Materials frontier for transparent advanced electronics II (E-MRS / MRS-J bilateral symposium)

Resume : Development of a practical alternative to indium tin oxide (ITO) has been the critical hurdle for realizing stretchable optoelectronics. Here, we present a new kind of substrate-embedded highly tall (~350nm) and thin (~30nm) three-dimensional (3D) metal grid mesh structure which exhibits excellent optoelectronic performance (Sheet resistance of 9.8Ω/□ at the transmittance of 85.2%), high stretchability (no significant change in resistance < applied strain of 15%), high density of metal mesh (< open space of 1µm), flat surface, no haze, and strong substrate adhesion on the polymer substrate. It is important to note that our approach satisfy most of essential requirements of transparent electrode for practical application in future electronics, all of which features arise from advantages of substrate-embedded unique 3D structure. For demonstration of practical suitability, our transparent electrode is successfully applied in flexible touch screen panel. We believe that our approach opens up opportunity for practical application in wearable electronics.

Resume : We report on an indium-free and cost-effective Cu2O/Cu/Cu2O multilayer mesh electrode grown by room temperature roll-to-roll sputtering as a viable alternative to ITO electrodes for the cost-effective production of large-area flexible touch screen panels (TSPs). By using a low resistivity metallic Cu interlayer and a patterned mesh structure, we obtained Cu2O/Cu/Cu2O multilayer mesh electrodes with a low sheet resistance of 15.1 Ohm/square and high optical transmittance of 89% as well as good mechanical flexibility. Outer/inner bending test results showed that the Cu2O/Cu/Cu2O mesh electrode had a mechanical flexibility superior to that of conventional ITO films. Using the diamond-patterned Cu2O/Cu/Cu2O multilayer mesh electrodes, we successfully demonstrated TSPS of the flexible film-film type and rigid glass-film-film type TSPs. The TSPs with Cu2O/Cu/Cu2O mesh electrode were used to perform zoom in/out functions and multi-touch writing, indicating that these electrodes are promising cost-efficient transparent electrodes to substitute for conventional ITO electrodes in large-area flexible TSPs.

Resume : Nowadays the development of wireless communications and wireless sensors requires emerging technologies to improve the network antenna performance, especially in dense urban areas. An attractive possibility concerns the development of optically transparent antennas to improve their location in the cities by accessing new surfaces, namely glazed surfaces such as building or car windows. Materials belonging to the transparent conducting oxide (TCO) family are of great interest. Nonetheless the most popular TCO, namely indium tin oxide, is based on a critical raw material (indium). In this study, we present an alternative to TCOs: a micrometric mesh silver/titanium bilayer deposited on glass substrate at room temperature by RF sputtering technique. This transparent conducting layer is critical raw material-free, exhibits high performance: optical transparency >80%; sheet resistance <0.05 ohm/sq; and is non-visible for the ordinary human vision acuteness. The performance of this new technology is compared with that of TCO single layer and TCO/metal multilayer. Moreover the influence of the mesh pattern on optical transparency, sheet resistance and microwave performance (input impedance; gain) is investigated through the reported results of transparent frequency agile mesh antennas (tunability=11%) operating at 10 GHz. The ability to solder easily sub-millimeter-size active components on such material gives access to transparent active antennas and thereby new emerging electronics.

Resume : Nowadays paper substrates are considered as a potential “electronic” material. Many research groups have being working on the optimization of cellulose based substrates for electronic applications either by using paper as a support for devices or by functionalizing it with conductor/semiconductor materials. In our previous work we reported for the first time field-effect transistors (FETs) in an “interstrate” structure where paper was used as gate dielectric. This work reports the possibility of producing high performance flexible planar dual-gate oxide-based field effect transistors (DGFETs), where the paper acts simultaneously as substrate and dielectric. The paper-based devices were produced at room temperature and patterned with shadow masks using gallium indium zinc oxide (GIZO) as channel layer and exhibit an ION/IOFF ratio above to 104 and a saturation mobility of ≈ 3 cm2V-1s-1. Moreover, it is possible to tune by almost 20 V the VON and change the operation mode from depletion to enhancement mode for a VDS = 1 V. In the saturation regime the device works exclusively in depletion mode with VON modulation from -30 to -12.5 V, for a VGS2 of 5 and -15 V, respectively. This demonstrates the possibility of having eco-friendly flexible low-cost devices with tunable switching characteristics which is particularity attractive for logics or biosensors applications.

Resume : Oxides are truly fascinating materials for numerous reasons. One of them is certainly their multifunctionality: researchers can build their transistors entirely with oxide materials, from the substrate to the passivation/encapsulation shielding oxide conductors, insulators and semiconductors. The current database of oxide materials is already so vast that it is possible to find candidates combining excellent electrical and optical properties tailored for each application. But the possibilities are even broader when considering that most of these oxides can be combined with great success with other material classes: probably the most striking example is the recent wave of hybrid devices, combining the mechanical flexibility of organics with the superior electrical performance of inorganic materials. This is also potentiated by the large range of techniques that can be used for processing oxides, ranging from the conventional PVD tools to low-cost screen-printing, or even hydrothermal synthesis of nanostructures. The presentation will cover the most recent oxide electronics concepts being developed at CENIMAT having this background in mind. It will focus particularly sputtered zinc-tin oxide and hybrid oxide/organic TFTs and circuits on flexible foils for x-ray sensors, smart packaging and OLED displays. Other innovative concepts such as flexible oxide electrochromic transistors with simultaneous optical and electrical modulation will also be shown.

Resume : Zn₃N₂ was reported to be an n-type wide-bandgap conductor [1]. We found that oxygen-doped Zn₃N₂ (Zn₃N_2-xO_x) films exhibited resistivities on the order of 10⁻⁴ Ω cm [2]. Accordingly, we have directed our attention to this material as a potential transparent conductor. Notably, Zn₃N₂ films with high electron mobilities ~100 cm²/(Vs) can be easily grown. However, the reason for high mobility has not been elucidated yet. Therefore we have investigated the electron transport of electrical properties in Zn₃N₂ films. In this presentation, we discuss the electron transport properties of Zn₃N_2-xO_x films in detail. We prepared both polycrystalline and epitaxial films of Zn₃N₂. Those films were grown by employing reactive radio-frequency magnetron sputtering technique. Alkali-free glass and (100)-oriented yttria-stabilized zirconia substrates were used to grow polycrystalline and epitaxial films, respectively. Film growth conditions were described in detail elsewhere [2,3]. Film depositions were carried out at a working pressure of 2 Pa using a metal Zn target. Growth of phase-pure polycrystalline and epitaxial films were confirmed by X-ray diffraction. The epitaxial films were grown in a cube-on-cube manner. X-ray photoelectron spectroscopy (XPS) was adopted to determined the oxygen concentrations, x, in Zn₃N_2-xO_x films. XPS results confirmed that a series Zn₃N_2-xO_x films with x = 0.04–0.19 were obtained. From Hall measurements, we observed an increase in carrier density (n) with increasing x values, suggesting that oxygen serves as an electron donor in Zn₃N₂. The same tendencies were observed in both polycrystalline and epitaxial films. n reached the maximal values of 1.2 × 10²⁰ cm⁻³ at x = 0.19 for polycrystalline film and 9.8 × 10¹⁹ cm⁻³ at x = 0.11 for epitaxial film. Consequently, lowest resistivities of 6.2 × 10⁻⁴ Ω cm for the polycrystalline film and 1.1 × 10⁻³ Ω cm for the epitaxial film were achieved. Zn₃N₂ having n > 10¹⁹cm⁻³ was identified as a degenerate conductor. Significant difference of mobility (μ) between polycrystalline and epitaxial films was not seen when these films had n > 5× 10¹⁹ cm⁻³. Even in polycrystalline films, a high μ value of 86 cm²/(Vs) was obtained. The effective mass was deduced from the Drude analysis of transmittance and reflectance spectra. As a result, the effective mass at the bottom of the conduction band was extracted to be (0.08 ± 0.03)m₀, which is as small as that in InN. These results suggested that the small effective mass value is responsible for high electron mobility in Zn₃N₂ films. The bandgap values of Zn₃N₂ polycrystalline and epitaxial films were estimated to be 2.5 ± 0.2 eV, which resulted in average visible transmittances of 40%−60%. The transmittance is insufficient for a transparent conductor. Therefore, bandgap widening is needed to establish this material as a truly transparent conductor. In summary, we experimentally proved Zn₃N₂ has high electron mobility. Optical analysis suggested that high mobility in Zn₃N₂ is mainly attributed to the small effective mass. If bandgap widening is achieved, this material can be a high-mobility transparent conductor which consists of earth-abundant elements. [1]K. Kuriyama, et al., Phys. Rev. B 1993, 48, 2781−2782 [2]X. Cao, et al., J. Phys. Chem. C 2015, 119, 5327−5333. [3]X. Cao, et al., J. Appl. Phys. 2016, 119, 025104.

Resume : Over the last decade transparent conductive materials (TCMs) had a dramatic increase in interest in the field of optoelectronics. Most attention has been paid to n-type wide bandgap semiconductors including In2O3, SnO2, ZnO, TiO2 and their doped versions. However, the lack of suitable p-type TCMs has been the main obstacle in front of either passivate or active electronic applications. Copper iodide in the zincblende ground-state phase (γ-CuI) has p-type conductivity with a high Hall mobility (>40 cm2V-1s-1 in bulk), a wide band gap (3.1 eV) with a direct band structure, and a large exciton binding energy (62 meV). These advantageous properties make it one of the most promising p-type TCMs [1]. It is a challenge to grow epitaxial thin films of γ-CuI due to the lack of lattice-matched substrates. Here, we report the room-temperature heteroepitaxial growth of γ-CuI on various substrates by reactive sputtering technique. In such heteroepitaxial growth the formation of rotation domains is observed and hereby systematically investigated. The controllable epitaxy of CuI thin films allows for the combination of p-type CuI with suitable n-type TCMs with the purpose to fabricate epitaxial thin film heterojunctions. The obtained epitaxial thin film heterojunction of p-CuI(111)/n-ZnO(00.1) exhibits a high rectification up to 2×109 (±2 V), a 100-fold improvement compared to diodes with disordered interfaces [2]. These results prove the great potential of epitaxial CuI as a promising p-type optoelectronic material. References [1] M. Grundmann et al., phys. stat. sol. (a) 210(9), 1671-1703 (2013). [2] F.-L. Schein et al., Appl. Phys. Lett. 102(9), 092109 (4 pages) (2013).

Resume : Copper and nickel oxides are well-known p-type transparent conductive materials that can be used in various devices such as solar cells, TFT, electrochromic… The efficiency of such devices is strongly driven by the preferred orientation of the layers and by the structural quality of the interfaces. This communication aims to present an original way to control the texture of copper and nickel oxides that is independent of the deposition conditions. Oxide thin films have been deposited at room temperature on glass and silicon substrates using a reactive magnetron sputtering process. Depending on the oxygen flow rate introduced into the deposition chamber, it is possible to selectively grow Cu2O or Cu4O3 films. For both materials, the texture of the films is mainly governed by the deposition pressure. Then, a two-step deposition procedure is detailed to tune the film texture independently of the deposition conditions. We have demonstrated that the texture of the top layer is only determined by that of the bottom layer. The bottom layer acts as a seed layer for the growth of the top one. Transmission electron microscopy analyses in cross-section show the top layer is epitaxially grown on the columns of the seed layer, indicating the existence of local homoepitaxial growth. The same kind of results has been obtained for NiO thin films. Finally, we have shown that the texture of NiO can be tuned using a seed layer of Cu2O.

Resume : High conductivity in the off-state and low field-effect mobility compared to bulk properties are widely observed in p-type thin-film transistors of polycrystalline Cu2O, especially when processed at moderate temperature. This work presents results on sputter-deposited Cu2O, obtained from in situ conductance and XPS measurements at thicknesses from sub-nm to above 200 nm. A matrix-model is adjusted to the electrical data to show the presence of highly conductive grain boundaries, which at least partially short-circuit conduction through the grain interior. Parallel in situ XPS analysis reveals that the effect is related to a segregation of Cu(II) in the grain boundaries of Cu2O. The influence of the substrate material is discussed for quartz glass, Al2O3 grown by atomic layer deposition and Bi2O3 by sputter deposition. The analysis of grain boundaries is possible since the entire process is conducted in ultra-high vacuum, which excludes surface oxidation. This bottom-up approach reveals phenomena which are commonly not observable after device fabrication, but clearly dominate electrical properties of polycrystalline thin films.

