**About the Editors**

**Erwan Rauwel** received his PhD degree fromthe University of Caen in materials science in 2003. He continued with postdoctoral studies in collaboration with STMicroelectronics at Minatec, Grenoble, France and at the University of Aveiro (Marie Curie IE Fellowship), Portugal. He worked as a senior researcher at the university in collaboration with Statoil and then worked at Taltech as a professor in Estonia for 5 years. He is now a professor at the Institute of Technology of the Estonian University of Life Science in Estonia where his team investigates the properties of nanoparticles for water purification, the development of biocidal coating to fight AMR and COVID-19, and the development of nanocomposites for photocurrent generation and energy harvesting and they also investigate new possible treatments against cancer with the LBN (Montpellier). He has more than 65 peer-reviewed publications with an H-index of 22, 3 books, 5 book chapters, and 6 patents. He is chief scientist in his start-up company specializing in nanomaterials (PRO-1 NANOSolutions). More information about his recent research activities can be found at www.rauwel.eu.

**Protima Rauwel** is Head of Energy Applications Engineering Chair at the Institute of Technology of the Estonian University of Life Sciences. She obtained her PhD in 2005 from the University of Caen, France in condensed matter and materials science with a special focus on nitride thin films with quantum dots and quantum wells. She has postdoc experience from the University of Aveiro, Portugal and the University of Oslo, Norway, where she continued her research on thin films and freestanding nanomaterials. Her research focus is presently on nanomaterials for water remediation, optoelectronic applications, and anti-microbial applications with a focus on synthesis, characterization, and application. She has co-authored more than 85 publications, 3 books, and 5 book chapters. She is an inventor on 2 patents and has a Google Scholar H-index of 28.

#### *Editorial* **Editorial for the Special Issue on 'Metal Oxide Thin Film s: Synthesis, Characterization, and Application s'**

**Erwan Rauwel \* and Protima Rauwel**

Institute of Technology, Estonian University of Life Sciences, Kreutzwaldi 56/1, 51014 Tartu, Estonia; protima.rauwel@emu.ee

**\***Correspondence: erwan.rauwel@emu.ee

The last two decades have witnessed the development of new technologies for thinfilm deposition and coating. A marked improvement in thin-film deposition methods, enabling the precise coating of surfaces at the nanoscale on complex nanostructure and on large surfaces is viable. These include new deposition methods like atomic layer deposition (ALD) that enables the coating on complex nanostructures with high conformality and homogeneity. Other more conventional methods based on coating in solution have also been improved in order to achieve a high quality conformal coating. These deposition processes also enable the synthesis of multilayers and the development of synergistic effects through the mutual interaction of such layers. At the same time, a need for new tools to characterize these films has emerged. This Special Issue is dedicated to these recent developments and contains 10 manuscripts reporting thin-film deposition and coating and their characterization. It compiles works on vapor-phase deposition processes including atomic-layer deposition (ALD) [1,2], mist-chemical-vapor deposition [3] and atmosphericpressure chemical-vapor deposition (APCVD) [4] methods in which authors have studied the stabilization of metastable phases and the effect of growth conditions on thin film quality, epitaxy, and crystallinity. The Special Issue also includes liquid-phase coatings like sol-gel [5], solution immersion [6], micro-arc oxidation [7], Plasma Electrolytic Oxidation Processing [8], and single pulse anodization [9] where the authors were more focused on the coating of aluminum. The last paper reports on a new metal oxide thin film characterization method [10] that can extract the majority charge carriers' type, their concentration, and mobility and is extendable to the study of other wide-bandgap semiconductors. Two of these manuscript reports studies are on Transparent Conductive Oxide thin films [4,10] and in one manuscript, the authors investigate InGaZnOx Thin Film Transistors [6].

The first four manuscripts of this Special Issue consist of thin-film depositions in gas phase or solution. The stabilization of NiTiO3 metastable phase using atomic layer deposition (ALD) was studied by Bratvold et al., in the manuscript "Phase and Orientation Control of NiTiO3 Thin Films" [1]. They demonstrated that it is possible to stabilize NiTiO3 disordered phase (R −3c) on α-Al2O3 substrates. They also investigated the influence of substrate on the symmetry and orientation of NiTiO3 thin films under epitaxial growth conditions on LaAlO3(100), SrTiO3(100) and MgO(100) substrates. They have demonstrated that the orientation and symmetry of NiTiO3 thin films deposited by ALD can be controlled by the choice of a suitable substrate. Using ALD, Duparc et al., studied the deposition of GdCoO3 and Gd0.9Ca0.1CoO3 thin films on LaAlO3(100) and YAlO3(100/010) substrates in "Atomic Layer Deposition of GdCoO3 and Gd0.9Ca0.1CoO3" [2]. These coatings may find applications in the field of oxidation catalysis. They showed that crystal orientations of the films can be tuned by the selection of the substrate and mitigated through the interface via solid-face epitaxy upon annealing. In the third manuscript, Xu et al. report on the influence of carrier gases in mist chemical vapor deposition method on the quality of epitaxial corundum-structured α-Ga2O3 films [3]. The thin films were grown on c-plane sapphire substrates with single (0006) plane orientation for all carrier gases, but the authors

