The Electrochemical Atomic Layer Epitaxy (ECALE) method is based on the alternate electrodeposition of atomic layers of the elements that form the compound semiconductor at underpotential [1]. The phenomenon is surface limited, so that the resulting deposit is generally limited to one atomic layer. A monolayer of the compound is obtained by alternating underpotential deposition of the metallic element with the underpotential deposition of the non-metallic element in a cycle. The number of cycles, which determines the thickness of the deposits, can be repeated as many times as desired to obtain deposits of practical importance. By limiting depositions to single atomic layers, no three dimensional growth is promoted and epitaxy should result. The substrates used are single crystals to increase the probability for the epitaxial growth. The experimental conditions for deposition are strictly dependent on the compound which one wants to form and on the substrate used. In the last years we found the conditions to obtain all II-VI compound semiconductors and a typical II-V compound such as InAs. Then, we extended the method to the attainment and characterization of ternary compounds such as CdxZn(1-x)S, CdxZn(1-x)Se and CdSxSe(1-x). The interest for the ternary compounds arises from the possibility of varying the bandgap with composition. As an example, the binary CdS and ZnS are n-type semiconductors that form a continuous series of solid solutions with different stoichiometries and bandgap values included in the range 2.42 eV (CdS) and 3.66 eV (ZnS). Different surface science techniques have been used to investigate the composition, morphology and structure of our samples. In particular, XPS confirmed that a compound with the right stoichiometry is obtained. Then, ex-situ AFM investigations were performed to study the morphological evolution from the bare silver single crystal to deposits formed with an increasing number of deposition cycles up to 200 cycles. The structural investigation of the compound obtained were performed in-situ by STM, and ex-situ by XPD (X-ray Photoelectron Diffraction) and SXRD (Surface X-ray Diffraction). These latter measurements were carried out at the synchrotron beamlight in Grenoble. Finally the thin films were characterized by photoelectrochemical techniques. The photoresponse of the material, evaluated in alkaline polysulphide medium, shows a spectral dependence in very good agreement with literature reports. 1.. B.W. Gregory and J.L. Stickney, J. Electroanal. Chem., 1991, 300,543-561.
Department of Chemistry, University of Florence