  The strong spin$-$orbit coupling (SOC) and numerous crystal phases in
few$-$layer transition metal dichalcogenides (TMDCs) MX$_2$ (M$=$W, Mo, and
X$=$Te, Se, S) has led to a variety of novel physics, such as Ising
superconductivity and quantum spin Hall effect realized in monolayer 2H$-$ and
Td$-$MX$_2$, respectively. Consecutive tailoring of the MX$_2$ structure from
2H to Td phase may realize the long$-$sought topological superconductivity in
one material system by incorporating superconductivity and quantum spin Hall
effect together. In this work, by combing Raman spectrum, X-ray photoelectron
spectrum (XPS), scanning transmission electron microscopy imaging (STEM) as
well as electrical transport measurements, we demonstrate that a consecutively
structural phase transitions from Td to 1T$'$ to 2H polytype can be realized as
the Se-substitution concentration increases. More importantly, the
Se$-$substitution has been found to notably enhance the superconductivity of
the MoTe$_2$ thin film, which is interpreted as the introduction of the
two$-$band superconductivity. The chemical constituent induced phase transition
offers a new strategy to study the s$_{+-}$ superconductivity and the possible
topological superconductivity as well as to develop phase$-$sensitive devices
based on MX$_2$ materials.
