Infrared ellipsometry study of the confined electrons in a high-mobility <i>γ</i> -Al ₂ O ₃ /SrTiO ₃ heterostructure

Abstract

With infrared ellipsometry we studied the response of the confined electrons in γ-Al2O3/SrTiO3 (GAO/STO) 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 SrTiO3, we derive the sheet carrier density, N s , the mobility, μ, and the depth profile of the carrier concentration. Notably, we find that N s and the shape of the depth profile are similar as in LaAlO3/SrTiO3 (LAO/STO) heterostructures for which the itinerant carriers are believed to arise from a polar discontinuity. Despite an order of magnitude higher mobility in GAO/STO, as obtained from transport measurements, the derived mobility in the infrared range exhibits only a twofold increase. We interpret this finding in terms of the polaronic nature of the confined charge carriers in GAO/STO and LAO/STO which leads to a strong, frequency-dependent interaction with the STO phonons.