Infrared ellipsometry study of the confined electrons in a high-mobility γ-Al₂O₃/SrTiO₃ heterostructure

¹ University of Fribourg, Department of Physics and Fribourg Center for Nanomaterials Chemin du Musée 3, CH-1700 Fribourg, Switzerland
² Department of Condensed Matter Physics, Faculty of Science and Central European Institute of Technology, Masaryk University - Kotlářska 2, 61137 Brno, Czech Republic
³ Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus 4000 Roskilde, Denmark

Received: 17 February 2016
Accepted: 3 March 2016

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.