In the field of semiconductors, gallium nitride (GaN) has become a key material for high-frequency, high-power, and optoelectronic device applications due to its excellent electrical and optical properties. The epitaxial substrate, as the foundation for supporting the growth of gallium nitride, plays a decisive role in the performance of the final device. Besides the commonly used sapphire and silicon, materials such as silicon carbide (SiC), aluminum nitride (AlN), and zinc oxide (ZnO) also show great potential as epitaxial substrates for gallium nitride, each with unique advantages and suitable application scenarios.
Silicon carbide exhibits good compatibility with gallium nitride in terms of lattice structure. The lattice mismatch between them is only about 3.5%, which is much lower than that of sapphire and silicon substrates. The relatively low mismatch fundamentally reduces the probability of dislocation generation in gallium nitride during the epitaxial growth process, facilitating the acquisition of high-quality and high-crystallinity epitaxial layers. In terms of electrical properties, this advantage is particularly crucial as it can effectively suppress leakage current and improve electron mobility, meeting the stringent performance requirements of high-power and high-frequency devices.
In terms of thermal properties, silicon carbide has a high thermal conductivity of 490 W/(m·K), making it an excellent "heat dissipater". In high-power semiconductor lasers, which have extremely high requirements for heat dissipation, the silicon carbide substrate can quickly dissipate heat, maintain the stability of the device's operating temperature, avoid performance degradation or even failure caused by overheating, and ensure the long-term stable operation of the device.
The chemical stability endows the silicon carbide substrate with another advantage. Under high temperatures and complex chemical environments, it can create a stable and reliable condition for the epitaxial growth of gallium nitride, facilitating precise control of the growth parameters and quality of the epitaxial layer. Currently, silicon carbide-based gallium nitride materials have been deeply integrated into key fields such as power amplifiers in 5G communication base stations and radio frequency devices in radar systems. Leveraging their high-frequency and high-power advantages, they significantly improve signal transmission and power processing efficiency.
2. Aluminum Nitride (AlN) Substrate
The aluminum nitride substrate is an almost perfect match for gallium nitride, with a lattice mismatch of less than 1%. The nearly ideal lattice matching paves a smooth path for the growth of high-quality gallium nitride epitaxial layers. When manufacturing optoelectronic devices with extremely high requirements for crystal quality, such as deep ultraviolet light-emitting diodes (UV - LED), the small lattice mismatch helps improve the luminous efficiency, enhance the device stability, reduce non-radiative recombination caused by lattice defects, and extend the service life of the device.
Aluminum nitride has a thermal conductivity of about 320 W/(m·K), which is sufficient to meet the heat dissipation requirements of high-power gallium nitride devices. Its stable chemical properties ensure that it can maintain a stable chemical environment throughout the epitaxial growth of gallium nitride, facilitating the fine-tuning of key parameters such as the thickness and doping concentration of the epitaxial layer to produce high-quality products. Therefore, aluminum nitride substrates are mostly used in the manufacturing of high-performance optoelectronic devices and high-power electronic devices, such as high-brightness LEDs and laser diodes, demonstrating their excellent performance.
3. Zinc Oxide (ZnO) Substrate
Zinc oxide has a similar lattice structure to gallium nitride, and the lattice mismatch between them is relatively small, creating favorable conditions for the growth of gallium nitride epitaxial layers and facilitating the growth of epitaxial layers with excellent crystal quality, laying a solid foundation for the research and development of high-performance electronic devices.
Although its thermal conductivity is not as high as that of silicon carbide and aluminum nitride, it can basically meet the heat dissipation needs of some devices and maintain the normal operating temperature range of the devices. In terms of chemical stability, by precisely adjusting the growth process, zinc oxide can provide a relatively stable chemical environment for the epitaxial growth of gallium nitride, avoiding unnecessary interference from chemical reactions. Currently, ZnO-based gallium nitride materials are emerging in the fields of optoelectronic devices and sensors. For example, in the preparation of ultraviolet detectors, the sensitivity of the detectors can be significantly improved by leveraging the advantages of lattice matching and photoelectric properties.
In conclusion, substrate materials such as silicon carbide, aluminum nitride, and ZnO, with their respective unique advantages, have broadened the selection range of epitaxial substrates for gallium nitride. With the continuous progress of materials science and semiconductor processes, different substrate materials will be more precisely matched to the corresponding device requirements, promoting gallium nitride-based semiconductor devices to achieve higher performance levels and unlock more emerging application possibilities.
