Abstract
The rapid advance of femtosecond laser technology has offered superior time resolution for studying transient carrier transport phenomena, and many studies of these transport processes have been performed [1]. In order of time scale, carrier transport process can be divided into four distinct stages: In the first stage, when a biased semiconductor excited by an ultrashort light pulse, an instantaneous polarization of the photoinjected electron-hole pairs is created [2-4]. In the second stage, carriers undergo ballistic acceleration without scattering by the electric field for times shorter than the mean free scattering time (~10−13s). In the third stage, as scattering processes set in, the acceleration of the carriers stops, and the velocity reaches a maximum. Velocity-overshoot occurs for materials such as GaAs, when hot electrons are scattered into satellite L, X valleys with lower mobility. In the fourth stage, the velocity is maintained at a lower leve1 in steady-state drift. These regimes, while easily defined, have not been clearly separated to date because scattering times limit the ballistic regime to ~ 50 fs for typical conditions.
© 1995 Optical Society of America
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