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Scanning Tunneling Microscopy and Photoelectron Spectroscopy Studies of Si(111) and Ge(111) Surfaces: Clean and Modified by H or Sn Atoms
Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
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: urn:nbn:se:liu:diva-17371ISBN: 978-91-7393-705-4 (print)OAI: oai:DiVA.org:liu-17371DiVA: diva2:208851
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
List of papers
1. STM study of site selective hydrogen adsorption on Si(111) 7×7
Open this publication in new window or tab >>STM study of site selective hydrogen adsorption on Si(111) 7×7
2005 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 23, 235411- p.Article in journal (Refereed) Published
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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17366 (URN)10.1103/PhysRevB.71.235411 (DOI)000230276800100 ()
Available from: 2013-03-27 Created: 2009-03-20 Last updated: 2013-03-27Bibliographically approved
2. Electronic structure of Ge(111)c(2x8): STM, angle-resolved photoemission, and theory
Open this publication in new window or tab >>Electronic structure of Ge(111)c(2x8): STM, angle-resolved photoemission, and theory
Show others...
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()).

Keyword
band theory, elemental semiconductors, germanium, low energy electron diffraction, photoelectron spectra, scanning tunnelling microscopy, surface states
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-19417 (URN)10.1103/PhysRevB.79.205410 (DOI)
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
3. Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)
Open this publication in new window or tab >>Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)
2006 (English)In: Applied Surface Science, ISSN 0169-4332, Vol. 252, no 15, 5300-5303 p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2006
Keyword
Scanning tunneling microscopy; Angle-resolved photoelectron spectroscopy; Hydrogen adsorption; Ge(1 1 1) c(2 × 8)
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17368 (URN)10.1016/j.apsusc.2005.12.062 (DOI)
Available from: 2009-03-20 Created: 2009-03-20 Last updated: 2012-02-06Bibliographically approved
4. Electronic structure of H/Ge(111)1x1 studied by angle-resolved photoelectron spectroscopy
Open this publication in new window or tab >>Electronic structure of H/Ge(111)1x1 studied by angle-resolved photoelectron spectroscopy
(English)Manuscript (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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17369 (URN)
Available from: 2009-03-20 Created: 2009-03-20 Last updated: 2010-01-14
5. STM studies of the Sn/Ge(111)√3x√3 and 3x3 surfaces
Open this publication in new window or tab >>STM studies of the Sn/Ge(111)√3x√3 and 3x3 surfaces
(English)Manuscript (Other academic)
Abstract [en]

Scanning tunneling microscopy (STM) was used to study the room temperature (RT) √3x√3 and the low temperature (LT) 3x3 surfaces of Sn/Ge(111). The Sn/Ge(111)√3x√3 surface was prepared under different conditions. Nine sample preparations were performed with various Sn coverage and annealing treatments. The conditions that produced the best √3x√3 surface (low defect density and minimal area covered by islands and disorder) were: i) Sn coverage of 0.38 ML, ii) sample temperature slightly above that corresponding to the c(2x8) to 1x1 transition. This optimum preparation was used for the STM study of the LT 3x3 phase. The apparent height distribution of the Sn atoms in the 3x3 phase was analysed in detail and discussed in relation to the Sn 4d core-level line shape. Two peaks were observed in the apparent height distribution of the Sn atoms corresponding to the up and down atoms constituting the 3x3 reconstruction. Simulated Sn 4d core-level spectra were generated from the distribution by assuming a linear relation between the apparent height and the core-level binding energy. The simulated spectra are compared to experimental spectra appearing in the literature.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17370 (URN)
Available from: 2009-03-20 Created: 2009-03-20 Last updated: 2014-03-13Bibliographically approved

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