CURRENT OSCILLATIONS IN THIN METAL-OXIDE-SEMICONDUCTOR STRUCTURES OBSERVED BY BALLISTIC-ELECTRON-EMISSION MICROSCOPY

Quantum interference oscillations of electrons in a thin SiO2 layer we re observed by ballistic electron emission microscopy (BEEM). With BEE M, electrons are injected across the gate of a metal-oxide-semiconduct or (MOS) structure and directly into the conduction band of the SiO2. The MOS capacitor consisted of a 5 nm thick Pd film deposited on a 2.8 +/-0.2 nm oxide thermally grown on Si(100). Oscillations with up to fo ur peaks in an energy range of 0-3 eV above the injection threshold we re noted. Their magnitude is of the order of 30% of the underlying BEE M current. The oscillations were most salient and their energy locatio n repeatable at points of the sample that were previously not exposed to the electron beam. Even modest exposures caused a buildup of positi ve charge. This charge resulted in energy shifts, as well as a weakeni ng of the oscillations, both of which are a consequence of the added s cattering and local field inhomogeneities associated with the random d istribution of the positive charge. Solutions of the Schrodinger equat ion that included a built-in oxide potential of 0.20 V and image force effects at both interfaces gave excellent fits to the experimental da ta for an effective electron mass in the oxide m(ox) = 0.63 +/- 0.09m( o). The uncertainty in m(ox) arises from an uncertainty of +/-0.2 nm i n the determination of the oxide thickness by ellipsometric methods. N evertheless, the obtained value is well above the generally accepted v alue of 0.5m(o). (C) 1998 American Vacuum Society.