Electronic and surface properties of Ga-doped In                         2                         O                         3                          ceramics

A. Regoutz; R. G. Egdell; D. J. Morgan; R. G. Palgrave; H. Téllez; S. J. Skinner; D. J. Payne; G. W. Watson; D. O. Scanlon

doi:10.1016/j.apsusc.2015.04.106

Electronic and surface properties of Ga-doped In ₂ O ₃ ceramics

A. Regoutz^*, R. G. Egdell, D. J. Morgan, R. G. Palgrave, H. Téllez, S. J. Skinner, D. J. Payne, G. W. Watson, D. O. Scanlon

^*Corresponding author for this work

Chemistry

Research output: Contribution to journal › Article › peer-review

Abstract

The limit of solubility of Ga ₂ O ₃ in the cubic bixbyite In ₂ O ₃ phase was established by X-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepared at 1250 °C. Density functional theory calculations suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga ₂ O ₃ as compared to In ₂ O ₃ . However both diffuse reflectance spectroscopy and valence band X-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga ⁺ surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addition photoemission reveals a broadening of the valence band edge. Core X-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chemically shifted structure in Ga 2p core XPS associated with Ga ⁺ . However experiments on ion bombarded Ga ₂ O ₃ , where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga ⁺ at the surface, show that the chemical shift between Ga ⁺ and Ga ³⁺ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chemically shifted structure associated with Ga ⁺ is not inconsistent with the proposal that band gap narrowing is associated with lone pair states at surfaces and interfaces.

Original language	English
Pages (from-to)	970-982
Number of pages	13
Journal	Applied Surface Science
Volume	349
DOIs	https://doi.org/10.1016/j.apsusc.2015.04.106
Publication status	Published - 15 Sept 2015

Bibliographical note

Funding Information:
The work at TCD was supported by SFI through the PI program (06/IN.I/I92). Calculations at TCD were performed on the Lonsdale and Kelvin clusters as maintained by TCHPC, and the Stokes and Fionn clusters as maintained by ICHEC. The UCL/Diamond work presented here made use of the UCL Legion HPC Facility, the IRIDIS cluster provided by the EPSRC funded Centre for Innovation (Grants No. EP/K000144/1 and No. EP/K000136/1), and the ARCHER supercomputer through support by the UK's HPC Materials Chemistry Consortium, which is funded by EPSRC Grant No. EP/L000202. RGP acknowledges EPSRC Grant EP/K014099/1. DJP, DOS and RGP acknowledge the Materials Design Network. D.J.P. acknowledges support from the Royal Society (UF100105). HT acknowledges financial support from the Marie Curie Intra European Fellowship within the 7 th European Community Framework Programme the European Union (PIEF-GA-2010-274999). X-ray photoelectron spectroscopy was provided through the EPSRC “Access to Research Equipment Initiative: Cardiff XPS” (Grant No. EP/F019823/1). AR thanks Trinity College Oxford for financial support.

Publisher Copyright:
© 2015 Elsevier B.V. All rights reserved.

Keywords

Band bending
Ceramic
Dopant solubility
Transparent conducting oxide

ASJC Scopus subject areas

General Chemistry
Condensed Matter Physics
General Physics and Astronomy
Surfaces and Interfaces
Surfaces, Coatings and Films

Access to Document

10.1016/j.apsusc.2015.04.106

Cite this

Regoutz, A., Egdell, R. G., Morgan, D. J., Palgrave, R. G., Téllez, H., Skinner, S. J., Payne, D. J., Watson, G. W., & Scanlon, D. O. (2015). Electronic and surface properties of Ga-doped In ₂ O ₃ ceramics. Applied Surface Science, 349, 970-982. https://doi.org/10.1016/j.apsusc.2015.04.106

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title = "Electronic and surface properties of Ga-doped In 2 O 3 ceramics",

abstract = " The limit of solubility of Ga 2 O 3 in the cubic bixbyite In 2 O 3 phase was established by X-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepared at 1250 °C. Density functional theory calculations suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga 2 O 3 as compared to In 2 O 3 . However both diffuse reflectance spectroscopy and valence band X-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga + surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addition photoemission reveals a broadening of the valence band edge. Core X-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chemically shifted structure in Ga 2p core XPS associated with Ga + . However experiments on ion bombarded Ga 2 O 3 , where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga + at the surface, show that the chemical shift between Ga + and Ga 3+ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chemically shifted structure associated with Ga + is not inconsistent with the proposal that band gap narrowing is associated with lone pair states at surfaces and interfaces.",

keywords = "Band bending, Ceramic, Dopant solubility, Transparent conducting oxide",

author = "A. Regoutz and Egdell, {R. G.} and Morgan, {D. J.} and Palgrave, {R. G.} and H. T{\'e}llez and Skinner, {S. J.} and Payne, {D. J.} and Watson, {G. W.} and Scanlon, {D. O.}",

