BALLISTIC ELECTRON-EMISSION SPECTROSCOPY OF NOBLE METAL-GAP(110) INTERFACES

Ballistic electron emission microscopy (BEEM) was used in an UHV environment to investigate the current-voltage spectra of Cu, Ag, and Au films on cleaved n-type GaP(110). Current thresholds, which occur when the applied tip-to-metal bias exceeds the Schottky barrier height, were remarkably uniform to within +/- 0.02 V for any particular metal-GaP interface, but varied for interfaces with different metals. The fitted threshold voltage V(o) depends on the model used for the interface current transport. Its value varies by approximately 0.05 V between a prior model that predicts a (V - V(o))2 dependence near threshold and a new (V - V(o))5/2 dependence proposed here. The latter includes an energy dependent transmission coefficient for transport across the metal-semiconductor interface. Significant BEEM current variations and fluctuations were observed, which are associated with topographic gradients of the metal surface and sudden changes in the tunneling geometry during the experiment. The transport model is further developed to include average angles of incidence of the injected electrons that are off-normal at the metal-semiconductor interface, a condition that occurs when the tunneling tip is scanning over nonplanar surface features. Model predicted current variations are consistent with the range of surface gradients observed in the topographic images of the noble metal films on GaP.