Phase Engineering of Ga₂O₃ Hetero- and Homo- Epitaxial Growth by Mist Chemical Vapor Deposition

Gallium oxide (Ga₂O₃) has a larger band gap (4.4~5.3eV) than conventional wide bandgap material such as SiC and GaN, so it has attracted attention as a next-generation power semiconductor material [1]. Gallium oxide has five phases (α, β, γ, δ, and ε) having different energy band gaps and crystal structures, and is applied according to various uses such as a Schottky device, a MOSFET in a power device, and a solar blind photodetector [2][3][4]. Among these polymorphs, metastable alpha phase has an ultra-wide band gap (~5.3 eV) and exhibits hexagonal structure the similar as sapphire (Al₂O₃), enabling hetero-epitaxial single crystal thin film growth. In addition, beta phase which has monoclinic structure, is most thermodynamically stable and is capable of bulk growth, most application studies are being conducted on power devices.<br/>In this study, alpha and beta gallium oxide epitaxy were grown on each different substrate using low cost, non-vacuum and non-toxic mist chemical vapor deposition (MIST-CVD). For α-Ga₂O₃, it was hetero-epitaxially grown on a corundum structured sapphire. In order to further reduce the lattice mismatch between the substrate and the epitaxy layer to induce single crystal growth and reduce FWHM to improve the quality of the epitaxy, an Al-doped (Al_1-XGa_X)₂O₃ buffer layer was inserted into the interlayer. At less than 500°C, monocrystalline α-Ga₂O₃ was grown on a sapphire substrate, and in the 500-800°C range, α, β, and ε phases were combined in the form of polycrystalline structures. Above 800 degrees, preferred orientation of β-Ga₂O₃was investigated, but the quality of the epitaxy film is low due to the severe lattice mismatch between the sapphire substrate and β-Ga₂O_3. Therefore, β-Ga₂O₃(001) substrate was used for homo-epitaxial growth of β-Ga₂O_3. At temperatures above 900°C, the value of the FHWM was lower to a similar degree to that of the β-Ga₂O₃substrate, indicating that high-quality single beta gallium oxide epitaxy had successfully grown. As a results, we developed process techniques for MIST-CVD based α, β-phase single crystal, and high-quality epitaxial thin film growth.<br/><b>Reference</b><br/>[1] D. Shinohara, S. Fujita, Heteroepitaxy of corundum-structured a-Ga2O3 thin films on a-Al2O3 substrates by ultrasonic mist chemical vapor deposition, Jpn. J. Appl. Phys. 47 (2008) 7311.<br/>[2] Higashiwaki M, Sasaki K, Kuramata A, et al. Gallium oxide (Ga2O3 ) metal–semiconductor field-effect transistors on single-crystal βGa2O3 (010) substrates. Appl Phys Lett, 2012, 100(1), 013504<br/>[3] Zhao B, Wang F, Chen H, et al. Solar-blind avalanche photodetector based on single ZnO–Ga2O3 core-shell microwire. Nano Lett, 2015, 15(6), 3988<br/>[4] Sasaki K, Higashiwaki M, Kuramata A, et al. Ga2O3 Schottky barrier diodes fabricated by using single-crystal β-Ga2O3 (010) substrates. IEEE Electron Device Lett, 2013, 34(4), 493<br/><b>Acknowledgment</b><br/>This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1A2C1013693), Korea Institute for Advancement of Technology (KIAT) grant funded by By the Ministry of Trade, Industry & Energy (MOTIE, Korea) (P0012451, The Competency Development Program for Industry Specialist) and the Technology Innovation Program - (20016102, Development of 1.2kV Gallium oxide power semiconductor devices technology) funded by MOTIE.