Highly efficient gate-tunable photocurrent generation in vertical heterostructures of layered materials

Abstract

Layered materials of graphene and MoS₂, for example, have recently emerged as an exciting material system for future electronics and optoelectronics. Vertical integration of layered materials can enable the design of novel electronic and photonic devices. Here, we report highly efficient photocurrent generation from vertical heterostructures of layered materials. We show that vertically stacked graphene-MoS₂-graphene and graphene-MoS₂-metal junctions can be created with a broad junction area for efficient photon harvesting. The weak electrostatic screening effect of graphene allows the integration of single or dual gates under and/or above the vertical heterostructure to tune the band slope and photocurrent generation. We demonstrate that the amplitude and polarity of the photocurrent in the gated vertical heterostructures can be readily modulated by the electric field of an external gate to achieve a maximum external quantum efficiency of 55% and internal quantum efficiency up to 85%. Our study establishes a method to control photocarrier generation, separation and transport processes using an external electric field.

Figure 1. Photocurrent generation in vertical heterostructures of graphene–MoS₂–graphene

a, Schematic illustration of the three-dimensional device layout. b, Schematic illustration of the side view of the device, with the semiconducting multilayer MoS₂ sandwiched between the Gr_T and Gr_B electrodes. Red and blue colours indicate electrons and holes, respectively. The silicon substrate can be used as a back-gate electrode with 300 nm SiO₂ as the dielectric layer. c, Experimental current–voltage characteristic of the vertical device in the dark (blue) and under illumination (red) by a focused laser beam (wavelength, 514 nm; power, 80 µW; spot size, 1 µm).

Figure 2. Field-effect modulated photocurrent generation in single-gated graphene–MoS₂–graphene heterostructures

a, Optical image of the vertical heterostructure with a multilayer MoS₂ flake (~50 nm thick) sandwiched between the Gr_T and Gr_B electrodes. b, SEM image of the same device with the Gr_T (yellow), Gr_B (red) and intermediate MoS₂ layer (blue) labelled with different false colours. c, I–V characteristics of the device under laser illumination (on white dot in b) at V_BG varying from −60 V to +60 V in steps of 20 V. Inset: variation of I_sc and V_oc with V_BG. d, Scanning photocurrent images taken at gate biases between −60 V and +60 V under a 514 nm laser (excitation power, 80 µW; spot size, 1 µm). Dashed and solid lines indicate the edges of the Gr_B and Gr_T electrodes, respectively. e–h, Schematic band diagrams of the vertical heterostructure with zero (e), negative (f), positive (g) and large positive (h) bias on the silicon back gate. Blue dots, black circles and green arrows indicate electrons, holes and photons, respectively. i, EQE of the vertical heterostructure device as a function of back-gate voltage under 80 µW, 514 nm laser excitation. Inset: EQE of the device as a function of excitation laser power at V_BG = −60 V. j, Excitation laser power-dependent EQE of another graphene-MoS₂ (16 nm)-graphene device under various excitation wavelengths at V_BG = −60 V. k, Wavelength-dependent EQE (black line with squares), absorbance (red line with circles) and IQE (blue line with triangles) of the device at V_BG = −60 V under a focused laser power of 5 µW.

Figure 3. Field-effect switchable photocurrent generation in dual-gated graphene–MoS₂–graphene heterostructures

a, Schematic illustration of the side view of the device, with a multilayer MoS₂ flake sandwiched between the Gr_T and Gr_B electrodes, the silicon substrate as a back-gate electrode with a 300 nm SiO₂ dielectric layer, and another graphene layer as the top-gate electrode (Gr_G) with a 60 nm HfO₂ gate dielectric layer. b, Optical image of a dualgated device. c, Transfer characteristics of a dual-gated vertical heterostructure device as a function of V_BG (−60 V to +60 V) at various V_TG (−6 V to +4 V) at V_DS = 100 mV. Blue and red arrows indicate the Dirac points of the Gr_B and Gr_T electrodes, respectively. d, Two-dimensional colour plot of the transconductance (dI_DS/dV_BG) of the dual-gated vertical heterostructure device. The Dirac points of the Gr_T and Gr_B electrodes are shown by solid and dashed lines, respectively. e–g, Schematic band diagrams of the dual-gated vertical heterostructure device with negative, zero and positive top-gate bias at a back-gate bias near the bottom graphene Dirac point (−30 V). The red dashed arrow in f indicates the recombination of electrons and holes. h, Schematic band diagram of the device with negative top-gate bias and more positive back-gate bias than the Gr_B Dirac point −30 V to further enhance the band slope and overall photocurrent. i, Scanning photocurrent images taken at V_BG = −30 V and variable V_TG (−6 V to +6 V) (under 80 µW, 514 nm laser excitation, with a spot size of 1 µm). Dashed and solid lines indicate the edges of the Gr_B and Gr_T electrodes, respectively. j, Scanning photocurrent images taken at fixed V_TG = −6 V and variable V_BG (−30 to +60 V).

Figure 4. Field-effect modulated photocurrent generation in a single-gated graphene–MoS₂–metal heterostructure

a, Schematic three-dimensional illustration of the device layout. b, Schematic illustration of the side view of the device, with a multilayer MoS₂ flake sandwiched between a Gr_B electrode and a top metal electrode (Ti). The device was fabricated on transparent ITO on a glass substrate (as the back gate) with 30-nm-thick Al₂O₃ as the gate dielectric to allow illumination to transmit through the back side of the substrate to reach the graphene–MoS₂ contact. c, Optical image of a graphene–MoS₂–metal heterostructure device. d,e, Schematic band diagrams of the graphene–MoS₂–metal heterostructure at negative and positive back-gate voltages, respectively. f, Scanning photocurrent images taken at V_BG = −1, 0 and +1 V (80µW, 514 nm excitation, with a spot size of 1 µm). Dashed and solid lines indicate the edges of the Gr_B and top metal electrodes, respectively. g, I–V characteristics of the device under laser illumination (on red dot in c) with V_BG varying from −1 V to 1 V in steps of 0.5 V. The I–V curve obtained in the dark at V_BG = −1 V passes through the origin, indicating that gate leakage is negligible. h, Excitation laser power-dependent EQE under various excitation wavelengths at V_BG = −1 V. i, Wavelength-dependent EQE at a laser power of 5 µW.