【International Papers】Metalorganic vapor phase epitaxy of β-(AlₓGa₁₋ₓ)₂O₃  (x = 0 – 0.55) and multilayer structure on (100) β-(Al₀.₂₄Ga₀.₇₆)₂O₃ substrates

      Researchers from the Leibniz-Institut für Kristallzüchtung (IKZ) have published a dissertation titled " Metalorganic vapor phase epitaxy of β-(Al_xGa_1−x)₂O₃ (x = 0 – 0.55) and multilayer structure on (100) β-(Al_0.24Ga_0.76)₂O₃ substrates " in Journal of Vacuum Science & Technology A.

Background

      Beta-gallium oxide (β-Ga₂O₃) is an emerging semiconductor that has drawn significant interest for next-generation power electronic devices, primarily because of its ultra-wide bandgap of ∼4.9 eV. and its high theoretical breakdown electric field of up to 8 MV/cm. In addition, the monoclinic β-phase of (Al_xGa_1−x)₂O₃ has attracted growing attention, as alloying with Al enables bandgap tuning to wider energies, making it particularly promising for applications in the ultraviolet spectral range. However, the synthesis of high-aluminum-composition β-(Al_xGa_1−x)₂O₃ films of sufficient quality poses significant challenges. These include a progressively increasing lattice mismatch with the underlying β-Ga₂O₃ substrate, which promotes strain and defect formation.

      The epitaxial growth of β-(Al_xGa_1−x)₂O₃ on β-Ga₂O₃ substrates has been investigated using molecular beam epitaxy (MBE), metalorganic vapor phase epitaxy (MOVPE), and mist chemical vapor deposition. Epitaxial growth of β-(Al_xGa_1−x)₂O₃ on (010) β-Ga₂O₃ substrates is typically limited to aluminum compositions below 27%, as higher concentrations induce phase segregation. However, the use of alternative substrate orientations, including (100) and (-201), represents a promising pathway to achieve substantially higher Al incorporation (52%) in a phase-pure β-structure. However, the incorporation of a high aluminum concentration results in substantial lattice mismatch, which often leads to defect formation and cracking.

Abstract

      We successfully achieved the growth of β-(Al_xGa_1−x)₂O₃ (x = 0–0.55) on a (100) β-Al_0.24Ga_0.76O₃ substrate using metalorganic vapor phase epitaxy (MOVPE). A stacked layer composed of β-(Al_0.47Ga_0.53)₂O₃/β-Ga₂O₃ on a β-Al_0.24Ga_0.76O₃ substrate was demonstrated. High-resolution x-ray diffraction and reciprocal space mapping analysis verified the coherent epitaxial growth of phase-pure β-(Al_xGa_1−x)₂O₃ thin films up to an Al composition of x = 0.55. Films with higher Al content exhibited partial plastic strain relaxation. The multilayer structure of the β-(Al_0.47Ga_0.53)₂O₃/β-Ga₂O₃ thin film exhibited good quality and coherent growth. Atomic force microscopy measurements proved a surface roughness well below 0.42 nm for the fully strained films and about 1 nm for the relaxed ones. These results indicate that the MOVPE technique has significant potential for the fabrication of β-Ga₂O₃-based heterojunctions suitable for device applications.

Conclusion

      In summary, coherent epitaxial growth of β-(Al_xGa_1−x)₂O₃ (x = 0–0.55) thin films on (100) β-Al_0.24Ga_0.76O₃ substrates was successfully achieved by metalorganic vapor phase epitaxy. Structural characterization confirmed that fully strained epitaxial layers were maintained up to x = 0.55, while higher Al compositions led to partial strain relaxation. The β-(Al_0.47Ga_0.53)₂O₃/β-Ga₂O₃ multilayer structure exhibited high crystalline quality and a coherent epitaxial relationship with the substrate. Atomic force microscopy revealed atomically smooth surfaces, with root-mean-square roughness values of 0.33–0.42 nm for strained films and approximately 1 nm for relaxed layers. These results demonstrate that MOVPE provides a controllable and reliable route for the synthesis of high-quality β-(Al_xGa_1–x)₂O₃ heterostructures. The demonstrated growth control and interface quality establish a foundation for further optimization of Al incorporation and strain management. Future work may explore higher Al compositions, intentional doping, and device-oriented heterostructures, advancing the development of (Al_xGa_1−x)₂O₃/β-Ga₂O₃-based electronic and optoelectronic devices.