2014 Fall Meeting
Transparent Conducting Oxides and Related Materials
Transparent conducting oxides and related materials are at the forefront of contemporary materials science with emerging novel materials and experimental and computational methods. The key challenge is to unravel the detailed microscopic structure – property relationships to be exploited targeting the lowest resistivity and highest transparency.
This symposium will address fundamental and applied aspects of transparent conducting oxides and related materials and will highlight recent developments in both experimental and theoretical/computational approaches. Special attention will be paid to:
- Advances in basic science of traditional transparent conducting oxides, covering both traditional n-type (e.g. doped ZnO, SnO2, In2O3 as well as their ternary and quaternary counterparts including IZO and ITO) and p-type materials (e.g. delafossite CuAlO2, SrCu2O2)
- Progress in our understanding of the electronic structure and properties of strongly defected, doped, or degenerate semiconducting oxides (e.g. a simple rock salt CdO or perovskite structured oxides such as BaSnO3 or rutile PbO2) as transparent conducting materials
- Progress in development Mo and W based transparent conducting oxides (e.g. MoOx) and related materials, tuning their performance, electrochromic applications
- Transition metal doping in nonmagnetic semiconducting oxides for TCO (e.g. titania).
- Multicomponent semiconducting glasses as TCO
- Non-oxide based transparent conducting materials (e.g. chalcogenides and pnictides) and their properties
- Redox processes in bulk and at the surfaces of pure and doped TCO and related materials. Defect - charge carrier formation, reaction and transport
- Nanostructuring as an approach to control and performance tuning of TCO
- Experimental techniques focused on TCO, structure characterization, spectroscopy, electronic response and transport, reactivity
- Development, validation and application of new computational techniques to TCO. Structure, prediction. Thermodynamics and kinetics
Hot topics to be covered by the symposium
- Novel Transparent Conducting Oxides and related materials
- P-type TCO
- Defects and charge carriers in TCO and related materials
- Predictive modelling of electronic properties for TCO and related materials
- New Structure Characterisation Techniques
- Atomic Structure prediction for TCO and related materials
- Synchrotron based studies of TCO
- Surfaces and interfaces of TCO and related Materials
- Devices based on TCO and related Materials
List of invited speakers:
- Roberts Eglitis, University of Latvia, Latvia
- Su-Huai Wei, NREL, USA
- Ivan Parkin, UCL, UK
- Elvira Fortunato, CENIMAT-I3N, FCT-UNL, Portugal
- Andrzej Suchoki, Polish Academy of Sciences, Poland
- Tim Veal, University of Liverpool, UK
- Margareta Wagner, TU Wien, Austria
- Aline Rougier, ICMCB Bordeaux, France
- Igor Schvets, Trinity College Dublin, Ireland
- Filip Tuomisto, Aalto University, Finland
Tentative list of scientific committee members:
- Russel Egdell (Oxford University) email@example.com
- Martin Albrecht (Leibnitz Institute of Crystal Growth) firstname.lastname@example.org
- John Vager (Oregon State University) email@example.com
- Scott Woodley (UCL) firstname.lastname@example.org
- David Scanlon (Diamond) email@example.com
- Alexey Sokol (UCL) firstname.lastname@example.org
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Session I : -
Authors : Ivan P. Parkin*, Andreas Kafizas, Davinder Bachu and Nuruzzaman Noor
Affiliations : Department of Chemistry, University College London. 20 Gordon Street, London, WC1H OAJ
Resume : Thin film transparent conducting oxides (TCOs) are used in a variety of building (e.g. Low Emissivity windows) and opto-electronic applications from flat panel displays to thin film photovoltaics. These films are deposited onto glass substrates and have the dual role of acting as a window layer (to allow transmission of light) and as an electrically conducting contact layer. In building applications the conductivity also indicates Infrared reflectivity, offering alternatively insulation or cooling enhanced glazing systems, depending on design and properties. Optimising the materials properties to maximise either transmission or electrical conductivity can be conflicting and depends on the control of film composition, stoichiometry, structure, defects and doping concentration. Here we report on the formation of thin films of TCO materials formed by Aerosol assisted CVD and combinatorial CVD. We show that by varying the carrier solvent in AACVD enables optimised F-SnO2 films to be grown with figures of merit significantly beyond current commercial products. The best films were grown at 600°C using dioxane as a carrier solvent and had a sheet resistance of 3.9 ohm/square, a transmission at 550 nm of 80%, a charge carrier density of 65 x 1020 and electron mobility of 30 cm2V-1s-1. The films were grown from monobutyltin trichloride in an air atmosphere, the F was introduced from ammonium fluoride. It was noted that a partial metathesis reaction where F partially substituted Cl in the precursor solvent prior to the reaction and was a key step in forming films with enhanced properties. Combinatorial CVD was used to grow a series of new TCIO materials over a graded set of films. By using multiple precursor entry points films with graded properties were produced. These films enabled rapid assessment of phase space to optimise TCO performance. A series of metal doped SnO2, Nb doped TiO2 and Al doped ZnO films were formed using this method.  N. Noor and I. P. Parkin, J. Mater. Chem C. 2013, 1, 984
Session IV : -
Authors : Aline Rougier
Affiliations : CNRS, University of Bordeaux, ICMCB, 87 avenue du Dr. Albert Schweitzer, 33608 Pessac, France.
Resume : Aiming at fast switching time and homogeneous optical properties, transparent electrodes are key layers of electrochromic devices, ECDs. In the visible, Transparent Conducting Oxides remains the most commonly used in a standard ECDs configuration, based on a 5 layers stack, schematized by TCO/EC/Electrolyte/CE/TCO, in which EC stands for Electrochromic material and CE for Counter Electrode. If In2O3:Sn (so-called ITO) is still the state of the art for n-type TCOs, the scarcity of In as well as its high cost has led to the development of other alternative materials. Among them, ZnO is highly suitable as a result of being abundant, safe for the environment, and exhibiting interesting physical properties. Conductivity in ZnO thin films can be achieved both by intrinsic doping (i.e. excess Zn) or by extrinsic doping. For the latter, the most common dopants remain by far, trivalent aluminum and gallium cations with resistivity in the order of 10−4 Ωcm for films deposited at room temperature. Screening the group of IV elements, we demonstrated the promising properties of silicon doped ZnO thin films [1-2]. Herein, the influence of dopant concentration on the structure, morphology of ZnO based thin films deposited by PVD techniques will be discussed in relationship with their optical and electrical properties.  J. Clatot, G. Campet, A. Zeinert, C. Labrugère, M. Nistor, and A. Rougier. Solar Energy Materials and Solar Cells, 95(8), 2357-2362 (2011).  C. Faure, J. Clatot, L. Teulé-Gay, G. Campet, C. Labrugère, M. Nistor, A. Rougier, Thin Solid Films, 524, 151-156 (2012).
