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  • 1.
    Eriksson, Johan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Adell, M.
    MAX-lab Lunds universitet.
    Sakamoto, Kazuyuki
    Graduate School of Advanced Integration Science Chiba University, Chiba, Japan.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Origin of a surface state above the Fermi level on Ge(001) and Si(001) studied by temperature-dependent ARPES and LEED2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no L8, p. 085406-1-085406-5Article in journal (Refereed)
    Abstract [en]

    Variable temperature photoemission studies in the literature have revealed the presence of a surface state above the Fermi level on clean Ge(001). We present photoemission and low energy electron diffraction results from Ge(001) obtained between 185 and 760 K. Our measurements show a peak above the Fermi level with a maximum intensity at a sample temperature of around 625 K. At higher temperatures, we observe a gradual decrease in the intensity. Angle resolved spectra show that the surface state has a k̅ dependence and is therefore not attributed to defects. Very similar results were obtained on both an intrinsic (30 Ω cm) and a 10 m Ω cm n-type sample. The overall appearance of the spectral feature is found to be quite insensitive to sample preparation. Low energy electron diffraction investigations show how the sharp c(4×2) pattern becomes streaky and finally turns into a 2×1 pattern. The onset of the structure above the Fermi level takes place just before all c(4×2) streaks have disappeared which corresponds to a temperature of around 470 K. On Si(001), we also observe photoemission intensity above the Fermi level. It is weaker than on Ge(001) and appears at higher temperature. We find that the emission above the Fermi level can be explained by thermal occupation of the π* band derived from a 2×1 ordering of asymmetric dimers on the surface.

  • 2.
    Eriksson, Johan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Osiecki, Jacek
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Sakamoto, Kazuyuki
    Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Atomic and electronic structures of the ordered 2√3 × 2√3 andthe molten 1×1 phase on the Si(111):Sn surface2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 23, p. 235410-Article in journal (Refereed)
    Abstract [en]

    The Si(111) surface with an average coverage of slightly more than one monolayer of Sn, exhibits a 2√3 × 2√3 reconstruction below 463 K. In the literature, atomic structure models with 13 or 14 Sn atoms in the unit cell have been proposed based on scanning tunneling microscopy (STM) results, even though only four Sn atoms could be resolved in the unit cell. This paper deals with two issues regarding this surface. First, high resolution angle resolved photoelectron spectroscopy (ARPES) and STM are used to test theoretically derived results from an atomic structure model comprised of 14 Sn atoms, ten in an under layer and four in a top layer [Törnevik, et al., PRB 44, 13144 (1991)]. Low temperature ARPES reveals six occupied surface states. The calculated surface band structure only reproduces some of these surface states. However, simulated STM images show that certain properties of the four atoms that are visible in STM are reproduced by the model. The electronic structure of the Sn atoms in the under layer of the model does not correspond to any features seen in the ARPES results. New STM images are presented which indicate the presence of a different under layer consisting of eight Sn atoms, that is not compatible with the model. These results indicate that a revised model is called for. The second issue is the reversible transition from a 2√3 × 2√3 phase below 463 K to a 1×1 phase corresponding to a molten Sn layer, above that temperature. It is found that the surface band structure just below the transition temperature is quite similar to that at 100 K. The surface band structure undergoes a dramatic change at the transition. A strong surface state, showing a 1×1 periodicity, can be detected above the transition temperature. This state resembles parts of two surface states which, already before the transition temperature is reached, has begun a transformation and lost much of their 2√3×2√3 periodicities. Calculated surface band structures obtained from 1×1 models with 1 ML of Sn are compared with ARPES and STM results. It is found that the strong surface state present above the transition temperature shows a dispersion similar to that of a calculated surface band originating from the Sn-Si interface with the Sn atoms in T1 sites.

  • 3.
    Eriksson, Johan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Sakamoto, Kazuyuki
    Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Electronic structure of the thallium induced 2×1 reconstruction on Si(001)Manuscript (preprint) (Other academic)
    Abstract [en]

    With a Tl coverage of one monolayer, a 2×1 reconstruction is formed on the Si(001) surfaceat room temperature. In this study, low temperature angle resolved photoelectron spectroscopy(ARPES) data reveal four surface state bands associated with this Tl induced reconstruction.Calculated surface state dispersions, obtained using the “pedestal + valley-bridge” model, are foundto be similar to those obtained using ARPES. Inclusion of spin-orbit coupling in the calculations isfound to be important to arrive at these results. A known effect of the strong spin-orbit coupling isthe reluctance of the Tl 6s2 electrons to participate in the bonding, i.e., the inert pair effect. In thecalculations, inclusion of spin-orbit coupling results in a ~5 eV downshift of the Tl 6s2 electrons.

  • 4.
    Eriksson, Johan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Sakamoto, Kazuyuki
    Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Electronic structure of the thallium-induced 2x1 reconstruction on Si(001)2010In: PHYSICAL REVIEW B, ISSN 1098-0121, Vol. 81, no 20, p. 205422-Article in journal (Refereed)
    Abstract [en]

    With a Tl coverage of one monolayer, a 2 x 1 reconstruction is formed on the Si (001) surface at room temperature. In this study, low-temperature angle-resolved photoelectron spectroscopy (ARPES) data reveal four surface state bands associated with this Tl induced reconstruction. Calculated surface state dispersions, obtained using the "pedestal + valley-bridge" model, are found to be similar to those obtained using ARPES. Inclusion of spin-orbit coupling in the calculations is found to be important to arrive at these results. A known effect of the strong spin-orbit coupling is the reluctance of the Tl 6s(2) electrons to participate in the bonding, i.e., the inert pair effect. In the calculations, inclusion of spin-orbit coupling results in a similar to 5 eV downshift of the Tl 6s(2) electrons.

  • 5.
    Eriksson, Johan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Sakamoto, Kazuyuki
    Graduate School of Advanced Integrated Science Chiba University, Japan.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Lithium-induced dimer reconstructions on Si(001) studied by photoelectron spectroscopy and band-structure calculations2007In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 75, no 20, p. 205416-Article in journal (Refereed)
    Abstract [en]

    The electronic and atomic structure of Si(001) with 0.5 and 1 ML of lithium have been studied experimentally using angle resolved ultraviolet photoelectron spectroscopy, Si 2p core level spectroscopy, and low energy electron diffraction. Experimental surface state dispersions are compared with recent theoretical results in the literature and with results from additional density functional theory calculations. Four adsorption configurations for the 0.5 ML 2×2 surface and three configurations for the 1 ML 2×1 surface are compared. Fittings of Si 2p core level data support the alternation of strongly and weakly buckled Si dimers of the 2×2 models and symmetric Si dimers of the 2×1 models based on the relative intensities of the surface components. As a tool to differentiate between the different 2×2 and 2×1 models surface state dispersions are used since they are sensitive to the positions of the Li adatoms.

  • 6.
    Eriksson, Johan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Surface core-level shifts on clean Si(001) and Ge(001) studied with photoelectron spectroscopy and DFT calculations2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 12, p. 125443-Article in journal (Refereed)
    Abstract [en]

    The Si 2p and Ge 3d core-levels are investigated on the c(4×2) reconstructed surfaces of Si(001)and Ge(001), respectively. Calculated surface core-level shifts are obtained both with and withoutfinal state effects included. Significant core-level shifts are found within the four outermost atomiclayers. A combination of the theoretical results and high-resolution photoemission data facilitatea detailed assignment of the atomic origins of the various components identified in the core-levelspectra of both Si(001) and Ge(001).

