| 1. |
- Lang, J. J. K., et al.
(författare)
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Unveiling and controlling the electronic structure of oxidized semiconductor surfaces: Crystalline oxidized InSb(100)(1 x 2)-O
- 2014
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 90:4
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Tidskriftsartikel (refereegranskat)abstract
- The exothermic nature of oxidation causes nearly all semiconductor applications in various fields like electronics, medicine, photonics, and sensor technology to acquire an oxidized semiconductor surface part during the application manufacturing. The significance of understanding and controlling the atomic scale properties of oxidized semiconductor surfaces is expected to increase even further with the development of nanoscale semiconductor crystals. The nature of oxidized semiconductor layers is, however, hard to predict and characterize as they are usually buried and amorphous. To shed light on these issues, we pursue a different approach based on oxidized III-V semiconductor layers that are crystalline. We present a comprehensive characterization of oxidized crystalline InSb(100)(1 x 2)-O layers by ab initio calculations, photoelectron spectroscopy, scanning tunneling microscopy, and spectroscopy, and demonstrate the electronic band structures of different oxidized phases of the semiconductor, which elucidate the previous contradictory semiconductor-oxidation effects. At 0.5 monolayer (ML) oxidation, oxygen atoms tend to occupy subsurface Sb sites, leading to metallic states in the semiconductor band gap, which arise from top dimers. When the oxidation is increased to the 1.0-2.0 ML concentration, oxygen occupies also interstitial sites, and the insulating band structure without gap states is stabilized with unusual occupied In dangling bonds. In contrast, the 2.5-3.0 ML oxide phases undergo significant changes toward a less ordered structure. The findings suggest a methodology for manipulating the electronic structure of oxidized semiconductor layers.
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| 2. |
- Uusitaloa, R. -R, et al.
(författare)
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Order-disorder transition of Pd0.5Ag0.5 alloys
- 2016
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Ingår i: PHILOSOPHICAL MAGAZINE. - : Taylor & Francis. - 1478-6435 .- 1478-6443. ; 96:36, s. 3697-3710
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio total-energy calculations, based on the exact muffin-tin orbitals method, are used to determine the equilibrium chemical configuration of Pd0.5Ag0.5 alloy as a function of disorder and temperature. The transition from a substitutionally disordered face-centred-cubic crystallographic phase to an ordered L1(1) phase is monitored using the coherent potential approximation which allows us to continuously scan the order parameter and thermodynamics of the alloy. We find signs of a first-order phase transition from a substitutionally disordered state to a partially ordered state around 210 K temperature and the completely ordered L1(1) state is predicted around 90 K.
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| 3. |
- Ahola-Tuomi, M., et al.
(författare)
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Properties of self-assembled Bi nanolines on InAs(100) studied by core-level and valence-band photoemission, and first-principles calculations
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:24, s. 245401-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied self-assembled bismuth (Bi) nanolines on the Bi-terminated InAs(100) surface by core-level and valence-band photoelectron spectroscopy, and ab initio first-principles calculations. A structural model for this intriguing surface is suggested based on the comparison of the measured and calculated core-level shifts. Also, the atomic origins for the core-level shifts are proposed based on the calculations. A clear peak related to this surface was observed in the valence band 0.34 eV below the Fermi level, which can be used as a "fingerprint" of a well-ordered Bi/InAs(100) nanoline surface.
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| 4. |
- Kokko, K., et al.
(författare)
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Atomistic study of surfaces and interfaces of Fe-Cr and Fe-Cr-Al alloys
- 2013
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Ingår i: Physical and numerical simulation of materials processing VII. - : Trans Tech Publications Inc.. - 9783037857281 ; , s. 728-733
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Konferensbidrag (refereegranskat)abstract
- Surface and interface properties of Fe-Cr, Fe-Al, and Fe-Cr-Al are studied using Exact Muffin-Tin Orbitals and Monte Carlo methods and with x-ray photoelectron and Auger electron techniques. Surface composition is investigated as a function of oxidation (heating) time. Hard x ray photoelectron spectroscopy (HAXPES) is used to scan non destructively the compositions below the surface. It is found that Cr boosts the Al segregation to the surface.
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| 5. |
- Kuronen, A., et al.
