ICMCTF2017 Session F4-2: Functional Oxide and Oxynitride Coatings

As reported in [1], Ta-O-N material can provide appropriate properties (i.e., band gap width and alignment) for splitting of water into H₂ and O₂ under visible light irradiation (without any external voltage). This could bring a great possibility to convert the solar light into a useful chemical energy. However, it is still impossible to prepare the Ta-O-N electrodes by conventional (chemical) methods at the temperatures less than 500°C without post-annealing. Moreover, the efficiency of this material for water splitting is limited due to fast recombination rate of photogenerated electrons and holes.

Recently, we have demonstrated [2] in our laboratory that high-power impulse magnetron sputtering is a suitable technique for low-temperature (less than 250 °C) and high-rate (higher than 150 nm/min) deposition of Ta-O-N coatings with tunable elemental composition and optical band gap width. In this work, we focus on a further optimization of deposition conditions (e.g., average pulse target power density, working gas pressure, substrate bias and temperature) in order to reach proper crystal and electronic structures of Ta-O-N coatings with respect to the water splitting application. Moreover, we propose to modify the surface of the coatings by Cu nanoclusters in order to enhance the efficiency of water splitting due to a reduced recombination rate of electrons and holes. For this purpose, we have designed a unique dual magnetron-based system combining the reactive high power impulse magnetron sputtering with a source of metallic nanoclusters. The results of our experiments including the coating properties investigated using atomic force microscopy, spectroscopic ellipsometry and high-resolution SEM and preliminary data on photocatalytic activity are presented in detail.

[1] R. Abe, J. Photochem. Photobiol. C Photochem. Rev. 11 (2010) 179.

[2] J. Rezek et al., Thin Solid Films. 566 (2014) 70.

I present an overview of our experimental and theoretical investigations of Ca₃Co₄O₉- and CaMnO₃-based systems by reactive magnetron sputtering for thermoelectrics and amorphous oxynitride M-Si-O-N coatings as hard, transparent coatings. We have introduced a two-step sputtering/annealing method for the formation of highly textured virtually phase-pure Ca₃Co₄O₉ thin films by reactive co-sputtering from Ca and Co targets followed by an annealing process at 730 °C under O₂-gas flow.The thermally induced phase transformation mechanism was investigated by in-situ time-resolved annealing experiments using synchrotron-based 2D x-ray diffraction as well as ex-situ annealing experiments and standard lab-based x-ray diffraction [1] . By tuning the proportion of initial CaO and CoO phases during film deposition, the method enables synthesis of Ca₃Co₄O₉ thin films as well as Ca_xCoO₂. The same approach is used to synthesize CaMnO₃ and CaMn_xNb_1-xO₃ perovskite oxides on Al₂O₃ (0001), (1-100) and (1-102). Furthermore, amorphous thin films in the Mg/Ca-Si-O-N systems were deposited by reactive RF magnetron co-sputtering from Mg(Ca) and Si targets in Ar/N₂/O₂ gas mixtures [2]. The films were found to be homogeneous and transparent in the visible region with high hardness of 21 GPa and elastic modulus of 166 GPa.

Within this work, niobium oxide thin films were deposited on silicon and glass substrates by reactive dc magnetron sputtering from niobium targets prepared by cold gas spraying. The oxygen partial pressure in the oxygen/argon atmosphere was varied while the overall gas pressure and the applied target current were kept constant. The applied pulsed d.c. substrate bias voltage was set to -50 V and several pulse conditions were used. Scanning electron microscopy was used to investigate the topography of film surface and fracture cross-section. To examine the crystalline structure, X-ray diffraction and Raman spectroscopy was applied. Further, X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy were used to measure the oxygen content within the films and to illuminate the chemical bond structure. The optical properties of the films were determined by their reflexion and transmission spectra, while the electrical thin film resistivity was measured by four point probe. With increasing oxygen partial pressure the film growth rate decreases, while the oxygen content within the films increases. All films grown show a smooth surface and their amorphous microstructure is reflected by both, the fracture cross-sections and the X-ray diffractograms. The increasing oxygen content within the films leads to a transition from non-transparent films to nearly fully-transparent films. This transition is linked to an increase of the electrical resistivity, where films grown at the highest oxygen content show insulating properties.

