Crystal defects and optical emissions of pulse electrodeposited ZnO

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ZnO has been widely studied in the last decades as an n-type semiconductor due to its wide application range, for example, in optoelectronics, solar cells, light-emitting diodes, thermoelectrics, amongst others. The material efficiency for certain applications is highly dependent on the presenting film morphology. Electrodeposition is well-known as a technique with precise control over the structural and morphological properties of the obtained materials. When the structural and morphological properties are tuned, it is possible to find a wide variety of defects in the ZnO structure. In this study, ZnO films were grown using pulsed electrodeposition with variation of the reduction potential. The crystal order, structural defects and optical emissions of the films have been analyzed by X-Ray Diffraction (XRD), X-ray Absorption Near-Edge Structure (XANES), Extended X-ray Absorption Fine Structure (EXAFS) and Photoluminescence (PL). ZnO film grown at less negative reduction potential presents a stronger texture along [0001] by XRD, higher crystalline order, and more zinc vacancies by XANES and EXAFS. The films obtained at less negative potential present less OH - trapped in the ZnO structure and a relatively higher level of defects (O_i )^0, (O_Zn)^0, (O_i)^(-/2-) and (O_Zn)^(0/-) than those grown at higher reduction potentials by PL. This will be related to the fact that at less negative potentials there is less concentration of OH- at the film surface than at more negative potentials. The combination of X-ray absorption spectroscopy and photoluminescence reveals the complicated nature of the atomic defect in electrodeposited ZnO films. Allowing to evidence the preferential presence of atomic defect as a function of the reduction potential. In this work, we have also compared those defects with reference compounds such as a Zn foil and ZnO polycrystalline powder.
©2020 Elsevier The authors would like to acknowledge financial support from MAT2017-86450-C4-1-R, MAT2017-86450-C4-3-R, and RTI2018095303-A-C52. C.V.M acknowledges financial support from Juan de la Cierva Incorporacion grants IJCI-2017-31350. A.S. and A.M.N. acknowledge the financial support from the Comunidad de Madrid for an "Atraccion de Talento Investigador" contract No. 2017-t2/IND5395 and 2018-T1/IND-10360, respectively. We acknowledge The European Synchrotron (ESRF), MICIU, and CSIC for provision of synchrotron radiation facilities in using the BM25-SpLine beamline. We also thank the BM25-SpLine staff for the technical support beyond their duties. We acknowledge the service from the MiNa Laboratory at IMN, and funding from CM (project SpaceTec, S2013/ICE2822), MINECO (project CSIC13-4E-1794) and EU (FEDER, FSE).