ECALE technique

The electrochemical atomic layer epitaxy (ECALE) technique, also known as electrochemical atomic layer deposition (EC-ALD), is based on layer-by-layer electrodeposition. Each constituent of the thin him are deposited separately using underpotential deposition (UPD) of that element. UPD is a process wherein an atomic layer of one element is deposited on the surface of a different element at a potential under that needed to deposit the element on itself. ECALE has been used to grow mainly II-VI and III-V compounds. A thorough review of ECALE research has been published by Stickney.144 A summary of the materials deposited using ECALE are given in Table 8.4, with a more detailed discussion for a few select examples given below. 

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... 

Several review articles have been published about SILAR-grown films.4-7 The SILAR technique, including its advantages and disanvantages and the equipment employed, is presented in Section 8.2. Materials that have been prepared by SILAR are reviewed in Section 8.3. Short descriptions of the related ILGAR, ECALE, and other sequential solution-phase techniques follow in Sections 8.4- 8.6. 

The deviations from the ideal evident in the smdies above may be very important and real, or they may be a result of problems with the operator, with quantification, or with the experimental set-up. They do, however, provide a direction for an initial ECALE cycle to be used in the next stage an automated deposition system to form thin films that can be analyzed with techniques other than coulometric stripping. 

Atomic layer epitaxy (electrochemical) — Electrochemical atomic layer epitaxy (ECALE) is a self-limiting process for the formation of structurally well-ordered thin film materials. It was introduced by Stickney and coworkers [i] for the layer by layer growth of compound semiconductors (CdTe, etc.). Thin layers of compound semiconductors can be formed by alternating - underpotential deposition steps of the individual elements. The total number of steps determines the final thickness of the layer. Compared to flux-limited techniques... 

A similar UPD technique called electrochemical atomic layer epitaxy" (ECALE) was introduced by Stickney et al. [6.170-6.172] to form mbced 2D MexMey UPD overlayers using simultaneously electrochemical reduction and oxidation of different electrolyte components. 

Metal compound semiconductors such as CdS, CdSe, Hg(i CdjTe, BijScj, and InAs can also be produced by electrodeposition. Films of these materials have been produced by codeposition of the elements,or by a technique known as electrochemical atomic layer epitaxy (ECALE). 2-5s In codeposition, the films or nanostructures are produced from solution precursors by EC reactions, such as those shown in Equations 17.3 through 17.5. ECALE is the electrochemical equivalent of atomic layer epitaxy (ALE), because the material is assembled monolayer-by-monolayer using surface-limited reactions. The alternating layers in ECALE are deposited by UPD. 

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