  • 7.
    He, Jiangping
    et al.
    Linköping University, Department of Physics, Chemistry and Biology.
    Hansson, Göran
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Atomic structure of the carbon induced Si(001)c(4x4) surface2006In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 252, p. 5284-5287Article in journal (Refereed)
    Abstract [en]

       

  • 8. Jemander, S.T.
    et al.
    Lin, N.
    Kern Department, Max-Planck-Institute FKF, 70569, Stuttgart, Germany.
    Zhang, Hanmin
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Hansson, Göran
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    STM study of the surface defects of the (v3×v3)-Sn/Si(1 1 1) surface2001In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 475, no 1-3, p. 181-193Article in journal (Refereed)
    Abstract [en]

    The (v3×v3)-Sn/Si(1 1 1) surface has been studied with scanning tunneling microscopy (STM) and Auger electron spectroscopy, and it is found that it contains basically 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 (v3×v3) adatom neighbour. Close to some defect constellations, a very local (3×3) 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. We also provide an explanation why the linearity breaks down when large perturbations are probed with small tip biases.

  • 9.
    Jemander, S.T.
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Zhang, Hanmin
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Hansson, Göran
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    STM study of the C-induced Si(100)-c(4×4) reconstruction2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, no 11, p. 115321-Article in journal (Refereed)
    Abstract [en]

    We report a direct and reliable way to produce the Si(100)-c(4×4) reconstruction by submonolayer deposition from a SiC source and subsequent annealing. Auger electron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy (STM) investigations reveal that a C amount equivalent to 0.07 monolayers (ML’s) is sufficient to obtain full coverage of the c(4×4) reconstruction. A deposition of 0.035 ML’s C produces a c(4×4) coverage of only 19%, indicating that C is not only present in the c(4×4) areas, but also in the 2×1 areas. There is not enough C to make it a regular part of the c(4×4) reconstruction and we therefore conclude that the c(4×4) reconstruction is strain induced. We find that a combination of the mixed ad-dimer and buckled ad-dimer models explains all main features observed in the STM images. Images of freshly prepared c(4×4) surfaces exhibit a decoration of approximately 50% of the unit cells, which is attributed to perpendicular ad-dimers. Long exposures (>20 h) to the UHV background gas quench these features and the c(4×4) reconstruction appears as if more homogeneous.

  • 10. Jemander, S.T.
    et al.
    Zhang, Hanmin
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Hansson, Göran
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Surface structure of Si(100) with submonolayer coverages of C2002Conference paper (Refereed)
    Abstract [en]

    We report a combined STM, LEED and AES study of Si(100) surfaces prepared by coevaporation from two MBE-sources with Si and SiC, respectively. The flux from the SiC source contains 10% C, enabling studies of deposited layers with C-concentrations in the range 0-10%. After room temperature deposition, the structures have been annealed at 600 °C to generate a c(4 × 4) reconstruction. This has previously been reported to contain from 0.0 to 0.5 monolayers (ML) of C. Annealing at 800 °C irreversibly transforms the c(4 × 4) surface to a 2 × 1-reconstructed surface that contains precipitates of SiC. Since only 0.07 ML of C is needed to have 98% of the surface covered with the c(4 × 4) reconstruction, we conclude that the c(4 × 4) reconstruction is impurity-induced rather than having C-atoms in well defined positions within each unit cell. The c(4 × 4) reconstruction is attributed to a basic structure containing buckled parallel ad-dimers, which in approximately 50% of the reconstructed unit cells is decorated with perpendicular dimers. © 2002 Elsevier Science B.V. All rights reserved.

  • 11.
    Magnuson, Martin
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Fahlman, Mats
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Johansson, Leif
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Total of 100 authors in alphabetical orders, -
    MAX IV Conceptual Design Report (CDR)2006Report (Other academic)
  • 12.
    Muhammad Sohail, Hafiz
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Umklapp induced surface band structure of Ag/Ge(111)6 x 62015In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 635, p. 55-60Article in journal (Refereed)
    Abstract [en]

    This study focuses on the electronic structure of a 6 x 6 surface which is formed by 0.2 monolayer of Ag on top of the Ag/Ge(111) root 3 x root 3 surface. The 6 x 6 periodicity was verified by low energy electron diffraction. Angle resolved photoelectron spectroscopy was employed to study the electronic structure along the (Gamma) over bar-(M) over bar-(Gamma) over bar and (Gamma) over bar-(K) over bar-(M) over bar high symmetry lines of the 6 x 6 surface Brillouin zone. There are six surface bands in total. Out of these, three were found to be related to the 6 x 6 phase. The surface band structure of the 6 x 6 phase is significantly more complex than that of the,root 3 x root 3 surface. This is particularly the case for the uppermost surface band structure which is a combination of a surface band originating from the underlying root 3 x root 3 surface and umklapp scattered branches of this band. Branches centered at neighboring 6 x 6 SBZs cross each other at an energy slightly below the Fermi level. An energy gap opens up at this point which contains the Fermi level. The complex pattern of constant energy contours has been used to identify the origins of various branches of the surface state dispersions. (C) 2014 Elsevier B.V. All rights reserved.

  • 13.
    Osiecki, Jacek
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Sohail, Hafiz Muhammad
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Eriksson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Experimental and Theoretical Evidence of a Highly Ordered Two-Dimensional Sn/Ag Alloy on Si(111)2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 5, p. 057601-Article in journal (Refereed)
    Abstract [en]

    The existence of a highly ordered, two-dimensional, Sn/Ag alloy on Si(111) is reported in this study. We present detailed atomic and electronic structures of the one atomic layer thick alloy, exhibiting a 2 x 2 periodicity. The electronic structure is metallic due to a free-electron-like surface band dispersing across the Fermi level. By electron doping, the electronic structure can be converted into a semiconducting state. A rotated Sn trimer constitutes the key structural element that could be identified by a detailed analysis of constant energy contours derived from the free-electron-like band.

  • 14.
    Osiecki, Jacek
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Alloying of Sn in the surface layer of Ag(111)2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 7Article in journal (Refereed)
    Abstract [en]

    It is found that 1/3 monolayer (ML) of Sn forms a surface alloy with 2/3 ML of Ag on Ag(111). This highly ordered alloy layer shows a √3×√3 structure. By employing experimental and theoretical tools (scanning tunneling microscopy [STM], angle resolved photoelectron spectroscopy, low-energy electron diffraction, and density functional theory), an atomic model has been obtained that reproduces the experimental electronic structure in both real and reciprocal space. Detailed surface band dispersions, constant energy contours, and STM images, obtained experimentally and theoretically, are compared in order to verify the model. Similar, 1-layer-thick alloys on Ag(111) with Pb, Bi, or Sb exhibit measurable spin–orbit interactions. However, no such spin split could be detected in the case of Sn in this study.

  • 15.
    Osiecki, Jacek
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Formation of a root 3 x root 3 surface on Si/Ge(111) studied by STM and LEED2009In: SURFACE SCIENCE, ISSN 0039-6028, Vol. 603, no 16, p. 2532-2536Article in journal (Refereed)
    Abstract [en]

    We have performed a detailed study of the formation and the atomic structure of a root 3 x root 3 surface on Si/Ge(111) using both scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Both experimental methods confirm the presence of a root 3 x root 3 periodicity but unlike the Sn/Ge(111) and the Sn/Si(111) surfaces, the Si/Ge(111) surface is not well ordered. There is no long range order on the surface and the root 3 x root 3 reconstruction is made up of double rows of silicon atoms separated by disordered areas composed of germanium atoms.

