Advanced Semiconductor Crystal Materials

Silicon carbide (SiC) has become one of the most strategic materials in power electronics, RF devices, and next-generation semiconductor platforms. Among all available crystal growth technologies, Physical Vapor Transport (PVT) remains the dominant industrial method for producing high-quality bulk SiC single crystals.

In the PVT process, high-purity SiC powder is thermally sublimed in a sealed growth chamber, and the vapor species are transported and re-condensed onto a seed crystal, forming a single-crystal SiC boule. A typical PVT growth system consists of three tightly coupled subsystems: temperature control, pressure control, and crystal growth assembly.

1. Architecture of the PVT Growth System

1.1 Temperature Control System

Two heating modes are commonly used in SiC PVT furnaces:

• Induction heating (10–100 kHz):
  A water-cooled double-layer quartz coil induces eddy currents in the graphite crucible, generating heat. The crucible is surrounded by graphite felt for thermal insulation.

• Resistance heating:
  A graphite heater produces Joule heat, which is transferred to the crucible by radiation and then to the SiC powder by conduction.

Compared with resistance heating, induction heating offers higher efficiency, lower maintenance cost, and a simpler furnace design, but it is more sensitive to external disturbances and requires more sophisticated thermal field control.

1.2 Pressure Control System

The pressure system first evacuates the chamber to high vacuum, then introduces a controlled amount of inert gas (typically argon). The growth pressure must be precisely regulated, as SiC sublimation, vapor transport, and condensation are strongly pressure-dependent. High-quality growth requires tight coupling of temperature and pressure control.

1.3 Crystal Growth Assembly

The core growth region consists of:

• Graphite crucible

• SiC source powder

• Seed crystal

At high temperature, SiC powder decomposes into vapor species such as Si, Si₂C, and SiC₂. These gaseous species migrate toward the cooler seed crystal region, where they recombine and crystallize into single-crystal SiC.

2. Crucible and Internal Structure Engineering

The internal geometry of the crucible strongly affects crystal size, growth uniformity, and defect density.

Early work by SiCrystal (Germany) used graphite partitions to force parasitic nucleation onto sacrificial surfaces, allowing the main crystal to grow larger. DENSO introduced movable shielding plates and conical flow guides to control vapor transport and improve edge uniformity.

Later developments include:

• Gas filtering partitions (II-VI, SiCrystal)

• Source purification layers (TankeBlue, China)

• Movable seed holders and adjustable growth zones (Institute of Physics, CAS; SKC; Showa Denko; Tianyue Advanced)

More recently, attention has shifted to dynamic growth-zone control, such as lifting either the seed or the source powder to maintain a stable temperature difference and enable larger boule diameters.

3. Seed Crystal Design and Orientation

SiC growth is highly anisotropic. The crystallographic orientation of the seed directly determines growth rate, defect formation, and polytype stability.

Key historical developments include:

• Siemens (1989): (0001) polar face

• Toyota (1997): off-axis faces tilted 20°–55°

• Wolfspeed (2005): small tilt between c-axis and thermal gradient

• Bridgestone (2008): convex seed surfaces to suppress micropipes

Surface engineering further reduces defects:

• Grooves and periodic textures (Nippon Steel, HOYA, Fuji Electric)

• Hollow microstructures to control step flow

4. Large-Diameter Seed Engineering

Large SiC boules require large seeds. Since native large seeds are unavailable, mosaic seed technology is widely used.

• TankeBlue (2016): bonded small seeds → 150 mm boules

• Shandong University (2019): mosaic + lateral and surface epitaxy → ≥8-inch seeds

This approach is now central to 200 mm SiC wafer development.

5. Critical SiC Growth Parameters

5.1 Temperature Gradient

Because direct measurement inside the crucible is impossible, numerical simulation tools (e.g., Virtual Reactor) are used to estimate internal temperature fields. The axial and radial gradients determine vapor transport direction, supersaturation, and crystal morphology.

5.2 Growth Rate

The SiC growth rate increases when:

• Temperature rises

• Source–seed temperature gradient increases

• Chamber pressure decreases

• Source–seed distance decreases

However, excessive growth rates can induce defects, polytype instability, and stress.

5.3 Vapor Chemistry

The C/Si ratio is the most critical thermodynamic parameter:

• Low C/Si → favors 3C-SiC

• Carbon-rich vapor → stabilizes 4H-SiC

Gas composition, dopants, and inert gas pressure jointly determine supersaturation, polytype, and doping uniformity.

6. Outlook

Modern SiC single-crystal growth is a multi-physics optimization problem, involving:

• Powder purity and particle size

• Crucible and guide design

• Seed orientation and surface topology

• Dynamic thermal and pressure control

To increase boules beyond 200 mm, the main strategies are growth-zone enlargement and large-area mosaic seeds. To improve crystal quality, the focus shifts to pressure-temperature scheduling, vapor chemistry control, and source engineering.

As electric vehicles, AI power modules, and high-voltage grids drive SiC demand, mastery of PVT crystal growth physics will remain the core competitive advantage in the global wide-bandgap semiconductor industry.