Investigating the Formation of MoSe₂ and TiSe₂ Films from Artificially Layered Precursors

The reaction of ultrathin layers of Mo and Ti with Se was investigated, and significantly different reaction pathways were found. However, in both systems postdeposition annealing results in smooth dichalcogenide films with specific thicknesses determined by the precursor. X-ray diffraction (XRD) patterns of as-deposited Mo|Se films around a 1:2 ratio of Mo to Se contain weak, broad reflections from small and isolated MoSe₂ crystallites that nucleated during deposition and a sharper intensity maximum resulting from the composition modulation created from the alternating deposition of Mo and Se layers. In contrast, as-deposited Ti|Se films around a 1:2 ratio of Ti to Se contain narrow and intense 00l reflections from TiSe₂ crystallites and do not contain a Bragg reflection from the sequence of deposited Ti|Se layers. The as-deposited TiSe₂ crystallites have a larger c-axis lattice parameter than was previously reported for TiSe₂, however, which suggests a poor vertical interlayer registry and/or high defect densities including interstitial atoms. In-plane XRD patterns show the nucleation of both TiSe₂ and Ti₂Se during deposition, with the Ti₂Se at the substrate. For both systems, annealing the precursors decreases the peak width and increases the intensity of reflections from crystalline TiSe₂ and MoSe₂. Optimized films consist of a single phase after the annealing and show clear Laue oscillations in the specular XRD patterns, which can only occur if a majority of the diffracting crystallites in the film consist of the same number of unit cells. The highest quality films was obtained when an excess of ∼10% Se was deposited in the precursor, which presumably acts as a flux to facilitate diffusion of metal atoms to crystallite growth fronts and compensates for Se loss to the open system during annealing.