  Semiconductor heterostructures have played a critical role as the enabler for
new science and technology. The emergence of transition metal dichalcogenides
(TMDs) as atomically thin semiconductors has opened new frontiers in
semiconductor heterostructures either by stacking different TMDs to form
vertical heterojunctions or by stitching them laterally to form lateral
heterojunctions via direct growth. In conventional semiconductor
heterostructures, the design of multi-junctions is critical to achieve carrier
confinement. Analogously, we report successful synthesis of monolayer
WS2/WS2(1-x)Se2x/WS2 multi-junction lateral heterostructure via direct growth
by chemical vapor deposition. The grown structures are characterized by Raman,
photoluminescence, and annular dark-field scanning transmission electron
microscopy to determine its lateral compositional profile. More importantly,
using microwave impedance microscopy, we demonstrate that the local
photoconductivity in the alloy region can be tailored and enhanced by 2 orders
of magnitude over pure WS2. Finite element analysis confirms that this effect
is due to the carrier diffusion and confinement into the alloy region. Our work
exemplifies the technological potential of atomically thin lateral
heterostructures in optoelectronic applications.
