LOW COVERAGES OF LITHIUM ON SI(001) STUDIED WITH STM AND ARUPS

We have used scanning tunneling microscopy (STM), scanning tunneling s pectroscopy, and synchrotron based angle-resolved ultraviolet photoele ctron spectroscopy (ARUPS) to study the initial stages of the adsorpti on of lithium onto the Si(001)-2x1 surface. Our STM study shows that t he half-missing dimer defects (C defects) act as sites for Li adsorpti on on this surface. Once these are occupied the Li adsorbs on top of t he dimer atoms and forms small clusters, which stabilize the dimer buc kling seen by the STM into areas of c(4x2) symmetry. This is observed with ARUPS as a peak at the Fermi level, originating from partial occu pation of an empty surface band associated with the c(4x2) reconstruct ion. On p-type Si substrates, STM shows that small amounts of Li cause further dramatic changes to the electronic structure of the surface. We observe negative differential conductance (NDC) on both Li sites an d clean Si dimers, as well as digital switching in the tunnel current over the Li sites. We propose that this trapping noise is caused by el ectrons emptying and filling thermally activated traps close to the Fe rmi level and that the NDC has its origin in coulombic repulsion by oc cupied traps on the surface.