In this work, we study the thermal stabilization of metastable α-Ga2O3 in growth experiments. Gallium oxide films are grown on c- and r-plane sapphire substrates by halide vapor phase epitaxy (HVPE) at the temperature range of 450-690 oC. The surface morphology is investigated by scanning electron microscopy. The structural quality and phase composition of the grown films is studied by X-ray diffraction. It is found that the use of r-plane sapphire substrates prevents the formation of the orthorhombic κ-Ga2O3 and monoclinic β-Ga2O3 and thus extends the growth process window for the deposition of the rhombohedral α-phase of gallium oxide.