Resume : The capacity to modulate the properties of Transparent Conducting Oxide (TCO) materials is of great importance for the optimisation of device efficiency. Tuning of the refractive index to reduce mismatch between two materials is highly desirable for the reduction of internal interface reflections. Previous works on ZnO:Al have utilised substitutional doping with magnesium as a means to alter the bulk refractive index. However, the increased effects of neutral impurity scattering from the randomly distributed Mg leads to a reduction in the mobility of the material and high level inclusions were capped by dephasing at Mg concentrations of >33% [1,2]. We present a superlattice structure of layered, conductive amorphous InGaZnO (a-IGZO) and a dielectric material (SiO2) as an alternative method of reducing the refractive index without altering the carrier mobility significantly. These superlattice structures are grown with conventional RF magnetron sputtering from separate IGZO and SiO2 targets on both glass and sapphire substrates. By varying the IGZO to SiO2 layer thickness ratio the dielectric function of the superstructure was tuned from n=2 to n=1.8 with a reduction in the Hall mobility from 16.5cm2/Vs to 6cm2/Vs. This is an improvement over the results obtained via bulk substitution in ZnO:Al, wherein a reduction from n=1.93 to n=1.81 resulted in a reduction in mobility from 6cm2/Vs to 0.1cm2/Vs [2]. [1] J. G. Lu; S. Fujita, et al., Appl. Phys. Lett. 89, (2006), 262107 [2] K. Fleischer; E. Arca, et al., Appl. Phys. Lett. 101, (2012), 121918

Resume : Indium is a key element in high-performance transparent conducting oxides (e.g. ITO) and amorphous oxide semiconductors (e.g. IGZO). However, it faces resource shortage and unstable supply problems, thus alternative materials are strongly desired. In this presentation, our recent studies on Indium-free transparent conductors and amorphous semiconductors will be introduced. The first topic is fabrication of high-performance TiO2–based transparent conducting films and their application to organic electronics devices. An important breakthrough for obtaining “device-grade” Nb:TiO2 thin films by sputtering is the two-step annealing process, which drastically suppresses generation of cracks. The organic polymer solar cells with the Nb:TiO2 transparent electrodes achieved comparable efficiency to those with ITO electrodes. I will also talk about the fine tuning of the band alignment and refractive index by solid-solution of anatase TiO2 and anatase TaON. The second topic is mixed-anion amorphous semiconductor, which has been stimulated by the reports on high mobility amorphous ZnOxNy. We fabricated ZnOxNy thin films by less-energetic plasma-assisted pulsed laser deposition than conventional reactive sputtering, and achieved much higher mobility. The relationship between the mobility and surface morphology suggest that suppression of nanocrystals is the key for the high mobility.

Resume : The present study aims to investigate the optical, electronic and structural properties of titanium oxide thin films doped with nitrogen. The films deposition is made by Ion Beam Deposition (IBD) by bombarding a pure titanium target with argon ions in oxygen atmosphere. The films are deposited on an amorphous quartz substrate at 400 and at 500 °C. Afterwards, the films are doped by ion implantation with low-energy ion beam mixed of nitrogen and hydrogen ions at 150 eV and under the same temperature of the growth for times ranging from 10 to 60 minutes. After implantation, Hall measurements indicated that the majority carrier density in the nitrogen doped anatase films reaches up to 10^(19) cm^(-3) (while the undoped films have a carrier density of 10^(12) cm^(-3)). The resistivity of the doped films is as low as 10^(-1) Ohms cm while maintaining good optical transmission. Indeed, depending on the doping time and substrate temperature, light transmission of up to 90% could be obtained at 550 nm with this resistivity. X-ray photoelectron spectroscopy (XPS) performed in situ shows that the surface composition is compatible with N:TiO_(2-x) with nitrogen concentrations of up to ˜ 20%. Small angle x-ray diffraction measurements (SAXRD) confirmed the anatase crystal structure of the films before and after the low energy ion implantation. This study indicates that it is indeed possible to dope anatase thin films with nitrogen by low energy ion beam. This approach is interesting for allowing a greater control of doping concentration with respect to what is usually obtained by reactive sputtering. Acknowledgments: FAPESP funding 2012/10127-5.

Resume : Amorphous Cd-Ga-O is one of the interesting amorphous oxide semiconductors (AOSs), because its band-gap energy can be tuned from ~2.5 to 4.3 eV by controlling the cation ratio though its electron mobility is as high as ~10 cm²V^–1s^–1. In this study, we investigated effects of post-annealing conditions on amorphous Cd-Ga-O thin films. Amorphous Cd-Ga-O thin films whose Cd concentrations were 30%, 50% and 70% were deposited on silica glass substrates by RF-magnetron sputtering method. The Cd concentrations were controlled by changing mixing ratios of compacted CdO and Ga₂O₃ mixed powders which were employed for sputtering targets. Post thermal annealing in vacuum, Ar, and O₂ were conducted from 100 to 800 °C for 30 minutes. The films whose Cd concentration was 30% and 50% were not crystallized up to 800 °C by the vacuum annealing, however 70% films were crystallized at 400 °C. For 50% films, the vacuum annealing at 500 °C effectively increased their Hall mobility, for example, from 2.7 to 18 cm²V^–1s^–1. The mobility of 70 % films also reached 15 cm²V^–1s^–1 by 300 °C annealing. On the other hand, enhancement of the mobility of 30% films was not significant. Annealing above 600 °C decreased Cd concentration in the 30% and 50% films and their mobilities were decreased by further annealing.

Resume : Delafossite materials are thought to hold one of the highest potential of p-type transparent oxides semiconductor, and among them CuCrO2 has exhibited strong potential in terms of optical vs. electrical figure of merits. Recently, chemical vapour deposition has demonstrated the synthesis of highly conductive CuCrO2 without intentional doping[1-3]. In particular, we have found pure delafossite CuCrO2 phase synthesis, with pulsed injection Metal-Organic Chemical Vapour Deposition[1,3]. CuCrO2 thin films exhibited a large Cu/Cr non-stoichiometry and an electrical conductivity as high as 17 S.cm-1, which is in the range of Mg-doped CuCrO2 obtained with other techniques. In order to investigate the origin of the large conductivity, here we studied thermoelectric properties of thin films with different synthesis parameters. Seebeck coefficient and electrical conductivity have been measured simultaneously in a temperature range of 140-400 K. The datas are directly compared with different transport models, in order to determine the temperature-dependence of the carrier density and the mobility. [1] J. Crêpellière, N. Bahlawane, S. Siebentritt and D. Lenoble, in 5th International Symposium on Transparent Conductive Materials, Chania (Crete), 2014 [2] L. Farrell, E. Norton, C. M. Smith, D. Caffrey, I. Shvets and K. Fleischer, J. Mater. Chem. C, 2015, 4, 126–134 [3] J.Crêpellière, P. Lunca Popa, N. Bahlawane, R. Leturcq, F. Werner, S. Siebentritt, D. Lenoble, J. Mater. Chem. C (submitted)

Resume : The electronic surface properties of Indium Oxide (In2O3) are of great interest, due to its application in Seebeck effect based and conductometric gas sensors. In2O3 typically exhibits a high surface electron concentration in the as grown state, which is influenced by an oxygen plasma treatment [1]. The bulk electron concentration of In2O3 can be reduced by Mg-doping or annealing in oxygen, and increased by vacuum annealing [2]. In this study we characterize the surface composition and electronic properties of undoped and Mg-doped In2O3 films grown by PAMBE or MOCVD using photoelectron spectroscopy (XPS and UPS). We characterize different preparation methods, such as thermal annealing in vacuum or O2, oxygen plasma treatment and exposure to O3 to identify their influence on the formation of adsorbates, oxygen vacancies and defects as well as on variation in surface band bending, electron concentration and electric dipoles at the In2O3 surface. Oxidative surface treatments (e.g. oxygen plasma conditioning) provoke attachment of negative O adsorbates, which form an effective negative surface dipole. Thereby the barrier for electron emission increases and the surface electron density decreases via charge transfer between substrate surface and adsorbate, which can be reversed by adsorbate desorption. [1] O. Bierwagen, et al., Appl. Phys. Lett. 98, 172101 (2011) [2] O. Bierwagen, J. S. Speck, Appl. Phys. Lett. 101, 102107 (2012)

Resume : Indium Tin Oxide (ITO) thin films have attracted much attention in the recent years because of a large variety of technological applications in the fields of optoelectronics (transparent electronics). ITO thin films are deposited by GLancing Angle Deposition in a sputtering deposition chamber as a function of sputtered ions and thickness. The morphology of the columns can be controlled by changing the substrate orientation in respect of the sputtered particles flow. These parameters greatly influence the nanostructure and the optical properties. The results show that as the glancing angle increases, the column angle and the porosity of the ITO films increase due to the shadow effects. This work is focused on the relationship between the nanostructuration, and the optical properties. Thin films elaborated at low angle optically behave like classical degenerate semi-conductors, described by the Drude Model, whereas at high angle, thin films have a different optical behavior modeled by the Extended Drude Model; well-known in strongly correlated materials. Improvements made on optical modeling provided a better understanding of the nanostructure and the porosity and dopant distribution within the films. Therefore, it has been proved that a dense layer grows prior to the porous layer which contains a porosity and dopant gradient. This complex multilayer structure obtained in a single growth process provides multifunctionality: conductive TCO and anti-reflective function.

Resume : The demand for transparent conductive electrodes (TCE) is a critical issue in many strategic technological areas. In the last decade, the employ of reliable TCEs in flexible devices has also become a hot topic. TCO-metal-TCO multilayer structures have been proposed as good candidates compared to standard TCOs because of the good conductivity at low temperature process, reduced thickness and good mechanical flexibility. However, TCO films embedding metal nanostructures are still far from being implemented in industrial processes. Among the important aspects, laser patterning (scribing) for electrical isolation and robustness during bending cycles, must be taken into account. In this work, we show how the energy density for laser scribing is significantly reduced when the standard TCO single layer (700 nm of AZO) is replaced with a 10 times thinner AZO/Ag/AZO multilayer structure. Moreover, the same material grown onto flexible PEN substrate, has shown a much better mechanical and electrical reliability during bending cycles with respect to single AZO films of higher or equivalent thickness. Thin films of of AZO/Ag/AZO (45nm/10nm/45nm) were grown on soda lime and PEN substrates by RF magnetron sputtering at RT. Nd:YAG laser treatments were done by a single pulse (12 ns) at 1064 nm. Samples grown on PEN underwent up to 100 bending cycles with different curvature radii. Our experimental results, supported by computer simulation, show the key role of the Ag in lowering the laser power for cutting and helping the flexibility during bending cycles.

Resume : Transparent electrodes offer simultaneously high transparency and low sheet resistance and are an essential part in modern optoelectronic devices such as flat panel displays and photovoltaic cells. The material most commonly used for transparent electrodes is tin-doped indium oxide (ITO) which contains the critical raw material indium. The substitution of this material is of paramount importance due to its high price and scarcity. Ultrathin metals embedded in high refractive index dielectrics offer an efficient alternative for substitution of ITO. In the current work, several combinations of flexible and rigid substrates, metals and dielectrics will be presented. The transparent electrodes were simulated with a transfer matrix algorithm and optically characterized by transmittance and reflectance measurements. The comparison between simulation and experiment reveals significant differences which originate from increased absorption in the ultrathin metal film. To achieve more realistic prediction of the performance and account for the increased absorption, we include the roughness of the interfaces by an effective medium approximation (EMA) in the simulation. The model including roughness was subsequently applied for the design optimization and a good agreement of simulation and experiment was achieved. Samples with maximum transmittance at 550 nm of T>0.87 and average transmittance in the visible wavelength range of T>0.80 with a sheet resistance of 15 Ω have been experimentally achieved.

Resume : The United States’ Materials Genome Initiative (MGI), announced in 2011, helped launch the era of Materials-by-Design which combines theory, computation, experiment and data sciences to accelerate materials development and deployment. So far, Materials-by-Design research has primarily focused on the intrinsic properties of equilibrium materials. However, many technologically relevant materials are metastable and, further, roughly 1/3 or more of new materials discovered each year are metastable rather than equilibrium materials. Therefore, for new materials to help in addressing energy and other overarching societal challenges as envisioned by the MGI, we must directly address the research challenges of both including metastable materials and moving from Materials-by-Design to Solutions-(or Devices)-by-design. Our approach to and progress towards this challenge will be demonstrated by examples taken from current research on transparent conductors and opto-electronic semiconductors.

Resume : We report on our recent findings on novel transparent conductive layer stacks for CIGS thin film photovoltaics. We addressed this subject from the viewpoint of high mobility TCOs to minimize the impact of free carrier absorption in conventional TCOs such as ZnO:Al (AZO). Based on amorphous In2O3:H (IOH), we developed process technologies for low temperature annealing compatible to CIGS thin film photovoltaics where low substrate temperature below 200 °C is mandatory. High mobility exceeding 130 cm2/(Vs) has been achieved which allows to push up the short circuit current by more than 2 mA/cm2 without detrimental effects on other parameters. Current work is on upscaling of the IOH technology from small scale RF sputtering to large area pulsed DC in-line sputtering. Furthermore, we implement the large area serial co-sputtering of TCOs for CIGS thin film photovoltaics in order to achieve advantageous performance due to tailored material composition in the case of IZO and tailored doping in the case of ZnO:Al. The ultimate goal is to shift the pn-junctions towards the CIGS bulk material by means of a thin heavily doped interface layer. We report on our recent achievements on that subject.

Resume : Al-doped ZnO thin films deposited by sputtering are known to exhibit non-negligible inhomogeneity in their electrical properties. This has been explained by a non-uniform spatial distribution of bombarding high-energy negative ions, which has been shown to be correlated to the film resistivity. Several other works have also found a relationship between the spatial distribution of the resistivity and of other quantities, such as lattice shift, grain size, and composition. While correlations are indeed evident in specific cases, they are often in contradiction from one work to another, and some works admittedly find that different spatial correlations exist when different deposition parameters are used. We therefore attempt to distinguish case-specific correlations from more general correlations by characterizing ZnO:Al films grown at three pressures (where different spatial distributions are clearly observed) by both RF and DC magnetron sputtering. We investigate the spatial distribution of the film composition, as well as of several structural, morphological, and optical properties. In this process, we find that it is especially important to elucidate the origin of x-ray diffraction peak broadening; hence we separate size-, strain-, and instrument-related broadening with a standard procedure. By electron microscopy, we show that this separation is necessary, for example, to extract the average crystallite size, which can otherwise be underestimated by up to a factor of three.

