The rising momentum of research and developments on Ga2O3 has broaden the area of material exploration even for metastable phases. In the field of metastable Ga2O3, we have focused on defective-spinel-structured γ-phase and established some milestones: epitaxial stabilization of single crystalline films on MgAl2O4 [T. Oshima,et al., J. Cryst. Growth 359, 60 (2012).], carrier generation by impurity doping [T. Oshima et al., J. Cryst. Growth 421, 23 (2015).], and band-gap engineering by alloying γ-Al2O3 [T. Oshima et al., Appl. Phys. Express 10, 051104 (2017).]. We consider their results endorse further semiconductor engineering studies on γ-Ga2O3-related materials. Therefore, as a successive study, we have attempted to fabricate first γ-(AlxGa1−x)2O3-based heterostructures, particularly the superlattices (SLs) comprised with the end members of the alloy, to consider the possibility of obtaining coherent heterojunctions for future heterojunction device applications.
10-period γ-Al2O3/Ga2O3 SLs on (001) MgAl2O4 substrate were fabricated by plasma-assisted molecular beam epitaxy. By controlling the each layer thickness, we tuned the average Al composition (x_ave) of the coherent SLs from 0.26 to 0.86, and obtained nearly-lattice-matched SLs to the substrate at x_ave ~ 0.5. The lattice-matched SLs maintained coherent interfaces up to a period length of 7.2 nm (3.2/4.0 nm for γ-Al2O3/Ga2O3 layers) in spite of a large lattice mismatch between the end members (−3.6%). These successful fabrication of γ-Al2O3/Ga2O3 SLs means wide flexibility in designing γ-(AlxGa1−x)2O3-based heterostructures including superlattices for future development of functional heterojunction devices.