Authors : Tvarit Patel, Chetan Singh, Emila Panda
Affiliations : Department of Materials Science and Engineering, Indian Institute of Technology Gandhinagar, Ahmedabad 382424, Gujarat, India
Resume : 2 wt.% Al doped ZnO (AZO) is a promising transparent conducting oxide due to its low cost, earth abundance and non-toxicity. However, up to now, studies have been mostly restricted to the macroscopic properties of these films without giving insight into their microscopic details. In this work, AZO films were grown on cleaned soda lime glass substrates by RF magnetron sputtering by varying the substrate temperature (Ts) from room temperature (RT) until 400oC, whereas, keeping all other process parameters constant. The film microstructure was characterized using AFM, XRD and FE-SEM, whereas, their optical and electrical properties were measured by UV-VIS spectroscopy and Four point probe respectively. The optical properties of these grown AZO films are found to be only weakly influenced by Ts (as evidenced by transmission spectra, which varies from ~84% at RT to ~91% at 400oC in the visible region). However, the macroscopic resistivity seems to be strongly influenced by Ts (decreasing from 508×10-4 Ω.cm at RT to 7.85×10-4 Ω.cm at 400oC). Thus, as a next step, conducting probe atomic force microscopy was used to simultaneously capture the topography and local nanoscale conductivity profiles. The average conductivity is found to increase with increasing Ts, following similar trend as that of the macroscopic one with the non-conducting and highly conducting regions being mainly associated to the grain boundaries and bulk of the grains respectively. The increase in conductivity with increasing Ts can be associated to higher crystallinity in the film. Further, I-V characteristics reveal the areas below 50nA current regions showing FowlerNordheim tunnelling behaviour, whereas those above 50nA regions showing direct tunnelling behaviour.
Authors : H. Teisseyre1,2, A. Kaminska1, S. Birner3, A. Suchocki1, A. Kozanecki1.
Affiliations : 1 Institute of Physics, Polish Academy of Sciences, Al. Lotnik?w 32/46, 02-668 Warsaw, Poland 2 Institute of High Pressure Physics ?Unipress?, Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland 3Walter Schottky Institut, Physik-Department, Technische Universit?t M?nchen, Am Coulombwall 4, 85748 Garching, Germany
Resume : ZnO/Zn0.84Mg0.16O multi-quantum wells (MQWs) structures oriented along c-direction (polar direction) were grown by plasma-assisted molecular beam epitaxy (PAMBE) on a-plane sapphire. Due to spontaneous and piezoelectric polarizations which exist in wurtzite structure at ambient pressure, we observed a red shift of the emission related to the quantum-confined Stark effect. In the hydrostatic pressure experiments, the pressure coefficients (dE/dp) of the QW PL peaks are smaller than that of the ZnO and show a clear trend with respect to well width: they decrease from 19.4 meV/GPa for the 1.5 nm wells to 8.9 meV/GPa for the 8 nm wells. This observation of the pressure-induced reduction of these coefficients is related to pressure induced increase of the built-in electric field. These could be explained by taking into account increase of the strains both in the barriers and in the QWs, which leads to a rise of the piezoelectric field inside the structure. In addition, we performed theoretical calculations that allow us to take into account the strain in multilayer heterostructures calculated for different pressures. Then the software (nextnano3) calculates piezo and pyroelectric polarization, and solves the Poisson equation and the single-band effective-mass Schr?dinger equations for electrons and holes. As a result of these calculations, we were able to get values of the built-in electric fields and compare them with values obtained in experiments.
Authors : David Mora-Fonz, Richard Catlow
Affiliations : University College London, University College London
Resume : Due to its interesting chemical and physical properties, zinc oxide has been studied in depth over the last few decades. One of the most puzzling features of this material is the stability of its polar surfaces, which is not evidenced in other oxides. We investigate the origin of this stability using global search techniques coupled with methods of interatomic potentials and density functional theory. Morphology and mechanisms of the stabilisation of the polar surfaces are related to thermodynamic and optical properties of ZnO, the latter characterised using hybrid QM/MM simulations. To model a polar surface it is essential to remove the inherent dipole, which is typically achieved in simulations by altering the number of atoms in the terminating layers. We employ one-sided 3D-periodic slab surface models, with the reconstruction done by removing Zn or/and O atoms from the surface and spreading compensating charge uniformly over the slab bottom. According to our calculations, the optimum ratio of Zn (or O) vacancies is very close to 0.24. The global search using Monte Carlo routines as implemented in our in-house Knowledge-Led Master Code probed more than 10, 000 different reconstructions of the (5x5) polar zinc and oxygen-terminated surfaces of ZnO. The lowest energy configurations reveal definitively triangular patterns on the Zn-terminated surface in excellent agreement with experiment and DFT. Further work on other surfaces is in progress. We show the counterpart O-terminated surface on reduction to be an important source of green luminescence, where oxygen vacancies play a crucial role.
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Session V : -
Authors : Igor Shvets, Elisabetta Arca, Karsten Fleischer.
Affiliations : School of Physics, Trinity College Dublin, University of Dublin, Ireland
Resume : Transparent conducting oxides are critical components of numerous devices essential to modern life (flat screens, solar cells, optoelectronics). For this reason they have been at the focus of extensive research in the past decades. Whereas n-type TCOs are commonly used by industry, the use of p-type TCOs in commercial devices has been inhibited by their poor performance, in particular by the low carrier mobility, poor transparency and high process temperature. Much effort has been made by the research community to overcome these problems, leading to the investigation of multicomponent oxides, oxynitrides, oxychalcogenides, etc. In this contribution, we would like to give overview of the state-of-the-art, the main limitations affecting p-type TCOs and approaches being undertaken to overcome them. We also would like to present our own research on engineering band structure and defect chemistry in Cr2O3, simultaneously improving optical and electrical properties of this oxide by doping it. A fundamental study of correlation between the dopants and the observed changes in the physical properties of Cr2O3 will be presented.
