Using ballistic electron emission microscopy to investigate the metal-vacuum interface

Abstract

This dissertation investigates the possibility of using the ballistic electron microscope (BEEM) to study the metalvacuum interface. In order to do that, we have designed and built a novel experimental set-up which consists of an STM tip from which electrons tunnel into a thin ([ 60 nm), free-standing metal film in vacuum ambient. When the tunnel bias exceeds the work function of the metal, some small fraction of the tunneling electrons traverses through the film without any energy loss, and emits into the vacuum through the back side of the film. The rate of emission of such ballistic electrons, which is called the collector current, is measured by a channel electron multiplier. One of the major challenges for this investigation was preparing free-standing thin films. We found that we can prepare good-quality free-standing Au thin films by the following steps: a) evaporating Au onto a (100) face of NaCl at room temperature, b) dissolving the NaCl in a 50-50 mixture of ethyl alcohol and distilled water, and c) catching the Au film that floats on the surface of the solvent onto a Cu grid. Subsequent annealing increased the grain size, and improved the bonding of the film onto the grid. We have succeeded in observing ballistic electron emission through these free-standing thin films, even though the collector current tended to decay in a time interval of a few tenths of a seconf. The exact cause of this decay is not known, however we have suggested some possibilities. By ramping the bias voltage from about 0.2 V to about 10.5 V, we find the threshold bias voltage at which the collector current begins. This threshold voltage is an upper limit for the work function of Au. From our data we obtained a value of 5.2 V for this upper limit. We also have plotted the collector current, that was averaged over an scan area of 375 nm X 375 nm, against the tunnel bias. This plot shows that, for this region, the lowest threshold bias voltage for ballistic electron emission is between 3.5 V and 4.5 V. We have obtained also a collector-current image that showed spatial variations of the collector current.