DENSITY-OF-STATES AND HOT-ELECTRON EFFECTS IN BALLISTIC-ELECTRON-EMISSION SPECTROSCOPY

Ballistic electron emission microscopy (BEEM) and BEEM spectroscopy are reported for thin (almost-equal-to 50 angstrom) Cr layers on n-type GaP(110). BEEM images reveal that current transport across the interface occurs primarily in the vicinity of grain boundaries where the Cr is thinnest, and on small (< 200 angstrom in diameter) surface patches that are relatively smooth and parallel to the interface. For the conventional electron injection mode, the collector current I(c) exhibits pronounced but broad structures near tip biases V(T) of 4 and 6 V, whose origin is attributed to density of states effects in the semiconductor. For V(T) > 7 V, I(c) was frequently observed to exceed the (constant) scanning tunneling microscope tip current. Under reverse bias (electron extraction) a novel hole current was observed for V(T) greater than or similar to 1.7 V, which, following an initial rise with increasing bias, would saturate and then decay, often leading to a polarity reversal for V(T) > 5.5 V. This behavior, as well as the large I(c) observed at high forward bias, is attributed to impact ionization in the depletion region of the semiconductor. In addition to the conventional threshold in I(c) at 1.31 V, attributed to electron transmission across the Schottky barrier into the X1 conduction band minima, a second, more pronounced threshold at 1.69 V was observed for the first time for GaP. It is assigned to electron transfer into the higher lying L1 and X3 conduction band minima.