Band offsets at amorphous-crystalline Al₂O₃-SrTiO₃ oxide interfaces

2D electron gases (2DEGs) formed at oxide interfaces provide a rich testbed for fundamental physics and device applications. While the discussion of the physical origins of this phenomenon continues, the recent discovery of oxide 2DEGs at non-epitaxial interfaces between amorphous and crystalline oxides provides useful insight into this debate. Furthermore, using amorphous oxides offers a low-cost route towards realizing 2DEGs for device applications. In this work, the band offsets of a simple model system of an amorphous-crystalline oxide interface are investigated. The model system consists of amorphous Al₂O₃ grown on single-crystalline (001) SrTiO₃. X-ray photoelectron spectroscopy is employed to study the chemical states, bandgap, and band offsets at the interface. The density of ionic defects near the interface is found to be below the detection limit, and the interface is found to be insulating. Analysis of the relative band structure yields significant interfacial barriers, exceeding 1.05 eV for holes and 2.0 eV for electrons. The barrier for holes is considerably larger than what is known for related material systems, outlining the promise of using amorphous Al₂O₃ as an effective and simple insulator, an important building block for oxide-based field effect devices.