Origin of the 2D electron gas at the SrTiO₃ surface

Since the discovery of the 2D electron gas (2DEG) at the SrTiO₃ (001) surface in 2011 via angle-resolved photoemission spectroscopy (ARPES), there have been several theories proposing its origin. In separate studies, scientists have demonstrated that many of reconstructions at the SrTiO₃ surface are TiO₂-rich. Others using X-ray photoelectron spectroscopy (XPS) have demonstrated the presence of oxygen vacancies at the surface, correlating them with a surface electronic state (in-gap state, IGS). Here we employed in situ synchrotron X-ray methods during growth by molecular beam epitaxy (MBE), ARPES, and XPS to study the origin of the 2DEG and its evolution. We discover that the 2DEG only appears when the surface has TiO₂ double layer structure [1], for the first time linking the atomic surface structure to its 2DEG electronic states. Interestingly, by deposition of single layer of SrO or the subsequent growth of a TiO₂ monolayer, we are able to completely control the disappearance/reappearance of both the 2DEG and the IGS, potentially providing a means of manipulating 2DEG-behavior in oxide electronics.

[1] S. Cook et al., Sci. Adv. 5, eaav0764 (2019).