Role of Intra‐Band Relaxation of Holes and Tunneling of Electrons in Carrier Relaxation in AlGaAs/GaAs Quantum Well

Abstract

The role of intra‐band hole relaxation and tunneling of electrons in the carrier relaxation in Al0.7Ga0.3As/GaAs quantum well (QW) is studied by performing pump‐probe transient reflectivity measurements with a femtosecond laser at room temperature. The ultrafast optical response of the QW sample is measured with an aim of understanding various ultrafast processes occurring on the time scale of ∼100 fs to 10 ps. Carriers are injected into the QW layer via a resonant photoexcitation of electron–hole pairs enabling a sharp rise of the signal. Carrier relaxation inside QW is dominated by non‐radiative processes like phonon‐mediated intra‐band relaxation of holes and tunneling of electrons out of the QW plane and their subsequent capture by the midgap states in the AlGaAs barrier. Subsequent to the initial thermalization within 100 fs, carrier relaxation is mainly driven by the tunneling of electrons and trapping by the midgap states over a time scale of 3–8 ps. In between, photoexcited holes are seen to relax from light to heavy hole states of QW within 1–2 ps, leading to a delayed rise of transient reflectivity signal. Systematic efforts are made to isolate various contributions which showed phonon re‐absorption leading to saturation effect in intraband relaxation.