  We investigated the optical properties of epitaxial \textit{n}- and
\textit{p}-type ZnO films grown on lattice-matched ScAlMgO$_4$ substrates.
Chemical doping yielded a severe inhomogeneity in a statistical distribution of
involved charged impurities. Two approaches are adopted to treat the
inhomogeneity effects; Monte Carlo simulation technique for the
\textit{n}-doped films, and the fluctuation theory for the \textit{p}-ZnO. The
broadening of PL band of \textit{n}-ZnO was significantly larger than predicted
by theoretical results in which the linewidths of each individual emissions
have been determined mainly from the concentration fluctuation of donor-type
dopants by the simulation. Moreover, the rather asymmetrical line shape was
observed. To explain these features, a vibronic model was developed accounting
for contributions from a series of phonon replicas. In case of \textit{p}-type
ZnO:N, analysis of excitation-intensity dependence of the peak shift of
donor-acceptor luminescence with a fluctuation model has also proven the
importance of the inhomogeneity effect of charged impurity distribution, as in
the case of ZnO:Ga. We extracted the inhomogeneity in the sample and acceptor
activation energy prepared under the various growth conditions. It is shown
that the theoretical results are in good agreement with the experimental 5-K
time-resolved luminescence for the systems in a fluctuation field. Finally,
localized-state distributions have been studied in N-doped ZnO thin films by
means of transient photocurrent measurement.
