CdS deposition

Fig. 3.1 Model of CdS deposition and recrystaUization. The changes in film structure are related to the features of the cyclic voltammogram and the capacitance plot broken line). The interpretation of the capacitance data in this way leads to a mean value of ffcds = 17 for the relative permittivity of the film. (Reprinted from [34], Copyright 2009, with permission from Elsevier)...

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. 

Gerischer H, Liibke M (1986) A particle size effect in the sensitization of TiOa electrodes by a CdS deposit. J Electroanal Chem 204 225-227... 

CdS growth, by EC-ALE, has been studied by more groups than any other compound (Table 1) [111, 123, 143, 145, 154, 163, 165, 167-169, 172, 186], Initial EC-ALE studies by this group of CdS were performed with a TLEC (Figure 13), to determine potentials for a cycle [145]. Cd and S coverages were determined coulometrically for deposits as a function of the numbers of cycles performed. The dependence of thickness on the Cd deposition potential, for CdS deposits, revealed a plateau between —0.3 and —0.55 V, with the best deposits formed at —0.5 V, using a 10 mM CdSCL solution, pH 5.9 and an 11 mM, pH 11 Na2S solution. Reductive UPD was used for the Cd atomic layers and oxidative UPD for S. 

CdTe Eledrodeposltlan-Te Deposition First at +0.15OV, Followed by Cd Deposition... 

Fig. 67. Graphs of the Cd/Te ratios from Auger electron spectroscopy as a function the potentials used to deposit the second atomic layer in the formation of CdTe monolayers on Au(lll). The Auger signals have been corrected for sensitivity factors, so the ratios are an approximation of the atomic ratios. A) Te deposited first at 0.15 V, B) Te deposited first at 0.10 V, C) Cd deposited first at —0.6 V, D) Cd deposited first at —0.7 V. Adapted from ref. [236],...

CdTe Elect rod epos it io n - Cd Deposition First at -0.600V, Followed by Te Deposition... 

Cd deposition -0.600V 2nd-Te deposition -0.700V 3ld- Cd deposition -0.600V... 

Sasagawa, M. Nosaka, Y. 2001. Studies on the effects of Cd ion sources and chelating reagents on atomic layer CdS deposition by successive ionic layer adsorption and reaction (SILAR) method. Phys. Chem. Chem. Phys. 3 3371-3376. 

Codeposition produces some of the better II-VI electrodeposits and, as can be seen in Table 1, has been used and studied extensively. Aqueous codeposition of CdTe serves as a good example of the method. The deposition is usually performed at an underpotential for Cd, at a potential where the Cd deposits exclusively on previously deposited Te. Te, on the other hand, is more noble than Cd and is thus deposited at an overpotential. The tellurite concentration, however, is kept far below that of the Cd+ so there is a large excess of Cd+l As soon as Te deposits, Cd quantitatively underpotentially deposits on top, providing control over deposit stoichiometry. 

FIG. 27. Cd/Te ratio observed using EPMA for deposits formed using 200 cycles as a function of the Cd deposition potential. 

Also evident in Fig. 64B are small light-colored clusters close to 1/2 nm in size. At present it appears that these clusters are extra CdSe formed because too much Se was present on the surface initially (Fig. 64A). As mentioned above, the Se coverage prior to Cd deposition appeared close to 0.6 ML—significantly greater than the 0.44 needed for the structure diagrammed in Fig. 63B. Studies have shown that Cd will react nearly stoichiometrically with up to several monolayers of Se [107]. These results are consistent with the discussions of CdTe formation in Fig. 41C, where a second layer begins to form and results in island on the surface. 

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