  • 16.
    Razado Colambo, Ivy
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Zhang, Hanmin
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Electronic structure of H/Ge(111)1×1 studied by angle-resolved photoelectron spectroscopy2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 19, p. 193403-Article in journal (Refereed)
    Abstract [en]

    The electronic structure of H/Ge(111)1×1 was investigated using angle-resolved photoelectron spectroscopy. Spectra were measured along the high-symmetry lines of the 1×1 surface Brillouin zone. In the Γ̅ −K̅ −M̅ direction, two surface states, labeled a and a, were found in the lower and upper band-gap pockets. The a and a surface states are associated with the Ge-H bonds and the Ge-Ge backbonds, respectively. In the Γ̅ −M̅ direction, only the Ge-H surface state, a, can be identified. It is found in the band-gap pocket around the M̅ point. The two hydrogen-induced surface states on H/Ge(111)1×1 show strong similarities with the corresponding surface states on H/Si(111)1×1. Results from H/Ge(111)1×1 and H/Si(111)1×1 are compared in this Brief Report.

  • 17.
    Razado, Ivy
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    He, Jiangping
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Zhang, Hanmin
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Hansson, Göran
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Electronic structure of Ge(111)c(2x8): STM, angle-resolved photoemission, and theory2009In: PHYSICAL REVIEW B, ISSN 1098-0121, Vol. 79, no 20, p. 205410-Article in journal (Refereed)
    Abstract [en]

    The surface electronic structure of Ge(111)c(2x8) was studied by experimental techniques [low-energy electron diffraction, scanning tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES)] and theoretical band-structure calculations. Bias-dependent STM images exhibit two different types of adatoms (A(T),A(R)) and rest atoms (R-T,R-R) confirming the presence of asymmetries within the c(2x8) cell. The ARPES study resulted in a more detailed picture of the surface electronic structure of the Ge(111)c(2x8) surface compared to earlier studies. The energy dispersion curves showed the presence of seven surface bands labeled A1, A2, A2(), A3, A4, A4(), and A5. The experimental surface bands were compared to the calculated band structure of the full c(2x8) unit cell. The most important results are (i) we have identified a split surface-state band in the photoemission data that matches a split between R-T and R-R derived rest atom bands in the calculated surface band structure. This allows us to identify the upper A2 band with the R-R and the lower A2() band with the R-T rest atoms. (ii) The uppermost highly dispersive band (A1) originates from states below the adatom and rest atom layers and should not be confused with rest atom bands A2 and A2(). (iii) The bias-dependent changes in the adatom/rest atom contrast in the experimental STM images were closely reproduced by simulated STM images generated from the calculated electronic structure. (iv) A split was observed in the back-bond derived surface band at higher emission angles (A4 and A4()).

  • 18.
    Razado-Colambo, Ivy
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Zhang, Hanmin M.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Hansson, Göran
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)2006In: Applied Surface Science, ISSN 0169-4332, Vol. 252, no 15, p. 5300-5303Article in journal (Refereed)
    Abstract [en]

    We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.

  • 19.
    Razado-Colambo, Ivy
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Zhang, Hanmin M.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Electronic structure of H/Ge(111)1x1 studied by angle-resolved photoelectron spectroscopyManuscript (Other academic)
    Abstract [en]

    The electronic structure of H/Ge(111)1x1 was investigated using angleresolved photoelectron spectroscopy (ARPES). Spectra were measured along the high symmetry lines of the 1x1 surface Brillouin zone (SBZ). In the Γ − Κ −M direction, two surface states, labelled a and a’, were found in the lower and upper band gap pockets. The a and a’ surface states are associated with the Ge-H bonds and the Ge- Ge backbonds, respectively. In the Γ − Μ direction, only the Ge-H surface state, a, can be identified. It is found in the band gap pocket around the Μ -point. The two hydrogen induced surface states on H/Ge(111)1x1 show strong similarities with the corresponding surface states on H/Si(111)1x1. Results from H/Ge(111)1x1 and H/Si(111)1x1 are compared in the paper.

  • 20.
    Razado-Colambo, Ivy
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Zhang, Hanmin M.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger I. G.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Hansson, Göran V.
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, The Institute of Technology.
    STM study of site selective hydrogen adsorption on Si(111) 7×72005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 23, p. 235411-Article in journal (Refereed)
    Abstract [en]

    Adsorption of atomic hydrogen has been studied by scanning tunneling microscopy (STM) and photoelectron spectroscopy with a focus on the different adsorption sites provided by the Si(111) 7×7 surface. At low temperature, the hydrogen atoms adsorb preferentially on adatoms while at elevated temperatures the rest atoms are the first to become hydrogen terminated. The hydrogen-terminated rest atoms are no longer visible in the STM images and the surrounding adatoms appear brighter compared to the clean 7×7 surface. This indicates that there is a local charge transfer back to the adatoms from the rest atoms. Three kinds of modified triangular subunit cells of the 7×7 reconstruction have been identified corresponding to one, two, and three hydrogen-terminated rest atoms, respectively. A detailed study of the apparent height using STM line profiles through the adatom and rest atom positions on the surface is presented. These line profiles show a characteristic and reproducible variation of the apparent heights of the adatoms for the different kinds of triangular subunit cells and the changes are interpreted in terms of charge transfer. The very local nature of the charge transfer is concluded from the fact that only the hydrogen termination of neighboring rest atoms is significantly affecting the apparent height of an adatom.

  • 21. Sakamoto, K.
    et al.
    Hirano, M.
    Department of Physics, Grad. Sch. Sci., Tohoku U., Sendai, Japan.
    Takeda, H.
    Department of Physics, Grad. Sch. Sci., Tohoku U., Sendai, Japan.
    Jemander, S.T.
    Matsuda, I.
    Department of Chemistry, Grad. Sch. Sci., Univ. T., Tokyo, Japan.
    Amemiya, K.
    Research Center for Spectrochemistry, The University of Tokyo, 113-0033, Tokyo, Japan.
    Ohta, T.
    Department of Chemistry, Grad. Sch. Sci., Univ. T., Tokyo, Japan, Research Center for Spectrochemistry, The University of Tokyo, 113-0033, Tokyo, Japan.
    Uchida, W.
    Department of Physics, Grad. Sch. Sci., Tohoku U., Sendai, Japan.
    Hansson, Göran
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Interaction of metastable molecular oxygen with the dangling bonds of a Si(111)-(7×7) surface2001In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 114-116, p. 489-494Article in journal (Refereed)
    Abstract [en]

    We have investigated the interaction between metastable molecular oxygen and the adatom dangling bonds modified by the adsorption of atomic oxygen into the back-bonds of a Si(111)-(7×7) surface. Ultraviolet photoelectron spectroscopy shows that the metastable states increase in intensity faster but with a decrease in its saturated intensity as the coverage of atomic oxygen increases. This result suggests that the number of modified dangling bonds is not the only important factor for the adsorption process of metastable oxygen. Taking into account the observation of modified dangling bonds with different density of states in scanning tunneling microscopy, we conclude that the adsorption of the metastable oxygen species correlates closely with the density of states of the dangling bond.