Authors : Elisabetta Arca, Cormac McGuinness, Daragh Mullarkey, Stephen Callaghan, Leo Farrell, Igor Shvets
Affiliations : School of Physics, Trinity College Dublin, Ireland
Resume : p-type TCO show remarkably lower properties in comparison to their n-type counterpart. To overcome the existing gap, it is important to understand the fundamental properties of the band structure and how this is affected by the presence of defects. As a case study, we chose Cr2O3 and its further modification by Mg doping to assess the evolution of the electronic, electrical and optical properties and correlate them to the presence of defects. Epitaxial films were deposited and defects were introduced in a controlled and selective manner by using molecular beam epitaxy as deposition technique. Both undoped and Mg-doped films were grown and characterized. For undoped Cr2O3, the energetic position and composition of both the VBM and CBM was determine. Following native stoichiometric defects were introduced in a controlled manner and their presence was probe by x-ray diffraction (XRD). The characteristic of the VBM and CBM of perfectly stoichiometric Cr2O3 and well as their evolution upon introduction of defects has been followed by spectroscopic techniques such as x-ray photoelectron spectroscopy (XPS), x-ray emission (XES) and absorption (XAS). Following, doping with Mg was performed. Conductivity of the films was observed to change over several orders of magnitude depending both on the material used as a source (Cr versus Cr2O3) and the oxygen partial pressure. The origin of these differences was investigated by a combination of structural (XRD) and electrical measurements.
Authors : G. Hautier, J. Varley, A. Miglio, V. Lordi
Affiliations : Université catholique de Louvain Lawrence Livermore National Laboratory
Resume : Transparent conducting oxides (TCOs) are essential to many technologies from solar cell to transparent electronics. While n-type TCOs (using electrons as carriers) are widespread in current applications (e.g., indium tin oxides or ITO), their p-type counterparts have been much more challenging to develop and still exhibit carrier mobilities an order of magnitude lower. In this talk, we will report on a potential p-type transparent conducting oxide: boron suboxide (B6O). This material was found by high-throughput computational search for oxides with low hole effective mass and wide band gap. We will discuss the peculiar electronic structure of B6O leading to low hole effective mass and large band gaps. We will report on a full defect computational analysis within the hybrid functional (HSE) scheme, including intrinsic defects but also many potential extrinsic defects. We show that the material can be p-type doped and does not show any intrinsic hole-killing mechanism. We also identify an extrinsic shallow acceptor that is likely to be responsible in the previously reported p-type behavior. Finally, the potential for ambipolar doping of boron suboxide will be discussed. 1. Hautier, G., Miglio, A., Ceder, G., Rignanese, G.-M. & Gonze, X. Nat. Commun. 4, 2292 (2013) 2. Akashi, T., Itoh, T., Gunjishima, I., Masumoto, H. & Goto, T. Mater. Trans. 43, 17191723 (2002).
Authors : E. Przeździecka1, W. Lisowski2, M. Stachowicz1, A. Reszka1, R. Jakieła1, J. W. Sobczak2, M. Krawczyk2, A. Wierzbicka1, M. A. Pietrzyk1, A. Jablonski2, A. Kozanecki1
Affiliations : 1 Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland 2 Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland
Resume : Zinc oxide with its Eg=3.37 eV energy gap and large exciton binding energy is a prospective material for a number of applications such as UV emitters and detectors. For applications p-type doping of ZnO is necessary. It is known, however, that it is a difficult task due to the background n-type doping. Group V elements are often used for obtaining the p-type. Understanding of how they build into ZnO matrix is important for successful p-type doping. The As, As-N, As-Sb and As-N-Sb -doped ZnO films were formed by PA-MBE method. The concentration of V-group elements atoms was high ~1E18-1E21 at/cm3 whereas a hole concentrations depending on the annealing were ~1E15-1E19 cm-3. The quality of the samples were examined by PL, SEM, XRD, SIMS, Hall measurements and X-ray photoelectron spectroscopy (XPS). The As 3d high resolution (HR) XPS spectra, recorded on ZnO:As films, reveal two or tree chemical states of As associated with As-O, As-Zn and probably with AsZn-2VZn complexes. Changes of the PL spectra after annealing in different atmospheres and doping reflect changes to local environment around As atoms. This is also confirmed by rearrangement of the As 3d HR XPS spectra as a result of annealing of the As-N-Sb -doped ZnO films in various gas atmosphere. One author (E.P) would like to acknowledge the support by the NCN project DEC-2013/09/D/ST3/03750, three authors (W.L., J.WS and A.J.) would like to acknowledge the support by the NCN project DEC-2011/01/B/ST4/00959.
Authors : Mark Buckwell, Luca Montesi, Adnan Mehonic, Manveer Munde, Stephen Hudziak, Richard Chater, Sarah Fearn, David McPhail, Anthony J. Kenyon
Affiliations : Department of Electronic and Electrical Engineering, University College London; Department of Materials, Imperial College London
Resume : Resistive switches offer the prospect of improved performance, efficiency and scalability over current data storage methods. Many device architectures have been proposed, reliant upon a wide variety of materials whose conductance switches in a non-volatile manner with the application of an applied field. Silicon-based switching materials are of particular interest in these devices as they offer the added potential for integration into existing CMOS infrastructures. It is of great importance that the underlying physics of switching is well-understood, such that device optimisation and integration into commercial hardware may be realised. Our devices employ a non-stoichiometric, silicon-rich layer of silica, a suboxide, sandwiched between conductive electrodes. We report on the material changes leading to reversible resistive switching in silicon suboxide using secondary ion mass spectroscopy (SIMS), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and conductive atomic force microscopy (cAFM). Analysis with a range of techniques serves to highlight the broad dynamics of device behaviour, and supports the model of a defect-dependent switching mechanism.
