Characteristics of ultrafast optical responses originating from non-equilibrium carrier transport in undoped GaAs/ n -type GaAs epitaxial structures

We have investigated the characteristics of ultrafast optical responses originating from a carrier transport process in undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial structures with the use of a reflection-type pump-probe technique at room temperature. The built-in electric field in the i-GaAs top layer, whose strength is controlled by its thickness d, accelerates the transit of photogenerated carriers through the i-GaAs layer. We systematically observed that the decay time of a carrier-induced reflectivity change shortens with an increase in built-in electric field strength resulting from a decrease in d: 6.1, 12, and 28 kV/cm for d = 1200, 500, and 200 nm, respectively. In the i-GaAs/n-GaAs sample with d = 200 nm, which has the highest built-in electric field strength, the decay time is much shorter than the oscillation period of longitudinal optical (LO) phonon. From the spectrally resolved detection of the reflected light, it was found that the energy relaxation of the photogenerated carriers by the LO-phonon scattering hardly occurs in the i-GaAs layer, which indicates a quasiballistic transport. This finding demonstrates that the i-GaAs/n-GaAs structure with the non-equilibrium carrier transport process is useful for ultrafast optical applications.