  • 22.
    Sakamoto, K.
    et al.
    Department of Physics, Graduate School of Science, Tohoku University, Japan.
    Pick, A.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Surface electronic structures of the Eu-induced Si(111)-(3×2) and -(2×1) reconstructions2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 4, p. 045310-Article in journal (Refereed)
    Abstract [en]

    The electronic structures of the Eu∕Si(111)-(3×2) and (2×1) surfaces have been investigated by angle-resolved photoelectron spectroscopy. On the (3×2) surface, we identify six surface states in the gap and a pocket of the bulk band projection. Among the five surface states observed in the bulk band gap, the dispersions of three of them agree well with those of the surface states of monovalent atom adsorbed Si(111)-(3×1) surfaces. The dispersions of the two other surface states observed in the band gap agree well with those observed on the Ca∕Si(111)-(3×2) surface, which has basically the same structure as that of monovalent atom adsorbed Si(111)-(3×1) surfaces. Taking these results into account, we conclude that the five surface states observed in the band gap originate from the orbitals of Si atoms that form a honeycomb-chain-channel structure. In the case of the (2×1) surface, two semiconducting states are observed in the bulk band gap. The difference in binding energy of these two states at the Γ̅ point agrees well with that of the surface states obtained theoretically for a clean Si(111)-(2×1) surface with a Seiwatz structure, and the dispersion of the upper state shows good agreement with the corresponding theoretical surface state. These observations indicate that the two surface states in the band gap originate from Si atoms that form a Seiwatz chain. The present results support the structures of the Eu∕Si(111)-(3×2) and (2×1) surfaces proposed in the literature.

  • 23.
    Sakamoto, K.
    et al.
    Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan.
    Takeyama, W.
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
    Zhang, Hanmin
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    High-resolution core-level study of the Ca/Si(1 1 1)-(2×1) surface2003In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 428, no 1-2, p. 115-118Article in journal (Refereed)
    Abstract [en]

    We have investigated the geometric structure of the Ca/Si(1 1 1)-(2×1) surface using low-energy electron diffraction (LEED) and high-resolution core-level photoelectron spectroscopy. A clear (2×1) periodicity was observed in LEED after annealing the (7×1) phase formed at room temperature in LEED. In the Si 2p core-level spectra, three surface components were observed. By considering the energy shift and intensity of each surface component and the Ca 3p core-levels of the two phases, we regard the (2×1) phase to be formed by p-bonded Seiwatz Si chains with a Ca coverage of 0.5 ML. © 2002 Elsevier Science B.V. All rights reserved.

  • 24.
    Sakamoto, K.
    et al.
    Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japen.
    Takeyama, W.
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
    Zhang, Hanmin
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    High-resolution Si2p core-level and low-energy electron diffraction studies of the Ca/Si(1 1 1)-(3 × 2) surface2003In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 532-535, p. 628-632Article in journal (Refereed)
    Abstract [en]

    We have investigated the surface structure of the Ca/Si(1 1 1)-(3 × 2) surface using low-energy electron diffraction (LEED) and high-resolution core-level photoelectron spectroscopy. Weak ×2 streaks were observed in LEED at 300 K. After cooling the sample to 100 K, ×2 spots, which originate from both (3 × 2) and c(6 × 2) periodicities, appeared. By considering the energy shift and intensity of each surface component observed in the Si2p core-level spectra, we conclude that the structure of the (3 × 2) surface is basically the same as that of the honeycomb-chain-channel model with a Ca coverage of 1/6 ML. Further, we propose that the weak ×2 streaks at 300 K result from thermally induced disorder. © 2003 Elsevier Science B.V. All rights reserved.

  • 25. Sakamoto, K.
    et al.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Atomic and electronic structures of metal induced Si(111)-(3x1) reconstructed surfaces2004In: e-Journal of Surface Science and Nanotechnology, ISSN 1348-0391, Vol. 2, p. 210-221Article in journal (Refereed)
    Abstract [en]

      In this review, we present a series of photoemission studies performed on the so-called metal induced Si(111)-(3×1) surface, which is one of the most well known 1D structure formed on a Si(111) surface. Three different kinds of metals have been used as adsorbate, K, Ca, and Ag.On the K/Si(111)-(3×1) surface, five surface components were observed in the Si 2p core-level spectra. The energy shift and intensity of each surface component indicates that this surface has the honeycomb-chain-channel (HCC) structure with a K coverage of 1⁄3 ML. The extra spots observed in the LEED pattern of the so-called Ca/Si(111)-(3×1) surface suggest that this surface has a (3×2) periodicity instead of the (3×1) periodicity reported in the literature. By considering the energy shift and intensity of each Si 2p surface component, we conclude that the structure of the (3×2) phase is basically the same as that of the HCC model, but with a Ca coverage of 1⁄6 ML. Regarding the valence-band, five surface states, none of which crosses the Fermi level, were observed in the bulk band gap. The dispersion features of three of them agree well with those of monovalent atom adsorbed Si(111)-(3×1) surfaces along the chain direction. The two other states observed in the band gap have not been reported in the literature, and they are interpreted as surface states that are peculiar to the Ca/Si(111)-(3×2) surface due to the 1⁄6 ML coverage. Regarding the Ag/Si(111) surface, a new c(12×2) phase is observed in LEED after cooling the room temperature (6×1) phase to 70 K. In the Si 2p core-level spectra and in the valence band spectra, no significant difference is observed between the two surfaces. Further, the origins of the Si 2p surface components and the surface states of these surfaces are well explained using the HCC model. These results indicate that the basic structure of this Ag/Si(111) surface is quite similar with the HCC model but with a c(12×2) periodicity, and that the (6×1) structure results from thermal vibrations of the surface atoms.

  • 26. Sakamoto, K.
    et al.
    Zhang, Hanmin
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    High-resolution Si 2p core-level study of the K/Si(111)-(3×1) surface2002Conference paper (Refereed)
    Abstract [en]

    The structure of the K/Si(111)-(3 × 1) surface was studied by high-resolution core-level photoelectron spectroscopy. Five surface components were observed in the Si 2p core-level spectra. Compared to the bulk component, three components are shifted to lower and two to higher binding energies. The two components with the lowest binding energies are assigned to the top-layer Si atoms bonded to the K atoms with different configurations. The component with highest binding energy has a contribution from the p-bonded Si atoms of the top layer. The two other components originate from the Si atoms of the second and third layers.

  • 27.
    Sakamoto, K.
    et al.
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
    Zhang, Hanmin
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Initial oxidation process of an Si(111)-(7 × 7) surface studied by photoelectron spectroscopy2004In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 464-465, p. 10-13Article in journal (Refereed)
    Abstract [en]

    We have investigated the initial oxidation stage of an Si(111)-(7 × 7) surface using valence-band photoemission measurements. As the oxygen exposure increases, the intensities of the dangling bond states of adatoms (S 1) and rest atoms (S2) decrease. Among the four oxygen-induced states, three originate from the orbitals of adsorbed oxygen species, and one originates from the dangling bonds of adatoms with more than one oxygen atom adsorbed into its back-bond. Taking the dosage-dependent intensity of this modified dangling bond state into account, we conclude that the first adsorption site of oxygen is the back-bond of an adatom. © 2004 Elsevier B.V. All rights reserved.

