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Instruments / Technical Area	Publication Details	Published
I09-Surface and Interface Structural Analysis	A hard x-ray photoemission study of transparent conducting fluorine-doped tin dioxide Jack E. N. Swallow , Benjamin A. D. Williamson , Max Birkett , Alex Abbott , Mark Farnworth , Thomas J. Featherstone , Nianhua Peng , J. Kieran Cheetham , Paul Warren , Anna Regoutz , David A. Duncan , Tien-lin Lee , David O. Scanlon , R. Vin Dhanak , Tim D. Veal 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC) DOI: 10.1109/PVSC.2018.8547950 Show Abstract ▼ Abstract: Fluorine-doped tin oxide (FTO) is a commercially successful transparent conducting oxide with very good electrical (resistivities < 1 × 10 3 Ω⋅ cm) and optical properties (transmittance >85%). These properties coupled with cheap and large-scale deposition on float-glass lines means FTO has found commercial use in, for example, low emissivity windows and solar cells. However, despite its widespread application, a detailed understanding is lacking of the doping and defects in FTO. Recent work [1] has suggested that the fluorine interstitial plays a major role in limiting the conductivity of FTO. Here we present synchrotron radiation high energy x-ray photoemission spectroscopy (XPS) of the fluorine 1s core level of FTO films without in situ surface preparation. This probes deeper than standard XPS and shows that the fluorine interstitial is present not just at the surface of the films and is not an artefact of argon ion sputtering for surface preparation.	Nov 2018
	Valence band modification of Cr 2 O 3 by Ni-doping: creating a high figure of merit p-type TCO Elisabetta Arca , Aoife B. Kehoe , Tim D. Veal , Aleksey Shmeliov , David O. Scanlon , Clive Downing , Dermot Daly , Daragh Mullarkey , Igor V. Shvets , Valeria Nicolosi , Graeme W. Watson Journal Of Materials Chemistry C 96 DOI: 10.1039/C7TC03545D Open Access Show Abstract ▼ Abstract: p-Type transparent conductors and semiconductors still suffer from remarkably low performance compared to their more widespread n-type counterparts, despite extensive investigation into their development. In this contribution, we present a comparative study on the defect chemistry of potential p-type transparent conducting oxides Mg-doped and Ni-doped Cr2O3. Conductivities as high as 28 S cm−1 were achieved by Ni-doping. By benchmarking crystallography and spectroscopy characterization against density functional theory calculations, we show that the incorporation of Ni into Cr2O3 contributes to the composition of the valence band, making the formed holes more delocalized, while Mg states do not interact with the valence band in Mg-doped Cr2O3. Furthermore, it is experimentally proven that Ni has a higher solubility in Cr2O3 than Mg, at least in the highly non-thermodynamic deposition conditions used for these experiments, which directly translates into a higher acceptor concentration. The combination of these two effects means that Ni is a more effective acceptor in Cr2O3 than Mg and explains the improved conductivity observed for the former.	Nov 2017
	Self-Compensation in Transparent Conducting F-Doped SnO 2 Jack E. N. Swallow , Benjamin A. D. Williamson , Thomas J. Whittles , Max Birkett , Thomas J. Featherstone , Nianhua Peng , Alex Abbott , Mark Farnworth , Kieran J. Cheetham , Paul Warren , David O. Scanlon , Vin R. Dhanak , Tim D. Veal Advanced Functional Materials 515 DOI: 10.1002/adfm.201701900 Open Access Show Abstract ▼ Abstract: The factors limiting the conductivity of fluorine-doped tin dioxide (FTO) produced via atmospheric pressure chemical vapor deposition are investigated. Modeling of the transport properties indicates that the measured Hall effect mobilities are far below the theoretical ionized impurity scattering limit. Significant compensation of donors by acceptors is present with a compensation ratio of 0.5, indicating that for every two donors there is approximately one acceptor. Hybrid density functional theory calculations of defect and impurity formation energies indicate the most probable acceptor-type defects. The fluorine interstitial defect has the lowest formation energy in the degenerate regime of FTO. Fluorine interstitials act as singly charged acceptors at the high Fermi levels corresponding to degenerately n-type films. X-ray photoemission spectroscopy of the fluorine impurities is consistent with the presence of substitutional FO donors and interstitial Fi in a roughly 2:1 ratio in agreement with the compensation ratio indicated by the transport modeling. Quantitative analysis through Hall effect, X-ray photoemission spectroscopy, and calibrated secondary ion mass spectrometry further supports the presence of compensating fluorine-related defects.	Nov 2017
