Femtosecond laser spectroscopy of In0.53Ga0.47As/InP multiple quantum wells: interfacial roughness and photoexcited carrier relaxation

Interfacial properties and relaxation dynamics of photoexcited carriers in In0.53Ga0.47As/InP multiple quantum wells (MQWs) have been investigated by means of cross-sectional scanning tunneling spectroscopy and optical spectroscopy methods (luminescence, absorption and pump–probe experiments). The MQW structure consists of 125 periods of 10-nm-wide well layers and 40-nm-wide barrier layers on an InP(0 0 1) substrate. The observed interfacial roughness of the InGaAs-on-InP is 1–2 monolayers (ML), while that of the InP-on-InGaAs is 3–4 ML. The Stokes shift observed in luminescence and absorption spectra at 77 K corresponds well to the well-width distribution observed by the cross-sectional STM. Differential absorption spectra measured by pump–probe spectroscopy show that relaxation of hot carriers in conduction and valence bands followed by exciton formation takes place in 17–30 ps depending on the excitation photon energy. The excitons formed at the band bottom are localized at thicker areas within a quantum well in ∼500 ps.