Infrared ellipsometry study of the confined electrons in a high-mobility $\gamma$-Al$_2$O$_3$/SrTiO$_3$ heterostructure

TL;DR

This study uses infrared ellipsometry to analyze the properties of confined electrons in $\gamma$-Al$_2$O$_3$/SrTiO$_3$ heterostructures, revealing similarities to LaAlO$_3$/SrTiO$_3$ but with higher mobility and stronger confinement.

Contribution

First application of infrared ellipsometry to characterize confined electrons in $\gamma$-Al$_2$O$_3$/SrTiO$_3$ heterostructures, providing detailed depth profiles and mobility data.

Findings

Sheet carrier density similar to LaAlO$_3$/SrTiO$_3$ heterostructures

Higher electron mobility in $\gamma$-Al$_2$O$_3$/SrTiO$_3$

Electrons are more strongly confined in $\gamma$-Al$_2$O$_3$/SrTiO$_3$

Abstract

With infrared ellipsometry we studied the response of the confined electrons in a $\gamma$-Al$_2$O$_3$/SrTiO$_3$ heterostructures in which they originate predominantly from oxygen vacancies. From the analysis of a so-called Berreman mode, that develops near the highest longitudinal optical phonon mode of SrTiO$_3$, we derive the sheet carrier density, ${N_{s}}$, the mobility, $\mu$, and also the depth profile of the carrier concentration. Notably, we find that ${N_{s}}$ and the shape of the depth profile are similar as in LaAlO$_3$/SrTiO$_3$ heterostructures for which the itinerant carriers are believed to arise from a polar discontinuity. The main differences concern the higher mobility and a relatively stronger confinement of the electrons in $\gamma$-Al$_2$O$_3$/SrTiO$_3$