  Compact optical detectors with fast binary photoswitching over a broad range
of wavelength are essential as an interconnect for any light-based parallel,
real-time computing. Despite of the tremendous technological advancements yet
there is no such single device available that meets the specifications. Here we
report a multifunctional self-powered high-speed ultrabroadband (250-1650 nm)
photodetector based on g-C3N4/Si hybrid 2D/3D structure. The device shows a
novel binary photoswitching (change in current from positive to negative) in
response to OFF/ON light illumination at small forward bias (<0.1 V) covering
250-1350 nm. At zero bias, the device displays an extremely high ON/OFF ratio
of 1.2 x 10^5 under 680 nm (49 microWcm-2) illumination. The device also shows
an ultrasensitive behaviour over the entire operating range at low light
illuminations, with highest responsivity (1.2 AW-1), detectivity (2.8 x 10^14
Jones) and external quantum efficiency (213%) at 680 nm. The response and
recovery speeds are typically 0.23 and 0.60 ms, respectively, under 288 Hz
light switching frequency. Dramatically improved performance of our device is
attributed to the heterojunctions formed by the ultrathin g-C3N4 nanosheets
embedded in the Si surface.
