Control of Schottky and ohmic interfaces by unpinning Fermi level

Abstract

We propose the first systematical method to control Schottky barrier heights of metal/semiconductor interfaces by controlling the density of interface electronic states and the number of charges in the states. The density of interface states is controlled by changing the density of surface electronic states, which is controlled by surface hydrogenation and flattening the surface atomically. We apply establishing hydrogen termination techniques using a chemical solution, pH controlled buffered HF or hot water. Also, slow oxidation by oxygen gas was used to flatten resultant semiconductor surfaces. The density of interface charges is changeable by controlling a metal work function. When the density of surface states is reduced enough to unpin the Fermi level, the barrier height is determined simply by the difference between the work function of a metal φ m and the flat-band semiconductor φ FB s. In such an interface with the low density of interface states, an ohmic contact with a zero barrier height is formed when we select a metal with φ m < φ FB s. We have already demonstrated controlling Schottky and ohmic properties by changing the pinning degree on silicon carbide (0001) surfaces. Further, on an atomically-flat Si(111) surface with monohydride termination, we have observed the lowering of an Al barrier height. Moreover, we found the recovery of an ohmic property after TiC formation at Ti/6H-SiC interface at 700°C whereas conventional 5% HF rinsed Schottky Ti/6H-SiC interfaces still have Schottky properties after TiC formation.