(författare)
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Segregation, precipitation, and alpha-alpha ' phase separation in Fe-Cr alloys
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:21
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Tidskriftsartikel (refereegranskat)abstract
- Iron-chromium alloys, the base components of various stainless steel grades, have numerous technologically and scientifically interesting properties. However, these features are not yet sufficiently understood to allow their full exploitation in technological applications. In this work, we investigate segregation, precipitation, and phase separation in Fe-Cr systems analyzing the physical mechanisms behind the observed phenomena. To get a comprehensive picture of Fe-Cr alloys as a function of composition, temperature, and time the present investigation combines Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy. In order to obtain a general picture of the relation of the atomic interactions and properties of Fe-Cr alloys in bulk, surface, and interface regions several complementary methods have to be used. Using the exact muffin-tin orbitals method with the coherent potential approximation (CPA-EMTO) the effective chemical potential as a function of Cr content (0-15 at. % Cr) is calculated for a surface, second atomic layer, and bulk. At similar to 10 at. % Cr in the alloy the reversal of the driving force of a Cr atom to occupy either bulk or surface sites is obtained. The Cr-containing surfaces are expected when the Cr content exceeds similar to 10 at. %. The second atomic layer forms about a 0.3 eV barrier for the migration of Cr atoms between the bulk and surface atomic layer. To get information on Fe-Cr in larger scales we use semiempirical methods. However, for Cr concentration regions less than 10 at. %, the ab initio (CPA-EMTO) result of the important role of the second atomic layer to the surface is not reproducible from the large-scale Monte Carlo molecular dynamics (MCMD) simulation. On the other hand, for the nominal concentration of Cr larger than 10 at. % the MCMD simulations show the precipitation of Cr into isolated pockets in bulk Fe-Cr and the existence of the upper limit of the solubility of Cr into Fe layers in Fe/Cr layer systems. For high Cr concentration alloys the performed spectroscopic measurements support the MCMD simulations. Hard x-ray photoelectron spectroscopy and Auger electron spectroscopy investigations were carried out to explore Cr segregation and precipitation in the Fe/Cr double layer and Fe0.95Cr0.05 and Fe0.85Cr0.15 alloys. Initial oxidation of Fe-Cr was investigated experimentally at 10(-8) Torr pressure of the spectrometers showing intense Cr2O3 signal. Cr segregation and the formation of Cr-rich precipitates were traced by analyzing the experimental atomic concentrations and chemical shifts with respect to annealing time, Cr content, and kinetic energy of the exited electron.
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| 6. |
- Kuzmin, M., et al.
(författare)
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Atomic structure of Yb/Si(100)(2X6) : Interrelation between the silicon dimer arrangement and Si 2p photoemission line shape
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:11, s. 113302-
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Tidskriftsartikel (refereegranskat)abstract
- Combining photoelectron spectroscopy and density-functional theory calculations, we have studied the atomic geometry of Yb/Si(100)(2 X 6) reconstruction and the mechanisms responsible for its stabilization as well as the influence of this reconstruction on Si 2p core-level photoemission. The analysis of measured and calculated surface core-level shifts supports the recently proposed model of the Yb/Si(100)(2 X 6). It involves, in agreement with valence-band measurements, unbuckled (symmetrical) silicon dimers, leading to unusually narrowed Si 2p line shape as compared to those of related systems. The origin of the symmetrical dimers in the (2 X 6) structure is discussed in the context of previous results in literature.
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| 7. |
- Kuzmin, M., et al.
(författare)
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Photoemission and density functional theory study of Ge(100) : Clean surface and Yb-induced (2x4) reconstruction
- 2013
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Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 615, s. 88-96
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Tidskriftsartikel (refereegranskat)abstract
- Clean and metal-adsorbed (100) surfaces of group-IV semiconductors, such as Si and Ge, often exhibit electronically and structurally similar reconstructions. However, the fundamental bulk properties of group-IV materials can have an impact on particular features of such systems, which are related, e.g., to final-state relaxation in photoemission and thus determine their spectral line shape. Here we have studied Yb/Ge(100)(2 x 4) reconstruction as well as clean Ge(100) surface by high-resolution photoelectron spectroscopy and ab initio calculations. An atomic geometry of both surfaces is thoroughly investigated. A detailed analysis of Ge 3d core-level photoemission, atomic origins of surface-shifted components, and final-state screening effects is presented. In particular, it is demonstrated that the core-hole screening plays an essential role in Ge 3d measurements, and that its amount in the complete screening model correlates well with the core-level binding energy of respective Ge atoms in the initial state. The results are discussed in the proper context of related reconstructions on Si(100).
