Transparent conducting and semiconducting oxides and their applications

Resume : In recent years we have grown arrays of ZnO, TiO2 and InGaN nanorods for solar cell and water splitting applications. Transmission electron microscopy (TEM), carried out in cross-section (perpendicular to the growth direction) and in plan-view (along the growth axis), has led to a detailed understanding of structure and growth. Studies of (0001) InGaN nanorods grown by MBE, of interest as absorbers in solar cells and water splitting applications, show core-shell structures with a low In-content shell surrounding a high In core. Atomic resolution imaging and microanalysis reveal that both core and shell are further separated into In-rich and In-poor platelets which alternate over periods of a few nanometers, while diffraction studies show local strains of several percent which are accommodated elastically. Arrays of ZnO and TiO2 nanostructures have been investigated for use as solar cell electrodes, and TEM has been used to understand their growth and the structure of the interface with absorber and sensitiser layers. For example, studies of chemically deposited ZnS sensitiser layers on single crystal (0001) ZnO nanorods, show non-uniform growth, proceeding first on the polar (0001) surface, and by island nucleation on the non-polar facet intersections and surface steps, such that comparatively thick films are needed to form continuous layers. The paper will illustrate some of these results and show how they provide insight into the resulting optical and device properties

Resume : We report on fabrication and characterization of Cu(In,Ga)Se2(CIGS)–based thin film solar cell using transparent conductive oxide(TCO) as back contact. Different TCOs were investigated: ZnO:Al(AZO), SnO2:F(FTO) and In2O3:Sn(ITO), with particular emphasis on AZO and FTO being In-free. Electrical characterization of FTO and ITO revealed a carrier concentration >5•1020 cm-3 and a sheet resistance lower than 7 Ω/square, which we assumed as reference for optimizing CIGS–based devices on AZO. We found out that the Al fraction in AZO must be at least 5% in order to achieve such high carrier concentration: films deposited via Pulsed Electron Deposition using a ZnO:Al 5%wt target showed an average carrier density of 5.5•1020 cm-3 and a sheet resistance of 5 Ω/square for a thickness of about 1 micron. Preliminary results show efficiencies of 11.7% (front) and 1.6 (rear) using ITO and of 14.7% (front) and 3% (rear) using FTO for CIGS–based devices. Although efficiencies using AZO as back contact are somewhat lower, we managed to obtain preliminary values of 9.3% (front) and 5% (rear) which are higher than any other results reported in literature. Therefore, our low temperature deposition process for CIGS(250°C) seems to inhibit the formation of unwanted Ga2O3 at the interface between CIGS and back-AZO as already reported in previous works. In addition, we investigated the effects of using different dopants in ZnO, Gallium and Indium, and their impact on CIGS–based devices’ efficiencies.

Resume : Numerical calculations indicate that reducing the conduction band offset (∆Ec) appearing in n-ZnO/p-Si heterojunction solar cells strongly reduces the impact of recombination centers at the interface between the two semiconducting materials, thus enabling high efficiency solar cells. By introducing Mg to reduce the band offset we have experimentally shown that there is a clear relation between higher efficiency n-ZnO/p-Si based heterojunction solar cells and lower ∆Ec (with Mg varying in the 0-4 at.%). In detail, by decreasing ∆Ec from ~0.63 eV to ~0.48 eV a corresponding rise in the solar cell efficiency from ~3.7% to ~6.0% has been observed if the ideality factor, n, is below ~2. A further ~1.1% efficiency raise is obtained by increasing the deposition temperature to 300 oC from 160 oC with approximately the same Mg content. Finally a ~10 % efficiency has been obtained on a solar cell including an anti-reflecting coating by introducing an HF pretreatment step. The increase appeared to be related to a larger VOC (~ 480 mV) pointing to an additional interfacial improvement. Acknowledgments. This work was partially supported by the National Science Centre (dec. No. DEC-2012/D6/A/ST7/00398). The author T. A. K. acknowledge support by the grant DEC 2013/09/D/ST5/03879 of NSC of Poland. The author R.S. was supported by the EU 7th Framework Programme project REGPOT-CT-2013-316014 (EAgLE).

Resume : Hybrid structures of organic semiconductors and transparent conductive oxides (TCO) offer wide possibilities for new opto-electronic devices as they can take advantage of different functionalities of the inorganic and the organic components. For example, in hybrid photovoltaics the high carrier mobility of TCOs can be combined with the high absorption coefficient of organic conjugated molecules. The performance of hybrid photovoltaic devices is determined by the efficiency of the charge separation process. Excitons generated in the organic and inorganic part diffuse to the interface of the heterojunction and dissociate. Our experiments show that prior generation of free charge carriers, a hybrid charge transfer exciton (HCTE) is formed, i.e. a coulombically bound charge pair residing on both sides of the interface. Only after dissociation of such a pair a photocurrent is generated. The formation of HCTE and their properties are elucidated in planar ZnMgO/poly(3-hexylthiophene) (P3HT) heterojunction devices by photoemission spectroscopy, electroluminescence (EL), photovoltaic EQE and I-V measurements. The energy offset ΔE_io between the conduction band minimum of ZnMgO and the P3HT highest unoccupied molecular orbital is systematically tuned by varying the Mg content. Both, the HCTE EL transition energy and the open circuit voltage V_OC scale linearly with ΔE_io indicating the correlation of both quantities. The photovoltaic performance is thus closely interlinked with the properties of the HCTE. Combined analysis of HCTE EL and photovoltaic performance provides thus valuable input for the optimization of interfaces between TCOs and organic semiconductors to fully exploit the potential of hybrid devices.

Resume : Amorphous zinc-tin-oxide (ZTO) consists of naturally abundant, nontoxic elements only and can be deposited at room temperature with an electron density and a mobility as high as 10^19 cm^-3 and 10 cm^2/Vs, respectively [1]. Therefore, ZTO is a suitable material for low-cost, transparent transistors and generally low-cost, transparent electronic applications. We present our results on the influence of the Zn/Sn ratio on the electrical properties of thin films grown by pulsed laser deposition on glass substrates using a continuous composition spread method [2]. Amending previous reports on only a limited number of discrete Zn/Sn ratios [3], we obtained a comprehensive data set for almost the entire composition range. Using energy dispersive X-ray analysis the spatial dependence of the Zn/Sn ratio was mapped. Charge carrier concentration and resistivity were determined in dependence on the composition [4]. Further, the properties of all-amorphous pn-heterojunctions using zinc-cobalt-oxide (ZCO) as p-type electrode will be discussed in dependence on the thin film stoichiometry. An improvement of the diode characteristics regarding the ideality factor and the rectification ratio is observed with aging. [1] Jayaraj et al., J. Vac. Sci. & Technol. B, 26, 2 (2008) [2] von Wenckstern et al., CrystEngComm, 15, 10020 (2013) [3] Görrn et al., Applied Physics Letters, 91, 193504 (2007) [4] Bitter et al., ACS Comb. Sci., 18, 188 (2016)

Resume : The material system of CuCrO2, a delafossite, is well known as one of the best performing p-type transparent conducting oxides. We report details on a low temperature facile growth method for CuCrO2 by spray pyrolysis. The dependence of the growth on the precursors, the temperature and oxygen partial pressure is examined. The decomposition routes are critical to obtain the best performing films. The thermopower and electrical measurements indicate p-type films with conductivity ranging from 1-12 Scm?1 depending on the growth conditions. This p-type conductivity is retained despite the nanocrystallinity of the films. The figure of merit of these films can be as high as 350 ?S, which is the best performing p-type TCO by solution methods to date. Comprehensive analysis of the thin films by X-Ray Diffraction, Raman spectroscopy, X-ray absorption spectroscopy and X-ray photoelectron spectroscopy was used to identify the material as a highly Copper deficient CuxCrO2, with best performance in terms of the figure of merit found for x=0.4. Interestingly, the material?s low growth temperature is compatible with flexible substrates, glass (Corning Willow® glass) and plastic sheets (Kapton® polyimide). We discuss the optical and electrical properties and morphology of films grown on variety of glass and flexible substrates as well as their bending stability.

