Automated electrochemical deposition

An automated electrochemical deposition system making use of a simple distribution valve is described. 

Define electros5mthetic routes amenable to the automated electrochemical deposition system for synthesizing for doped and mixed metal... 

Fig. 6.9 An automated electrochemical deposition system used for the production of metal oxides. The custom-made well plate shown has been designed fora 120-sample array, each sealed by 0-rings underneath a patterned Teflon block, and each containing a unique electrolyte composition. Reprinted with permission from [86], copyright 2005 American Chemical Society...

This paper describes ongoing studies of the electrodeposition thin films of the compound semiconductors CdTe and InAs, using the method of electrochemical atomic layer epitaxy (ALE). Surface limited electrochemical reactions are used to form the individual atomic layers of the component elements. An automated electrochemical flow deposition system is used to form the atomic layers in a cycle. Studies of the conditions needed to optimize the deposition processes are underway. The deposits were characterized using X-ray diffraction, scanning probe microscopy, electron probe microanalysis and optical/infrared absorption spectroscopy. 

Most E-ALD deposits are formed using some type of electrochemical flow cell that allows for the rapid exchange of solutions, in combination with automation (Electrochemical ALD L.C., Athens, GA), where the cycle can be... 

Potentiometric stripping analysis (PSA) is another commonly used technique in water analysis. This technique can usually be applied directly to the analysis of water samples without previous treatment, and it is virtually free from interferences of dissolved oxygen. Both, PSA and ASV techniques are based on the same principle the anal) e is first deposited on the electrode surface while the solution is stirred, and then stripped back to the solution in the measurement step [14,22,196]. The ASV technique works on a film electrode (electrochemically deposited mercury or gold on a glassy carbon support). One advantage of PSA is that it requires simpler equipment than ASV, and can compete with nonelectroanalytical techniques in terms of price, and possibility of automation [247-249]. This method has been applied to determine metals in tap water and rainwater samples [250-253], coupled with FIA to determine copper in natural waters [254,255], etc. In addition, portable PSA instruments have also been developed, and demonstrated to be useful for metals determination in aquatic samples [256-259]. 

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. 

The ellipsometer used in this study is described elsewhere(3). It consists of a Xenon light source, a monochromator, a polarizer, a sample holder, a rotating analyzer and a photomultiplier detector (Figure 1). An electrochemical cell with two windows is mounted at the center. The windows, being 120° apart, provide a 60° angle of incidence for the ellipsometer. A copper substrate and a platinum electrode function as anode and cathode respectively. Both are connected to a DC power supply. The system is automated with a personal computer to collect all experimental data during the deposition. Data analysis is carried out by a Fortran program run on a personal computer. 

As already pointed out in our previous papers [48-50], the high stability is probably the result of the newly developed chemical modification procedure which may lead to a stronger adsorption of the PB particles on the electrode surface. In contrast to the PB layer obtained with the more commonly used electrochemical procedures, these modified electrodes are in fact more stable at basic pH and their continuous use is possible with a minimal loss of activity after several hours. Moreover, with respect to the electrochemical procedure, our chemical deposition is much more suitable for mass production since no electrochemical steps are required and a highly automated process could be adopted (see Procedure 17 in CD accompanying this book). 

Given the repetitive nature of compound formation using electrochemical ALE. an automated deposition system was constructed to form films of a reasonable thickness (Figure I) [24"). The cell is a... 

Colletti LP, Stickney JL (1998) Optimization of the growth of CdTe thin films formed by electrochemical atomic layer epitaxy in an automated deposition system. J Electrochem Soc 145 3594... 

Here, m andp are the oxidation states of UPD metal M and the more noble metal P. The factors b, 9, and q are introduced to accurately express the amount of deposited metal P in ML units with respect to atomic areal density of the substrate S h, k, /). They represent, respectively, the number of full UPD MLs, the UPD ML coverage, and the packing density of M atoms in complete UPD ML with respect to the substrate S h, k, /). The subscripts s and solv indicate the physical state of the metal (solv = solution phase and s = deposited). If sequence A-E (Fig. 7) is repeated an arbitrary number of times, a multilayer homo- or heteroepitaxial films can be obtained. The thin film growth using this method can be completely automated with experimental apparatus for Electrochemical Atomic Layer Epitaxy developed by Stickney et al. ... 

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