Optimization of the formation of bismuth telluride thin film by using ECALE W. Zhu, J.Y. Yang * , J. Hou, X.H. Gao, S.Q. Bao, X.A. Fan State Key Laboratory of Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Received 18 May 2005; received in revised form 12 July 2005; accepted 21 July 2005 Available online 29 August 2005 Abstract The formative optimization of Bi 2 Te 3 VA–VIA group compound thin film by using electrochemical atomic layer epitaxy (ECALE) was reported. Cyclic voltammograms were used to analyze electrochemical aspects of tellurium and bismuth. The voltam- metric analysis of underpotential shift demonstrates that the first Te UPD on Bi-covered Au and Bi UPD on Te-covered Au fit UPD dynamics mechanism. Using the integrated Faradaic charges of anodic stripping currents, the UPD region of the first Bi UPD on Te-covered Au and Te UPD on Bi-covered Au were determined. An optimized deposition program was developed to form 200 cycles deposits. X-ray diffraction indicated the deposits were Bi 2 Te 3 compound. EDX quantitative analysis gave a 2:3 stoichiometric ratio of Bi to Te, which is consistent with the XRD result. The surface morphology was determined by field emission scanning elec- tron microscope (FESEM). It showed that the deposits consist of 30–100 nm crystallites and are conformal with the Au substrate, which suggested an epitaxial growth mechanism for Bi 2 Te 3 thin films. Ó 2005 Elsevier B.V. All rights reserved. Keywords: ECALE; UPD; Bismuth telluride; Thermoelectric material; Thin film 1. Introduction The VA–VIA group compound Bi 2 Te 3 and its relevant solid solution are widely used for thermoelectric and optoelectronic devices, for example in solid-state refriger- ation, heat pumps, subminiature electronic devices, infra- red sensors and high efficiency photovoltaic solar cells. The performance of these devices depends on the figure of merit (ZT) of the material. Several possible approaches to enhance ZT have been investigated [1,2]. In compari- son with bulk materials, nanostructured thin film materi- als offer tremendous scope for ZT enhancement. The enhancement is achieved by controlling the transport of phonons and electrons in nanomaterials. A Bi 2 Te 3 / Sb 2 Te 3 superlattice structure via the route of metal–or- ganic chemical vapor deposition (MOCVD) with a high ZT value of 2.4 at 300 K, has been reported [3], which can compete to a kitchen refrigerator. However, for a Bi 2 Te 3 compound thin film, MOCVD means very expen- sive precursors, complicated process and devices, high temperature condition and even some toxic byproducts. It is of significance to develop more competitive and economical process for them. Recently, a method named as electrochemical atomic layer epitaxy (ECALE), which originates from a combi- nation of electrochemical deposition and atomic layer epitaxy, was brought forward by Gregory and Stickney [4]. It is a valid alternate underpotential deposition (UPD) of the elements that form the compound semi- conductor in a cycle [5,6]. Epitaxial deposition is referred to as underpotential deposition by the means 0022-0728/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.07.016 * Corresponding author. Tel.: +86 27 87540944; fax: +86 27 87543776. E-mail addresses: wennarhust@yahoo.com.cn (W. Zhu), jyyang@ public.wh.hb.cn (J.Y. Yang). www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 585 (2005) 83–88 Journal of Electroanalytical Chemistry