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• 1.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Atomic and Electronic Structures of Clean and Metal Adsorbed Si and Ge Surfaces: An Experimental and Theoretical Study2010Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)

In this work, a selection of unresolved topics regarding the electronic and atomic structures of Si and Ge surfaces, both clean ones and those modified by metal adsorbates, are addressed. The results presented have been obtained using theoretical calculations and experimental techniques such as photoelectron spectroscopy (PES), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM).

Si(001) surfaces with adsorbed alkali metals can function as prototype systems for studying properties of the technologically important family of metal-semiconductor interfaces. In this work, the effect of up to one monolayer (ML) of Li on the Si(001) surface is studied using a combination of experimental and theoretical techniques. Several models for the surface atomic structures have been suggested for 0.5 and 1 ML of Li in the literature. Through the combination of experiment and theory, critical differences in the surface electronic structures between the different atomic models are identified and used to determine the most likely model for a certain Li coverage.

In the literature, there are reports of an electronic structure at elevated temperature, that can be probed using angle resolved PES (ARPES), on the clean Ge(001) and Si(001) surfaces. The structure is quite unusual in the sense that it appears at an energy position above the Fermi level. Using results from a combined variable temperature ARPES and LEED study, the origin of this structure is determined. Various explanations for the structure that are available in the literature are discussed. It is found that all but thermal occupation of an ordinarily empty surface state band are inconsistent with our experimental data.

In a combined theoretical and experimental study, the surface core-level shifts on clean Si(001) and Ge(001) in the c(4×2) reconstruction are investigated. In the case of the Ge 3d core-level, no previous theoretical results from the c(4×2) reconstruction are available in the literature. The unique calculated Ge 3d surface core-level shifts facilitate the identification of the atomic origins of the components in the PES data. Positive assignments can be made for seven of the eight inequivalent groups of atoms in the four topmost layers in the Ge case. Furthermore, a similar, detailed, assignment of the atomic origins of the shifts on the Si surface is presented that goes beyond previously published results.

At a Sn coverage of slightly more than one ML, a 23 × 23 reconstruction can be obtained on the Si(111) surface. Two aspects of this surface are explored and presented in this work. First, theoretically derived results obtained from an atomic model in the literature are tested against new ARPES and STM data. It is concluded that the model needs to be revised in order to better explain the experimental observations. The second part is focused on the abrupt and reversible transition to a molten 1×1 phase at a temperature of about 463 K. ARPES and STM results obtained slightly below and slightly above the transition temperature reveal that the surface band structure, as well as the atomic structure, changes drastically at the transition. Six surface states are resolved on the surface at low temperature. Above the transition, the photoemission spectra are, on the other hand, dominated by a single strong surface state band. It shows a dispersion similar to that of a calculated surface band associated with the Sn-Si bond on a 1×1 surface with Sn positioned above the top layer Si atoms.

There has been extensive studies of the reconstructions on Si surfaces induced by adsorption of the group III metals Al, Ga and In. Recently, this has been expanded to Tl, i.e., the heaviest element in that group. Tl is different from the other elements in group III since it exhibits a peculiar behavior of the 6s2 electrons called the “inert pair effect”. This could lead to a valence state of either 1+ or 3+. In this work, core-level PES is utilized to find that, at coverages up to one ML, Tl exhibits a 1+ valence state on Si(111), in contrast to the 3+ valence state of the other group III metals. Accordingly, the surface band structure of the 1/3 ML 3 x √3 reconstruction is found to be different in the case of Tl, compared to the other group III metals. The observations of a 1+ valence state are consistent with ARPES results from the Si(001):Tl surface at one ML. There, six surface state bands are seen. Through comparisons with a calculated surface band structure, four of those can be identified. The two remaining bands are very similar to those observed on the clean Si(001) surface.

