Atomic Layer Epitaxy and UPD Revisited

There has been an increasing number of studies of the UPD of main group elements, including S, Se, Te, I, Br, Cl, and As, on metal substrates, whereas studies of UPD processes on the surface of semiconductors and semimetal substrates are significantly less. Presently, most interesting in this connection is the combined use of photoexcitation of a semiconductor substrate and/or an immobilized precursor, and electrodeposition, as will be discussed in a subsequent paragraph. 

Before examining selenium in some detail, as an illustration of the phenomena under consideration, let us note here that although oxidative UPD of sulfur can be achieved from sulfide solutions, the oxidative UPDs of Se and Te are complicated by the fact that, unlike sulfides, the Se and Te species are not very stable in aqueous solutions they tend to react with even traces of O2, to form the corresponding element. 

The electrochemistry of Se adsorbate and bulk layers has been studied (and debated) by many research groups (e.g., [81, 82] see also references connected 

STM studies of the Au(llO) surface indicated that only the Se (2x3) structure was formed at coverages much below one monolayer and that it was formed homogeneously. At monolayer and higher coverages, a honeycomb structure composed of chains of Se atoms was observed, which at still higher coverages filled in to complete a second Se layer. 

Numerous works have been implemented on tellurium electrochemistry and its adsorption at metal surfaces. The morphological structures of electrodeposited Te layers at various stages of deposition (first UPD, second UPD, and bulk deposition) are now well known [88-93]. As discussed in the previous paragraphs, Stickney and co-workers have carried out detailed characterizations of the first Te monolayer on Au single-crystal surfaces in order to establish the method of electrochemical atomic layer epitaxy of CdTe. 

---