Effect of Nitrogen and Aluminum Doping on the Stability of SiC Polytypes
The study of the stability of silicon carbide (SiC) crystal polytypes and the influence of doping is a key technical direction for improving the performance of semiconductor devices. Silicon carbide is widely used in high-temperature, high-power, high-frequency microwave, and radiation resistant devices due to its excellent characteristics such as a bandgap width of over 3.2 eV, thermal conductivity exceeding 4.9 W/cm · K, and breakdown field strength of up to 3 MV/cm. By regulating the concentration distribution of dopants such as nitrogen (N) and aluminum (Al), it is possible to selectively induce the growth of specific crystal forms (such as N dopant promoting the formation of 4H phase), while also suppressing the generation of crystal defects such as stacking faults, enabling the control of carrier concentration in the range of 1014~1019 cm-3. This multidimensional doping effect directly affects the on resistance and switching losses of power devices, and provides a material basis for the development of high-voltage IGBTs with a withstand voltage of over 20kV, microwave devices with operating frequencies up to GHz, and high-temperature (>600 ℃) sensors. PAM-XIAMEN can provide SiC wafers for IGBT, microwave devices and sensors, for more information please consult [email protected] .
1. Effect of Nitrogen Dopingon the Stability of SiC Crystal Polytype
Researchers studied the effect of nitrogen doping on the stability of SiC crystals through first principles analysis and found that during the growth process of Al doped p-type 4H SiC crystals, when N doping is too high, the originally stable hexagonal crystal structure will shift towards the 6H phase. Even with 4H seed crystals, it is difficult to avoid the phenomenon of polymorphic inclusions. In order to solve the problem of crystal drift, researchers have proposed a co doping scheme of N and Al, which can significantly improve the structural stability of 4H phase while maintaining p-type conductivity.
In addition, during the doping process, nitrogen atoms cause lattice distortion due to their small atomic radius. By forming specific defect states, the system energy of the 3C SiC crystal polytype is significantly reduced, thereby enhancing its structural advantage in polytopes such as 4H/6H. This doping effect synchronously affects the electrical properties of the material. The additional electrons introduced in the conduction band reduce the bandgap width of the crystal, improve the conductivity of the material, and make it suitable for high current devices. It is worth noting that the presence of Al impurities can interact with nitrogen atoms. When the two are adjacent in space, Al atoms capture N donor electrons to form charge neutral complexes, resulting in a weakened or even disappeared effect of N doping on the stability of 3C crystal structure. Experiments have shown that there is a strong attraction between N atoms and Al atoms, which means they often appear in pairs. This pairing effect further affects the stability of SiC crystals under N doping.
2. Effect of Aluminum Doping on the Stability of SiC Polytype
Research on PVT growth has shown that lattice distortion caused by Al atoms during doping significantly enhances the hexagonal structure of the material, manifested by an increase in the c-axis lattice constant and c/na ratio. This anisotropic enhancement directly changes the crystal growth orientation. In addition, aluminum dopants can cause a decrease in the absolute value of surface bilayer stacking energy, thereby interfering with the stacking process during bilayer formation. Due to the interference of stacking energy, aluminum doped 4H single crystal SiC is difficult to maintain its polymorphic stability, and is more prone to the conversion of 6H and 3C polymorphs, and even the formation of inclusions. Therefore, under PVT growth, 6H and 3C polymorphs are more stable under Al doping conditions, while there are no suitable growth conditions to ensure the stability of 4H-SiC.
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