Fig. 1. (a) Cross-sectional transmission electron microscopy (TEM) image of the epitaxial material grown on the SOI substrate; the inset above is the selected-area diffraction pattern of the GeSn layer. (b) High-resolution TEM (HR-TEM) image of interface between epitaxial GeSn and Ge-buffer. (c) HR-TEM image of interface between epitaxial Ge-buffer and top-Si substrate. (d) The SIMS depth profile analysis of various elements in the as-grown sample. (e) X-ray diffraction reciprocal space map (XRD-RSM) around the asymmetric (−2-24) reflection of the material.

Fig. 2. (a) 3D structure schematic of the normally illuminated p-i-n Ge0.951Sn0.049 photodetector. (b) Top-view SEM image of the device with a 10 μm diameter mesa.

Fig. 3. Typical I-V characteristics of the Ge0.951Sn0.049 photodetectors (D=10, 12, 15, 18, 20, 25, and 30 μm); the inset is the dark current densities of devices at −1  V versus 1/D.

Fig. 4. (a) C-V characteristics of the Ge0.951Sn0.049 photodetectors with various diameters. (b) The capacitances of different devices at −3  V versus area.

Fig. 5. (a) I-V characteristics of the GeSn photodetector with a diameter of 10 μm with/without light incidence. (b) Spectrum response and optical responsivity of the Ge0.951Sn0.049 photodetector as functions of wavelength under zero-bias. Devices were measured by an FTIR optical spectrometer and lasers (two tunable lasers, one is located between 1260 nm and 1360 nm, and the other is located between 1500 nm and 1630 nm, and a laser at 2000 nm). The data are shown as a blue curve and a scatter plot, respectively.

Fig. 6. Illustration of an optical heterodyne beat frequency measurement system.

Fig. 7. (a) Normalized frequency responses of the photodetectors with various diameters at 2 μm (D=10, 15, 20, 25, and 30 μm). (b) The theory RC-limited bandwidth, transit-time-limited bandwidth, and combined bandwidth of devices with different diameters.

Fig. 8. Comparison of 3 dB bandwidth of high-speed photodetectors for 2 μm-wavelength light detection in different groups.

Table 1. Summary of Lattice Constant, In-Plane Strain and Sn Concentration

Table 2. Summary of Dark Current Density, Capacitance, and 3 dB Bandwidth of Devices with Different Diameters