Nonpolar n-type doped a-plane GaN films were grown by metal-organic vapor phase epitaxy on r-plane sapphire substrates using silane and isobutylgermane as Si and Ge dopants, respectively. It is found that Ge-doping acts as a surfactant enabling the growth of fully coalesced and mirror-like a-plane GaN films with electron concentrations above 1020 cm−3. Si-doping in excess of 2×1019 cm−3 shows an antisurfactant behavior leading to surface degradation. No significant impact on strain was found for any dopant. Results on the ordinary and extraordinary dielectric functions as determined by ellipsometry prove the high optical quality of the Ge-doped films.