  Monolayer transition metal dichalcogenides (TMDCs) constitute the core group
of materials in the emerging semiconductor technology of valleytronics. While
the coupled spin-valley physics of pristine TMDC materials and their
heterstructures has been extensively investigated, less attention was given to
TMDC alloys, which could be useful in optoelectronic applications due to the
tunability of their band gaps. We report here our experimental investigations
of the spin-valley physics of the monolayer and bilayer TMDC alloy,
MoS$_{2x}$Se$_{2(1-x)}$, in terms of valley polarization and the generation as
well as electrical control of a photocurrent utilising the circular
photogalvanic effect. Piezoelectric force microscopy provides evidence for an
internal electric field perpendicular to the alloy layer, thus breaking the
out-of-plane mirror symmetry. The experimental observation is supported by
first principles calculations based on the density functional theory. A
comparison of the photocurrent device, based on the alloy material, is made
with similar devices involving other TMDC materials.
