The controlled deposition of single atomic layers of matter is known under the general name of "Atomic Layer Epitaxy" (ALE) and can be obtained either by gas-phase or liquid phase methods. In the latter case, when the deposition is obtained by electrochemical methods it is referred to as electrochemical atomic layer epitaxy (ECALE). ALE and ECALE techniques hold the promise of being able to provide low-cost, structurally well-ordered solids whose composition can be controlled at the nanoscopic level along the direction perpendicular to the substrate. In particular, in the present communication we'll report about the growth by underpotential depostion and characterization by surface science techniques of nanolayered semiconductors such as cadmium and zinc chalcogenides on single-crystal silver substrates.
The ECALE method of deposition is based on the alternate underpotential deposition (upd) of atomic layers of both elements forming the compound semiconductor. Underpotential deposition (upd) is the well-known phenomenon whereby the potential necessary to deposit one element onto a second element occurs before that necessary to deposit the element on itself. In the ECALE method, the upd of the metallic element is alternated with that of the non-metallic element to form a single monolayer of the compound per cycle.
The number of cycles determines how thick the deposit will be. Films of various thicknesses were deposited by this method on single crystal Ag substrates oriented along the (111) direction. In order to determine the characteristcs of composition and thickness distribution, the as deposited films were examined by different surface science techniques, in particular by X-ray photoelectron spectroscopy (XPS).
Measurements were performed ex-situ transferring the samples after deposition to the XPS spectrometer in ultra-high vacuum. The results obtained so far indicate that the elements present on the surface correspond to the intended composition of the deposit. We observed by XPS also the presence of substrate signal (Ag) even for relatively thick (> 15 atomic layers) films. Further work is in progress for the structural characterization of the deposit by electron diffraction techniques.