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ABSTRACT: Extreme electron accumulation with sheet density greater than 10^{13} cm^{-2} is almost universally present at the surface of indium nitride (InN). Here, x-ray photoemission spectroscopy and secondary ion mass spectrometry are used to show that the surface Fermi level decreases as the Mg concentration increases, with the sheet electron density falling to below 10^{8} cm^{-2}. Surface space-charge calculations indicate that the lowering of the surface Fermi level increases the density of unoccupied donor-type surface states and that these are largely compensated by Mg acceptors in the near-surface hole depletion region rather than by accumulated electrons. This is a significant step towards the realization of InN-based optoelectronic devices.
Physical Review Letters 12/2012; 109(24):247605. · 7.37 Impact Factor
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ABSTRACT: The majority of InN doping studies have primarily focused on Mg, as it has previously been used to successfully realize p-type GaN. Here, we consider an alternative dopant-Mn-as a possible acceptor candidate in InN. Magnetotransport, x ray photoelectron spectroscopy, and photoluminescence were used to investigate electrical and optical properties of a series of Mn-doped InN thin films grown using molecular beam epitaxy. Evidence of acceptor behavior was observed only for moderate (10(17) cm(-3)) doping levels. At a doping level around 10(17) cm(-3), light hole features appear in the quantitative mobility spectrum analysis, the surface Fermi level shifts downwards towards the valence band, and low energy features appear in the low temperature photoluminescence spectra. (C) 2012 American Vacuum Society. [DOI:10.1116/1.3687903]
Journal of Vacuum Science & Technology B. 01/2012; 30(2).
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ABSTRACT: Metal oxides such as ZnO, Ga2O3, CdO, In2O3, and SnO2 exhibit high degree of transparency to visible light while supporting high levels electrical conductivity. The causes of
the conductivity and the role played by the surface are current topics of research. This chapter presents a systematic study
of the electronic structure and electrical properties of these post-transition metal oxides (PTMO) using a combination of
X-ray photoelectron spectroscopy, angle-resolved photoelectron spectroscopy, Hall effect, infrared reflectivity, and optical
absorption spectroscopy measurements. Evidence of surface electron accumulation in these PTMO is presented. It is found that
for CdO and In2O3, electron accumulation is observed even in the absence of extremely high doping levels. The results also indicate that despite
the strong tendency to exhibit surface electron accumulation, these materials can also exhibit an electron depletion layer
under the appropriate surface stoichiometry conditions or when certain anions are adsorbed. The proclivity towards surface
electron accumulation shown by the PTMOs is discussed in terms of bulk band structure, surface states, and the position of
their band edges in an absolute energy scale. The electronic properties of thin films and bulk crystals of the PTMO surfaces
also provide information vital for the interpretation of conductivity measurements of PTMO nanostructures, which are often
dominated by surface effects.
12/2011: pages 127-145;
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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: Electron mobility in degenerate CdO thin films has been studied as a function of carrier concentration. The “optical” mobility has been determined from infrared reflectance measurements of the conduction band plasmon lifetime. The acquired values vary from ∼ 209 to ∼ 1116 cm <sup>2</sup> V <sup>-1</sup> s <sup>-1</sup> for carrier concentrations between 2.5× 10<sup>20</sup> and 2.6× 10<sup>19</sup> cm <sup>-3</sup> . Ionized impurity scattering is shown to be the dominant effect reducing the intra-grain mobility of the electrons at room temperature. The transport mobilities from Hall effect measurements range between ~ 20 and ~ 124 cm <sup>2</sup> V <sup>-1</sup> s <sup>-1</sup> which are much lower than the optical mobilities. Simulation of grain boundary scattering-limited mobility is commonly based on models that assume a depletion layer at the boundaries which causes an inter-grain potential barrier. These models are found not to be applicable to CdO as it has been previously shown to have surface electron accumulation. Therefore, simulation of the transport mobility has been performed using the Fuchs-Sondheimer and Mayadas-Shatzkes models to take into account the grain boundary and surface scattering mechanisms, in addition to intra-grain scattering. The results indicate that electron scattering at grain boundaries with ~95 % reflection is the dominant mechanism in reducing the mobility across the layer. The effect of surface scattering plays only a minor role in electron transport.