  • 28.
    Sakamoto, Kazuyuki
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Ashima, Hidenori
    Department of Physics, Graduate School of Science, Tohoku University, Japan .
    Zhang, Hanmin
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Identification of the basic structure of the Ag/Si(111)-(6×1) surface: Observation of a low-temperature c(12×2) phase2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, no 4, p. 045305-Article in journal (Refereed)
    Abstract [en]

    The surface structure of the so-called Ag/Si(111)-(6×1) surface is studied by low-energy electron diffraction (LEED), high-resolution core-level photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy. A c(12×2) phase is observed in LEED after cooling the room-temperature (6×1) phase to 100 K. In the Si 2p core-level spectra, no significant difference is observed between the two surfaces. In the valence-band spectra, five surface states are observed on both the (6×1) and c(12×2) surfaces. None of these surface states crosses the Fermi level. The binding energies and dispersions of the surface states observed on the (6×1) surface are quite similar to those of the c(12×2) surface. These results indicate that the basic structure of this Ag/Si(111) surface has a c(12×2) periodicity, and that the (6×1) structure results from thermal vibrations of the surface atoms. Moreover, we assign two of the surface states to bonding states between Ag and Si atoms, and one of them to a π-bond state.

  • 29.
    Sakamoto, Kazuyuki
    et al.
    Chiba University, Japan.
    Eriksson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Mizuno, Seigi
    Kyushu University, Japan.
    Ueno, Nobuo
    Chiba University, Japan.
    Tochihara, Hiroshi
    Kyushu University, Japan.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Core-level photoemission study of thallium adsorbed on a Si(111)-(7×7) surface: Valence state of thallium and the charge state of surface Si atoms2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 7, p. 075335-Article in journal (Refereed)
    Abstract [en]

    The coverage-dependent valence state of Tl adsorbed on a Si(111)-(7×7) surface and the coverage dependence of the charge states of surface Si atoms have been investigated by high-resolution core-level photoelectron spectroscopy. Although two different reconstructions were observed in low-energy electron diffraction at different coverages, a (1×1) pattern at a Tl coverage of 1 monolayer (ML) and a (√3×√3) pattern at a coverage of 1∕3 ML, the binding energy of the Tl 5d core-level was the same at Tl coverages up to 1 ML. Taking the valence state on a (1×1) surface reported in the literature into account, we conclude that the valence state of Tl is 1+, and that the 6s2 electrons of Tl are inactive as an inert pair in the Tl-Si bonding on a Si(111) surface at a coverage of 1 ML and below. In the Si 2p core-level spectra, one surface component was observed on the (1×1) surface, and three surface components were observed on the (√3×√3) surface. The binding energies and intensities of the Si 2p surface components indicate that the charge state of the surface Si atoms on Tl∕Si(111)-(1×1) is the same as that of the (√3×√3) surfaces induced by the other group III metals, but they are different on the Tl∕Si(111)-(√3×√3) surface.

  • 30.
    Sakamoto, Kazuyuki
    et al.
    Chiba University, Japan.
    Eriksson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Pick, A.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Ueno, Nobuo
    Chiba University, Japan.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Surface electronic structures of the Eu- and Ca-induced so-called Si(111)-(5×1) reconstructions2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 23, p. 235311-Article in journal (Refereed)
    Abstract [en]

    We have investigated the electronic structures of the so-called Eu- and Ca-induced Si(111)-(5×1) surfaces by using angle-resolved photoelectron spectroscopy (ARPES) and low-energy electron diffraction (LEED). The LEED patterns of these surfaces indicate that the periodicities of both surfaces are actually (5×4). In the ARPES study, seven surface states were observed on each (5×4) reconstruction. Of these surface states, the dispersions of five of them show good agreement with those of the Eu- and Ca-induced (3×2) honeycomb-chain-channel (HCC) surfaces and the dispersions of the two other states agree well with those of the Eu- and Ca-induced (2×1) Seiwatz surfaces along the [1̅ 10] direction—i.e., the direction parallel to the adsorbate chain. Taking the dispersion behavior of these surface states into account, we conclude that the interaction between the nearest-neighbor HCC chain and Seiwatz chain is quite small and that the electronic structure of one chain hardly affects the electronic structure of its neighboring chain. We also discuss the atomic structure of the Eu- and Ca-induced Si(111)-(5×1) reconstructions based on their electronic structures.

  • 31.
    Sakamoto, Kazuyuki
    et al.
    Graduate School of Advanced Integrated Science Chiba University, Japan.
    Eriksson, Johan
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Ueno, Nobuo
    Spectroscopy, Applied Physics Chiba University.
    Uhrberg, Roger
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Photoemission study of the thallium induced Si(111)-´`3x´`3 surface2007In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 601, no 22, p. 5258-5261Article in journal (Refereed)
    Abstract [en]

    We have investigated the surface electronic structure of the Tl induced Si (1 1 1) - (sqrt(3) × sqrt(3)) surface by using angle-resolved photoelectron spectroscopy. Three semiconducting surface states were observed in the gap of the bulk band projection. Of these three states, the one, whose binding energy is approximately 0.3 eV, hardly disperses. Regarding the two other states, we discuss their properties by comparing their dispersion behaviors with those of the surface states of the other group III metal (Al, Ga and In) induced (sqrt(3) × sqrt(3)) reconstructions. The split observed at the over(G, -) point and the smaller dispersion widths of these two states indicate that the origins of the surface states of the Tl induced (sqrt(3) × sqrt(3)) reconstruction are not the same as those of the Al, Ga and In induced (sqrt(3) × sqrt(3)) reconstructions. These results support the idea that the atomic structure of the Tl / Si (1 1 1) - (sqrt(3) × sqrt(3)) surface is different from that of the (sqrt(3) × sqrt(3)) reconstructions induced by other group III metals, which was proposed in the literature.

  • 32.
    Sakamoto, Kazuyuki
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Jemander, S. Torbjörn
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hansson, Göran
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Bias-dependent scanning tunneling microscopy study of the oxygen-adsorbed Si(111)-(7×7) surface: Observation of metastable molecular oxygen2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, no 15, p. 155305-Article in journal (Refereed)
    Abstract [en]

    We have observed the initial stage of oxygen adsorption on a Si(111)-(7×7) surface using scanning tunneling microscopy. Among the bright sites observed after exposing the surface to oxygen in occupied state images, there are differences in the bias dependence of the brightness. Taking into account the local density of states of the oxygen-adsorbed Si(111)-(7×7) surface, we conclude that the sites appearing brightly only with a tip bias of >~+2.1 V are the molecular oxygen. The preferred adsorption site of this molecular species is a corner adatom, which has an oxygen atom adsorbed into its backbond, of the faulted half of the (7×7) unit cell.

  • 33.
    Sakamoto, Kazuyuki
    et al.
    Chiba University, Japan .
    Kim, Tae-Hwan
    Pohang University of Science and Technology, South Korea .
    Kuzumaki, Takuya
    Chiba University, Japan .
    Mueller, Beate
    Chiba University, Japan .
    Yamamoto, Yuta
    Chiba University, Japan .
    Ohtaka, Minoru
    Chiba University, Japan .
    Osiecki, Jacek
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Miyamoto, Koji
    Hiroshima University, Japan .
    Takeichi, Yasuo
    University of Tokyo, Japan .
    Harasawa, Ayumi
    University of Tokyo, Japan .
    Stolwijk, Sebastian D.
    University of Munster, Germany .
    Schmidt, Anke B.
    University of Munster, Germany .
    Fujii, Jun
    CNRS, Italy .
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Donath, Markus
    University of Munster, Germany .
    Woong Yeom, Han
    Pohang University of Science and Technology, South Korea .
    Oda, Tatsuki
    Kanazawa University, Japan .
    Valley spin polarization by using the extraordinary Rashba effect on silicon2013In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 4, no 2073Article in journal (Refereed)
    Abstract [en]

    The addition of the valley degree of freedom to a two-dimensional spin-polarized electronic system provides the opportunity to multiply the functionality of next-generation devices. So far, however, such devices have not been realized due to the difficulty to polarize the valleys, which is an indispensable step to activate this degree of freedom. Here we show the formation of 100% spin-polarized valleys by a simple and easy way using the Rashba effect on a system with C-3 symmetry. This polarization, which is much higher than those in ordinary Rashba systems, results in the valleys acting as filters that can suppress the backscattering of spin-charge. The present system is formed on a silicon substrate, and therefore opens a new avenue towards the realization of silicon spintronic devices with high efficiency.

