Transition Metal Dichalcogenide TiS₂Prepared by Hybrid Atomic Layer Deposition/Molecular Layer Deposition: Atomic-Level Insights with in Situ Synchrotron X-ray Studies and Molecular Surface Chemistry

In this work, a two-dimensional (2D) titanium disulfide (TiS₂) film was grown using tetrakis(dimethylamido)titanium and 1,2-ethanedithiol on a 100 nm thick amorphous SiO₂/Si substrate. The first step of the process relied on the growth of an amorphous film of Ti-amidothiolate by hybrid atomic layer deposition/molecular layer deposition (ALD/MLD) at 50 °C. Such thiolate converted into TiS₂upon subsequent thermal annealing under H₂(4%)/Ar(96%) at 450 °C. The final lamellar TiS₂layers tend to be parallel to the substrate surface, as observed by transmission electron microscopy and confirmed at a larger scale by X-ray absorption linear dichroism at the Ti K-edge. The crystalline quality of the resulting films was assessed by Raman scattering. Angle-resolved X-ray photoelectron spectroscopy and hard X-ray photoelectron spectroscopy confirmed the stoichiometry of the TiS₂layers. Repetitive and self-limiting growth behavior on the thermal SiO₂/Si substrate was displayed from the early stages of the growth using in situ synchrotron radiation, yielding the Ti and S X-ray fluorescence, as well as in situ ellipsometry and X-ray reflectivity. Modeling the initial ALD and MLD half-cycles on high-surface-area silica beads afforded characterization by more analytical techniques and provided insights into the growth chemistry that agreed with observations on the SiO₂/Si substrate. Four-point probe resistivity measurements and spectroscopic ellipsometry strongly suggest that the thin films grown on SiO₂/Si substrates behave as heavily doped semiconductors.