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

Abstract

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 $1.4\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }{\text{cm}}^{\ensuremath{-}3}$. Mobilities of $\ensuremath{\sim}2000\text{ }{\text{cm}}^{2}\text{ }{\text{V}}^{\ensuremath{-}1}\text{ }{\text{s}}^{\ensuremath{-}1}$ are measured and do not vary significantly with temperature over 60--120 K.