In this work, we investigate the electronic structure and interfacial band alignment of β-$({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3/{\mathrm{Ga}}_2{\mathrm{O}}_3$ heterojunctions using a combination of synchrotron-based hard x-ray photoemission spectroscopy (HAXPES) and first-principles hybrid density functional theory calculations. β-$({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ films with $\mathrm{Al}$ compositions of x = 0.12, 0.19, and 0.29 were grown on $\mathrm{Fe}$-doped β-${\mathrm{Ga}}_2{\mathrm{O}}_3$ (010) substrates via pulsed laser deposition. The band gap of β-$({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ increases from (4.83 ± 0.05) eV (x = 0) to (5.37 ± 0.08) eV (x = 0.29), primarily driven by an upward shift of the conduction band edge due to hybridization between $\mathrm{Al}$ 3s and $\mathrm{Ga}$ 4s states, while the valence band edge exhibits a slight downward shift. Both experimental HAXPES data and theoretical calculations confirmed the formation of a “type I” (straddling) band alignment in the β-$({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3/{\mathrm{Ga}}_2{\mathrm{O}}_3$ heterojunctions. For instance, at x = 0.29, the conduction band offset and valence band offset are approximately 0.33 and 0.21 eV, respectively. These findings provide valuable insights for designing modulation-doped β-$({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3/{\mathrm{Ga}}_2{\mathrm{O}}_3$ heterostructures, enabling the realization of a two-dimensional electron gas and its application in high-frequency electronic devices.

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• Electronic structure
• Interfaces
• Junctions
• Transparent conducting oxides
• Density functional calculations
• Epitaxy
• Hard x-ray photoelectron spectroscopy
• Physical deposition