STM studies of epitaxial overlayers formed by metal deposition : Mo on MgO, Ni on SiC and Sn on Si

Abstract: Magnetron sputtering and molecular beam epitaxy (MBE) have been used to deposit metallic overlayers on semiconducting and insulating materials such as Mo on MgO, Ni on SiC(0001) and Sn on Si(111). The layers have been annealed and characterized with scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), low energy electron diffration (LEED), refiecti ve high energy electron diffraction (RHEED) and atomic force microscopy (AFM).Thin Mo films were grown on MgO(OOI) substrates by ultra-high vacuum DC magnetron sputter deposition. The generated surface was characterized with LEED, RHEED and STM and it was found that oxygen is present on the surface during the initial stages of growth. Oxygen-derived reconstructions of the l\Io(lOO) surfaces are found below film thicknesses of ~ 50 nm and it is shown that the oxygen originates from the MgO substrate. The growing film continuously reconstructs, with R26°33', p(2 x 2) and c(2 x 2) reconstructions on l\Io(lOO) indicating oxygen coverages ranging from ~ 0.8 to 1 ML. Models for the p(2 x 2) and c(2 x 2) reconstructions are proposed. There is also a discovery that the Mo surface is interspersed with 2 x 2 holes not visible with space averaging techniques such as LEED or RHEED.Ni has been deposited on 6H-SiC(OOOI) substrates and subsequently annealed to 600, 800 and 1000°C. At 800 and 1000°C, islands are formed. The islands feature extremely sharp edges and exceptionally fiat tops containing a peculiar "stiched" pattern on top. The islands were ~300 nm in diameter and ~30 nm high and separated ~5μm from each other. A second type of island was also found: ~1.5 μm in diameter, ~10 nm high and separated by ~10 μm. AES revealed that the first type contains Ni and C while the second type contains Ni, C and Si. In both types is C graphitically bound, as opposed to the substrate where it is carbidically bound. From comparison to literature, it is plausible that the first type is a graphite intercalation compond (GIC) with the Ni as the intercalant. An indexing of Ni on the top graphite sheets is presented for each annealing temperature.The -Sn/Si(111) surface has been studied with STM and AES, and it is found that it contains five different types of defects originating from substitutional atoms and vacancies. The influence the defects have on their immediate neighbourhood is investigated and found to extend to the third nearest  adatom neighbour. Close to some defect constellations, a very local (3 x 3) periodicity in the apparent height in the STM images is found. This periodicity is shown to be a linear superposition of the perturbations generated by the individual defects in the constellations. The superposition of the height modulation is accurate and linear for a wide range of tip biases, as for combinations of different types of defects. Its linearity is explained in terms of small perturbations in the surface electronic structure. vVe also provide an explanation why the linearity breaks down when large perturbations are probed with small tip biases.