Atomic layer epitaxy of CdTe using an automated electrochemical thin-layer flow deposition reactor

Abstract

A number of different cycle chemistries, along with an automated thin-layer flow cell electrodeposition system, are described for the formation of CdTe thin film deposits using electrochemical atomic layer epitaxy (EC-ALE). Atomic layer epitaxy (ALE) involves the deposition of a compound one atomic layer at a time, via surface limited reactions, in a repeating cycle. In EC-ALE, underpotential deposition (upd) is used to form the atomic layers. Previous studies of the EC-ALE growth of CdTe have involved a cycle where Cd was deposited by reductive upd, followed by oxidative upd of Te from an acidic (pH 2.0) solution. In the present study, basic (pH 10.2) tellurium solutions were investigated in an attempt to use direct reductive upd of Te, as well as reductive upd of Cd. The idea was to simplify the cycle. The deposition in the basic solution is shifted dramatically negative, such that surface limited reductive deposition of Te appears to coincide with potentials used for reductive Cd upd, thus allowing both elements to be reductively deposited in a cycle at similar potentials. Improvements have been made relative to previous deposits reported by this group, such as an increase in the amount deposited per cycle. The old cycle and the H-cell design produced only 0.4 ML per cycle, while our new cycle deposits the expected 1 ML per cycle. However, there were some drawbacks to the new cycle, which was based on the reductive upd formation for both Cd and Te. Even though voltammetry for Te deposition on Au suggests that Te deposits by a surface limited process, it in fact deposits at an overpotential. Therefore, some bulk Te is inevitably deposited along with each Te atomic layer. The amount of bulk deposited is a function of convection in the cell, and thus leads to inhomogeneity in the deposit, something not expected for a purely surface limited process. In order to avoid the traces of bulk Te, the best deposits were formed when the reductive deposition of Te was combined with a bulk Te stripping step to remove excess material. This process is referred to here as oxidative Te upd. The resulting deposits evidenced a predominant [111] orientation for zinc blende CdTe (from XRD), and a band gap of 1.55 eV (from reflection adsorption measurements), consistent with the literature bandgap for CdTe.