  Undoped and Si-doped GaN films were grown epitaxially on sapphire by reactive
rf sputtering of GaAs (and Si) in Ar-N2 mixture. The resistivity of undoped GaN
film grown at 100% N2 was ~2 x 105 {\Omega} cm, which reduced to ~1 {\Omega} cm
in Si-doped film, revealing the effect of Si doping. With decrease of N2 from
100% to 75%, the carrier concentration of Si-doped films increased from ~7 x
1018 cm-3 to ~2 x 1019 cm-3, but remained practically unchanged as N2 was
decreased to 20%, which is explained by effects due to saturation of Si doping
and increase of Ga interstitials as well as compensation by N interstitials and
Ga vacancies. Undoped and Si-doped films grown below 20% N2 displayed similar
carrier concentrations (~1020 cm-3), due to dominance of Ga interstitials. Both
undoped and Si-doped films were degenerate and displayed increase of mobility
with carrier concentration and temperature, which was analyzed by the combined
effect of ionized impurity and dislocation scattering, using compensation ratio
as fitting parameter. At carrier concentrations below 1019 cm-3, the mobility
was governed by both ionized impurity and dislocation scattering, while at
higher carrier concentrations, ionized impurity scattering was found to
dominate, limited by compensation due to acceptors. In spite of the degenerate
character, the films displayed a small decrease of carrier concentration with
temperature, along with a nearly linear decrease of mobility, which are
explained by a marginal increase of compensation ratio with decrease of
temperature, taking into account the band edge fluctuation effects.
