Theory of Si/transition-metal silicide schottky barriers

Abstract

The height of the Schottky barrier formed at transition-metal/Si interfaces varies over a very small range (≈0.4 eV) considering the wide range of electronic structures possible from one end of the transition-metal series to the other. Furthermore, the barriers are observed to form within a few monolayers of metal coverage, suggesting that the barrier is a property of the local bonding and that the true metallic states play only a minor role. A model has been developed to explain these facts in terms of the Fermi-level pinning mechanism of Schottky barrier formation. The physics contained in the model is that of a Si dangling bond sheltered from the transition-metal-silicide by an interfacial vacancy. Since (i) the dangling-bond is sheltered from the metallic-silicide and (ii) the atomic energy levels of the transition metal are out of resonance with Si, the dangling bond (which forms a level in the Si band gap) will be only weakly perturbed by the silicide. Thus this interfacial dangling bond can pin the Fermi level at nearly the same energy for all the transition-metal-silicides. A tight-binding calculation of the electronic structure of this defect at the NiSi 2/Si(111) interface has been performed for an infinite interface using the transfer-matrix technique. The results of this calculation are described in terms of a very simple molecular model.