| 1. |
- Laukkanen, P., et al.
(författare)
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Anomalous bismuth-stabilized (2x1) reconstructions on GaAs(100) and InP(100) surfaces
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 100, s. 086101-
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Tidskriftsartikel (refereegranskat)abstract
- First-principles phase diagrams of bismuth-stabilized GaAs- and InP(100) surfaces demonstrate for the first time the presence of anomalous (2 x 1) reconstructions, which disobey the common electron counting principle. Combining these theoretical results with our scanning-tunneling-microscopy and photoemission measurements, we identify novel (2 x 1) surface structures, which are composed of symmetric Bi-Bi and asymmetric mixed Bi-As and Bi-P dimers, and find that they are stabilized by stress relief and pseudogap formation.
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| 2. |
- Laukkanen, P., et al.
(författare)
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Bismuth-stabilized c(2X6) reconstruction on a InSb(100) substrate : Violation of the electron counting model
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 81:3
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Tidskriftsartikel (refereegranskat)abstract
- By means of scanning tunneling microscopy/spectroscopy (STM/STS), photoelectron spectroscopy, and first-principles calculations, we have studied the bismuth (Bi) adsorbate-stabilized InSb(100) substrate surface which shows a c(2X6) low-energy electron diffraction pattern [thus labeled Bi/InSb(100)c(2X6) surface] and which includes areas with metallic STS curves as well as areas with semiconducting STS curves. The first-principles phase diagram of the Bi/InSb(100) surface demonstrates the presence of the Bi-stabilized metallic c(2X6) reconstruction and semiconducting (4X3) reconstruction depending on the chemical potentials, in good agreement with STS results. The existence of the metallic c(2X6) phase, which does not obey the electron counting model, is attributed to the partial prohibition of the relaxation in the direction perpendicular to dimer rows in the competing reconstructions and the peculiar stability of the Bi-stabilized dimer rows. Based on (i) first-principles phase diagram, (ii) STS results, and (iii) comparison of the measured and calculated STM and photoemission data, we show that the measured Bi/InSb(100)c(2X6) surface includes metallic areas with the stable c(2X6) atomic structure and semiconducting areas with the stable (4X3) atomic structure.
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| 3. |
- Laukkanen, P., et al.
(författare)
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Core-level shifts of the c(8 x 2)-reconstructed InAs(100) and InSb(100) surfaces
- 2010
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Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 177:1, s. 52-57
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Tidskriftsartikel (refereegranskat)abstract
- We have studied In-stabilized c(8 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) semiconductor surfaces, which play a key role in growing improved III-V interfaces for electronics devices, by core-level photoelectron spectroscopy and first-principles calculations. The calculated surface core-level shifts (SCLSs) for the zeta and zeta a models, which have been previously established to describe the atomic structures of the III-V(1 00)c(8 x 2) surfaces, yield hitherto not reported interpretation for the As 3d, In 4d, and Sb 4d core-level spectra of the III-V(1 00)c(8 x 2) surfaces, concerning the number and origins of SCLSs. The fitting analysis of the measured spectra with the calculated zeta and zeta a SCLS values shows that the InSb spectra are reproduced by the zeta SCLSs better than by the zeta a SCLSs. Interestingly, the zeta a fits agree better with the InAs spectra than the zeta fits do, indicating that the zeta a model describes the InAs surface better than the InSb surface. These results are in agreement with previous X-ray diffraction data. Furthermore, an introduction of the complete-screening model, which includes both the initial and final state effects, does not improve the fitting of the InSb spectra, proposing the suitability of the initial-state model for the SCLSs of the III-V(1 0 0)c(8 x 2) surfaces. The found SCLSs are discussed with the ab initio on-site charges.
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| 4. |
- Punkkinen, M. P. J., et al.
(författare)
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Bismuth-stabilized (2x1) and (2x4) reconstructions on GaAs(100) surfaces : Combined first-principles, photoemission, and scanning tunneling microscopy study
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78, s. 195304-
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Tidskriftsartikel (refereegranskat)abstract
- Bismuth adsorbate-stabilized (2x1) and (2x4) reconstructions of the GaAs(100) surfaces have been studied by first-principles calculations, valence-band and core-level photoelectron spectroscopies, and scanning tunneling microscopy. It is demonstrated that large Bi atom size leads to the formation of the pseudogap at the Fermi energy and to the lower energy of an adsorbate-derived surface band, which contributes to the stabilization of the exceptional Bi/GaAs(100)(2x1) reconstruction. It is proposed that the Bi/GaAs(100)(2x4) reconstructions include asymmetric mixed Bi-As dimers, in addition to the Bi-Bi dimers. Based on the calculations, we solve the atomic origins of the surface core-level shifts (SCLSs) of the Bi 5d photoemission spectra from the Bi/GaAs(100)(2x4) surfaces. This allows for resolving the puzzle related to the identification of two SCLS components often found in the measurements of the Bi 5d and Sb 4d core-level emissions of the Bi/III-V and Sb/III-V(100)(2x4) surfaces. Finally, the reason for the absence of the common (2x4)-beta 2 structure and additional support for the stability of the (2x1) structure on the Bi/III-V(100) surfaces are discussed in terms of Bi atom size and subsurface stress.
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| 5. |
- Lang, J. J. K., et al.
(författare)
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Tin-stabilized (1 x 2) and (1 x 4) reconstructions on GaAs(100) and InAs(100) studied by scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations
- 2011
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 605:9-10, s. 883-888
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Tidskriftsartikel (refereegranskat)abstract
- Tin (Sn) induced (1 x 2) reconstructions on GaAs(100) and InAs(100) substrates have been studied by low energy electron diffraction (LEED), photoelectron spectroscopy, scanning tunneling microscopy/spectroscopy (STM/STS) and ab initio calculations. The comparison of measured and calculated STM images and surface core-level shifts shows that these surfaces can be well described with the energetically stable building blocks that consist of Sn-III dimers. Furthermore, a new Sn-induced (1 x 4) reconstruction was found. In this reconstruction the occupied dangling bonds are closer to each other than in the more symmetric (1 x 2) reconstruction, and it is shown that the (1 x 4) reconstruction is stabilized as the adatom size increases.
