On the alleviation of Fermi-level pinning by ultrathin insulator layers in Schottky contacts

With a few exceptions, metal-semiconductor or Schottky contacts are rectifying. Intimate n-Ge Schottky contacts are the most extreme example in that their barrier heights are almost independent of the metal used. Such behavior is characterized as pinning of the Fermi level. Quite recently, ultrathin insulator layers placed between the metal and the semiconductor were found to lower the barrier heights of Schottky contacts and to increase their dependence on the metals used. In this way ohmic behavior was achieved without alloying. The barrier heights of intimate Schottky contacts and the valence-band offsets of heterostructures are well described by the intrinsic interface-induced gap states (IFIGS). Insulators fit in this concept because they are large-gap semiconductors. This article demonstrates that the IFIGS concept also explains the experimentally observed alleviation of the Fermi-level pinning or, as it is also addressed, the Fermi-level depinning in metal-ultrathin insulator-semiconductor or MUTIS structures. Their barrier heights are determined by the IFIGS branch-point energy of the semiconductor and the dependence of the barrier heights of the insulator Schottky contacts on the metals used. Furthermore, saturation of the semiconductor dangling bonds by, for example, sulfur or hydrogen adatoms prior to the deposition of the metals also reduces or increases the barrier heights of Schottky contacts irrespective of the metals applied. In other words, no alleviation of the Fermi-level pinning or depinning occurs. These modifications of the barrier heights are explained by the partial ionic character of the covalent bonds between the adatoms and the semiconductor atoms at the interface, i.e., by an extrinsic electric-dipole layer. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3699180]