  Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit
unprecedented tunability of the short-range spin exchange interaction. In the
context of large exciton binding energies and electron-hole exchange
interaction in ZnO, this tunability results from the competition between
quantum confinement and giant quantum confined Stark effect. By using
time-resolved photoluminescence we identify, for well widths under 3 nm, the
redistribution of oscillator strengths between the A and B excitonic
transitions, due to the enhancement of the exchange interaction. Conversely,
for wider wells, the redistribution is cancelled by the dominant effect of
internal electric fields, which dramatically reduce the exchange energy.
