Unpinning the GaAs Fermi level with thin heavily doped silicon overlayers
A theoretical investigation is made into the unpinning of the GaAs Fermi level (E/sub f/) at Schottky contacts by thin interfacial layers of heavily doped Si. The pinning mechanism is assumed to be a plane of interface states in the GaAs. The method used is to solve Poisson's equation numerically as a two-point boundary problem across the semiconductor. The results show that E/sub f/ can be moved from its pinned position to near the edge of the silicon valence band maximum or conduction band minimum at the Si/GaAs heterojunction with heavily doped p-Si or n-Si overlayers, respectively. This unpinning is observed with and without thick metallization on the Si. Exactly analogous results are obtained for Ge interfacial layers. The results are in good agreement with the experimental results obtained by J.R. Waldrop and R.W. Grant (1988). Although the unpinning is seen to occur for interface-state densities sufficient to pin E/sub f/ at the free GaAs surface, interface-state densities high enough to result in significant E/sub f/ pinning at metal/GaAs contacts are seen to prevent such unpinning by Si interfacial layers. It is therefore suggested that Si (or Ge) deposition gives rise to fewer interface states in the GaAs than does metal deposition. >