  We investigated the optical properties of epitaxial \textit{n}-type ZnO films
grown on lattice-matched ScAlMgO$_4$ substrates. As the Ga doping concentration
increased up to $6 \times 10^{20}$ cm$^{-3}$, the absorption edge showed a
systematic blueshift, consistent with the Burstein-Moss effect. A bright
near-bandedge photoluminescence (PL) could be observed even at room
temperature, the intensity of which increased monotonically as the doping
concentration was increased except for the highest doping level. It indicates
that nonradiative transitions dominate at a low doping density. Both a Stokes
shift and broadening in the PL band are monotonically increasing functions of
donor concentration, which was explained in terms of potential fluctuations
caused by the random distribution of donor impurities.
