Direct Subpicosecond Measurement of Carrier Mobility of Photoexcited Electrons in GaAs

Abstract

Hot carrier phenomena in semiconductors play an important role in high-speed devices like ultrafast photodetectors and hot-electron transistors, but they are also an intrinsic phenomena in submicron devices due to the large internal electric fields. Hot carrier effects have been investigated by direct heating of electrons by an applied electric field, and transport parameters like the drift velocity were determined by microwave time-of-flight techniques [1] or optical pump-and-probe techniques involving picosecond laser pulses[2l. Hot electrons can also be studied by photoinjection of carriers with excess energy, thereby creating hot electrons directly. The hot-electron dynamic can then be probed by a second ultrashort pulse[2]. Because of the availability of ultrashort laser pulses in the picosecond and even femtosecond time regime, the relaxation phenomena of hot carriers can be traced directly in time and extremely valuable information has been extracted from these experiments. On a femtosecond time scale, both absorption saturation[3,4] and time resolved photoluminescence[5] have been used to probe the ultrafast relaxation of injected carriers. All these studies probe the carrier distribution in the bands after injection. An important property for high speed devices is, however, the time dependent mobility or drift velocity. These transport parameters can be studied by microwave conductivity and time-of-flight techniques[l]. However, the time resolution of these techniques is not sufficient to observe transient transport properties on a picosecond or even femtosecond time scale as they occur in a semiconductor after photoinjection of hot carriers.