Resume : ZnO-based transparent conductive thin films (TCO) have attracted much attention as a promising alternative material to the currently used Sn doped In2O3 films in various optoelectronic applications, such as flat panel display and organic light emitting diodes [1-3]. In this study, we studied the effect of V and Ga dopant on the structural and electrical properties of ZnO films. Magnetron sputtering was used to prepare the V and Ga codoped ZnO (VGZO) films by placing V metal pellet on a Ga2O3-ZnO ceramic target (Ga: 5.7 wt.%). V concentration (V/(V Ga Zn)) was adjusted by changing the number of V metal pellet. The films were deposited on the synthetic quartz substrate heated at room temperature and 130 °C. With V concentration increasing, the crystallinity of VGZO films degraded, and the surface roughness decreased. The increase in V concentration in the VGZO films led to an increase in the resistivity and optical band gap, where the change of optical band gap with carrier density could not be explained by Burstein-Moss effect. It was also found that the VGZO films almost kept the constant moisture resistivity after the introduction of V dopant, which was measured at 60 °C and 95% RH for > 500 hours, and such behavior may be attributed to the effect of the grain. We will report the relationship between the film structure and electrical properties in detail. [1] J. Jia, A. Takasaki, N. Oka, and Y. Shigesato, J. Appl. Phys. 112, 013718, 2012. [2] K. Nagamoto, T. Hara, S. Naganawa, T. Kondo, H. Sakuma, and K. Ishii, IEICE Transactions on Electronics, J98-C, 244, 2015. [3] K. Nagamoto, T. Hara, H. Sakuma, and K. Ishii, Proc. CPMT Symposium Japan (ICSJ), 2014 IEEE, pp. 27-30

Resume : Al-doped ZnO (AZO) finds tremendous applications in technological domains because of its higher optical transmittance and lower electrical resistivity. Unlike optical properties, electrical properties are strongly dependent on the fabrication conditions with their underlying mechanisms being always not known. Thus, in this work, attempts have been made to understand the origin of these electrical properties, i.e.,carrier concentration and carrier mobility. To this end, thin AZO films were deposited on SLG substrates by varying a range of experimental parameters in RF magnetron sputtering. Then a detailed microstructural and optoelectronic characterizations of these films were conducted using a range of experimental techniques, like XRD, AFM, FESEM, UV-Vis-NIR spectrophotometer, Hall probe measurement system, PL, XPS and HR-TEM. All these films are found to have grown in ZnO hexagonal wurtzite structure with strong (002) orientation of the crystallites and with an average transmittance of above 85% in the visible range. However, electrical resistivities were found to vary from 420×10-4 Ω-cm to 0.872×10-3 Ω-cm with carrier concentration and carrier mobility varying from 0.506×1020 cm-3 to 5.21×1020 cm-3 and 2.94 cm2 V-1s-1 to 16.32 cm2 V-1s-1 respectively. In all cases, a relatively larger carrier concentration is found to be related to the presence of shallow level defects in the band gap because of the presence of Zn interstitials. On contrary, PL emissions due to the presence of deep level defects are found to be associated with oxygen vacancies/adsorbed oxygen on surface, resulting in lower carrier concentration. Additionally, higher carrier mobility in these films are found to be associated with the most oriented and larger crystallite growth. Understanding developed through this will help in better designing the TCO for any relevant application.

Resume : Nowadays, application areas of Transparent Conductive Oxides (TCO?s) become widen from low-emission windows to solar cells. The interest on selection of zinc oxide based materials for these applications increased rapidly due the enhanced properties such as: high optical transmittance with low resistivity characteristics and excellent chemical stability. Mainly, aluminum and gallium are used as doping elements in order to improve the conductivity by one or two orders of magnitude. TCO films can be achieved by using various deposition techniques and magnetron sputtering is one of them that provide low cost production at low deposition temperature conditions. In sputtering, deposition parameters such as deposition rate, power, target-to-substrate distance, substrate temperature, process pressure etc. have to be optimized in detail to achieve a high quality film. Therefore, the present study depends on understanding the process pressure influence on gallium doped zinc oxide (ZnO:Ga) films. 5 wt. % Ga2O3 doped zinc oxide (99.99% purity, 100 mm diameter, Kurt J. Lesker) thin films were deposited on glass substrates by using r.f. magnetron sputtering technique, at room temperature. The sputtering power was maintained at 230 W and process pressure was changed from 0.2 Pa to 0.6 Pa. After determination of the process conditions, ZnO:Ga thin films were textured to improve both the conversion efficiency and light scattering capability are necessary for thin film solar cells. Therefore, ZnO:Ga thin films textured by a post-deposition wet-chemical etching in a 0.1% diluted HCl solution at different times. Textured ZnO:Ga thin films exhibit rough textured surface, high optical transmittances in the visible and near infrared region and excellent electrical properties. Achieved results were analyzed with the help of x-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), four point probe and optical transmittance measurement techniques. According to XRD results, majority of the thin films were oriented with the crystallographic c-(002) axis. The average transmittance value was achieved approximately 86 % in the visible spectrum while, minimum resistivity value was obtained as 1.43 ×10-3 ?.cm. Moreover, the surface roughness (RMS) values were in the range of 3.57 nm ? 12.80 nm and the maximum figure of merit (FOM) value was obtained around 59.2800 ×104 (?.cm)-1. Key Words: r.f magnetron sputtering, transparent conductive oxide, ZnO:Ga, etching

Resume : Mg-doped ZnO (ZnMgO) attracts a growing interest due to its implementation as buffer layer in different types of thin film heterojunction solar cells, such as chalcogenides and copper oxides. The interest stems from the fact that the bandgap of the material can be tuned in a wide range through the Mg content. In this work, atomic layer deposition (ALD) is employed for growing Zn(1-x)MgxO films of variable doping concentration, x. The incorporation of Mg into the ZnO films is controlled by the variation of the number of MgO cycles during the deposition, obtaining dopant concentration in the range between 0 and 25 at. %, as measured by XPS. The structural and optical properties are analyzed by means of XRD and Fourier transform spectroscopy, respectively. A direct correlation is found between the doping concentration and the optical bandgap. To elucidate the effect on the energy band structure, XPS investigation was used to extract the valence band maximum and conduction band minimum positions, showing significant shifts with the doping concentration. These changes are further discussed in the context of the ZnMgO employment as buffer layer in an all-oxide heterojunction solar cell.

Resume : The prediction of electronic structure, doping, defects, and interface properties based on first principles calculations becomes increasingly instrumental for the design and discovery of oxide semiconductors for a wide range of applications from transparent electronics to solar energy conversion. Relative recent developments in electronic structure calculations have enabled much more quantitative predictions than in the past. A few examples from our recent research activities illustrate these developments, including the multivalency of group 14 elements in SnO2 [1], defect phase diagram in Ga2O3, and design of MnO-ZnO alloys for solar water splitting applications [2]. [1] H. Peng, J.D. Perkins, S. Lany, Chem. Mater. 26, 4876 (2014). [2] H. Peng, P. Ndione, D.S. Ginley, A. Zakutayev, S. Lany, Phys. Rev. X 5, 021016 (2015).

Resume : The most commercially successful transparent conducting oxide (TCO) so far is tin doped indium oxide (Indium Tin Oxide – ITO), which has become the industrial standard TCO for many optoelectronics applications; the ITO market share was 93% in 2013. Its widespread use stems from the fact that lower resistivities have been achieved in ITO than in any other TCO; resistivities in ITO have reached as low as 7.2 × 10-5Ω cm, while retaining >90% visible transparency. In recent years, the demand for ITO has increased considerably, mainly due to the continuing replacement of cathode ray tube technology with flat screen displays. However, indium is quite a rare metal, having an abundance in the Earth’s crust of only 160 ppb by weight, compared with abundances for Zn and Sn of 79000 ppb and 2200 ppb respectively, and is often found in unstable geopolitical areas. The overwhelming demand for ITO has led to large fluctuations in the cost of indium over the past decade. There has thus been a drive in recent years to develop reduced-indium and indium-free materials which can replace ITO as the dominant industrial TCO. Recent research has therefore focused both on developing alternative TCOs, such as SnO2, ZnO, and BaSnO3, which are all more abundant and less expensive. In this presentation I will outline how the electronic structure of La-doped BaSnO3 represents a nearly ideal case for high mobility n-type TCO materials, and represents a new paradigm in the field.

Resume : Transparent conducting oxides (TCOs) are critical to many technologies (e.g., thin-film solar cells, flat-panel displays or organic light-emitting diodes). TCOs are heavily doped (\emph{n} or \emph{p}-type) oxides that satisfy many design criteria such as high transparency to visible light (i.e., a band gap $>$ 3 eV), high concentration and mobility of carriers (leading to high conductivity), ... In such (highly doped) systems, optical transitions from the conduction band minimum to higher energy bands in \emph{n}-type or from lower energy bands to the valence band maximum in \emph{p}-type are possible and can degrade transparency. In fact, it has been claimed that a high energy ($>$ 3eV) for any of these transitions made possible by doping, commonly referred as a high ``second gap'', is a necessary design criterion for high performance TCOs. Here, we study the influence of this second gap on the transparency of doped TCOs by using \emph{ab initio} calculations within the random phase approximation (RPA) for several well-known \emph{p}-type and \emph{n}-type TCOs. Our work highlights how the second gap affects the transparency of doped TCOs, shining light on more accurate design criteria for high performance TCOs.

Resume : Despite a tremendous technological appeal of amorphous oxide semiconductors (AOSs) and a large body of experimental and theoretical publications in the area, the structure-property relationships in AOSs have not been fully understood. Complex behavior associated with the crystalline-to-amorphous transition arises from factors like (i) size and distribution of nanocrystalline inclusions; (ii) spatial distribution and clustering of incorporated cations in multicomponent oxides; (iii) formation of scattering and trap defects; and (iv) internal strains that lead to piezoelectric effects. In this work, ab-initio molecular dynamics and accurate density-functional approach are employed to understand how the electronic, optical, and mechanical properties of amorphous ternary and quaternary oxides depend on quench rates, cation compositions, and oxygen stoichiometries. The theoretical results, combined with thorough experimental characterization, reveal the importance of medium-range structural characteristics, i.e., how the Metal-Oxygen polyhedra are integrated into a continuous network. The proposed models of AOSs help explain the observed peculiar transport properties (carrier mobility; thermal conductivity) and suggest ways to broaden the AOSs phase space toward materials with tunable properties and novel functionalities.

Resume : Amorphous indium gallium zinc oxide (a-IGZO) is in focus because of its high mobility values (10-60 cm2/V-sec) when compared to amorphous silicon a:Si-H that exhibits only modest mobility values (1-2 cm2/V-sec). As a result, this system is being studied widely and defects and dopants in this system are being extensively investigated. However, because of structural disorder in an amorphous network it is difficult to identify the equivalent defect and doping sites. To confront this problem, a method of identifying similar polyhedral sites is proposed in this first principles based study. A large number of samples (ten) of a-IGZO were prepared by ab initio melt-and-quench molecular dynamics with different initial conditions and cooling cycles to sample the amorphous landscape at distinct locations. Using a method of comparing bond angles between metal and oxygen atoms the amorphous polyhedra were matched to the polyhedral motifs present in the related crystalline systems, such as, InGaZnO4, In2O3, Ga2O3 and ZnO,. Subsequently, we find that a-IGZO is mainly composed of the following polyhedra: a tetrahedron of space group 199 and an octahedron of space group 206 of In2O3; a tetrahedron of space group 12 and an octahedron of space group 167 of Ga2O3; a tetrahedron of space group 186 of ZnO; zinc and gallium trigonal bipyramids of c-IGZO; and one zinc 4-fold, one zinc 5-fold and one indium 5-fold coordination polyhedra that occur only in the amorphous phase. Thus, instead of 360 distinct polyhedra the a-IGZO network can be described with only 10 groups of polyhedra. Hence, we believe we have identified independent doping sites in a-IGZO.

Resume : Amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) is a promising material for active channels in thin film transistors. In addition to be processable directly on plastic substrate, it offers better electronic and mechanical characteristics than amorphous silicon, which is wildly used for large area application such as in displays. Nevertheless, long time bias stress of a-IGZO based transistor can lead to large shifts of the transfer curve (Ids-Vgs curve). This instability is further enhanced in presence of blue or near UV light and is problematic for the operation of complex circuitry. Its origin is often associated to the presence of oxygen vacancies and the phenomena arising upon illumination are assumed to have the same origin. Based on first-principles simulations and experimental results, we will argue in this work that these are two separated phenomena and propose a new explanation for the negative bias stress instability. Negative bias stress in presence of light will also be discussed. We will argue that this phenomena is incompatible with the nature of isolated oxygen vacancies in a-IGZO.

