Ballistic Electron Emission Microscopy Studies of Gold-Silicon Interfaces

Ballistic electron emission microscopy (BEEM) has been used to study electron transport and scattering in gold and at the gold-silicon interface. The BEEM technique utilizes a scanning tunneling microscope (STM) to provide high spatial resolution at the interface (~ 10 A) in addition to a variable electron injector. The energy of the electrons from the tunneling tip (0.5 -5.0 eV) can be adjusted independently of the tunnel current. In BEEM, the buried Schottky barrier acts as a 'filter' for the electrons incident upon it, allowing only those satisfying energy and transverse momentum constraints to pass. Transverse momentum constraints are calculated for the Au-Si interface, and are consistent with the experimental observation that the BEEM current vs. tip voltage spectra (BEEM spectra) are independent of the silicon face upon which the interface is constructed. The gold-silicon interface exhibits large variation in the scaling of the BEEM spectra (ballistic transmittance). This is studied as a function of interface preparation (doping) and time. It is attributed to a combination of variation in the scattering by and thickness of the dopant layer due to composition changes and terraces on the interfacial surface of the gold, respectively. The shape of the BEEM spectra is found to be independent of the interfacial chemistry, but dependent on the defect properties of the gold. Reasons for the dependence are discussed. A high bias can be used to locally modify the interface and, if higher still, the bulk gold film. Real time studies of the ballistic transmittance enhancement and degradation indicate the importance of inelastic scattering. The hot electrons injected by the tunneling tip produce vacancy adatom pairs at the interfacial surface of the gold. The adatoms form terraces which produce enhancement in the ballistic transmittance. A limit on the adatom production rate can be found. At higher sample-tip bias, a second gold terrace begins to form. This brings the gold in close proximity to the silicon, and gold-silicon intermixing ensues. The additional scattering accounts for the ballistic transmittance degradation.