Guiding principles for the design of a chemical vapor deposition process for highly crystalline transition metal dichalcogenides

Abstract

Two-dimensional transition metal dichalcogenides (TMDs) for advanced logic transistor technologies are deposited by various modifications of the chemical vapor deposition (CVD) method using a wide variety of precursors. Being a major electrical performance limiter, the TMD crystal grain size strongly differs between the various CVD precursor chemistries from nano- to millimeter-sized crystals. However, it remains unclear how the CVD precursor chemistry affects the nucleation density and resulting TMD crystal grain size. This work postulates guiding principles to design a CVD process for highly crystalline TMD deposition using a quantitative analytical model benchmarked against literature. The TMD nucleation density reduces favorably under low supersaturation conditions, where the metal precursor sorption on the starting surface is reversible and the corresponding metal precursor desorption rate exceeds the overall deposition rate. Such reversible precursor adsorption guarantees efficient long-range gas-phase lateral diffusion of precursor species in addition to short-range surface diffusion, which vitally increases crystal grain size. As such, the proposed model explains the large spread in experimentally observed TMD nucleation densities and crystal grain sizes for state-of-the-art CVD chemistries. Ultimately, it empowers the reader to interpret and modulate precursor adsorption and diffusion reactions through designing CVD precursor chemistries compatible with temperature sensitive application schemes.