Correlation of Metal-Semiconductor Barrier Height and Metal Work Function; Effects of Surface States

Abstract

The validity of the Schottky theory of metal-semiconductor contacts is examined. This is accomplished by deriving values of metallic work function for several metals on Si, CdS, GaAs, and GaP through the use of published values of barrier heights. Values of electron affinity for the four semiconductors are also derived. Close agreement is obtained between the work function values of a given metal on different semiconductors. The derived work function values also agree in most cases with values measured recently under conditions that correspond at least approximately to the conditions existing for evaporated-metal-semiconductor systems. The derived values of electron affinity do not agree, however, in general with the values measured by photoelectric means. It is shown that this disagreement can be accounted for on the basis of a high density of surface states near the valence band edge of the semiconductor. In the case of GaAs, it appears that these surface states move up to the Fermi level for the high work function metals and ``pin'' the barrier at about 0.85 to 0.90 eV.