  Transparent and conductive ZnO:Ga thin films are prepared by laser
molecular-beam epitaxy. Their electron properties were investigated by the
temperature-dependent Hall-effect technique. The 300-K carrier concentration
and mobility were about $n_s \sim 10^{16}$ cm$^{-3}$ and 440 cm$^{2}$/Vs,
respectively. In the experimental `mobility vs concentration' curve, unusual
phenomenon was observed, i.e., mobilities at $n_s \sim 5\times$ 10$^{18}$
cm$^{-3}$ are significantly smaller than those at higher densities above $\sim
10^{20}$ cm$^{-3}$. Several types of scattering centers including ionized
donors and oxygen traps are considered to account for the observed dependence
of the Hall mobility on carrier concentration. The scattering mechanism is
explained in terms of inter-grain potential barriers and charged impurities. A
comparison between theoretical results and experimental data is made.
