Local analysis of the two-dimensional electron systems in oxide heterostructures by nanooptical microscopy and spectroscopy

• Applicants: Professor Dr. Thomas Taubner; Professor Dr.-Ing. Rainer Waser, since 1/2019
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 315025796

Owing to their unique physical properties, novel 2-dimensional electron gases (2DEGs) arising at interfaces between insulating oxides bear high potential for fundamental research in physics as well as for electronic applications.These 2DEGs are observed when a polar oxide (e.g. LaAlO3) is grown a non-polar transition metal oxide (e.g. SrTiO3) such that the two materials are interfacing each other with atomic precision. These 2DEGs arise because of an electronic-ionic interface reconstruction caused by the polarity discontinuity at the interface associated with an electrical dipole that grows with increasing layer thickness of the polar oxide.In this project, we aim to investigate and to understand the relation between microscopic structure and electronic properties of such interfacial 2DEGs with lateral resolution on the nanometer scale using scattering-type scanning near-field optical microscopy (s-SNOM), complemented by X-ray spectroscopy (XPEEM) and alternative scanning probe techniques.In particular, we aim to image directly inhomogeneities and local fluctuations in the 2DEG. A special focus of the project is the investigation of the local formation process of the electron gas (1) when locally traversing the critical layer thickness and (2) when locally varying the chemical interface termination. Furthermore, the project addresses (3) the local dielectric properties associated with the local defect structure of the involved materials adjacent to the interfacial 2DEG.These tasks are especially challenging since the 2DEG is located at an interface and is thus buried underneath an insulating oxide layer. s-SNOM is a unique technique that allows imaging the optical properties of such buried electron systems laterally resolved on the nanoscopic scale in a non-destructive manner. At same time, s-SNOM provides access to the local dielectric properties of the interface region through optical spectroscopy on the nanoscale.Complementing s-SNOM analysis of the buried 2DEG through the capping layer, this project addresses (4) the electron gas in cross-sectional sample geometry in order to probe the shape of the potential well at the interface.Further objectives of this project are methodical advances in the description of the optical near-field response of low-dimensional electron systems using the conducting oxide heterointerface as model system. In particular, a general model for the near-field interaction between s-SNOM probe and 2DEG has to be established (5). Moreover, a new (short wavelength) quantum cascade laser will be put into s-SNOM operation (6).The results of this project are likely to improve substantially the general understanding of interface reconstructions at oxide hetero-interfaces on the local scale. Methodical progress furthermore improves the general understanding of the optical near-field response of such systems and enables the s-SNOM technique to access other low dimensional systems in the future.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Dr. Felix Gunkel, until 1/2019