Interband carrier recombination mechanism in Al-doped ZnO

Due to strong nonlinear optical effects and ultrafast response properties, transparent conductive oxides are promising candidate materials for next-generation THz modulation devices and other photonic applications. However, the mechanisms of photon-induced ultrafast carrier relaxations in those materials have yet to be fully understood. Here we investigated the interband carrier dynamics in a prototype of this family, Al-doped ZnO, with different excitation photon energies through femtosecond optical transmittance measurements. We found that the interband relaxation of the photon-induced carriers is mainly through second-order recombination, which is essentially the direct band-to-band recombination with the assistance of intraband electron relaxation. The high density of the doped electrons provides superior intraband relaxation channels through electron-phonon interaction and electron-impurity scattering, which contribute to the overall ultrafast response, especially under indirect and band tail transitions. This study provides a new insight into the ultrafast response mechanism of Al-doped ZnO and the analyzing method may also be generalized to other heavily doped semiconductors.