Experimental and Theoretical Studies of Metal Adsorbates Interacting with Elemental Semiconductor Surfaces
Abstract: Metal adsorbates on semiconductor surfaces have been widely studied over the last few decades. The main interest is focused on various one or two-dimensional structures that exhibit interesting electronic and atomic properties. This thesis focuses on metal adsorbates interacting with the Si(111) and Ge(111) surfaces. The main experimental techniques used in the thesis include angle resolved photoelectron spectroscopy (ARPES), core-level spectroscopy, scanning tunneling microscopy (STM), and low energy electron diffraction (LEED). The experimental studies have, in some cases, been complemented by theoretical electronic structure investigations based on density functional theory (DFT).Silver (Ag), a noble metal, gives rise to several reconstructions on the (111) surfaces of Si and Ge. The Ag/Si(111) surface has been extensively studied, but the Ag/Ge(111) surface has not been given similar attention, and there are no detailed experimental nor calculated electronic band structures available in the literature. Thus, a detailed ARPES investigation of the electronic structure of the Ag/Ge(111) surface, with nominally 1 monolayer (ML) of Ag, is presented in the thesis together with its atomic structure.The Ag/Si(111) and Ag/Ge(111) surfaces were also studied by first principles DFT based calculations (WIEN2k). Two atomic models have been suggested for the surfaces in the literature, i.e., the honeycomb-chained-trimer (HCT) and the in-equivalent trimer (IET) models. Band structure calculations were performed for both models, and comparisons between calculated and experimental surface band structures are presented for the Si and Ge cases.Adding approximately 0.2 ML of Ag to Ag/Ge(111) results in a 6×6 phase. The electronic structure of the surface is presented in detail. Several new bands appear in the energy region close the Fermi level, which can all be explained by umklapp scattering by reciprocal lattice vectors of the 6×6 lattice. A metal to semiconductor transition, associated with the to 6×6 structural change, is explained by gaps opening up where the umklapp scattered bands cross.After having established sufficient understanding of the Ag/Si(111) and Ag/Ge(111) surfaces, they were used as substrates for the formation of binary surface alloys. An amount of 0.45 ML of Sn, in combination with the Ag of the Ag/Ge(111) surface, forms a well-defined xbinary alloy. The surface band structure shows some modifications compared to that of Ag/Ge(111) surface. The STM results show clearly the x periodicity.A Sn coverage of 0.75 ML on the Ag/Ge(111) surface results in a very wellordered 3×3 surface alloy. This alloy shows a very rich surface band structure in which the upper band exhibits peculiar splits. Two-dimensional constant energy contour data reveal the existence of two rotated contours which is related to the presence of split bands in certain directions. STM images show a hexagonal or a honeycomb structure depending on sample to tip bias.A similar amount of Sn (0.75 ML) was also evaporated onto the Ag/Si111) surface, with the purpose to form a surface alloy on Si(111). This resulted in a very well-ordered Sn/Ag/Si(111)2×2 periodicity. The surface shows an interesting free electron like band which crosses the Fermi level. STM images reveal clear, but differently looking, protrusions in the 2×2 unit cell when comparing empty and filled state images. The atomic structure of the surface alloy was modelled by DFT calculations using structural information provided by the STM images. The modelling resulted in a structure consisting of Sn and Ag trimers and a fourth Ag atom located at the corner of the 2×2 cell. In addition, the calculated electronic structure based on the proposed model is consistent with experimental results, which verifies the atomic model.Another combination of metals, 1.33 ML of Pb and 0.85 ML of In, resulted in the formation of a well-defined In/Pb/Ge(111)3×3 surface alloy. The 3×3 surface exhibits an interesting band structure where five surface bands were identified of which four cross the Fermi level resulting in a metallic character of the surface. Two-dimensional constant energy data reveal the presence of intricate rotated hexagon like contours which intersect each other along the − and − directions of the surface Brillouin zone. The STM results reveal nine bright protrusions per 3×3 unit cell.
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