Precise Determination of Thermal Expansion Coefficients Observed in 4H-SiC Single Crystals

The coefficient of thermal expansion (CTE) of SiC single crystals is important, in particular, for both designing device assembly and controlling stress distributions in heteroepitaxial thin film structures grown onto SiC substrates. We have performed very precise measurements of the CTEs for SiC single crystals comprising of single 4H polytype PVT-grown in NIPPON Steel Corporation for a temperature range from 123 K to 473 K using a laser interferometry method. This method allows us to directly measure the temperature dependent variation in thermal expansion of the crystal volume with much higher accuracy, and enables us to straightforwardly obtain practical information of CTE data. Furthermore in order to discuss the CTE behavior for a wider temperature range the CTEs at higher temperatures up to 1573 K have been also measured using dilatometer method. The CTE obtained for a nitrogen-doped 4H-SiC single crystal increases continuously from 0.8 ppm/K to 3.1 ppm/K for temperatures of 273 K and 423 K respectively, and further increases to 5.4 ppm/K at 1273 K. We conclude from our data that the CTE variations are likely to be almost independent of the crystal axis directions of SiC from 123 K up to 1573 K.