Split-Gate Photodiode Based on Graphene/HgTe Heterostructures with a Few Nanosecond Photoresponse

Hopping transport associated with the granular nature of nanocrystal arrays has led to the thought that nanocrystal-based devices might be incompatible with fast operation. Here, we explore the design of HgTe nanocrystal-based sensors operating in the short-wave infrared and with a very fast time response down to a few nanoseconds. To reach this goal, the design relies on a planar geometry to reduce the device capacitance. A strong in-built electric field is tailored via electrostatic control from two bottom split-gate electrodes, which promote the charge extraction. Through the use of graphene electrodes patterned over the two gate electrodes, we optimize the control on the electrostatic design of the p–n junction inside the nanocrystal array. Taking advantage of a high-k dielectric spacer, we demonstrate that the device can be operated under a low gate bias (<6 V). The split-gate photodetector are versatile as they can be used in either the phototransistor or diode mode, with the two gates voltages that are set to design isotype or diode-type heterojunctions. We finally highlight that the time response enabled by the planar diode configuration can be made much faster than the one associated with the conventional vertical geometry.