Silicon-germanium (SiGe) alloys have emerged as a critical material system for optoelectronic applications, particularly in the near-infrared (NIR) spectrum. Their tunable bandgap, compatibility with silicon technology, and unique optical properties make them suitable for photodetectors, modulators, and light-emitting devices. This article examines the optical absorption, refractive index, and luminescence characteristics of SiGe alloys, with a focus on their behavior in the NIR range.

The optical properties of SiGe alloys are strongly influenced by the germanium composition. The bandgap of SiGe varies with the Ge fraction, following Vegard's law for indirect bandgap materials. Pure silicon has a bandgap of 1.12 eV, while pure germanium has a bandgap of 0.66 eV. For Si1-xGex alloys, the bandgap decreases nearly linearly with increasing x, enabling absorption edge tuning from 1.1 µm (Si) to 1.8 µm (Ge). This tunability is particularly useful for NIR applications, where wavelengths between 1.2 µm and 1.6 µm are critical for telecommunications and sensing.

Optical absorption in SiGe alloys is dominated by indirect transitions, similar to pure Si and Ge. The absorption coefficient increases with Ge content due to the reduced bandgap and higher density of states near the conduction band minimum. For example, at a wavelength of 1.3 µm, the absorption coefficient of Si0.7Ge0.3 is approximately 100 cm-1, significantly higher than that of pure Si but lower than Ge. The absorption spectrum shows a characteristic Urbach tail below the bandgap, attributed to disorder-induced states from alloy fluctuations. Strain further modifies the absorption profile; tensile strain increases the bandgap, while compressive strain reduces it.

The refractive index of SiGe alloys is another critical parameter for waveguide and photonic device design. The index increases with Ge concentration, following a near-linear relationship. At 1.55 µm, the refractive index of Si is around 3.48, while Ge has an index of approximately 4.0. For Si0.5Ge0.5, the refractive index is about 3.7. This gradation allows for index engineering in heterostructures, enabling efficient light confinement in waveguides. The thermo-optic coefficient of SiGe is also composition-dependent, with higher Ge fractions exhibiting stronger temperature sensitivity.

Luminescence in SiGe alloys is generally weak due to their indirect bandgap nature, but it becomes measurable at high Ge concentrations or under strain. Photoluminescence (PL) spectra of relaxed SiGe alloys show a peak near the bandgap energy, broadened by alloy disorder. For strained SiGe layers grown on Si, the luminescence efficiency improves due to strain-induced splitting of the valence band. At room temperature, the PL intensity is typically low, but at cryogenic temperatures, distinct peaks associated with excitonic transitions can be observed. Electroluminescence has been demonstrated in SiGe heterostructures, though external quantum efficiencies remain below 1% for most devices.

NIR applications of SiGe alloys leverage their tunable absorption and compatibility with silicon photonics. SiGe photodetectors can cover the entire NIR range, with cutoff wavelengths adjustable by composition. For instance, Si0.2Ge0.8 detectors are sensitive up to 1.6 µm, suitable for fiber-optic communications. Waveguides using SiGe cores provide tighter mode confinement than pure Si, reducing bending losses in compact photonic circuits. Modulators based on the plasma dispersion effect benefit from the higher free-carrier absorption in Ge-rich alloys, enabling faster switching speeds.

Strain engineering plays a pivotal role in enhancing the optical performance of SiGe alloys. Compressively strained SiGe layers exhibit a reduced effective mass for holes, improving absorption and emission characteristics. Strain also shifts the light-hole and heavy-hole bands, altering the polarization dependence of optical transitions. Pseudomorphic growth of SiGe on Si substrates introduces biaxial compression, while relaxed buffers allow for strain customization. These effects are exploited in quantum well structures, where carrier confinement boosts luminescence efficiency.

The temperature dependence of optical properties in SiGe alloys is another consideration for practical applications. The bandgap shrinks with increasing temperature at a rate of approximately -0.4 meV/K for Si and -0.5 meV/K for Ge, with intermediate compositions following a linear interpolation. This shift affects the absorption edge and luminescence peak positions. The thermo-optic coefficient, around 1.8x10-4 K-1 for SiGe, influences the thermal stability of photonic devices, requiring careful thermal management in integrated systems.

Challenges remain in optimizing SiGe alloys for high-performance NIR devices. The indirect bandgap limits luminescence efficiency, prompting research into nanostructured SiGe or hybrid systems with direct-gap materials. Defect-mediated non-radiative recombination at high Ge concentrations reduces carrier lifetimes, necessitating improved growth techniques. Strain relaxation and interdiffusion at elevated temperatures pose reliability concerns for long-term operation.

Recent advances in epitaxial growth have enabled precise control of SiGe alloy composition and strain profiles. Molecular beam epitaxy and chemical vapor deposition techniques produce high-quality layers with abrupt interfaces, essential for quantum-confined structures. Selective epitaxy allows for localized growth of Ge-rich regions, facilitating monolithic integration with Si photonics. These developments support the realization of SiGe-based light sources, detectors, and modulators on silicon platforms.

The following table summarizes key optical parameters for selected SiGe alloy compositions at 1.55 µm:

Composition | Absorption Coefficient (cm-1) | Refractive Index | PL Efficiency
Si0.9Ge0.1 | 50 | 3.52 | Low
Si0.7Ge0.3 | 100 | 3.62 | Moderate
Si0.5Ge0.5 | 300 | 3.72 | Moderate
Si0.3Ge0.7 | 800 | 3.85 | High

In conclusion, SiGe alloys offer a versatile platform for NIR optoelectronics, with optical properties that can be systematically tuned through composition and strain. While challenges related to indirect bandgap limitations persist, ongoing materials engineering efforts continue to expand their application space in integrated photonics and sensing systems. The compatibility with mainstream silicon technology ensures that SiGe will remain a cornerstone material for cost-effective optoelectronic solutions.