**Citation:** Rauwel, E.; Rauwel, P. Editorial for the Special Issue on 'Metal Oxide Thin Films: Synthesis, Characterization, and Applications'.*Materials* **2021**, *14*, 1834. https:// doi.org/10.3390/ma14081834

Received: 31 March 2021 Accepted: 6 April 2021 Published: 7 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

demonstrated that the O2 gas enables the growth of better quality epitaxial films with increased growth rate (10.3 nm/min vs. 5.3 nm/min and 2.4 nm/min) and better crystalline quality. The fourth manuscript reports the study of the influence of La doping in BiFeO3 thin films grown on Pt/TiO2/SiO2/Si substrates by the sol-gel method [5]. The authors have shown that it is possible to increase the grain size and self-polarization and promote (102) texture of the film by La doping and a higher growth temperature. These characteristic are important for future piezoelectric applications. They show that piezoelectric properties are connected to the films' growth conditions and La doping, emphasizing that if both are thoroughly controlled BiFeO3 -based films could be used in micromechanical applications.

Manuscripts reporting on thin films grown by APCVD and solution immersion follow these studies. In "Origin of Magnetotransport Properties in APCVD Deposited Tin Oxide Thin Films" by Jurai´c et al., the structural and magnetotransport studies of tin oxide thin films prepared by the cost-effective Atmospheric Pressure Chemical Vapor Deposition method deposited on soda-lime glass substrates [4]. In particular, they studied the charge carrier density and mobility of the films that are typical values of SnO2 films manifesting a temperature dependence in their case. This behavior is attributed to grain boundary scattering and crystallites preferred orientation. In the second paper, "Optimizing the Properties of InGaZnOx Thin Film Transistors by Adjusting the Adsorbed Degree of Cs+ Ions", Zhang et al. investigated surface structure modifications and increased oxygen vacancy concentrations of a-IGZO thin films through the introduction of Cs+ ions into the surface [6]. They observed an improvement of transfer properties and stability of the films coated by solution immersion method at low temperature. They found that the optimized performance of Cs-doped InGaZnOx Thin Film Transistors was obtained for a Cs+ ion concentration of 2% mol/L, with thin films that exhibit a carrier mobility of 18.7 cm2·V−1·s<sup>−</sup>1, OFF current of 0.8 × 10−<sup>10</sup> A and a threshold voltage of 0.2 V.

The last set of manuscripts focus more particularly on aluminum coating using liquid phase coating. In these investigations, the authors mainly investigated how the coating methods could be improved and identified the main parameters that control the growth quality. Song et al. studied the tribological performances of micro-arc oxidation of aluminum 6061 alloy with the addition of 0.75 g/L of cellulose into the electrolyte [7]. They observed an improvement of the coating with a decrease of the roughness along with an increase of cellulose concentration in the electrolyte. They indicate that cellulose fills the micro-cracks and micro-pores and a part of cellulose is cross-linked with the Al3+. This study is followed by the report of Yang et al., in which the authors investigated a novel self-adaptive control method for large scale coating of aluminum alloys using plasma electrolytic oxidation processing [8]. They showed that coating can be controlled via feature information (Vb, Ib, Vi, and Vu) and the growth process can be tuned via the voltage, current, and processing time parameters. The coating exhibited good uniformity and compactness without visible cracks and large pores. For optimal growth process conditions on 6061 aluminum alloy samples, they measured a specific energy consumption of 1.8 kWh·m<sup>−</sup>2· μm<sup>−</sup>1. The same group also studied the discharge behavior and dielectric breakdown of oxide films deposited on aluminum in an alkaline silicate electrolyte using single-pulse anodization of aluminum micro-electrodes [9]. They measured voltage and current waveforms of applied pulses and studied the morphology of the coating. They found a good correlation between the pulse parameters and shape of discharge channels and showed that by increasing potential and pulse width it is possible to form circular opened pores on the surface.

The last paper entitled "Hot-Probe Characterization of Transparent Conductive Thin Films" describes the dynamic hot-probe measurement system that replaces or supplements existing methods like Hall effect measurements or the Haynes–Shockley experiments [10]. The replacement of indium–tin oxide by more cost-effective materials like ZnO doped by Al, Mn, or Sb requires mobile and low-cost evaluation methods. The paper shows that Dynamic Hot-Probe Characterization can extract the main parameters of metal oxide

thin films like the majority charge carriers type, their concentration, and mobility and this method may also be applied to other wide-bandgap semiconductors.

We hope that this special issue will serve as a valuable support for researchers in the field of metal oxide thin-film deposition and characterization and we wish you a pleasant read.

**Funding:** This research was funded by European Regional Development Fund gran<sup>t</sup> number "Emerging orders in quantum and nanomaterials" and Eesti Maaülikool (EMÜ) Bridge Funding gran<sup>t</sup> number P200030TIBT.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The gues<sup>t</sup> editors would like to thank the authors for submitting their work to the special issue and its successful completion. A special thank you to all the reviewers participating in the peer-review of the manuscripts submitted. We are also grateful to Tiffany Wan and the editorial assistants who made the whole special issue gues<sup>t</sup> editing a smooth and efficient process.

**Conflicts of Interest:** The authors declare no conflict of interest.
