High-responsivity graphene photodetectors integrated on silicon microring resonators

Abstract

Graphene integrated photonics provides several advantages over conventional Si photonics. Single layer graphene (SLG) enables fast, broadband, and energy-efficient electro-optic modulators, optical switches and photodetectors (GPDs), and is compatible with any optical waveguide. The last major barrier to SLG-based optical receivers lies in the current GPDs' low responsivity when compared to conventional PDs. Here we overcome this by integrating a photo-thermoelectric GPD with a Si microring resonator. Under critical coupling, we achieve >90% light absorption in a ~6 μm SLG channel along a Si waveguide. Cavity-enhanced light-matter interactions cause carriers in SLG to reach ~400 K for an input power ~0.6 mW, resulting in a voltage responsivity ~90 V/W, with a receiver sensitivity enabling our GPDs to operate at a 10^-9 bit-error rate, on par with mature semiconductor technology, but with a natural generation of a voltage, rather than a current, thus removing the need for transimpedance amplification, with a reduction of energy-per-bit, cost, and foot-print.

Fig. 1. Si ring resonator integrated GPD.

a Sketch of device. b Transmission spectra of a ring cavity with SLG on top, indicating the effects of varying W on ER and Q. For clarity, the spectra are aligned at the resonance wavelength closest to 1553.55 nm. c Corresponding transmission at resonance. The black line is the calculated transmission for κ = 10%. The coloured dots mark the transmission for various loss coefficients.

Fig. 2. Device fabrication.

a Assembly of hBN/SLG/hBN. b Stack placement on photonic circuit and interface cleaning. c hBN etching in SF₆ plasma. d SLG etching in O₂ plasma to define channel geometry. e Metallization (Cr/Au) for drain-source contacts. f Al seed layer evaporation and ALD of Al₂O₃. g Wet transfer of CVD SLG. h Split-gate fabrication. i Metallization (Cr/Au) for gate contacts.

Fig. 3. LMH characterisation.

a Microscope image of hBN/SLG/hBN on ring resonator. The black dashed line indicates the area over which the Raman map in d is measured. Scale bar, 10 μm. b Raman spectrum measured at the position of the final device. c AFM image of LMH. The yellow dashed line indicates the area of top and bottom hBN. The blue line indicates the SLG area. Scale bar, 10 μm. d Raman map of FWHM(2D). The red box marks the position of the final device in a, c, d.

Fig. 4. GPD characterisation.

a False-colour SEM image of our GPD, showing Cr/Au contacts (yellow), Si WG (green), contacted hBN/SLG/hBN (blue, dashed line under contacts), and SLG gates (red). Scale bar: 2 μm. b Resistance map demonstrating independent tunability of charge carrier concentration in the SLG channel via V_G1 and V_G2. c Electrical characterization at homogeneous channel doping (solid lines, measured data; dashed lines, model). d Calculated S based on the electrical data in c. e, f) Resistance vs. inverse carrier concentration for e electron and f hole doping. g Photoresponse at zero bias on resonance (λ = 1555.87 nm). h Frequency response. The 3-dB cutoff frequency, marked by intersecting dashed lines, is ~12 GHz.

Fig. 5. Wavelength and power dependence.

a Transmitted power for various P_in. b Photovoltage for a fixed gate voltage combination (V_G1 = 1 V, V_G2 = − 1 V) for various input powers (same color code as for the transmission in a). c T_e calculated from the photoresponse in b and S in Fig. 4d. d Power-dependent R_[V/W].