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Investigating the Formation of MoSe2 and TiSe2 Films from Artificially Layered Precursors
journal contribution
posted on 2020-08-12, 20:30 authored by Aaron
M. Miller, Danielle M. Hamann, Erik C. Hadland, David C. JohnsonThe 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.