Chemical vapor deposition (CVD) is a versatile and scalable technique for depositing high-quality thermoelectric thin films, enabling precise control over composition, microstructure, and electronic properties. This method is particularly advantageous for fabricating thermoelectric materials such as bismuth telluride (Bi₂Te₃) and silicon-germanium (SiGe) alloys, which are widely used in energy harvesting and solid-state cooling applications. The ability to tailor film stoichiometry, crystallinity, and defect density through CVD allows optimization of the Seebeck coefficient, electrical conductivity, and thermal conductivity—key parameters determining thermoelectric efficiency.

**Precursor Chemistry and Reaction Mechanisms**
The deposition of Bi₂Te₃ via CVD typically employs organometallic or halide precursors. Commonly used precursors include trimethylbismuth (Bi(CH₃)₃) and diethyltelluride (Te(C₂H₅)₂) in a hydrogen or argon carrier gas. The reaction proceeds through gas-phase decomposition and surface reactions, with the following simplified pathway:
Bi(CH₃)₃ + Te(C₂H₅)₂ → Bi₂Te₃ + volatile byproducts.
The growth temperature ranges between 350°C and 450°C, ensuring sufficient precursor decomposition while avoiding undesirable secondary phases. For SiGe deposition, silane (SiH₄) and germane (GeH₄) are the standard precursors, often diluted in hydrogen. The deposition occurs at higher temperatures (500°C–700°C) to promote crystalline film formation. The Ge content in SiGe alloys is controlled by adjusting the precursor flow ratios, enabling bandgap engineering for optimal thermoelectric performance.

**Stoichiometry and Doping Control**
Achieving the correct stoichiometry is critical for thermoelectric properties. In Bi₂Te₃, even slight deviations from the ideal 2:3 Bi:Te ratio can lead to significant changes in carrier concentration and mobility. Excess Te, for example, introduces n-type doping due to Te antisite defects, while Bi-rich conditions result in p-type behavior. Dopants such as antimony (Sb) for p-type or selenium (Se) for n-type can be introduced using precursors like trimethylantimony (Sb(CH₃)₃) or diethylselenide (Se(C₂H₅)₂).

For SiGe alloys, the Ge fraction determines the lattice thermal conductivity, which decreases with higher Ge content due to increased phonon scattering. Phosphine (PH₃) or diborane (B₂H₆) are used for n-type and p-type doping, respectively. Precise control of gas-phase ratios and deposition pressure ensures uniform doping profiles, which are essential for maintaining high power factors (S²σ, where S is the Seebeck coefficient and σ is electrical conductivity).

**Film Microstructure and Thermoelectric Properties**
The microstructure of CVD-grown thermoelectric films significantly impacts their performance. Bi₂Te₃ films often exhibit layered growth along the (00l) planes, which is favorable for high electrical conductivity along the in-plane direction. However, the cross-plane thermal conductivity is reduced due to weak van der Waals bonding between Te layers. Post-deposition annealing can further enhance crystallinity and reduce defects, improving carrier mobility.

SiGe films grown by CVD tend to form columnar grains with nanoscale compositional fluctuations, which scatter phonons effectively while maintaining reasonable electrical conductivity. The thermal conductivity of SiGe alloys can be as low as 3–5 W/m·K at room temperature for Ge-rich compositions, compared to ~150 W/m·K for pure Si. The Seebeck coefficient of optimally doped SiGe ranges between 200–300 μV/K, depending on carrier concentration and temperature.

**Applications in Energy Harvesting and Cooling**
CVD-deposited thermoelectric films are integral to miniature energy harvesters and micro-cooling devices. Thin-film Bi₂Te₃ modules are used in wearable electronics to convert body heat into electricity, with reported power densities of ~10 μW/cm² for small temperature gradients. The compatibility of CVD with patterned substrates allows integration into on-chip cooling solutions for high-power electronics, where localized thermal management is critical.

SiGe-based thermoelectric generators are employed in space applications due to their high-temperature stability. Radioisotope thermoelectric generators (RTGs) using SiGe alloys have been used in NASA missions, leveraging their ability to operate at temperatures exceeding 900°C. The combination of low thermal conductivity and high thermoelectric efficiency makes these materials ideal for environments where reliability and longevity are paramount.

**Challenges and Future Directions**
Despite its advantages, CVD faces challenges in scaling up while maintaining uniformity over large areas. Precursor costs and toxicity (e.g., tellurium-based compounds) also pose economic and environmental concerns. Advances in precursor design, such as less toxic alternatives or liquid injection systems, could mitigate these issues. Additionally, the development of hybrid CVD techniques, combining plasma enhancement or atomic layer control, may further improve film quality and device performance.

The continued optimization of CVD processes for thermoelectric materials will enable broader adoption in both niche and mainstream applications. By refining precursor chemistries, doping strategies, and microstructure control, researchers can push the boundaries of thin-film thermoelectrics, unlocking new possibilities for energy-efficient technologies.