Introduction

4H silicon carbide (4H-SiC) has emerged as a crucial material for next-generation high-power electronic devices, RF/microwave electronics, and quantum information technology due to its exceptional physical properties, including a wide bandgap (~3.26 eV), high carrier mobility (~1000 cm²/V·s), high thermal conductivity (~490 W/m·K), and excellent chemical stability. However, the presence of dislocation densities as high as 10³ ~ 10⁴ cm⁻² significantly impairs its performance and represents a major bottleneck to fully realizing its potential. This paper discusses the characteristics, formation mechanisms, transformation behaviors, and impacts of dislocations in 4H-SiC single crystals. It also analyzes their evolution during processing and epitaxy and explores future research directions and application prospects.

1. Types and Distribution of Dislocations in 4H-SiC Single Crystals

1.1 Threading Dislocations (TDs)

Threading dislocations are the most common type of dislocations in 4H-SiC single crystals, with dislocation lines extending along the crystal’s \(c\)-axis. Based on Burgers vector characteristics, TDs are classified as follows:

-Micropipes (MPs):An extreme form of screw dislocations with large Burgers vectors (typically > 1 lattice period). Despite a low density (< \(0.1 \, ext{cm}^{-2}\)), MPs are highly detrimental to device performance.

-Threading Screw Dislocations (TSDs): Dislocations with Burgers vectors parallel to the \(c\)-axis. Their density ranges from \(300 \sim 500 \, ext{cm}^{-2}\), and they are primary contributors to leakage current and reduced breakdown voltage.

-Threading Mixed Dislocations (TMDs): Combining screw and edge characteristics, with densities similar to TSDs.

-Threading Edge Dislocations (TEDs): Dislocations with Burgers vectors perpendicular to the dislocation line, exhibiting densities up to \(2000 \sim 5000 \, ext{cm}^{-2}\).

1.2 Basal Plane Dislocations (BPDs)

BPDs glide within the \( (0001) \) basal plane and can be further divided into:

- Integrated BPDs: Groups of dislocation lines bound together.

- Decomposed BPDs: Individual dislocation lines gliding freely within the basal plane.

The typical BPD density is \(500 \sim 1000 \, ext{cm}^{-2}\). During high-power device operation, BPDs can transform into TEDs or expand to form other types of defects, leading to performance degradation.

2. Formation Mechanisms and Transformation Behaviors of Dislocations

2.1 Formation Mechanisms

-Thermal Stress: Temperature gradients during crystal growth are the primary drivers of dislocation formation.

-Seed Crystal Inheritance: TDs in seed crystals propagate in the growth direction, showing significant inheritance.

-Inclusions and Voids: Stress concentration induced by inclusions and voids during growth triggers dislocations.

-2D Nucleation Islands: Uneven growth steps may lead to lattice misalignment and defect generation.

2.2 Transformation Behaviors

-Interconversion between Dislocations:

   MPs and TSDs can transform into each other.

   TSDs may evolve into Frank-type stacking faults under specific conditions.

-Annihilation of Dislocations:

  TSDs may annihilate through mutual interactions during growth, reducing overall defect density.

-BPD Evolution:

  Under thermal stress, some BPDs can glide to other planes and convert into TEDs.

3. Dislocation Evolution during Processing and Epitaxy

3.1 Processing

4H-SiC single crystal processing involves multiple steps such as wire cutting, grinding, and chemical mechanical polishing (CMP). Mechanical stresses during these steps result in:

- Elastic and Plastic Deformation: Extension and interaction of dislocations.

- Crack Formation and Propagation: Localized structural deterioration.

- Dopant Effects: High doping concentrations (up to \(10^{18} \, ext{cm}^{-3}\)) significantly interfere with BPD nucleation kinetics.

3.2 Homogeneous Epitaxy

During homogeneous epitaxy, substrate dislocations heavily influence epitaxial layer quality:

- Over 95% of TSDs propagate into the epitaxial layer, forming threading dislocations.

- Most BPDs transform into TEDs during initial stages, though some persist as BPDs, posing latent risks to device performance.

4. Impacts of Dislocations on Material Performance

4.1 Electrical Properties

- TSDs: Serve as leakage paths, exacerbating leakage effects and reducing device breakdown voltage.

- BPDs: Act as electron-hole recombination centers, impairing forward conduction and diminishing reverse blocking capability.

4.2 Optical Properties

Dislocations can act as radiative recombination centers, causing photoluminescence phenomena. By modulating dislocation-induced emission wavelengths, researchers can investigate defect distribution and characteristics within crystals.

4.3 Microscopic Analysis

Melt-alkali etching is an effective method for studying dislocations. The morphology and distribution of etch pits reveal local lattice distortions, providing valuable insights into defect density measurement and dislocation type identification.

5. Future Prospects and Conclusions

As research on dislocations in 4H-SiC single crystals deepens, comprehensive analysis of their generation, propagation, and transformation mechanisms will provide strong support for material optimization. Future research directions include:

- Development of High-Quality Seed Crystals: To reduce initial defect density.

- Improved Epitaxial Growth Techniques: Optimizing chemical vapor deposition (CVD) processes to effectively mitigate dislocations.

- Stress Control and Thermal Treatments: Employing annealing techniques to promote dislocation annihilation.

- Novel Device Designs: Developing fault-tolerant device architectures to mitigate the negative impacts of dislocations.

Through these approaches, the performance of 4H-SiC materials in high-power and high-frequency applications will be further enhanced, facilitating their broad adoption in the semiconductor industry and paving the way for a more promising future.