Carrier Thermalization and Biexciton Formation in a Polar ZnO/Zn$_{0.84}$Mg$_{0.16}$O Quantum Well Probed by Ultrafast Broadband Spectroscopy

Daniel O. Siebadji Tchuimeni; Marc Ziegler; Olivier Crégut; Pierre Gilliot; Christian Morhain; Andrea Balocchi; Mathieu Gallart

arXiv · 2505.14056

Carrier Thermalization and Biexciton Formation in a Polar ZnO/Zn$_{0.84}$Mg$_{0.16}$O Quantum Well Probed by Ultrafast Broadband Spectroscopy

Daniel O. Siebadji Tchuimeni·Marc Ziegler·Olivier Crégut·Pierre Gilliot·Christian Morhain·Andrea Balocchi·Mathieu Gallart

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Abstract

We investigate the ultrafast dynamics of excitons in a 2.6 nm-thick $\mathrm{ZnO/Zn_{0.84}Mg_{0.16}O}$ 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.

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Daniel O. Siebadji Tchuimeni, Marc Ziegler, Olivier Crégut, Pierre Gilliot, Christian Morhain, Andrea Balocchi, Mathieu Gallart. 2025-05-20. Carrier Thermalization and Biexciton Formation in a Polar ZnO/Zn$_{0.84}$Mg$_{0.16}$O Quantum Well Probed by Ultrafast Broadband Spectroscopy. https://arxiv.org/abs/2505.14056

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