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Experimental and Theoretical Studies of Metal Adsorbates Interacting with Elemental Semiconductor Surfaces
Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics. Linköping University, The Institute of Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

Metal adsorbates on semiconductor surfaces have been widely studied over the last few decades. The main interest is focused on various one or two-dimensional structures that exhibit interesting electronic and atomic properties. This thesis focuses on metal adsorbates interacting with the Si(111) and Ge(111) surfaces. The main experimental techniques used in the thesis include angle resolved photoelectron spectroscopy (ARPES), core-level spectroscopy, scanning tunneling microscopy (STM), and low energy electron diffraction (LEED). The experimental studies have, in some cases, been complemented by theoretical electronic structure investigations based on density functional theory (DFT).

Silver (Ag), a noble metal, gives rise to several reconstructions on the (111) surfaces of Si and Ge. The Ag/Si(111)  surface has been extensively studied, but the Ag/Ge(111)  surface has not been given similar attention, and there are no detailed experimental nor calculated electronic band structures available in the literature. Thus, a detailed ARPES investigation of the electronic structure of the Ag/Ge(111)  surface, with nominally 1 monolayer (ML) of Ag, is presented in the thesis together with its atomic structure.

The Ag/Si(111)  and Ag/Ge(111)  surfaces were also studied by first principles DFT based calculations (WIEN2k). Two atomic models have been suggested for the  surfaces in the literature, i.e., the honeycomb-chained-trimer (HCT) and the in-equivalent trimer (IET) models. Band structure calculations were performed for both models, and comparisons between calculated and experimental surface band structures are presented for the Si and Ge cases.

Adding approximately 0.2 ML of Ag to Ag/Ge(111)  results in a 6×6 phase. The electronic structure of the surface is presented in detail. Several new bands appear in the energy region close the Fermi level, which can all be explained by umklapp scattering by reciprocal lattice vectors of the 6×6 lattice. A metal to semiconductor transition, associated  with the  to 6×6 structural change, is explained by gaps opening up where the umklapp scattered bands cross.

After having established sufficient understanding of the Ag/Si(111)  and Ag/Ge(111)  surfaces, they were used as substrates for the formation of binary surface alloys. An amount of 0.45 ML of Sn, in combination with the Ag of the Ag/Ge(111)  surface, forms a well-defined $3\sqrt{3}$x$3\sqrt{3$binary alloy. The surface band structure shows some modifications compared to that of Ag/Ge(111)  surface. The STM results show clearly the $3\sqrt{3}$x$3\sqrt{3$ periodicity.

A Sn coverage of 0.75 ML on the Ag/Ge(111)  surface results in a very wellordered 3×3 surface alloy. This alloy shows a very rich surface band structure in which the upper band exhibits peculiar splits. Two-dimensional constant energy contour data reveal the existence of two rotated contours which is related to the presence of split bands in certain directions. STM images show a hexagonal or a honeycomb structure depending on sample to tip bias.

A similar amount of Sn (0.75 ML) was also evaporated onto the Ag/Si111)  surface, with the purpose to form a surface alloy on Si(111). This resulted in a very well-ordered Sn/Ag/Si(111)2×2 periodicity. The surface shows an interesting free electron like band which crosses the Fermi level. STM images reveal clear, but differently looking, protrusions in the 2×2 unit cell when comparing empty and filled state images. The atomic structure of the surface alloy was modelled by DFT calculations using structural information provided by the STM images. The modelling resulted in a structure consisting of Sn and Ag trimers and a fourth Ag atom located at the corner of the 2×2 cell. In addition, the calculated electronic structure based on the proposed model is consistent with experimental results, which verifies the atomic model.

Another combination of metals, 1.33 ML of Pb and 0.85 ML of In, resulted in the formation of a well-defined In/Pb/Ge(111)3×3 surface alloy. The 3×3 surface exhibits an interesting band structure where five surface bands were identified of which four cross the Fermi level resulting in a metallic character of the surface. Two-dimensional constant energy data reveal the presence of intricate rotated hexagon like contours which intersect each other along the $\overline{K}$ and  directions of the surface Brillouin zone. The STM results reveal nine bright protrusions per 3×3 unit cell.

##### Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1575
Natural Sciences
##### Identifiers
ISBN: 978-91-7519-399-1 (print)OAI: oai:DiVA.org:liu-105223DiVA: diva2:704921
##### Supervisors
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-03-14Bibliographically approved
##### List of papers
1. Electronic and atomic structures of the Ag induced √3x√3 superstructure on Ge(111)
Open this publication in new window or tab >>Electronic and atomic structures of the Ag induced √3x√3 superstructure on Ge(111)
2014 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 625, 23-29 p.Article in journal (Refereed) Published
##### 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.