Resume : The influences of atomic defects and structural disorder on thermodynamic and electronic properties of amorphous indium based (IGZO, ITO, IZO), indium-free (ZTO) and related oxides are investigated by density-functional-theory (DFT) calculations with supercell models containing atomic and extended defects. For vacancies, cation dopants substituting Zn or Sn, and anion dopants substituting O as atomic defects, defect levels in the electronic band structures are analyzed in terms of densities of states, which are calculated by means of the LDA with a self-interaction-correction (SIC). The important outcome of this study is detailed microscopic information on how much positions and shapes of electronic defect levels can be altered in amorphous structures with respect to doped single crystals. Based on our extensive set of DFT results for In-, Sn-, Zn- based oxides we develop a general concept of the subgap states which is applicable to these systems. Electronic defect levels in the lower half of the band gap are created by undercoordinated oxygen atoms while local oxygen deficiencies cause defect levels in the upper part of the band gap. W. Körner, D. F. Urban, und C. Elsässer, Origin of subgap states in amorphous In-Ga-Zn-O, J. Appl. Phys. 114, 163704 (2013). W. Körner and C. Elsässer, DFT study of stability and subgap states of crystalline and amorphous Zn-Sn-O, Thin Solid Films 555, 81-86 (2014). W. Körner, D. F. Urban and C. Elsässer, Generic origin of subgap states in transparent amorphous semiconductor oxides illustrated for the cases of In-Zn-O and In-Sn-O Phys. Status Solidi A 212, 1476–1481 (2015)

Resume : Flame aerosol technology dominates the manufacture of nanostructured materials at tons/h, albeit rather simple ones (carbon blacks, fumed silica, alumina, titania etc.). Recent advances in combustion and aerosol science, including cluster dynamics, extend now the use of this undisputably scalable technology to synthesis of far more sophisticated materials and even devices. Here I focus on portable gas sensors for breath analysis1. So capitalizing on the capacity of capturing stably nanostrusctured metastable phases and solid solutions by combustion, flame aerosol deposition and in-situ annealing of gas sensor films is presented as it had led, first, to optimally-doped SnO2 for ethanol and CO sensing and, most notably to epsilon-WO3 for selective detection of acetone (a diabetes tracer in the breath) at the ppb level & 90% RH. In tandem with PTR-MS, such a portable breath sensor was used for online and offline testing of humans benchmarked with standard glucose tests on humans (finger pricking) 2. Last year a prototype was assembled by industry for clinical testing3 while new MoO3 –based sensors are developed for kidney disease monitoring4. Emphasis now is placed in development of an E-nose with focus on formaldehyde (FA) as it is a potential breath marker for lung cancer and a tracer for indoor air quality monitoring. Typical FA concentrations are below 100 ppb posing a sensitivity and selectivity challenge to current portable sensor systems. Here, I present a highly sensitive, selective and compact electronic nose (E-nose) for real-time quantification of FA in realistic gas mixtures. This E-nose consists of four nanostructured and highly porous Pt-, Si-, Pd- and Ti-doped SnO2 sensing films directly deposited onto silicon wafer-based microsubstrates by flame spray pyrolysis (FSP). The constituent sensors offer stable responses and detection of FA down to 5 ppb (signal-to-noise ratio > 30) at breath-realistic 90% relative humidity. Each dopant induces different analyte selectivity enabling selective detection of FA in gas mixtures by multivariate linear regression. In simulated breath (FA with higher acetone, NH3 and ethanol concentrations), FA is detected with an average error ≤ 9 ppb using the present E-nose and overcoming selectivity issues of single sensors. This device could facilitate easy screening of lung cancer patients and monitoring of indoor FA concentrations. . 1. Aerosol-based Technologies in Nanoscale Manufacturing: from Functional Materials to Devices through Core Chemical Engineering, AIChE J., 56, 3028-3035 (2010). 2. M. Righettoni, A. Schmidt, A. Amman, S.E. Pratsinis, “Correlations between blood glucose & breath components from portable gas sensors and PTR-TOF-MS”, J. Breath Res., 7, 037110 (2013). 3. M. Righettoni, A. Ragnoni, A.T. Güntner, C. Loccioni, S.E. Pratsinis, T.H. Risby, "Monitoring breath markers under controlled conditions", J Breath Res., 9, 047101 (2015). 4. A.T. Güntner, M. Righettoni, S.E. Pratsinis, "Selective sensing of NH3 by Si-doped α-MoO3 for breath analysis", Sensors and Actuators B: Chemical, 223, 266–273 (2016).

Resume : ZnO nanowires are promising in a wide range of nanoscale devices for future applications in photocatalysis, solar cells, optical devices and chemical sensing. ZnO features a 3.37 eV band gap and a 60 meV exciton binding energy at room temperature. Thanks to its large exciton emission in the UV and lower Auger recombination (i.e. lower efficiency droop at high current density), ZnO represents a promising indium-free alternative to III-nitride light emitting diodes. Moreover, since ZnO is historically used as gas sensing materials, ZnO nanowires find also application into either optical or electrical gas sensing devices. The single crystal nature of ZnO nanowires can be exploited to ensure great stability as well as high sensitivity of the device. We growth ZnO nanowires by vapour phase technique in a tubular furnace. For application into different fields, is of utmost importance the study of the growth habit of the material which is strongly dependent on the substrate: growth on GaN , and alumina substrate by vapour phase growth will be reviewed. Application of ZnO nanowires as chemical sensors exploiting novel architectures (single nanowires, optical gas sensors, hybrid structure) for environmental monitoring will be reviewed as well as other application like food safety by a novel Electronic Nose. Acknowledgements The research leading to these results has received funding from the European Community’s FP7-ICT-2013-10, MSP— Multi-Sensor-Platform for Smart Building Manage

Resume : Zinc and Tin oxide is a promising In and Ga-free transparent semiconductors that demonstrated great potential as transparent conducting oxides in application like photovoltaic devices, flat panel displays, solar cells, due to its high electron mobility, high electrical conductivity, and low visible absorption [1]. Zn2SnO4 can also be applied to the field of gas sensing, if the resistance of the material is appropriate, that is not too conducting. We prepared Zn2SnO4 thin films by RF sputtering starting from sintered ceramic target with Zn:Sn ratio 2:1, investigating the influence of deposition parameters (oxygen content and substrate temperature) on the thin film properties. Amorphous films are obtained for deposition temperature up to 400°C, but they become crystalline (Zn2SnO4) after proper annealing treatment at temperatures higher than 600°C. Annealing in air and in Ar resulted in porous film that are suitable for gas sensing. All films are transparent before and after annealing. We investigated optical properties of thin films by UV-VIS, Raman and photoluminescence spectroscopy and functional properties of thin films for gas sensing applications. The material demonstrated to have high potential for ethanol sensing of at 400°C and acetone at 300°C. Acknowledgements The research leading to these results has received funding from the European Communities 7th Framework Programme under grant agreement NMP3-LA-2010-246334 ORAMA (Oxide Materials Towards a Matured Post-silicon Electronics Era). References [1] Coutts TJ, Young DL, Li X, Mulligan WP, Wu X: Search for improved transparent conducting oxides: a fundamental investigation of CdO, Cd2SnO4, and Zn2SnO4. J Vac Sci Technol 2000, A 18:2646–2660

Resume : The improvement of solar cell performance in the near-infrared (IR) region is of great importance to achieve ultrahigh efficiency solar cells. In this respect, colloidal quantum dots (CQDs) are promising absorbers for the solar cells because the absorption band can be tuned so as to cover a wider range of solar spectrum. Wide band gap semiconductors such as ZnO also play important roles as electron transporter and selective contact for next-generation solar cells. Moreover, crystalline ZnO nanostructures can be formed in various shape and size by a hydrothermal synthesis method. Widely studied CQD-based solar cells (planar-type) have a typical structure of CQD layer deposited on top of dense ZnO or TiO2 layer. Although the power conversion efficiency has been steadily increasing, there are still many issues to be addressed to enhance the performance. Transport limitation in the CQD layer is an issue associated with short carrier diffusion length. We have then focused on PbS CQD/ZnO nanowire (NW) structures (NW-type), aiming at simultaneous enhancement in carrier transport and light harvesting (LH) efficiency. The morphology of ZnO NW arrays was systematically investigated. The NW-type solar cell with densely-grown thin ZnO NWs about 1.2 microns long was found to yield a maximum EQE of ca 60% in the near-IR region (at 1020 nm) and over 80% in the visible region. Our recent study with modulation spectroscopy revealed that NW-type solar cells give a carrier diffusion length of over 1 micron, which is about one order of magnitude longer than that of the planar-type one (J. Phys. Chem., 119, 27265 (2015)). Implementation of spatially separated carrier pathways is a strategy for high efficiency CQD-based solar cells. Our light management approaches to boost LH in the near-IR region are also reported.

Resume : Inorganic halide based perovskite solar cells have recently attracted significant attention due to their excellent absorption properties across the visible and infrared electromagnetic spectrum, close to ideal band gap and resultant power conversion efficiencies. However, they are not without problems, issues with reliability, lead content and difficulties in scale up still require significant effort. In this paper we focus on optimization of the TCO layers where low temperature (< 200°C) is required to prevent damage to the under laying active layer. Using atomic layer deposition (ALD) we have studied amorphous zinc oxide where aluminum, titanium and hafnium were employed as dopants (1-5 at %) using an ALD super cycle to generate nano-laminate type doping profiles. The material properties are discussed with respect to solar cell performance relative to traditional fluorine doped tin oxide (FTO) based structures.

Resume : SnO2 as a standard TCO material have recently attracted special attention, thanks to the particular dual valency of tin and to the reversible transformation of the material that can occurs from p-type SnO to n-type SnO2 with excellent transport properties. Besides, SnO2 can be functionalized with Rare Earth elements (RE), which gives rise to new properties. In the field of solar cells, this class of materials can be used as conversion layer to adapt the incident solar spectrum to the solar cell absorption, reducing the losses due to carriers’ thermalization. In this work we present properties of SnO2 films doped with Yb (1.30 at.%) and synthesized by reactive magnetron sputtering. The films were elaborated at 100°C for the sake of compatibility with low temperature used for devices fabrication. The XRD analysis showed structural change in the polycrystalline SnO2 films as a function of the oxygen gas flow during elaboration. We gain better insight into the oxides proportions through chemical analysis by XPS spectroscopy. Yb ions are well inserted in the structure and possess the 3+ valence state. Analysis by PL spectroscopy shows that under UV excitation of 325 nm, the SnO2: Yb films exhibit a wide and intense emission lines in the infrared region characteristic of Yb3+ ions. The Yb-related PL was correlated to the SnO2 phase. Thanks to PLE measurements, an efficient energy transfer from the SnOx host matrix to the Yb rare earth has been identified. The films exhibit transparency laying between 80-90% with excellent transport properties, resistivities as low as 0,006 Ohm.cm and mobilities as high as 50 cm2/V.s were measured. Such optical and electrical results are of potential interest to solar cells using Yb doped SnO2 films as TCO and photon down shifter.

Resume : Photoelectrochemical energy conversion based on the natural systems such as enzymes or photosystems is an attractive and intensively explored concept, which could benefit from the evolution-optimized conversion efficiency of the biological units. The development of bioelectronic devices requires connecting the biological systems in an optimized way to the electrodes and understanding the fundamental issues in the context of their interfacing. We have developed transparent electrodes with custom designed porous 3D-electrode architectures enabling incorporation of large amounts of biomolecules and featuring optical transparency allowing interaction with light. Our approach is based on the assembly of nanoparticles of transparent conducting oxides such as antimony-doped tin oxide (ATO) and indium tin oxide (ITO) directed by different templates for porosity such as novel amphiphilic polymers (PEO-b-PHA) or PMMA beads [1-2]. The high crystallinity of the nanoparticles serving as building blocks enables the formation of fully crystalline porous transparent scaffolds with high electrical conductivity and accessible porosity, which can incorporate various redox moieties ranging from small redox proteins to large protein complexes and show greatly enhanced electrochemical response proportional to the electrode surface area [3-4]. References [1] K. Peters, P. Zeller, G. Stefanic, V. Skoromets, H. N?mec, P. Ku?el, D. Fattakhova-Rohlfing, Chem. Mater. 2015, 27, 1090?1099. [2] Liu, Y.; Peters, K.; Mandlmeier, B.; Müller, A.; Fominykh, K.; Rathousky, J.; Scheu, C.; Fattakhova-Rohlfing, D. Electrochim. Acta 2014, 140, 108. [3] Sarauli, D.; Peters, K.; Xu, C.; Schulz, B.; Fattakhova-Rohlfing, D. and Lisdat, F. ACS Appl. Mater. & Interfaces 2014, 6, 17887?17893. [4] Sarauli, D.; Wettstein, C.; Peters, K.; Schulz, B.; Fattakhova-Rohlfing, D.; Lisdat, F. ACS Catal. 2015, 5, 2081?2087.

Resume : We have succeeded in glow discharging of nitrogen plasma generated near atmospheric pressure with high electron temperature above 4000 K and low gas temperature below 400 K using dielectric barrier discharge. The self-limited nitridation of Si at around 1.8nm was recognized at room temperature by using this nitrogen nonequilibrium plasma in which the emission corresponding to N2 2nd positive system was dominantly observed. The leakage current of the nitride film is lower than two digits than that nitrided using RF plasma in which atomic nitrogen active species are dominantly observed. By introducing only 1ppb O2 gas, oxidation is recognized. We have studied the effect of active species on the growth of oxide films such as ZnO and Ga2O3 in nitrogen nonequilibrium plasma generated near atmospheric pressure using home-made chemical vapor deposition systems. Highly resistive ZnO epitaxial films were fabricated even at 200 ºC. The residual donor concentration was recognized to be below 1x1013 cm-3 that should be quite useful for the p-type ZnO and piezoelectric applications. The reaction mechanisms occurred at the surface of oxide films such as ZnO and Ga2O3 and the advantages on the decrease in the residual donner concentration and N2 doping are discussed as well.

Resume : Vanadium-dioxide-based thermochromic thin films and nanoparticle composites can show significant luminous transmittance and let through more solar energy below a "critical" temperature than above this temperature. It has been realized for decades that these properties are of interest, in principle, for windows in energy-efficient buildings, but the technology has meen slow to mature. This paper summarizes the state-of-the-art for thermochromic glazings and emphasizes that many advances have been made recently. Specifically, this paper treats the important roles of doping and nanostructuring of vanadium dioxide. New deposition techniques to make superior vanadium dioxide films are discussed. Finally, very recent results are presented on thermochromic light scattering.