Journal of Applied Physics 05/2011; · 2.17 Impact Factor
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ABSTRACT: Significant polarity-related effects were observed in the near-surface atomic composition and valence band electronic structure of ZnO single crystals, investigated by x-ray photoemission spectroscopy using both Al Kα (1486.6 eV) and synchrotron radiation (150 to 1486 eV). In particular, photoemission from the lowest binding energy valence band states was found to be significantly more intense on the Zn-polar face compared to the O-polar face. This is a consistent effect that can be used as a simple, nondestructive indicator of crystallographic polarity in ZnO and other wurtzite semiconductors.
Applied Physics Letters 03/2011; 98(10):101906-101906-3. · 3.84 Impact Factor
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ABSTRACT: Ultra-low-energy secondary ion mass spectrometry has been used to undertake a structural analysis of GaN-In(x)Ga(1-x)N (x approximately 0.25) quantum wells used in optoelectronic devices. The high resistivity of intrinsic GaN-In(x)Ga(1-x)N restricts the necessary electrical path between the analyzed area and the instrument ground potential resulting in surface charge accumulation. Consequently, unstable and unrepresentative depth profiles tend to be produced. A technique known as optical conductivity enhancement (OCE) has been used during depth profiling to reduce the material resistivity. This creates an electrical path between the sample and holder, eliminating charge build up and resulting in accurate depth profiles.
Rapid Communications in Mass Spectrometry 07/2010; 24(14):2122-6. · 2.79 Impact Factor
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ABSTRACT: An energy gap between the valence and the conduction band is the defining property of a semiconductor, and the gap size plays a crucial role in the design of semiconductor devices. We show that the presence of a two-dimensional electron gas near to the surface of a semiconductor can significantly alter the size of its band gap through many-body effects caused by its high electron density, resulting in a surface band gap that is much smaller than that in the bulk. Apart from reconciling a number of disparate previous experimental findings, the results suggest an entirely new route to spatially inhomogeneous band-gap engineering.
Physical Review Letters 06/2010; 104(25):256803. · 7.37 Impact Factor
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ABSTRACT: Magnetotransport measurements and x-ray photoemission spectroscopy were used to investigate the surface conductivity of ZnO. Near-surface downward band bending, consistent with electron accumulation, was found on the polar and nonpolar faces of bulk ZnO single crystals. A significant polarity effect was observed in that the downward band bending was consistently stronger on the Zn-polar face and weaker on the O-polar face. The surface electron accumulation layer was found to significantly influence the electrical properties of high resistivity, hydrothermally grown bulk ZnO crystals at temperatures below 200 K, and is largely responsible for the anomalously low electron mobility reported for this material.
Physical review. B, Condensed matter 01/2010; 81:075211.
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ABSTRACT: The effects of treatment with ammonium sulfide ((NH4)2Sx) solution on the electronic properties of InN surfaces have been investigated with high resolution x-ray photoemission spectroscopy. The valence band, In 3d, and N 1s x-ray photoemission spectra show that the surface Fermi level decreases by approximately 0.15 eV with (NH4)2Sx-treatment. This corresponds to a reduction of the downward band bending with the surface sheet charge density decreasing by 30%.
Applied Physics Letters 11/2009; 95(19):192111-192111-3. · 3.84 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 influence of Sn doping on the growth of In <sub>2</sub> O <sub>3</sub> on Y-stabilized ZrO <sub>2</sub>(100) by oxygen plasma assisted molecular beam epitaxy has been investigated over a range of substrate temperatures between 650 and 900 ° C . The extent of dopant incorporation under a constant Sn flux decreases monotonically with increasing substrate temperature, although the n -type carrier concentration in “overdoped” films grown at 650 ° C is lower than in films with a lower Sn concentration grown at 750 ° C . The small increase in lattice parameter associated with Sn doping leads to improved matching with the substrate and suppresses breakup of the films into square islands observed in high temperature growth of undoped In <sub>2</sub> O <sub>3</sub> on Y-stabilized ZrO <sub>2</sub>(100) . Plasmon energies derived from infrared reflection spectra of Sn-doped films are found to be close to satellite energies in core level photoemission spectroscopy, but for a nominally undoped reference sample there is evidence for carrier accumulation at the surface. This influences both the In 3d core line shape and the intensity of a peak close to the Fermi energy associated with photoemission from the conduction band.
