One-step electrodeposition of CdS/ZnS bilayer from an aqueous mixture of Cd 2+ and Zn 2+
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One-step electrodeposition of CdSyZnS bilayer from an aqueous mixture of Cd21 and Zn21 Koichi Yamaguchi, Tsukasa Yoshida, Takashi Sugiura, and Hideki Minoura Department of Chemistry, Faculty of Engineering, Gifu University, Yanagido 1-1, Gifu 501-11, Japan (Received 17 September 1997; accepted 6 January 1998)
Electrochemical thin film deposition in aqueous mixtures containing Cd21 , Zn21 , and thioacetamide has been investigated. It has been found that the atom-by-atom growth of well-crystallized CdS thin film preferentially takes place at the early stage of the reaction, followed by the cluster-by-cluster growth of the poorly crystallized ZnS outer layer, thus resulting in the formation of CdSyZnS bilayer by a one-step synthesis.
I. INTRODUCTION
Materials preparation by solution processing has enjoyed increasing attention in recent years because of its advantages in lowering the cost as well as the stress to the environment upon the production, as compared to the conventional gas phase techniques which unavoidably require high temperature and/or vacuum.1 II-VI compound semiconductors such as CdS or ZnS find applications in various opto-electronic devices. Thin film preparation of such materials by employing chemical2–9 and electrochemical9–14 reactions in aqueous solutions has been intensively studied in the past few decades. However, the chemistry in the solutions responsible for the growth of the thin films is still under active discussion and yet to be clarified. For example, the chemical bath deposition (CBD) of CdS and ZnS thin films has been achieved in very similar systems using aqueous mixtures of corresponding metal salt and sulfur source molecules such as thiourea or thioacetamide (TAA), although the crystallinities of the resultant films are remarkably different.2 It has been claimed by Lincot and co-workers that the growth of CdS crystals proceeds in an atom-by-atom fashion through the heterogeneous reaction between the Cd(OH)2 like surface sites and thiourea, while the deposition of ZnS films is achieved by the accumulation of ZnS fine particles formed in the bulk of the solution, namely, in a cluster-by-cluster manner.2 It is important to elucidate the differences in behavior among the chemical species which are responsible for the film growth, in order to further improve the solution phase techniques for the deposition of compound thin films. In this work, we have examined the film growth in an acidic aqueous mixture containing both Cd21 and Zn21 . Although the CBD of CdS thin films does not take place in acidic solutions, we have reported that an electrochemical assistance, namely, to cathodize the conductive substrate, can yield a CdS thin film with an excellent crystallinity owing to the electrochemical J. Mater. Res., Vol. 13, No. 4, Apr 1998
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formation of the Cd(OH)2 surface sites.15 In contrast, we have found that ZnS can be chemically deposited even in acidic media. It is interesting to inv
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