Thin film formation using ECALE

Another frequently raised concern about purity involves the fact that electrodeposition takes place in a condensed phase, with a solvent in contact with the substrate and deposit. The solvent used for the present studies is water, and it is used copiously in the processing of compounds and devices. The electronics industries are well aware of how to obtain very high-purity water. The point is that the purity issues in an electrodeposition method are the same issues being addressed in presently used methodologies. There does not appear to be anything inherently dirty about electrodeposition. 

The logistics of forming thin films using ECALE revolve around alternating the solutions and potentials in a cycle. As stated earlier, manually forming deposits with much over 10 cycles proved tedious. Some work in 

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Initially, a thin layer flow cell (Fig. 19) was used in this group to study the EC ALE formation of compounds [158] and in studies of electrochemical digital etching [312,313], Wei and Rajeshwar [130] used a flow cell system to deposit compound semiconductors as well, however, the major intent of that study was to form superlattices by modulating the deposition of CdSe and ZnSe. Their study appears to be the first example of the use of a flow electrodeposition system to form a compound semiconductor superlattice. 

Besides the electrochemical flow cells just mentioned, other hardware for sequential solution-deposition scenarios has been developed to form 

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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. 

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