Transparent Conducting Oxides and Related Materials

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.

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.

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.[1]⁠ 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.[2]⁠ 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, 1719–1723 (2002).

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.

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.

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.

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. [1] 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. [2] 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

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.

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 [4]. 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. [1] J. Carrasco, F. Illas and S. T. Bromley, PRL 99, 235502 (2007). [2] M. A. Zwijnenburg, F. Illas and S. T. Bromley, PRL 104, 175503 (2010). [3] D. Stradi, F. Illas, S. T. Bromley, PRL 105, 045901 (2010). [4] I. Demiroglu, S. Tosoni, F. Illas, S. T. Bromley, Nanoscale, 6, 1181 (2014)

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.

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 [1]. 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. [1] N.O.Korsunska, L.V.Borkovska, B.M.Bulakh, L.Yu.Khomenkova, V.I.Kushnirenko, I.V.Markevich, J.Lumin., 102-103 (2003) 733.

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.

Resume : Zinc oxide (ZnO) is a commonly used material as non-toxic transparent conductive oxide (TCO) [1]. 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. [1] K. Ellmer et al., Transparent Conductive Zinc Oxide. Springer-Verlag, 2008 [2] Janotti et al., Nat mater 6, 2007 [3] Kim et al., Solid State Commun152, 2012

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.

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).

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.

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) p–n junction and transistors. CuAlO2 is the most perspective p-type transparent semiconductor with optical band gap 2.1–4.5 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.

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 [1] 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. [1] 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.

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.

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.

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.

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).

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.

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.

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.

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.

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 [1] 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 [2]. 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