Solution-Processed Deposition

Ki, W. Huang, X. Li, J. Young, D. L. Zhang, Y. 2007. Highly conductive group VI transition metal dichalcogenide films by solution-processed deposition. J. Mater. Res. 22 1390-1395. 

In addition, EC-ALE offers a way of better understanding compound electrodeposition, a way of breaking it down into its component pieces. It allows compound electrodeposition to be deconvolved into a series of individually controllable steps, resulting in an opportunity to learn more about the mechanisms, and gain a series of new control points for electrodeposition. The main problem with codeposition is that the only control points are the solution composition and the deposition potential, or current density, in most cases. In an EC-ALE process, each reactant has its own solution and deposition potential, and there are generally rinse solutions as well. Each solution can be separately optimized, so that the pH, electrolyte, and additives or complexing agents are tailored to fit the precursor. On the other hand, the solution used in codeposition is a compromise, required to be compatible with all reactants. 

As mentioned, in the case of most electroless processes, deposition tends to be accompanied by H2 gas evolution. The efficiency of this reaction tends to be <100%, most notably in certain electroless Pd solutions where competing reactions involving H oxidation appear to occur, e.g. ... 

As the concentration of Ge(IV) was increased in the electroless solution, the deposition rate decreased rapidly, while the Ge content in the deposits increased (Fig. 11(b)). In contrast to a Ni-Ge-P solution employing a complexant such a citrate, increasing the concentration of Ge(IV) in the aspartate solution does not significantly alter the concentration of uncomplexed Ni2+ species through consumption of aspartate by Ge(IY). Thus, the decrease in deposition rate is associated with the soft character of the latter, which manifests itself in terms of strong adsorption on the Ni-Ge-P surface thereby inhibiting the electroless deposition process. 

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. 

Figure 4.6. Cross-sectional SEM images of an A1PO film deposited on Si02 and cured at (a) 275 °C, and (b) flash annealed to 600 °C. [Reproduced with permission. Meyers, S. T. Anderson, J. T. Hong, D. Hung, C. M. Wager, J. F. Keszler, D. A. 2007. Solution processed aluminum oxide phosphate thin-film dielectrics. Chem. Mater. 19 4023-4029. Copyright 2007 American Chemical Society.]...

Figure 5.14 and Table 5.4 show the electrical characteristics of the fabricated TFTs (W/L = lOpm/lOpm). TFT-4 and 5 (Gox, UDL and channel Si are solution-processed) have the mobility values, 23,0cm2/Vs and 9.9cm2/Vs, respectively. They are lower than that of TFT-6 (only the channel silicon was solution-processed). In this experiment, however, the mobility of the reference TFT (TFT-6) is also relatively poor, as expected, because the laser power and other conditions under which the channel silicon was solution-processed were not optimized. Thus, the mobilities of TFT-4 and TFT-5 were also affected by the channel silicon and were much lower than the mobilities of TFT-1 and TFT-2. With optimization of the conditions under which the channel silicon is deposited, we believe that higher mobility values can be achieved in the devices with solution-processed Gox, UDL, and channel Si. 

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