Temperature-dependent Raman property of Al-doped 6H-SiC crystals

Al-doped and un-doped 6H-SiC crystals were grown by physical vapor transport (PVT) method. Raman spectra were measured from room temperature to 400 degrees C. The Raman frequencies were shifted to lower wavenumber and Raman peaks were broadened with increasing temperature for both samples due to thermal expansion and optical phonons decay. Free carrier concentration in Al-doped sample was increased with increasing temperature due to increasing of the plasma concentration, which correspondingly enhanced more strong coupling interactions among LO phonons, plasma, and free carriers. Therefore, the intensity of A(1) (LO) mode for Al-doped sample decreased obviously and maintained unchangeable for un-doped sample. The Al activation behavior and its contribution to free carriers in high temperatures were analyzed theoretically and demonstrated experimentally.