Authors : David O. Scanlon1,2* and Aron Walsh3
Affiliations : 1. University College London, Kathleen Lonsdale Materials Chemistry, Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, UK. 2. Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK. 3. Centre for Sustainable Chemical Technologies and Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
Resume : CuMO2 semiconductors crystalize naturally in the delafossite mineral structure, and have been shown to possess large optical band gaps and to display p-type conductivity. These attributes led to an explosion of interest in these materials for p-type transparent conducting oxide (TCO) applications, but also for water-splitting applications. It is now realised that these materials are limited by indirect band gaps and deep acceptor levels, which limits their range of utility. Recently, however, β-CuGaO2 which crystalizes in a wurtzite-derived orthorhombic β-NaFeO2 structure has been synthesized, displaying a direct band gap of 1.5 eV, ideal for visible light conversion. The discovery of a direct band gap, highly absorbing oxide is extremely exciting, as at present, the only current oxide solar absorber is Cu2 O, which currently displays photovoltaic conversion efficiencies of ~5%. In this presentation we investigate the electronic structure of a range of possible β-CuMO2 materials. We analyse the structure-property relationships between the delafossite and β-NaFeO2 structure, and rationalise the experimentally observed change in properties between β-CuGaO2 and delafossite CuGaO2 . Finally, we identify other β-CuMO2 compounds that could have interesting technological applications.
Session VII : -
Authors : Elvira Fortunato* and Rodrigo Martins
Affiliations : Materials Science Department, CENIMAT|I3N, FCT-UNL and CEMOP-UNINOVA, 2829-516, Caparica, Portugal
Resume : Transparent electronics has arrived and is contributing for generating a free real state electronics that is able to add new electronic functionalities onto surfaces, which currently are not used in this manner and where silicon cannot contribute [1,2]. The already high performance developed n- and p-type TFTs have been processed by physical vapour deposition (PVD) techniques like rf magnetron sputtering at room temperature which is already compatible with the use of low cost and flexible substrates (polymers, cellulose paper, among others). Besides that a tremendous development is coming through solution-based technologies very exciting for ink-jet printing, where the theoretical limitations are becoming practical evidences. In this presentation we will review some of the most promising new technologies for n- and p-type thin film transistors based on oxide semiconductors and its currently and future applications.  E. Fortunato, P. Barquinha, and R. Martins, "Oxide Semiconductor Thin-Film Transistors: A Review of Recent Advances," Advanced Materials, vol. 24, pp. 2945-2986, Jun 2012.  P. Barquinha, R. Martins, L. Pereira and E. Fortunato, Transparent Oxide Electronics: From Materials to Devices. West Sussex: Wiley & Sons (March 2012). ISBN 9780470683736. Acknowledgments This work was funded by the Portuguese Science Foundation (FCT-MCTES) through projects PTDC/CTM/103465/2008, PTDC/EEA-ELC/099490/2008, CMU-PT/SIA/0005/2008, PEst-C/CTM/LA0025/2011 (Strategic Project - LA 25 - 2011-2012) and E. Fortunato ERC 2008 Advanced Grant (INVISIBLE contract number 228144) and by EU-FP7 Project ORAMA CP-IP 246334-2
Authors : John Buckeridge*, K. T. Butler, C. R. A. Catlow, A. J. Logsdail, D. O. Scanlon, A. A. Sokol, S. M. Woodley, and A. Walsh
Affiliations : University College London, Kathleen Lonsdale Materials Chemistry, Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, United Kingdom
Resume : TiO2 is a widely used transparent conducting oxide with applications in solar cell technology as well as photocatalysis, for which Mixed anatase- and rutile-phase samples have been found to outperform the individual polymorphs, a consequence of their energy band alignment. In this work we use a state-of-the-art hybrid quantum mechanical/molecular mechanical (QM/MM) embedded cluster approach to determine the bulk ionisation potentials of eight TiO2 polymorphs, four of which are naturally occuring, and four of which are 'high pressure' phases. We also perform plane-wave hybrid-density functional theory (DFT) calculations to determine their energy band gaps, thereby providing the band alignments for a range of TiO2 polymorphs. We predict that improved photocatalytic activity should result from mixing brookite and anatase, or, if achievable experimentally, hollandite and brookite. We justify our results in the context of the effect of local oxygen coordination on the Madelung potential.
Authors : Stefan T. Bromley
Affiliations : Departament de Química Física and Institut de Química Teòrica i Computacional, Universitat de Barcelona (IQTCUB), 08028 Barcelona, Spain AND Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain
Resume : Following our previous studies [1-3], we have mined databases of 4-connected nets to generate a wide range of novel nanoporous ZnO bulk polymorphs. Using density functional theory and GW calculations, we then calculate the energetic stability and band gaps of >100 distinct nanoporous ZnO solids. We find that the degree and type of nanoporosity is inextricably linked with band gap magnitude. Increasing the degree of nanoporosity tends reduce energetic stability and increase the band gap. Within this tendency, we also find significant variations in band gap (~0.5 eV) for structures with very similar densities or energetic stabilities but different types of nanoporosity (e.g. pore size). We present a general physical interpretation of this phenomenon which should be applicable to many other transparent conducting oxides and related materials . For ZnO We estimate that altering the degree/type of nanoporosity could allow tailored band gap values up to ~4.2 eV. This proposed nanomorphological approach to band gap engineering potentially opens the door to optoelectronically tunable sensors, solar cells and other unforeseen devices which could take advantage of this versatile combination.  J. Carrasco, F. Illas and S. T. Bromley, PRL 99, 235502 (2007).  M. A. Zwijnenburg, F. Illas and S. T. Bromley, PRL 104, 175503 (2010).  D. Stradi, F. Illas, S. T. Bromley, PRL 105, 045901 (2010).  I. Demiroglu, S. Tosoni, F. Illas, S. T. Bromley, Nanoscale, 6, 1181 (2014)
Authors : I.V.Markevich, T.V.Stara
Affiliations : V.Lashkaryov Institute of Semiconductor Physics of NASU, 41 Pr.Nauky, Kyiv 03028, Ukraine
Resume : Doped with Cu, Ag and Li as well as undoped ZnO ceramics were sintered at 1000C in air. Photoluminescence (PL) and PL excitation (PLE) spectra of obtained samples were measured at 300K. In undoped samples, a weak broad green-yellow PL band was observed. PLE spectrum of this band had the only maximum at 380 nm, which corresponded to the position of free exciton. In doped samples, bright PL bands related to CuZn, AgZn and LiZn acceptors and peaked at 540, 580 and 600 nm accordingly were present. In PLE spectra of doped samples, in addition to the maximum at 380 nm, a distinct intense peak at about 395nm arose. This extrinsic excitation can be accounted for by Auger-type interaction between luminescent centers and some exciting complexes that were formed in the vicinity of the impurity acceptors under doping. Since the intrusion of impurity atoms into Zn sites leads to the appearance of zinc interstitials and/or oxygen vacancies, one can assume that the exciting complexes contain these native defects. To verify this assumption undoped ceramics were sintered under conditions that are known to produce Zn excess in ZnO. It was found that such treatments resulted in the appearance of 395 nm peak in PLE spectra of undoped samples and the intensity of this peak could be controlled by the change of treatment conditions. The analysis of obtained results led to the conclusion that, in all probability, oxygen vacancies were defects taking part in the formation of exciting centers.
