Understanding and Controlling Electronic Structures in Transition Metal Oxides: Insights from Photoemission Spectroscopy on SrVO₃ and LaTiO₃ Films

Due to electronic correlations, transition metal oxides (TMOs) exhibit a rich phase diagram associated with specific functionalities. To utilize them in devices, the understanding and control of the microscopic electronic structures has yet to be improved, especially for thin films and heterostrcutures. High-energy photoemission spectroscopy has emerged as a powerful tool, being able to differentiate between surface and bulk electronic structure and give access to both core-level and valence band information. We present two case studies: (1) Using the prototypical correlated metal SrVO₃ as an example, we demonstrate that the usual description of TMO films as ideally terminated with stoichiometric composition overlooks an essential ingredient: oxygen adsorbing at the surface apical sites. The oxygen adatoms, which are present even if the films are kept in an ultrahigh vacuum environment and not exposed to air, are shown to severely affect the intrinsic electronic structure of a TMO film. Their presence leads to the formation of an electronically dead surface layer but also alters the band filling and the electron correlations in thin films. These findings highlight that it is important to consider the specific oxygen configuration imposed by a capping layer to predict the behavior of ultrathin films of TMOs near the 2D limit.¹ (2) In LaTiO₃ films, we examine the electronic structure across the band-filling induced metal-insulator transition, which could be of use in future Motttronic devices.² While LaTiO₃ films tend to grow oxygen-rich, the oxygen content can be controlled (reduced) by X-ray irradiation, thus changing the electronic configuration towards d¹ (=Mott insulator). This allows us to observe mass renormalization as in a Brinkman- Rice scenario by angle-resolved photoemission, consistent with thermodynamic data of (La,Sr)TiO₃. Additionally, we infer different correlation strengths for the surface and bulk of the films from our photoemission data, while Brinkman-Rice-like behavior is observed in both cases.

¹Adv. Electron. Mater. 2022, 0, 2101006

² Adv. Mater. 2018, 30, 1706708