Electrochemical Flow Deposition Systems

There are a number of vendors that sell solenoid actuated Teflon valves, which are easily interfaced to a computer. Care must be taken to chose a design where the internal volume at the valve outlet can be flushed easily between steps, however [110], Rotary selection valves have been used as well, but given the number of rotations needed for a 200 cycle deposit, various failure modes revealed themselves. 

As deposition of most of the relevant atomic layers involves reduction at relatively low potentials, oxygen has proven to be a major problem. It has been repeatedly shown that if oxygen is not rigorously excluded, deposits are thinner or not formed at all. For this reason, extensive sparging of the solution reservoirs is critical. 

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Figure 1 is a schematic diagram of a basic electrochemical flow deposition systems used for electrodepositing thin-films by EC-ALE, and Figure 2 is a picture showing the solution reservoirs, pumps, valves, and electrochemical cell. 

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. 

Figure 1 is a schematic diagram of a basic electrochemical flow-deposition system used for electrodepositing thin films using EC-ALE, and Fig. 2 is a picture showing the solution reservoirs, pumps, valves, electrochemical cell, potentiostat, and computer. A number of elechochemical cell designs have been tried. A larger thin-layer electrochemical flow cell is now used (Fig. 3c) [40], with a deposition area of about 2.5 cm and a cell volume of 0.1 mL, resulting in a two order of magnitude drop in solution volume, compared with the H-cell (Fig. 3b). The cell includes an indium tin oxide (ITO) auxiliary electrode, as the opposite wall of the cell from...

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. 

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. 

Fig. 3. Diagrams of electrochemical cells used in flow systems for thin film deposition by EC-ALE. A) First small thin layer flow cell (modeled after electrochemical liquid chromatography detectors). A gasket defined the area where the deposition was performed, and solutions were pumped in and out though the top plate. Reproduced by permission from ref. [ 110]. B) H-cell design where the samples were suspended in the solutions, and solutions were filled and drained from below. Reproduced by permission from ref. [111]. C) Larger thin layer flow cell. This is very similar to that shown in 3A, except that the deposition area is larger and laminar flow is easier to develop because of the solution inlet and outlet designs. In addition, the opposite wall of the cell is a piece of ITO, used as the auxiliary electrode. It is transparent so the deposit can be monitored visually, and it provides an excellent current distribution. The reference electrode is incorporated right in the cell, as well. Adapted from ref. [113],...

There are numerous applications that depend on chemically reacting flow in a channel, many of which can be represented accurately using boundary-layer approximations. One important set of applications is chemical vapor deposition in a channel reactor (e.g., Figs. 1.5, 5.1, or 5.6), where both gas-phase and surface chemistry are usually important. Fuel cells often have channels that distribute the fuel and air to the electrochemically active surfaces (e.g., Fig. 1.6). While the flow rates and channel dimensions may be sufficiently small to justify plug-flow models, large systems may require boundary-layer models to represent spatial variations across the channel width. A great variety of catalyst systems use... 

Under open circuit conditions, the PEVD system is in equilibrium after an initial charging process. The equilibrium potential profiles inside the solid electrolyte (E) and product (D) are schematically shown in Eigure 4. Because neither ionic nor electronic current flows in any part of the PEVD system, the electrochemical potential of the ionic species (A ) must be constant across both the solid electrolyte (E) and deposit (D). It is equal in both solid phases, according to Eqn. 11, at location (II). The chemical potential of solid-state transported species (A) is fixed at (I) by the equilibrium of the anodic half cell reaction Eqn. 6 and at (III) by the cathodic half cell reaction Eqn. 8. Since (D) is a mixed conductor with non-negligible electroific conductivity, the electrochemical potential of an electron (which is related to the Eermi level, Ep) should be constant in (D) at the equilibrium condition. The transport of reactant... 

The current relation in Eqn. 20 is achieved by adjusting the chemical potential of (A) at (II) under closed circuit conditions. Since the gradient in electrochemical potential is the driving force for the flow of charged particles in the multiphase PEVD system, the current density carried by (A+) in either the solid electrolyte (E) or deposit (D) can be written as ... 

To demonstrate unidirectional charge flow via electron mediation, Murray s group electrochemically polymerized complexes [Ru(bpy)2(vpy)2] " ", A, and [Ru(bpy)2(vpy)Cl]+, B, on Pt electrodes in CH3CN. (vpy is 4-vinylpyridine.) The order of deposition of the fllms is crucial of course since Eg f [Ru +z2+(a)] = +1.23 and Eg j. [Ru " " (B) ] = +.76 V vs SSCE and the inner film mediator (poly(A)) would not be expected to move electrons uphill. The results are summarized in Figure 2, where it is clear that both redox waves associated with the outer film couple (poly (B)) are missing in the dual layer system (Fig 2(b)). The (A + B) copolymerized single film electrode (Fig 2(c)) shows the electronic presence of both couples at the Pt/polymer interface. 

Does a soil-fluid-chemical system behave as an active electrochemical system or a passive electrical conductor under the influence of a DC electric field This is a fundamental question of significant implications. The evaluation criterion that can be used to differentiate the two systems of completely different nature is vested in Faraday s laws of electrolysis, as the transfer of electrons from the electrodes to the system and vice versa in an ideal electrochemical system is invariably associated with chemical reactions obeying Faraday s laws of electrolysis (Antropov, 1972). The two important fundamental laws of electrolysis can be simply expressed as follows (a) the amount of chemical deposition is proportional to the quantity of electric charges flowing through the system in an electrolytic process, and (b) the masses of different species deposited at or dissolved from electrodes by the same quantity of electric charges are directly proportional to their equivalent weights (Crow, 1979). 

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