1. Lithium-induced dimer reconstructions on Si(001) studied by photoelectron spectroscopy and band-structure calculations
Öppna denna publikation i ny flik eller fönster >>Lithium-induced dimer reconstructions on Si(001) studied by photoelectron spectroscopy and band-structure calculations
2007 (Engelska)Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 75, nr 20, s. 205416-Artikel i tidskrift (Refereegranskat) Published
##### 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.

##### Nationell ämneskategori
Naturvetenskap
##### Identifikatorer
urn:nbn:se:liu:diva-39513 (URN)10.1103/PhysRevB.75.205416 (DOI)000246890900117 ()49105 (Lokalt ID)49105 (Arkivnummer)49105 (OAI)
2. Origin of a surface state above the Fermi level on Ge(001) and Si(001) studied by temperature-dependent ARPES and LEED
Öppna denna publikation i ny flik eller fönster >>Origin of a surface state above the Fermi level on Ge(001) and Si(001) studied by temperature-dependent ARPES and LEED
2008 (Engelska)Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, nr L8, s. 085406-1-085406-5Artikel i tidskrift (Refereegranskat) Published
##### 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.

##### Nationell ämneskategori
Naturvetenskap
##### Identifikatorer
urn:nbn:se:liu:diva-41610 (URN)10.1103/PhysRevB.77.085406 (DOI)000253764300089 ()58313 (Lokalt ID)58313 (Arkivnummer)58313 (OAI)
3. Surface core-level shifts on clean Si(001) and Ge(001) studied with photoelectron spectroscopy and DFT calculations
Öppna denna publikation i ny flik eller fönster >>Surface core-level shifts on clean Si(001) and Ge(001) studied with photoelectron spectroscopy and DFT calculations
2010 (Engelska)Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, nr 12, s. 125443-Artikel i tidskrift (Refereegranskat) Published
##### 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).

##### Nationell ämneskategori
Naturvetenskap
##### Identifikatorer
urn:nbn:se:liu:diva-54177 (URN)10.1103/PhysRevB.81.125443 (DOI)000276248900160 ()
##### Anmärkning
Original Publication: Johan Eriksson and Roger Uhrberg, Surface core-level shifts on clean Si(001) and Ge(001) studied with photoelectron spectroscopy and DFT calculations, 2010, Physical Review B. Condensed Matter and Materials Physics, (81), 12, 125443. http://dx.doi.org/10.1103/PhysRevB.81.125443 Copyright: American Physical Society http://www.aps.org/ Tillgänglig från: 2010-03-01 Skapad: 2010-03-01 Senast uppdaterad: 2017-12-12
4. Atomic and electronic structures of the ordered 2√3 × 2√3 andthe molten 1×1 phase on the Si(111):Sn surface
Öppna denna publikation i ny flik eller fönster >>Atomic and electronic structures of the ordered 2√3 × 2√3 andthe molten 1×1 phase on the Si(111):Sn surface
2010 (Engelska)Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, nr 23, s. 235410-Artikel i tidskrift (Refereegranskat) Published
##### 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.

##### Nationell ämneskategori
Naturvetenskap
##### Identifikatorer
urn:nbn:se:liu:diva-54178 (URN)10.1103/PhysRevB.81.235410 (DOI)000278482800003 ()
##### Anmärkning
Original Publication: Johan Eriksson, Jacek Osiecki, Kazuyuki Sakamoto and Roger Uhrberg, Atomic and electronic structures of the ordered 2√3 × 2√3 andthe molten 1×1 phase on the Si(111):Sn surface, 2010, Physical Review B. Condensed Matter and Materials Physics, (81), 23, 235410. http://dx.doi.org/10.1103/PhysRevB.81.235410 Copyright: American Physical Society http://www.aps.org/ Tillgänglig från: 2010-03-01 Skapad: 2010-03-01 Senast uppdaterad: 2017-12-12
5. Electronic structure of the thallium induced 2×1 reconstruction on Si(001)
Öppna denna publikation i ny flik eller fönster >>Electronic structure of the thallium induced 2×1 reconstruction on Si(001)
(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
##### 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.

