Abstract

Ballistic Electron Emission Spectroscopy (BEEM) is a promising new variant of STM spectroscopy that allows the determination of Schottky barrier heights with high lateral resolution for relatively thick (≃100 Å) metal overlayers.^1,2 The technique encompasses the injection of electrons or holes with an STM, which then reach the interface without scattering (ballistically). At the interface they will be reflected unless sufficient bias is applied between the tunneling tip and the metal overlayer to overcome the Schottky barrier height (see Fig. 1). Once the electrons reach the conduction band of the semiconductor they will be detected as a collector current I_c. Representative I_c vs V_T, where V_T, is the tip-to-metal bias, are shown in Fig. 2 for Ag and Au films on GaP(110). The Schottky barrier height is associated with the voltage threshold V_o beyond which a current can be detected. Because of the "soft" turn on of I_c, V_o is poorly defined unless the appropriate shape of the I-V curve is known from theoretical considerations. The imaging of the variations in I_c as a function of latcral position is referred to as a BEEM image, and has been associated with variations in Schottky barrier heights across the interface.¹_’³ We will show here that another and perhaps more dominant source of contrast in BEEM images is the surface topography, as surface gradients may result in current injections that reach the interface at angles substantially off-normal, a condition which drastically reduces the collector current intensities. The current variations in Fig. 2 are attributed to such variations in the injection angles. In order to quantify this notion we have also developed a model for the interface transport that includes non-classical transmission across the metal-semiconductor interface, as well as off-normal angles of incidence. An outline of these concepts will be presented here, as well as a quantitative comparison of the the model with topographic and BEEM data for several metals on GaP(110).

© 1991 Optical Society of America