【Member Papers】Tunable electronic properties of κ-(Al, In)₂O₃/Ga₂O₃ digital alloys via superlattice design

      Researchers from the Hong Kong University of Science and Technology (Guangzhou) have published a dissertation titled "Tunable electronic properties of κ-(Al, In)₂O₃/Ga₂O₃ digital alloys via superlattice design" in Journal of Applied Physics.

Project Support

      This work was supported by the C. K. Tan start-up fund from the Hong Kong University of Science and Technology (Guangzhou), the Guangzhou Municipal Science and Technology Project (Nos. 2023A03J0003, 2023A03J0013, 2023A04J0310, and 2023A03J0152), the Department of Education of Guangdong Province (No. 2024ZDZX1005), and the State Administration of Foreign Experts Affairs (No. Y20240005). The authors also acknowledge financial support from the Shenzhen Science and Technology Innovation Commission (No. 20231115111658002). This work was supported by the Materials Characterization and Preparation Facility (MCPF) and the Green Materials Laboratory at the Hong Kong University of Science and Technology (Guangzhou). The work was also supported by the Matching Funding for Selected Talent of National Programs (No. CZ118SC24007), National Major Talent Project (No. CZ118SC25005), and Excellent Young Scientists Fund (overseas): Nos. RK118QN24006 and RK118QN25006. The numerical calculations in this paper have been done using computational resources from Guangzhou Tanoxi Company.

Background

      Gallium oxide (Ga₂O₃) has emerged as a highly promising wide-bandgap semiconductor due to its exceptional material properties, such as a large breakdown field (∼8 MV/cm) and tunable bandgaps across polymorphs. Among its various phases, the metastable orthorhombic phase (κ-Ga₂O₃) has garnered significant attention due to its high spontaneous electrical polarization property, which surpasses that of conventional III-nitrides, such as AlN and GaN. This property enables polarization-induced doping at heterointerfaces, facilitating the formation of two-dimensional electron gases (2DEGs) without extrinsic dopants, makes it suitable for high-electron-mobility transistors (HEMTs) and quantum-well infrared photodetectors (QWIPs). 

      In addition, forming κ-(Al_xGa_1−x)₂O₃ and κ-(In_xGa_1−x)₂O₃ alloys would be a promising strategy to adjust the material properties and modify the polarization differences to increase localize 2DEGs with higher sheet charge carrier densities. Tahara et al. demonstrated the ε-(Al_xGa_1−x)₂O₃ grown by mist chemical vapor deposition. Nishinaka et al. reported the ε-(In_xGa_1−x)₂O₃ on c-plane sapphire for bandgap tuning. The stabilized pure-phase κ-(Al_xGa_1−x)₂O₃ and κ-(In_xGa_1−x)₂O₃ thin film was also revealed by pulsed laser deposition (PLD). However, experimental feasibility remains a critical challenge at high Al or In concentrations in the random alloy, particularly regarding phase separation and high threading dislocation densities. Furthermore, controlling the phase stability between the metastable κ-polymorph and the competing α-phase or the thermodynamically stable β-phase presents a significant hurdle in bulk heteroepitaxy. Therefore, it is necessary to find an alternative strategy to enhance crystalline and interface quality.

Abstract

      κ-Ga₂O₃-based digital-alloy superlattices offer a promising alternative to traditional random alloys for high performance heterostructure devices. Using density functional theory, we model (Al₂O₃/κ-Ga₂O₃) and (In₂O₃/κ-Ga₂O₃) superlattices with varying monolayer (ML) thicknesses (1 ML/3 ML, 2 ML/2 ML, and 3 ML/1 ML). Our calculations reveal precise tuning of lattice parameters and bandgaps dependent on layer thickness. The incorporation of Al₂O₃ introduces tensile strain and widens bandgap up to 6.65 eV, while In₂O₃-rich structures exhibit compressive strain with bandgap reduction down to 3.32 eV. Element-projected band structures confirm quantum confinement effects and interfacial contributions to electronic states. Notably, intersubband transition energies are controllable via ML thickness, enabling absorption in the telecom-compatible wavelength range (∼1.55 μm). Band alignment analysis reveals significant conduction band offsets (up to 3.71 eV for Al₂O₃/Ga₂O₃), which is vital for polarization-induced 2DEG (two-dimensional electron gas) formation. This work demonstrates the feasibility of κ-Ga₂O₃ digital-alloy superlattices for tailored high-electron-mobility transistors and quantum-well infrared photodetectors.

Conclusion

      In summary, we performed first-principles DFT calculations to investigate the structural and electronic properties of a κ-Ga₂O₃-based digital-alloy superlattice. We demonstrated that the bandgap can be tuned over a wide range (3.32–6.65 eV) by varying the ML thickness of Al₂O₃ and In₂O₃ insertions, overcoming the phase separation challenges typical of random ternary alloys. The calculations reveal that intersubband transition energies can be engineered to target the telecommunication wavelength (1.55 μm). Furthermore, the adjustable conduction band offset (up to 3.71 eV) in Al₂O₃/κ-Ga₂O₃ systems highlights their potential for high confinement 2DEGs in HEMT application. These results establish κ-phase digital alloys as a viable and flexible platform for next-generation oxide electronics and optoelectronics.