  Vertically stacking two dimensional (2D) materials can enable the design of
novel electronic and optoelectronic devices and realize complex functionality.
However, the fabrication of such artificial heterostructures in wafer scale
with an atomically-sharp interface poses an unprecedented challenge. Here, we
demonstrate a convenient and controllable approach for the production of
wafer-scale 2D GaSe thin films by molecular beam epitaxy. In-situ reflection
high-energy electron diffraction oscillations and Raman spectroscopy reveal a
layer-by-layer van der Waals epitaxial growth mode. Highly-efficient
photodetector arrays were fabricated based on few-layer GaSe on Si. These
photodiodes show steady rectifying characteristics and a relatively high
external quantum efficiency of 23.6%. The resultant photoresponse is super-fast
and robust with a response time of 60 us. Importantly, the device shows no sign
of degradation after 1 million cycles of operation. Our study establishes a new
approach to produce controllable, robust and large-area 2D heterostructures and
presents a crucial step for further practical applications.
