On-chip graphene photodetectors with a nonvolatile p-i-n homojunction

Abstract

Graphene's unique photothermoelectric (PTE) effect, combined with its compatibility for on-chip fabrication, promises its development in chip-integrated photodetectors with ultralow dark-current and ultrafast speed. Previous designs of on-chip graphene photodetectors required external electrical biases or gate voltages to separate photocarriers, leading to increased power consumption and complex circuitry. Here, we demonstrate a nonvolatile graphene p-i-n homojunction constructed on a silicon photonic crystal waveguide, which facilitates PTE-based photodetection without the need for electrical bias or gate voltages. By designing an air-slotted photonic crystal waveguide as two individual silicon back gates and employing ferroelectric dielectrics with remnant polarization fields, the nonvolatile p-i-n homojunction with a clear gradient of Seebeck coefficient is electrically configured. Hot carriers in the graphene channel generated from the absorption of waveguide evanescent field are separated by the nonvolatile p-i-n homojunction effectively to yield considerable photocurrents. With zero-bias and zero-gate voltage, the nonvolatile graphene p-i-n homojunction photodetector integrated on the optical waveguide exhibits high and flat responsivity of 193 mA W^-1 over the broadband wavelength range of 1560-1630 nm and an ultrafast dynamics bandwidth of 17 GHz measured in the limits of our instruments. With the high-performance on-chip photodetection, the nonvolatile graphene homojunction directly constructed on silicon photonic circuits promises the extended on-chip functions of the optoelectronic synapse, in-memory sensing and computing, and neuromorphic computing.

Fig. 1. Schematic of the waveguide-integrated nonvolatile graphene p–i–n homojunction photodetector.

a Schematic of the graphene p–i–n homojunction photodetector constructed on a PC waveguide with two air slots, showing optical mode propagating in the central waveguide and two separated back gates provided by the silicon PC slab with the P(VDF-TrFE) dielectric layer. b Gradients of Seebeck coefficient (ΔS) and hot-carrier temperature (T_e) over the nonvolatile graphene p–i–n homojunction. A downward (upward) polarization field P_down (P_up) of the P(VDF-TrFE) layer causes the graphene layer to be doped in p−type (n−type). Different doping levels of graphene gated by air-slotted PC waveguide result in the gradient of Seebeck coefficient ΔS = S₁ − S₂. The Fermi level of graphene is also provided for guiding the Seebeck coefficient variations related to the doping of graphene along the channel. Hot-carrier temperature T_e distributions of the graphene layer correspond to the guiding mode in the PC waveguide

Fig. 2. Electrical characteristics of nonvolatile graphene p–i–n homojunction constructed on the air-slotted silicon PC waveguide with the dielectric layer of P(VDF-TrFE).

a Top panel: optical microscope image of a fabricated device. Scale bar, 50 μm. Bottom panel: the zoomed image of the active section of the device, consisting of graphene flake and P(VDF-TrFE) coated on the PC waveguide substrate. Scale bar, 10 μm. b Transfer curve of the graphene channel with the global gate voltages V_G = V_G1 = V_G2, and V_DS = 0.01 V. c Source-drain current I_DS map at V_DS = 0.01 V with varying V_G1 from −30 V to 30 V, V_G2 from −30 V to 30 V. d Source-drain current I_DS map at V_DS = 0.01 V with varying V_G1 from 30 V to −30 V, V_G2 from −30 V to 30 V, as indicated by the white arrows

Fig. 3. Spatially resolved photocurrent distribution of nonvolatile graphene p–i–n homojunction constructed on the air-slotted silicon PC waveguide with the dielectric layer of P(VDF-TrFE).

a Scanning photocurrent map of the graphene p–i–n junction at V_DS = 0 V after a programming pulse gate voltage of V_G1 = −15 V, V_G2 = +15 V. b The corresponding scanning region is outlined by the black dashed line in the top panel. Scale bar, 10 μm. A zoomed view of the PC waveguide captured a part of the outlined region in the optical image of the top panel is shown in the bottom panel. Scale bar, 1 μm. c Scanning photocurrent map of the graphene n–i–p junction at V_DS = 0 V after a programming pulse gate voltage of V_G1 = +15 V, V_G2 = −15 V

Fig. 4. Steady-state photoresponses of the nonvolatile graphene p–i–n homojunction integrated on silicon PC waveguide with a laser at 1620 nm guided in the waveguide.

a Measured photocurrent map at zero bias after the configurations with different gate voltages on G1 and G2. b Optical power dependence of the photocurrents at V_DS = 0 V after pulsed gate voltages of V_G1 = −15 V and V_G2 = +15 V to configure the graphene p–i–n homojunction. c Monitored photocurrents for 28 h after the removal of the gate voltages, displaying excellent endurance and retention performance

Fig. 5. Wavelength-dependent and high-speed photoresponse of the waveguide-integrated nonvolatile graphene p–i–n homojunction photodetector at zero-bias after the configuration of V_G1 = −15 V and V_G2 = +15 V.

a Wavelength-dependent photocurrent ranging from 1560 to 1630 nm. b Measured impulse response, showing a 3 dB bandwidth of 17 GHz limited by the employed oscilloscope