Ballistic Electron Emission Microscopy

Abstract

Ballistic Electron Emission Microscopy (BEEM) is a scanning-tunneling-microscopy-based technique which gives a plan view, nanometer scale image of electronic properties of a buried interface. Among the properties which can be imaged are barrier height, the edges of multiple conduction and valence bands, and carrier scattering. These properties can be correlated with strain, defects, and stoichiometry variations at the interface. A topographic image of the sample surface is aquired simultaneously with the interface image. BEEM data has been reported from six laboratories to date, including images of Au/Si, Au/GaAs, NiSi2/Si, and Au/CdTe. Several new systems, some designed for ultra-high-vacuum operation, are under construction.In the simplest implementation, electrons or holes are injected from a tunnel tip into the metal electrode of a metal-semiconductor diode (figure 1). The carriers are ballistically transported across the electrode and, if momentum, energy, and scattering criteria are satisfied, are collected in the semiconductor base. Momentum conservation requires the ballistic electron or hole beam to cross the interface within a small cone of solid angle, and hence the collected current only reflects injection into a small region of the Brillouin zone. This small critical angle cone is responsible for the high spatial resolution of BEEM. By varying the potential of the initial injected beam (i.e. tip voltage with respect to electrode), the electronic dispersion relation of the substrate can be spectroscopically probed.