Silicon carbide (SiC) is a typical representative of the third generation of semiconductor materials. Its unique polymorphic structure gives the material rich physical and chemical properties. There are more than 250 known SiC polymorphs. Common polymorphs such as 4H, 6H and 3C show completely different physical properties due to the difference in the stacking order of Si-C diatomic layers in the crystal structure. These polymorphs will undergo phase transitions under certain conditions. The phase transition process not only involves rearrangement at the atomic level, but also has a profound impact on the macroscopic properties of silicon carbide bushings. Analyzing its phase transition mechanism from the perspective of crystal structure has become a key breakthrough in optimizing material performance.
The essence of the phase transition of silicon carbide polymorphs is the redistribution of the positions of atoms in the lattice. This transition is usually triggered by external factors such as temperature, pressure and stress. Under high temperature and high pressure environment, the vibration of Si-C bonds intensifies. When the energy accumulation exceeds the phase transition barrier, atoms begin to break through the original lattice constraints and achieve a new stacking order through diffusion and migration. For example, in the high temperature zone above 1500℃, during the transformation of 6H-SiC to 4H-SiC, some atoms in the hexagonal lattice will shift along the c-axis and reconstruct into a tetragonal lattice structure. This transformation is not completed instantly, but undergoes a metastable phase transition, accompanied by the generation and annihilation of lattice distortion and defects, and its dynamic process is controlled by the atomic diffusion rate and interface energy.
The evolution of crystal structure during phase transition directly affects the physical properties of silicon carbide bushings. From the perspective of electrical properties, there are significant differences in the band structures of different polytypes: the indirect band gap of 4H-SiC is about 3.26eV, while the band gap of 6H-SiC fluctuates between 2.86-3.03eV. This difference leads to changes in key parameters such as electron mobility and breakdown electric field strength. If a phase transition occurs during the bushing preparation process, inconsistent electrical properties in local areas will cause uneven current distribution and reduce the reliability of power devices. In terms of mechanical properties, the volume change associated with phase change will generate residual stress inside the bushing. When the stress exceeds the yield strength of the material, it may cause microcracks to initiate, especially under cyclic load conditions, the risk of crack propagation increases significantly.
Thermal properties are also profoundly affected by polymorph phase change. The thermal conductivity of silicon carbide polymorphs is closely related to its phonon scattering mechanism. Changes in the lattice structure will adjust the propagation path and scattering probability of phonons. Studies have shown that the thermal conductivity of 4H-SiC can reach 490W/(m·K) at room temperature, which is significantly higher than 370W/(m·K) of 6H-SiC. This difference is due to the more regular atomic arrangement in the 4H structure that reduces phonon-phonon scattering. When the bushing undergoes phase change during high-temperature service, local fluctuations in thermal conductivity will lead to uneven temperature field distribution, induce thermal stress concentration, and accelerate material failure. In addition, the latent heat release associated with the phase change process itself will also cause local temperature surges in a short period of time, further exacerbating the difficulty of thermal management.
Defects play a complex and critical role in the polymorph phase change process. On the one hand, crystal defects such as dislocations and stacking faults can serve as fast channels for atomic diffusion, reduce the activation energy required for phase transition, and promote the occurrence of phase transition; on the other hand, the stress generated during the phase transition will induce the generation of new defects, forming a vicious cycle. For example, when 6H-SiC transforms to 4H-SiC, atoms in the stacking fault region are more likely to migrate, but the lattice distortion caused by the phase transition will produce edge dislocations in the nearby area. These defects not only affect the intrinsic properties of the material, but also become adsorption centers for impurity atoms, changing the electrical and chemical stability of silicon carbide bushings. How to control the phase transition process by controlling the defect density and distribution has become an important research direction for improving the performance of the bushing.
From the perspective of preparation technology, precise control of the phase transition of silicon carbide polymorphs is the core challenge to achieve high-performance bushings. During the chemical vapor deposition (CVD) process, parameters such as the ratio of reactive gases, growth temperature and pressure play a decisive role in the selection of polymorphs. For example, lower growth temperature and higher carbon-silicon ratio are conducive to the formation of 4H-SiC, while high temperature and low carbon-silicon ratio tend to generate 6H-SiC. In addition, the crystal form, surface roughness of the substrate material and the pretreatment method of the seed crystal will also affect the polymorphic structure of the subsequent growth layer through the lattice matching effect. In recent years, by introducing buffer layers and optimizing the growth rate gradient, researchers have achieved partial control of the polymorphic structure, but how to achieve precise control of phase transition still requires more in-depth research on thermodynamic and kinetic mechanisms.
As wide bandgap semiconductor devices develop towards high frequency, high pressure and high temperature, the performance requirements of silicon carbide bushings continue to increase. It is particularly important to deeply understand the polymorphic phase transition mechanism and achieve effective control of it. Combining theoretical tools such as first-principles calculations and molecular dynamics simulations, as well as in-situ characterization techniques such as transmission electron microscopy (TEM) and X-ray diffraction (XRD), it is expected to reveal the dynamic behavior of the atomic scale during the phase transition process. On this basis, the development of new processes and equipment to achieve controllable growth of polymorphic structures of silicon carbide bushings will lay a solid foundation for improving its application performance in power devices, aerospace, new energy vehicles and other fields. At the same time, exploring the design of polymorphic composite structures and leveraging the performance advantages of different phases to achieve complementarity may also become a new direction for breakthroughs in silicon carbide material performance.
