Electronic charge transport in sapphire studied by optical-pump/THz-probe spectroscopy

THz time-domain spectroscopy (THz TDS) with ultrafast photo-excitation is applied to probe the complex conductivity of the charge carriers in sapphire over the temperature range of 40 - 350 K. A comparison of the measured complex conductivity to the Drude model yields the carrier scattering rate and density. The dependence of the carrier scattering rate on temperature and sample purity is used to identify the scattering mechanisms in sapphire. In the higher temperature range, scattering is determined by intrinsic phonon processes, but impurity scattering becomes dominant at low temperatures in typical optical-grade samples. In high-purity samples, however, impurity scattering remains negligible down to 40 K, and carrier mobilities exceeding 10,000 cm ²/Vs can be achieved.