##### Nationell ämneskategori
Naturvetenskap
##### Identifikatorer
urn:nbn:se:liu:diva-54179 (URN)
6. 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 atoms
Öppna denna publikation i ny flik eller fönster >>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 atoms
2006 (Engelska)Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, nr 7, s. 075335-Artikel i tidskrift (Refereegranskat) Published
##### 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.

##### Nationell ämneskategori
Naturvetenskap
##### Identifikatorer
urn:nbn:se:liu:diva-36299 (URN)10.1103/PhysRevB.74.075335 (DOI)000240238800086 ()30915 (Lokalt ID)30915 (Arkivnummer)30915 (OAI)
7. Photoemission study of the thallium induced Si(111)-´`3x´`3 surface
Öppna denna publikation i ny flik eller fönster >>Photoemission study of the thallium induced Si(111)-´`3x´`3 surface
2007 (Engelska)Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 601, nr 22, s. 5258-5261Artikel i tidskrift (Refereegranskat) Published
##### 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.

##### Nyckelord
Angle-resolved photoemission, Low-energy electron diffraction, Silicon, Surface states, Surface structure, Thallium
##### Nationell ämneskategori
Naturvetenskap
##### Identifikatorer
urn:nbn:se:liu:diva-39514 (URN)10.1016/j.susc.2007.04.245 (DOI)000251603200033 ()49106 (Lokalt ID)49106 (Arkivnummer)49106 (OAI)
• 2.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
MAX-lab Lunds universitet. Graduate School of Advanced Integration Science Chiba University, Chiba, Japan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Origin of a surface state above the Fermi level on Ge(001) and Si(001) studied by temperature-dependent ARPES and LEED2008Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, nr L8, s. 085406-1-085406-5Artikel i tidskrift (Refereegranskat)

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.

• 3.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Atomic and electronic structures of the ordered 2√3 × 2√3 andthe molten 1×1 phase on the Si(111):Sn surface2010Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, nr 23, s. 235410-Artikel i tidskrift (Refereegranskat)

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.

• 4.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Electronic structure of the thallium induced 2×1 reconstruction on Si(001)Manuskript (preprint) (Övrigt vetenskapligt)

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.

• 5.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Electronic structure of the thallium-induced 2x1 reconstruction on Si(001)2010Ingår i: PHYSICAL REVIEW B, ISSN 1098-0121, Vol. 81, nr 20, s. 205422-Artikel i tidskrift (Refereegranskat)

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.

• 6.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Graduate School of Advanced Integrated Science Chiba University, Japan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Lithium-induced dimer reconstructions on Si(001) studied by photoelectron spectroscopy and band-structure calculations2007Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 75, nr 20, s. 205416-Artikel i tidskrift (Refereegranskat)

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.

• 7.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Surface core-level shifts on clean Si(001) and Ge(001) studied with photoelectron spectroscopy and DFT calculations2010Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, nr 12, s. 125443-Artikel i tidskrift (Refereegranskat)

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).

• 8.
KTH Royal Institute of Technology, Material Physics, Kista, Sweden. KTH Royal Institute of Technology, Material Physics, Kista, Sweden. KTH Royal Institute of Technology, Material Physics, Kista, Sweden. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
Monolayer iron oxide grown on porous platinum sensing layers of carbon monoxide sensors2015Manuskript (preprint) (Övrigt vetenskapligt)

Mono-layer iron oxide has been deposited through e-beam evaporation on a silica supported poly-crystalline platinum (Pt) model catalyst and its CO oxidation characteristics obtained from mass spectrometry measurements under various CO and O2 concentrations (ranging from 100 to 900 ppm and 3 to 7 %, respectively) as well as at different temperatures (ranging from 130 to 220 °C) and compared to the CO oxidation on corresponding non-coated Pt samples. Fabricating the model system as a Metal Oxide Semiconductor (MOS) structure from 4H-SiC with a top layer of SiO2 (as the support material) and a thin, discontinuous polycrystalline Pt film as the metal (the active catalyst material) also provided the possibility to investigate whether changes in catalyst surface conditions could be electronically monitored through the changes in capacitance they induce across the MOS structure.

