Phonon‐Enhanced Near‐Field Spectroscopy to Extract the Local Electronic Properties of Buried 2D Electron Systems in Oxide Heterostructures

J. Barnett, M.‐A. Rose, G. Ulrich, M. Lewin, B. Kästner, A. Hoehl, R. Dittmann, F. Gunkel and T. Taubner

Advanced Functional Materials, Early View (2020)
In the family of functional oxide materials, the interface between LaAlO3 and SrTiO3 (LAO/STO) is an interesting example, as both materials are large‐bandgap insulators in their bulk state but give rise to a confined 2D electron gas (2DEG) when combined through thin‐film deposition. While this 2DEG exhibits remarkable properties, its experimental investigation is mostly limited to destructive or non‐local (i.e. averaging over larger areas) methods until recently. Scanning near‐field optical microscopy is shown to overcome this limitation, detecting buried 2DEGs by using highly confined optical near‐fields. Here, a full spectroscopic approach with phonon‐enhancement and simulations based on the finite dipole model is combined to extract quantitative electronic properties of the interfacial LAO/STO 2DEG. This threefold improvement compared to previous work will enable the quantitative nanoscale, non‐destructive, sub‐surface analysis of complex oxide thin films and interfaces, as well as similar heterostructures.