Authors : I.V. Markevich, M. Osipyonok, L. Khomenkova
Affiliations : V.Lashkaryoy Institute of Semiconductor Physics of NASU, 41 Pr.Nauky, Kyiv 03028, Ukraine
Resume : ZnO layers were prepared by screen-printed approach on sapphire substrate and sintered at 800-1200C during 1 h in air. Photoluminescence (PL) spectra were measured under 365-nm excitation light at 20C. To study the effect of oxygen adsorption/desorption on PL spectra, additional heating at 200C for 30min and cooling down to 20C either under illumination with focused light of Xenon lamp or in dark was performed. Defect-related PL emission exhibited itself as a bright green band peaked at ~540nm. The decrease of its intensity was found after illumination at 200C, whereas following heating in dark resulted in its recovering. Observed effect is believed to result from the change of concentration of zinc interstitials Zni in the near-surface region of the film due to photodesorption of adsorbed oxygen under illumination and its readsorption in dark. It was earlier shown that the intensity of green PL band correlated with concentration of Zni which were highly mobile in ZnO lattice at elevated temperatures . Drift of ZnI+ ions in depletion band-banding electric field created by adsorbed oxygen results in their accumulation near the surface. After oxygen desorption this electric field disappeared, which leads to the drift of excessive Zni from the surface and the decrease of green PL intensity. After oxygen readsorption the initial state restored.  N.O.Korsunska, L.V.Borkovska, B.M.Bulakh, L.Yu.Khomenkova, V.I.Kushnirenko, I.V.Markevich, J.Lumin., 102-103 (2003) 733.
Authors : Fengyan Zhang, Xuan Huang
Affiliations : Institute of Solar Energy, College of Energy, Xiamen Univeristy
Resume : A multilayer aluminum-doped zinc oxide (ZnO:Al) thin films were deposited on low-cost flexible polymer substrates at room temperature using chemical bath deposition (CBD) and magnetron sputtering. The effects of inserted silver layer on multilayer ZnO:Al top eletrode properties were investigated. And the morphological structure and characteristic of ZnO:Al thin films, was studied using modern methods such as: X-ray photoelectron spectroscopy (XPS) and Scanning Electron Microscope (SEM). A simple mechanism had been proposed to interpret the superior performance of the multilayer ZnO:Al films applied on Cu(In,Ga)Se2 solar cell.
Authors : J. Laube, D. Nübling, S. Gutsch, D. Hiller, M. Zacharias
Affiliations : Laboratory for Nanotechnology, Department of Microsystems Engineering - IMTEK, University of Freiburg, Georges-Koehler-Allee 103, 79110 Freiburg im Breisgau, Germany
Resume : Zinc oxide (ZnO) is a commonly used material as non-toxic transparent conductive oxide (TCO) . Usually it is a n-type semiconductor, which is commonly attributed to hydrogen based defects [2,3]. In order to allow for conformal, low-temperature and low-impact TCO fabrication, thermal atomic layer deposition (ALD) is the method of choice, in contrast to e.g. sputtering. We present a systematic study of ALD-ZnO films where the amount of incorporated hydrogen is related with the conductivity. It is shown that post processing at elevated temperatures (annealing) causes the effusion of the hydrogen accompanied by a loss of conductivity. Furthermore, even the long term storage at ambient conditions leads to deterioration of the conductivity. In order to circumvent such stability problems we evaluate a method to encapsulate the hydrogen in the ZnO by an in-situ deposited ALD aluminum oxide (Al2O3) diffusion barrier.  K. Ellmer et al., Transparent Conductive Zinc Oxide. Springer-Verlag, 2008  Janotti et al., Nat mater 6, 2007  Kim et al., Solid State Commun152, 2012
Authors : E. Lampin, P . L. Palla, P. A. Francioso and F. Cleri
Affiliations : IEMN UMR CNRS 8520 and University of Lille - CS 60069 - 59625 Villeneuve d'Ascq Cedex- France
Resume : Molecular dynamics (MD) is a statistical mechanics computational approach that provides the opportunity to access basics phenomena involved in the heat transfer at the nanoscale. MD is generally used either to extract bulk conductivities from the heat current fluctuations during a NVE simulation (Green-Kubo or EMD approach), or bulk conductivities and interface resistances from the temperature profile once the stationary regime between a hot and a cold reservoir is reached (direct method or NEMD). We have developed an alternative framework of MD simulations for the study of thermal conductivities and interface resistances. The method, called approach-to-equilibrium MD (AEMD), relies on the Fourier-based exploitation of the transient regime to equilibrium, i.e. flat temperature profile. It is computationally faster than the two other approaches and can be applied to a range of systems. We will present in particular application to bulk Si, Ge and a-quartz and discuss the length dependence originating from micrometric phonon mean free paths in Si. We will also propose different solutions to exploit the temperature transient in the case of interfaces and nanoconstrictions depending on the weight of the interface resistance compared to the conductivity of the materials at both side. Finally we will open perspectives of the approach.
