<i>In Situ</i>, Monolayer Control over the 2D Electron Gas on SrTiO₃

SrTiO₃ (001) is the principal material in the field of oxide electronics, due to both its properties and the fact that the composition of its surface can be controlled, leading to TiO₂ surface termination. Employing in situ synchrotron X-ray methods during growth by molecular beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we discover the existence of a 2D electron gas (2DEG) at the as-prepared, TiO₂-terminated SrTiO₃ (001) surface. Interestingly, the 2DEG appears correlated with the presence of a TiO₂ double layer [S. Cook et al., Sci. Adv. 5, eaav0764 (2019).], as deposition of single layer of SrO leads to island formation and complete elimination of the 2DEG. The subsequent growth of a TiO₂ monolayer leads to smoothing and return of the 2DEG. We will provide an atomic-level description of the evolving surface and how it relates to the surface electronic structure.