Carrier thermalization and biexciton formation in a polar ZnO/Zn0.84Mg0.16O quantum well probed by ultrafast broadband spectroscopy

We investigate the ultrafast dynamics of excitons in a 2.6 nm-thick ZnO/Zn0.84Mg0.16O quantum well grown on a c-axis sapphire substrate, using non-degenerate time-resolved pump–probe spectroscopy. A pump pulse at 266 nm generates photocarriers within the ZnMgO barriers, and their dynamics is monitored through time-resolved differential reflectance measurements using a supercontinuum probe spanning the 345–400 nm spectral range. Photocarriers generated in the barriers rapidly relax into the quantum well, where they form excitons within sub-picosecond timescales. These excitons quickly thermalize and become localized, likely due to interface disorder or well-width fluctuations, as supported by photoluminescence measurements showing a clear Stokes shift and the absence of free exciton emission. A phonon-assisted absorption process, leading to the effective thermalization of excitons, is observed and analyzed. We identify moreover a negative differential reflectance feature as a photoinduced absorption into a biexciton state, with a binding energy ranging from 18 to 22 meV depending on temperature.