  Recently the developments in the field of II-VI-oxides have been spectacular.
Various epitaxial methods has been used to grow epitaxial ZnO layers. Not only
epilayers but also sufficiently good-quality multiple quantum wells (MQWs) have
also been grown by laser molecular-beam epitaxy (laser-MBE). We discuss mainly
the experimental aspect of the optical properties of excitons in ZnO-based MQW
heterostructures. Systematic temperature-dependent studies of optical
absorption and photoluminescence in these MQWs were used to evaluate the
well-width dependence and the composition dependence of the major excitonic
properties. Based on these data, the localization of excitons, the influence of
exciton-phonon interaction, and quantum-confined Stark effects are discussed.
  The optical spectra of dense excitonic systems are shown to be determined
mainly by the interaction process between excitons and biexcitons. The
high-density excitonic effects play a role for the observation of
room-temperature stimulated emission in the ZnO MQWs. The binding energies of
exciton and biexciton are enhanced from the bulk values, as a result of
quantum-confinement effects.