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| 8. |
- Kuzmin, M., et al.
(författare)
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Stability, structural, and electronic properties of atomic chains on Yb/Ge(111)3X2 studied by STM and STS
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:15, s. 155312-
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Tidskriftsartikel (refereegranskat)abstract
- By means of scanning tunneling microscopy and spectroscopy (STM/STS), we have investigated the stability and the structure of atomic chains on Yb/Ge (111) 3 X 2. STM allows the identification of different building blocks of this reconstruction, depending on the bias polarity and voltage, and validates the honeycomb chain-channel (HCC) structure with the Ge=Ge double bond and metal coverage of 1/6 ML for Yb/Ge (111)3 X 2, in agreement with the recent photoemission study [Kuzmin et al., Phys. Rev. B 75, 165305 (2007)]. The Yb atoms are found to be adsorbed on similar sites in the well-defined X 2 rows. Locally, such rows are distorted, leading to the X 4 periodicity, where the Yb atoms are adsorbed on two different sites that are well consistent with T4 and H3 sites. It is also assumed that Yb atoms can fluctuate rapidly between the neighboring T4 and H3 sites, leading to continuous rows observed together with the X 2 rows in STM images. The stability of Ge honeycomb chain is controlled by the presence of Yb atom per two (3 X 1) surface units in average, which results in the donation of one electron from Yb to the surface per (3 X 1) unit. When this density is locally changed, the Ge honeycomb chain is found to be broken. The inner structure of the Ge honeycomb chain is visualized in STM and shows dimerized features without any apparent buckling. The STM observations also account for why the double periodicity is missing in the low-energy electron diffraction pattern from Yb/Ge (111)3X2. The local electronic structure of this reconstruction, namely the Yb rows and Ge honeycomb chains, is studied by STS. The results support the HCC structure with the Ge=Ge double bond. It is believed that the present study elucidates the difference between the (3X2) reconstructions of Yb and Eu on Ge (111) and those of alkaline-earth and rare-earth metals on Si (111).
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| 9. |
- Kuzmin, M., et al.
(författare)
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Surface core-level shifts on Ge(111)c(2 x 8) : Experiment and theory
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:24, s. 245319-
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Tidskriftsartikel (refereegranskat)abstract
- Combining high-resolution photoelectron spectroscopy and density functional theory (DFT) calculations, 3d photoemission line shape and surface core-level shifts have been reinvestigated on the Ge(111)c(2 x 8) surface. It is found that 3d spectra include, in addition to the bulk and three surface-shifted components reported in literature, a component that was not identified in earlier measurements with a lower resolution. The detailed interpretation of these spectra and their line shape is made on the basis of DFT calculations. It is shown that the lowest binding energy component is due to the rest atoms. The higher binding energy emission is caused by the adatoms and the third-layer atoms that are below the adatoms. Finally, the two other surface components originate from the first- and second-layer atoms. The screening effects in the Ge(111)c(2 x 8) are discussed.
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| 10. |
- Kuzmin, M., et al.
(författare)
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Yb-induced (2x3) and (2x4) reconstructions on Si(100) studied by first-principles calculations and high-resolution core-level photoelectron spectroscopy
- 2008
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 78:4
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Tidskriftsartikel (refereegranskat)abstract
- We report here a combined theoretical and experimental study of Yb/Si(100)-(2x3) and -(2x4) reconstructions by means of first-principles calculations and high-resolution core-level photoelectron spectroscopy. Energetically stable atomic structures are presented for these reconstructions. Yb atoms are found to occupy the cave sites in the structures, and the Si substrate is strongly rearranged due to Yb adsorption. It is shown that scanning tunneling microscopy images and surface core-level shifts (SCLSs) calculated for these atomic configurations agree with experimental data, giving further support to the models. In addition, by comparing our theoretical and experimental Si 2p results, we discuss and interpret the atomic origins of SCLSs measured for the YbSi(100)-(2x3)/(2x4). Finally, the results presented are helpful in the analysis of (2x3) and (2x4) structures induced by other rare earth metals on Si(100).
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