Metal chalcogenides, solution-processed

Froment M, Lincot D (1995) Phase formation processes in solution at the atomic level Metal chalcogenide semiconductors. Electrochim Acta 40 1293-1303... 

Particularly desirable among film deposition processes are solution-based techniques, because of the relative simplicity and potential economy of these approaches. However, the covalent character of the metal chalcogenides, which provides the benefit of the desired electronic properties (e.g., high electrical mobility), represents an important barrier for solution processing. Several methods have been developed to overcome the solubility problem, including spray deposition, bath-based techniques, and electrochemical routes, each of which will be discussed in later chapters. In this chapter, a very simple dimensional reduction approach will be considered as a means of achieving a convenient solution-based route to film deposition. 

Milliron, D. I Mitzi, D. B. Copel, M. Murray, C. E. 2006. Solution-processed metal chalcogenide films for p-type transistors. Chem. Mater. 18 587-590. 

As with the n-TiCte and n-SrTiCh counterparts discussed earlier in Section 6.2 of this Chapter (see also Ref. 407), luminescence probes have proven to be very useful for unraveling the mechanistic details of the cathodic processes both at n-type (e.g., n-GaAs)556 and p type (e.g., p InP)557,558 Group III V semiconductor surfaces. Finally, these semiconductors share another trend with those discussed earlier (metal chalcogenides) in that the majority of the studies since 1990 have been directed at solid solutions (alloys of GaP and InP, GaAs and InAs etc.). These newer studies will be addressed in Section 12 of this Chapter. 

The hydrothermal method has been used to prepare monodispersed ZnS (6 nm) [10] and CdS nanocrystals (16 nm) [11]. By hydrothermal polymerization and simultaneous sulfidation processes, nanocomposites CdS/poly(vinyl acetate) nanorods [12] and nanospheres [13] were synthesized. In aqueous hydrazine solutions, nonstoichiometric metal telluride nanocrystallites such as Cu2.86Te2, CuyTes, Cuy-xTe, and Ag7Te4 [14], and cubic CogSg were hydrothermally synthesized [15]. Other transition metal chalcogenides, such as single-molecular-layer M0S2 [16] and MoSey [17] were also prepared under hydrothermal conditions. 

The chemical bath deposition (CBD) process uses a controlled chemical reaction to achieve the deposition of a thin film by precipitation. CBD has been used to prepare ZnS films (by route C). In a typical experiment, substrates are immersed in an alkaline solution containing the chalcogenide source, the metal ion and added base. A chelating agent is also added, in order to control the hydrolysis of the metal ioa The process rehes on the slow release of ions into an alkaline solution in which the free metal ion is buffered at a low concentration. 

Solution pH is an important variable, as it controls solubility, the deposition potential, and precursor speciation [140], As noted, one of the advantages of using an ALE process is that very different solutions can be used for each step in the cycle. For instance, pH 4 Cd solutions have been used with pH 10 Te solutions to form CdTe. Some care must be taken rinsing between solutions, however. On the other hand, Foresti et al. have used the same pH for both the metal and chalcogenide in the formation of II-VI compounds, such as CdS and CdSe on Ag single crystals [115, 116, 123, 143], To keep the Cd from precipitating in the basic solution, they complexed it with pyrophosphate. 

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