Modification of ZnO Layers by Molecular Adsorbates During Electrochemical Deposition
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Modification of ZnO Layers by Molecular Adsorbates During Electrochemical Deposition Thomas Loewenstein1, Christian Neumann2, Bruno K. Meyer2, Tsukasa Yoshida3, and Derck Schlettwein1 1 Institute of Applied Physics, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, Giessen, D- 35392, Germany 2 Institute of Experimental Physics 1, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, Giessen, D- 35392, Germany 3 Graduate School of Engineering, Gifu University, Yanagido 1-1, Gifu, 501-1193, Japan
ABSTRACT ZnO and ZnO/EosinY hybrid materials were electrodeposited from aqueous zinc salt solutions on (0001) GaN and on (0001) ZnO. Crystalline ZnO was deposited as proven by X-ray diffraction (XRD). The intensity pattern for ZnO/EosinY showed a preferential orientation with the c- plane of ZnO parallel to GaN (0001) or ZnO (0001). XRD rocking curves with FWHM=0.25° indicated a high level of in-plane orientation of the deposited ZnO crystalline domains. The peak position of (0002) ZnO was shifted by 2Θ=1.3°. This difference and the corresponding simultaneous shift of (0004) ZnO were explained by a lattice expansion by 3.6 % in the c- direction. This clearly indicated the strong influence of the Eosin Y molecules adsorbed during the electrodeposition of ZnO. Scanning electron microscopy (SEM) revealed the formation of domains with different crystal sizes pointing at a varying density of nucleation sites on the substrate. INTRODUCTION Electrochemical deposition of ZnO films from aqueous Zn salt solutions is an attractive deposition method because of a wide accessible range of crystal size and film texture [1,2]. If molecular adsorbates are added to the deposition bath as structure- directing agents (SDA), films of high porosity but still maintained high crystallinity can be prepared [3]. Eosin Y (2’,4’,5’,7’Tetrabromofluorescein, C20H6Br4Na2O5) is a very suitable molecule to serve this purpose [4]. The reactions are compatible with a large number of conductive substrates which makes electrodeposition an attractive deposition method for nanostructured crystalline ZnO as electrode material. Single crystalline substrates allow to investigate the structure and texture of such electrodeposited materials to a far higher extent than typical polycrystalline or amorphous substrates. (0001) GaN offers good possibilities of hetero- epitaxial growth of ZnO as proven in electrochemical depositions [1,5,6] or by chemical vapor deposition (CVD) [7]. Homoepitaxial growth of ZnO films on (0001) ZnO substrates was also reported for electrochemical [8] or CVD growth of films [9]. The deposition of porous hybrid materials like ZnO/EosinY films on ordered ZnO substrates as aimed at by the present study was not published yet.
The preparation of porous ZnO films is of interest to utilize the sensitization of ZnO as wide-bandgap semiconductor for photoelectrochemical photovoltaic cells [10]. The influence of SDA on the film growth, their structure and morphology represents an important optimization parameter. In this study
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