  Avalanche photodiodes (APDs) are the semiconducting analogue of
photomultiplier tubes offering very high internal current gain and fast
response. APDs are interesting for a wide range of applications in
communications1, laser ranging2, biological imaging3, and medical imaging4
where they offer speed and sensitivity superior to those of classical p-n
junction-based photodetectors. The APD principle of operation is based on
photocurrent multiplication through impact ionization in reverse-biased p-n
junctions. APDs can either operate in proportional mode, where the bias voltage
is below breakdown, or in Geiger mode, where the bias voltage is above
breakdown. In proportional mode, the multiplication gain is finite, thus
allowing for photon energy discrimination, while in Geiger mode of operation
the multiplication gain is virtually infinite and a self-sustaining avalanche
may be triggered, thus allowing detection of single photons5. Here, we
demonstrate APDs based on vertically stacked monolayer MoS2 and p-Si, forming
an abrupt p-n heterojunction. With this device, we demonstrate carrier
multiplication exceeding 1000. Even though such multiplication factors in APDs
are commonly accompanied by high noise, our devices show extremely low noise
levels comparable with those in regular photodiodes. These heterostructures
allow the realization of simple and inexpensive high-performance and low-noise
photon counters based on transition metal dichalcogenides.
