AVS 45th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS1-WeM

Paper SS1-WeM6
Ballistic Electron Emission Microscopy Studies of Tunneling to Surface and Bulk States on Cu (111) Thin Films

Wednesday, November 4, 1998, 10:00 am, Room 308

Session: Physics of Semiconductors
Presenter: W.H. Rippard, Cornell University
Authors: W.H. Rippard, Cornell University
M.K. Weilmeier, Cornell University
R.A. Buhrman, Cornell University
Correspondent: Click to Email
Thin bilayer films consisting of a thin Au layer and an overlayer of Cu have been grown on the surface of hydrogen passivated Si (111) and Si(100) wafers. UHV STM measurements reveal that these bilayers consist of ~10 nm grains with surfaces that are either atomically flat or transversed by only a few atomic step-edges. X-ray diffraction indicates that these films, for both substrates, consist nearly completely of (111) normal oriented grains. When ballistic electron emission microscopy (BEEM) measurements are made on these systems, a strong contrast is seen between the BEEM current when the STM tip is over a flat surface and when it is positioned at a grain boundary or step edge, with the higher BEEM current occurring in the latter case. This contrast in current can be as much as a factor of three, depending on the sharpness of the STM tip, and occurs for both (100) and (111) oriented substrates. STM spectroscopy measurements indicate the presence of pronounced surface states on the flat areas of the Cu surface and an absence of thes states at grain boundaries and steps. We conclude that the low BEEM current which occurs when the STM tip is positioned over a Cu surface mesa is the result of tunneling predominately to empty surface states, while at grain boundaries and step edges the STM tip is able to tunnel more strongly to the empty bulk states of the Cu, which enhances the BEEM signal. This observation provides a strong demonstration of the role of the metal band structure in determining the amplitude of the BEEM signal, even in the case of the noble metals, and demonstrates a new means by which the degree of coupling between bulk and surface states on the (111) normal surface of Cu, Au and Ag can be determined by BEEM.