Authors : G. Luka1, K. Gościński1, E. Lusakowska1, L. Wachnicki1, B. S. Witkowski1, R. Jakiela1, M. Godlewski1,2
Affiliations : 1 Institute of Physics, Polish Acad. of Sciences, Warsaw, Poland; 2 Dept. of Mathematics and Natural Sciences College of Science, Cardinal S. Wyszyński University, Warsaw, Poland
Resume : Transparent conductive oxides (TCOs) are nowadays widely investigated mainly due to a possible replacement of indium tin oxide as transparent electrodes. Among various TCOs, doped zinc oxide films, ZnO:(Al,Ga), are very promising for such applications. More recently, niobium-doped titanium dioxide films (TiO2:Nb) were reported as a TCO material. TiO2 films in anatase structure and doped with Nb reveal very low resistivities, metallic conductivity and relatively high optical transparency in the visible range. However, for TiO2:Nb TCO growth by such methods as magnetron sputtering or pulsed laser deposition, high growth temperatures or additional post-deposition annealing are necessary to obtain low resistivity films. One of the methods offering low temperature growth is atomic layer deposition (ALD) by the use of very reactive reagents. In our study, we concentrated on the ALD growth conditions that led to anatase TiO2 growth. The growth temperature was 220 °C. We investigated the Nb doping mechanism. We show optical and electrical properties of ALD-grown TiO2:Nb films and indicate the optimal growth conditions leading to the TiO2:Nb films with the lowest resistivities. The as-grown films on LaAlO3 substrates have the lowest resistivities ~10-3 Ωcm and metallic conductivity. This work was supported by Polish institutions: NCN (decision No. PBS1/A5/27/2012 and by NCBiR (decision No. DEC-2012/06/A/ST7/00398).
Authors : Woo Hyun Nam, Bo Bae Kim, Young Soo Lim, Won-Seon Seo, Jeong Yong Lee
Affiliations : KAIST; Korea Institute of Ceramic Engineering and Technology; Korea Institute of Ceramic Engineering and Technology; Korea Institute of Ceramic Engineering and Technology; Institution for Basic Science (IBS) & KAIST
Resume : ZnO is one of the most promising semiconductor materials for various applications due to its controllable electrical conductivity and high optical transparency. To improve its performance, the hybrid strategies of ZnO with graphene have been widely implemented. Synergistic effects of ZnO-graphene hybrid system have been reported in various applications, such as thin film transistor, photocatalyst, supercapacitor, and light-emitting diode. These properties were mainly influenced by electron transport in the ZnO-graphene hybrid system. Therefore, interpretation of electron transport in the ZnO-graphene hybrid system is considerably important. In this work, we report the charge transport properties in Al-doped ZnO (AZO)-reduced graphene oxide (RGO) nanocomposite thin films. The AZO-RGO nanocomposite thin films were prepared by electrospray deposition of RGO coated AZO nanoparticle suspension. Microstructures of the films were characterized by transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy. In addition, charge transport properties of the AZO-RGO nanocomposite thin films were characterized by various methods. The electron transport in AZO thin film without graphene was strongly influenced by grain boundary scattering, but the AZO-RGO nanocomposite thin films displayed enhanced charge transport properties. Detailed mechanism for the enhanced charge transport properties in the AZO-RGO nanocomposite thin film will be presented.
Authors : Aleksandras Iljinas, Vytautas Stankus
Affiliations : Kaunas University of Technology
Resume : Transparent conductive oxides (TCOs) are widely used in optoelectronic devices, such as transparent electrodes in flat panel displays, solar cells, touch panels and have potential to make all-oxide (transparent) pn junction and transistors. CuAlO2 is the most perspective p-type transparent semiconductor with optical band gap 2.14.5 eV, but the resistivity of the p-type CuAlO2 film is few orders of magnitude larger than n-type semiconductors. The physical properties of the sputtered films mainly depend on the deposition parameters such as oxygen partial pressure, substrate temperature, sputtering power and sputtering pressure. Furthermore, the origin of the p-type carriers in undoped CuAlO2 has not been well enough understood. This study aims is to analyze the structural characteristics and optoelectronic properties of p-type CuAlO2 films. The formation of CuAlO2 thin films using reactive magnetron layer-by-layer deposition on substrate temperature (300-500oC) were investigated in this work. The thin film stoichiometry was accurately controlled by deposition of individual layers with the required (few nm) thickness. X-Ray diffraction patterns revealed the nanometric microstructure of synthesized films. The size of crystallites in oxidized films depends on the substrate temperature. The surface morphology was investigated by SEM microscopy and AFM microscopy. The optical transmittance (T) and reflectance (R) of the films was recorded. Resistivity of thin films were measured by four probe method. The experimental results support that the CuAlO2 thin films are homogeneous in structure and composition. The crystallinity of the films increased with increasing the substrate temperature.
Authors : E. Axente1, G. Socol1, G. Dorcioman1, D. Craciun1, L.M. Trinca2, A.C. Galca2, D. Pantelica3, P. Ionescu3, J. Hermann4, and V. Craciun1,*
Affiliations : 1Laser-Surface-Plasma Interactions Laboratory, Lasers Department, National Institute for Lasers, Plasma and Radiation Physics, RO - 077125, Măgurele-Bucharest, Romania 2Laboratory of Multifunctional Materials and Structures, National Institute of Materials Physics, RO - 077125, Măgurele-Bucharest, Romania 3National Institute of Physics and Nuclear Engineering Horia Hulubei, RO - 077125, Măgurele-Bucharest, Romania 4LP3, CNRS - Aix-Marseille University, Luminy, Marseille, France
Resume : High-permittivity HfO2 thin films were introduced into commercial C-MOS technology only several years back as a revolutionary solution to continued scaling of devices. A further increase of the dielectric permittivity of HfO2 thin films could be achieved by doping with Ti, Ta, Zr, La or Y, while Al or Si doping has been used to increase the crystallization temperature and to maintain the deposited amorphous phase. More recently, it has been shown that Hf doped indium zinc oxide is a good alternative for channel materials used in TFTs. To this purpose, a combinatorial pulsed laser deposition (C-PLD) technique will be used for controlled doping and synthesis of advanced combinatorial libraries in a single-step process to facilitate the search for new dopants and optimum concentration values. The composition of the HfO2-doped films along their longitudinal direction will be monitored by calibration-free Laser-Induced Breakdown Spectroscopy, based on plasma modeling  and complementary measurements by Energy Dispersive X-ray Spectroscopy. The thickness profile and the dielectric function will be inferred from Spectroscopic Ellipsometry, X-ray reflectivity and X-ray diffuse scattering, while the structural properties will be determined by Grazing Incidence X-ray Diffraction techniques.  E. Axente, J. Hermann, G. Socol, L. Mercadier, S. Beldjilali, M. Cirisan, C. R. Luculescu, C. Ristoscu, I. N. Mihailescu, V. Craciun, J. Anal. At. Spectrom. 29/3 (2014), 553-564.
