Electrodeposition Compounds

The quality of an elemental deposit is a function of the deposition rate, surface diffusion, the exchange current and the substrate structure. Electrodeposition of a compound thin-film not only requires all these things, but stoichiometry as well. Under ideal conditions, the mass transfer rates and discharge rates of two elemental precursors can be tuned to produce a deposit with the correct overall stoichiometry for a compound. Whether the two elements will form the right compound, or a compound at all, is another question. 

Several methods and variations have been developed to electrodeposit compounds. Most of the work described in this article concerns the formation of nonoxide compounds such as II-VI and in Vs. Oxides are probably the largest group of electrodeposited compounds (aluminum anodization for example), but will not be discussed here. The electrodeposition of H-VI compounds has been extensively studied and is well reviewed in a number of articles [24-29], The most prominent compound electrodeposition methods include codeposition, precipitation, and various two-stage techniques. 

In general, annealing has been used to either form or improve the structures of compound films formed by the electrodeposition methods described above. This severely limits applications in systems where more complex structures are involved, structures where interdiffusion is a problem nanostructured materials. 

In the first chapter, on electrochemical atomic layer epitaxy, Stickney provides a review of experimental methodology and current accomplishments in the electrodeposition of compound semiconductors. The experimental procedures and detailed fundamental background associated with layer-by-layer assembly are summarized for various compounds. The surface chemistry associated with the electrochemical reactions that are used to form the layers is discussed, along with challenges and issues associated with device formation by this method. 

O Sullivan describes the fundamental theory, mechanistic aspects and practical issues associated with autocatalytic electroless metal deposition processes. Current approaches for gaining fundamental understanding of this complex process are described, along with results for copper, nickel and various alloys. Emphasis is placed on microelectronic applications that include formation of structures that are smaller than the diffusion layer thickness which influences structure formation. 

The electrodeposition of transition metal - aluminum alloys from chloroalumi-nate molten salts is reviewed by Stafford and Hussey. The chemical, electrochemical, 

Edited by Richard C. Alkire, and Dieter M. Kolb 2001 Wiley-VCH Verlag GmbH ISBNs 3-527-29830-4 (Hardcover) 3-527-60026-4 (Electronic) 

Electrochemical Atomic Layer Epitaxy (EC-ALE) Nanoscale Control in 

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Copper-indium diselenide, CuInSe2 (CIS), is a widely electrodeposited compound, due to its significance in thin film photovoltaics. 

The thrust of our w ork is to better understand the limits of electrodeposition as a methodology for compound thin film formation what controls the structure, composition, and morphology of an electrodeposited compound. 

Several methods and variations have been developed to electrodeposit compounds. Most of the work described in this article concerns the formation of nonoxide compounds such as II to VI and III to Vs. Oxides are probably the largest group of electrodeposited compounds (e.g. 

Only about 10 elements, ie, Cr, Ni, Zn, Sn, In, Ag, Cd, Au, Pb, and Rh, are commercially deposited from aqueous solutions, though alloy deposition such as Cu—Zn (brass), Cu—Sn (bronze), Pb—Sn (solder), Au—Co, Sn—Ni, and Ni—Fe (permalloy) raise this number somewhat. In addition, 10—15 other elements are electrodeposited ia small-scale specialty appHcations. Typically, electrodeposited materials are crystalline, but amorphous metal alloys may also be deposited. One such amorphous alloy is Ni—Cr—P. In some cases, chemical compounds can be electrodeposited at the cathode. For example, black chrome and black molybdenum electrodeposits, both metal oxide particles ia a metallic matrix, are used for decorative purposes and as selective solar thermal absorbers (19). 

The electrodeposition of Cr in acidic chloroaluminates was investigated in [24]. The authors report that the Cr content in the AlCr deposit can vary from 0 to 94 mol %, depending on the deposition parameters. The deposit consists both of Cr-rich and Al-rich solid solutions as well as intermetallic compounds. An interesting feature of these deposits is their high-temperature oxidation resistance, the layers seeming to withstand temperatures of up to 800 °C, so coatings with such an alloy could have interesting applications. 