A low-temperature shift in the activity to CO oxidation for the iron oxide modified compared to bare Pt catalysts similar to what has previously been reported on single-crystalline Pt was found also for the near-realistic MOS model catalyst. This low-temperature shift was furthermore reflected in the electrical measurements, strongly indicating a correlation between the MOS capacitance and the CO oxidation characteristics, both in the case of iron oxide coated and non-coated Pt samples. By monitoring the MOS capacitance during more than 200 hours of continuous operation and analyzing the iron oxide coated samples by photo electron spectroscopy it could also be concluded that the iron oxide coated model catalyst seemingly retains its CO oxidation characteristics and chemical/compositional integrity over time. These findings might not only point to the applicability of iron oxide modified Pt in practical applications but may also open up new possibilities regarding the utilization of MOS model systems in studying and understanding as well as tailor CO oxidation (and other) catalysts and/or gas sensors for specific applications.

• 9.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Experimental and Theoretical Evidence of a Highly Ordered Two-Dimensional Sn/Ag Alloy on Si(111)2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 5, s. 057601-Artikel i tidskrift (Refereegranskat)

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.

• 10.
Development of FETs and resistive devices based on epitaxially grown single layer graphene on SiC for highly sensitive gas detection2012Ingår i: Materials Science Forum Vols 717 - 720, Trans Tech Publications Inc., 2012, Vol. 717-720, s. 687-690Konferensbidrag (Refereegranskat)

Epitaxially grown single layer graphene on silicon carbide (SiC) resistive sensors were characterised for NO2 response at room and elevated temperatures, with an n-p type transition observed with increasing NO2 concentration for all sensors. The concentration of NO2 required to cause this transition varied for different graphene samples and is attributed to varying degrees of substrate induced Fermi-level (E-F) pinning above the Dirac point. The work function of a single layer device increased steadily with increasing NO2 concentration indicating no change in reaction mechanism for high and low concentrations despite a change in sensor response direction. Epitaxially grown graphene device preparation is challenging due to poor adhesion of the graphene layer to the substrate. A field effect transistor (FET) device is presented which does not require wire bonding to contacts on graphene.

• 11.
Chiba University, Japan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. Kyushu University, Japan. Chiba University, Japan. Kyushu University, Japan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
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 atoms2006Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, nr 7, s. 075335-Artikel i tidskrift (Refereegranskat)

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.

• 12.
Chiba University, Japan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. Chiba University, Japan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Surface electronic structures of the Eu- and Ca-induced so-called Si(111)-(5×1) reconstructions2006Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, nr 23, s. 235311-Artikel i tidskrift (Refereegranskat)

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.

• 13.
Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Spectroscopy, Applied Physics Chiba University. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik.
Photoemission study of the thallium induced Si(111)-´`3x´`3 surface2007Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 601, nr 22, s. 5258-5261Artikel i tidskrift (Refereegranskat)

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.

• 14.
Chiba University.
Kanazawa University. Hiroshima University. Hiroshima University. Kanazawa University. Chiba University. Chiba University. Hiroshima University. Hiroshima University. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Abrupt Rotation of the Rashba Spin to the Direction Perpendicular to the Surface2009Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 102, nr 9, s. 096805-Artikel i tidskrift (Refereegranskat)

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.

• 15.
Chiba University.
National Institute of Material Science, Japan. Tohoku University. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. National Institute of Material Science, Japan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Electronic structure of the Si(110)-(16×2) surface: High-resolution ARPES and STM investigation2009Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, nr 4, s. 045304-Artikel i tidskrift (Refereegranskat)

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.

• 16.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Yt- och Halvledarfysik. Linköpings universitet, Tekniska högskolan.
First principles study of electronic and atomic structures of a √3x√3 superstructures induced by Ag on Si(111) and Ge(111)2014Manuskript (preprint) (Övrigt vetenskapligt)

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) .

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