  Photodetectors converting light signals into detectable photocurrents are
ubiquitously in use today. To improve the compactness and performance of
next-generation devices and systems, low dimensional materials provide rich
physics to engineering the light matter interaction. Photodetectors based on
two dimensional (2D) material van der Waals heterostructures have shown high
responsivity and compact integration capability, mainly in the visible range
due to their intrinsic bandgap. The spectral region of near-infrared (NIR) is
technologically important featuring many data communication and sensing
applications. While some initial NIR 2D material-based detectors have emerged,
demonstrating doping junction based 2D material photodetectors with the
capability to harness the charge separation photovoltaic effect are yet
outstanding. Here, we demonstrate a 2D p-n van der Waals heterojunction
photodetector constructed by vertically stacking p type and n type few layer
indium selenide (InSe) 2D flakes. This heterojunction charge separation based
photodetector shows a three fold enhancement in responsivity at near infrared
spectral region (980 nm) as compared to a photoconductor detector based on p or
n only doped regions, respectively. We show, that this junction device exhibits
self-powered photodetection operation and hence enables few pA-low dark
currents, which is about 4 orders of magnitude more efficient than state of the
art foundry based devices.
