REVERSE SURFACE PHOTOVOLTAIC EFFECTS IN GAAS SURFACE QUANTUM-WELLS

We observed that GaAs surface quantum wells on undoped AlGaAs substrat es experienced a reverse surface photovoltage (RSPV) that caused the b ands to bend upward when illuminated by femtosecond laser pulses for t ime-resolved two-photon photoemission spectroscopy (TRPES). The RSPV e ffect is created by trapping of photoexcited electrons in the quantum well. Secondary illumination creates a surface photovoltage opposite i n magnitude that flattens the bands. The secondary illumination does n ot affect the width of the energy spectrum or the ultrafast dynamics w ith respect to the conduction band minimum. A simple model based on th e recombination current from the AlGaAs fits the data excellently.