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.

No abstract

No 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.Many II-VI and a few III-V compounds have been formed by EC-ALE. TI-VI films such as CdSe, CdS, and CdTe have been successfully formed. In addition, deposition of III-V compounds of InAs and InSb are being explored, along with initial studies of GaAs deposition. Depositions of the I-VI systems are better understood so this report will focus on the III-V's, particularly InAs and InSb.Building compounds an atomic layer at a time lends electrochemical-ALE to nanoscale technology. Deposited thickness ranged from a few nanometers to a few hundred. The films are typically characterized by atomic-force microscopy (AFM), Xray diffraction (XRD), electron microprobe analysis (EPMA) and ellipsometry. InAs deposits are also characterized by infrared reflection absorption.

This chapter concerns the state of development of electrochemical atomic layer epitaxy (EC‐ALE), the electrochemical analog of atomic layer epitaxy (ALE). EC‐ALE is being developed as a methodology for the electrodeposition of compound semiconductors with nanoscale control. ALE is based on the formation of compounds, one monolayer (ML) at a time, using surface‐limited reactions. An atomic layer of one element can be electrodeposited at a potential under that needed to deposit the element on itself, and this process is referred to as underpotential deposition (UPD). EC‐ALE is the use of UPD for the surface‐limited reactions in an ALE cycle. Electrodeposition is generally performed near room temperature, avoiding problems with interdiffusion and mismatched thermal expansion coefficients. This makes EC‐ALE a good candidate to form superlattices, where the compound deposited is modulated on the nanometer scale. This chapter describes the basics of the EC‐ALE cycle, the elements that have been used to form deposits, as well as the solutions, rinsing, and potential changes. It describes the hardware presently being used by this group and other research laboratories, the compounds that have been formed, and the development of deposition cycles for various compounds. It describes the status of device formation using EC‐ALE, and goes over some of the problems and issues involved in developing cycles and growing films. Finally, given that EC‐ALE is based on surface‐limited electrochemical reactions, studies of relevant electrodeposit surface chemistry are discussed.