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 TiS₂ (metallic) and TiS₃ (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. TiS₂ was prepared at 100 °C using H₂S gas as coreactant and was also observed using H₂S plasma as a coreactant at growth temperatures between 150 and 200 °C. TiS₃ was synthesized only at 100 °C using H₂S plasma as the coreactant. The S₂ species in the H₂S plasma, as observed by optical emission spectroscopy, has been speculated to lead to the formation of the TiS₃ phase at low temperatures. The control between the synthesis of TiS₂ and TiS₃ 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-TiS₂ (1T-phase). Postdeposition annealing of the TiS₃ 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 TiS₃. The current study opens up a new ALD-based synthesis route for controlled, scalable growth of transition-metal di- A nd tri-chalcogenides at low temperatures.