Elsevier, 2014
##### Keyword
Ge(111); Ag; Monolayer; Atomic structure; Electronic band structure; Free electron like band; STM; LEED; ARPES
Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105216 (URN)10.1016/j.susc.2014.02.013 (DOI)000336355200005 ()
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-06-23Bibliographically approved
2. First principles study of electronic and atomic structures of a √3x√3 superstructures induced by Ag on Si(111) and Ge(111)
Open this publication in new window or tab >>First principles study of electronic and atomic structures of a √3x√3 superstructures induced by Ag on Si(111) and Ge(111)
##### 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 $\overline{K}$- 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) .

Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105218 (URN)
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-03-14Bibliographically approved
3. Origin of the metal to semiconductor transition associated with the √3x√3 and 6×6surfaces of Ag/Ge(111)
Open this publication in new window or tab >>Origin of the metal to semiconductor transition associated with the √3x√3 and 6×6surfaces of Ag/Ge(111)
##### 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 $\overline{M}$ and $\overline{K}$$\overline{M}$ 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.

Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105220 (URN)
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-03-14Bibliographically approved
4. Electronic and atomic structures of a Sn induced 3√3x3√3 superstructure on the Ag/Ge(111) √3x√3 surface
Open this publication in new window or tab >>Electronic and atomic structures of a Sn induced 3√3x3√3 superstructure on the Ag/Ge(111) √3x√3 surface
2016 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 644, 29-33 p.Article in journal (Refereed) Published
##### 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 $3\sqrt{3}x3\sqrt{3}$ 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 $\overline{M}$ and $\overline{K}$$\overline{M}$ high symmetry lines of the $3\sqrt{3}x3\sqrt{3}$ surface Brillouin zone (SBZ). The $3\sqrt{3}x3\sqrt{3}$ surface has a metallic character since there is a strong surface band crossing the Fermi level near -points coinciding with $\overline{K}$-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 $3\sqrt{3}x3\sqrt{3}$ periodicity.

Elsevier, 2016
Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105221 (URN)10.1016/j.susc.2015.08.037 (DOI)000367489000005 ()
##### Note

Funding agencies: Swedish Research Council (VR); Knut and Alice Wallenberg Foundation (KAW)

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Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2016-02-02Bibliographically approved
5. 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 studies
Open this publication in new window or tab >>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 studies
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 20, 205409- p.Article in journal (Refereed) Published
##### 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.

##### Place, publisher, year, edition, pages
American Physical Society, 2012
##### National Category
Engineering and Technology
##### Identifiers
urn:nbn:se:liu:diva-77726 (URN)10.1103/PhysRevB.85.205409 (DOI)000303655800006 ()
##### Note

Funding Agencies|Swedish Research Council (VR)||Knut and Alice Wallenberg Foundation (KAW)||

Available from: 2012-05-30 Created: 2012-05-28 Last updated: 2014-03-14
6. Experimental and Theoretical Evidence of a Highly Ordered Two-Dimensional Sn/Ag Alloy on Si(111)
Open this publication in new window or tab >>Experimental and Theoretical Evidence of a Highly Ordered Two-Dimensional Sn/Ag Alloy on Si(111)
2012 (English)In: Physical Review Letters, ISSN 0031-9007, Vol. 109, no 5, 057601- p.Article in journal (Refereed) Published
##### 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.

##### Place, publisher, year, edition, pages
American Physical Society, 2012
##### National Category
Engineering and Technology
##### Identifiers
urn:nbn:se:liu:diva-80784 (URN)10.1103/PhysRevLett.109.057601 (DOI)000306995700019 ()
##### Note

Funding Agencies|Swedish Research Council|621-2010-37462008-6582|Linkoping Linnaeus Initiative for Novel Functional Materials (LiLi-NFM)||Knut and Alice Wallenberg foundation (KAW)||

Available from: 2012-08-30 Created: 2012-08-30 Last updated: 2014-03-14Bibliographically approved
7. Experimental studies of an In/Pb binary surface alloy on Ge(111)
Open this publication in new window or tab >>Experimental studies of an In/Pb binary surface alloy on Ge(111)
2016 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 649, 146-151 p.Article in journal (Refereed) Published
##### 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.

Elsevier, 2016
Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105222 (URN)10.1016/j.susc.2016.02.016 (DOI)000379097000021 ()
##### Note

Funding agencies:Technical support from Dr. Johan Adell and Dr. T. Balasubramanian at MAX-lab is gratefully acknowledged. Financial support of the research work was provided by the Swedish Research Council (VR) and the Knut and Alice Wallenberg Foundation (KAW).

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Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2016-08-19Bibliographically approved

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