Resume : Spatial ALD is emerging as an industrially scalable deposition technique at atmospheric pressure which combines the advantages of temporal ALD, i.e. excellent control of film composition and uniformity on large area substrates, with high growth rates (up to nm/s). We present the S-ALD of indium-free transparent conductive oxides (TCOs): ZnO, ZnSnO and Al:ZnO. Temporal ALD of multicomponent oxides is carried out by alternating the growth of different binary oxides, so that the film composition and morphology changes along the growth direction. This results typically in a low carrier mobility and poor doping efficiency in Zn-based TCOs. In S-ALD a moving substrate is sequentially exposed to the oxygen precursor and metal precursor vapors (i.e. DEZ, TDMA-Sn, TMAl, DMAI), which are pre-mixed and co-injected in the same deposition zone. The successful application and the limitations of this new method will be presented for different precursors. A much higher doping efficiency of Al-atoms (~ 90 %) is achieved in ZnO:Al when DMAI, instead of TMA, is co-injected with DEZ. A minimum resistivity of 0.5 mOhm cm and an optical transparency of ~ 90 % in the visible range are measured in 750 nm thick Al:ZnO. The morphology of the ZnSnO and ZnOAl varies from polycrystalline to amorphous with increasing the dopant concentration, as measured by X-ray diffraction. The application of S-ALD indium-free transparent conductors in c-Si, CIGS solar cells and OLEDs is currently under investigation.

Resume : The stable p-type conductivity of ZnO is still the main obstacle in a wide application of this material in optoelectronics. It has been reported that doping ZnO with group V elements results in enhancement of luminescence intensity around 3.30-3.32 eV. However, it has been shown that in epitaxial films the acceptor-related 3.31 eV luminescence comes from stacking faults [1]. We report on a photo- and cathodoluminescence on polycrystalline ZnO and ZnO:N films grown at 100^oC by Atomic Layer Deposition. Photoluminescence spectra show two emission bands in the excitonic region. Apart from the DoX line at 3.36 eV, a characteristic emission at 3.30 – 3.32 eV appears both in undoped and nitrogen-doped samples, however the RTP annealing in oxygen atmosphere leads to a considerable enhancement of this band only in samples intentionally doped with nitrogen. This enhancement is accompanied by shifting the conductivity towards p-type. Spatially resolved low-temperature cross-section CL studies show that the area showing donor-related 3.36 eV emission is complementary to the area showing acceptor-related ~3.3 eV band, which suggests that both p-type and n-type regions simultaneously coexist in this material. This points out that the 3.31 eV band cannot be clearly associated with stacking faults as was suggested before [1]. Acknowledgements. The work was partially supported by the Polish NCN project DEC-2012/07/B/ST3/03567. References [1] M. Schirra et al., Phys. Rev. B77, 125215 (2008).

Resume : Among transparent conductive materials (usually known as TCOs), Zn(O,S) represents an attractive alternative to ITO or other doped ZnO. The objective stands in finding the best compromise between good conductive properties and high transparency. Published works on ZnO:S thin films have widely shown how the targeted TCO properties are strongly dependant on the oxygen content [1][2]. In this study, the deposition of Zn(O,S) thin films were performed by radiofrequency magnetron sputtering in reactive Ar-O2 plasma, starting from two different ZnS targets, with oxygen atomic content in the 10% and 30% range. The properties of Zn(O,S) layers, i.e. chemical, structural, morphological and optical properties, have been investigated by XPS, XRD, SEM, TEM, ellipsometry and UV-visible spectrometry. In our presentation, we will illustrate how the target composition modifies the process characteristics and the layers properties. Finally, we will focus on process characteristics allowing to reach the best TCO properties in ZnO:S films. [1] B-K. Meyer et al, Applied Physics Letters vol. 85 4929-4931(2004) [2] G. Baldissera and C. Persson, Journal of Applied Physics, vol 119, 045704 (2016)

Resume : The formation of oxygen vacancies and their effect on the n-type conductivity of transparent conducting oxides (TCOs) has remained a controversial topic. They have been determined to be deep donors in SnO2 and ZnO, while it has been proposed that their formation on the surface of In2O3 would lead to intrinsic n-type conduction. Numerous computational studies on this subject have tried to correlate the calculated defect properties with available experimental spectroscopic data (luminescence etc.), but, notably, often contradicting each other in the defect assignment. We use state-of-the-art hybrid quantum mechanical/molecular mechanical solid state embedding to determine the formation energy, electronic and optical properties of the oxygen vacancy in the three archetypal TCOs In2O3, ZnO and SnO2. To ensure a high level of accuracy in our simulations, we employ three hybrid exchange and correlation density functionals (PBE0, B97-2 and BB1k) and compare their predictions with previous plane-wave pseudopotential based calculations and experiment. Our results using the PBE0 functional are in excellent agreement with calculations done using a plane-wave basis set, which predict the oxygen vacancy to be a deep donor in all three systems. Using BB1k, which has been developed specifically to treat both reaction energies and kinetic barriers accurately, we find, in contrast to previous studies, that the oxygen vacancy is a resonant shallow donor in bulk In2O3. In SnO2 and ZnO we show that the vacancy is indeed a deep donor, but shallower than previously proposed. Our results are in excellent agreement with available deep level transient spectroscopy measurements and other relevant experimental data.

Resume : Zinc oxide thin films were deposited from the aerosol assisted chemical vapour deposition reaction of zinc acetate dihydrate in methanol on to glass substrates. The addition of acetic acid and / or water into the reaction flask led to large changes in the films microstructure, texturing and roughness. By careful tailoring of the reaction mixture columnar, hexagonal pyramid or hexagonal plate structures could be deposited. the changes in structure led to large changes in both optical and electrical properties of the films, specifically an increase in haze (up to 95%) or a decrease in film resistance (60 ohm.sq-1). Aerosol assisted chemical vapour deposition shows great promise as a simple and flexible method to produce high quality zinc oxide thin films.

