Silicon-Germanium (SiGe) alloys have emerged as a promising material system for photodetection applications, particularly in the near-infrared (NIR) and short-wave infrared (SWIR) spectral ranges. The tunable bandgap of SiGe, achieved by adjusting the Ge composition, allows for tailored spectral response, making it suitable for applications such as optical communications, imaging, and sensing. Unlike pure silicon, which has a limited cutoff wavelength of around 1.1 µm, SiGe extends the detectable wavelength range due to its narrower bandgap, enabling detection up to approximately 1.6 µm for Ge-rich alloys.

The spectral response of SiGe photodetectors is primarily determined by the alloy composition and the device architecture. For instance, a SiGe layer with 30% Ge exhibits an absorption edge near 1.3 µm, aligning with the telecommunication O-band. Increasing the Ge content shifts the absorption edge to longer wavelengths, but this also introduces challenges such as higher lattice mismatch with silicon substrates, leading to increased defect densities. To mitigate this, strain-engineered heterostructures and graded buffer layers are employed to reduce threading dislocations and improve material quality.

Dark current is a critical parameter in photodetectors, as it directly impacts the signal-to-noise ratio and detection sensitivity. In SiGe-based photodetectors, dark current arises primarily from thermal generation of carriers, defect-assisted tunneling, and surface leakage. Several strategies have been developed to minimize dark current. One approach involves using heterojunction designs, such as p-i-n structures with intrinsic SiGe absorption regions, which reduce bulk recombination. Passivation techniques, including hydrogenation and dielectric capping, are also employed to suppress surface states that contribute to leakage currents.

Another effective method for dark current reduction is the implementation of avalanche photodiodes (APDs) with separate absorption and multiplication regions. In such devices, the SiGe layer serves as the absorption region, while the multiplication occurs in a silicon layer with lower noise characteristics. This configuration leverages the high absorption coefficient of SiGe while maintaining low dark current through optimized electric field distribution. Reported dark current densities for SiGe p-i-n photodiodes typically range from 1 to 100 mA/cm² at room temperature, depending on Ge content and device geometry.

Temperature plays a significant role in dark current performance. Cooling the detector can substantially reduce thermal generation rates, but this is often impractical for consumer or portable applications. Instead, advanced doping profiles and bandgap engineering are used to achieve acceptable dark current levels at room temperature. For example, delta-doping techniques can create sharp potential barriers that suppress minority carrier diffusion, thereby lowering dark current without compromising quantum efficiency.

Recent advancements in SiGe photodetectors include the integration of waveguide-coupled structures for enhanced light absorption in thin active layers. By confining light within a SiGe waveguide, the effective absorption length is increased, allowing for high responsivity even at wavelengths where SiGe has weaker absorption. Such devices have demonstrated responsivities exceeding 0.5 A/W at 1.55 µm, making them competitive with III-V-based photodetectors for certain applications.

The compatibility of SiGe with silicon CMOS fabrication processes is a key advantage, enabling monolithic integration of photodetectors with readout electronics. This integration reduces parasitic capacitances and enables high-speed operation, with bandwidths exceeding 10 GHz reported for waveguide-integrated SiGe detectors. Furthermore, the use of silicon substrates lowers manufacturing costs compared to exotic materials like InGaAs, making SiGe an attractive option for large-scale deployments.

Despite these advantages, challenges remain in achieving uniform material quality across large wafers, particularly for high-Ge-content alloys. Non-uniform strain relaxation and defect clustering can lead to variations in device performance, necessitating precise growth control during epitaxy. Techniques such as chemical-mechanical polishing (CMP) and cyclic annealing have shown promise in improving layer uniformity and reducing defect densities.

Looking ahead, ongoing research focuses on extending the operational wavelength of SiGe photodetectors further into the SWIR range by incorporating Sn into the alloy (forming SiGeSn). This ternary system offers additional bandgap tunability, with potential cutoff wavelengths beyond 2 µm. However, material quality and stability remain areas of active investigation, particularly for Sn concentrations above 10%.

In summary, SiGe-based photodetectors offer a compelling combination of tunable spectral response, CMOS compatibility, and cost-effectiveness. Advances in strain engineering, passivation, and device architecture continue to push the boundaries of performance, making them viable alternatives to traditional III-V detectors in specific wavelength regimes. As growth techniques mature and new alloy systems like SiGeSn are explored, the applicability of SiGe photodetectors is expected to expand further into emerging fields such as LiDAR, biomedical imaging, and quantum sensing.

The table below summarizes key performance metrics for representative SiGe photodetectors:

| Ge Content (%) | Cutoff Wavelength (µm) | Dark Current Density (mA/cm²) | Responsivity (A/W) | Bandwidth (GHz) |
|----------------|------------------------|-----------------------------|--------------------|-----------------|
| 20 | 1.2 | 10 | 0.3 | 5 |
| 30 | 1.3 | 50 | 0.4 | 8 |
| 50 | 1.5 | 100 | 0.5 | 10 |

These values are indicative and can vary based on specific device designs and fabrication processes. Continued optimization of material quality and device engineering will further enhance the performance and applicability of SiGe photodetectors in next-generation optoelectronic systems.