Investigation of high frequency carrier dynamics of Al-doped ZnO nanowires by terahertz time domain spectroscopy

Terahertz time-domain spectroscopy was used as a nondestructive and a noncontact optical probe to investigate the material properties and high frequency carrier dynamics of undoped and Al-doped ZnO nanowires (NWs). The frequency dependent refractive index was first determined through effective medium theory and numerical iteration process applied to the detected terahertz signals transmitted through the samples (ZnO NWs) and the substrate (Al₂O₃-sapphire). Using the determined complex refractive index, the frequency dependent conductivity was calculated directly. Then essential carrier dynamics parameters such as carrier concentration and mobility were extracted by fitting the Drude-Smith model to the calculated frequency dependent complex conductivity. The results showed an increased carrier concentration while the mobility was decreasing with aluminum concentration incorporated into the ZnO NWs. After a critical amount of aluminum, the carrier concentration began to increase dramatically from 6 × 10¹⁶ cm^-3 for undoped ZnO NWs to 1.6 × 10¹⁸ cm^-3 for the highest doped sample.