Impact of strain-field interference on the coexistence of electron and hole gases in SrTiO3/LaAlO3/SrTiO3

Abstract

The renowned two-dimensional electron gas at the interface of insulating ${\mathrm{LaAlO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ poses a grand challenge in engineering the hole counterpart and its coexistence with the electron gas. The room-temperature realization of such coexisting states was recently achieved in the ${\mathrm{SrTiO}}_{3}$-capped ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$, and the electron and hole gases in the ${\mathrm{SrTiO}}_{3}/{\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ reside at the respective bottom and top interfaces, which are separated by a few tens of the interlayer-${\mathrm{LaAlO}}_{3}$ unit cells. What happens if the two interfaces are brought to the extreme proximity of just a few lattices away? Using atomic-resolution electron microscopy and spectroscopy, we tackled this problem in ${\mathrm{SrTiO}}_{3}/{\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ with ${\mathrm{LaAlO}}_{3}$ of 3 and 5 unit cells, respectively. Notably, both ${\mathrm{SrTiO}}_{3}/{\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures are dominated by electron conductivity at room temperature. The microscopic evaluation at picometer accuracy unveils head-to-head (tail-to-head) ferroelectriclike polarizations across the bottom (top) interface, in comparison to the established notion of characteristic strain-induced head-to-head polarizations in primitive metallic ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ that form a potential well and confine the accompanied screening electrons into two-dimensional electron gas. The atomic-scale spectral investigation of the ${\mathrm{SrTiO}}_{3}/{\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ reveals the unexpected depletion (resurrection) of electrons at the head-to-head bottom (tail-to-head top) interface and the ${\mathrm{LaAlO}}_{3}$ hosts coexisting localized electrons and holes, casting an electric field with tens of meV/\AA{} and serving as the depolarization field to the ${\mathrm{LaAlO}}_{3}$ dipole. The landscape of the ${\mathrm{SrTiO}}_{3}/{\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ turns out to be the polarity-neutral ${\mathrm{LaAlO}}_{3}$ sandwiched by the head-to-head ${\mathrm{SrTiO}}_{3}$ polarizations, with the potential gradient due to the unbalanced polarizations pointing to the top interface and resurrecting the screening electrons thereby. The dominant electron conductivity in the thin ${\mathrm{SrTiO}}_{3}/{\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ is profoundly related to this polarization pattern and the intricate strain-field interference across the two closely spaced interfaces, accounting for the pattern onset, was atomically revealed.