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

TL;DR

This study uses ultrafast broadband spectroscopy to explore exciton dynamics, thermalization, and biexciton formation in a ZnO/ZnMgO quantum well, revealing rapid carrier relaxation and biexciton binding energies of 18-22 meV.

Contribution

It provides new insights into ultrafast exciton thermalization and biexciton formation in ZnO-based quantum wells using time-resolved spectroscopy.

Findings

Photocarriers relax into the quantum well within sub-picoseconds.

Excitons become localized due to interface disorder or well-width fluctuations.

Biexciton binding energy ranges from 18 to 22 meV depending on temperature.

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.