Resume : Most of the existing photovoltaic (PV) solar cells are already optimized in terms of their absorption and conversion efficiency, however any strategy that can help to raise their efficiency, even slightly, is welcome, and all the more if it is cheap and does not require any modification of the solar cell fabrication technology. One possibility to increase solar cell’s efficiency is the use of a material that could convert the high energy photons from the sun spectrum, namely UV and blue light, which are otherwise inefficiently absorbed by the amorphous Si, CdTe, CIGS and organic PV cells, and re-emit them as lower energy photons, for which the conversion efficiency of these cells is optimal. This so-called “down-shifting” strategy belongs to the “add-on” technology, as the aim is to fabricate a thin layer of the down-shifting material on top of the existing solar cells and all this at low cost. The criteria which a good down-shifting material needs to fulfill are: having a large Stoke shift (i.e. a discrepancy between the absorption and emission energies) and possessing high photoluminescence quantum yield (PL QY). Furthermore, this material has to be environmental-friendly and cheap. Several attempts were reported in the literature aiming at the fabrication of such a material. For example, CdS and CdSe nanoparticles embedded in polymers or silica have proved to be efficient but not necessarily cheap and non-toxic. However, high PL QY, stable green/yellow emission and easy scale–up process are expected for industrial applications. We study down-shifting materials based on ZnO nanoparticles. ZnO is a low cost, abundant and non-toxic material. It naturally absorbs the blue and UV light thanks to a wide band gap of about 3.37 eV at room temperature. ZnO can also emit visible light, from yellow to red, depending on the nature of the crystalline and surface defects involved in the emission process. Although the nature of these defects is still under debate, it is widely admitted that reducing the size down to the nanoscale enhances the presence of the defects and the luminescence efficiency of ZnO nanostructures in the visible. We present a quick and convenient chemical solution approach to get unique mesospheric self-assembly hybrid ZnO system with intense photoluminescent quantum yield of 40-75 % and stable visible emissions. We develop luminescent layers of ZnO nanoparticles dispersed and embedded in a polymer matrix to be used as down-shifting materials in the structures of solar cells. Our luminescent ZnO layers are fabricated using an easy scale–up process, which is easily adaptable for industrial applications. We will also present the parameters influencing the luminescent efficiency of chemically synthesized ZnO nanoparticles used as down-shifting material. The issue of the optimization of the luminescence EQE, in conjunction with the nano- and mesostructure of the material will also be addressed. We will show how the chemical synthesis parameters influence the ZnO nanoparticle’s EQE and how they can be adjusted to reach EQE as high as 75 %. The possibility to disperse and embed the nanoparticles in polymers (for example PMMA) will also be discussed. Furthermore, the possibility of application of other ZnO nanostructures, namely zinc oxide nanowires, in gas sensors, is presented. These nanowires are fabricated by a simple and low-cost process - chemical bath deposition. They have similar luminescent properties as studied ZnO nanoparticles, i.e. they emit in the UV and in the visible spectral range, thanks to the defects and surface states. Their excitonic and visible emission is studied in the presence of gas vapors and the results demonstrate the change of the visible photoluminescence of ZnO nanowire array and that the material is able to adsorb gases. Finally, the mechanism of gas sensing is discussed.

Resume : Transparent oxide semiconductors nanostructures are promising systems to expand the significant achievements of plasmonics into the infrared (IR) range [1,2]. We report on tunable mid IR plasmon induced in degenerate Al and Ga doped ZnO nanostructures considering their wide range of possible doping levels and thus of plasmon tuning.4 The plasmon resonances of nanocrystal(s) and nanowires can clearly be tuned between 2000 cm-1 and 3000 cm-1 by introducing donor dopants. We thus investigate the reasons for this large damping and identify two distinct mechanisms. We first show that by embedding the GZO nanocrystals in Al2O3 matrix has prevented the OA, and hence the damping was reduced along with the broadening. The second mechanism is related to the partial activation of the dopants. The partial activation is the consequence of two causes. First, depending on the synthesis conditions, acceptors defects can be created among the nanoparticles or nanowires and compensate the intentionally introduced donors. We have shown that the activation of Al is enhanced when it is introduced in ZnO nanocrystals synthesized in O-poor conditions rather than in O-rich conditions [3]. In this case, acceptors complexes related to O vacancies are minimized. The second reason is partial activation of the dopants. We have observed that less than half of the dopants actually participate to the electron gas. The cause of the partial activation is related to the position of the dopants within the particles. It has been proposed that if the dopants are homogeneously distributed the damping is larger . Via STEM EELS, we map the spatial distribution of dopants within the nanocrystals at the atomic scale. We subsequently anneal the nanocrystals to let them reach the thermodynamic equilibrium and map again the distribution of dopants to see whether the latter have segregated through a self-purification process. The effect of the return to the thermodynamic equilibrium on the Plasmon resonance is concomitantly investigated. References [1] G.V. Naik, V.M. Shalaev, A. Boltasseva, “Alternative Plasmonic Materials: Beyond Gold and Silver”, Adv. Mat. 2013, 25, 3264-3294. [2] M. K. Hamza, J.-M. Bluet, K. Masenelli-Varlot, B. Canut,O. Boisron, P. Melinon and B. Masenelli.”Tunable mid IR plasmon in GZO nanocrystals”. Nanoscale, 2015, 7, 12030 [3]S. D. Lounis, E.L. Runnerstrom, A. Llordes, D. J. Milliron, “Defect chemistry and plasmon physics of colloidal metal oxide nanocrystals”, Phys. Chem. Lett.2014, 5, 1564-1574.

Resume : The interaction of metals with electromagnetic radiation gives rise to collective charge excitations called surface plasmon polaritons (SPPs). The potential of these coupled light-matter states for creating nano-scale photon-based circuits is the core of what is summarized today by the term "plasmonics". We will show that strongly n-type ZnO is an excellent plasmonic material in the infrared spectral range. Using molecular beam epitaxy, we are able to generate free carrier concentrations of almost 10E21 cm-3 by Ga-doping of ZnO without significant deterioration of the crystal perfection. In this way, a metallic dielectric function is created with a negative-to-positive crossover of the real part tunable from mid infrared up to telecommunication wavelengths. The losses are at least one order of magnitude lower than for traditional metals. Fabrication of epitaxial multilayer structures with different doping level enables the formation of novel SPP dispersions that can be engineered in a unique way. Coupling of SPPs at adjacent interfaces allows for almost arbitrarily shaping of their dispersion curves for achieving, e.g., phase matching for nonlinear processes or even anomalous dispersion. Moreover, the controllable stacking of alternate subwavelength-thin metallic and dielectric layers in a multilayer structure opens a direct route for realization of hyperbolic metamaterials. We demonstrate realization of ZnO-based hyperbolic metamaterials operating at wavelengths covering the entire telecommunication band.

Resume : In this work detailed investigation on the surface and bulk electrical properties of the Al-doped ZnO (AZO) films fabricated under various experimental conditions in RF magnetron sputtering was carried out using Scanning Tunnelling Microscopy/Scanning Tunnelling Spectroscopy (STM/STS), Conductive Atomic Force Microscopy (C-AFM) and Hall probe measurement system. Both these properties showed substantial variation with respect to their microstructure, a consequence of the growth conditions, hence altering their surface and bulk chemistries. An increase in the carrier concentration is found to be strongly influenced by the presence of the shallow donor level defects, whereas, deep donor level defects were not found to contribute to it. The highest carrier mobility of the film is found to occur for the most oriented and larger crystallite growth. The combination of an increased concentration of the shallow donor level defects and an absence of the deep donor level defects are found to correspond with the best values of carrier concentration and carrier mobility leading the lowest electrical resistivity of 0.913×10-3 Ω-cm. The local surface conductivities and their spreads are found to be larger for the films with lower bulk electrical resistivity. Moreover, this study sheds light on the use of specific experimental tools (like, Hall probe measurement system, Photoluminescence, C-AFM and STM/STS) as an indicator to estimate the overall film quality in a stand-alone manner, hence making the extensive use of a wide range of characterization techniques for optimizing the film quality of the transparent conducting oxide (TCO) redundant.

Resume : The atomistic details of metal oxide surfaces control their electronic and catalytic properties. In the case of ZnO, rich phase diagrams are predicted by ab-initio calculations, with the considered structures allowing for surface reconstructions, adsorbates, and relaxation of surface-near layers. Since these characteristics are interrelated, a clear identification of a given surface structure is significantly assisted by experimental techniques that yield structural information with chemical sensitivity. In addition, such experiments help identifying important chemical species and preparation conditions that have hitherto not been considered, thus allowing to refine the computational studies. With X-ray standing waves (XSW), atomic positions can be independently resolved for the species found in an X-ray photoelectron spectrum. We collected XSW data for ZnO single crystals treated ex-situ only and investigated the effect of several in-situ treatments. By variation of temperature and H2O partial pressure we determined structures for selected points in the phase diagram. In addition, we evidenced deterioration of the surface-near crystallinity by Ar incorporation during Ar+ sputtering and studied the annealing behavior.

