Density of States and Hot Electron Effects in Ballistic Electron Emission Spectroscopy

Abstract

Ballistic electron emission microscopy (BEEM) and BEEM spectroscopy are reported for thin (≈50 A) 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 A in diameter) surface patches that are relatively smooth and parallel to the interface. For the conventional electron injection mode, the collector current Icexhibits pronounced but broad structures near tip biases VT of 4 and 6 V, whose origin is attributed to density of states effects in the semiconductor. For VT> 7 V, Icwas frequently observed to exceed the (constant) scanning tunneling microscope tip current. Under reverse bias (electron extraction) a novel hole current was observed for VT≳ 1.7 V, which, following an initial rise with increasing bias, would saturate and then decay, often leading to a polarity reversal for VT> 5.5 V. This behavior, as well as the large Icobserved at high forward bias, is attributed to impact ionization in the depletion region of the semiconductor. In addition to the conventional threshold in Icat 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 L1and X3 conduction band minima. © 1993, American Vacuum Society. All rights reserved.