  We demonstrate experimentally the realization and the characterization of a
chip scale integrated photodetector for the visible and the near infrared
spectral regime based on the integration of a MoSe2/WS2 heterojunction on top
of a silicon nitride waveguide. This configuration achieves high responsivity
of ~1 A/W at the wavelength of 780 nm (indicating an internal gain mechanism),
while suppressing the dark current to the level of ~50 pA, much lower as
compared to a reference sample of just MoSe2 without WS2. We have measured the
power spectral density of the dark current to be as low as ~1x10^-12 A/Hz^0.5 ,
from which we extract the noise equivalent power (NEP) to be ~1x10^-12
W/Hz^0.5. To demonstrate the usefulness of the device, we use it for the
characterization of the transfer function of a microring resonator that is
integrated on the same chip as the photodetector. The ability to integrate
local photodetectors on chip and to operate such devices with high performance
at the visible and the near-infrared regime is expected to play a critical role
in future integrated devices in the field of optical communications, quantum
photonics, biochemical sensing and more.
