【Domestic Papers】Bandgap nonlinearity and composition-dependent bowing in α-(AlₓGa₁₋ₓ)₂O₃ epilayers

      Researchers from the Nanjing University have published a dissertation titled "Bandgap nonlinearity and composition-dependent bowing in α-(Al_xGa_1−x)₂O₃ epilayers" in The Journal of Chemical Physics.

Project Support

      This work was supported by the National Key R&D Program of China (Grant No. 2022YFB3605403) and the National Natu ral Science Foundation of China (Grant Nos. 62425403, 62234007, 62293521, U21A20503, and U21A2071).

Background

      Ultrawide-bandgap semiconductors, such as Ga₂O₃ and AlN, hold significant promise for high-power, high-frequency electronic devices and deep-ultraviolet photodetectors. However, β-Ga₂O₃ faces phase instability and strain-induced issues at high aluminum content, limiting further performance enhancements. In contrast, α-Ga₂O₃ enables stable epitaxial growth across the full range of Al composition, offering a pathway for high-performance power and optoelectronic devices. Precise alloy engineering and bandgap tailoring of α-(Al_xGa_1−x)₂O₃ allow optimization of crystalline quality, thermal stability, and electronic/optical properties, providing a solid foundation for the design and application of next-generation ultrawide-bandgap devices.

Abstract

      Designing functional materials with tailored properties often involves alloying different semiconductors, yet the nonlinear bandgap bowing effect complicates precise bandgap engineering, particularly in ultrawide-bandgap systems, such as Ga₂O₃ ternary alloys. In this work, we examined the bandgap nonlinearity and composition-dependent bowing in pure-phase α-(Al_xGa_1−x)₂O₃ epilayers (0 < x < 0.62) grown via laser molecular beam epitaxy on m-plane sapphire substrates. The variations in the x-ray rocking curve full width at half maximum and surface roughness of epilayers with increasing Al composition x follow the trend predicted by the theoretical formation enthalpy of α-(Al_xGa_1−x)₂O₃ alloys. The high crystalline quality of the α-(Al_xGa_1−x)₂O₃ epilayers was further confirmed by x-ray diffraction and transmission electron microscopy characterizations. While lattice constants adhered to Vegard’s law, the optical bandgap (5.28–7.22 eV) exhibited nonlinearity, with a bowing factor of 1.33 eV, aligning closely with theoretical predictions. Our findings suggest that the observed optical bandgap nonlinear effect in the α-(Al_xGa_1−x)₂O₃ alloy primarily stems from charge exchange, rather than volume deformation or strain relaxation effects, providing a pathway for precise bandgap tuning in Ga₂O₃ for high-performance power electronics.

Conclusion

      In summary, we successfully grew single-crystal α-(Al_xGa_1−x)₂O₃ (0 < x < 0.62) epilayers on m-plane sapphire substrates by LMBE. HRXRD and XPS analyses confirm that the lattice constants of α-(Al_xGa_1−x)₂O₃ follow Vegard’s law with no phase transition as Al composition increases. Combining trans mission spectroscopy and detailed XPS analysis, it was observed that the bandgaps of α-(Al_xGa_1−x)₂O₃varied continuously in the range of 5.28–7.33 eV, displaying nonlinearity characteristics. The bowing parameters for the nonlinear bandgap variation of α-(Al_xGa_1−x)₂O₃ were calculated, and their physical origins were thoroughly investigated. Using three distinct sources and employing Zunger’s approach, it was confirmed that the primary mechanism responsible for the nonlinear bandgap shift in α-(Al_xGa_1−x)₂O₃ alloys is charge exchange within the lattice. These results would pave the way for application on high-power electronic devices and vacuum ultraviolet photonic devices for α-(Al_xGa_1−x)₂O₃’s bandgaps are wider than those of all materials available to date.