Extending the metal-induced gap state model to include silicides

The metal-induced gap states (MIGS) model has successfully explained the Schottky barrier height and its variation with metal work function in many different semiconductors and insulators. It states that the Schottky barrier height is determined by the consistently induced electronic charge accumulated at the metal-semiconductor interface. However, it fails in the area of silicides, where the question of anomalous Fermi-level pinning has endured for more than 30 years.

Three years ago, Robertson et al. figured out half the answer when they explained how silicides have a face – or orientation – dependence, which goes against the MIGS model. Now, they’ve found the missing link: a localized defect interface state around the Fermi level that leads to both weaker Fermi-level pinning and a face dependence. Their explanation of silicide’s Fermi-pinning behavior solves a long-standing mystery in semiconductor theory.

The change in Schottky barrier height with respect to metal work function, known as the slope parameter, is much larger for silicides than for elemental metals on silicon. To investigate why, the authors employed density functional theory supercell calculations to study the wavefunctions around the Fermi level for NiSi₂/Si and other silicide and arsenide interfaces.

They find that the apparent Fermi-level “depinning” in silicides is, in fact, not a reduction in gap states as its name suggests. Instead, the interfaces are found to contain bonding configurations such as dangling bonds, lateral bonds or mis-coordinated sites that create the addition of defect gap states. This new type of gap state, which causes Fermi-level pinning at a sequence of energies across the gap, should contribute to a more complete MIGS model.

Source: “Extending the metal-induced gap state model of schottky barriers,” by John Robertson, Yuzheng Guo, Zhaofu Zhang, and Hongfei Li, Journal of Vacuum Sciences & Technology B (2020). The article can be accessed at http://doi.org/10.1116/6.0000164 .