Researchers from the University of Illinois Urbana-Department Champaign’s of Material Science and Engineering have found the answer to a long-standing mystery regarding cubic silicon carbide (3C-SiC) bulk crystals’ lower measured thermal conductivity values in comparison to the structurally more complex hexagonal phase SiC polytype (6H-SiC).
Only diamond has a higher thermal conductivity among inch-scale big crystals than the newly observed thermal conductivity of bulk 3C-SiC.
Professor David Cahill (Grainger Distinguished Chair in Engineering and co-director of the IBM-Illinois Discovery Accelerator Institute) and Dr. Zhe Cheng (Postdoc) report an isotropic high thermal conductivity of 3C-SiC crystals that exceeds 500 W m-1K-1.
To grow high-quality crystals, the team worked with Air Water, Inc. of Japan; the thermal conductivity tests were completed at UIUC in the MRL Laser and Spectroscopy suite. Their results were recently published in Nature Communications.
There are several crystalline forms of silicon carbide (SiC), a wide bandgap semiconductor that is frequently employed in electronic applications. Thermal management of high localized heat flux, which can cause device overheating and long-term loss of device performance and reliability, is a critical concern in power electronics.
Materials with high thermal conductivity (κ) are critical in thermal management design. In contrast to the cubic phase SiC polytype (3C), which has the potential to have the best electrical characteristics and higher, hexagonal phase SiC polytypes (6H and 4H) are more frequently utilized and the subject of substantial research. According to κ. Cahill and Zhe, the measured thermal conductivity of 3C-SiC in the literature has long baffled scientists because it is less than that of the structurally more complicated 6H-SiC phase and is lower than the theoretically expected value.
The measured thermal conductivity of 3C-SiC bulk crystals in this work is ~50% higher than the structurally more complex 6H-SiC, consistent with predictions that structural complexity and thermal conductivity are inversely related. Moreover, the 3C-SiC thin films grown on Si substrates have record-high in-plane and cross-plane thermal conductivities, even higher than that of diamond thin films with equivalent thicknesses.
Dr. Zhe Cheng
This goes against the notion that predicts an inverse relationship between structural complexity and thermal conductivity (as structural complexity goes up, thermal conductivity should go down).
Zhe says that 3C-SiC is “not a new material, but the issue researchers have had before is poor crystal quality and purity, causing them to measure lower thermal conductivity than other phases of silicon carbide.”
The 3C-SiC crystals’ thermal conductivity is greatly reduced as a result of the extremely potent resonant phonon scattering caused by the presence of boron impurities.
Air Water Inc. created wafer-scale 3C-SiC bulk crystals with exceptional crystal quality and purity using low-temperature chemical vapor deposition. The researchers discovered that the high purity and high crystal quality 3C-SiC crystals had a high thermal conductivity.
Zhe says that “the measured thermal conductivity of 3C-SiC bulk crystals in this work is ~50% higher than the structurally more complex 6H-SiC, consistent with predictions that structural complexity and thermal conductivity are inversely related. Moreover, the 3C-SiC thin films grown on Si substrates have record-high in-plane and cross-plane thermal conductivities, even higher than that of diamond thin films with equivalent thicknesses.”
In terms of inch-scale crystals, the high thermal conductivity this study measured places 3C-SiC second to single-crystal diamond, which has the greatest among all natural materials. Diamond’s use in heat management materials is, however, constrained by its high price, tiny wafer size, and challenging semiconductor integration.
A good thermal management material or an outstanding electronic material with high thermal conductivity for scalable manufacturing, 3C-SiC is less expensive than diamond, is simple to integrate with other materials, and can be grown to enormous wafer sizes.
Cahill says, “The unique combination of thermal, electrical, and structural properties of 3C-SiC can revolutionize the next generation of electronics by using it as active components (electronic materials) or thermal management materials,” since 3C-SiC has the highest thermal conductivity among all SiC polytypes and helps facilitate device cooling and reduce power consumption.
Applications including power electronics, radio-frequency electronics, and optoelectronics may be affected by 3C-high SiC’s thermal conductivity.
