Gallium oxide (Ga2O3) is attracting attention as a next-generation semiconductor material for power device because it has a wide energy band gap and high breakdown electric field. We deposited a Sn polymer, poly-tetraallyl tin, on Ga2O3 samples using a disclosed initiated chemical vapor deposition (iCVD) process. The Sn dopant of the Sn polymer layer is injected into the Ga2O3 through a heat treatment process. Diffusion model of Sn into the Ga2O3 is proposed through secondary ion mass spectroscopy analysis and bond dissociation energy. The fabricated device exhibited typical n-type field-effect transistor (FET) behavior. Ga2O3 Sn-doping technology using iCVD will be applied to 3D structures and trench structures in the future, opening up many possibilities in the Ga2O3-based power semiconductor device manufacturing process.