Electrochemical atomic layer epitaxy

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

A variety of compound semiconductors have been successfully prepared by this technique. Much of the work concerning ECALE has been concentrated on the deposition of CdTe on An substrates. Notwithstanding the inherent problems of the system (for instance, a 10% lattice mismatch), the formation of CdTe epitaxial layers became a model example of ECALE synthesis. In their pioneering studies, Stickney and co-workers [27, 28] have focused on the deposition of the compound on 

Ultrathin films of CdS ranging in coverage from 25 to 200 ML were grown also by the previous method on Au substrates (of non-specified nature) and were characterized by quantitative Raman resonance [41], It was found that the electronic structure of the films in this coverage regime corresponds to that of bulk CdS. It was concluded also that ECALE does not involve growth by random precipitation of CdS onto the Au surface the thin deposited layers of the material were contiguous. 

Changing the substrate from gold to silver has been shown to strongly affect the structure of the first few layers of CdS grown by ECALE. STM measurements carried out on the first CdS layer on Ag(lll) revealed a much less compact structure than the one found on Au(lll). This disparity was tentatively attributed to the different structure of the first S layer on Ag(l 11), as obtained by oxidative UPD from sulfide ion solutions, due to a higher affinity of sulfur for silver than for gold. The Cd layers were attained on S by reductive UPD from cadmium ion solutions. Precursors for both elements were dissolved in pyrophosphate/NaOH at pH 12 [43 5], 

Significant improvements in ECALE deposit morphology and quality were reported as achieved by switching from a thin layer cell to a thick layer H-form cell, integrated in an automated deposition system [46]. Thin epitaxial films of zinc blende CdTe, CdSe, and CdS with predominate (111) orientations were grown. 

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Mathe MK, Cox SM, Venkatasamy V, Uwe Happek, Stickney JL (2005) Formation of HgSe thin films using electrochemical atomic layer epitaxy. J Electrochem Soc 152 C751-C755... 

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. 

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Varazo K, Lay MD, Sorenson TA, Stickney JL (2002) Formation of the first monolayers of CdTe on Au(l 11) by electrochemical atomic layer epitaxy (EC-ALE) studied by LEED, Auger, XPS, and in-situ STM. J Electroanal Chem 522 104-114... 

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Colleti LP, Thomas S, WUmer EM, Stickney JL (1997) Thin layer electrochemical studies of ZnS, ZnSe, and ZnTe formation by Electrochemical Atomic Layer Epitaxy (ECALE). Mater Res Soc Symp Proc 451 235. 

Venkatasamy V, Mathe MK, Cox SM, Happek U, Stickney JL (2006) Optimization studies of HgSe thin film deposition by electrochemical atomic layer epitaxy (EC-ALE). Electrochim Acta 51 4347-4351... 

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Torimoto T, Takabayashi S, Mori H, Kuwabata S (2002) Photoelectrochemical activities of ultrathin lead sulfide films prepared by electrochemical atomic layer epitaxy. J Electroanal Chem 522 33-39... 

Qiao Z, Shang W, Wang C (2005) Fabrication of Sn-Se compounds on a gold electrode by electrochemical atomic layer epitaxy. J Electroanal Chem 576 171-175... 

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Electrochemical Atomic Layer Epitaxy Nanoscale Control in the Electrodeposition... 

The focus of the work described here is on understanding the mechanisms of compound electrodeposition and how to control structure, morphology and composition. The primary tool for understanding compound electrodeposition and for improving control over the process has been the methodology of electrochemical atomic layer epitaxy (EC-ALE) [29, 73-75],... 

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