Low temperature phase-controlled synthesis of titanium di- and tri-sulfide by atomic layer deposition

摘要： Phase-controlled synthesis of two-dimensional (2D) transition metal chalcogenides (TMCs) at low temperature 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) nano-layers by atomic layer deposition (ALD) with precise thickness control. The phase-control has been obtained by carefully tuning the deposition temperature and co-reactant 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 co-reactant, and was also observed using H2S plasma as co-reactant at growth temperatures between 150 and 200 °C. TiS3 was only synthesized at 100 °C using H2S plasma as co-reactant. 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 temperature. The control between synthesis of TiS2 and TiS3 was elucidated by Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution electron microscopy, and Rutherford back scattering studies. Electrical transport measurements showed the low resistive nature of ALD grown 2D-TiS2 (1T-phase). Post-deposition 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 bandgap) nature of TiS3. 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.