Thin films of beta-Ga2O3 doped with Si are grown on Al2O3 (0001) and beta-Ga2O3 (100) substrates by metal organic vapor phase epitaxy. Homoepitaxial growth of single-phase, smooth beta-Ga2O3 layers doped with Si, with a dislocation density not exceeding those of the melt grown substrate, was demonstrated. The interplay between growth conditions, structural and electrical properties of the Ga-oxide layers is studied. XRD and HRTEM show that Si-doping in the concentration range 10(17)-10(18) cm(-3) does not deteriorate the quality of the Ga-oxide layers compared to undoped ones. The different nature of defects in undoped and doped material is investigated by HRTEM. It is found out that the density of twins and stacking faults on a-planes are an order of magnitude lower in beta-Ga2O3:Si than in undoped material, while defects on inclined planes have rarely been observed in beta-Ga2O3. (C) 2013 Elsevier B.V. All rights reserved.