  Two-dimensional (2D) MoS$_2$ has been intensively investigated for its use in
the fields of microelectronics, nanoelectronics, and optoelectronics. However,
intrinsic 2D MoS$_2$ is usually used as the n-type semiconductor due to the
unintentional sulphur vacancies and surface gas adsorption.The synthesis and
characterization of 2D MoS$_2$ semiconductor of p-type are crucial for the
development of relevant p-n junction devices, as well as the practical
applications of 2D MoS$_2$ in the next-generation CMOS integrated circuit.
Here, we synthesize high-quality, wafer-scale, 2D p-type MoS$_2$
(Mo$_{1-x}$Nb$_x$S$_2$) with various niobium (Nb) mole fractions from 0 to 7.6%
by a creative two-step method. The dielectric functions of 2D Mo1-xNbxS2 are
accurately determined by spectroscopic ellipsometry. We find that the
increasing fraction of Nb dopant in 2D MoS$_2$ can modulate and promote the
combination of A and B exciton peaks of 2D MoS$_2$. The direct causes of this
impurity-tunable combination are interpreted as the joint influence of
decreasing peak A and broadening peak B. We explain the broadening peak B as
the multiple transitions from the impurity-induced valance bands to the
conductive band minimum at K point of Brillouin zone by comparing and analyzing
the simulated electronic structure of intrinsic and 2D Nb-doped MoS$_2$. A
p-type FET based on the 2D Nb-doped MoS$_2$ was fabricated for
characterization, and its working performance is expected to be adjustable as a
function of concentration of Nb dopant according to our theoretical research.
Our study is informative for comprehending optical and electronic properties of
extrinsic 2D transitional metal dichalcogenides, which is important and
imperative for the development and optimization of corresponding photonics and
optoelectronics devices.