Resume : The development of flexible substrates that are based on organic polymers is of critical importance to electrochromics, display and photovoltaic applications. Flexible substrates offer the advantages of mechanical flexibility, design freedom, optical transparency, lightweight, and cost-effectiveness. A number of inorganic functional coatings on polymers have critical properties such as control of transmittance, electronic conductance and water vapor permeability which are required for electrochromic, display and photovoltaic applications. In this study WO3 and transparent conductive oxide (TCO) hybrid films were deposited on polymer substrates by E-beam evaporation for WO3 and Pulsed DC magnetron reactive sputtering method for TCO. The influences of oxygen partial pressure on the microstructure, surface roughness and electrical properties of this hybrid coating films were studied. The high quality WO3/TCO films were deposited at RT with a resistivity of 2.5x10E-4Ωcm and the transmissivity of batter than 90%. Change of structural properties according to the deposition rate and plasma conditions was also observed with XRD, TEM, and XPS. Very flat surface roughness could be obtained at RT, while surface roughness of the films was increased due to the formation of grains over than 100℃. The mechanical failure of WO3/TCO coatings on flexible polymers is a serious issue for the applications of flexible electrochromic devices. The formation of defects, such as cracks and debonding in WO3/TCO films may be inevitable in situations in which bend geometries are required, because of the large difference between the elastic properties of TCO and polymers. The initiation and subsequent developments of defects mainly depend on the coating material, thickness, and interfacial adhesion. Thus, it is important to have a clear understanding of the failure behavior of coatings in the case of bend geometries, in order to prevent the destruction of devices in which they are used. Flexibility of WO3/TCO films was measured as functions of the bending radius and bending cycle in bend geometries. And we carry out the electrochromic performances such as a device life time, response time and coloring status for this hybrid coating layers. It will be considered that these experimental results can be applied to the WO3/TCO substrate of flexible electrochromic devices.

Resume : A new worldwide challenge consists in the development of new renewable energy sources for powering micro-devices, such as sensors or nomad electronics. Thanks to their specific properties,piezoelectric nanowires (NWs)are excellent candidatesto fabricate a new generation of piezogenerators. From now few years, III-NitrideNWs are explored due to their higher piezoelectric response with respect to other piezoelectric semiconductor materials. This is especially true if we compare the piezoelectric potential of nitrides with that of ZnO, which is today the most widely used semiconductor in the NW-based piezogenerators. We demonstrate the first piezogenerator integrating a vertical array of GaN NWsgrown by molecular beam epitaxy on Si(111) substrate.Our approach is based on multi-scale analyses going from single wire properties to macroscopic device fabrication and characterization. The piezo-conversion of individual GaN NWs was assessed by atomic force microscopy equipped with a modified Resiscope module, yielding an average output voltage of 228 ± 120 mV and a maximum value of 350 mV, generated per NW. The fine understanding of the piezo-generation mechanism in the NWs gained from AFM analyses was applied to design thepiezogenerator.Our prototype delivers a power density of 12.7 mW/cm3 for a large number of cycles. This result defines the new state of the art and offers promising prospects for the development of high-efficiency and ultra-compact piezo-generators based on GaN NWs.

Resume : Oxide semiconductors provide a strong materials basis for environmental technologies. For example, indium oxide (In2O3) and gallium oxide (Ga2O3) are well-known materials for conductometric gas sensors, and recently, Ga2O3 has been recognized as a promising material for new generation power electronics and deep UV detectors. While gas sensors have been traditionally realized with polycrystalline material, single crystalline material is required for power electronics and will be shown to facilitate identification of the gas sensing mechanism. Defined surface orientation can help optimizing gas sensing and defined doping is required for electronics and will also be shown to improve gas sensing. A well accepted method to prepare high-quality, single crystalline semiconductor layers is molecular beam epitaxy (MBE). In the first part, this presentation will discuss two fundamental aspects of oxide MBE: At first, the role of parasitic sub oxide formation and it’s desorption will be discussed with focus on Ga2O3. Secondly, the control of the surface morphology will be demonstrated for In2O3 by modifying the facets’ surface free energy through flux stoichiometry. In the second part, the fundamentals of In2O3-based ozone sensors will be discussed using MBE-grown layers with well defined surface orientation as model systems with reduced complexity. The pivotal role of the surface electron accumulation layer is shown, its transport properties are estimated, and bulk doping with the deep acceptor Mg is shown to improve sensitivity.

Resume : We have studied the influence of 1 nm thin Al2O3 layers grown by atomic layer deposition onto magnetron sputtered Sn-doped In2O3 (ITO) thin films. The film thickness of the ITO layers was varied from 10-200 nm and substrate temperatures during deposition from room temperature to 400°C. The samples were analysed using X-ray photoelectron spectroscopy, 4-point conductivity and Hall effect measurements. In addition, 4-point conductivity measurements in Ar atmosphere during ramping temperature from room temperature up to 500°C and subsequently cooling down to room temperature was conducted. In these experiments samples with and without Al2O3 were measured simultaneously. The Al2O3 deposition leads to a reduction of the ITO surfaces, which is accompanied by a significant increase of conductivity for film thickness up to 50 nm and substrate temperatures up to 200°C. ITO films deposited at room temperature show an increase of conductivity by up to a factor of 2 after Al2O3 deposition. For lower film thickness, the conductivity is comparable to that of films deposited at 400°C. In contrast to uncoated samples, the conductivity of low temperature films with Al2O3 coating increased after temperature ramping, which is associated to a blocking of oxygen incorporation by the Al2O3 film. Thin Al2O3 coatings therefore offer a new approach to manipulate and stabilize electrical conductivity of TCO thin films.

Resume : Transparent conductive oxide (TCO) films, such as Sn-doped In2O3 (ITO) and In2O3-ZnO (IZO) have been widely used as transparent electrodes for various applications including flat panel displays (FPDs), solar batteries and organic light-emitting diodes (OLEDs), because ITO and IZO films show both excellent electrical conductivity and high transparency. In recent years, there is a strong commercial demand for high quality transparent electrodes deposited on various polymer substrates for flexible devices. However, it is not so easy to deposit the high-quality ITO or IZO films on polymer substrate, because of the degassing, adsorbed water, low thermal stability or low surface energy of the polymer substrates [1]. We have reported that the reactive sputtering using the alloy target should be an effective approach to reduce the influence of adsorbed water at the substrate surface, because the oxidation states of the films can be controlled precisely in the wide range [2,3]. In this study, ITO and IZO films were deposited on polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) substrates to compare with those deposited at the quartz substrates by using reactive sputtering. The In-Sn (Sn: 10 wt.%) and In-Zn (Zn: 20 at.%) alloy targets were used for the deposition with the plasma emission feedback system. ITO and IZO films were successfully deposited in transition region by reactive sputtering. The lowest resistivity of ITO and IZO films on the glass were 4.5×10-4 [Ωcm] and 3.0×10-4 [Ωcm], respectively. These films showed over 80% of transmittance in visible region. [1] Y. Shigesato, R. Koshi-ishi, T. Kawashima, J. Ohsako, Vacuum 59 (2000) 614. [2] M. Kusayanagi, Yuzo Shigesato, et al., Thin Solid Films 555 (2014) 93. [3] N. Tsukamoto, Y. Shigesato, Thin Solid Films 559 (2014) 4.

Resume : Vanadium dioxide (VO2) undergoes the metal-insulator transition at around 340 K1). Because of different carriers of heat conduction between the metal and insulator phases, thermal conductivity of VO2 changes as the metal-insulator transition. In this study, we investigated the temperature dependence of thermal conductivity and electrical conductivity of VO2 thin films through the phase transition temperature, and discussed its thermal conduction mechanism. The VO2 films with the thickness of 300 nm were deposited on quartz glass substrates heated at 645 K by rf magnetron sputtering using a V2O3 target and Ar-O2 mixture gas2)3). All the films were confirmed as a single-phase of VO2 by electron beam diffraction patterns. At the elevated temperature, the electrical conductivity of the films rapidly increased from 6.0×103 Sm-1 to 1.9×105 Sm-1 at 325 ~ 340 K. To measure the thermal conductivity, we deposited the Mo(100 nm)/VO2(300 nm)/Mo(100 nm) three layered film. The sample was measured by a pulse light heating thermoreflectance technique. The thermal conductivity of the 300 nm-thick increased from 3.6 Wm-1K-1 at 336 K for the insulator phase to 5.4 Wm-1K-1 at 351 K for the metal phase. The thermal conductivity carried by free electrons (λel) was estimated by Wiedemann-Franz law using the measured electrical conductivity of VO2 film. As a result, the jump of measured thermal conductivity (=1.4 Wm-1K-1) is well agreement with Δλel (=1.5 Wm-1K-1), indicating the change of the thermal conductivity of VO2 film corresponds to that of the electrical conductivity. 1) F. Morin, Phys. Rev. Lett., 3, 34 (1959), 2) Y. Shigesato, et al., Jpn. J. Appl. Phys., 39, 6016 (2003), 3) K. Kato, Y. Shigesato, et al., Jpn. J. Appl. Phys., 42, 6523 (2003).

Resume : Gamma-phase CuI (γ-CuI) is a p-type semiconductor with a bandgap of 3.1 eV [1]. The highest hole mobility in a single crystal was reported to be ~44 cm² V^-1 s^-1 [2], which is very large for widegap p-type semiconductors. Thus, γ-CuI is expected to be a good p-type transparent semiconductor. γ-CuI thin films have been fabricated by reacting copper metal thin films with iodine vapor. However, those films usually have very rough surface and thus frosted-glass-like appearance. Namely, these films are not satisfactorily transparent, owing to the light scattering on the rough surface. We have developed a new method for the fabrication of truly transparent γ-CuI films. We found that highly transparent γ-CuI thin films with smooth surface can be fabricated by reacting copper nitride (Cu₃N) thin films with solid-phase iodine (solid iodination method): 2Cu₃N 3I₂(s) → 6CuI N₂(g). In this presentation, we show structural, electrical, and optical properties of γ-CuI thin films produced by the solid iodination method. Precursor Cu₃N thin films were reactively sputtered on unheated glass substrates. The deposition was carried out in a mixture gas of Ar and N₂ (Ar : N₂ = 1 : 1) at a working pressure of 2 Pa. The Cu₃N films were reacted with solid-phase iodine at 298 K in a nitrogen filled glove box. The chemical reaction proceeded immediately, and completed in 70 min. The dark-brown Cu₃N films were converted to transparent films. X-ray diffraction measurements confirmed that the resultant films were polycrystalline γ-CuI. No impurity phase was observed. From scanning electron microscope observations, we found that the films have smooth surface and were composed of crystalline grains with a size of a few hundreds nanometers. Rutherford backscattering spectrometry (RBS) was used to determine the film composition. The Cu/I atomic ratio was determined to be 1.0, indicating that stoichiometric γ-CuI films were obtained. Residual nitrogen in the γ-CuI films was not detected in the RBS spectra. These results suggested that high purity γ-CuI films can be obtained by the solid iodination method. The films had shiny transparent appearance, suggesting that the light scattering on the film surface was suppressed. Indeed, the internal transmittance of the films was higher than 90% in the visible region. The specular transmittance of our samples was higher than that of γ-CuI films produced via conventional method mentioned above, particularly in the wavelength region of 400-500 nm. The bandgap values were estimated to be 3.0 eV, which was consistent with the single crystal value [2]. The Seebeck and Hall coefficients of the γ-CuI films were always positive, indicating p-type conductivity. An as-prepared γ-CuI film had a resistivity (ρ) of 3.1 × 10^-2 Ω cm, hole density (n) of 8.6 × 10¹⁹ cm^-3, and mobility (μ) 2.4 cm² V^-1 s^-1 at room temperature. The large n value implied that the holes in the film were nearly degenerated. To reduce hole density, as-prepared films were annealed under inert atmosphere at various temperature ranging from 50 to 150 ºC. As the annealing temperature was increased up to 150 ºC, n decreased down to 9.2 × 10¹⁷ cm^-3 and μ increase up to 10 cm² V^-1 s^-1. The μ value of 10 cm² V^-1 s^-1 is comparable to that of a bulk single crystal [3] and comparably higher than those of p-type transparent oxide semiconductors fabricated at high temperatures. These results clearly indicated that the solid iodination-based method is a good method for the synthesis of high performance γ-CuI transparent p-type semiconducting film. In addition, this process does not need high temperature. This fact may lead to the fabrication of transparent p-type semiconducting films on flexible plastic substrates. REFERENCES [1] Grundmann et al. Phys. Status Solidi Appl. Mater. Sci. 2013, 210, 1671-1703. [2] Chen et al. Cryst. Growth Des. 2010, 10, 2057-2060. [3] Lv et al. CrystEngComm 2015, 17, 862-867.

Resume : Homologous In2O3(ZnO)m (m=integer) compounds have received much attention as the high temperature thermoelectric conversion material because of their structural and chemical stability at high temperature. The present work is to investigate the defect formation mechanism of homologous In2O3(ZnO)m at the high temperature (>673K). Using in-situ Hall effect measurement, we measured the electrical properties of the sputtered epitaxial In2O3(ZnO)m film at different oxygen partial pressure (pO2). The results yielded a power law dependence of conductivity versus pO2 of -1/4 and -2/5 at 873 K and 673 K, respectively. This cannot be solely explained by the doubly-charged oxygen vacancies (O¨), which is the dominant defect species in amorphous In2O3-ZnO film [1, 2]. The Zn k-edge EXAFS (extended X-ray absorption fine structure) measurement results showed an increase in peak intensity of Zn-O and Zn-Zn spectra after in-situ Hall effect measurement, which implied the existence of zinc and oxygen vacancy in the sputtered epitaxial In2O3(ZnO)m film. Whereas, Zn k-edge EXAFS data cannot provide enough evidence on the formation of In anti-site defect, which consists of an In atom occupying a Zn lattice site. Thus, it is considered that both Zn and oxygen vacancy may act as important defect species at high temperature in the In2O3(ZnO)m film. [1] J. Jia, N. Oka, Y. Shigesato, J. Appl. Phys. 113, 163702 (2013). [2] S. Moffitt, A. Adler, T. Gennett, D. Ginley, J. Perkins, and T. Mason, J. Am. Ceram. Soc., 98, 2099, (2015).

