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Electronic structure of Ge(111)c(2x8): STM, angle-resolved photoemission, and theory
Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. (Yt- och Halvledarfysik, Surface and Semiconductor Physics)
Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. (Yt- och Halvledarfysik, Surface and Semiconductor Physics)
Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. (Yt- och Halvledarfysik, Surface and Semiconductor Physics)
Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. (Yt- och Halvledarfysik, Surface and Semiconductor Physics)
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2009 (English)In: PHYSICAL REVIEW B, ISSN 1098-0121, Vol. 79, no 20, 205410- p.Article in journal (Refereed) Published
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()).

Place, publisher, year, edition, pages
2009. Vol. 79, no 20, 205410- p.
Keyword [en]
band theory, elemental semiconductors, germanium, low energy electron diffraction, photoelectron spectra, scanning tunnelling microscopy, surface states
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-19417DOI: 10.1103/PhysRevB.79.205410OAI: oai:DiVA.org:liu-19417DiVA: diva2:224932
Note
Original Publication: Ivy Razado, Jiangping He, Hanmin Zhang, Göran Hansson and Roger Uhrberg, Electronic structure of Ge(111)c(2x8): STM, angle-resolved photoemission, and theory, 2009, PHYSICAL REVIEW B, (79), 20, 205410. http://dx.doi.org/10.1103/PhysRevB.79.205410 Copyright: American Physical Society http://www.aps.org/ Available from: 2009-08-17 Created: 2009-06-22 Last updated: 2012-02-06Bibliographically approved
In thesis
1. Scanning Tunneling Microscopy and Photoelectron Spectroscopy Studies of Si(111) and Ge(111) Surfaces: Clean and Modified by H or Sn Atoms
Open this publication in new window or tab >>Scanning Tunneling Microscopy and Photoelectron Spectroscopy Studies of Si(111) and Ge(111) Surfaces: Clean and Modified by H or Sn Atoms
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The (111) surfaces of Si and Ge were studied by scanning tunneling microscopy (STM) and photoelectron spectroscopy (PES) that are complementary techniques used to obtain structural and electronic properties of surfaces. The (111) surfaces have been of great interest because of the complex reconstructions formed by annealing. Adsorption of different types of atoms on these surfaces has been widely explored by many research groups. In this thesis work, both clean and modified Si(111) and Ge(111) surfaces were extensively studied to gain information about their atomic and electronic structures.

Hydrogen plays a significant role in surface science, specifically in passivating dangling bonds of semiconductor surfaces. There has been a significant number of studies performed on hydrogen exposure of the Si(111)7x7 surface. However, most studies were done after higher exposures resulting in a 1x1 surface. In this thesis work, low hydrogen exposures were employed such that the 7x7 structure was preserved. STM images revealed that the hydrogen atoms preferentially adsorb on the rest atoms at elevated temperatures. A hydrogen terminated rest atom dangling bond is no longer visible in the STM image and the surrounding adatoms become brighter. This implies that there is a charge transfer back to the adatoms. Three types of Htermination (1H, 2H and 3H) were studied in detail by analysing the line profiles of the apparent heights.

There are still unresolved issues regarding the electronic structure of the Ge(111)c(2x8) surface. By combining STM, angle-resolved photoelectron spectroscopy (ARPES), and theoretical calculations, new results about the electronic structure of the clean surface have been obtained in this thesis. A more detailed experimental surface band structure showing seven surface state bands is presented. A split surface state band in the photoemission data matched a split between two types of rest atom bands in the calculated surface band structure. A highly dispersive band close to the Fermi level was identified with states below the adatom and rest atom layers and is therefore not a pure surface state. The bias dependent STM images which support the photoemission results were in agreement with simulated images generated from the calculated electronic structure of the c(2x8) surface.

Many studies have been devoted to hydrogen adsorption on Si(111)7x7 but only a few have dealt with Ge(111)c(2x8). In this work, hydrogen adsorption on Ge(111)c(2x8) has been studied using STM and ARPES. The preferred adsorption site is the rest atom. As a consequence of the adsorption on the rest atom there is a reverse charge transfer to the adatoms, which makes them appear brighter in the filled-state STM images. Photoemission results showed that for the H-exposed surface, the surface states associated with the rest-atom dangling bonds decreased in intensity while a new peak appeared in the close vicinity of the Fermi level which is not present in the spectrum of the clean surface. This is a clear evidence of a semiconducting to metallic transition of the Ge(111)c(2x8) surface. A higher H exposure on the Ge(111)c(2x8) surface was also done which resulted in a 1x1 surface. The electronic structure was investigated using ARPES. Two surface states were observed that are related to the Ge-Ge backbonds and the Ge-H bonds.

Sn/Ge(111) has attracted a lot of attention from the surface science community because of the interesting phase transition from the RT-(√3x√3) phase to the LT-(3x3) phase. Previously, the Sn/Ge(111)√3x√3 surface was considered to be just a simple α- phase surface on which the Sn atoms sit on the T4 sites. However, a core-level study of the RT-(√3x√3) surface showed two components in the Sn 4d core-level spectra which implies that there are two inequivalent Sn atoms. The transition was later on explained by the dynamical fluctuation model. There have been different models proposed for the Sn/Ge(111)3x3 structure such as the 2U1D, 1U2D and IDA models. In this thesis work, the surface was studied using STM. The optimum √3x√3 surface was determined by performing different sample preparations. The LT STM images of the 3x3 surface were investigated and they showed that there are different types of Sn atoms such as up and down atoms. A histogram of the apparent height distribution revealed two peaks, a sharper peak associated with the up atoms and a broader peak for the down atoms. The height distribution was used to produce simulated Sn 4d core-level spectra and the line shape was compared to that of experimental spectra.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2009. 46 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1236
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17371 (URN)978-91-7393-705-4 (ISBN)
Public defence
2009-03-20, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2009-03-20 Created: 2009-03-20 Last updated: 2009-09-04Bibliographically approved

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Razado, IvyZhang, HanminHansson, GöranUhrberg, Roger

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