They form a monolayer that is rich in defects, but no second monolayer is observed. The interpretation of these results is not straightforward from a chemical point of view both the electrodeposition of low-valent Ge Iy species and the formation of Au-Ge or even Au Ge h compounds are possible. A similar result is obtained if the electrodeposition is performed from GeGl4. There, 250 20 pm high islands are also observed on the electrode surface. They can be oxidized reversibly and disappear completely from the surface. With Gel4 the oxidation is more complicated, because the electrode potential for the gold step oxidation is too close to that of the island electrodissolution, so that the two processes can hardly be distinguished. The gold step oxidation already occurs at -i-lO mV vs. the former open circuit potential, at h-485 mV the oxidation of iodide to iodine starts. 

In addition to the alloys in Table 4.21, Ni-Sn and Ni-Ti alloys also possess useful corrosion resistance. Ni-Sn alloys are extremely brittle and, because of this, are used only as electrodeposited coatings. Ni-Ti alloys over a wide range of compositions have been studied, of which perhaps the intermetallic compound NiTi (55 06Ni-44-94Ti) has attracted the most interest. 

Fig. 12.16 Increase in porosity of an electrodeposit caused by mechanical polishing. Left, 7-5/im unpolished coating right, polished with lime finishing compound. The average thickness removed by abrasian was 0-1 /im...

The hot-dipped coatings are distinct from the others in having practical thickness limits and in possessing an inner layer of intermetallic compound, usually described as the alloy layer. The flow-melted electrodeposited coatings also have an alloy layer, which is somewhat thinner than that obtained in hot dipping. 

There is no intermetallic compound formation and the electrodeposit behaves as a simple mixture of the two metals. It can be considered as basically a stable wick of tin through which zinc is fed to be consumed at a rate lower than its consumption from a wholly zinc surface. If the conditions are such that zinc is rapidly consumed, and no protective layer of corrosion products is formed, the coating may break down, but in mildly corrosive conditions some of the benefits of a zinc coating, without some of its disadvantages, are obtained. 

A series of Be-Pt intermetallic compounds arc prepared during the electrodeposition of Be on Pt from a solution of BeCl2 in an equimol NaCl-KCl mixture at 710°C. X-Ray diffraction of the electrode surface shows the presence of BePt, BcjPt. Electrolytic methods are also used to extract single crystals of Be,V from alloys prepared by arc melting Be and the transition metal in the proportion 15 1. 

The induced co-deposition concept has been successfully exemplified in the formation of metal selenides and tellurides (sulfur has a different behavior) by a chalcogen ion diffusion-limited process, carried out typically in acidic aqueous solutions of oxochalcogenide species containing quadrivalent selenium or tellurium and metal salts with the metal normally in its highest valence state. This is rather the earliest and most studied method for electrodeposition of compound semiconductors [1]. For MX deposition, a simple (4H-2)e reduction process may be considered to describe the overall reaction at the cathode, as for example in... 

Generally, the experimental results on electrodeposition of CdS in acidic solutions of thiosulfate have implied that CdS growth does not involve underpotential deposition of the less noble element (Cd), as would be required by the theoretical treatments of compound semiconductor electrodeposition. Hence, a fundamental difference exists between CdS and the other two cadmium chalcogenides, CdSe and CdTe, for which the UPD model has been fairly successful. Besides, in the present case, colloidal sulfur is generated in the bulk of solution, giving rise to homogeneous precipitation of CdS in the vessel, so that it is quite difficult to obtain a film with an ordered structure. The same is true for the common chemical bath CdS deposition methods. 

If the electrolysis parameters (precursor concentrations, pH, temperature, cur-rent/potential, substrate) be defined in a precise manner, a self-regulated growth of the compound can be established, and highly (111 )-oriented zinc blende (ZB) deposits up to several p,m thickness are obtained at potentials lying at the anodic limit of the diffusion range (Fig. 3.3) [60]. Currently, the typical method of cathodic electrodeposition has been developed to yield quite compact and coherent, polycrystalline, ZB n-CdSe films of well-defined stoichiometry. The intensity of the preferred ZB(f 11) orientation obtained with as-deposited CdSe/Ni samples has been quite high [61]. 

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