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Bandwidth Enhancement in GeSn-on-Si Avalanche Photodiodes for High-Speed Short-Wave Infrared Detection

Introduction

The demand for high-performance detectors capable of detecting weak light signals in the short-wave infrared (SWIR) region has increased significantly in recent years. Applications such as eye-safe imaging, medical diagnostics, lidar, and fiber-optic telecommunications require detectors that can operate at high frequencies while maintaining high sensitivity. Avalanche photodiodes (APDs) fulfill these requirements, with Ge-on-Si and InGaAs-based APDs being prominent candidates. However, incorporating Sn into the Ge crystal lattice reduces the material's bandgap, increasing the absorption for wavelengths of 1,550 nm and above, leading to an enhanced sensitivity compared to Ge-based devices. Despite this promising prospect, the challenge of growing high-quality GeSn has limited the research efforts on GeSn-on-Si APDs.

Device Structure and Fabrication

The GeSn-on-Si APDs presented in this work employ the established separate absorption, charge, and multiplication (SACM) structure. A 500 nm thick layer of intrinsic Si serves as the avalanche material, offering a low ratio of ionization coefficients for electrons and holes. The charge layer, with a nominal doping of 1.5 × 10^17 cm^-3, reduces the internal electrical field strengths in the narrow bandgap absorption material, suppressing carrier multiplication and tunneling. The 300 nm thick GeSn absorption layer is deposited at a low temperature of 200°C using molecular beam epitaxy. After growth, vertical-illumination-type mesa diodes are fabricated using a standard process.

Electro-optical Characterization

The electro-optical characteristics of the GeSn-on-Si APDs were evaluated through current-voltage (I-V) measurements with and without illumination at 1,550 nm. Figure 1 shows the I-V measurements and gain characteristics for an 80 μm diameter diode.

layer structure of the GeSn-on-Si APD
Fig. 1: Dark, illuminated current and gain as a function of voltage. The inset shows the layer structure of the GeSn-on-Si APD.

The dark currents obtained are comparatively low, with the surface current being the limiting factor in the reverse region, as revealed by measurements on different device sizes. The breakdown voltage (VBD) is defined as the voltage at which the current reaches 100 μA, which in this case is -21.9 V. The punch-through voltage (VPT), at which the electrical field penetrates through the p-doped charge layer into the GeSn absorption layer, is -15 V.

For an optical power of 18 μW, a responsivity of 2.85 A/W is achieved, corresponding to a maximum gain of ~26 when normalized to the primary responsivity of a Ge0.98Sn0.02 reference pin diode. The gain is strongly dependent on the optical power due to the space-charge effect, with gains of ~22 and ~30 observed for optical powers of 31.8 μW and 9.7 μW, respectively. Beyond VBD, the gain starts to decrease due to thermal heating and carrier injection effects.

High-Frequency Characteristics

The high-frequency behavior of the GeSn-on-Si APDs was investigated using an impulse response method. The devices were contacted via ground-signal-ground (GSG) probes, a bias tee, and a DC source. A ps-pulsed laser with a 7.8 MHz repetition rate was used to illuminate the devices through a single-mode fiber, and a sampling oscilloscope with a 16 GHz bandwidth was used to observe the impulse response.

Figure 2(a) shows the impulse responses for different bias voltages at an average illumination power of 18 μW for an 80 μm diameter diode. At voltages ≥ VBD, the pulse becomes significantly shorter and changes its overall shape while maintaining a high amplitude. This phenomenon is attributed to resonance caused by the appearance of inductive components in the avalanche region, making these operating voltages suitable for high-frequency operation.

Pulse response at different bias voltages while illuminated by 18 µW at 1,550 nm
Fig. 2: (a) Pulse response at different bias voltages while illuminated by 18 µW at 1,550 nm. The inset shows a microscopic image of the diode with the GSG probe and the single-mode fiber, and (b) the normalized frequency response showing an increase in bandwidth for higher VR. Measurements were done on an 80 µm diameter device.

The frequency response, obtained by Fourier transforming the impulse response, is shown in Figure 2(b). An increase in bandwidth from 580 MHz to 3.58 GHz is achieved by increasing the voltage from -21.6 V to -22.6 V. The bandwidth enhancement is more pronounced at lower optical powers due to the stronger resonance effect.

Gain-Bandwidth Product

Figure 3 illustrates the bandwidth as a function of gain for different optical powers. As the reverse voltage increases, the bandwidth increases while the gain initially increases until it reaches its maximum and then decreases again. A small increase in bandwidth is observed for decreasing optical powers at large reverse voltages.

Bandwidth as a function of gain at different optical powers, measured at 1,550 nm.
Fig. 3: Bandwidth as a function of gain at different optical powers, measured at 1,550 nm.

The maximum gain-bandwidth product (GBP) achieved is 60 GHz, obtained at a voltage of -22.4 V and an optical power of 9.7 μW. This remarkable GBP demonstrates the excellent performance of GeSn-on-Si APDs for low-light detection at 1,550 nm.

Conclusion

This work presents GeSn-on-Si APDs with a Sn concentration of 2.1%, exhibiting a bandwidth enhancement at high reverse voltages. The devices achieve bandwidths up to 3.9 GHz and high gains at 1,550 nm, resulting in a 60 GHz gain-bandwidth-product. The bandwidth enhancement is attributed to resonance caused by inductive components in the avalanche region, making these APDs an attractive choice for applications that require high-speed, low-light detection in the SWIR region, such as lidar and fiber-optic telecommunications.

Reference

[1] M. Wanitzek, D. Schwarz, J. Schulze, and M. Oehme, "Bandwidth enhancement in GeSn-on-Si avalanche photodiodes with a 60 GHz gain-bandwidth-product," Institute of Semiconductor Engineering, University of Stuttgart, Stuttgart, Germany, and Chair of Electron Devices, Friedrich-Alexander-University Erlangen, Erlangen, Germany, 2024.

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