Resume : We investigated characteristic of Ti-doped In2O3 (TIO) films grown by direct current (DC) sputtering to use as transparent anode for heterojunction organic solar cells (OSCs). The electrical, optical, structural and morphological properties of the TIO films were comprehensively investigated as a function of Ti composition in the TIO targets and film thickness. The in-situ substrate heating during DC sputtering led to an effective activation of Ti dopants and crystallization of the TiO films. At optimized conditions, the TIO film showed a resistivity of 2.4 × 10-4 Ohm-cm and optical transmittance of 85 % comparable to crystalline ITO film. In particular, the TIO films showed a high near infrared transparency due to effect of the Ti dopant with high Lewis acid strength (3.06). The heterojunction OSCs fabricated on TIO anode exhibited a good cell-performance with a fill factor of 62.53 %, a short circuit current of 8.785 mA/cm2, an open circuit voltage of 0.607 V, and a power conversion efficiency of 3.333 %, which are comparable to OSCs with ITO anodes (PCE 3.382%).

Resume : Sn-doped In2O3 (ITO) is known as one of the most commonly used transparent conducting oxide materials for various applications such as flat panel display, solar cell, etc due to its high transparency and superior electrical conductivity. However, it is also well known that the oxygen in oxygen interstitial position, Oi (16c position in base-centered lattice), of ITO films plays important role in its electrical properties. Previously, Frank et al suggested the pO2 and Sn doping content can determine the carrier concentration variation in ITO using defect model [1]. Furthermore, G. B. González et al. showed experimentally the detailed structure of pO2 varied ITO using EXAFS [2]. In this presentation, we will explain the chemical state of post-annealed ITO films to examine the correlation between defect and electrical properties based on defect model. DC sputtered ITO films were annealed in various environments such as O2, in-Air, N2, Ar, Vacuum at 400 oC for 10 min. The electrical conductivity and carrier concentration reduced for the ITO film annealed under oxygen environments (for example, O2 and in-Air). On the other hand, the electrical conductivity and carrier concentration improved for films annealed under less oxygen environments such as N2, Ar, Vacuum indicating the existence of correlation between electrical properties and oxygen contents in the film. Interestingly, the mobility of ITO film increased after post annealing regardless of annealing environments. In particular, for the film annealed under O2 environment exhibits the highest mobility (~ 56 cm2/Vs) because of the lower carrier concentration. The spectroscopic measurement also confirmed that oxygen vacancies and Sn4+ contents were decreased with additional oxygen content in the film. Therefore, our observation suggested that the oxygen contents in ITO is key parameter to control the electrical properties of the film. This research is supported in part by NRF Korea (NRF-2015R1D1A1A01058672), Korea Basic Science Institute (E35800) and Korea Atomic Energy Research Institute. Also, J. W. Kim is supported by Global PhD Fellowship Program through the NRF Korea funded by the Ministry of Education (2015H1A2A1034200).

Resume : The top gate typed transparent indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) have been demonstrated with high performance of high field effect mobility and high reliability using sputtering method. The most common bottom gate typed IGZO TFT structure with SiO2 as gate insulator, has the difficulty of both side transparency and low device performance due to the low electrical properties of SiO2 and device structure. The key issue of top gate typed IGZO TFT is the application of robust and stable gate insulator. However, the sputtered SiO2 has the low electrical properties and SiO2 grown by the other method causes to the complex fabrication of TFT structure. In this presentation, all of fabrication process of top gate typed IGZO TFT, such as electrodes, active IGZO layer, and gate insulator, has been used by sputtering method. High dielectric oxides as gate insulator are one of the effective solution to reduce leakage current and improve on/off current ratio. Sputtered HfO2 and Al2O3 were applied as gate insulator and their TFT performance was compared to the top and bottom gate typed device using gate insulator of SiO2. The device reliability under the bias, depending on gate insulator, was compared as well. Finally, based on the electronic structure, the effects of gate insulator with SiO2, Al2O3, and HfO2 on the performance and reliability in IGZO TFTs are evaluated.

Resume : Amorphous-InGaZnO (a-IGZO) thin films were deposited using by a radio frequency (RF) sputtering system as a function of oxygen partial pressure. In order to modulate the electrical properties, such as carrier concentration and mobility of a-IGZO films, the hydrogen plasma treatment applied to as-deposited a-IGZO films. The resistivity of a-IGZO films dramatically improved after the hydrogen plasma treatment by the increase of carrier concentration. We investigated the origins of change of electrical properties using the hydrogen plasma treatment, in terms of the electronic structure including chemical bonding states, band alignment and band edge states below the conduction band. Comparing to as-deposited IGZO film, the film with the hydrogen plasma treatment represents the increase of oxygen deficient bonding states, the decrease of conduction band offset, and the increase of band edge states below the conduction band.

Resume : Tungsten (W) doped InZnO thin film transistors (WIZO-TFTs) were fabricated by co-sputtering of WO3 and InZnO (1:1 at %) sputtering target using a radio frequency (RF) sputtering system as a function of W doping concentrations. In order to explain the improvement in device instability characteristics of WIZO TFTs depending on W doping concentration, we investigated the correlations between the device performance and electronic structure such as chemical bonding states, band alignment and band edge states below the conduction band. W-doped at ~4 at % showed the lowest threshold voltage shift (ΔVth), which resulted in changes of oxygen deficient bonding states and band edge stats below conduction band.

Resume : High infrared (IR) transmission TCOs have the potential to provide a means for significantly increasing the efficiency of tandem photovoltaic devices by allowing a greater proportion of the IR spectrum to reach the lower junction. Previous work by the US National Renewable Energy Laboratory showed that doping indium oxide with titanium resulted in the formation of a high IR transmission material. The rising cost of indium means that finding alternatives is increasingly important. This study looks at the effects of doping zinc oxide with titanium to produce a high IR transmission material using a combinatorial methodology. Films were deposited through a mask at room temperature by co-sputtering. Substrate rotation was disabled to give a dopant concentration gradient across the sample. The optimum dopant concentration was determined by using X-ray photoelectron spectroscopy to measure the sample location with the best combination of low resistivity and high IR transmission. A set of non-combinatorial samples were deposited using the identified dopant concentration at a range of temperatures to determine the impact of temperature on the optical and electrical properties. The optimum dopant concentration was found to be 1.5 at. % titanium, which gave a resistivity of 1.26x10-3Ω.cm and an average transmission of 80% over the measured IR spectrum (from 1500nm to 950nm). The average transmission in the visible range was 75%.

Resume : In optoelectronic devices, such as light-emitting diodes (LED), the challenge is to find an acceptable trade-off between the need to increase the light emitting/absorbing area and the need for low series resistance of the metal contact grid. ZnO heavily doped with Gallium (GZO) is becoming a very attractive candidate for future generation TCOs. The optical and electrical behaviors of GZO thin layers are strongly dependent on the structural properties (thickness, density, roughness and elemental depth profiles). In this work we have investigated the physical properties of a series of Ga-doped ZnO thin films. Considering samples with different thicknesses we compare the physical properties of as-deposited samples and annealed samples. The transport properties, performed by four probes (ρ<1µΩ.cm-1) and Hall effect measurements, confirm the potential use of GZO layer as a candidate in optoelectronic devices. We carried out wide spectral range (far-UV to near-IR) spectroscopic ellipsometry measurements and an in-situ analysis reveals that the annealing process affects the optical parameters in the near-IR region corresponding to a variation of the conduction properties. Furthermore, we observe a significant difference in physical properties while increasing the thickness of the TCO layer. In order to understand this evolution we use a recently developed technique, the XRR-GIXRF combined analysis, which gives new insight on the elemental depth profile of the Ga along the thickness and helps to interpret the observed physical properties.

Resume : In this study, we investigate the potential for excimer laser annealing ELA using KrF (248 nm) excimer laser to enhance the electrical characteristics of IGZO TFTs. First, a-IGZO thin films were deposited by a radio frequency RF magnetron sputtering system at room temperature and various growth parameters including applied RF power, O2 in Ar %, working pressure. Then, the IGZO films were ELA-treated at room temperature in air. 1 pulse was applied at moderate 50 to 125 mJ/cm2. The electrical conductivity of IGZO is strongly dependent on the deposition parameters. ELA is very effective to tune the resistivity of IGZO samples over a wide range, while maintained the amorphous structure. Thereafter, IGZO-TFT devices were fabricated on Si and PEN substrates. ELA with one pulse at rather low energy densities 15 – 60 mJ/cm2 was applied to the TFTs at room temperature in air. The ELA-treated TFTs demonstrated enhanced electrical characteristics such as field-effect mobility, on/off ratio, and sub-threshold swing compared to as-deposited TFTs. This improvement could be attributed to reducing the microstructural defects and tuning electrical properties of the IGZO channel layer, as well as reducing the contact resistance between IGZO and the source/drain electrodes after processing.

Resume : In the last few years, TiO2 nanopaticles were used in a large range of applications due to their multifunctional properties such as photocatalytic activity, photoelectric and photochemical properties. This paper reports the self-cleaning, self-sterilizing and anti-fogging applications of TiO2 nanostructures used for civil constructions. We present the preparation of nanostructured titanium dioxide, by an acid-catalyzed sol–gel technique using titanium butoxide (Ti(OBu)4) as precursor, which was hydrolyzed with 2-propanol (C3H8O) and distilled water under the catalytic effect of nitric acid (HNO3). The sample was sintering at temperature of 400 °C for 4h, in order to obtain TiO2 powder as material with photocatalytic and antibacterial properties.The structure and morphology of TiO2 nanopaticles were analyzed. Phase identification was performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fourier Transform Infrared (FTIR) and Raman Spectroscopy were used to study the bond configuration of titanium dioxide obtained by sol-gel process. Also the analysis of morphology and microstructure of the sintered powder, was performed by field emission scanning electron microscopy (FE-SEM).

Resume : Transparent conducting oxides (TCOs) have a wide variety of applications in optoelectronics, including photovoltaic cells and flat panel displays. TCO materials have a band gap of greater than 3 eV, thus can transmit visible light, and conductivity greater than 1000 S cm^–1, with carrier concentrations in the region of 10^20-10^21 cm^–3. Currently only n-type materials exhibit high electronic conductivity and optical transparency required for use in these devices. Doped LaCrO3 (LCO) has recently been suggested as a potential p-type TCO. Sr doping is reported to dope holes into the top of the valence band, giving p-type conductivity.[1] Thin films of Sr-doped LCO showed an increase in conductivity with increase in Sr content, and hole concentrations of 10^20 to 10^21 cm^–3 were observed. The optical transmission of the Sr-doped LCO films was found to decrease with increasing Sr content. Defect analysis of pure and LCO containing Sr defects has been carried out using PBEsol + U calculations.[2] All possible intrinsic and extrinsic defects have been assessed, in order to investigate the improvement in electronic conductivity and optical properties that have been observed experimentally when LCO is doped with Sr. The formation energies and transition levels have been calculated for each defect. This allows the potential of Sr-doped LCO as a p-type TCO to be assessed. [1] Zhang et al., Adv. Mater, 27, 5191-5195 (2015) [2] J. P. Perdew et al., Phys. Rev. Lett., 2008, 100, 136406

Resume : The composite ceramics with bright visible emission in wide spectral range were fabricated. The samples were formed of the mixture of ZnO, MgO and TiO2 powders (all of 99.99% purity) and sintered for 3 hours in air at 1100 oC. Photoluminescence (PL) and PL excitation (PLE) spectra of prepared ceramics were measured at room temperature. In the PL spectra, a broad band in blue-orange spectral range consisting of some overlapping components and a distinct red one were observed. The relative intensity of the red band with respect to that of the broad one was controlled by the change of TiO2 content. It was found that PLE spectra of all broad band components were the same and coincided with that of visible PL in ZnMgO alloys, while PLE spectrum of the red band was quite different and similar to that in zinc and magnesium titanates. Based on obtained results, the formation of zinc and magnesium titanate inclusions in ZnMgO alloy under sintering was stated.

Resume : Nanostructuring of ZnO has been extensively studied due to its potential for optoelectronic applications. To realize this potential in low dimensional structures, size control in the range of a few nanometres is needed. Anodic alumina (AA) can be readily used as a template for growth of ZnO nanowires and nanotubes providing high aspect ratios in an optically transparent framework. In this study, the challenge of fabricating ultrathin ZnO nanowires within AA templates is addressed by utilizing the uniform, conformal and self-limiting growth attainable by atomic layer deposition (ALD). Controlled deposition of Al2O3 has been successfully used to reduce the pore diameter of the AA template beyond reported values, while maintaining the uniformity throughout the entire length of the AA channels. Subsequently, the pores have been filled with ZnO by ALD, allowing for fabrication of either nanowires or nanotubes in the AA framework with precisely defined thickness due to the self-limiting growth of ALD. The uniformity and crystallinity of the nanostructures has been assessed by Secondary Ion Mass Spectrometry, Transmission Electron Microscopy, X-ray diffraction and optical emission and absorption spectroscopy. Our results suggest that the demonstrated template method is a promising route for realizing oxide semiconductor nanostructures with high aspect ratios and uniform thickness down to a few nanometres.

