Causes of Carrier Lifetime Degradation (Part 4 of 10)

Carrier lifetime degradation is closely related to components such as quartz tubes and flanges, especially in semiconductor, photovoltaic, or high-temperature material processes. The following is an analysis of key influencing factors and their interactions:

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1. Influence of Quartz Tubes

(1) Material Purity and Impurities

• Metal impurities (Fe, Cu, Na, etc.):
  Metal ions in quartz tubes may diffuse into silicon wafers or epitaxial layers at high temperatures, forming carrier recombination centers and significantly reducing lifetime.
  Key indicators: The metal impurity content should be controlled (e.g., ≤1 ppm, and for ultra-high-purity quartz tubes ≤0.1 ppm).
• Hydroxyl (OH⁻) content:
  Hydroxyl groups absorb energy in the ultraviolet band, potentially affecting photo-generated carrier generation, especially in photovoltaic or UV sensor applications.
  Recommendation: Select low-hydroxyl quartz tubes (e.g., synthetic quartz, OH⁻ < 5 ppm).

(2) Structural Defects and Thermal Stability

• Micro-cracks or devitrification:
  At high temperatures, quartz tubes may devitrify (e.g., transform to cristobalite) or develop thermal stress cracks, releasing particles and contaminating the process environment.
  Relation to carrier lifetime: Particles adhering to the silicon wafer surface increase interface recombination rate.
  Solution: Use ultra-high-purity quartz tubes or titanium-doped quartz tubes (anti-devitrification, withstand >1200°C) and optimize heating/cooling rates (avoid thermal shock).

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2. Influence of Flanges and Sealing Components

(1) Material Compatibility

• Metal flange contamination:
  Stainless steel or nickel-based flanges may release metal vapors (e.g., Cr, Ni) at high temperatures, contaminating the inner wall of the quartz tube or the sample through gas-phase transport.
  Case: In SiC epitaxial growth, metal contamination can increase interface state density, leading to reduced carrier lifetime.
  Alternative: Use ceramic flanges (e.g., Al₂O₃) or flanges with a platinum coating.

(2) Sealing Performance

• Leakage causing oxidation/contamination:
  Poor flange sealing can introduce oxygen or water vapor, which may react with silicon at high temperatures to form defective SiO₂ layers, increasing surface recombination.
  Detection method: Use a helium mass spectrometer leak detector to verify sealing performance (leak rate <1×10⁻⁹ mbar·L/s).

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3. System-Level Interactions

(1) Quartz Tube–Flange Interface

• Coefficient of thermal expansion (CTE) mismatch:
  Quartz (CTE ~0.55×10⁻⁶/°C) and metal flanges (e.g., stainless steel, CTE ~16×10⁻⁶/°C) can undergo stress deformation at high temperatures, potentially causing micro-leaks or particle shedding.
  Improved design: Use gradient sealing structures (e.g., graphite gasket transitions) or elastic sealing materials (e.g., Viton fluoro rubber, temperature limit <200°C).

(2) Gas Flow Disturbance

• Turbulence caused by flange structure:
  Improper flange inner diameter or sharp-edge design can disturb process gas flow, leading to local temperature non-uniformity in quartz tubes, which affects doping uniformity (and indirectly carrier lifetime).

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4. Customer Problem Diagnosis Recommendations

If a customer reports carrier lifetime degradation, guide them to check the following aspects:

• Quartz tube batch inspection: Request ICP-MS reports (metal impurities) and FTIR reports (hydroxyl content) from the supplier.
• Flange and seal inspection: Confirm flange material, sealing ring temperature resistance, and check for high-temperature discoloration (signs of metal vaporization).
• Process parameter review: Compare whether the drop in carrier lifetime coincides with changes in quartz tube/flange batch or process temperature adjustments.

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5. Recommended Solutions

Root Cause	Improvement Measures
Metal contamination from quartz tube	Use ultra-high-purity synthetic quartz tubes (e.g., Heraeus Suprasil®, metal impurities <0.1 ppm).
Metal evaporation from flange	Replace with ceramic flanges or platinum-coated metal flanges.
Seal leakage	Use double O-rings + helium leak testing, or adopt metal seals (e.g., copper gaskets for UHV).
Thermal stress devitrification	Select ultra-high-purity quartz tubes or titanium-doped quartz tubes, and control heating/cooling rate (≤5°C/min).

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Zaključak

Carrier lifetime degradation may result from a combination of quartz tube impurities, flange contamination, and system design defects. To address the issue fundamentally, optimization must be performed in three aspects: material purity, sealing reliability, and thermal compatibility. It is recommended that customers provide more detailed process data (such as temperature curves and gas types) for accurate component recommendations.