Integrated avalanche photodetectors for visible light

Abstract

Integrated photodetectors are essential components of scalable photonics platforms for quantum and classical applications. However, most efforts in the development of such devices to date have been focused on infrared telecommunications wavelengths. Here, we report the first monolithically integrated avalanche photodetector (APD) for visible light. Our devices are based on a doped silicon rib waveguide with a novel end-fire input coupling to a silicon nitride waveguide. We demonstrate a high gain-bandwidth product of 234 ± 25 GHz at 20 V reverse bias measured for 685 nm input light, with a low dark current of 0.12 μA. We also observe open eye diagrams at up to 56 Gbps. This performance is very competitive when benchmarked against other integrated APDs operating in the infrared range. With CMOS-compatible fabrication and integrability with silicon photonic platforms, our devices are attractive for sensing, imaging, communications, and quantum applications at visible wavelengths.

Fig. 1. Device structure and doping configurations.

a Schematic of the APD device, consisting of a Si rib waveguide end-fire coupled to an input SiN waveguide. The yellow arrow denotes the propagation direction of input light. The inset shows the simulated optical mode in the Si rib waveguide. b Cross-sectional view of the Si rib waveguide with a lateral doping profile. The junction placed at a distance Δj from the left edge of the waveguide core with a width W. A reverse bias voltage V_B is applied via metal contacts deposited on top of heavily doped p⁺⁺ and n⁺⁺ regions. c Top view of the Si rib waveguide, showing the lateral and interdigitated doping profiles. a–c are not drawn to scale. d Scanning electron microscope (SEM) image of a fabricated device without the top SiO₂ cladding and metal contacts. e, f Fabricated devices imaged under an optical microscope, showing the lensed fiber coupling and Si APD regions, respectively. The red glow is due to the scattering of the 685-nm input light.

Fig. 2. DC characteristics of a laterally doped device with width W = 900 nm.

a Current–voltage measurements at different input optical powers P_opt. The reverse bias voltage V_B is swept till the avalanche breakdown voltage V_br ≈ 15.5 V, where the dark current I_dark reaches 10 μA. Each sweep takes a few seconds; prior to each sweep, the device is reset with the application of a forward bias voltage. b The avalanche gain G at different P_opt. The inset is a magnified view of the area marked by the rectangle, showing the curves at larger P_opt on a linear scale. Both plots in this figure share the same legend for P_opt.

Fig. 3. Comparison of DC performance for lateral and interdigitated doping profiles with different widths W.

a Photocurrent I_ph versus input power P_opt at the unity gain point of reverse bias V_B = 2 V. Straight lines are linear fits, from which we extract the primary responsivity R_p, see Table 1. b Dark current I_dark measurements at varying V_B. c Avalanche gain G at varying V_B with a fixed input power P_opt = −63.7 ± 0.7 dBm. b, c share the same legend on the right.

Fig. 4. Optical–electrical bandwidth measurements.

An input power of P_opt = −24.5 dBm is used throughout. a Frequency response of a W = 900 nm laterally doped device at various bias voltages V_B. The 3 dB bandwidth is obtained from a smoothing fit to the data points (see Methods). b, c The 3 dB bandwidth and gain-bandwidth product (GBP), respectively, for different device types. Both plots share the same legend shown in c. Each data point and error bar in both plots represent the mean and standard deviation, respectively, of several measurements.

Fig. 5. Measured eye diagrams for the different device types.

Lateral devices show open eyes at data rates of up to 56 Gbps at V_B = 20 V, where the maximum GBP is observed. The results for interdigitated devices are obtained at the highest data rate where open eyes could be measured for each device. The signal-to-noise ratio (SNR) is obtained from the sampling oscilloscope.