Large-size and high-quality gallium nitride （GaN） bulk crystals are a fundamental requirement for next-generation power electronics and optoelectronic devices. Hydride vapor phase epitaxy （HVPE） is currently the dominant commercial technology for GaN substrate fabrication. Due to the formation of solid by-products such as NH4Cl，readily block exhaust systems，the long-term growth of GaN have been greatly limited.
In recent years，oxide vapor phase epitaxy （OVPE） has attracted attention as a promising alternative，since it employs Ga2O vapor as the gallium source and produces only gaseous H2O as a reaction by-product，thereby avoiding solid-phase residues. However，the lack of an efficient，stable，and chemically clean method for generating Ga2O vapor has restrained the development of OVPE. The Ga2O generation routes currently used in OVPE，such as carbon reduction of Ga2O3，hydrogen reduction of Ga2O3，and water vapor oxidation of metallic Ga，have greatly limited the effective supplement of Ga2O as well as the growth rate of GaN.
Here，based on the high-temperature reaction between metallic Ga and Ga2O3，a novel Ga2O generation strategy with chemically clean is proposed as a Ga2O source for OVPE growth of GaN for the first time. Compared with the conventional approaches，this strategy does not require external reactants and produces no additional by-products. In this work，a thermodynamic analysis was carried out to evaluate the feasibility of the proposed reaction. And the results show that the Gibbs free energy of the Ga-Ga2O3 reaction remains negative in the temperature range of 1 073~1 273 K，indicating that the reaction is thermodynamically spontaneous. The saturated vapor pressure of Ga2O increases exponentially with temperature and approaches nearly 1 000 Pa at 1 273 K，which is suitable for vapor-phase transport. The theoretical growth rate of ~5 400 μm/h for a 2-inch GaN could be achieved at 1 573 K when the Ga2O was complete conversion.
GaN films were grown on sapphire substrates using a home-made 6-inch vertical OVPE reactor. The Ga source temperature was varied from 900 to 1 070 ℃ while the growth-zone temperature was kept at 1 050 ℃. The results indicate that the GaN growth rate increases exponentially with Ga source temperature，from 0.223 μm/h （950 ℃） to 2.02 μm/h （1 070 ℃），which agrees well with the theoretical calculations. However，the theoretical value is approximately five times the experimental value，leading to an estimated actual conversion efficiency of about 20%. The higher Ga source temperatures also has led to the smoother surface morphology and higher crystalline quality from the observation of XRD and SEM images. Moreover，the growth rate of GaN could be up to ~1 080 μm/h for 2-inch substrate and ~120 μm/h for 6-inch substrate when the Ga source temperatures set as 1 300 ℃ under the standard reaction stoichiometry of 1 mol Ga2O3 to 4 mol Ga. At present，experiments in the high-temperature range of 1 100~1 300 ℃ are in progress，and the specific growth behavior will be reported in future work.
In summary，a novel Ga2O generation strategy based on the Ga-Ga2O3 high-temperature reaction is proposed and demonstrated for OVPE growth of GaN. Based on the thermodynamic analysis and growth experiments，this work establishes a new technical foundation for high-rate，long-term，and large-size GaN epitaxy.
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