Evidence of Momentum Conservation at a Nonepitaxial Metal/Semiconductor Interface Using Ballistic Electron Emission Microscopy

L. D. Bell ,,'_/2 "-_Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology,Pasadena, Cal_m_ia 91109(Received 13 May 1996) f/V_. _7,_-- (_"/_.Ballistic-electron-emission microscopy (BEEM) spectroscopy has been performed on Au/Si(III)('_j)Z_,,AI/M<_)structures as a function of Au thickness and temperature. At 77 K a direct signature of parallel "-".-'I "- _momentum conservation at the Au/Si interface is observed in the BEEM spectra. The variationin spectral shape with both Au thickness and temperature places restrictions on allowable values of _ _ 7 _'/-_inelastic and elastic mean-free paths in the metal, and also requires the presence of multiple electronpasses within the Au layer. An independent indication of multiple reflections is directly observed inthe attenuation of BEEM current with Au thickness. [S0031-9007(96)01516-5]PACS numbers: 73.50.Gr, 61.16.Ch, 73.30.+y, 73.6t.AtThe verification of parallel momentum (kit) conser-vation at a metal/semiconductor (M/S) interface is offundamental importance to the operation of many noveldevice structures, such as metal-base transistors and quan-tum well photodetectors, and to transport models used tocharacterize them. In epitaxial M/S structures the questionis less controversial, since atomically abrupt interfaces canbe achieved between materials with matching lattice nets.The problem of identifying ktl conservation in the case ofa nonepitaxial evaporated metal film has been the subjectof much research and is conceptually less straightforward.Contributing to the difficulty is the insensitivity of con-ventional measurements to this process. In general, theproblem is one of separating a lack of kll conservation, in=the form of elastic interface scattering, from other scatter-ing processes.Ballistic-electron-emission microscopy [1] (BEEM),based on scanning tunneling microscopy [2] (STM), pro-vides a nanometer-scale method for probing transport andinterface properties. BEEM uses an STM tip to inject hotelectrons into a M/S structure, where the collector currenti(1,) which enters the semiconductor is measured. EvenBEEM measurements, however, have been inconclusivein determining the degree of kll conservation at nonepi-taxial M/S interfaces. One early prediction [3] basedon this idea was the BEEM spectrum for metal/Si(I 11).this was expected to differ dramatically from that ofnetal/Si(100), showing a much more slowly increasinghreshold region and a substantially different spectral;hape. This is due to the mismatch between the off-axis;i conduction-band minima and the forward-focused:lectron distribution in the metal produced by tunneling.This predicted behavior was not observed for Au/;i(lll). Instead, previously reported BEEM spectra [3]xe nearly identical to those for Au/Si(100). This obser-:ation, while not confirming klL conservation, does not dis-,rove it. Elastic scattering in the metal film can provideae necessary momentum for electrons to enter the Si(l I 1)onduction band, even if