  Epitaxial growth and controllable doping of wafer-scale single-crystal
transition-metal dichalcogenides (TMDCs) are two central tasks for extending
Moore's law beyond silicon. However, despite considerable efforts, addressing
such crucial issues simultaneously under two-dimensional (2D) confinement is
yet to be realized. Here we design an ingenious epitaxial strategy to
synthesize record-breaking 4-inch single-crystal Fe-doped TMDCs monolayers on
industry-compatible c-plane sapphire without miscut angle. In-depth
characterizations and theoretical calculations reveal that the introduction of
Fe significantly decreases the formation energy of parallel steps on sapphire
surfaces and contributes to the edge-nucleation of unidirectional TMDCs domains
(>99%). The ultrahigh electron mobility (~86 cm2 V -1 s-1) and remarkable
on/off current ratio (~108) are discovered on 4-inch single-crystal Fe-MoS2
monolayers due to the ultralow contact resistance and perfect Ohmic contact
with metal electrodes. This work represents a substantial leap in terms of
bridging the synthesis and doping of wafer-scale single-crystal 2D
semiconductors without the need for substrate miscut, which should promote the
further device downscaling and extension of Moore's law.