	Core-Levels, Band Alignments, and Valence Band States in CuSbS 2 for Solar Cell Applications Thomas J. Whittles , Tim D. Veal , Christopher N. Savory , Adam W. Welch , Francisco Willian De Souza Lucas , James T. Gibbon , Max Birkett , Richard J. Potter , David O. Scanlon , Andriy Zakutayev , Vinod R. Dhanak Acs Applied Materials & Interfaces DOI: 10.1021/acsami.7b14208 Show Abstract ▼ Abstract: The earth-abundant material CuSbS2 (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core-levels of the constituent elements, surface contaminants, ionization potential, and valence band spectra are studied by x-ray photoemission spectroscopy (XPS). The ionization potential and electron affinity for this material (4.98 eV and 3.43 eV) are lower than for other common absorbers, including CuInxGa(1-x)Se2 (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the VBM is raised by the lone pair electrons from the antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies.	Nov 2017
	Band Gap Dependence on Cation Disorder in ZnSnN 2 Solar Absorber Tim D. Veal , Nathaniel Feldberg , Nicholas Quackenbush , Wojciech M. Linhart , David O. Scanlon , Louis Piper , Steven M. Durbin Advanced Energy Materials 5 (24) DOI: 10.1002/aenm.201501462 Open Access Show Abstract ▼ Abstract: The band gap of earth-abundant ZnSnN2 can be tuned between 1 and 2 eV by varying the growth conditions and resulting cation disorder. The optical absorption edges and carrier densities fall between model curves for cation-ordered orthorhombic and disordered wurtzite ZnSnN2. Hard X-ray photoemission spectra suggest different degrees of cation disorder from comparison with hybrid DFT-calculated densities of states.	Dec 2015
B18-Core EXAFS	Origin of High Mobility in Molybdenum-Doped Indium Oxide Davinder Bhachu , David O. Scanlon , Gopinathan Sankar , T. D. Veal , Russell George Egdell , Giannantonio Cibin , Andrew Dent , Caroline E. Knapp , Claire J. Carmalt , Ivan Parkin Chemistry of Materials 27 (8), 2788 - 2796 DOI: 10.1021/cm503896h Diamond Proposal Number(s): [4944] Show Abstract ▼ Abstract: Molybdenum-doped indium oxide (IMO) thin films prepared by aerosol-assisted chemical vapor deposition (AACVD) show significantly improved charge carrier mobilities as compared to nominally undoped films prepared by the same technique. The basis for this very unusual behavior has been investigated by density functional theory calculations using a hybrid Hamiltonian, mobility modeling, X-ray photoemission, and X-ray absorption spectroscopies. In contrast to previous claims that Mo acts as a three-electron donor, it is shown that substitutional Mo traps two electrons in localized states falling within the bulk bandgap and thus Mo is a simple one-electron donor. At the same time, there is very little hybridization of Mo 4d states with In 5s states at the bottom of the conduction band. This results in conduction that is spatially separated to some degree from the donors, giving rise to significantly reduced ionized impurity scattering, enhancing the carrier mobility. This is in contrast to Sn-doped In2O3 where the conduction band minimum has significant Sn 5s character, resulting in regular ionized impurity scattering.	Apr 2015
	Growth and properties of GaSbBi alloys M. K. Rajpalke , W. M. Linhart , M. Birkett , K. M. Yu , David O. Scanlon , J. Buckeridge , T. S. Jones , M. J. Ashwin , T. D. Veal Applied Physics Letters 103 DOI: 10.1063/1.4824077 Show Abstract ▼ Abstract: Molecular-beam epitaxy has been used to grow GaSb 1− x Bi x alloys with x up to 0.05. The Bi content, lattice expansion, and film thickness were determined by Rutherford backscattering and x-ray diffraction, which also indicate high crystallinity and that >98% of the Bi atoms are substitutional. The observed Bi-induced lattice dilation is consistent with density functional theory calculations. Optical absorption measurements and valence band anticrossing modeling indicate that the room temperature band gap varies from 720 meV for GaSb to 540 meV for GaSb 0.95Bi0.05, corresponding to a reduction of 36 meV/%Bi or 210 meV per 0.01 Å change in lattice constant.	Oct 2013

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