  • 34.
    Sakamoto, Kazuyuki
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Matsui, Fumihiko
    Graduate School of Material Science, Nara Institute of Science and Technology, Japan.
    Hirano, Masumi
    Department of Physics, Graduate School of Science, Tohoku University, Japan.
    Yeom, Han Woong
    Atomic-Scale Surface Science Research Center and Institute of Physics and Applied Physics, Yonsei University, Seoul, Korea.
    Zhang, Hanmin
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Determination of the bonding configuration of the metastable molecular oxygen adsorbed on a Si(111)-(7×7) surface2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, no 20Article in journal (Refereed)
    Abstract [en]

    The initial adsorption stage of oxygen on a Si(111)-(7×7) surface has been investigated using real time high-resolution core-level measurements at 100 K. Among the six O 1s components observed on an O2 exposed Si(111) surface, the three components, whose intensities saturate at the same dosage, have the same lifetime. These results lead to a conclusion that three oxygen atoms with different chemical environments compose the metastable species. Based on a comparison with our calculated O 1s binding energies, we have determined the bonding configuration of the metastable oxygen.

  • 35.
    Sakamoto, Kazuyuki
    et al.
    Chiba University.
    Oda, Tatsuki
    Kanazawa University.
    Kimura, Akio
    Hiroshima University.
    Miyamoto, Koji
    Hiroshima University.
    Tsujikawa, Masahito
    Kanazawa University.
    Imai, Ayako
    Chiba University.
    Ueno, Nobuo
    Chiba University.
    Namatame, Hirofumi
    Hiroshima University.
    Taniguchi, Masaki
    Hiroshima University.
    Eriksson, Johan
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Abrupt Rotation of the Rashba Spin to the Direction Perpendicular to the Surface2009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 102, no 9, p. 096805-Article in journal (Refereed)
    Abstract [en]

    The polarization vector of the Rashba spin, which must be parallel to the two-dimensional (2D) plane in an ideal system, is found to change abruptly and definitely to the direction perpendicular to the surface at the K̅ point of the Brillouin zone of a real hexagonal system, the Tl/Si(111)-(1×1) surface. This finding obtained experimentally by angle-resolved and spin-resolved photoemission measurements is fully confirmed by a first-principles theoretical calculation. We found that the abrupt rotation of the Rashba spin is simply understood by the 2D symmetry of the hexagonal structure.

  • 36.
    Sakamoto, Kazuyuki
    et al.
    Chiba University, Japan.
    Oda, Tatsuki
    Kanazawa University, Japan.
    Kimura, Akio
    Hiroshima University, Japan.
    Takeichi, Yasuo
    University of Tokyo, Japan.
    Fujii, Jun
    CNR, Italy.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, Faculty of Science & Engineering.
    Donath, Markus
    University of Munster, Germany.
    Woong Yeom, Han
    Pohang University of Science and Technology, South Korea; Pohang University of Science and Technology, South Korea.
    Symmetry induced peculiar Rashba effect on thallium adsorbed Si(111) surfaces2015In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 201, p. 88-91Article in journal (Refereed)
    Abstract [en]

    The geometric symmetry of the surface plays an important role for the spin-orbit-induced spin texture of two-dimensional electronic states. This article reviews the peculiar Rashba spins induced by a C-3 symmetry, including the completely spin polarized surface states with the polarization vector oriented perpendicular to the surface, i.e. a direction that is not expected in a typical Rashba system. This review also describes that this peculiar Rashba situation has possibility to suppress backscattering and therefore to greatly improve the efficiency of spin transport, which is an essential issue in the development of high-performance semiconductor spintronic devices. (C) 2014 Elsevier B.V. All rights reserved.

  • 37.
    Sakamoto, Kazuyuki
    et al.
    Graduate school of Advanced Integrated Science, Chiba University, Japan.
    Pick, Alexander
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Structural investigation of the quasi-one-dimensional reconstructions induced by Eu adsorption on a Si(111) surface2005In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 72, no 19, p. 195342-Article in journal (Refereed)
    Abstract [en]

    The surface structures of the (quasi-)one-dimensional reconstructions induced by the adsorption of Eu on Si(111) have been investigated by low-energy electron diffraction (LEED) and high-resolution core-level photoelectron spectroscopy. Different phases were observed in LEED depending on the Eu coverage. The lowest coverage phase has a (3×2) periodicity, and the highest coverage phase has a (2×1) one. Of the intermediate phases, the LEED pattern of the so-called (5×1) surface indicates that this surface has actually a (5×4) periodicity. The Eu 4f core-level spectra show that the Eu coverages of the (3×2), (5×4), and (2×1) phases are 1∕6 monolayer (ML), 0.3 ML, and 0.5 ML, respectively, and that the valence state of the adsorbate is 2+ in all these three phases. In the Si 2p core-level spectra, three surface components were observed in both the lowest and highest coverage phases. By considering the energy shift and intensity of each surface component, we conclude that the structure of the (3×2) phase is basically the same as that of the honeycomb-chain-channel model, and that the (2×1) phase is formed by π-bonded Seiwatz Si chains. Regarding the (5×4) phase, two extra Si 2p surface components were observed together with the three components observed in the two end phases. Taking the energy shifts and intensities of the extra surface components into account, we propose a structural model of the (5×4) phase.

  • 38.
    Sakamoto, Kazuyuki
    et al.
    Chiba University.
    Setvin, Martin
    National Institute of Material Science, Japan.
    Mawatari, Kenji
    Tohoku University.
    Eriksson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Miki, Kazushi
    National Institute of Material Science, Japan.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Electronic structure of the Si(110)-(16×2) surface: High-resolution ARPES and STM investigation2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 4, p. 045304-Article in journal (Refereed)
    Abstract [en]

    The electronic structure of a single domain Si(110)-(16×2) surface has been investigated by high-resolution angle-resolved photoelectron spectroscopy and scanning tunneling microscopy (STM). Four semiconducting surface states with flat dispersions, whose binding energies are 0.2, 0.4, 0.75, and 1.0 eV, were observed in the bulk band gap and more than six states were observed within the projected bulk band at binding energies less than 5.2 eV. The origins of the four surface states and of one state at a binding energy of approximately 1.5 eV at the Γ̅ point are discussed based on the local density of states mappings obtained by STM. Further, a structural model that can explain all these five states is proposed.

