Electronic and atomic structure of metal induced reconstructionson Si(lll) and Ge(lll) surfaces studied by PES and STM

Sammanfattning: Deposition of metal atoms on semiconductors leads in many cases to reconstructed surfaces. The lack of symmetry in the direction normal to the surface allows for a large variety in the atomic arrangements of the surface reconstructions. In the case of silicon, more than 300 kinds of adsorbate-induced surface reconstructions have been found. The atomic and electronic structures of these surfaces have attracted a lot of attention in the field of surface science. Detailed investigations of some surfaces of current interest are presented in this thesis.The interest in the Sn/Ge(111) system increased very much a few years ago when it was realized that the room temperature ✓3x✓3 phase changes to a 3x3 phase at low temperature. This transition was described as a charge density wave transition in an early report. However, photoemission data gave a different picture since the room temperature phase shows clear similarities with the low temperature phase regarding both valence band and core-level spectra. By employing low energy electron diffraction (LEED), core-level, and valence bandspectroscopy the correct Sn 4d line shape for the Sn/Ge(111) surface was established. At 70 K three 4d components were identified on the 3x3 phase. The two major components are attributed to the two types of Sn atoms in the 3x3 unit cell. The results presented in this study put an end to the discussion about the core-level line shape.An apparent 3x3 LEED pattern was observed for the Sn1-xSix/Si(111)✓3x✓3 surface. The origin of this additional diffraction has been investigated in detail by scanning tunneling microscopy (STM). The 3x3 diffraction, which appears after annealing, is associated with the arrangement of Sn and Si substitutional atoms in the surface layer, forming many local structures such as honeycombs, hexagons, and atomic lines. As revealed by Fourier-transformsof the STM-images, these local structures are the origins of the 3x3 diffraction and a weak 2✓3x✓3 streaky background superposed on the ✓3x✓3 LEED pattern.Metal to semiconductor transitions have recently attracted much attention by the surface physics community. Except for the well-known temperature-dependent transitions, there exist other types such as coverage-dependent transitions. The surface electronic structures of the ✓3x✓3, ✓39x✓39, and 6x6 phases of Ag/Ge(111) have been studied by angle-resolved photoelectron spectroscopy and LEED. An interesting transition from the metallic ✓39x✓39 phase to a semiconducting 6x6 phase was observed. On the 6x6 surface, the upper metallic band is missing in the photoemission spectra. This change results in a gap of around 0.2 eV with respect to the Fermi level.Despite the extensive research on Ag/Si(111) surfaces (like ✓3x✓3 and ✓21x✓21-Au), the surface electronic structures of the Ag-induced ✓21x✓21 and 6x6 phases were reported for the first time in this study. The basic character of these reconstructions formed by Ag on theSi(111) or Ge(111) surfaces is very similar although a different periodicity is observed for the intermediate phases. In contrast to the two metallic bands discussed in earlier reports on✓21x✓21-Au, only one surface metallic band was found on the ✓21x✓21-Ag surface. The 6x6surface has four surface state bands without any band crossing the Fermi level. The result is a semiconducting character of the 6x6 phase. Thus, by depositing small amounts of Ag atoms onthe ✓3x✓3 surface, the surface electronic structure shows an interesting change from a semiconducting to a metallic and back to a semiconducting state.A crystal to glass-like transition was investigated in the Au/Si(111) system by means of LEED, core-level and valence photoelectron spectroscopy. The α-✓3x✓3 phase shows Si 2p spectra that are different from the quenched β-✓3x✓3 and 6x6 phases. A similarity between the 6x6 and quenched β-✓3x✓3 phases is evident from the decomposition of the Si 2p spectra. Eight surface state bands are found on the 6x6 surface in contrast to two surface state bands reported in the literature. The reconstructions of the α-✓3x✓3 phase, the 6x6 and the quenched β-✓3x✓3 phases are discussed in terms of extra Au adatoms on the ✓3x✓3 surface described by the honeycomb-chain-trimer (HCT) model. Seven surface bands are broadened into three bands, when the 6x6 phase is transformed into the quenched β-✓3x✓3 just by annealing followed by quick cooling. This behavior indicates a crystal to glass-like transition between the 6x6 surface and the quenched β-✓3x✓3 surface.The ✓3x✓3, ✓39x✓39 and 6x6 phases of Ag/Ge(111) have been studied by STM. Four types of configurations that contain 6 additional Ag adatoms per unit cell are found on the 6x6 surface, which show mirrored and complementary relations. On the ✓39x✓39 surface, the protrusions from extra Ag atoms are extended into neighboring Ag trimers. The surface structure models of the ✓39x✓39 and 6x6 phases have been proposed based on the HCT model. An interesting transition between the different types of 6x6 configurations was observed. By applying a combination of negative and positive tip biases, one can transform the surface reconstruction between the ✓39x✓39 and the 6x6 phases. Two types of domains were found on the ✓39x✓39 surface, and transformations between these two phases are also possible. Thus byusing different tip biases, it is possible to manipulate the different types of surface reconstructions.

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