Structural characterization and transistor properties of thickness-controllable MoS₂ thin films

We report an optimized multi-step chemical vapor deposition process for growing MoS₂ thin films. This process enables large-area processing, film patterning simply by using shadow masks, and precise control of the final thickness by changing the initial thickness of the first step of MoO₃ film deposition. The structural characterization of the MoS₂ films is performed with transmission electron microscopy and X-ray diffraction as a function of film thickness. MoS₂ film with a thickness of 3 nm possesses a highly crystalline structure with a spacing of 0.62 nm. The crystallinity and orientation of the films are degraded with increased film thickness. Careful analysis by time-of-flight secondary ion mass spectroscopy reveals that a film with a thickness of 9 nm is not completely sulfurized, and unreacted MoO₃ is left at the bottom of the film. These fundamental analyses coincide with the thickness dependence of thin-film transistor (TFT) performance. A TFT with the optimal film thickness of 3 nm achieves high performance, namely a carrier mobility of 0.57 cm² V⁻¹ s⁻¹ and an on/off ratio of ~ 10².