Carrier dynamics in bulk ZnO. I. Intrinsic conductivity measured by terahertz time-domain spectroscopy

Far-infrared absorption and refractive index of a ZnO wafer were measured as a function of temperature below 120 K using terahertz (THz) time domain spectroscopy. The accompanying frequency-dependent complex conductivity gives an accurate picture of electron dynamics because the measured range of 0.2-2.5 THz brackets the scattering rate. The frequency-dependent conductivity shows the general trends predicted by the Drude model but with significant deviations that are better fit by the generalized Drude model, which allows for a distribution of carrier relaxation times. Conductivity increases with increasing temperature as electrons are thermally activated from shallow donor states, with calculated donor energy of 27 meV and density of 1.4x10(17) cm(-3). Mobilities of similar to 2000 cm(2) V(-1) s(-1) are measured and do not vary significantly with temperature over 60-120 K.