Interface Carriers and Enhanced Electron-Phonon Coupling Effect in Al₂O₃/TiO₂ Heterostructure Revealed by Resonant Inelastic Soft X-Ray Scattering

The electronic structure and the electron-phonon couplings in a novel mass-production-compatible Al₂O₃/TiO₂ 2D electron system (2DES) are investigated using resonant inelastic soft X-ray scattering. The experimental data from the samples of various TiO₂ thicknesses unequivocally show that the Ti³⁺ state indeed exists at the deep interface to serve as an n-type dopant for the 2DES. The electronic structure of Ti³⁺ species is scrutinized as entirely separated from that of the Ti⁴⁺ host lattice. Furthermore, features of sub-eV energy loss phonon modes are clearly observed, indicating substantial electron-phonon coupling effects. Such low energy loss features are enhanced in thinner TiO₂ samples, implying that polaronic local lattice deformation is enhanced due to the presence of Ti³⁺. These findings suggest that the 2DES properties can be controlled via well-established TiO₂ engineering, thereby enthroning the binary oxide heterostructure as a promising candidate for 2DES device applications.