Resume : In recent years, DUV light sources using wide band gap semiconductor have been getting huge attention as an alternative of gaseous light sources for their low-cost, safety and having long product lifetime. Among wide band gap semiconductors, Mg1?xZnxO which is an alloy of magnesium oxide (MgO) and zinc oxide (ZnO) is an unexplored and promising candidate material because its bandgap can be tuned from 3.3 eV to 7.8 eV, that is, the maximum band-gap can be larger than that of AlN. However, there are few reports on DUV luminescence from Mg1?xZnxO. Almost reports are studied on wurtzite-structured Mg1?xZnxO for the band gaps smaller than about 5 eV. We focused attention on rocksalt-structured Mg1?xZnxO having wider band gaps. Mg1?xZnxO alloy films were grown on MgO (001) substrates by mist CVD method. Growth temperatures were settled at 500 - 800 C. Molar ratios of Zn source were changed from 10 to 40 % in precursor sources. In an x-ray diffraction (XRD) profile, Mg1?xZnxO and MgO 002 peaks were confirmed and other crystal phases were not detected for entire range of Zn concentartions. It was found that oriented grown Mg1?xZnxO thin films were obtained on MgO substrates at 600 and 700 C for each growth conditions of various molar ratios of Zn in source solutions. A cathodoluminescence (CL) spectra of a Mg0.64Zn0.36O thin film on a MgO substrate, DUV emission peaks were observed around 5.3 eV (234 nm) at low temperature of 11 K, though the intensity rapidly decreased with increasing temperature to 300 K. The MgO substrate was confirmed to exhibit no emission around 5.3 eV. Since the band gap of Mg0.64Zn0.36O is calculated as 6.2 eV, it may be concluded that the DUV emission at 5.3 eV is originating from a near-band-edge (NBE) emission of the Mg0.64Zn0.36O film.

Resume : In the first part of this presentation it will be demonstrated that Al-doped ZnO grown by atomic layer deposition (ALD) in 'nanolaminate' mode whereby the dopant Al species are introduced via an ALD cycle that interrupts the ALD growth of ZnO itself, exhibits resistivities as low as 0.002 ohm.cm while maintaining transparency at levels around 70%, despite the fact that the dopant atoms are effectively placed at specific points within the ZnO layer. It is suggested that this unconventional approach to dopant incorporation, which might be termed true 'delta-doping', offers a number of exciting possibilities in terms of potential applications, not the least being as the transparent conducting electrodes in perovskite-based solar cells. In the second part the potential use of Co-doped ZnO as a photoanode for the solar splitting of water will be outlined while the photocatalytic activity of this material will be demonstrated with a view to it forming the basis of simple sensor and water splitting devices. In addition we demonstrate that these materials, which are grown using a combination of ALD and hydrothermal processes, exhibit unusual optically-active defects following thermal processing which could also facilitate their use in a range of interesting new applications.

Resume : Amorphous zinc tin oxide (ZTO) can be fabricated at room temperature and exhibits electron mobilities of more than 10 cm^2/Vs [1]. This makes its use interesting for channel layers in pixel driving thin film transistors for active matrix displays. Bipolar (hetero-)diodes [2] as well as Schottky barrier diodes are viable options for switching gate electrodes for low voltage operation. They can additionally be employed for material characterization by space charge spectroscopy. We present Schottky barrier diodes on ZTO using platinum contacts. The semiconducting films are grown by pulsed laser deposition, the Schottky contacts by reactive direct current sputtering. Diodes exhibit rectification ratios of more than 5 orders of magnitude. Temperature dependent current voltage characteristics were obtained and modelled using thermionic emission model. A mean barrier heigth of about 1.3 eV is obtained for diodes with an ultrathin semi-insulating layer between semiconductor and Schottky metal. Additionally, capacitance-voltage and thermal admittance spectroscopy were performed. We found two defect levels in amorphous ZTO, one deep level at about 200 meV and one shallow level near the conduction band minimum who's properties are discussed following the method of Pautrat et al. [3]. [1] Jayaraj et al., J. Vac. Sci. Technol. B 26, 495 (2008) [2] Schlupp et al., Adv. Electron. Mater 1, 140023 (2015) [3] Pautrat et al., Solid-State Electron. 23, 1159 (1980)

Resume : Transparent oxide La:BaSnO3 demonstrates great potential as a high-mobility electron transport layer for applications in flat panel displays, solar cells, light-emitting diodes, and multi-functional perovskite-based electronic devices. While an extraordinary high room-temperature mobility of 320 cm2/Vs has been reported for La:BaSnO3 single crystals[1], epitaxial La:BaSnO3 thin films grown on SrTiO3 substrates suffer from the presence of dislocations and domain boundaries that cause electron scattering. This work presents epitaxial growth of high mobility La:BaSnO3 thin films by pulsed laser deposition (PLD) on NiO-buffered MgO substrates of less than 1.4% lattice mismatch. The maximum Hall mobility of 69 cm2/Vs is among the highest for films prepared by PLD on SrTiO3 or BaSnO3 substrates[2] and solid phase epitaxy[3]. High resolution transmission electron micrographs show a coherent La:BaSnO3/NiO interface free of dislocations that may affect the electronic transport. By variation of the carrier concentration from 8.3 × 10^18 cm-3 to 4.9 × 10^20 cm-3, the increase in the electron effective mass is evaluated from the free carrier absorption observed in Fourier transform IR spectra. The 170 meV widening of the direct optical band gap is described by an analytical model taking into account band gap narrowing by heavy doping and the non-parabolic La:BaSnO3 conduction band. From temperature-dependent electronic transport properties the prevailing carrier scattering mechanisms are discussed for design of low-defect density, high mobility epitaxial films at moderate doping levels of about 1 × 10^20 cm-3, since ionized impurity scattering limits carrier transport at all doping levels beyond. [1] H. J. Kim et al. Appl. Phys. Express 5, 061102 (2012). [2] W. J. Lee et al. Appl. Phys. Lett. 108, 082105 (2016). [3] C. A. Niedermeier et al. Appl. Phys. Lett. 108, 172101 (2016).

Resume : Rutile and anatase TiO2 films are widely used in various industrial applications [1,2]. Rutile TiO2 film is utilized as the optical coating material due to its high refractive index, and anatase TiO2 film is applied as photocatalysts and transparent electrodes. Many experimental techniques were investigated to control the crystal phase in TiO2 thin films. Impurity doping and the controlling of sputtering conditions are most useful techniques [3,4]. Recently, we revealed that Sn doping induced the transformation of TiO2 crystal structure from anatase to rutile phase [5]. In this study, we focused on the origin of rutile to anatase conversion of TiO2 thin film prepared by the sputtering process with Sn doping. We investigated the local structures of Sn ions in TiO2 thin film by X-ray absorption spectroscopy, indicating the Sn ions were substituted at Ti site in rutile TiO2 thin film. The first-principles calculations revealed that the formation energy of SnTi defect in rutile TiO2 was lower than that in anatase TiO2. We will discuss the mechanism on the effect of Sn doping on the crystal structure. [1] J. M. Bennett, et. al., Applied Optics 28, 3303 (1989), [2] Y. Furubayashi, et. al., Appl. Phys. Lett. 86, 252101 (2005), [3] S. S. Pradhan, et. al., Thin Solid Films 520,1809 (2012), [4] K. Okada, et. al., J. Am. Ceram. Soc. 84, 1591 (2001), D. Mardare, et. al., Appl. Surf. Sci. 156, 200 (2000), [5] H. Kotake, et. al., J. Vac. Sci. Technol. A 33, 041505 (2015).

