Low-Temperature Phase-Controlled Synthesis of Titanium Di- and Tri-sulfide by Atomic Layer Deposition.

Phase-controlled synthesis of two-dimensional (2D) transition-metal chalcogenides (TMCs) at low temperatures with a precise thickness control has to date been rarely reported. Here, we report on a process for the phase-controlled synthesis of TiS (metallic) and TiS (semiconducting) nanolayers by atomic layer deposition (ALD) with precise thickness control. The phase control has been obtained by carefully tuning the deposition temperature and coreactant composition during ALD. In all cases, characteristic self-limiting ALD growth behavior with a growth per cycle (GPC) of ∼0.16 nm per cycle was observed. TiS was prepared at 100 °C using HS gas as coreactant and was also observed using HS plasma as a coreactant at growth temperatures between 150 and 200 °C. TiS was synthesized only at 100 °C using HS plasma as the coreactant. The S species in the HS plasma, as observed by optical emission spectroscopy, has been speculated to lead to the formation of the TiS phase at low temperatures. The control between the synthesis of TiS and TiS was elucidated by Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution electron microscopy, and Rutherford backscattering study. Electrical transport measurements showed the low resistive nature of ALD grown 2D-TiS (1T-phase). Postdeposition annealing of the TiS layers at 400 °C in a sulfur-rich atmosphere improved the crystallinity of the film and yielded photoluminescence at ∼0.9 eV, indicating the semiconducting (direct band gap) nature of TiS. The current study opens up a new ALD-based synthesis route for controlled, scalable growth of transition-metal di- and tri-chalcogenides at low temperatures.