Introduction

Silicon carbide (SiC), as a wide-bandgap semiconductor material, is widely used in high-power, high-frequency, and high-temperature electronic devices due to its excellent physical and chemical properties, such as high breakdown electric field, high thermal conductivity, and high electron saturation velocity. The polytypes and various crystal orientations of SiC present different performance characteristics in practical applications. Understanding the differences in these crystal structures and orientations is crucial for designing and manufacturing high-performance SiC devices.

Crystal Structures of Silicon Carbide

Silicon carbide exhibits several polytypes, all of which are based on the covalent bonds between silicon and carbon atoms. However, different stacking sequences result in distinct crystal structures. The main SiC polytypes include 4H-SiC, 6H-SiC and 3C-SiC, each with unique physical properties and application scenarios.

1. 4H-SiC

4H-SiC belongs to the hexagonal crystal system and is the most commonly used polytype of silicon carbide. In its crystal structure, the Si-C layers repeat every four layers along the c-axis. 4H-SiC has a wide bandgap (~3.26 eV), with both electron and hole mobilities being relatively high, making it particularly suitable for high-frequency and high-power devices. Due to its excellent electrical properties, 4H-SiC has become the preferred material for power electronic devices.

2. 6H-SiC

6H-SiC is also part of the hexagonal crystal system, with Si-C layers repeating every six layers along the c-axis. Although the electron mobility of 6H-SiC is slightly lower than that of 4H-SiC, it exhibits greater crystalline anisotropy, which may be advantageous in certain mechanical and optical applications. While 6H-SiC is used in high-power devices, its application is gradually being replaced by 4H-SiC due to the latter's superior properties.

3. 3C-SiC (β-SiC)

3C-SiC belongs to the cubic crystal system, also known as β-SiC. Its structure is similar to that of diamond, and among all SiC polytypes, 3C-SiC has the highest electron mobility. However, it has a smaller bandgap (~2.36 eV). Due to the challenges in growing high-quality, large-area wafers of 3C-SiC, its applications are mainly limited to microelectromechanical systems (MEMS) devices and specific sensors.

Crystal Orientations of Silicon Carbide

In addition to crystal structures, the crystal orientation of silicon carbide is also a critical factor influencing its material properties. Crystal orientation refers to the relative alignment of crystal planes to an external coordinate system. Different crystal orientations affect the mechanical strength, chemical reactivity, and device performance of the material. In silicon carbide, the primary crystal orientations include:

1. (0001) Orientation (C-face)

The (0001) orientation is one of the most common orientations in silicon carbide. The C-face is perpendicular to the c-axis of the hexagonal crystal system, with a high atomic density and good chemical reactivity, making it ideal for epitaxial growth and device fabrication.

2. (11-20) Orientation (A-face)

The (11-20) orientation has a crystal plane parallel to the c-axis and perpendicular to the a-axis. The A-face may offer advantages in specific epitaxial growth processes, although its processing is more challenging, leading to less frequent industrial use.

3. (1-100) Orientation (M-face)

The M-face orientation is neither parallel to the c-axis nor the a-axis, creating a unique alignment. Although this orientation is less common in SiC substrates, it may be used in certain optoelectronic devices or specialized device structures.

4. (03-38) Orientation

The (03-38) orientation is a semi-polar plane, lying between the (0001) and (11-20) orientations. This orientation may be selected for the growth of materials like gallium nitride (GaN) to optimize epitaxial layer quality and device performance.

Conclusion

The crystal structure and orientation of silicon carbide substrates significantly influence the electrical, mechanical, and chemical properties of the material. Due to its excellent properties, 4H-SiC has become the mainstream material for power electronic devices, while other polytypes like 6H-SiC and 3C-SiC have their own advantages in specific applications. Additionally, the choice of crystal orientation is critical for optimizing device performance. Therefore, a deep understanding of the differences between the crystal structures and orientations of silicon carbide is essential for achieving high-performance devices.