Heteroepitaxial Growth of High-Mobility Ge-Doped β -Ga2O3 Films on Sapphire Substrate by Low-Pressure CVD

Abstract

In this work, high-quality Ge-doped (2̅01) β-Ga2O3 thin films were heteroepitaxially grown on c-plane sapphire substrates with offcut angles of 0°, 2°, 6°, and 8° using low-pressure chemical vapor deposition (LPCVD). Increasing the sapphire offcut promoted step-flow growth, resulting in improved terrace alignment, reduced surface roughness, and enhanced crystalline quality. Phase-pure monoclinic β-Ga2O3 with strong (2̅01) preferential orientation was confirmed by X-ray diffraction and Raman spectroscopy, while X-ray photoelectron spectroscopy revealed a near-stoichiometric composition with an O/Ga ratio of 1.48. Electrical transport properties exhibited a strong dependence on the substrate offcut angle, with room-temperature Hall mobility increasing from 15 to 117 cm2/V·s as the offcut angle increased from 0° to 6°, across carrier concentrations spanning 1.43 × 1017 to 2.75 × 1018 cm–3. The 6° offcut sample achieved a room-temperature mobility of 117 cm2/V·s at a carrier concentration of 1.43 × 1017 cm–3 and a peak low-temperature mobility of 337 cm2/V·s at 128 K with a carrier concentration of 8.96 × 1016 cm–3, representing the highest reported room-temperature and low-temperature mobilities for Ge-doped β-Ga2O3 films grown on sapphire substrates. Carrier concentration and mobility data were analyzed using charge-neutrality and Boltzmann transport models incorporating donor activation together with polar optical phonon, ionized impurity, neutral impurity, acoustic deformation potential, and dislocation scattering mechanisms. The fitting revealed shallow donor activation energies of 12.5–19 meV, a deeper donor level at 80 meV, low acceptor compensation (<5 × 1015 cm–3), and threading dislocation densities on the order of 109 cm–2. These results establish sapphire offcut engineering as an effective pathway for achieving high-mobility, LPCVD-grown Ge-doped β-Ga2O3 heteroepitaxy on scalable foreign substrates, which is critical for the development of high-performance β-Ga2O3 power devices.