  Substitutionally-doped 2D transition metal dichalcogenides are primed for
next-generation device applications such as field effect transistors (FET),
sensors, and optoelectronic circuits. In this work, we demonstrate
substitutional Rhenium (Re) doping of MoS2 monolayers with controllable
concentrations down to 500 parts-per-million (ppm) by metal-organic chemical
vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of
Re lead to a reduction in sulfur site defect density by 5-10x. Ab initio models
indicate the free-energy of sulfur-vacancy formation is increased along the
MoS2 growth-front when Re is introduced, resulting in an improved
stoichiometry. Remarkably, defect photoluminescence (PL) commonly seen in
as-grown MOCVD MoS2 is suppressed by 6x at 0.05 atomic percent (at.%) Re and
completely quenched with 1 at.% Re. Furthermore, Re-MoS2 transistors exhibit up
to 8x higher drain current and enhanced mobility compared to undoped MoS2
because of the improved material quality. This work provides important insights
on how dopants affect 2D semiconductor growth dynamics, which can lead to
improved crystal quality and device performance.
