Quasi-van der Waals Epitaxial of Single Crystal β-Ga₂O₃ on (100) Surface

Gallium oxide (β-Ga₂O₃), an emerging wide bandgap semiconductor, shows significant promise for power devices due to its high breakdown voltage and low on-resistance. β-Ga₂O₃ is one of the most thermodynamically stable polymorphs of Ga₂O₃, with the (100) surface being more readily obtainable in melt-based bulk crystal growth, offering the potential for producing high-quality, large-area single crystal substrates.<br/><br/>However, similar to the van der Waals surfaces of two-dimensional (2D) materials, the Ga₂O₃ (100) surface exhibits weak surface interactions, with a surface energy of 0.49 J/m², comparable to that of graphene.[1] The homoepitaxy of Ga₂O₃ on the (100) surface, akin to quasi-van der Waals epitaxy on 2D materials, is prone to lack of nucleation with epitaxial orientation.[2] Twinning of 180 degrees in-plane rotation formation occurs due to its small twin energy difference. Twins tend to grow in a three-dimensional island and prevent proper film coalescence, resulting in a large number of grain boundaries.[3] Twinning reduces breakdown voltage and decreases carrier mobility, severely impairing device quality. Achieving epitaxial films without twin structures on this surface has been challenging, significantly limiting the application potential of the (100) substrate.<br/><br/>In this study, we achieved quasi-van der Waals epitaxial growth of single-crystal Ga₂O₃ on (100) substrates by introducing excess indium as a catalytic metal during growth. The excess indium facilitates the surface diffusion of gallium adatoms on the substrate. The increased mobility helps gallium adatoms find energetically favorable sites, promoting the formation of a well-ordered crystalline structure.[4] By increasing the growth temperature to ensure indium evaporation and maintaining a high Group III-VI ratio, we shifted the growth mode from 3D island to 2D layer, and achieved β-Ga₂O₃ epitaxial growth on the (100) surface approaching thermal equilibrium conditions. This approach eliminates twin nucleation sites due to their unstable energy state near thermal equilibrium. Additionally, these conditions ensured a layer-by-layer growth mode, with single-crystal nucleation sites expanding into a film. Therefore, we achieved the epitaxial growth of twin-free single-crystal thin films with atomically flat surfaces on Ga₂O₃ (100) substrates. This advancement significantly improves the material quality of β-Ga₂O₃ epitaxially grown on (100) substrates and greatly expands the substrate choices for Ga₂O₃-based power semiconductor devices. With the recent successful development of 6-inch or even larger (100) β-Ga₂O₃ substrates, the pathway for developing high-performance, cost-effective Ga₂O₃-based power devices is now feasible.<br/><br/>Reference:<br/>[1] Barman, S. K., & Huda, M. N. (2019). Mechanism Behind the Easy Exfoliation of Ga2O3 Ultra-Thin Film Along (100) Surface. physica status solidi (RRL)–Rapid Research Letters, 13(5), 1800554.<br/>[2] Kim, H., Chang, C. S., Lee, S., Jiang, J., Jeong, J., Park, M., ... & Kim, J. (2022). Remote epitaxy. Nature Reviews Methods Primers, 2(1), 40.<br/>[3] Qiao, K., Liu, Y., Kim, C., Molnar, R. J., Osadchy, T., Li, W., ... & Kim, J. (2021). Graphene buffer layer on SiC as a release layer for high-quality freestanding semiconductor membranes. Nano letters, 21(9), 4013-4020.<br/>[4] Kum, H., Lee, D., Kong, W., Kim, H., Park, Y., Kim, Y., ... & Kim, J. (2019). Epitaxial growth and layer-transfer techniques for heterogeneous integration of materials for electronic and photonic devices. Nature Electronics, 2(10), 439-450.