Physical vapor transport (PVT) is a widely used method for growing bulk silicon carbide (SiC) crystals, particularly for applications requiring high-quality substrates in power electronics, RF devices, and optoelectronics. The process involves sublimation of a SiC source material and subsequent condensation on a seed crystal under controlled temperature and pressure conditions. Key aspects of PVT growth include seed preparation, temperature gradient optimization, polytype control, and defect mitigation.

The growth process begins with the preparation of a high-quality seed crystal, typically a single-crystal SiC wafer with a specific orientation and polytype. The seed is placed at the cooler end of a graphite crucible, while the source material, often high-purity SiC powder, is positioned at the hotter end. The crucible is heated to temperatures between 2000°C and 2400°C under reduced pressure, typically in the range of 1 to 100 mbar. Sublimation of the source material produces vapor species such as Si, Si2C, and SiC2, which migrate toward the seed due to the imposed temperature gradient. The vapor species then condense on the seed, leading to crystal growth.

Temperature gradients play a critical role in determining growth rate and crystal quality. A steep gradient can enhance mass transport but may introduce thermal stress, leading to defects. Conversely, a shallow gradient may reduce stress but slow growth rates. Optimal gradients are typically in the range of 15°C/cm to 50°C/cm, depending on the desired growth conditions. Precise control of the axial and radial temperature profiles is necessary to maintain stable growth fronts and minimize defects.

Polytype control is another crucial aspect of SiC PVT growth. SiC exists in numerous polytypes, with 4H-SiC and 6H-SiC being the most common for electronic applications. The polytype is determined by the stacking sequence of Si-C bilayers along the c-axis. 4H-SiC, with its higher electron mobility and larger bandgap, is preferred for high-power devices, while 6H-SiC is often used in optoelectronic applications. The polytype can be controlled by adjusting the growth temperature, pressure, and seed orientation. Lower growth temperatures (around 2000°C to 2100°C) and higher argon partial pressures favor 4H-SiC formation, whereas higher temperatures (above 2200°C) tend to promote 6H-SiC growth. The seed crystal’s polytype and off-cut angle also influence the resulting crystal structure.

Defect mitigation is essential for producing high-quality SiC substrates. Micropipes, threading dislocations, and stacking faults are among the most common defects in PVT-grown SiC crystals. Micropipes, which are hollow-core screw dislocations with large Burgers vectors, can severely degrade device performance. Their density can be reduced by optimizing growth conditions, such as lowering thermal stress and ensuring stoichiometric vapor composition. Threading dislocations, including edge and mixed types, are inherited from the seed or generated during growth. Techniques such as repeated a-face growth and modified seed attachment have been shown to reduce dislocation densities below 1000 cm^-2. Stacking faults, which disrupt the periodic stacking sequence, are often minimized by maintaining stable growth conditions and avoiding rapid temperature fluctuations.

The use of in-situ doping during PVT growth allows for controlled electrical properties. Nitrogen is commonly used for n-type doping, while aluminum or boron can be introduced for p-type doping. The doping concentration is influenced by the vapor phase composition and temperature, with typical nitrogen doping levels ranging from 1e17 cm^-3 to 1e19 cm^-3 for n-type SiC. Uniform doping distribution is achieved by ensuring a stable vapor phase and consistent temperature profiles.

Post-growth processing further improves crystal quality. Wafer slicing, lapping, and chemical-mechanical polishing (CMP) are employed to produce substrates with low surface roughness and minimal subsurface damage. Etching techniques, such as molten KOH etching, are used to reveal and quantify defects for quality assessment.

PVT remains the dominant method for bulk SiC crystal growth due to its scalability and ability to produce large-diameter crystals. Advances in process control, including real-time monitoring and automated feedback systems, continue to enhance crystal quality and yield. Future developments may focus on further reducing defect densities, improving polytype uniformity, and scaling up to larger wafer sizes while maintaining cost-effectiveness. The ongoing refinement of PVT techniques ensures that SiC substrates meet the stringent requirements of next-generation semiconductor devices.