Thin films of CdTe, CdSe, and CdS have been electrodeposited by electrochemical atomic layer epitaxy (ECALE), using an automated electrochemical deposition system. Previous reports of an automated system for forming ECALE deposits involved use of a small thin layer flow cell, which revealed several drawbacks. Conversion of the thin layer cell to a thick layer design resulted in greatly improved deposit quality and reproducibility. Deposits were analyzed using electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and grazing incident X-ray diffraction (XRD). The results were consistent with a layer by layer growth mode and the principles of atomic layer epitaxy. CdTe films were grown using up to 1000 ECALE cycles, and were stoichiometric through 500. The 1000 cycle films were a few percent rich in Te, under the conditions used. CdSe and CdS films formed also contained some excess chalcogenide, probably the result of less then ideal deposition parameters. Increasing amounts of particulates and surface roughening were observed for the 500 and 1000 cycle CdTe and CdSe films, relative to the 200 cycle deposits normally formed. This roughening may result from the excess chalcogenide. XRD of the films indicated cubic crystal structures with preferred (111) orientations for all three compounds.

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This paper is the first report of the formation of thin films, thicker than ten monolayers, using electrochemical atomic layer epitaxy (ECALE). Thin films of CdTe have been electrodeposited on polycrystalline gold substrates in an electrochemical thin-layer flow-cell deposition system using the ECALE methodology. Studies of the deposit morphology have been performed using scanning electron microscopy and atomic force microscope. Significant improvements in deposit morphology are reported as a result of changes to the ECALE cycle program and deposition hardware. Deposit compositions analyzed using electron probe microanalysis and inductively coupled plasma atomic emission spectrometry, were found to be stoichiometric and nearly independent of the number of cycles and the Cd deposition potential. In addition, the deposition rate was shown to be one CdTe monolayer per cycle (half monolayer of Cd and half monolayer of Te per ECALE cycle).

Abstract. The electrochemical analog of atomic layer epitaxy (ALE) is being studied.ALE is a methodfor growing thin films of materialsusing a cycle of surface limitedreactions.The surface limitedreactionscontrol the depositionby limitingthe growth to an atomiclayer at a time. In electrochemistry, a surface limited deposition is generallyreferred to as underpotential deposition(UPD), and UPD is used to form the atomic layers in electrochemicalALE (ECALE).The work presented here is an atomiclevel study of the depositionof the first few monolayersof CdSe via ECALE: by the alternatedUPD of atomic layers of Se and Cd on the low index planes of Au. UPD of Se resulted in the formation of ordered structures on each of the low index planes of Au, as observed by low energy electron diffraction (LEED) and scanning tunneling microscopy(STM). The subsequentUPD of Cd resulted in CdSe deposits which exhibited1:1 stoichiometry,as determinedby coulometryand Auger electron spectroscopy (AES). The following LEED patterns were observed for the CdSe monolayers: Au(111)(v'7xv'7)R19.10, Au(111)(3x3), Au(110)(2x3), and the Au(100)(v'2x2v'2)R45°. Similar LEED patterns were observed on each surface for deposits formed using up to three ECALE cycles. In situ STM studies of Cd deposition on Se-covered Au(111) indicated the formation of a (3x3) structure, consistent with LEED results, and with previous TEM studies. The same LEED patterns were also observed for CdSe monolayers where Cd was deposited as the first atomic layer. AES indicated that the element deposited first remained on the bottom, and that deposited second remained on top.

This paper describes the deposition of CdTe thin films by electrochemical atomic layer epitaxy (ALE). ALE involves the formation of compounds an atomic layer at a time, using surface-limited reactions. That is, atomic layers of the elements making up a compound are deposited in a cycle, where each cycle produces a monolayer of the compound. In electrochemical ALE, the surface-limited reactions that produce the atomic layers are referred to as underpotential deposition (UPD). This article describes the dependence of the deposit structure, morphology and composition on a number of the steps in the deposition cycle. Separate optimized solutions and potentials are used to deposit each of the elements. Specifically, a variety of deposition and stripping potentials have been examined, resulting in a broad range of deposit compositions and morphologies. The dependence of the deposits on the potential used to form Cd atomic layers is a good example. If the potential was too positive, no CdTe deposits were formed, as no Cd was deposited, so there was nothing for Te UPD to form on. If the potentials were too far negative, bulk Cd began to deposit, and Cd-rich three-dimensional growth predominated. There was a 0.2 V plateau for the Cd deposition potential where stoichiometric films were deposited; however, the highest quality films were formed within a 0.1 V wide plateau, between -0.55 and -0.65 V The optimal Te deposition potentials appear to be between -0.7 and -0.8 V At more positive potentials, the Cd atomic-layers stripped, while at more negative potentials Te dendrites were formed and the surface roughened badly. Potentials of -1.2 V and below should be used for the Te stripping step, in order to remove all excess Ta, above an atomic layer. If more positive potentials were used, some excess Te remained and three-dimensional growth resulted. Te stripping should be performed f or at least 20 s to completely remove the excess Ta. Neither substrate orientation, nor annealing to 300°C had much effect on the quality of the deposited films.