Ultrafast carrier dynamics and terahertz conductivity of photoexcited GaAs under electric field

摘要

Ultrafast carrier dynamics in semiconductors has attracted much attention due to the application in high speed devices. Compared to the conventional experimental techniques, such as the time-resolved optical transmission technique and the all-optical pump-probe spectroscopy, the optical pump-terahertz probe spectroscopy has a plethora of advantages to provide the ability to temporally resolve phenomena at the fundamental timescales of carrier motion. The distinct advantage of OPTP is being able to directly measure the photo-induced changes in the photoconductivity, which contains the information of carrier density and mobility, with a temporal resolution of sub-picosecond. The ultrafast carrier dynamics and transient terahertz photoconductivity in semi-insulating GaAs have been investigated under electric field by using optical pump-terahertz probe technique with an unchanged pump power irradiating on the GaAs surface. One-dimensional pump scan at the maximum value of the THz pulse under electric fields of 0 kV/cm, 6 kV/cm, and 15 kV/cm, respectively. The measurements indicate that the terahertz transmission change induced by the pump pulses at high electric field is smaller than that without electric field. It is obvious that the threshold value of E, which begins to enhance the transmission, is about 3-4 kV/cm. We attribute this phenomenon to carrier scattering into the L valley or even X valley, which leads to a drop in carrier mobilities due to the large effective masses in those satellite valleys. The calculated transient photoconductivities fit well with the Drude-Smith model, which attributes the negative imaginary conductivity to the backward scattering of electrons. The negative value of cl in our fitting implies that a fraction, but not all, of the backward scattering is a result of the electron reflecting from surfaces. It could also result from a Coulombic scattering between carriers. Due to the low mobilities of electrons in the L valley, the average mobility of all electrons will decrease under high E. These fitting results are consistent with our intervalley scattering model. Our investigation suggests that the OPTP technique is a very promising method for detecting the ultrafast dynamics in those materials.