We studied the photoluminescence taken at 5 K from ZnO/Mg0.27Zn0.73O quantum wells on lattice-matched substrates which were grown by laser molecular-beam epitaxy. Well-width dependence of the coupling between localized excitons and longitudinal-optical phonons was estimated experimentally. It is found that the Huang-Rhys factor S, which determines the distribution of luminescence intensities between the phonon replicas and the zero-phonon peak, increases significantly when the well width increases. We assign this variation to (i) the fact that electric field present across the well layer by polarization effects tends to push electrons and holes to the opposite side of well layer, and (ii) the localization of the excitons in the plane of the wells due to potential fluctuations that are induced by well width and barrier height fluctuations.
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