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Design and development of the growth-process for the production of wafer-scale spatially
homogeneous thickness controlled atomically-thin transition metal dichalcogenides (TMDs)
is one of the key challenges to realize modern electronic devices. Recently our group has
demonstrated a rapid and scalable fabrication of atomically-thin MoS 2 and WS 2 films with
precise thickness control via gas-phase chemical vapor deposition (CVD) approach. We
showed that a monolayer MoS 2 can be synthesized over a 2-in. sapphire wafer in a growth
time as low as 4 min. With a linear growth rate of 1-layer per 4 min, MoS 2 films with
thicknesses varying from 1- to 5-layers with monolayer precision were produced. In addition,
a viable solution for the large-scale production of MoS 2 and WS 2 thin films directly on
SiO 2 /Si with relatively larger growth rates was demonstrated via the gas-phase CVD
approach. Comprehensive Raman and photoluminescence measurements revealed the
excellent spatial homogeneity and high optical quality of these TMD thin films. The electrical
properties of the TMD layers were tested by fabricating arrays of back-gated monolayer
field-effect transistors. Our findings suggest that the electrical properties are influenced by
the grain size of the TMD monolayers. Very recently, we have demonstrated the growth of
Mo 1−x W x S 2 ternary alloy monolayers and precise compositional tuning for the entire range of
x from 0 to 1 using the gas-phase precursors. By means of Raman spectroscopy we showed
that W alloying in MoS 2 lattice can lead to a tensile strain of ∼0.8%. The alloying-induced
tensile strain plays a key role in observing redshift in optical absorption and
photoluminescence (PL) bands and resulted an unusual bandgap bowing. The coupling of
tensile strain and alloying effect allowed us to tune the overall PL emission energy to as large
as 185 meV. Our optical spectroscopy results indicated a three different phase-regions for the
Mo 1−x W x S 2 alloy system. For x < 0.37, the alloys exhibit MoS 2 -like nature, whereas, WS 2 -like
behavior is observed for x > 0.64, and a mixed behavior for 0.37 ≤ x ≤ 0.64.
References:
1. Nitin et al., APL Mater. 7, 081113 (2019).
2. Vijaykumar et al., Appl. Phys. Lett. 118, 013102 (2021).
3. Nitin et al., ACS Appl. Nano Mater. 8, 7371-7376 (2020).
4. Nitin et al, Appl. Surf. Sci. 568, 150908 (2021).
5. Nitin et al, J. Phys. Chem. Lett., 12, 6197 (2021). |