Resume : Amongst the different dopants studied for In2O3 thin films, Sn is still the one which results in the highest electrical conductivity. With other dopants such as H, Zr or Ti significantly higher carrier mobilities are obtained but the maximum carrier concentrations with these dopants are about one order of magnitude lower than those possible with Sn doping. This is supposedly related to their lower solubility. The solubility of Sn is, however, also limited. Particularly under reducing atmosphere, Sn segregates to the surface and to grain boundaries. The latter leads to an enhanced grain boundary scattering which reduces the carrier mobility. Solubility is therefore probably the most critical issue in obtaining higher carrier concentrations. As an alternative dopant we have prepared Ge-doped In2O3 thin films by magnetron sputtering from ceramic targets with different dopant concentrations. Substrate temperature and oxygen pressure during deposition were varied systematically. The films are characterised using in-situ photoelectron spectroscopy, Hall-effect and optical transmission as well as X-ray diffraction. Electrical and optical properties are comparable to those obtained with Sn-doped films. All films exhibit an enhanced Ge concentration at the surface, which is, in contrast to ITO, more pronounced under oxidising atmospheres.

Resume : Amorphous oxide semiconductor based thin-film transistors (TFTs) are promising components for the next generation of large area, transparent and flexible electronics. Indium-Gallium-Zinc-Oxide (IGZO) is the most used oxide material as active channel layer in TFTs due to the excellent uniformity, electrical performance and low temperature processing. However, Indium and Gallium are recognized as critical raw materials. For this reason, Zinc-Tin-Oxide (ZTO) has attracted much attention on the last years as an alternative oxide semiconductor material. In this work we report the role of deposition parameters on sputtered ZTO TFTs using a low thermal budget (180°C). Hydrogen incorporation during the ZTO deposition process is investigated, in order to evaluate the promising results predicted in literature by simulation models. In fact, relevant improvements on electrical performance of the TFTs are noticed and post-processing treatments using a hydrogen atmosphere revealed to be crucial to stabilize devices behavior over time. Electrical characterization of TFTs incorporating different dielectric layers was also performed, including thermal SiO2 and sputtered multilayer Ta2O5+SiO2-based materials. This last insulator resulted in very good performing transistors on PEN substrates, exhibiting Von= 0 V, On/Off ratio >10e4, S=0.4 V/dec and µFE=6 cm2/Vs. The results show how ZTO is getting close to IGZO performance, opening doors to low temperature indium free oxide electronics.

Resume : The Printed electronics revolution has led to the development of new low-temperature methods for flexible materials processing. Solution processed dielectrics are a vital part of this new era, their development meet requirements on electrical properties a challenging task. High-κ oxides offer appealing electrical properties and stability for application in TFTs. Solution processing, such as sol-gel methods, has been reported for several high κ dielectrics, however these methods often require an annealing step at high temperature, incompatible with flexible substrates. Solution combustion synthesis is a method that provides energy to convert precursors into oxides at low temperatures that has been explored for solution based electronics; however toxic organic solvents are used. In this work we report on the production of solution based aluminum oxide (AlOx) gate dielectric in an environmental friendly water-based process at a maximum temperature of 200 ᵒC, using solution combustion synthesis. Since increased dielectric thickness is crucial for the reduction of the leakage current on TFTs, an AlOx multilayer was spin-coated from a solution of AlOx with 0.1 M concentration and annealed at 200 ᵒC for 1 h. Dielectric films that demonstrated high capacitance, were integrated into sputtered a-IGZO TFTs yielding good electrical performance. The devices demonstrated low hysteresis, close to zero VON, low VT and a µsat of 10.3 cm2/Vs at low operation voltages for 48 nm film thickness.

Resume : Nano-crystalline ZnO:Al thin films were deposited by pulsed laser deposition (PLD) in oxygen and in RF excited oxygen atmosphere at gas pressures between 10 and 70 Pa, and also in vacuum. Grazing incidence small angle x-ray scattering (GISAXS) and grazing incidence x-ray diffraction (GIXRD) showed that the roughness of film surfaces increases with increasing gas pressure, and the density decreases due to the formation of nano-voids. The nano-crystals sizes estimated from GIXRD were around 20 nm, while the sizes of the nano-voids increased from 1 to 2 nm with the oxygen pressure. The addition of excited oxygen particles from a RF plasma improves the structural ordering by lowering the defect level, which is evident from GISAXS and luminescence (PL) measurements. The PL consisted of 3 well defined peaks, UV emission that corresponds to a band-to-band transition, blue emission that appeared due to Zn vacancies, V_Zn, and red emission that is probably due to oxygen interstitials, O_i. For all pressures the RF excitation lowered the defect level related to blue emission and resulted in a narrower UV luminescence peak, indicating better structural ordering. The red emission peak is only seen using a RF excited oxygen atmosphere at 70Pa. The observed influence of the pressure and RF excitation on the films properties is a consequence of two main effects: the variation of the energy transfer from the laser plume to the growing film due to a different collision rate in the gaseous phase and changes of the growth chemistry due to various concentrations of active oxygen species.

Resume : Thin films of SnOx have been prepared at room temperature by reactive magnetron sputtering from a metallic Sn target. The oxygen content in the process gas was varied between 0 and 15% resulting in a variation of film composition from almost pure metallic Sn to fully oxidized SnO2. Composition and chemical state were analysed using in-situ X-ray photoelectron spectroscopy. Further film analysis is carried out using electrical conductivity, Hall-effect, optical transmission and X-ray diffraction. Four different regions depending on oxygen content in the process gas can be identified. At lowest oxygen contents a highly conductive mixture of metallic Sn and SnO is present. For higher oxygen contents an insulating mixture of SnO and SnO2 is observed. Further increasing the oxygen content results in highly conductive sub-stoichiometric SnO2 films with a conductivity of up to 100 S/cm. At highest oxygen content nearly stoichiometric SnO2 with a low electrical conductivity is obtained. We neither find pure SnO nor amorphous SnO2. None of the films exhibit a low Fermi energy, which would an indication of p-type conductivity.

Resume : IGZO TFTs appear as prime candidates for flexible and transparent electronics due to their high performance, stability, large-area uniformity and compatibility with low-temperature techniques. Shortening IGZO TFT dimensions, especially channel length (L), is of great importance both in display application (to keep pixel aperture ratio in UHD displays) and in application for circuit building blocks, to achieve higher integration level and speed enhancement. However, below 5 μm channel length IGZO TFTs have been recently reported with significant or not significant short channel effects (SCE), that lead to degradation of the devices’ performance. It is not clearly understood yet the mechanisms for showing, or not, these effects. To investigate this in more detail, sputtered IGZO TFTs with different channel lengths were fabricated at low-temperatures (T ≤ 180 ºC). TFTs with L < 5 μm showed SCE as Channel Length Modulation, Drain Induced Barrier Lowering and Charge Sharing. Simulation with Silvaco’s ATLASTM was performed on IGZO TFTs with different L. SCE were observed for L below 5 μm. Device parameter extraction with TCAD simulation was used to understand the physical mechanisms behind performance degradation with the scaling down of the devices. Different source and drain electrodes (e.g. aluminum and molybdenum) were also used in simulation to understand their effect on SCE as schottky barrier and contact resistance might play a role in these effects.

Resume : Rare earth (RE) doping of ZnO is often used to achieve efficient, controllable and stable emission in a specific spectral range. In the present work, a comparative study of ZnO films doped with various RE ions (Eu, Tb, Sm or Ho) with concentrations of about 2x1019-4x1020 cm-3 is considered. The films were produced on sapphire or silicon substrates either by RF magnetron sputtering or screen printing methods using corresponding ZnO:RE composite source. The samples were studied with X-ray diffraction, transmission electron microscopy, Raman scattering and photoluminescence (PL) methods. The as-deposited films were found to be polycrystalline with the grain sizes dependent on the type of substrate used. Thermal treatment in air at about 900°C stimulated the increase of ZnO grain sizes and affected PL intensity. Specifically, both exciton and native defect-related ZnO emission bands were found to decrease while RE-related PL bands to arise upon the annealing for most samples. This was accompanied by the formation of RE oxide phase detected by XRD and Raman methods. Besides, in the screen-printed (Sm,Ho)-co-doped ZnO films (with high RE concentrations), the reduction of Sm3 to Sm2 was observed demonstrating 5D0→7FJ radiative transitions. The mechanism of PL and PL excitation is discussed in terms of the formation of rare-earth complexes as well as energy transfer towards them from the ZnO host.

Resume : Most popular transport electrode is ITO (In-doped In2O3). However, Indium is one of the rare elements, and expensive. Moreover, supply of indium is considerd to be unstable in the future. ZnO has attracted much attention as a substitute of ITO because ZnO is a low cost and abundant material. Its direct optical bandgap of 3.4 eV at room temperature is wide enough to transmit most of the useful solar radiation in ZnO/CuInSe2 based solar cells. Recently, a spray pyrolysis has been attracting attention to make thin-film devices, as it is a low cost process. This process is an atmospheric fabrication process without using a vacuum device, which is capable of relatively inexpensive fabrication [1, 2]. In our previous works [3, 4], ZnO and Ga-doped ZnO (GZO) films were successfully grown at low temperature (100 oC) by a conventional spray pyrolysis. The ZnO films had high transmittance (over 80%), and it has been found that the sheet resistance is drastically reduced by performing UV irradiation on these samples. In this work, annealing effect in each oxygen and hydrogen atmosphere was investigated on GZO film/glass grown by splay pyrolysis at 150 °C using diethylzinc (DEZ)-based solution. To the GZO/glass obtained by the splay pyrolysis, annealing treatments in each oxygen and hydrogen atmosphere were also carried out. The annealing time was 0 ~ 120 min., and annealing temperature was 100 ~ 500 °C. The sheet resistivity of the GZO film was decreased to 15 /sq. by annealing for 20 min at 450 oC in hydrogen atmosphere. By contrast, increasing of the sheet resistivity of the GZO thin film was observed by annealing at 450 oC in oxygen atmosphere. In results of the Raman spectroscopy, there was a tendency that the peak intensity due to oxygen vacancies was increased by annealing in hydrogen. Therefore, it is assumed that the phenomenon of decreasing sheet resistivity of the GZO film by hydrogen atmosphere annealing is due to the increasing of oxygen vacancies in the GZO. [1] H. Nisihinaka, T. Kawaharamura, S. Fujita, Jpn. J. Appl. Phys. 46 (2007) 6811. [2] K. Yoshino, S. Oyama, M. Oshima, T. Ikari, M. Yoneta, Jpn. J. Appl. Phys. 47 (2008) 8170. [3] K. Yoshino, Y. Takemoto, M. Oshima, K. Toyota, K. Inaba, K. Haga, K. Tokudome, Jpn. J. Appl. Phys. 50 (2011) 040207. [4] Y. Takemoto, M. Oshima, K. Yoshino, K. Toyota, K. Inaba, K. Haga, K.Tokudome, Jpn. J. Appl. Phys. 50 (2011) 088001.

Resume : In this work the relationship between electro-optical properties, microstructure and internal stress behaviour of sputtered aluminium-doped zinc oxide (AZO) thin films was systematically investigated. AZO films were deposited onto soda-lime glass from a 0.5 wt % doped ceramic target at different substrate temperatures. The discharge power, chamber pressure and argon flow were maintained at 180W, 1mTorr and 5sccm. Films were characterized using UV-VIS spectrophotometry, X-ray diffractometry (XRD), scanning electron microscopy (SEM), four-point probe and Hall measurements. All measurements were done at room temperature. XRD analysis reveals that all films show a hexagonal wurtzite crystal structure with a (002) preferred orientation. Both XRD and SEM results show improved film crystallinity, grain size and morphology as the growth temperature increases to 300C. Film microstructure deteriorates at higher temperatures. Both the sheet resistance and hall mobility display the same trend. However, optical properties seem to be somewhat less sensitive to growth temperatures above 150C. The total internal stress of the sputtered AZO films was found to be largely compressive and thickness dependent. Intrinsic stress dominates in the temperature range below 300C while thermal stress becomes increasingly relevant at higher temperatures. This characteristic behaviour may at least partially explain the electrical and microstructural evolution in sputtered AZO thin films.