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Authors : Paola Alippi (1), Maura Cesaria (2), and Vincenzo Fiorentini (3,4)
Affiliations : 1.CNR-ISM, Rome, Italy; 2. Dipartimento di Matematica e Fisica, U del Salento, Lecce, Italy; 3 Dipartimento di Fisica, Universita` di Cagliari; 4CNR-IOM, UOS Cagliari.
Resume : Based on hybrid density-functional calculations, we propose that ferromagnetism in the prototypical bixbyite sesquioxide In2O3 doped with Cr is due to Cr-oxygen vacancy complexes, while isolated Cr cannot support carrier-mediated magnetic coupling. Our proposal is consistent with experimental facts such as the onset of ferromagnetism in O-lean conditions only, the low or vanishing net moment in unintentionally doped material, and its increase upon intentional doping.
Authors : X. CASTEL1, J. HAUTCOEUR1, F. COLOMBEL1, M. HIMDI1
Affiliations : 1 IETR/UMR-CNRS 6164/Université de Rennes 1, 18 rue Henri Wallon, 22004 SAINT-BRIEUC & 263 avenue du Général Leclerc, 35042 RENNES, FRANCE Phone: 0033 296 609 653, Fax: 0033 296 609 652, E-mail: email@example.com
Resume : Nowadays the development of wireless communications and the expansion of the related antenna networks, especially in dense urban areas, are an environmental challenge requiring innovative technological solutions. To minimize the visual impact of such antennas and to improve their location in the city, an attractive possibility is to develop optically transparent antennas. Materials belonging to the transparent conducting oxide family and deposited on see-through substrates are of great interest. In this study, we report on various transparent and conducting films deposited on Corning glass substrates (50 mm x 50 mm) by r.f. sputtering technique at room temperature: ITO (tin-doped indium oxide) single layer, ITO/metal/ITO multilayer and mesh silver/titanium bilayer. For the latter, different square mesh pitches with a microstructuring approach are presented and discussed, targeting the lowest sheet resistance value (0.05 Ohm/sq) and the highest transparency (>80%) in the visible light spectrum. Finally we investigate the microwave performance of the transparent lozenge monopole antennas made from such materials with different sheet resistance values and levels of transparency. The performance at microwaves (close to 800 MHz), including input impedance and gain, is compared with that of a reference antenna made from a continuous (then opaque) silver/titanium film. The main factors causing the microwave loss of such antennas are investigated and discussed.
Authors : M. B. Maccioni, F. Ricci, and V. Fiorentini
Affiliations : Dept.Physics, U of Cagliari, Italy
Resume : We present a preliminary study of the local structure of (InGa)2O3 at low-In content, i.e. in the monoclinic beta-Ga2O3 structure, and report on the variation with composition of the fundamental gap and interface band offsets to Ga2O3. In atom tend to pair at short to mid-range distances in the (101) and (100) planes, but definitely avoids tetrahedral sites (i.e. 50% of the sites), making it unlikely to achieve In concentrations above 20% or so. The gap decreases at a rate of 17 meV/% In, comparing well with 20 meV/% experimentally. The band offset at low In content depends sensitively on the strain state of the superlattice, and can change from normal type-A to staggered type-B.
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Authors : M. Stachowicz, D. Jarosz, M. Pietrzyk, E. Przezdziecka, J. Dyczewski, A. Reszka, A. Wierzbicka, A. Kozanecki
Affiliations : Institute of Physics Polish Academy of Sciences, al. 32/46, 02-668 Warsaw, Poland
Resume : In this work ZnO/MgO superlattices were grown on a-plane ZnO substrates by plasma-assisted molecular beam epitaxy. In all samples the widths of the ZnO wells were 2nm and they were separated by 1-3 nm MgO barriers. Transmission electron microscopy reveals the very well defined barrier/well interfaces and Rutherford backscattering and channelling measurements confirm very good crystallographic quality of MQWs structure with the superlattices perfectly aligned with the ZnO substrate. Photoluminescence measurements revealed two types of excitonic transitions in the wells at 3.427 eV and 3.469 eV which we interpret as recombination of localized excitons and free exciton emission. The temperature dependence of the PL spectra has been measured in the temperature range between 6 and 300K. It is shown that at 300K free exciton emission is the only emission from these structures. Scanning electron microscopy measurements allow to assess the widths of structures and electron beam profiling allow to observe the depth evolution of the emissions from the structures. The results presented here show that the ZnO/MgO superlattices have hexagonal structure and are highly strained. Work supported in part within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).
Session XII : -
Authors : F. Ricci (1), A. Filippetti (1), V. Fiorentini (1), A. Baraldi (2), F. Boschi (2), R. Fornari (2), M. Higashiwaki (3), A. Kuramata (4), M. Bosi (5)
Affiliations : (1) Dept. Physics, U Cagliari (Italy); (2) Dept. Physics, U Parma (Italy), (3) National Institute of Information and Communications Technology, Tokyo (Japan), (4) A. Kuramata, Tamura Corporation, Saitama (Japan), (5) IMEM-CNR Institute, Parma (Italy)
Resume : Optical absorption measurements carried out on (010) wafers of beta-Ga2O3 showed that the bandgap edge is located at about 4.61 eV, independently of the free carrier concentration. Similar measurements carried out on epilayers deposited on c-oriented sapphire by Atomic Layer Deposition typically showed a blue shift for the fundamental absorption. Furthermore, polarized-light absorption measurements revealed a certain anisotropy, with the absorption edge varying according to the polarization of the light. These experimental findings were compared with self-interaction corrected density-functional calculations. The onset found theoretically is at 4.7 eV for field polarization in the a,c plane, which is in excellent agreement with experiment. For polarization along the b axis the theoretical onset is higher by about 0.4 eV, again in good agreement with experiments. The higher optical bandgap of epilayers is tentatively ascribed to strain and mosaicity.