  • 39.
    Sakamoto, Kazuyuki
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Takeyama, Wakabe
    Department of Physics, Graduate School of Science, Tohoku University, Japan.
    Zhang, Hanmin
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Structural investigation of Ca/Si(111) surfaces2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 16Article in journal (Refereed)
    Abstract [en]

    The surface structures of different Ca/Si(111) surfaces were studied by low-energy electron diffraction (LEED) and high-resolution core-level photoelectron spectroscopy. Five different phases were observed in LEED depending on the Ca coverage. The lowest coverage phase has both (3×2) and c(6×2) periodicities, and the highest coverage phase has a (2×1) periodicity. The LEED patterns of the three intermediate phases were (5×2), (7×1), and (9×1). In the Si 2p core-level spectra, three surface components were observed in both the lowest and highest coverage phases. These three surface components were also observed in each intermediate phase together with two other components. The presence of the two extra components indicates that the intermediate phases are not completely described by simple combinations of the two end phases as suggested in the literature. By considering the energy shift and intensity of each surface component, we conclude that the structure of the (3×2) phase is basically the same as that of the honeycomb-chain-channel model with a Ca coverage of 1/6 ML, and the (2×1) phase is formed by π-bonded Seiwatz Si chains with a coverage of 0.5 ML. Moreover, taking the energy shifts and intensities of the extra surface components into account, we propose a structural model of the (5×2) phase, whose Ca coverage is 0.3 ML.

  • 40.
    Sakamoto, Kazuyuki
    et al.
    Department of Physics, Graduate School of Science, Tohoku University, Japan.
    Zhang, Hanmin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Adsorption and reaction processes of physisorbed molecular oxygen on Si(111)-(7×7)2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 7, p. 075346-Article in journal (Refereed)
    Abstract [en]

    The adsorption and reaction processes of physisorbed oxygen molecules on a Si(111)-(7×7) surface have been investigated using time-resolved O 1s core-level photoemission measurements at 45 K. Physisorbed oxygen molecules are only observed at 45 K and lower temperatures on a Si(111)-(7×7) surface. At the dosage when the dangling bonds are saturated by chemisorbed oxygen, the coverage of the physisorbed species increases drastically. This result indicates that oxygen species, which are chemisorbed on top of adatoms, modifies the potential energy curve for an oxygen molecule approaching the surface such that physisorbed oxygen molecules are stabilized. Further, the longer lifetime at a higher dosage indicates that an intermolecular force plays a role for the stabilization of this species. Taking these results into account, an oxidation stage-dependent gas-surface interaction for an oxygen molecule approaching the Si(111) surface is suggested.

  • 41.
    Sakamoto, Kazuyuki
    et al.
    Graduate School of Advanced Integration Science, Chiba University, Japan.
    Zhang, Hanmin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Band structure of the Ca/Si(111)-(2×1) surface2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 24, p. 245316-Article in journal (Refereed)
    Abstract [en]

    We have investigated the electronic structure of the Ca/Si(111)-(2×1) surface using angle-resolved photoelectron spectroscopy. Two semiconducting surface states were clearly observed in the bulk band gap, and one was found in a pocket of the bulk band projection. Qualitatively, the dispersions of the two surface states observed in the band gap agree well with theoretical dispersions for a clean Si(111)-(2×1) surface with the Seiwatz structure. Taking this result into account, we conclude that the two surface states in the band gap originate from orbitals of Si atoms that form a Seiwatz structure, and that two electrons are donated from Ca to Si per (2×1) unit cell. This conclusion supports the structure of the Ca/Si(111)-(2×1) surface proposed in the literature.

  • 42.
    Sakamoto, Kazuyuki
    et al.
    Department of Physics, Graduate School of Science, Tohoku University, Japan.
    Zhang, Hanmin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Electronic structure of the Ca/Si(111)-(3×2) surface2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, no 12, p. 1253211-1253217Article in journal (Refereed)
    Abstract [en]

    The electronic structure of the Ca/Si(111)-(3×2) surface has been investigated by angle-resolved photoelectron spectroscopy. Five surface states, none of which crosses the Fermi level, were observed in the bulk band gap, and one surface state was observed in a bulk band pocket. The dispersion features of three of the surface states in the band gap agree well with results from monovalent atom adsorbed Si(111)-(3×1) surfaces along the chain direction. The close resemblance indicates that the origins of the surface states are the same as or quite similar to those of the (3×1) surface. The two other states observed in the band gap have not been reported in the literature, and they are interpreted as surface states that occur on Ca/Si(111)-(3×2) due to the lower coverage (1/6 monolayer of Ca). Further, based on the finite surface state dispersion in the direction perpendicular to the Ca chains, we conclude that the electronic character of this surface is not completely one dimensional.

  • 43.
    Sakamoto, Kazuyuki
    et al.
    Graduate School of Advanced Integration Science, Chiba University, Japan.
    Zhang, Hanmin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Observation of two metastable oxygen species adsorbed on a Si(111)-(7×7) surface: Reinterpretation of the initial oxidation process2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 7, p. 075302-Article in journal (Refereed)
    Abstract [en]

    Using high-resolution core-level photoemission measurements, we show that two different metastable species exist on a Si(111)-(7×7) surface, instead of only one as suggested in the literature. One metastable species has a finite lifetime and is composed of both molecular and atomic oxygen. The other one, which is formed by atomic oxygen only, is stable in terms of time at 300 and 100 K but disappears after annealing at 600 K. The present study reconciles the inconsistent former results and provides a detailed atomic-level understanding of the complex initial oxidation process of this surface.

  • 44.
    Sakamoto, Kazuyuki
    et al.
    Department of Physics, Graduate School of Science, Tohoku University, Japan.
    Zhang, Hanmin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Photoemission study of metastable oxygen adsorbed on a Si(111)-(7×7) surface2004In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 70, no 3, p. 035301-Article in journal (Refereed)
    Abstract [en]

    We have investigated the metastable oxygen species adsorbed on a Si(111)-(7×7) surface at room temperature using real time valence-band and core-level photoemission measurements. The dosage- and time-dependent changes in the intensity of the metastable 3.8 eV peak observed in the valence-band spectra were different from those of the metastable O 1s components. Further, although no metastable O 1s components were observed in the core-level measurement after annealing the sample at 600 K, the 3.8 eV peak was still observed. These results indicate that both the stable and the metastable oxygen species produce the 3.8 eV valence-band peak, and we therefore conclude that the contradicting lifetimes reported in the literature result from a misinterpretation of the 3.8 eV peak.

  • 45.
    Sohail, Hafiz Muhammad
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Eriksson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Osiecki, Jacek
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger I. G.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    First principles study of electronic and atomic structures of a √3x√3 superstructures induced by Ag on Si(111) and Ge(111)2014Manuscript (preprint) (Other academic)
    Abstract [en]

    We have employed first principles density functional theory (DFT) based calculations (WIEN2k) to study the electronic and atomic structures of the  reconstruction induced by Ag on Si(111) and Ge(111). The Ag/Si(111)  surface, in particular, has acted as a model system when it comes to the interaction between adsorbed metals and semiconductor surfaces. Two models have been studied, i.e., the honeycomb-chained-triangle (HCT) and the  in-equivalenttriangle (IET) model. The band structures of these models were calculated using density functional theory within the generalized gradient approximation (GGA) and the local density approximation (LDA). The band structures calculated from the fully relaxed versions of the two models were found to be quite similar except for the occupancy of the free electron like band at the - point. The IET model gives a slightly lower energy minimum compared to the HCT model for both Si and Ge. Further, we find that the energy minima are deeper for Ge when comparing the results with Si for the HCT and IET models, respectively. The theoretical surface band structure is qualitatively in good general agreement with the experimental dispersions of the main surface states, while the theoretical band widths are approximately half of the experimental ones. The calculated band structures show a gap between the two uppermost, fully occupied, bands at the - point only when the IET model is used to account for the electronic structure of Ag/Si(111) . Neither the IET nor the HCT model resulted in a gap when applied to Ag/Ge(111) .

