Two-dimensional electron gas at the atomically smooth LaAlO$_{3}$/SrTiO$_{3}$ (111) interface

The two-dimensional electron gas (2DEG) at the LaAlO$_{3}$/SrTiO$_{3}$ (001) heterointerface has been widely investigated due to its diverse functionalities such as conductivity, ferromagnetism, and superconductivity. In this orientation, the SrTiO$_{3}$ is nonpolar, with charge-neutral AO and BO$_{2}$ planes, while $+$e of charge is transferred between AO and BO$_{2}$ planes in the LaAlO$_{3}$ layer. The (111) orientation is, however, qualitatively different in that the AO$_{3}$ and B lattice planes in both materials exhibit charge transfer between layers, and both have in principle a polar character. We have found that LaAlO$_{3}$ deposited on the (111) SrTiO$_{3}$ polar surface also supports an interfacial 2DEG. An atomically smooth step and terrace structure of (111) SrTiO$_{3}$ surface was prepared by buffered-HF and heat treatment. The step height of the treated (111) SrTiO$_{3}$ is $\sim$2.25{\AA}, which is 1/3 of the diagonal of the cubic SrTiO$_{3}$ lattice along the [111] direction, consistent with the thickness of one AO$_{3}$/B (111) bilayer. LaAlO$_{3}$ was grown epitaxially in a layer-by-layer growth mode, with one oscillation of the reflection-high energy electron diffraction (RHEED) specular spot corresponding to this single step height. The (111) interfacial 2DEG shows a higher carrier concentration than LAO/STO (001) at room temperature. We find a LaAlO$_{3}$ critical thickness between 11.3 and 16 {\AA}, with the transition between insulating and conducting regimes broader than that of LAO/STO (001). Surface X-ray diffraction with COherent Bragg Rod Analysis (COBRA) has been carried out to explore the possible structural reconstruction of (111) SrTiO$_{3}$. We will discuss the origin of 2DEG at this polar-polar interface. This work has been done in collaboration with S. Ryu, C. W. Bark, T. Hernandez, M. S. Rzchowski, H. Zhou and D. D. Fong, T. R. Paudel and E.Y. Tsymbal.