In recent years, the development of electronic systems and products has skyrocketed. With the rapidly evolving industrial landscape, certain materials are emerging as game-changers, pushing the boundaries of technology and transforming various sectors. One such material is silicon carbide (SiC). With its exceptional properties and wide-ranging applications, SiC is becoming increasingly vital to modern industries. However, it is important to know how such material is fabricated into its useable form. One such way of utilizing silicone carbide is making a substrate, which refers to a base material made of silicon carbide that is used as a foundation for various electronic and semiconductor devices. The main function of a silicon carbide substrate is to provide a stable and reliable platform for the fabrication and integration of electronic components.

Owing to its reputation, SiC is famous for having exceptional electronic properties such as operating at a higher frequency, temperature, efficiency and voltage without damaging the system. Moreover, SiC substrates are used to fabricate homoepitaxial and faster power-switching devices, which increase overall efficiency when compared to traditional Si-based devices. Similarly, partially conducting SiC devices are used for heteroepitaxial growth in RF power devices. On the other hand, GaN-on-SiC HEMTs are extremely useful in the case of RF power amplifier technology, RF functions in defence radar, and the development of next-generation 5G technology,

Ⅰ. Early History of Silicon Carbide and Its Remarkable Journey from Ancient Discoveries to Modern Applications

The Discovery of SiC dated back centuries when a scientist observed dazzling sparkling crystals in the samples of the Canyon Diablo meteorite in Arizona. This scientist named Dr Ferdinand Moissan and several others made the first electric batch furnace to make silicon carbide abrasives. This only paved the way to deeply study the unique properties of SiC, however, it was still difficult to produce SiC single crystals for mass production of semiconductor electronics.

In 1978, a major breakthrough was achieved where with the use of seeded sublimation growth, a single 6H-SiC crystal was produced.

The early developments of SiC high-temperature sublimation growth proposed far too many issues and technical challenges like doping, micropipes, crystalline defects and diameter expansion. Unlike silicon, SiC cannot melt and attain a liquid phase at practical pressure, instead, SiC crystal growth is based on heating a polycrystalline source material under controlled atmospheric pressure where it subsequently sublimes and condenses on a SiC seed. The early choice of SiC polytrope was both 4H-SiC and 6H-SiC, however, 4H-SiC was more favoured due to its superior thermal conductivity, higher bandwidth and mobility which led to precise control of crystal growth conditions. On the other hand, Cree-Research showcased various ways to manipulate crystal growth conditions including doping p-n type elements to vary electrical properties or to achieve semi-insulating properties by incorporating impurities.

The development of SiC technology was mainly supported by the Title III Program support which helped give a much-needed boost to contracts like Litton-Airtron. This contract at that time was the leading producer of 150 mm diameter GaAs substrates. However, the industry demanded thinned substrates up to 100 mm for Defense Advanced Research Project Agencies which was difficult to research at that time. To achieve this, both Cree and Sterling companies were awarded contracts to reduce micropipe density to less than 1 cm-2 and reduce the diameter of the substrate. Cree’s superior quality of  4HN SiC substrates clearly outperformed its competitors in terms of unparalleled quality depicted in the cross-polar images taken as shown in Figure 1.

Fig 1: Cross-polarized images of Cree’s 75 mm diameter circa 2002

Ⅱ. Research growth of different companies in AFRL Sponsored Development

Apart from well-known contracts like Title III Program support and DARPA, AFRL has been a strong financial anchor for leading companies like ATMI, Cree-Research, Sterling, Westinghouse and DOW. The main aim of AFRL was to extend the capabilities of SiC substrates for domestic use, by increasing the diameter of 4HN SiC substrates to 100 mm by harnessing the Advanced Physical Vapor Transport crystal growth process. This technique helped to reduce X-ray crystal lattice curvature to o.o1°, moreover, the  Full Width Half Max was also reduced to approximately 17 arc seconds.

In 2010, AFRL granted its largest contract for the development of 4HN and semi-insulating 6H 150-mm substrates with extended diameters up to 200 mm and improved commercialization. Methods like APVT and Axial Gradient Transport were incorporated to improve the grade of crystal growth and procure a larger diameter substrate. The important aspect of this incorporation was to have limited carbon and polytype inclusions, micropipes and polycrystalline edge defects, moreover, it also enabled the crystals to prevent unwanted impurities and manage hot zone axial/radial thermal gradients. Figure 2 shows the crystal structure of 150-mm 4HN SiC substrate using these two processes.

Fig 2:  Cross-polar image and micropipe density of 150-mm 4HN SiC substrate using APVT and Axial Gradient Transport

The final contract issued by AFRL in 2017 helped researchers work on the improvement of quality, defect reduction, better efficiency, more diameter and lower cost for manufacturing and growth of crystals. The  Semi-insulating SiC substrates proved to be better for the fabrication of AlGaN/GaN RF transistors for high performance, using vanadium compensation for incorporating deep energy levels in the bandgap.

Ⅲ. Conclusion

SiC substrates have become increasingly sought-after in both commercial and military applications. The unique properties of SiC, such as high thermal conductivity, wide bandgap, and excellent electrical characteristics, make it ideal for power and radio frequency (RF) applications. Recognizing this potential, companies are investing heavily in expanding their manufacturing capabilities to meet the escalating demand. The SiC substrate market has witnessed the rise and fall of various companies over the past three decades. However, the remnants of these pioneers live on through Wolfspeed-Cree, II-VI, and SK Siltron. With the growing demand in both commercial and military sectors, the SiC industry is experiencing a transformative phase with better efficiency, lower impact of defects, and higher diameter of crystal substrates. The investments and technological advancements made by Wolfspeed-Cree have positioned them as leaders in the market, propelling the manufacturing of SiC substrates to new heights. As the industry continues to evolve, a strong domestic industrial base will be vital in meeting the ever-increasing demand for SiC substrates.