  Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs),
such as MoS$_2$, are promising candidates for nanoscale photonics because of
strong-light matter interactions. However, Fermi level pinning due to
metal-induced gap (MIGS) states at the metals-monolayer MoS$_2$ interface
limits the application of optoelectronic devices based on conventional metals
because of the high contact resistance of the Schottky contacts. On the other
hand, a semimetal-TMD-semimetal device can overcome this limitation, where the
MIGS are sufficiently suppressed and can result in ohmic contacts. Here we
demonstrate the optoelectronic performance of a bismuth-monolayer (1L)
MoS$_2$-bismuth device with ohmic electrical contacts and extraordinary
optoelectronic properties. To address the wafer-scale production, we grew full
coverage 1L MoS$_2$ by using chemical vapor deposition method. We measured high
photoresponsivity of 300 A/W in the UV regime at 77 K, which translates into an
external quantum efficiency (EQE) ~ 1000 or $10^5$%. We found that the 90% rise
time of our devices at 77 K is 0.1 ms, which suggests that the current devices
can operate at the speed of ~ 10 kHz. The combination of large-array device
fabrication, high sensitivity, and high-speed response offers great potential
for applications in photonics that includes integrated optoelectronic circuits.
