The thermal conductivity is a fundamental material property what is particular important in high-power/high-frequency microelectronic devices since the ability to dissipate heat is often the limiting factor that determines device performance. The new generation of microwave power devices will be with no doubt AlGaN/AlN high electron mobility transistors owing to their excellent high power handling capacity in high frequency operations. One of the major factors limiting the power performance and fabrication costs of these devices is the substrate material. Conventional sapphire and bulk silicon substrates come no longer into consideration because of low thermal conductivity. High-quality GaN-based epitaxial structures can only be grown on SiC or GaN substrates, which are expensive and limited in size. One possibility of reducing the wafer cost is the use of polycrystalline material. A novel approach recently proposed within the EC-founded HYPHEN Project (http://www.hyphen-eu.com) relies on the use of composite substrates made of thin single crystal “seed layer” transferred on the top of a thick semi-insulating polycrystalline material with efficient thermal dissipation.

SiC is a good thermal conductor, but thermal properties of SiC strongly depend on its crystal structure. Additionally, dopants cause decreasing of the thermal conductivity. Therefore, there is a need of having exact data on thermal diffusivity of various type of SiC (mono-, polycrystalline, doped), since the available literature and catalogue data present large discrepancy.

In this work we present the application of photothermal technique to determine thermal diffusivity α and thermal conductivity κ of polycrystalline 3C-SiC and, for comparison, monocrystalline 4H and 6H SiC. The applied method is based on generation of temperature field disturbance in the sample by modulated light beam and detection of harmonic component of the temperature over and under investigated sample. The probing beam from He-Ne laser is running parallel to the sample surface and is periodically deflected on the temperature field disturbance. This deflection is related to the thermal properties of the sample. The analysis consisting in fitting of theoretical curves which are the solution of the heat conduction equation to the dependences of the amplitude and the phase of the experimental signal on modulation frequency allows determination of the thermal diffusivity of the sample. The specific heat was determined using Differential Scanning Calorimeter DSC 1000.

We show, that thermal diffusivity of polycrystalline SiC wafers is on the level of 0.2 cm²s^-1 and this result conforms to the thermal conductivity κ of 50 Wm^-1K^-1. For comparison, measured thermal diffusivity of monocrystalline 4H and 6H SiC were 0.6 – 0.9 cm²/s (κ = 130 – 195 Wm^-1K^-1) and about 1.7 cm²s^-1 (κ = 373 Wm^-1K^-1), respectively.

Part of the research was supported by the grant from EC HYPHEN Contract Number: FP6-027455 and by the grant 3T11B 042 30 from Ministry of Science and Information Society Technologies, Poland.

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