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ABSTRACT: Angle-resolved photoemission spectroscopy reveals the presence of a two-dimensional electron gas at the surface of In_{2}O_{3}(111). Quantized subband states arise within a confining potential well associated with surface electron accumulation. Coupled Poisson-Schrödinger calculations suggest that downward band bending for the conduction band must be much bigger than band bending in the valence band. Surface oxygen vacancies acting as doubly ionized shallow donors are shown to provide the free electrons within this accumulation layer. Identification of the origin of electron accumulation in transparent conducting oxides has significant implications in the realization of devices based on these compounds.
Physical Review Letters 02/2013; 110(5):056803. · 7.37 Impact Factor
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ABSTRACT: The growth of Bi on Ag(111) induces different surface structures, including [Formula: see text] surface alloy, Bi-[Formula: see text] overlayer and Bi(110) thin film, as a function of increasing Bi coverage. Here we report the study of electronic states of these structures using core level and valence band photoemission spectroscopy at room temperature. The sp-derived Shockley surface state on Ag(111) is rapidly quenched upon deposition of Bi, due to the strong variation of the in-plane surface potential in the Ag(2)Bi surface alloy. The Bi 4f core levels of the [Formula: see text] alloy and Bi(110) thin film are shifted to lower binding energy by ∼0.6 eV and ∼0.3 eV compared with the Bi bulk value, respectively. Mechanisms inducing the core level shifts are discussed as due to a complex superposition of several factors. As Bi coverage increases and a Bi(110) overlayer forms on Ag(111), a new state is observed at ∼0.9 ML arising from electronic states localized at the Ag-Bi interface. Finally the change of work function as a function of coverage is discussed on the basis of a charge transfer model.
Journal of Physics Condensed Matter 10/2012; 24(43):435502. · 2.55 Impact Factor
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ABSTRACT: Oxygen plasma assisted molecular beam epitaxy was used to grow thin films of In2O3 on alpha-Al2O3(0001) over a range of substrate temperatures between 300 and 750 degrees C. The crystal structures and morphologies were examined by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. In all cases, the thermodynamically stable body-centered cubic phase bcc-In2O3 predominates in the films, with an epitaxial relationship In2O3(111)parallel to Al2O3(0001) and In2O3 [1 (1) over bar 10]parallel to Al2O3[10 (1) over bar0] determined by matching between the sublattice oxygen atoms in Al2O3(0001) and the In atoms in In2O3(111): this involves a 300 rotation of the epilayer unit cell relative to that of the substrate and a 3:2 coincidence structure. A minority fraction of metastable rhombohedral rh-In2O3(0001) can be stabilized for substrate temperatures below 550 degrees C due to the similarity in the bonding symmetries between rh-In2O3 and alpha-Al2O3. Despite the large mismatches between In2O3 and Al2O3 for the two epitaxial systems discussed above (-13.2% for bcc-In2O3 and +15.1% for rh-In2O3), we show that the epitaxy can be maintained in both cases by matching small but different integral multiples of lattice planes of the In2O3 and the substrate at the interface between the two. Thus, the strain is effectively released by dislocations localized at the interface. This so-called domain matching epitaxial growth mode may open up a new route to fabrication of high-quality crystalline thin films of oxides on highly mismatched substrates.
Crystal Growth & Design 02/2012; 12(2):1000-1007. · 4.72 Impact Factor
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ABSTRACT: Epitaxial films of In(2)O(3) have been grown on Y-stabilised ZrO(2)(111) substrates by molecular beam epitaxy over a range of thicknesses between 35 and 420 nm. The thinnest films are strained, but display a 'cross-hatch' morphology associated with a network of misfit dislocations which allow partial accommodation of the lattice mismatch. With increasing thickness a 'dewetting' process occurs and the films break up into micron sized mesas, which coalesce into continuous films at the highest coverages. The changes in morphology are accompanied by a progressive release of strain and an increase in carrier mobility to a maximum value of 73 cm(2) V(-1) s(-1). The optical band gap in strained ultrathin films is found to be smaller than for thicker films. Modelling of the system, using a combination of classical pair-wise potentials and ab initio density functional theory, provides a microscopic description of the elastic contributions to the strained epitaxial growth, as well as the electronic effects that give rise to the observed band gap changes. The band gap increase induced by the uniaxial compression is offset by the band gap reduction associated with the epitaxial tensile strain.
Journal of Physics Condensed Matter 08/2011; 23(33):334211. · 2.55 Impact Factor
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ABSTRACT: The deposition of 1/3 of a monolayer of Pb on Ag(111) leads to the formation of PbAg(2) surface alloy with a long range ordered (√3 × √3)R30° superstructure. A detailed analysis of this structure using low-energy electron diffraction (LEED) I-V measurements together with density functional theory (DFT) calculations is presented. We find strong correlation between experimental and calculated LEED I-V data, with the fit between the two data sets having a Pendry's reliability factor of 0.21. The Pb atom is found to replace one top layer Ag atom in each unit cell, forming a substitutional PbAg(2) surface alloy, as expected, with the Pb atoms residing approximately 0.4 Å above the Ag atoms due to their size difference. DFT calculations are in good agreement with the LEED results.
Journal of Physics Condensed Matter 07/2011; 23(26):265006. · 2.55 Impact Factor
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K. H. L. Zhang,
D.J. Payne,
R. G. Palgrave,
V. K. Lazarov,
W. Chen,
A. T. S. Wee,
C. F. McConville,
P. D. C. King,
T. D. Veal,
G. Panaccione,
P. Lacovig,
R. G. Egdell
10/2009;
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ABSTRACT: The deposition of 1/3 of a monolayer of Bi on Ag(111) leads to the formation of BiAg2 surface alloy with a long range ordered superstructure. A detailed analysis of this structure using LEED I–V measurements together with DFT calculations is presented. We find strong correlation between experimental and calculated LEED I–V data, with the fit between the two data sets having a Pendry's reliability factor of 0.16. The Bi atom is found to replace one top layer Ag atom in each unit cell, forming a substitutional BiAg2 surface alloy, with the Bi atoms residing approximately 0.6 Å above the Ag atoms due to their size difference. DFT calculations are in good agreement with the LEED results.
Surface Science. 604:1395-1399.
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K. H. L. Zhang,
I. M. McLeod,
Y. H. Lu,
V. R. Dhanak,
A. Matilainen,
M. Lahti,
K. Pussi,
R. G. Egdell,
X.-S. Wang,
A. T. S. Wee,
W. Chen
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ABSTRACT: The atomic structures that develop as a function of coverage during deposition of Bi on Ag(111) have been studied using low-temperature scanning tunneling microscopy, low-energy electron diffraction, and ab initio calculations. The growth process involves two sequential stages. At low coverage, Bi atoms are incorporated into the topmost layer of Ag(111), resulting in the formation of an Ag2Bi alloy confined to the surface and ordered (√3×√3)R30° Ag2Bi islands supported on Ag(111). This mode of accommodation of Bi was found to be energetically favorable based on ab initio total-energy calculations. At coverage above a critical value of 0.55 monolayers, the Ag2Bi alloy phase gradually converts into an ordered Bi (p×√3) overlayer structure supported on Ag(111). We postulate that the dealloying transition is likely driven by compressive strain induced by incorporation of large-size Bi atoms into Ag at a high coverage and the subsequent lack of miscibility of Ag and Bi bulk phases. After completion of the dealloying process, Bi(110) thin films can be grown epitaxially on top of Ag(111) with a chemically abrupt interface.
Phys. Rev. B. 83(23).
