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Instruments / Technical Area	Publication Details	Published
B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS	Understanding metal organic chemical vapour deposition of monolayer WS2 : the enhancing role of Au substrate for simple organosulfur precursors Ye Fan , Kenichi Nakanishi , Vlad P. Veigang-radulescu , Ryo Mizuta , J. Callum Stewart , Jack E. N. Swallow , Alice E. Dearle , Oliver J. Burton , Jack A. Alexander-webber , Pilar Ferrer , Georg Held , Barry Brennan , Andrew J. Pollard , Robert S Weatherup , Stephan Hofmann Nanoscale 7 DOI: 10.1039/D0NR06459A Diamond Proposal Number(s): [22123] Open Access Show Abstract ▼ Abstract: We find that the use of Au substrate allows fast, self-limited WS2 monolayer growth using a simple sequential exposure pattern of low cost, low toxicity precursors, namely tungsten hexacarbonyl and dimethylsulfide (DMS). We use this model reaction system to fingerprint the technologically important metal organic chemical vapour deposition process by operando X-ray photoelectron spectroscopy (XPS) to address the current lack of understanding of the underlying fundamental growth mechanisms for WS2 and related transition metal dichalcogenides. Au effectively promotes the sulfidation of W with simple organosulfides, enabling WS2 growth with low DMS pressure (<1 mbar) and a suppression of carbon contamination of as-grown WS2, which to date has been a major challenge with this precursor chemistry. Full WS2 coverage can be achieved by one exposure cycle of 10 minutes at 700 °C. We discuss our findings in the wider context of previous literature on heterogeneous catalysis, 2D crystal growth, and overlapping process technologies such as atomic layer deposition (ALD) and direct metal conversion, linking to future integrated manufacturing processes for transition metal dichalcogenide layers.	Oct 2020
I09-Surface and Interface Structural Analysis	Physics of existing and novel transparent conducting oxide semiconductors Jack E. N. Swallow DOI: 10.17638/03087042 Show Abstract ▼ Abstract: This thesis explores some of the fundamental properties of a rather special class of materials, the transparent conducting oxides. These ‘semiconductors’ combine the usually mutually exclusive properties of optical transparency with high electronic conductivity. Conventionally, TCOs are formed from metal-oxide structures doped with an element from the right hand column in the periodic table of either the anion (e.g. F in SnO2) or cation (Sn in In2O3). However, TCOs that use unconventional dopants (i.e. dopant elements not from the subsequent column in the periodic table) which display much improved optoelectronic properties are quite frequently reported in the literature. Most often, the host materials are doped with transition metal elements which can display unusual electronic configurations and many common oxidation states, making their properties as dopants hard to predict and understand. In this thesis, the properties of selected TCOs are presented in three case studies. The first study is on a conventionally doped and commercially available TCO, F:SnO2, in which the limitation of the electronic performance of the material is attributed to extrinsic defects associated with the dopant element. The second is a comparison between an In2O3 system doped with the transition metal Mo, and its conventionally doped and commercially relevant counterpart Sn:In2O3. In this study, the system with the novel dopant (Mo:In2O3) is shown to display much improved properties over the conventionally doped system. This is explained in the context of the band structure modifications due to choice of dopant. Finally, the surface properties of a relatively novel oxide semiconductor system, β-Ga2 O3 , are investigated. β-Ga2 O3 has gained a lot of attention in the literature recently, and is a rare case of a III-VI oxide whose properties are not well established. The natural surface electronic state of this material is determined and the effect that surface H adsorption has on the surface space charge is investigated.	Jun 2020
I09-Surface and Interface Structural Analysis	Resonant doping for high mobility transparent conductors: the case of Mo-doped In2O3 Jack E. N. Swallow , Benjamin A. D. Williamson , Sanjayan Sathasivam , Max Birkett , Thomas J. Featherstone , Philip A. E. Murgatroyd , Holly J. Edwards , Zachary W. Lebens-higgins , David A. Duncan , Mark Farnworth , Paul Warren , Nianhua Peng , Tien-lin Lee , Louis F. J. Piper , Anna Regoutz , Claire J. Carmalt , Ivan P. Parkin , Vin R. Dhanak , David O. Scanlon , Tim D. Veal Materials Horizons 6 DOI: 10.1039/C9MH01014A Diamond Proposal Number(s): [18428] Open Access Show Abstract ▼ Abstract: Transparent conductors are a vital component of smartphones, touch-enabled displays, low emissivity windows and thin film photovoltaics. Tin-doped In2O3 (ITO) dominates the transparent conductive films market, accounting for the majority of the current multi-billion dollar annual global sales. Due to the high cost of indium, however, alternatives to ITO have been sought but have inferior properties. Here we demonstrate that molybdenum-doped In2O3 (IMO) has higher mobility and therefore higher conductivity than ITO with the same carrier density. This also results in IMO having increased infrared transparency compared to ITO of the same conductivity. These properties enable current performance to be achieved using thinner films, reducing the amount of indium required and raw material costs by half. The enhanced doping behavior arises from Mo 4d donor states being resonant high in the conduction band and negligibly perturbing the host conduction band minimum, in contrast to the adverse perturbation caused by Sn 5s dopant states. This new understanding will enable better and cheaper TCOs based on both In2O3 and other metal oxides.	Sep 2019
I09-Surface and Interface Structural Analysis	Evidence of a second-order Peierls-driven metal-insulator transition in crystalline NbO 2 Matthew J. Wahila , Galo Paez , Christopher N. Singh , Anna Regoutz , Shawn Sallis , Mateusz J. Zuba , Jatinkumar Rana , M. Brooks Tellekamp , Jos E. Boschker , Toni Markurt , Jack E. N. Swallow , Leanne A. H. Jones , Tim D. Veal , Wanli Yang , Tien-lin Lee , Fanny Rodolakis , Jerzy T. Sadowski , David Prendergast , Wei-cheng Lee , W. Alan Doolittle , Louis F. J. Piper Physical Review Materials 3 DOI: 10.1103/PhysRevMaterials.3.074602 Diamond Proposal Number(s): [20647, 21430] Show Abstract ▼ Abstract: The metal-insulator transition of NbO 2 is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed NbO x that may be inherently different to crystalline NbO 2 . We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial NbO 2 thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at ∼ 810 ∘ C . The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline NbO 2 as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap.	Jul 2019
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
	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

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