Fig. 1. (a) Schematic diagram of the 2 μm RCE-PD; (b) according to the finite difference-time domain (FDTD) method, the simulation of the normalized electric field intensity |E/E0| inside photodetectors with and without RCE structure.

Fig. 2. High resolution 2θ-ω X-ray diffraction curves of GeSn alloys on Ge buffer with Si (100) substrate in the (a) (004) and (b) (224) directions. (c) Secondary ion mass spectroscopy (SIMS) depth profile analysis of various elements in the GeSn sample; background depicts the scanning electron microscope (SEM) image of the as-grown material. (d) FIB image of cross section of GeSn RCE-PD with a diameter of 12 μm.

Fig. 3. (a) I-V curves of the GeSn photodetector with a diameter of 20 μm: dark, photocurrent under 2 μm laser incidence with or without RCE structure. The incident light power was 1 mW. The inset shows responsivities of the photodetector with RCE structure under 0 V bias and −2  V bias at different output powers of tapered fiber. (b) Wide-spectrum responsivity curves of photodetector with a diameter of 70 μm under zero-bias with or without RCE structure. The inset shows the correlation between the gain and wavelength of the PD with RCE structure.

Fig. 4. (a) Normalized frequency responses of PDs with various mesa diameters at 2 μm incident light with −4  V bias. (b) Frequency response of an 8 μm diameter photodiode at different bias voltages.

Fig. 5. Comparison of 3-dB bandwidth of high-speed photodetectors at 2 μm band in different groups.

Fig. 6. (a) Demonstration link diagram of 2 μm communication system, consisting of 2 μm laser, polarization controller, Si carrier-depletion Mach–Zehnder modulator, bias-T, etc. Black and red lines represent the optical and electrical connections, respectively. (b) Eye diagrams of 20 Gbps, 30 Gbps, 40 Gbps, 50 Gbps GeSn photodetectors with a diameter of 8 μm at −4  V.

Table 1. Comparison of 3-dB Bandwidth and Responsivity of High-Speed Photodetectors at 2 μm Wavelength in Different Material Groups