In-situ Investigations for Possible Two-Dimensional Electron Gases on BiO₂-Terminated BaBiO₃ Thin Films

We analyzed the atomic configuration and the electronic structures of BaBiO₃ (BBO) thin films adopting in-situ photoemission. Two-dimensional electron gases (2DEGs) was theoretically predicted by density-functional-theory on BiO₂-terminated BBO surfaces. The so-called self-doping effect was suggested as the origin for 2DEG on BBO, which can be a generic mechanism to form 2DEGs on surfaces of charge-skipping oxides. However, direct visualization for surface atomic configuration using transmission electron microscopy has been restricted because BBO is too vulnerable to electron beam. In this work, we inferred depth-profile information for our BBO films using angle-dependent core-level spectra measurement, which is dramatically affected by finite escape depths of photoelectrons. We found that our BBO films were terminated by BiO₂ layers, rather than BaO or other possible mixtures of BaO and BiO₂. By measuring angle-resolved photoemission, we distinguished surface states and bulk states, and none of them showed any signatures from 2DEGs. Surface states were observed 4 eV below Fermi level, not near the Fermi level, which suggests that electronic structure description for BBO should accompany with sophisticated approaches.