Precursor stoichiometry

The SEL approach has some advantages over co-deposition, because it allows simpler coulometric control of the precursor stoichiometry. The SEL method may also be more suitable for large-scale fabrication since there is no requirement to balance the deposition rates of several different metals, so higher current densities can be used. If an alloyed precursor is preferred for the sulfur annealing step, a short (<5min) heat treatment at 200-350 °C is sufficient to completely alloy the stacked precursor. In this section, we highlight some of our recent work on the fabrication of CZTS solar cells by the SEL route [11]. 

Figure 1.10 shows surface micrographs of CoMo intermetallic alloys after carburization in an attempt to synthesize a bimetallic (ternary) carbide. Such phases are of increasing interest as catalysts for a variety of industrial processes, including hydro-treating to remove S, N, and O impurities from fossil fuels, ammonia synthesis, water gas shift reaction to produce hydrogen, and fuel cell catalysts [44 7]. They are typically formed by gas reactions of complex oxide precursors or by molecular precursor or chemical synthesis [48-50] routes. The use of intermetallic precursors offers the potential to leverage a different set of precursor stoichiometries and structures to form new complex... 

Weidmann MC, Beck ME, Hoffinarm RS, Prins F, Tisdale WA Monodisperse, air-stable PbS nanocrystals via precursor stoichiometry control, CS Nano 8 6363—6371, 2014. 

Prior to methanation, the gas product from the gasifier must be thoroughly purified, especially from sulfur compounds the precursors of which are widespread throughout coal (23) (see Sulfurremoval and recovery). Moreover, the composition of the gas must be adjusted, if required, to contain three parts hydrogen to one part carbon monoxide to fit the stoichiometry of methane production. This is accompHshed by appHcation of a catalytic water gas shift reaction. 

Linear step-growth polymerizations require exceptionally pure monomers in order to ensure 1 1 stoichiometry for mutually reactive functional groups. For example, the synthesis of high-molecular-weight polyamides requires a 1 1 molar ratio of a dicarboxylic acid and a diamine. In many commercial processes, the polymerization process is designed to ensure perfect functional group stoichiometry. For example, commercial polyesterification processes often utilize dimethyl terephthalate (DMT) in the presence of excess ethylene glycol (EG) to form the stoichiometric precursor bis(hydroxyethyl)terephthalate (BHET) in situ. 

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]. 

Upon reaction with various Ni° precursors, the rigid cA-l,2-bis(diphenylphosphino)ethen gives mononuclear (1032a) or polymeric (1032b), depending on the stoichiometry.2499 NiL2... 

Initially, the main disadvantage of using single-source precursors was their lack of volatility. However, the advent of ultralow-pressure and vacuum CVD techniques, along with the use of alternative delivery methods which do not rely primarily on precursor volatility, means that many compounds that are essentially nonvolatile can now be considered as potential CVD precursors. A major problem that remains, however, is that of stoichiometry control a molecular precursor containing two (or more) elements in a specific ratio, as desired within the thin film to be grown, will not necessarily deposit those elements in the same proportions. 

Zinc telluride, ZnTe, was deposited on quartz, silicon, InAs, and GaSb substrates using Zn[TeSi(SiMe3)3]2 at temperatures between 250 °C and 350 °C. On InAs (orientation not specified) a cubic ZnTe layer was obtained. Problems of stoichiometry are encountered at temperatures below 325 °C because decomposition of the precursor is incomplete, while at higher temperatures (above 350 °C) the deposited ZnTe decomposes into Zn (which evaporates) and involatile elemental tellurium which remains. The results with the analogous cadmium precursor (1.4 torr, 290 °C) indicate that the CdTe films may be of better stoichiometry than those of ZnTe, with XRD results indicating that on a Si substrate the hexagonal phase is predominantly... 

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