Recombination dynamics and screening of the internal electric field inZnO/ZnxMg1−xOmultiple quantum wells

We investigate the recombination dynamics within screened $c$-axis $\text{ZnO}/{\text{Zn}}_{0.7}{\text{Mg}}_{0.3}\text{O}$ quantum wells using time-resolved photoluminescence and femtosecond pump-probe spectroscopy. The relaxation of excited carriers restores the strength of the internal electric field, which we follow, via the decay time constant, as it increases from 180 ns to $5.8\text{ }\ensuremath{\mu}\text{s}$. Pump-probe spectroscopy reveals faster, initial decay times of 160--250 ps, which we attribute to additional recombination mechanisms, that become significant for carrier densities greater than $2\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\text{pairs}\text{ }{\text{cm}}^{\ensuremath{-}2}$. In addition, the time for screening of the internal electric field to be established is measured to be less than 1 ps. These measurements are followed by a self-consistent calculation which solves the Schr\"odinger and Poisson equations for pair densities up to $1\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{pairs}\text{ }{\text{cm}}^{\ensuremath{-}2}$, where there is no further substantial blueshifting of the transition energy with increasing pair density because the electron and hole charge distributions cancel. This calculation fits the measured recombination dynamics and can be used to determine the quantum-well properties at any time following excitation, including carrier densities, transition energies, and recombination lifetimes.