Journal of Applied Physics 08/2009; · 2.17 Impact Factor
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ABSTRACT: The valence-band density of states of single-crystalline rock-salt CdO(001), wurtzite c-plane ZnO, and rock- salt MgO(001) are investigated by high-resolution x-ray photoemission spectroscopy. A classic two-peak structure is observed in the VB-DOS due to the anion 2p-dominated valence bands. Good agreement is found between the experimental results and quasi-particle-corrected density-functional theory calculations. Occupied shallow semicore d levels are observed in CdO and ZnO. While these exhibit similar spectral features to the calculations, they occur at slightly higher binding energies, determined as 8.8 eV and 7.3 eV below the valence band maximum in CdO and ZnO, respectively. The implications of these on the electronic structure are discussed.
Physical Review B 05/2009; 79:205205. · 3.69 Impact Factor
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ABSTRACT: A three-region model for the high n-type conductivity in InN, including contributions from the bulk, surface and buffer layer interface of the sample, is considered. In particular, a parallel conduction analysis is used to show that this model can account for the carrier concentration and mobility variation with film thickness that has previously been determined from single-field Hall effect measurements. Microscopic origins for the donors in each region are considered, and the overriding tendency towards n-type conductivity is discussed in terms of the bulk band structure of InN.
Journal of Physics Condensed Matter 04/2009; 21(17):174201. · 2.55 Impact Factor
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ABSTRACT: The charge state of muonium has been investigated in p-type doped, nominally undoped (low n-type) and heavily n-type doped InAs. The donor Mu(+) state is shown to be the dominant defect in all cases. Consequently, muonium does not simply counteract the prevailing conductivity in this material. This is consistent with the charge neutrality level lying above the conduction band minimum in InAs.
Journal of Physics Condensed Matter 02/2009; 21(7):075803. · 2.55 Impact Factor
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ABSTRACT: High resolution X-ray photoemission spectroscopy is utilised to determine the variation in surface Fermi level pinning position for Mg-doped c -plane Inx Al1–xN alloys with x ≥ 0.4. For In-rich alloys, a transition from inversion to hole depletion was inferred at an alloy composition of x ∼ 0.7. For Al-rich alloys, insulating samples resulted due to the difficulty of p-type doping AlN and Al-rich alloys, resulting in approximately flat-band conditions, with little surface space-charge. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 02/2009; 246(6):1169 - 1172. · 1.32 Impact Factor
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P. D. C. King,
R. L. Lichti,
Y. G. Celebi,
J. M. Gil,
R. C. Vilão,
H. V. Alberto,
J. Piroto Duarte,
D. J. Payne,
R. G. Egdell,
I. McKenzie, C. F. McConville,
S. F. J. Cox,
T. D. Veal
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ABSTRACT: Muonium, and by analogy hydrogen, is shown to form a shallow-donor state in In2O3 and SnO2. The paramagnetic charge state is stable below ∼50 K in In2O3 and ∼30 K in SnO2 which, coupled with its extremely small effective hyperfine splitting in both cases, allows its identification as the shallow-donor state. This has important implications for the controversial issue of the origins of conductivity in transparent conducting oxides.
Phys. Rev. B. 01/2009; 80(8).
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Microelectronics Journal. 01/2009; 40:399-402.
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Phys. Rev. B. 01/2009; 79(20):205205.
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ABSTRACT: The electronic properties of clean and sulfur-terminated surfaces of InSb(001) and (111)B are investigated using x-ray photoemission spectroscopy and high-resolution electron energy loss spectroscopy. The clean surfaces exhibit upward band bending (electron depletion) consistent with the charge neutrality level in InSb lying at the valence band maximum. The surface Fermi level to valence band maximum separation is increased for the S terminated compared with the clean surface, leading to flat bands and downward band bending (electron accumulation) for the (001) and (111)B surfaces, respectively. This is discussed in terms of compensation of native acceptor surface states.
Journal of Applied Physics 11/2008; · 2.17 Impact Factor