Resume : The wide-band gap semiconductor SnO2 has been used for decades as polycrystalline material in gas sensors and, highly donor doped, in transparent contacts. Recent years brought a renewed interest in single crystalline semiconducting oxides for research into their basic physics, ultimate materials performance, and novel applications. This contribution benefits from unintentionally-doped rutile SnO2 bulk single crystals grown by physical vapor transport [1] to determine the impact of its anisotropic crystal structure [1,2] and effective electron mass [2] on the electron transport with direct relevance for In-free TCO and electronics applications. For this purpose van der Pauw-Hall measurements were performed with (110) and (100) oriented square shaped wafers contacted in the corners. The mobilities in a and c-direction were extracted by comparing the measurement results to finite element simulations of anisotropic transport for this geometry [3]. The extracted mobility anisotropy largely agrees with the effective mass anisotropy [2] for both, limiting phonon and impurity scattering. The increasing effective mass anisotropy with electron concentration [4] directly increases the benefits of a proper crystal(lite) orientation on electron mobility in SnO2-based TCO layers. [1] Galazka et al., Phys. Status Solidi A 211 (2014). [2] Button et al., Phys. Rev. B 4, 4539 (1971). [3] Bierwagen et al., Phys. Rev.B 70, 165307 (2004). [4] Feneberg et al., Phys. Status Solidi A 211 (2014).

Resume : Many applications of transparent conductive oxides (TCOs) would benefit from the ability to be able to deposit them as thin films at low temperatures. By removing the need for high temperature deposition techniques, the ability to use polymer substrates for flexible electronics is gained. However, low temperature methods for solution-deposited films (e.g. printed TCOs), still require high temperature, post-deposition, treatment to achieve transparency together with high conductivity. Here we demonstrate laser processing as a viable alternative to oven-based treatments. In particular, the rate of heating is explored as it is this that allows the laser-induced temperature rise to exceed the conventional upper working limit of the substrate. Thermal modelling has been carried out for laser irradiation of TCO films. Validation of these models has been achieved through the use of an IR camera recording live temperature data. Experimental laser irradiation of solution deposited TCO films has been conducted and preliminary data are presented here. The resistivity of TCO films deposited at low temperatures has been reduced with both UV and IR lasers without damaging the underlying substrate. Both photothermal and photochemical effects have been observed to improve the conductivity of the thin film. This work was conducted as part of the INFINITY project which has received funding from the European Union?s Horizon 2020 research and innovation programme under grant agreement No 641927.

Resume : Zinc oxide films deposited at temperature below 200°C have been intensively investigated because they are prospective candidates for novel electronic applications such as switch elements in three-dimensional memories and inorganic partners of hybrid organic-inorganic structures. Therefore control of conductivity and optical properties of such layers is of a great interest from both scientific and application points of view. These properties are significantly different than in ZnO films obtained at typical temperatures (500-700°C). We present optical and electrical properties of about 200 nm thick ZnO grown by Atomic Layer Deposition at low temperature range between 100 and 200°C. We demonstrate that ZnO films deposited in this temperature range reveal controllable electrical properties, which strongly depend on growth temperature. Ellipsometric measurements show that dielectric constants and energy gap of such films scale with growth temperature and the bandgap of films deposited at 100°C is about 100 meV narrower than the bandgap of ZnO films deposited at 200°C. This is correlated with low carrier concentration which is at the level of 1016 cm-3. Acknowledgements. The work was supported by the Polish National Science Centre (NCN) based on the decision No. DEC-2012/07/B/ST3/03567 and by the EU 7th FP REGPOT project INERA (GA3 16309).

Resume : Semi-insulating cadmium telluride (CdTe) is the most important material for widely used detectors of different types of radiation. It is known, the formation of an ohmic contact to a p-type CdTe crystal faces obstacles because in this case, it is necessary to use a metal with a too high work function. Molybdenum oxide (MoO) has found its wide practical application as an interlayer material for the fabrication of a high quality electrical contact to low resistance p-CdTe. Due to large work function (5.2-6 eV) MoO is a promising candidate for the role of the contact material to semi-insulating p-CdTe. This paper reports on the applications of molybdenum oxide thin films in combination with semiinsulating CdTe. The near-ohmic contacts were prepared by the DC magnetron sputtering of MoO thin films onto p-CdTe wafers produced by Acrorad Co., Ltd. Surface morphology and structural properties of the Mo-MoO/р+/р-CdTe near-ohmic contacts were investigated. The analysis of DC and AC electrical characteristics of the heterostructures was carried out. We have shown that the detectors under study are capable of detecting X-/γ-radiation. This research was supported by the NATO Science for Peace and Security Programme (Project SENERA SfP-984705).

Resume : Tin dioxide (SnO₂) is transparent in the visible spectrum and has good electrical conductivity. Therefore, it has a great potential for use as a transparent semiconducting oxide. In our previous work, we found the existence of a surface electron accumulation layer (SEAL) on air-exposed, single crystalline SnO₂ (101) [1] and (110) heteroepitaxial films. This SEAL likely plays an influential role in SnO₂–based gas sensors but hampers Schottky-contact based applications. Understanding the origin of the SEAL, e.g. intrinsic or due to defects or adsorbates, would bring these oxides into practical semiconductor device applications. In this work, we investigated electronic states of bulk SnO₂ single crystals by hard x-ray photoelectron spectroscopy (HAXPES, hν = 5.95 keV) at 300 and 50 K, which can probe the bulk due to the longer mean free path of photoelectrons. As-grown and annealed SnO₂ single crystals grown by physical vapor transport (PVT) method were measured with HAXPES at the undulator beamline BL15XU of SPring-8. The HAXPES results revealed that, in contrast to our epitaxial films, both as-grown and post-annealed samples showed no SEAL layer at the in-air cleaved (001) surface. The details of chemical bonding states and the temperature dependence of HAXPES will be discussed. The HAXPES measurements were performed under the approval of the NIMS Synchrotron X-ray Station (Proposal Nos. 2014A4600, 2015A4601). [1] T. Nagata et. al., Appl. Phys. Lett. 98, 232107 (2011).

Resume : The aim of this research is to use band gap engineering techniques to investigate the possibilities to reduce the thermal generation and thereby improve the signal-to noise ratio so that operation at room temperature becomes possible. The films with governed doping impurities are of much practical interest for this purpose. Films doping by means of diffusion from the substrate. The method of noise spectroscopy is one of the prospective techniques of impurities investigations in semiconductors. The study of the noise characteristic of ZnO doped with Co, Cr, Mn are presented here. The spectra of noise power were measured by using four-probe method. Noise spectra of ZnO doping different chemical elements, reveal the monotonous decrease: the noise value is given by the law 1/f^gamma (gamma= 0.9) in the frequency range 10^1 – 10^8 Hz. Nevertheless, in the range of middle frequencies the deviation with gamma <1 is observed for 1/f^gamma function. The level and spectral distribution of the noise density are weakly affected by the impurities type and concentration. We have also registered the higher noise level in the ZnO of solid ternary solution based on ZnO (for example, Zn1-xCoxO, x=0.4) in comparison to other film. Complex character of the spectra point out to the different sources of the noise: exceeding-low-frequency, white, generate-ion-recombination and fraktal-like. The increase of contribution caused by the last two sources is especially sufficient at the relatively high frequencies and clearly observed as a dependence Si(f)*f - f. This contribution increases in the ternary compounds. This fact can be cause by the enlargement of the lattice inhomogeneity (formation of Co-riched cluster is possible). The Hooge constant  is an informative parameter, which, in particular, depends on the type of charge scattering in the lattice. The temperature dependence of low-frequency noise can be presented as a sum of acoustic and optical components determined by two Hooge constants: a and o respectively. Shot noise is caused by the randomness in the flux of carriers crossing the active region of a given device and is associated with discreteness of the electric charge. Especially, the inhomogeneities lead to deviation low-frequency noise from the hyperbolic dependence. In the range of middle frequencies the noise power increases (dependence Si(f)*ff). Such a phenomena is observed also in semiconductors with grain boundaries and becomes (as it follows from the experiments) a typical one for films with macrodefects and inhomogeneities.

