Thickness effect on ultrafast thermalization of carriers in above-band-gap states in ZnO epitaxial films

Abstract

Energy-dependent free-carrier dynamics was investigated in 70 nm (thin) and 1 mu m (thick) ZnO epifilms using the optical pump-probe technique. The far-above-band-gap dynamics in the thin epifilm reveals the prolonged relaxation and the slow recovery of renormalized band gap. The band-gap renormalization (BGR) effect is affected by the inefficient carrier-phonon scattering. In addition, the loss of excited carrier density via surface trapping results in an energy-dependent BGR buildup time. However, the far-above-band-gap dynamics in the thick epifilm reveals fast relaxation followed by BGR recovery, which is independent of the photon energy. The near-band-gap dynamics shows an ultrafast carrier thermalization both in the thin and the thick epifilms.