Determination of unscreened exciton states in polar ZnO/(Mg,Zn)O quantum wells with strong quantum-confined Stark effect

Abstract

We report on recombination dynamics and screening effects due to photogenerated carriers in polar ZnO/(Mg,Zn)O single quantum wells. For small well widths ($\ensuremath{\le}$2.5 nm), the unscreened transition energy and decay time can be determined directly since the influence of the internal electric field is negligible due to strong confinement of the carriers. The energetic dependence of the decay time was reproduced by a relaxation model to describe the localization within potential minima due to well width fluctuations. For larger well widths, screening effects have to be taken into account even for moderate excitation densities in the 1 W/cm${}^{\ensuremath{-}2}$ range. It is very hard to determine the unscreened single-exciton state of these wells with strong quantum-confined Stark effect within reasonable time scales and a significant signal-to-noise ratio. We present an extended relaxation model to determine the decay time of the unscreened single-exciton state. A difference in polarization between ZnO and Mg${}_{x}$Zn${}_{1\ensuremath{-}x}$O of $\ensuremath{\Delta}P/x=0.026\phantom{\rule{0.28em}{0ex}}\text{C}/{\text{m}}^{2}$ is concluded from the dependence of the decay time on the well width.