Resume : Wurtzite ZnO with a band gap ~3.3eV at room temperature has attracted a great deal of attention for light emitter applications, notwithstanding the lack of stable p-doping. (0001)-GaN/c-sapphire templates have become good candidates for epitaxial ZnO films owing to their high structural quality with a moderate lattice mismatch of 1.9%. Zn-polar ZnO, which is preferred for the fabrication of ZnO-based heterojunction high mobility 2D electron gas transistors, can be achieved on Ga-polar GaN. In this work, we have succeeded in the two-dimensional growth of both O- and Zn-polar ZnO epilayers on Ga-polar GaN/c-sapphire templates by controlling oxygen to Zn flux ratio during ZnO nucleation. A detailed high resolution transmission electron microscopy investigation has been carried out in order to determine the mechanisms that underlie the polarity inversion occurring in O-polar ZnO on Ga-polar GaN. It was revealed that the change takes place within one to three monolayers at the GaN/ZnO interface. For clarifying the atomic bonding of interfacial Ga-N, Ga-O, O-Zn, Electron Energy Loss Spectrum (EELS) and elemental EDS mapping have been used. This experimental investigation is carried out along with an atomistic modelling of the interfacial phenomena using molecular dynamics using modified Stillinger-Weber potentials. We propose a chemical and electronic structure of this interfacial transformation.

Resume : NiO is a transparent semiconducting oxide with unintentional p-type conductivity. Currently, NiO is used in batteries, capacitors as well as gas sensors and is considered for future applications in UV-detectors, all-oxide hetero pn-diodes, and organic solar cells. For the latter, NiO is an excellent candidate as an interfacial layer between the ITO anode and the active organic layer, serving as electron blocking and hole transport layer. Here, unintentionally doped NiO thin layers were grown by plasma-assisted molecular beam epitaxy. For the well-defined epitaxy of NiO(100) layers and surfaces, MgO(100) was chosen as substrate due to their common crystal structure with low lattice mismatch. The resulting crystal structure and surface morphology were investigated by X-ray diffraction and atomic force microscopy as well as scanning electron microscopy, respectively. Raman spectroscopy was used to assess the content of symmetry-breaking lattice defects. The optical band gap and layer conductivity were determined by spectroscopic ellipsometry and DC current-voltage measurements, respectively. Near stoichiometric (green) bulk NiO was measured as a reference. The layer properties as a function of growth temperature and oxygen-to-Ni-flux will be discussed and conclusions on unintentional doping and lattice defects will be drawn.

Resume : The interest to study of the fundamental absorption edge of the annealed and unannealed In2O3 films is related to the discussion about the band structure and nature of observed "indirect" transitions in these materials. – This transitions probably associated with the imperfections of the material structure [1, 2]. The films produced by dc-magnetron sputtering on the substrates of Al2O3 (012). The deposition time was 1 h; current – 50 mA; voltage - 300 V, for all films. The temperature of substrates was varied in the range of 20 – 600 °C for different films. The unannealed films have smaller direct and "indirect" band gap when higher substrate temperature used (4.07 eV and 2.73 eV at 20 °C; 3.7 eV and 2.46 eV at 600 °C, accordingly). These results are quite natural – the films deposited on the "hot" (600 °C) substrates are less defective. Annealing leads films to unification in the band gap. The direct and "indirect" band gap decrease to 3.6 and 2.35, accordingly. This explained by the decrease of influence of the barriers in the annealed films. But the energy difference between the direct and "indirect" transitions weakly depends on the substrate temperature and annealing. The latter is hardly consistent with the conclusions of [2] and demonstrates the need for further research of electronic transitions in the indium oxide. [1] Erhart P., Klean A., Egdell R. G., Albe K., Phys. Rev. B, 75, 153205 (2007). [2] Klein A., Appl. Phys. Lett., 77, 2009 (2000)

Resume : Among transparent conductive oxide TCO materials, we can include zinc oxide (ZnO) with interesting physical properties, which places it among the most promising materials for use in various fields such as piezoelectricity, photovoltaic effect and optoelectronics. In this work, we have prepared two series, undoped and La doped ZnO thin films, with flowing concentrations by weight = 0, 1, 2, 3 and 5. LaCl3 was used as source of dopant in the precursor of Zn 2which was obtained by dissolving zinc acetate in 1:1 methanol-double distilled water. During deposition by spray pyrolysis technique, glass substrates were kept out at 375 °C. A fundamental study of their structural and opto-electrical properties such as crystallization, optical transmittance spectra, energy gap and grain size of these materials were realized using XRD and Vis. UV. X-ray diffraction analysis showed that all the films are polycrystalline with a hexagonal wurtzite structure with (002) preferred orientation. Spectrophotometric measurements in Vis. UV range have showed that all the films have a high transmission of about 85% in the visible zone with a band gap energy ranged in (3.28 -3.3) eV. It was found that the disorder of defects in the structure surveyed by deducting the Urbuch energy did not affect the energy band gap ranged in ((56.05-86.98 meV. FTIR measurements showed the presence of Zn-O vibration bands at the interval ( 400 -700 cm-1). The results showed that the increase in dopant concentration leads to a slight shift in the peaks positions. Keywords: TCO, zinc oxide ,spray pyrolysis , DRX, UV, FTIR

Resume : Transparent conductive oxides (e.g. ITO thin films), with thickness values in the range 230 – 370 nm, were prepared onto glass substrates using the rf magnetron sputtering technique. After deposition, the samples were RTA in air at temperatures up to 723K. In this study, a stylus profilometer (Ambios, XP–2) was used to measure the thickness of the oxide thin films. The structural (i.e. XRD, GIXRD, SEM and AFM), optical (i.e. transmission spectra ) and electrical (i.e. I-V characteristic) properties of both as-deposited and annealed samples were investigated. The surface morphology of the samples appeared as granular and polycrystalline with high optical transparency and a good electrical conductivity. Influences of post deposition thermal treatment on morphological properties of these oxides were discussed based on XRD measurements. Double-beam configuration transmittance spectra were recorded in the 190 – 3000 nm wavelength range and, consequently, several optical constants (i.e. Drude damping coefficient, Drude frequency, complex permittivity, refractive indices, extinction coefficients) were obtained for samples with various thicknesses. Optical properties of these oxide films in the near infrared (NIR) range were described by the Drude free electron model. A computational algorithm for oxides thin films using computational models (Swanepoel and Wemple DiDomenico model) was developed and optical properties were investigated. Depending on the preparation conditions and the annealing temperature, value of the optical bandgap, Eg, of the corresponding thin films ranged between 3.35 eV and 3.65 eV. The electrical conductivity was measured using the four points method. An electrical analysis of the conduction mechanisms specific for different voltage ranges was also performed. These oxide films present interesting optoelectronic properties in terms of basic research and related applications of plasmonic metamaterials.

