Electrodeposition of CU 2 SE thin films by Electrochemical Atomic Layer Epitaxy (EC-ALE).
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ELECTRODEPOSITION OF CU2SE THIN FILMS BY ELECTROCHEMICAL ATOMIC LAYER EPITAXY (EC-ALE). Raman Vaidyanathan*, Mkhulu K. Mathe*, Patrick Sprinkle**, Steve M Cox**, Uwe Happek** and John L Stickney*. *Department of Chemistry, **Department of Physics, University of Georgia, Athens, Georgia 30602. ABSTRACT Electrochemical atomic-layer epitaxy (EC-ALE) is an approach to electrodepositing thin-films of compound semiconductors. It takes advantage of underpotential deposition (UPD), deposition of a surface limited amount (a monolayer or less) of an element at a potential less negative than bulk deposition, to form a thin-film of a compound--one atomic layer at a time. Ideally, the 2-D growth mode should promote epitaxial deposition. We report the formation of compound Cu2Se, at room temperature by electrochemical atomic layer epitaxy (EC-ALE). Cyclic voltammograms were used to determine the deposition potentials of each element. An automated deposition program was used to form 750 cycles of Cu2Se thin films. Electron probe microanalysis was done to determine the stoichiometry of the thin films. X-ray diffraction of the 200 cycle deposit indicated the presence of polycrystalline Cu2Se. The atomic ratio of Cu/Se in the thin films was found to be 2. Band gap of the thin films were determined by reflection absorption measurements. The band gap of the 200 cycle Cu2Se films was found to be 1.6 eV. X-ray diffraction of 350 and 750 cycle Cu2Se films, indicated the deposits consisted of Cu3Se2 and Cu2Se. INTRODUCTION Electrodeposition of II-VI compounds such as CdTe and Chalcopyrite semiconductors CuInSe2 (CIS) has been studied for many years 1-4 and high efficiency photovoltaics, for instance, have been produced commercially. The standard compound electrodeposition methodology, codeposition, involves the use of a single solution, containing oxidized precursors for all the elements involved in the compound. The deposit is formed by reduction at a set potential or current density 5-7. However, reports of the electrodeposition of Cu2Se have been few, and have generally involved postelectrodeposition annealing or high temperature electrodeposition to produce the compound. CIS has also been formed by sequential evaporation and heat treatment of In2Se3 and Cu2Se layers 8-15. Cu2Se has been formed by co-deposition 16-19 and has also been reported to deposit with CuInSe2 during electrodeposition 20, 21, and it has also been used as a precursor for the formation of CIS. Post anneal treatment of the deposit is generally required to obtain stoichiometry and phase formation 17. As electrodeposited Cu2Se showed XRD peaks due to mixed phases of Cu3Se2 in the deposit and required post annealing and high temperature deposition. This highlights a major problem with compound electrodeposition: the need to maintain stoichiometry and form a phase pure material. Recently, the electrochemical analog of atomic-layer epitaxy (ALE) has been developed to form compound thin films 22-28. ALE refers to a methodology developed in the mid 70s for the for
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