Photoexcited carrier relaxation dynamics and terahertz response of photoconductive antennas made on proton bombarded GaAs materials

Abstract

We present a model reproducing the instrumental response of a time-domain spectrometer that integrates photoconductive transmitter and receiver antennas made on identical proton-bombarded GaAs substrates. This model is used to determine the ultrafast capture time of the photoexcited carriers by the ion-bombardment-induced traps. A 0.5 ps capture time can be extracted for a low laser pump fluence of 0.66 μJ/cm2 per pulse. This carrier trapping time gets longer as the pump fluence increases. This behavior is explained by a gradual filling of the traps that are distributed over a 1 μm depth from the GaAs surface. This interpretation is supported by time-resolved measurements obtained on the same photoconductive material using both an 820 nm pump/terahertz-probe transmission experiment and a degenerate 760 nm pump/probe reflectivity experiment. The differential transmission and reflectivity dynamics are reproduced using a biexponential function which correctly describes the photoexcited carrier relaxation and transport dynamics in this material. The strong agreement observed between these different measurements reinforces the validity of the theoretical model used to reproduce the instrumental response of the terahertz setup.