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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. |
- Punkkinen, M. P. J., et al.
(författare)
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Calculation of surface core-level shifts within complete screening : Problems with pseudohydrogenated slabs
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:24
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Tidskriftsartikel (refereegranskat)abstract
- By considering GaAs(110) and Si(100)(2x1) surfaces, it is shown that the use of the pseudohydrogen atoms to saturate the dangling bonds at one side of the slab modeling a free surface can lead to distorted surface core-level shifts within the complete screening picture. The effect is linked to the polarization of the slab and to the change in the bulklike electronic structure close to pseudohydrogenated part of the slab. It is demonstrated that these problems can be avoided if the pseudohydrogenated slab is large enough and the bulk reference layer is properly chosen. One easy way to control these errors is to monitor the planar-averaged potential energy curve of the slab.
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| 8. |
- Punkkinen, Marko P. J., et al.
(författare)
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Core-level shifts of InP(100)(2 x 4) surface : Theory and experiment
- 2009
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 603:16, s. 2664-2668
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Tidskriftsartikel (refereegranskat)abstract
- Surface core-level shifts (SCLSs) of the (2 x 4)-reconstructed InP(100) surface with the established mixed In-P dimer structure have been investigated by first-principles calculations and photoelectron spectroscopy. Theoretical values were calculated using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy functional. The obtained theoretical values are quite similar within both approximations. The found differences originate in the tiny structural differences. It is concluded that the expansion or contraction of the crystal lattice has smaller effect on the SCLSs than the geometrical details of the reconstruction, which suggests that the Madelung potential has the dominant effect on the SCLSs. The results support the presence of a P 2p peak at higher binding energy (BE) compared to bulk peak, as proposed with recent measurements [P. Laukkamen, J. Pakarinen, M. Ahola-Tuomi, M. Kuzmin, R. E. Perala, I. J. Vayrynen, A. Tukiainen, V. Rimpilainen, M. Pessa, M. Adell, J. Sadowski, Surf. Sci. 600 (2006) 3022], and reveal several hitherto not reported SCLSs. The calculated SCLSs reproduce the measured spectra within reasonable accuracy. Furthermore, the atomic origins of the InP(100)(2 x 4) SCLSs are solved. In particular, it is shown that the lowest SCLS of P 2p, of the InP(100)(2 x 4) arises from the topmost In-P dimers.
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| 9. |
- Punkkinen, Marko P. J., et al.
(författare)
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Stability and structure of rare-earth metal and Ba-induced reconstructions on a Si(100) surface
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 80:23
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Tidskriftsartikel (refereegranskat)abstract
- We have studied, by means of ab initio calculations, the energetics and the atomic and electronic structures of various reconstructions induced by rare-earth metals (RE=Eu, Nd, Sm, and Yb) and Ba on Si(100) in the coverage range up to 0.5 monolayer. It is shown that Si dimer buckling is an important structural element for such systems, leading frequently to oblique surface lattice symmetries. The strong metal atom-silicon binding favors the increased amount of metal atoms per unit surface area, i.e., the (2x3) reconstruction with two metal atoms per unit cell is found to be energetically unstable with respect to the (2x1) reconstruction with three metal atoms per the same surface area [Eu/Si(100) and Yb/Si(100)]. The influence of the atomic size and the valence of the adsorbates is also investigated. In particular, it is found that an increase in atomic size stimulates the metal-metal repulsion, stabilizing the (2x3) configuration [Ba/Si(100)]. In the case of trivalent metals, the stabilization of the (2x3) is mediated by the loss of semiconducting state in the competing phases [Sm/Si(100) and Nd/Si(100)]. Our results demonstrate the importance of many factors, which account for the abundance of RE/Si(100) reconstructions. Finally, prominent atomic models are proposed for (2x3) and (2x6) reconstructions, and the character of the wavy "(1x2)" reconstruction is discussed. The simulated scanning tunneling microscopy images for the proposed (2x6) reconstruction are in a particularly good agreement with the complex experimental images.
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| 10. |
- Punkkinen, M. P. J., et al.
(författare)
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Surface core-level shifts of GaAs(100)(2x4) from first principles
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:11
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations show that measured surface core-level shifts (SCLSs) of the GaAs(100)(2x4) surfaces can be described within the initial state effects. The calculated As 3d and Ga 3d SCLSs for the beta 2 and alpha 2 reconstructions of the GaAs(100)(2x4) surfaces are in reasonable agreement with recent measurements. In particular, the results confirm that both the lower and the higher binding energy SCLSs, relative to the bulk emission in the As 3d photoelectron spectra, are intrinsic properties of the GaAs(100)(2x4) surfaces. The most positive and most negative As shifts are attributed to the third layer As atoms, which differs from the previous intuitive suggestions. In general, calculations show that significant SCLSs arise from deep layers, and that there are more than two SCLSs. Our previously measured As 3d spectra are fitted afresh using the calculated SCLSs. The intensity ratios of the SCLSs, obtained from the fits, show that as the heating temperature of the GaAs(100)(2x4) surface is increased gradually, the area of the alpha 2 reconstruction increases on the surface, but the beta 2 phase remains within the whole temperature range, in agreement with previous experimental findings. Our results show that the combination of the experimental and theoretical results is a prerequisite for the accurate analysis of the SCLSs of the complex reconstructed surfaces.
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