  The carrier spin dynamics in ZnO is investigated by time-resolved optical
orientation experiments. We evidence a clear circular polarization of the
donor-bound exciton luminescence in both ZnO epilayer and non-intentionally
doped bulk ZnO. This allows us to measure the localized hole spin relaxation
time. We find $\tau^{s}_h$$\sim$350 ps at T=1.7 K in the ZnO epilayer. The
strong energy and temperature dependences of the photoluminescence polarization
dynamics are well explained by the fast free exciton spin relaxation time and
the ionization of bound excitons.