Authors : Hideyuki KAMISAKA*1, Koichi YAMASHITA*2, Tetsuya HASEGAWA*1
Affiliations : *1The University of Tokyo, Department of Chemistry ; *2The University of Tokyo, Department of Chemical System Engineering
Resume : The Nb-doped anatase TiO2 at doping rate of 6% (TNO) exhibits high electric conductivity and visible light transparency. On the contrary, the Nb-doped rutile TiO2 does not possess such conductive electrons, and the carrier activation ratio in F-doped anatase TiO2 (FTO) is reported to be 20 - 30% experimentally. In addition, the electric conductivity in TNO degrades when it is exposed to the O2 gas, which suggests the importance of the non-stoichiometry in this material. To clarify the microscopic mechanism of these observations, we conducted the spin-polarized GGA+U calculation of the doped TiO2 systems. The PBE functional was adapted for the GGA part, and the +U parameters were adjusted to meet the generalized Koopman's theorem (gKT). In Nb-doped rutile TiO2, the electron was trapped at a Ti 3d level adjacent to the Nb dopant. The formation energy of this trapping was as small as < 0.04 eV. A careful convergence check against the size of the unit cell was conducted. In F-doped anatase TiO2, the formation energies of F_O (F dopant substituting O) and F_O^+ is quite close, and the two crosses when the Fermi level is slightly above the conduction band minimum. Combining the Burstein-Moss effect and this crossing of the formation energies, a simple statistical analysis was made. The calculated activation ratio was about 10% - 32%, which agreed with the experimental data. We found that formation of interstitial oxygen in TiO2 is thermodynamically favored under the presence of O2 gas and carrier electrons in the conduction band. An electron trapping level was revealed in this interstitial oxygen, which explains the depletion of conductivity by the O2 gas annealing process.
Session XIV : -
Authors : Oliver Bierwagen
Affiliations : Paul-Drude-Institut, Berlin
Resume : In the last few years, transparent semiconducting oxides have been re-discovered as true wide-band gap semiconductors, beyond their traditional role as transparent contacts. Preparing the material to semiconductor quality standards in terms of crystal quality and purity, allowed the systematic investigation of the intrinsic material properties and physics. Molecular beam epitaxy, an established method to grow high-quality semiconductor thin films, has been used to grow single crystalline undoped and donor/acceptor doped SnO2 and In2O3 films. The transport properties of these films have been measured and investigated in terms of doping, defect, and surface contribution. This talk will survey our results. Special emphasis will be given to the growth of In2O3, addressing the control of surface free energy and related faceting, and nucleation influence on the film morphology -- continuous film or islands. Control of conductivity from the semi-insulating to the highly conductive regime of transparent contacts will be demonstrated for both materials in the context of semiconductor doping. Despite acceptor doping, p-type conductivity has not been achieved. The control of surface accumulation and its role for transport and contacts will be shown. For In2O3, the strong influence of defects on the conductivity of un-doped and doped films will be detailed. Thermoelectric transport properties are shown to be helpful in the interpretation of electron transport properties.
Authors : M. Chaves1, E. P da Silva1, S. F. Durrant1, J. H. D. da Silva2,T. F. da Silva3, J. R. R. Bortoleto1
Affiliations : 1 São Paulo State University (UNESP) Av.3 de Março 511, Sorocaba, SP, Brazil; 2 São Paulo State University (UNESP) Av. Eng. Luiz Edmundo Carrijo Coube, Bauru, SP, Brazil; 3 São Paulo University (USP) Rua do Matão, trav. R,187, São Paulo, SP, Brazil
Resume : Aluminum zinc oxide (AZO) has been used as transparent and conducting oxide film in solar cells and electrochromic devices. Also, AZO thin films have great potential for applications in flexible display technology. In this work, nucleation and final texture of ZnO:Al (2%at) polycrystalline films grown by reactive RF magnetron sputtering at temperatures lower than 250oC have been investigated. Surface morphology was measured with atomic force microscopy (XE-100, Park Instruments) operating in air. All AFM images of the films were analyzed using fractal and scaling concepts. The orientation and size of crystallites were estimated using X-ray diffraction (D/MAX-2100/PC, Rigaku). Optical transmittance was performed using Uv-Vis-NIR spectrometer (Lambda 750, Perkin Elmer) ranging from 190 nm to 3300 nm. The chemical composition and density of films were obtained by Rutherford backscattering spectroscopy (RBS) with 2.2 MeV He ions. Electrical resistivity was measured by the four-point probe method. In order to study the growth evolution, different ZnO films with thickness ranging from 0 up to 1250 nm were deposited. Our results indicate the formation of an AZO amorphous layer during the early growth stages. The surface roughness is driven by non-local shadowing mechanism. Furthermore, depending on temperature, surface diffusion between planes and between grains are comparable and a polycrystalline film with a mixed a-axis and c-axis final texture is produced.
Authors : Scott M. Woodley,* C. Richard A. Catlow, David O. Scanlon, Alexey A. Sokol
Affiliations : University College London, Kathleen Lonsdale Materials Chemistry, 20 Gordon Street, London WC1H 0AJ, United Kingdom
Resume : X-ray diffraction of particles of zinc oxide reveals a sub-stoichiometry that is inherent to this material at the nanoscale. Global optimisation methods  have been applied to identify the exact nature of this effect and showed that the zinc vacancies are disordered throughout the matrix of the wurtzite crystal, which can explain the observed drop in the second neighbour peak in the RDF. Next, global optimization and data-mining techniques have been used to generate the structures of Mg and Cd doped ZnO nanoclusters . The final electronic solutions have been used for the prediction of optical absorption spectra. The excitonic energies have been obtained using time dependent density functional theory including asymptotic corrections. The optical behaviour of most stable clusters considered is contrary to the quantum confinement model. 1. Knowledge Led Master Code, e.g. Woodley, SM, JOURNAL OF PHYSICAL CHEMISTRY C, 2013, Volume: 117 Issue: 45 Pages: 24003-24014 DOI: 10.1021/jp406854j, or http://www.ucl.ac.uk/klmc/Software 2. Structural and optical properties of Mg and Cd doped ZnO nanoclusters, Woodley, SB; Sokol, AA; Catlow, CRA; Al-Sunaidi, AA; Woodley, SM, JOURNAL OF PHYSICAL CHEMISTRY C, 2013, Volume: 117 Issue: 51 Pages: 27127-27145 DOI: 10.1021/jp4084635
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