  On-chip integration of two-dimensional (2D) materials offers great potential
for the realization of novel optoelectronic devices in different photonic
platforms. In particular, indium selenide (InSe) is a very promising 2D
material due to its ultra-high carrier mobility and outstanding
photo-responsivity. Here, we report a high-speed photodetector based on a
multilayer 90 nm thick InSe integrated on a silicon nitride (SiN) waveguide.
The device exhibits a low dark current of 10 nA at 1V bias, a remarkable
photoresponsivity of 0.38 AW-1, and high external quantum efficiency of 48.4%
measured at 5 V bias. This performance is tested at near-infrared (NIR) 976 nm
wavelength under ambient conditions. Furthermore, using numerical and
experimental investigations, the estimated absorption coefficient per unit
length is 0.11dB/um. To determine the dynamic response of the photodetector,
its small and large signal frequency response are also evaluated. A 3-dB
radiofrequency (RF) bandwidth of 85 MHz is measured with an open-eye diagram
observed at 1 Gbit/s data transmission. Given these outstanding optoelectronic
merits, active photonic devices based on integrated multilayer InSe can be
realized for a variety of applications including short-reach optical
interconnects, LiDAR imaging, and biosensing.