Resume : The transparent conductive oxide (TCO) indium oxide (In2O3) has widespread applications in optical displays and photovoltaic devices due to its high optical transparency in the visible region and its high electrical conductivity.1-3 Generally, the conductivity of In2O3 films can be tuned through doping.4, 5 In this work, films of intrinsic (i.e., not intentionally doped) and n-type doped (by Sn or H) In2O3 variants are prepared by RF magnetron sputtering, and their (optoelectronic) structures are investigated before and after annealing using lab-based x-ray diffraction, ellipsometry, synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES), and Hall measurements. The shape of the In 3d5/2 HAXPES line is observed to be asymmetric for all samples before and after annealing, which is interpreted in terms of a screening mechanism that depends on the amount of charge carriers. All valence band (VB) spectra show significant density of states (DOS) above the VB maximum (VBM) close to the Fermi level, which we ascribe to occupied defect and/or conduction band (CB) states. The intensity of these spectral features is higher (and before annealing similar) for the n-type doped compared to the intrinsic In2O3 material. Upon annealing the VB spectrum of the H-doped In2O3 changes most – in particular the above VBM DOS significantly decreases, indicating a change in defect chemistry and/or charge carrier concentration. Accompanying x-ray diffraction measurements indicate the crystalline and amorphous nature of the In2O3 and the Sn-doped In2O3 samples, respectively, independent of annealing. For the H-doped In2O3, we however see an annealing-induced crystallization of the material. At the same time, Hall measurements show that the carrier concentration is reduced by a factor of approx. 2 and the charge carrier mobility increases by a factor of approx. 3 to 127 cm2/Vs. In our contribution, we will discuss the implications for the optoelectronic properties (that are relevant – and thus need to be considered – for degenerative doped semiconductors, such as the Burstein-Moss [BM] shift6 and the band-gap renormalization [BGR] effect7) in order to relate the HAXPES derived VBM positions to the optically derived band gap values. We find that the BM shift is almost offset by the BGR effect due to the similar effective masses in the CB and VB. 1I. Hamberg and C. G. Granqvist, J. Appl. Phys. 60, R123 (1986). 2C. G. Granqvist and A. Hultäker, Thin Solid Films 411, 1 (2002). 3G. Thomas, Nature (London) 389, 907 (1997). 4J. H. W. de Wit, J. Solid State Chem. 13, 192 (1975). 5S. Limpijumnong, P. Reunchan, A. Janotti, and C. G. Van de Wall, Phys. Rev. B 80, 193202 (2009). 6E. Burstein, Phys. Rev. B 93, 632 (1954). 7P. Schmid, Phys. Rev. B 23, 5531 (1981).

Resume : Abstract Zinc oxide films are of great interest for electronic and optoelectronic device applications. Control of their electrical and optical properties is of great interest from scientific and applied point of view. We present spectroscopic ellipsometry study of zinc oxide films doped with aluminum (AZO), deposited on silicon substrates by Atomic Layer Deposition at 200°C deposition temperature. The thickness of the AZO layers is about 200 nm. The volume concentration of Al2O3 for the investigated layers varies from 0 to 3.39%. The calculated optical bang gap is around 3.31eV and increases with the Al content. The wavelength dispersion curves of the refractive index and the extinction coefficient of the ZnO layers as well as the dielectric functions will be presented. A shift of the ε1 peak toward the higher energies with increasing the Al content is observed due to the Moss-Burstein effect. Thickness maps of the ZnO films deposited on 4 inch Si wafer showing good thickness homogeneity will also be presented. Acknowledgements: This work was supported by the EU 7th FP INERA REGPOT-2012-2013-1 NMP Research and Innovation Capacity Strengthening of ISSP-BAS in Multifunctional Nanostructures

Resume : The effect of composition on interfacial polarization phenomena in polypyrrole/zinc oxide composites was investigated by complex permittivity measurements in the frequency range 10 mHz to 1 MHz Cooling from room temperature to 15 K results in the suppression of the dc conductivity and the identification of dielectric relaxation mechanisms. For 20, 30 and 40 wt % ZnO composites, a high frequency relaxation was detected in the range 10 - 100 k Hz.. Its characteristic mean relaxation time and its concentration dependence are compatible with theoretical redictions for interfacial polarization. For the last two composites mentioned above, an additional low-frequency relaxation appears about the frequency range 10 mHz - 10 Hz and is ascribed to space charge relaxation. The temperature evolution of the relaxation mechanisms studied at different isothermal conditions provides information about the dynamics of relaxing charge carriers. Composites loaded with 10 wt % ZnO are most proper optoelectronic material for electric charge percolation, since they combine good electrical conduction, poor capacitance effects and ohmic sample-electrode contacts.

Resume : To these days ZnO is one of the most widely investigated types of transparent conductive oxides except ITO. The electronic structure of most of the native defects in ZnO was studied both theoretically and experimentally using various methods. Based on simulation and experimental results, A. Sokol, et al., , have proposed a complex model for the electronic structure of different point defects in doped and undoped ZnO [Faraday Discuss., 134, 267-282 (2007)]. There is agreement that the zinc interstitial (Zni) is a shallow donor and is mainly responsible for the n-type conductivity of intrinsic ZnO. The oxygen interstitial (Oi) is neutral in ZnO, has the energy level located about 2.8 eV below the bottom of the conduction band, and is mainly responsible for the blue-green emission at 2.5 – 2.3 eV. But the energy levels of exited Zni* and Oi are controversial. Using density functional theory calculations, Yong-Sung Kim and C. H. Park have shown that Zni* should have an energy level above the conduction band but to this day it has not been proven experimentally and the exact energy position is unknown [Phys. Rev. Lett. 102, 086403 (2009)]. In order to verify their theory we have performed detailed optical and electrical investigations of ZnO films deposited on insulating Si wafers by reactive pulsed laser deposition. The defect engineering in ZnO was performed using non-equilibrium flash lamp annealing operated in the millisecond range with different annealing ambient. Temperature dependent photoluminescence (PL) emission and excitation (PLE) were utilised to determine the radiative transitions and excitation levels in ZnO, respectively. In order to determine the carrier concentration and conductivity type of processed ZnO films, Hall Effect measurements were performed in the temperature range from 3 to 300K. According to PLE the first excited levels of Zni* and Oi are located at 0.83 eV and 0.70 eV above the conduction band, respectively. The concentration of Zni* and Oi is determined by the annealing atmosphere. The oxygen-poor atmosphere promotes the Zn-interstitial formation while annealing in oxygen suppresses the n-type defects and increases the Oi concentration. Comparison of temperature dependent PL and Hall Effect data confirms that the Zni is a main intrinsic source of n-type conduction in ZnO.

Resume : Creating novel functionality utilizing abundant element is a major challenge in material research. 12CaO∙7Al2O3(C12A7) with a crystal structure composed of 3D-connected sub-nanometer-sized cages entrapping oxygen ions as the counter anion is an insulator with a band gap of ~7eV. We have attempted to realize novel functionalities by replacing these oxygen ions with unconventional anions such as O-, H- and electron. C12A7:O- and C12A7:H- exhibit high oxidation power enough to oxidize Pt and light-induced insulator-electronic conductor conversion, respectively. The striking results were obtained for C12A7:e-, a first RT electride ; the conductivity at RT is changed from ~E-10 to E+3 Scm-1 and metal-superconductor transition occurs at low temperatures. The most unique property is its low work function of 2.4eV, comparable to metal potassium, but chemically stable. This property led to the finding of high performance catalyst for ammonia synthesis at ambient pressure when Ru nanoparticles are loaded on the surface of C12A7:e-. A series of finding on electro-active functionality in C12A7 demonstrated the power of nanostructure composed of abundant elements and may be regarded as a pioneering research of “Element Strategy Initiative”, a Japan-original science and technology project. We expect the recent discovery of 2D-electride Ca2N and Y2C along with material design concept would open a new frontier. Recently, we have created amorphous C12A7:e thin films by sputtering. The resulting thin films are optically transparent but retain a low work function (~3.0eV). Such a unique properties meet the requirement for electron injection layers of inverted-type OLEDs. In this talk I show the recent advances in science and application of C12A7:e along with the background.