  • 46.
    Sohail, Hafiz Muhammad
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Osiecki, Jacek
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Electronic and atomic structures of a 3x3 surface formed by a binary Sn/Ag overlayer on the Ge(111)c(2x8) surface: ARPES, LEED, and STM studies2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 20, p. 205409-Article in journal (Refereed)
    Abstract [en]

    The electronic and atomic structures of a well-ordered 3x3 periodicity of a binary Sn/Ag overlayer on Ge(111) have been studied. The ordered binary overlayer was formed by depositing 0.75 monolayer of Sn on an Ag/Ge(111) root 3x root 3 surface. Annealing at 330 degrees C resulted in a low-energy electron diffraction pattern that exhibited sharp spots. A detailed electronic structure investigation was performed by angle-resolved photoelectron spectroscopy. The Sn/Ag/Ge(111) 3x3 surface shows a rich band structure. There are seven bands which are positively identified as 3x3 surface bands, all within 1.5 eV below the Fermi level (E-F). The upper two bands disperse across E-F exhibiting steep almost linear dispersions down to a minimum energy of approximate to 0.40 eV below E-F at the (Gamma) over bar point (approximate to 0.30 eV at the (K) over bar point). Constant energy contours have been mapped in the 3x3 surface Brillouin zone (SBZ) in order to study an intriguing split observed in the band structure related to the two upper bands. It turned out that the two upper bands are degenerate along the (Gamma) over bar - (K) over bar and (M) over bar - (K) over bar symmetry lines of the 3x3 SBZ but separated along (Gamma) over bar - (M) over bar. Scanning tunneling microscopy images obtained at approximate to 40 K show essentially a hexagonal structure except for a honeycomb structure in a limited bias range imaging empty states. Core-level spectroscopy shows a narrow Sn 4d spectrum consistent with the high degree of structural order.

  • 47.
    Sohail, Hafiz Muhammad
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger I. G.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Electronic and atomic structures of a Sn induced 3√3x3√3 superstructure on the Ag/Ge(111) √3x√3 surface2016In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 644, p. 29-33Article in journal (Refereed)
    Abstract [en]

    We have investigated sub-monolayer coverages of Sn on the Ag/Ge(111)  surface. It was found that ≈0.45 monolayer (ML) resulted in a new, well-defined, reconstruction with a periodicity. The periodic structure of the surface atoms was verified by low energy electron diffraction and scanning tunneling microscopy. The electronic structure was studied in detail using angle resolved photoelectron spectroscopy and core level spectroscopy at a temperature of 100 K. Several surface bands were identified and their dispersions are presented along the  and  high symmetry lines of the  surface Brillouin zone (SBZ). The  surface has a metallic character since there is a strong surface band crossing the Fermi level near -points coinciding with -points of the 1×1 SBZ. The Fermi contour of the metallic band showed a hexagonal shape in contrast to the circular shaped Fermi contour of the initial  surface. Both empty and filled state STM images showed a hexagonal arrangement of protrusions which show a local  periodicity and a superimposed modulation of the apparent heights with a  periodicity.

  • 48.
    Sohail, Hafiz Muhammad
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger I. G.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Electronic and atomic structures of the Ag induced √3x√3 superstructure on Ge(111)2014In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 625, p. 23-29Article in journal (Refereed)
    Abstract [en]

    The Ag/Ge(111) surface together with Ag/Si(111) constitutes a set of surfaces that is ideally suited for fundamental studies related to low dimensional physics. We here focus on the atomic and electronic structures of the two-dimensional superstructure induced by Ag on Ge(111), a surface that is significantly less studied than the Si counterpart. Extensive information on the surface band structure obtained by angle resolved photoelectron spectroscopy (ARPES) is presented, complemented by atomic information from scanning tunneling microscopy (STM). The results reveal new findings that are important for the understanding of the Ag induced structure, acting as a prototype for semiconductor/metal interfaces. i) We have identified a new occupied surface band near the -point of the surface Brillouin zone. ii) The Ag/Ge(111) surface exhibits a partially occupied surface band, S1, with a parabolic-like shape at Γ¯. At low temperature (≈ 100 K) this band splits into two bands, S1U and S1D. The identification of two bands is significantly different from the case of Ag/Si(111) for which just one band has been reported. Besides these specific results, our extensive ARPES study reveals four surface bands at room temperature (RT), while five surface bands were identified at ≈ 100 K (LT). Room temperature empty state STM images show, depending on the tunneling bias, both honeycomb and hexagonal periodicities which are consistent with the honeycomb chained trimer and the in-equivalent trimer models, respectively.

  • 49.
    Sohail, Hafiz Muhammad
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger I. G.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Experimental studies of an In/Pb binary surface alloy on Ge(111)2016In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 649, p. 146-151Article in journal (Refereed)
    Abstract [en]

    In this study, we present a binary In/Pb surface alloy on Ge(111) formed by evaporating0.85 monolayer (ML) of In on the Pb/Ge(111)  surface with 1.33 ML of Pb. A welldefined3×3 periodicity is formed after annealing at a temperature of ≈200 °C, as verified by bothlow energy electron diffraction (LEED) and scanning tunneling microscopy (STM). OverviewSTM images, obtained at 50 K, show a clear 3×3 periodicity. Detailed STM images reveal thatthe protrusions consist of atomic sized features with a local hexagonal arrangement. Each 3×3unit cell contains nine such features indicating a structure with 9 atoms per 3×3 cell. Based onangle resolved photoelectron spectroscopy (ARPES) data, we have identified five surface bandswithin the bulk band gap. Four of them cross the Fermi level leading to a metallic character of thesurface. The dispersions of these bands have been mapped in detail along the high symmetrydirections of the 3×3 surface Brillouin zone. Fermi contours, mapped in 2D k-space, showinteresting features. In particular, the occurrence of two differently rotated hexagon like contoursis discussed.

  • 50.
    Sohail, Hafiz Muhammad
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Uhrberg, Roger I. G.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
    Origin of the metal to semiconductor transition associated with the √3x√3 and 6×6surfaces of Ag/Ge(111)2014Manuscript (preprint) (Other academic)
    Abstract [en]

    This study focuses on the electronic structure of a 6×6 surface which is formed by 0.2 monolayer of Ag on top of the Ag/Ge(111)  surface. The 6×6 periodicity was verified by low energy electron diffraction. Angle resolved photoelectron spectroscopy was employed to study the electronic structure along the and high symmetry lines of the 6×6 surface Brillouin zone. There are six surface bands in total. Out of these, three were found to be related to the 6×6 phase. The surface band structure of the 6×6 phase is significantly more complex than that of the  surface. This is particularly the case for the uppermost surface band structure which is a combination of a surface band originating from the underlying  surface and umklapp scattered branches of this band. Branches centered at neighboring 6×6 SBZs cross each other at an energy slightly below the Fermi level. An energy gap opens up at this point which contains the Fermi level and thus making the 6×6 semiconducting. The complex pattern of constant energy contours have been used to identify the origins of various branches of the surface state dispersions.

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