Low coverages of lithium on Si(001) studied with STM and ARUPS

We have used scanning tunneling microscopy (STM), scanning tunneling spectroscopy, and synchrotron based angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) to study the initial stages of the adsorption of lithium onto the Si(001)-2x1 surface. Our STM study shows that the half-missing dimer defects (C defects) act as sites for Li adsorption on this surface. Once these are occupied the Li adsorbs on top of the dimer atoms and forms small clusters, which stabilize the dimer buckling 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 occupation of an empty surface band associated with the c(4x2) reconstruction. 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 and 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 electrons emptying and filling thermally activated traps close to the Fermi level and that the NDC has its origin in coulombic repulsion by occupied traps on the surface.