Resume : The transparent conducting oxides (TCO) such as ZnO doped Al or Mg are commonly used in solar cells, light emitting diodes, photodetectors and ultraviolet laser diode. In our work we focus on comparative study of structural, optical and electrical properties for highly ZnO doped Al (AZO) and Mg (MZO) films. These films are deposited on glass substrates by sol-gel spin coating method. The solutions were prepared using principal precursors (Zn(C2H3O2)2·2H2O), (Al(NO3)3·9H2O) and (Mg(CH3COO)2·4H2O). The doping concentrations for Al and Mg are fixed to 5 at 25 at %. The crystallinity of AZO and MZO thin films was determined by using the X¬-ray diffraction. The DRX spectra indicate that all samples are polycrystalline with hexagonal würtzite structure exhibiting a preferred orientation along (002) plan. We note that for the MZO layers, no degradation of würtzite ZnO phase was observed even when the Mg content was 25% as compared to Al-doped layers. This result is in agreement with other experimental results [1,2]. For the same concentration of Al and Mg dopant, we observe that the transmittance spectra for MZO thin films are wider than those of AZO indicating a shift toward the short wavelenghts with an optical gap energy equal to 3.67 eV. The electrical measurements of AZO and MZO thin films were determined by using I–V characteristic obtained by two probes method. All films present an Ohmic comportment with a better conductivity and high mobility for AZO films than MZO. [1] Xu Zi-qiang, Deng Hong, Li Yan, Cheng Hang, Materials Science in Semiconductor Processing 9 (2006) 132–135 [2] Dongyu Fang, Chaoling Li, Nan Wang, Pei Li, Pei Yao, Cryst. Res. Technol. 48, 5 (2013) 265–272

Resume : One of the challenges in ZnO-based technologies is to make stable bulk p-type ZnO. However, some reports [1,2], indicate that substituting Zn with Li can be a promising step in making p-type ZnO. Experimental studies approved that Lithium has a high solubility in ZnO [3], so substitutional Li occupying Zn-sites are possible in the material. In this study, we have performed density functional calculations on Li-doped ZnO using the Full Potential Linearized Augmented Plane Wave (FP-LAPW) formalism implemented in the Wien2k code. Lithium was considered at two different interstitial Lii sites, including octaedral Lii(oct) and tetrahedral Lii(tet) sites and also in substitutional zinc LiZn and oxygen LiO sites in the ZnO würtzite structure. We also examined the possibility of forming a complex pair of Lii with LiZn in ZnO structure. We found that Lii was most stable in octahedral positions than in tetrahedral positions in ZnO. The Li substitute at oxygen site behaves as a donor, and compared to Lii and LiZn it is not stable. In our calculations, LiZn states occur around 0.3 eV at the top of valence band maxima indicating a shallow acceptor in agreement with previous studies [ 2,4 ]. The formation of Lii(oct)- LiZn pair complex and its role on electronic properties of ZnO are discussed. We have shown that Lii(oct) donor-type character is more dominant than the LiZn acceptor-type character in ZnO and this could explain the difficulty in obtaining p-type conductivity in ZnO material. In this study, the structural, electronic, optical and electrical properties were investigated using The PBEsol functional, the TB-mBJ potential and BoltzTrap package as implemented in Wien2K code. [1] Park. C. H., Zhang. S. B., Wei. S. H., Physical Review B (2002) 66 [2] Wardle. M. G., Goss. J. P., and P. R. Briddon, Phys. Rev B 71(2005) 155205 [3] Onodera. A., Tamakani. N., Kawamura. Y., Sawada. T., Yamashita. H., Japanese Journal of Applied Physics 35 (1996) 5160 [4] Lee. C., Chang. K. J., Physica B 707 (2006) 376

Resume : A thin film transistor (TFT) has been fabricated using the amorphous Si doped zinc-tin-oxide (a- SZTO) with different amount of Si. Electrical performances of a-SZTO TFTs have been investigated and compared depending on Si contents. We derived electrical performance of thin film inverters with all n-type a-SZTO TFTs. Enhancement mode and depleted mode have been successfully implemented by using Si-controlled amorphous oxide thin film transistors.

Resume : Various semiconductor materials have been investigated due to the development of visible light active photocatalysts [1-6]. WO3 is expected to be a semiconductor photocatalyst driven visible light because its band gap ranges from 2.5 to 2.8 eV. We have already reported on photoinduced superhydrophilicity and oxidative decomposition of organic compounds under visible light irradiation on polycrystalline WO3 films deposited by reactive magnetron sputtering at the substrate temperature of 800 °C [7-9]. However deposition rate of the WO3 films by the conventional reactive magnetron sputtering was very low of around 10 nm/min. In this study, reactive gas flow sputtering (GFS) is adopted to deposit visible-light active photocatalytic WO3 films at high deposition rates and with high film quality. GFS encompasses two techniques, namely, hollow-cathode discharge and gas-flow-driven material transport. During the WO3 deposition, a hollow-cathode discharge occurs between a pair of facing rectangular W targets at gas pressures of 90 Pa. A stream of Ar gas was introduced, and directed between the facing targets (rate: 5000 sccm). The forced Ar stream transported sputtered W atoms to the substrate. O2 reactive gas was supplied in the vicinity of the substrate (rate: 0-100 sccm). W atoms, transported by Ar stream, reacted with O2 molecules to form WO3 on the substrate. The reactive GFS method employing metal targets offers great advantages over conventional reactive magnetron sputtering by providing stable high-rate deposition [10,11]. WO3 films were deposited on heated substrates at 700 oC by GFS. The WO3 films loaded with Pt (Pt/WO3) were also fabricated. The deposition rate for this process was over 10 times higher than that achieved by the conventional sputtering process. Furthermore, Pt nanoparticle- loaded WO3 films deposited by the GFS process exhibited much higher photocatalytic activity than those deposited by conventional sputtering, where the photocatalytic activity was evaluated by the extent of decomposition of CH3CHO under visible light irradiation. [12]. [1] H. Irie, K. Hashimoto, et al., Chem. Phys. Lett., 457 (2008) 202. [2] M. Miyauchi, Phys. Chem. Chem. Phys., 10 (2008) 6258. [3] R. Abe, B. Ohtani, et al., J. Am. Chem. Soc., 130 (25) (2008) 7780. [4] M. Ebihara, Y. Shigesato, et al., Proceedings of the 3rd ICCG , (2000) 137. [5] M. Kikuchi, Y. Shigesato, et al., Abstract of the 21st IUPAC SYMPOSIUM, (2006) 496. [6] M. Kikuchi, Y. Shigesato, et al., Proceedings of the 6th ICCG , (2006) 365. [7] A. Murata, N. Oka, S. Nakamura, Y. Shigesato, J. Nanosci. Nanotechnol. 12 (2012) 5082. [8] J. Takashima, N. Oka, Y. Shigesato, Jpn. J. Appl. Phys. 51 (2012) 055501. [9] M. Imai, N. Oka, Y. Shigesato, J. Vac. Sci. Technol. A 30 (2012) 031503. [10] Y. Kubo, Y. Shigesato, et al., J. Vac. Sci. Technol. A 26 (4) (2008) 893. [11] N. Oka, Y. Shigesato, et al., Thin Solid Films 532 (2013) 1. [12] N. Oka, Y. Shigesato, et al., APL MATERIALS 3 (2015) 104407.

Resume : Amorphous oxide semiconductors including Si have been deposited on glass substrate by RF sputtering method. Low emissive property has been observed by using multilayered amorphous oxide/Ag/amorphous oxide structures. Optical property of myltilayered structure has been systematically investigated depending on the thickness of each layeres. This low emissive propety will be effectively open the possibility for the application of energy efficiency window system for energy saving and conservation.

Resume : Graphene is one of the most appropriate transparent conductive materials. Using the graphene transparent conductive films, some applications, such as touch screen [1] and organic light emitting devices [2], have been demonstrated. For industrial mass productions, decreasing of temperatures and periods to form high quality graphene are remaining problems. In our previous studies, low temperature synthesizes of graphene using surface wave microwave plasma CVD were reported [3]. However, it is necessary to decrease the sheet resistances of the obtained graphene in order to use the practical transparent conductive films. Synthesis of graphene having low sheet resistance from solid carbon sources would be expected even low synthesis temperature [4]. Here, we report about graphene synthesis from PMMA, solid carbon sources, and compare its electrical properties with data of plasma CVD graphene. In addition, formation mechanism of graphene from PMMA is discussed. PMMA (50nm) was spin coated on Cu foils and H/Ar plasma treatment was carried out at 280ºC for 10min. After removal of Cu foils by wet-chemical etching, graphene was transferred onto SiO2/Si substrates. Then, van der Pauw configurations were formed using photolithography and EB evaporation. Plasma CVD graphene was also synthesized at about 300ºC of substrate tempeature to compare the electrical properties. Hall effects were measured from 80 to 500 K in He atmosphere. Both samples show positive Hall coefficient for all temperatures, suggesting both graphene are p-type conduction. Although sheet carrier densities of both samples are almost same (in the range of 1012 cm-2) and independent of temperatures, the sheet resistances of graphene from PMMA are two orders magnitude lower than those of plasma CVD graphene. Week temperature dependence of the sheet resistance of graphene from PMMA is obtained. Similar data are reported in mechanically exfoliated graphene [5]. The mobility also shows week temperature dependence and the highest mobility is 740cm2/Vs at 340K. For plasma CVD graphene, the sheet resistance and mobility increase with decreasing temperature and the estimated activation energy was bout 0.8meV which is almost same as the tunneling barrier heights at the grain boundaries of thermal CVD graphene (10meV) [6]. It is found, from the Hall effect measurements, relatively high quality graphene is formed by plasma treatment of PMMA. In order to understand the difference in the electrical properties of two samples, Raman signal maps were measured in van der Pauw configurations. It was clear from Raman maps that the average domain size of graphene from PMMA was ten times larger than those of plasma CVD graphene. The large domain size leads to high mobility. XPS was used to characterize the as-deposited graphene on Cu foils. Peaks due to Cu2O are obtained on graphene from PMMA, while no peak related copper oxide is confirmed on plasma CVD graphene. Oxygen in PMMA decomposed by plasma and then CuO2 was formed. From the electrical properties and characterizations, the oxidization of Cu increases domain size, which increase carrier mobility. Using PMMA as carbon source, graphene having high mobility was synthesized at low substrate temperature. Oxidization of Cu surface plays an important role for high quality graphene synthesis at low temperature using plasma techniques. This work was partially supported by a JSPS Grant-in-Aid for Scientific Research on Innovative Areas “Science of Atomic Layers. [1] S. Bae et al., Nat.Nanotechnol.5(2010)574. [2] C.R.Dean et al.,Nat.Nanotechnol.5(2010)722. [3] T.Yamada et al., J.Phys.D46(2013)063001. [4] Z.Sun et al., Nature 468 (2010)549. [5] Y.-W.Tan et al., Eur.Phys.J.Special Topics148)2007)15. [6] H.S.Song et al., Sci.Rep. 2(2012)337.

Resume : This presentation will review our recent work on the pulsed laser processing of graphene oxide (GO) nanosheets and transparent layers for organic photovoltaic (OPV) applications. Rapid and facile methodologies for the photochemical reduction, functionalization and doping of GO, based on pulsed laser irradiation of GO nanosheets in liquid or gas media, will be presented and discussed. In particular, we show the first GO reduction technique, compatible with temperature sensitive substrates, leading to the fabrication of large area flexible transparent reduced GO micromesh electrodes. Furthermore, we present a simple photochemical method for the simultaneous reduction and doping of GO nanometre layers. Using this technique Cl and N doping was rapidly carried out at room temperature in only few minutes. By changing the exposure time it is possible to tune the doping and reduction level and therefore the Work Function of GO nanolayers. The application of laser fabricated GO materials in all the three major components of OPV devices, i.e. in transparent electrodes, photoactive layer and interfacial (buffer) layers is demonstrated and discussed.

Resume : Figure of merit transparency and sheet resistance values of the metal nanowire networks are very close, if not better, than those of commercially available transparent and conducting thin films, such as indium tin oxide (ITO). In addition, nanowire networks carry the prominent advantage of solution processability. In this presentation, I will talk about our efforts on the utilization of silver (Ag) and copper (Cu) nanowire networks as transparent contacts for polymeric light emitting diodes, solar cells and transparent heaters. Following their synthesis and purification, nanowires are deposited onto various substrates in the form of networks through spray coating. Surface roughness and long-term stability of the networks are carefully monitored, where the latter is a particularly important problem for Cu nanowires. Infiltration of a conducting polymer decreased the roughness associated with the networks so that they can be used as anodes in polymeric light emitting diodes. In order to eliminate the shading losses associated with conventional screen-printed finger electrodes in single crystalline silicon solar cells, networks were utilized as top contacts. Through the application of a post-deposition treatment for the formation of the ohmic contacts, a relative enhancement in conversion efficiency of 21% was obtained with respect to the reference cells. When used as transparent heaters, reproducible heating characteristics and uniform temperature distribution was obtained from the networks. The effect of nanowire density on the temperature profile, applied voltage as well as turn on/off characteristics were investigated.

Resume : Recently, inorganic oxides such as InGaZnO (n-IGZO), InZnO (n-IZO) has revolutionized the thin film transistors (tft’s) technology, still its use in device heterojunctions hasn’t been studied well, which would need a suitable p-type semiconductor also. In the present work, along with n-IGZO and n-IZO, we also examined p-type cupric oxide (p-CuO) and fabricated n-p junction diodes using solution processing route. Processing temperature of n-IZO and n-IGZO is lowered to 325˚C by using ultraviolet (UV) curing treatment, making it amenable for flexible substrates such as, polyimide. The n-IGZO and n-IZO films are found amorphous, while pure monoclinic CuO phase is obtained for p-CuO films at an annealing temperature of 250˚C for 1 hour. The n-IGZO or n-IZO films are already well investigated. Hence, we carefully examined p-CuO films under varying annealing temperature and time conditions. At 200˚C for 30 minutes uniform film morphology is seen except for a banded structure in a small region. Increasing annealing time expands the banded region all over, but this banded structure again fades out as the annealing temperature and time is increased. The surface topological scans and compositional analysis suggests no compositional variation and the banded structure is due to topological differences arising from amorphous and crystalline regions of CuO. Rectifying behaviour in all structures is observed. In comparison to n-IGZO/p-CuO, n-IZO/p-CuO diodes show better performance with a rectification ratio of approximately 〖10〗^4 and low turn on voltage (Von) < 1.5 V. IZO films with low resistivity of 1.08×〖10〗^(-1)ohm cm, compared to 3.83×〖10〗^5 ohm cm in IGZO, offer low series resistance and hence better performance.

Resume : Semiconducting materials that combine superior charge transporting characteristics with processing versatility are currently in great demand due to their potential for application in the emerging field of printed electronics. Semiconducting metal oxides represent one such family of materials that promise to revolutionise next generation ubiquitous large-area opto/electronics. In this presentation I will discuss the development of solution-processed low-dimensional semiconducting metal oxide systems and devices. In particular, I will describe how ultra-thin layers of various oxides can be grown from solution-phase and how these layers can be combined to form complex superstructures with controlled dimensionality and electronic characteristics. Finally, I will discuss the application of these oxide systems in high-electron mobility transistors, quantum effect devices and ultra-sensitive biosensors.