New Results: Equilibrium thermodynamic modeling of CVD, using W(CO)6 and H2S as precursors in Ar, Ar/H2 and H2 ambient, was investigated by minimizing the total Gibbs free energy in the W-C-O-H-S system. The ranges of CVD parameters used for the present calculation were: T=100-1100°C, P=10-900 Torr, and flow rate ratio W(CO)6:H2S:(Ar and/or H2) =1:(2-50):(100-1000). The ability of these phase diagrams in predicting growth outcomes is corroborated by the Raman spectroscopy data, XRD and XPS from samples deposited over a range of process parameters.
A comprehensive CVD phase stability diagram for formation of solid phases in Ar ambient is demonstrated in Fig 1(a). It predicts the stability window for pure WS2 and indicates co-deposition of WS3 and C at lower T. Quantitative carbon contamination of WS2 films for wide variation in T and P is depicted in a contour plot in Fig. 1(b). Increasing carbon contamination of WS2 at lower T and higher P is validated by Raman spectra depicted in Fig. 1(c). This makes Ar unsuitable as carrier gas for growth, as higher P is required to achieve the reduction in supersaturation that is critical to obtain uniform layers of desired thickness.
Modeling predicts, however, that the deposition of carbon can be precluded by increasing the partial pressure of hydrogen, i.e., by using a mixture of Ar and H2 as the carrier gas, as shown in Fig 1(d). In Ar/H2 and in H2 ambient, the stability domain of carbon narrows as P is increased, meeting the criterion for reduction in supersaturation. Pure H2 as a carrier gas is shown to facilitate the largest contamination free process window. CVD phase stability diagram in H2 ambient, predicting the ranges of CVD conditions for growth of contamination-free WS2, is illustrated in Fig. 1(e). WC and W deposit in various combinations for process parameters beyond this range. Raman spectroscopic data [Fig. 1(f)] showing carbon-free WS2 for wide ranges of P and T validates the phase diagrams in H2 ambient. Given the multivariable nature of the problem, excellent agreement between theoretical and experimental results was observed.
Conclusion: A generic approach suitable to all TMDs, involving a combination of thermodynamic modeling and kinetic control for desired growth, is explored for the system W-C-O-H-S. Thermodynamic equilibrium calculations serve as a first step to determine process parameter windows for the desired end product. The ability to predict the completely different outcomes on using Ar versus Ar + H2 mixture as the carrier gas highlights the importance of such thermodynamic modeling. Thus, theoretical analysis of the CVD process can be employed to synthesize desired films in a predictive manner. This reduces the effort and resources needed for process development, and aims to bring a systemic approach to hitherto ad-hoc process.