The bismuth oxide-based materials have been gaining interest because their optical and electrical properties are suitable for applications, such as opto-electronic devices, catalysts in photo-induced processes and clean energy generation areas. The bismuth niobium oxides (BiNbO) system is one of this materials group, and they have been reported as possible candidates for solid electrolytes and high-k dielectric materials for capacitors. In such applications, their synthesis as thin films is desirable. In this work, we propose the use of the dual magnetron co-sputtering technique for the synthesis of the different compounds of the BiNbO system controlling the composition and structure by adjusting the deposition parameters of the two independently driven targets. The films were deposited from an α-Bi₂O₃ and Nb targets; the power applied to the ceramic target was fixed at 30 W (radio frequency), and the power applied to the Nb was varied between 20 and 150 W (DC). The deposition was done under Ar:O₂ (20 volume % O₂) reactive atmosphere. The substrates were borosilicate glass pieces, which were heated at 150 °C. The films deposited at Nb power above 50 W were amorphous, then they were annealed at 600 °C for 2 h in air. The identification of the obtained different phases was done correlating the structural and compositional results by x-ray diffraction and energy dispersive x-ray spectroscopy/x-ray photoelectron spectroscopy, respectively. Four different structures were obtained: solid solutions with different Nb concentrations and defective fluorite-based structure, Bi₃NbO₇, Bi₅Nb₃O₁₅ and BiNbO₄. The electrical conductivity of the films was measured as a function of the temperature by both DC and AC (impedance spectroscopy) two-probe methods, using Pt electrodes on the film surface and a ring electrode configuration. From the analysis of the data, it was possible to estimate the activation energy for the ionic and or electronic conduction processes. The films presenting the fluorite solid solution structure and the Bi₃NbO₇ phase showed ionic conductor behavior, while the other phases were insulators.

Acknowledgements: The research leading to these results has received funding from the BisNano project (125141), PHOCSCLEEN (318977) and the CONACYT (251279), DGAPA-PAPIIT (100116). ODR thanks to CONACYT for the scholarship during the PhD study.

Mixed oxides of iron with other metals like zinc, nickel, or manganese have already been used for a long time because of their magnetic properties, especially in electrical engineering as core material of coils and current transformers.

The fabrication of the bodies needed for these applications is usually carried out by sintering technique, in which the details of process management affect the characteristics of the finally produced material in decisive way.

Ferrites as thin film material so far have been little investigated but more and more come into focus of interest for potential applications as electromagnetic shielding or optical absorber.

In the work to be presented, manganese ferrite films with and without the addition of chromium have been produced by reactive pulsed magnetron sputtering of alloyed targets.

Primarily, the influence of substrate temperature and oxygen content in the gas atmosphere on structure and optical properties of the deposited layers have been investigated. The evaluation of the magnetic properties is of more complex nature and will be published at a later date.

Layers with thicknesses between one and two microns have been deposited onto flat samples of polished stainless steel and borosilicate glass. The substrate temperature was in the range from 150°C (without additional heating) to 600°C.

All deposited films show a dark anthracite or black appearance in reflected light and a more or less pronounced transmittance in the infrared region, where the transmission factor is determined by the oxygen content and the onset wavelength of transmission by the metal composition.

Whereas at lower substrate temperatures smooth amorphous films with distinct residual reflectivity are formed, at temperatures above 500°C matt crystalline layers with an absorption coefficient of more than 95% over the whole wavelength range of visible light arise.

The crystalline layers exhibit spinel structure and a surface roughness of more than 100 nanometers. XRD spectra, SEM pictures, and GD-OES profiles will be presented, and the mechanism of layer growth in the different temperature regions will be discussed.