Schottky barrier modulation at metal contacts to CdS and CdSe

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Abstract
Soft x-ray photoemission spectroscopy measurements have been used to study the pinning position of the surface Fermi level as well as the nature of the electrostatic band bending for single and interlayer metal contacts to ultrahigh-vacuum-cleaved CdSe and CdS (101̄0) surfaces. Single metal (Al,Au) contacts are found to exhibit a one-to-one correspondence between the pinning position and the effective Schottky barrier height as measured by in situ C–V and I-V analyses. However, a fundamentally different mechanism of barrier modulation is indicated for interlayer contacts, i.e., contacts formed by interspersing an ultrathin reactive metal interlayer (Al) between the semiconductor and a noble metal contact (Au). Core level broadenings as a function of photon energy are interpreted in terms of sharp band bending at the surface, leading to the possibility of quantum mechanical tunneling through the barrier. This barrier narrowing effect is attributed to an indirect doping effect as a consequence of metal–semiconductor interfacial reaction.

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