Low-temperature deposition methods for silicon-germanium (SiGe) alloys are critical for applications requiring precise control over material properties, such as advanced transistors, photonic devices, and thermoelectric systems. The reduced thermal budget of low-temperature processes minimizes unwanted diffusion, strain relaxation, and interfacial reactions, making them suitable for integration with temperature-sensitive substrates or pre-existing device layers. However, achieving high-quality epitaxial SiGe at low temperatures presents challenges, particularly in defect mitigation and maintaining stoichiometric control. This article examines key deposition techniques, their mechanisms, and strategies for optimizing epitaxial quality.

Molecular beam epitaxy (MBE) is a leading method for low-temperature SiGe growth due to its ultra-high vacuum environment and precise atomic control. MBE operates at temperatures typically between 200°C and 500°C, significantly lower than conventional chemical vapor deposition (CVD). The absence of carrier gases and chemical precursors reduces contamination risks, while the slow deposition rates (0.1–1 nm/s) enable layer-by-layer growth. Defect densities below 10^4 cm^-2 have been reported for SiGe films grown by MBE at 350°C, attributed to the minimization of thermal stress and interfacial misfit dislocations. To further suppress defects, substrate pre-treatment with atomic hydrogen or low-energy plasma can remove native oxides and passivate surface states, promoting smoother nucleation.

Plasma-enhanced chemical vapor deposition (PECVD) offers another pathway for low-temperature SiGe deposition, leveraging reactive plasma species to decompose precursors at reduced thermal energy. Typical PECVD processes operate between 100°C and 400°C using silane (SiH4) and germane (GeH4) as precursor gases. The plasma generates highly reactive radicals, enabling film growth at temperatures where thermal CVD would be impractical. However, PECVD-grown SiGe often exhibits higher defect densities (10^6–10^8 cm^-2) due to ion bombardment-induced damage and incomplete precursor dissociation. Mitigation strategies include optimizing plasma power density (0.1–1 W/cm^2) and employing dual-frequency excitation to balance radical generation and ion energy. Post-deposition annealing at temperatures below 450°C can also improve crystallinity without triggering strain relaxation or Ge segregation.

Atomic layer deposition (ALD) has emerged as a promising technique for conformal SiGe alloy growth at temperatures as low as 150°C. ALD relies on self-limiting surface reactions, alternating exposures to Si and Ge precursors with purge steps in between. This cyclic approach ensures atomic-level thickness control and excellent uniformity, even on high-aspect-ratio structures. Common precursors include chlorosilanes (e.g., Si2Cl6) and alkylgermanes (e.g., Ge(CH3)4), with hydrogen plasma or thermal energy driving the ligand-exchange reactions. The primary challenge for ALD is achieving sufficient Ge incorporation, as the reaction kinetics often favor Si-rich compositions. Modulating precursor pulse times and introducing Ge-selective reducing agents can enhance Ge content up to 30% while maintaining epitaxial alignment on Si substrates.

Defect mitigation in low-temperature SiGe deposition hinges on several interrelated factors. Strain management is paramount, as the 4.2% lattice mismatch between Si and Ge can generate threading dislocations if not properly controlled. Graded buffer layers, where the Ge concentration increases incrementally, have proven effective in redistributing strain energy. For example, a step-graded buffer with 10% Ge increments grown at 400°C can reduce threading dislocation densities to below 10^5 cm^-2. Another approach involves surfactant-mediated growth, where elements like antimony or bismuth segregate to the surface during deposition, altering adatom mobility and promoting two-dimensional growth. Surfactants can lower the critical thickness for defect-free SiGe layers by up to 30% compared to conventional growth.

Surface preparation and in-situ monitoring are equally vital for defect reduction. Substrate cleaning protocols using hydrofluoric acid (HF) last steps or remote plasma oxidation followed by thermal desorption can produce atomically clean surfaces essential for epitaxy. Real-time techniques like reflection high-energy electron diffraction (RHEED) allow for immediate feedback on surface morphology, enabling adjustments to growth parameters before defects propagate. For instance, RHEED intensity oscillations can detect the onset of three-dimensional islanding, prompting corrective measures such as growth interruption or temperature modulation.

The crystalline quality of low-temperature SiGe alloys is often assessed through high-resolution X-ray diffraction (HRXRD), with rocking curve full-width at half-maximum (FWHM) values serving as a key metric. High-quality epitaxial SiGe films grown at 300°C typically exhibit FWHM values below 100 arcseconds for the (004) reflection. Reciprocal space mapping further verifies strain state and relaxation levels, ensuring the material meets application-specific requirements. Transmission electron microscopy (TEM) provides direct imaging of defects, with cross-sectional analysis revealing dislocation propagation paths and interfacial abruptness. For optoelectronic applications, photoluminescence spectroscopy quantifies non-radiative recombination centers linked to point defects, with intensities correlating to overall material quality.

Electrical characterization complements structural analysis, particularly for devices where carrier mobility and leakage currents are critical. Hall effect measurements on low-temperature SiGe layers with 20% Ge content have shown room-temperature hole mobilities exceeding 200 cm^2/V·s, approaching values achieved at higher growth temperatures. Deep-level transient spectroscopy (DLTS) identifies trap states associated with growth-induced defects, such as vacancies or anti-site defects, which can be minimized through optimized deposition conditions and post-growth passivation.

Applications of low-temperature SiGe alloys span multiple domains. In CMOS technology, they enable strain engineering for mobility enhancement in p-channel transistors without compromising thermal stability of adjacent layers. SiGe heterojunction bipolar transistors (HBTs) benefit from the precise doping control achievable at low temperatures, improving high-frequency performance. Photonic integration leverages the tunable bandgap of SiGe for detectors and modulators compatible with silicon photonics platforms. Emerging applications include flexible electronics, where low-temperature growth on polymer substrates is essential, and quantum devices requiring atomically sharp interfaces.

Future advancements in low-temperature SiGe deposition will likely focus on precursor chemistry innovation, hybrid growth techniques combining MBE and ALD advantages, and machine learning-assisted process optimization. The development of novel germanium precursors with lower decomposition temperatures could further reduce thermal budgets while maintaining material quality. In-situ diagnostics paired with adaptive control systems may enable real-time defect correction during growth, pushing the boundaries of what is achievable in low-temperature epitaxy. As device architectures continue to shrink and diversify, the ability to deposit high-quality SiGe alloys at progressively lower temperatures will remain a cornerstone of semiconductor manufacturing.