note = "Funding Information: The work at TCD was supported by SFI through the PI program (06/IN.I/I92). Calculations at TCD were performed on the Lonsdale and Kelvin clusters as maintained by TCHPC, and the Stokes and Fionn clusters as maintained by ICHEC. The UCL/Diamond work presented here made use of the UCL Legion HPC Facility, the IRIDIS cluster provided by the EPSRC funded Centre for Innovation (Grants No. EP/K000144/1 and No. EP/K000136/1), and the ARCHER supercomputer through support by the UK's HPC Materials Chemistry Consortium, which is funded by EPSRC Grant No. EP/L000202. RGP acknowledges EPSRC Grant EP/K014099/1. DJP, DOS and RGP acknowledge the Materials Design Network. D.J.P. acknowledges support from the Royal Society (UF100105). HT acknowledges financial support from the Marie Curie Intra European Fellowship within the 7 th European Community Framework Programme the European Union (PIEF-GA-2010-274999). X-ray photoelectron spectroscopy was provided through the EPSRC “Access to Research Equipment Initiative: Cardiff XPS” (Grant No. EP/F019823/1). AR thanks Trinity College Oxford for financial support. Publisher Copyright: {\textcopyright} 2015 Elsevier B.V. All rights reserved.",

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doi = "10.1016/j.apsusc.2015.04.106",

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Regoutz, A, Egdell, RG, Morgan, DJ, Palgrave, RG, Téllez, H, Skinner, SJ, Payne, DJ, Watson, GW & Scanlon, DO 2015, 'Electronic and surface properties of Ga-doped In ₂ O ₃ ceramics', Applied Surface Science, vol. 349, pp. 970-982. https://doi.org/10.1016/j.apsusc.2015.04.106

TY - JOUR

T1 - Electronic and surface properties of Ga-doped In 2 O 3 ceramics

AU - Regoutz, A.

AU - Egdell, R. G.

AU - Morgan, D. J.

AU - Palgrave, R. G.

AU - Téllez, H.

AU - Skinner, S. J.

AU - Payne, D. J.

AU - Watson, G. W.

AU - Scanlon, D. O.

N1 - Funding Information: The work at TCD was supported by SFI through the PI program (06/IN.I/I92). Calculations at TCD were performed on the Lonsdale and Kelvin clusters as maintained by TCHPC, and the Stokes and Fionn clusters as maintained by ICHEC. The UCL/Diamond work presented here made use of the UCL Legion HPC Facility, the IRIDIS cluster provided by the EPSRC funded Centre for Innovation (Grants No. EP/K000144/1 and No. EP/K000136/1), and the ARCHER supercomputer through support by the UK's HPC Materials Chemistry Consortium, which is funded by EPSRC Grant No. EP/L000202. RGP acknowledges EPSRC Grant EP/K014099/1. DJP, DOS and RGP acknowledge the Materials Design Network. D.J.P. acknowledges support from the Royal Society (UF100105). HT acknowledges financial support from the Marie Curie Intra European Fellowship within the 7 th European Community Framework Programme the European Union (PIEF-GA-2010-274999). X-ray photoelectron spectroscopy was provided through the EPSRC “Access to Research Equipment Initiative: Cardiff XPS” (Grant No. EP/F019823/1). AR thanks Trinity College Oxford for financial support. Publisher Copyright: © 2015 Elsevier B.V. All rights reserved.

PY - 2015/9/15

Y1 - 2015/9/15

N2 - The limit of solubility of Ga 2 O 3 in the cubic bixbyite In 2 O 3 phase was established by X-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepared at 1250 °C. Density functional theory calculations suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga 2 O 3 as compared to In 2 O 3 . However both diffuse reflectance spectroscopy and valence band X-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga + surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addition photoemission reveals a broadening of the valence band edge. Core X-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chemically shifted structure in Ga 2p core XPS associated with Ga + . However experiments on ion bombarded Ga 2 O 3 , where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga + at the surface, show that the chemical shift between Ga + and Ga 3+ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chemically shifted structure associated with Ga + is not inconsistent with the proposal that band gap narrowing is associated with lone pair states at surfaces and interfaces.

AB - The limit of solubility of Ga 2 O 3 in the cubic bixbyite In 2 O 3 phase was established by X-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepared at 1250 °C. Density functional theory calculations suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga 2 O 3 as compared to In 2 O 3 . However both diffuse reflectance spectroscopy and valence band X-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga + surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addition photoemission reveals a broadening of the valence band edge. Core X-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chemically shifted structure in Ga 2p core XPS associated with Ga + . However experiments on ion bombarded Ga 2 O 3 , where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga + at the surface, show that the chemical shift between Ga + and Ga 3+ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chemically shifted structure associated with Ga + is not inconsistent with the proposal that band gap narrowing is associated with lone pair states at surfaces and interfaces.

KW - Band bending

KW - Ceramic

KW - Dopant solubility

KW - Transparent conducting oxide

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Electronic and surface properties of Ga-doped In 2 O 3 ceramics