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The method of electrochemical atomic layer epitaxy (ECALE) is described. It involves the alternated electrochemical deposition of atomic layers of elements to form compound semiconductors. It is being investigated as a method for forming epitaxial thin films. Presently, it appears that the method is applicable to a wide range of compound semiconductors composed of a metal and one of the following main group elements: S, Se, Te, As, Sb, or Bi. Initial studies have involved CdTe deposition. Factors controlling deposit structure and composition are discussed here. Preliminary results which show that ordered electrodeposits of CdTe can be formed by the ECALE method are also presented. Results reported here were obtained with both a polycrystalline Au thin-layer electrochemical cell and a single-crystal Au electrode with faces oriented to the (111), (110), and (100) planes. The single-crystal electrode was contained in a UHV surface analysis instrument with an integral electrochemical cell. Deposits were examined without their exposure to air using LEED and Auger electron spectroscopy. Coverages were determined using coulometry in the thin-layer electrochemical cell.

The oxidative chemisorption and cathodic stripping reductive desorption of iodine have been compared at smooth polycrystalline and well-defined Au( 1 1 1) singlacrystal electrodes. Experimental measurements were based upon cyclic voltammetry, thin-layer coulometry, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction. The results indicate that iodide is oxidatively adsorbed as zerovalent atomic iodine at potentials between -0.4 V and +0.4 V (Ag/AgCl reference); at lower potentials, surface iodine is reductively desorbed as aqueous iodide, while at considerably more positive potentials, it is oxidized to aqueous iodate. Studies with the Au( 11 1) electrode in dilute aqueous CsI solutions showed ordered adlayer structures at the selected potentials investigated. Below -0.4 V, the potential at which oxidative deposition of iodine starts to occur, a distinct (4x4) quarter-coverage CsI layer (0, = rCr/rAu = O1 rl/rAU -0.25) was formed. At -0.4 V < E < -0.2 V, increased to 0.33; this increase was coupled with the loss of adsorbed Cs, and the structure of this adlattice was Au( 11 1)(v'3Xt/3)R30°-I. At E > -0.2 V, the I coverage reached 0.4, a value made possible by a compression of the original (d3Xt/3)R30° structure in one dimension to form a nearly hexagonal iodine adlattice with a ( 5 x 4 3 ) unit cell. The amount of adsorbed iodine continues to increase as the potential is made still more positive until the surface is saturated with a monolayer of close-packed I atoms of coverage limited by van der Waals interactions; additional iodine forced into the already space-limited interfacial layer only leads to the formation of molecular iodine, which is evolved into the solution as I,(aq). The oxidative chemisorption process may be thought of as the oxidative underpotential deposition of I atoms, while the reductive desorption reaction may be viewed as the cathodic stripping of iodide ions.

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