Time-resolved studies of carriers dynamics in wide band gap materials

Abstract

Lasers delivering high intensity ultrashort pulses are adequate tools to study the dynamics of irradiation induced modifications in wide band gap materials. We present the results of two complementary time-resolved pump-probe optical experiments. We use a technique of interferometry in the frequency domain to probe the instantaneous modification of the refractive index of the material induced by a pump pulse. The presence of free carriers in the conduction band induces a negative phase shift of the probe pulse which is proportional to the excitation density. Comparison between different oxides reveals an extremely contrasted behaviour: the photoexcited carriers are trapped in 150 fs in SiO 2, while they remain quasi-free for more than 50 ps in MgO and Al 2O 3. These lifetimes are closely connected to the formation of radiation induced defects. In particular, using a conventional time-resolved absorption technique in the UV (5.2–5.6 eV), we have studied the intrinsic defects formation kinetics in SiO 2. Our results show that the formation of self-trapped excitons (STE) is occuring in the ultrashort time of 150 fs, in agreement with the free carrier lifetime measured in the above experiment. At low temperature, the STE recombine radiatively, while at temperature above the luminescence quenching temperature, a small proportion of the STE is converted into permanent defects (O vacancies).