) surfaces and capped with an amorphous selenium layer. The Se cap was thermally desorbed under ultrahigh vacuum to recover the (2 × 1) and c(2 × 2) reconstructed surfaces. Selected metal contact formation was monitored using core and valence level photoelectron emission spectroscopy by in-situ exposure of the surface to heated sources of Au, Ag and Pb. In each case, lineshape analysis of emission spectra indicated a low level of interfacial mixing and provided an insight into the metal layer growth model. Both Au and Ag were found to grow in closely spaced islands of approximately equal height. The morphology of Au and Ag layers was confirmed by cross-sectional transmission electron microscopy. Monitoring of core and valence level emission peak positions allowed the determination of the metal-n-ZnSe Schottky barrier height, for a sufficiently thick metallic layer. Measurements on this wide-gap semiconductor, even at 300 K, were influenced by the presence of a surface photovoltage, which could be identified and subtracted for a fully-formed metallic layer. The n-type ZnSe Schottky barrier heights inferred from the relative Fermi level shifts ( Φ BN( Au) = 1.74 eV , Φ BN( Ag) = 1.47 eV and Φ BN( Pb) = 1.25 eV ), were found to scale with the metal work function for these three unreactive interfaces.