coatings Article Ca-Doped ZnO:Al Thin Films: Synthesis and Characterization Anca-Ionela Istrate 1, *, Iuliana Mihalache 1,2 , Cosmin Romanitan 1 , Oana Tutunaru 1 , Silviu Vulpe 1 , Florin Nastase 1 and Lucia Monica Veca 1   Citation: Istrate, A.-I.; Mihalache, I.; Romanitan, C.; Tutunaru, O.; Vulpe, S.; Nastase, F.; Veca, L.M. Ca-Doped ZnO:Al Thin Films: Synthesis and Characterization. Coatings 2021, 11, 1023. https://doi.org/10.3390/ coatings11091023 Academic Editors: Angela De Bonis and Aomar Hadjadj Received: 21 July 2021 Accepted: 23 August 2021 Published: 26 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 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/). 1 National Institute for Research and Development in Microtechnologies, 126A Erou Iancu Nicolae Street, Ilfov County, 077190 Voluntari, Romania; iuliana.mihalache@imt.ro (I.M.); cosmin.romanitan@imt.ro (C.R.); oana.tutunaru@imt.ro (O.T.); silviu.vulpe@imt.ro (S.V.); florin.nastase@imt.ro (F.N.); monica.veca@imt.ro (L.M.V.) 2 Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, Gheorghe Polizu Street 1-7, 011061 Bucharest, Romania * Correspondence: anca.istrate@imt.ro Abstract: We unveiled the effect of doping on the morpho-structural and opto/electrical properties of Ca-doped ZnO:Al thin films obtained by RF magnetron sputtering. Scanning electron microscopy (SEM) was performed to reveal the surface morphology, while the composition and crystal structure were investigated by energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The correlation between the microstructure and the electrical conductivity identifies an increase in electrical conductivity up to 145 × 10 −3 Ω −1 ·m −1 at 5 wt.% Ca doping level with the decrease in the grain size. Furthermore, the presence of Ca dopant triggers the occurrence of the emission peak at 430 nm and an increase of the green emission peak in PL spectra. Corroborating the electrical measurements with X-ray diffraction and optical measurements, one can infer that the electrical conductivity is dominated by intrinsic defects developed during deposition and by the existence of dopants. Keywords: Ca-doped ZnO:Al; thin films; RF magnetron sputtering; chemical diffusion 1. Introduction Zinc oxide (ZnO) thin films prepared by physical methods such as radio frequency magnetron sputtering continue to be of high interest due to their low cost and low absorption of visible light, triggering an enhanced UV photoresponse of the photodetector, transparent conductive electrodes in solar cells, and fluorescence imaging performance [1–3]. In addition to good absorptivity, high conductivity is the prerequisite for their ap- plications in electro-optic devices. An effective method to achieve these properties is doping with metals, especially elements of group III (Al, Ga), which have been shown to substitute Zn or O in the ZnO structure to enable n-type doping [4–10]. Employing a physical deposition approach, Sun et al. [11] studied (Al, Co)–ZnO films cosputtered on glass substrate to reveal that (Co, Al) doping affects the carrier mobility due to the reduce crystallinity in the deposited films. Wang et al. [12] investigated Al and F co-doped zinc oxide (AFZO) thin films on glass substrates by radio frequency magnetron sputtering, when the co-doping induces a significant decrease of the film resistivity in comparison with either Al or F doping, revealing the effectiveness of Al and F co-doping on electrical properties of ZnO thin films. Besides these intensively investigated ZnO-based alloys, II–VI compounds with a rock salt structure and a band-gap above 7 eV (7.2 eV in the case of calcium oxide and 7.8 in the case of MgO) are known for enhancing the transmittance of the ZnO film [13]. Doped/co-doped ZnO films proved to be suitable materials for both gas sensing and as the constituent of a layered transparent conductive oxide electrode for organic electronics. An efficient transparent conductive gas diffusion barrier, with improved optical, electrical, Coatings 2021, 11, 1023. https://doi.org/10.3390/coatings11091023 https://www.mdpi.com/journal/coatings