Germanium-Tin (GeSn) alloys have emerged as a promising material system for mid-infrared (MIR) photodetectors due to their tunable bandgap, strong absorption characteristics, and compatibility with silicon-based technologies. These alloys bridge the gap between conventional group IV semiconductors and III-V or II-VI compounds, offering a cost-effective and scalable alternative for MIR applications. The ability to tailor their electronic and optical properties through Sn composition makes them particularly attractive for sensing, imaging, and communication systems operating in the 2–5 µm wavelength range.

Bandgap tuning in GeSn alloys is primarily achieved by adjusting the Sn concentration. Pure germanium has an indirect bandgap of approximately 0.66 eV, while the incorporation of Sn reduces the bandgap and can transition the material from an indirect to a direct bandgap semiconductor at sufficiently high Sn content. Theoretical and experimental studies indicate that the direct bandgap crossover occurs at around 6–10% Sn, depending on strain and temperature conditions. The bandgap energy (Eg) of relaxed Ge1-xSnx alloys follows a nonlinear relationship with Sn composition (x), described empirically by Eg ≈ (0.66 – 2.46x + 1.23x²) eV for x ≤ 0.2. This tunability enables the extension of optical absorption into the MIR region, with compositions of 10–20% Sn yielding bandgaps suitable for detection up to 5 µm.

The absorption characteristics of GeSn alloys are strongly influenced by their band structure. At Sn concentrations above the direct bandgap transition, the material exhibits enhanced absorption coefficients compared to indirect-gap Ge or GeSn with lower Sn content. For instance, Ge0.89Sn0.11 demonstrates an absorption coefficient exceeding 10⁴ cm⁻¹ at wavelengths around 3 µm, making it highly efficient for MIR detection. The absorption edge shifts to longer wavelengths with increasing Sn, but the trade-off involves higher defect densities and non-radiative recombination at elevated Sn levels. Strain engineering further modifies absorption properties; compressive strain in epitaxial GeSn layers on Ge or Si substrates can alter the valence band splitting, while tensile strain may enhance the direct transition probability.

Quantum efficiency (QE) in GeSn photodetectors depends on multiple material parameters, including minority carrier lifetime, defect density, and optical confinement. High Sn content improves the intrinsic QE by increasing the direct transition rate, but defects such as Sn vacancies, dislocations, and surface states can degrade performance. Minority carrier lifetimes in GeSn alloys typically range from nanoseconds to microseconds, with higher Sn compositions often exhibiting shorter lifetimes due to increased defect scattering. Surface passivation techniques and strain management are critical to maximizing QE, particularly for thin-film devices where surface recombination dominates.

Growth challenges for GeSn alloys stem from the large lattice mismatch between Ge and Sn (14.7%) and the low equilibrium solubility of Sn in Ge (<1%). These factors necessitate non-equilibrium growth techniques such as molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) to achieve metastable alloys with higher Sn content. Key growth parameters include substrate temperature, precursor fluxes, and post-growth annealing. Low-temperature growth (150–350°C) is often employed to suppress Sn segregation, but this can introduce point defects and amorphous phases. Alternatively, pulsed laser annealing or rapid thermal processing can improve crystal quality without excessive Sn diffusion.

Substrate compatibility is another critical consideration for GeSn epitaxy. The most common substrates are Ge, Si, and virtual substrates like relaxed Ge buffers on Si. Ge substrates provide near-lattice matching for low Sn compositions, reducing dislocation densities. However, the cost and limited size of bulk Ge wafers drive interest in Si-based solutions. Growing GeSn directly on Si introduces significant strain and threading dislocations due to the 4.2% lattice mismatch between Ge and Si. Intermediate buffer layers, such as graded SiGe or compliant substrates, help mitigate these issues but add complexity. Alternatively, strain-relaxed GeSn layers can serve as templates for higher Sn content epitaxy, though relaxation mechanisms must be carefully controlled to prevent cracking or roughening.

Thermal stability is a persistent challenge for GeSn alloys, particularly at Sn concentrations above 10%. Sn tends to segregate or precipitate during high-temperature processing, degrading optical and electrical properties. Strategies to improve stability include carbon doping, which pins Sn atoms, or the use of lower thermal budgets during device integration. Additionally, the metastable nature of high-Sn GeSn requires careful handling during subsequent processing steps to avoid phase separation.

In summary, GeSn alloys offer a versatile platform for MIR photodetectors through bandgap engineering and strong optical absorption. The direct bandgap transition at moderate Sn concentrations enhances quantum efficiency, while growth techniques and substrate innovations address material challenges. Continued advancements in defect reduction and thermal stability will further solidify their role in next-generation MIR optoelectronics.