From NiSi2 experiments to density functional theory calculations: How the Schottky barrier mystery was solved

Abstract

A cloud of mystery hung over the formation mechanism of the Schottky barrier height (SBH) for many decades. The experimental discovery of an insensitivity of the SBH of polycrystalline metal-semiconductor (MS) interfaces to the metal work function (WF), known as “Fermi-level pinning (FLP),” prompted the proposal of many empirical interface state models, which dominated the theoretical scenes of SBH research for decades. The reliance on empiricism in this field is curious because being a direct consequence of charge distribution at MS interfaces, the magnitude of the SBH should be predictable from principles that govern charge distribution in general, i.e., chemistry. With experimental work on epitaxial NiSi2/Si interfaces more than three decades ago that showed a sharp dependence of the SBH on atomic structure, the shroud covering the FLP mystery began to lift. Subsequently, SBHs of polycrystalline MS interfaces were widely recognized to be inhomogeneous, meaning that the FL was never “pinned” after all. The insensitivity of the SBH to metal WF was also shown to be in agreement with bond polarization that can be expected at the MS interface. Even though theoretical calculation was able to numerically reproduce SBHs for specific interfaces, it has not been possible to quantitatively predict/explain the SBH from chemical principles. That has remained the state of the affairs for SBH for the last two decades, until very recently, when density functional theory calculations demonstrated that SBH could be quantitatively predicted from basic chemical principles, provided the traditional analysis method, based on the Schottky–Mott model, was abandoned and a newly proposed neutral polyhedra theory was adopted. Through the same study, the cause for the FLP effect and the apparent experimental “pinning level” were also identified. It appears that the SBH mystery is finally over. This article chronicles important advances in the last four decades on different fronts of research that finally brought in a scientifically based understanding of the formation of the SBH.