Resume : Within the last years several concepts were developed for creating environmentally friendly energy sources, such as photovoltaics, fuel cells, and photo-electrochemical cells, which are based on novel nanostructured morphologies and material combinations. For these energy conversion systems semiconducting oxide nanostructures are of great interest since they can be used as e.g. electrode materials or photocatalysts. The occurring interfaces and defects within the nanostructures are the key parameters which determine the functionality and limit charge carrier separation and charge transport. To improve the performance, the inorganic nanostructures can be modified by e.g. doping and/or by creating core-shell structures. Annealing treatments can be performed to minimize defects and to induce phase transformation leading to crystal modifications with a more suitable band gap. Two examples will be presented in this talk, Nb3O7(OH) [1] and TiO2 [2,3] nanowire arrays which can be applied as electrode material in dye sensitized hybrid solar cells and in light induced water-splitting devices. The nanostructures were synthesized hydrothermally. Different advanced transmission electron microscopy (TEM) techniques were applied to study the crystal structure, atomic arrangement, chemical composition, bonding behavior and band gap of the individual nanowires and networks on the atomic scale The results were correlated to the synthesis conditions and used to construct growth models. In addition, the functional properties of the Nb3O7(OH) and TiO2 nanostructures were measured in photo-electrochemical and dye sensitized hybrid solar cells. The properties like charge carrier mobility and lifetime were evaluated and further improved by using modified systems developed with the insights obtained by TEM [3]. [1] S. B. Betzler, A. Wisnet, B. Breitbach, C. Mitterbauer, J. Weickert, L. Schmidt-Mende, and C. Scheu, J. Mater. Chem. A, 2014, 2, 12005 [2] A. Wisnet, S. B. Betzler, R. V. Zucker, J. A. Dorman, P. Wagatha, S. Matich, E.Okunishi, L. Schmidt-Mende, and C. Scheu, Cryst. Growth & Design, 2014, 14(9), 4658. [3] A. Wisnet, K. Bader, S. B. Betzler, M. Handloser, J. Weickert, A. Hartschuh, L. Schmidt-Mende, C. Scheu, J. A. Dorman., Adv. Funct. Mater., 2015, 25, 2601. [4] The author would like to thank the colleagues and co-workers who contributed to this work and the German Research Foundation (DFG) for financial support.

Resume : Perovskite oxides have received significant attention in recent years, in part due to their ability to generate very high density two-dimensional electron gases (2DEGs) at interfaces between polar and nonpolar materials [1]. Most of these complex oxides have degenerate conduction bands composed of transition-metal d states, leading to large effective masses and low mobilities, a detriment for electronic applications. BaSnO3 has emerged as an alternative: it crystallizes in the perovskite structure but its conduction band is nondegenerate and composed of Sn s states, resulting in high mobility, favorable for a transparent conductor. I will show how cutting-edge first-principles calculations shed light on the multiple aspects of this problem: band alignment and confinement of the 2DEG [1,2], mobility [3], and for applications requiring transparency, fundamental limits on absorption [4]. Work performed in collaboration with B. Himmetoglu, A. Janotti, E. Kioupakis, K. Krishnaswamy, and H. Peelaers, and supported by ONR, DOE, LEAST, and NSF. [1] L. Bjaalie, B. Himmetoglu, L. Weston, A. Janotti and C. G. Van de Walle, New J. Phys. 16, 025005 (2014). [2] K. Krishnaswamy, L. Bjaalie, B. Himmetoglu, A. Janotti, L. Gordon, and C. G. Van de Walle, Appl. Phys. Lett. 108, 083501 (2016). [3] B. Himmetoglu, A. Janotti, H. Peelaers, A. Alkauskas, and C. G. Van de Walle, Phys. Rev. B 90, 241204(R) (2014). [4] H. Peelaers, E. Kioupakis, and C.G. Van de Walle, Phys. Rev. B 92, 235201 (2015).

Resume : The stoichiometry of metal cations in complex oxides has a major impact on properties and on their application as functional materials. Synthetic methods must therefore aim at controlling composition. Atomic layer deposition (ALD) is a technique for depositing nanoscale thin films with unparalleled control of thickness, uniformity and conformality. However the control of stoichiometry is not so straightforward. In this talk we show how the cyclic nature of ALD can be exploited to formulate a 'rule of mixtures' for the stoichiometry of ternary oxides in thin film form. We then show how experimental deviations from this rule can be related to chemical reactivity of particular ALD precursors, computed at first principles level (1). This reveals what reactions are taking place at interfaces at the nanoscale. A second major factor affecting oxide properties is morphology, but so far there is little understanding of why certain ALD processes give amorphous films while others give a certain crystal phase. We investigate this question through first principles simulations of the likely surface structures that result from ALD reactions. The system that we study is the deposition of alumina from trimethylaluminium with water or oxygen plasma as co-reagent, where recent data from synchrotron experiments suggest that a nanocrystalline phase is present in the 'amorphous' as-deposited film, which directs the morphology towards the theta-alumina phase on annealing. Our results show that theta-like structural motifs are associated with the incorporation of protons into the film during growth.

Resume : The stoichiometry of metal cations in complex oxides has a major impact on properties and on their application as functional materials. Synthetic methods must therefore aim at controlling composition. Atomic layer deposition (ALD) is a technique for depositing nanoscale thin films with unparalleled control of thickness, uniformity and conformality. However the control of stoichiometry is not so straightforward. In this talk we show how the cyclic nature of ALD can be exploited to formulate a 'rule of mixtures' for the stoichiometry of ternary oxides in thin film form. We then show how experimental deviations from this rule can be related to chemical reactivity of particular ALD precursors, computed at first principles level (1). This reveals what reactions are taking place at interfaces at the nanoscale. A second major factor affecting oxide properties is morphology, but so far there is little understanding of why certain ALD processes give amorphous films while others give a certain crystal phase. We investigate this question through first principles simulations of the likely surface structures that result from ALD reactions. The system that we study is the deposition of alumina from trimethylaluminium with water or oxygen plasma as co-reagent, where recent data from synchrotron experiments suggest that a nanocrystalline phase is present in the 'amorphous' as-deposited film, which directs the morphology towards the theta-alumina phase on annealing. Our results show that theta-like structural motifs are associated with the incorporation of protons into the film during growth.

Resume : The stoichiometry of metal cations in complex oxides has a major impact on properties and on their application as functional materials. Synthetic methods must therefore aim at controlling composition. Atomic layer deposition (ALD) is a technique for depositing nanoscale thin films with unparalleled control of thickness, uniformity and conformality. However the control of stoichiometry is not so straightforward. In this talk we show how the cyclic nature of ALD can be exploited to formulate a 'rule of mixtures' for the stoichiometry of ternary oxides in thin film form. We then show how experimental deviations from this rule can be related to chemical reactivity of particular ALD precursors, computed at first principles level (1). This reveals what reactions are taking place at interfaces at the nanoscale. A second major factor affecting oxide properties is morphology, but so far there is little understanding of why certain ALD processes give amorphous films while others give a certain crystal phase. We investigate this question through first principles simulations of the likely surface structures that result from ALD reactions. The system that we study is the deposition of alumina from trimethylaluminium with water or oxygen plasma as co-reagent, where recent data from synchrotron experiments suggest that a nanocrystalline phase is present in the 'amorphous' as-deposited film, which directs the morphology towards the theta-alumina phase on annealing. Our results show that theta-like structural motifs are associated with the incorporation of protons into the film during growth.