Spectro-Microscopic Investigation of 2DEGs in KTaO₃ Interfaces

Emerging thin-film technology, and its electronic applications rise in parallel with the advances in their atomic-scale synthesis and characterization. Oxide based heterostructures are one key player in it, and they possess remarkable functionalities. The two-dimensional electron gases (2DEGs) in the 5d perovskite KTaO₃ (KTO) based heterostructures exhibits intriguing properties such as superconductivity [1-3], spin-charge interconversion which can have direct applications in spin-orbitronics [4], etc. Depending on the orientation of the substrate KTO, the physical properties in these systems are varied. For instance, the EuO/KTO (111) systems exhibit superconductivity below 2.2 K [1](0.9 K for an AlOx/KTO (111) [5]), while an EuO/KTO (110) exhibited superconductivity below 1.35 K [6]. The KTO (100) based systems were found to be superconducting only with ionic liquid gating, and at a very low transition temperature of 40 mK [2].<br/>In this path, here we study the Aluminium deposited- pristine KTO substrates in (111), (110), and (100) orientations and EuO/KTO (111) heterostructures with a focus on the emergence of 2DEG and how it influences the physical properties of interest in these. A microstructural and spectroscopic understanding of these interfaces is much appreciated in this direction. With this motivation, we conduct a study involving Scanning Transmission Electron Microscopy (STEM), Electron Energy Loss Spectroscopy (EELS) (Done at STEM Lab in LPS Orsay) and Standing Wave - Hard X-ray Photoemission Spectroscopy (SW-HAXPES) and Resonant Inelastic X-ray Scattering (RIXS) (Done at GALAXIES and SEXTANTS Beamlines in Synchrotron SOLEIL). STEM-EELS provides insights into the changes in the local chemistry and electronic structure together with an atomic scale resolution. On the other hand, SW-HAXPES is a depth selective and non-destructive technique, with a strong sensitivity to chemical differences in each layer. This combination enables us to extract the photoemission spectra from the interface separately. RIXS measurements with a linear polarized light sheds light on the orbital polarization, and also acts as a probe for the in-gap states. The objective is the characterization and determination of the spatial extension of 2DEG encompassing the possible Ferroelectric (FE) type distortion near the interface in these systems.<br/>STEM-EELS element map analysis on a superconducting AlOx/KTO (111) 2DEG system already shown less cation intermixing and a cleaner Al-KTO interface [5]. Our following studies on the structural aspect using STEM, 4D-STEM, geometrical phase analysis (GPA) and SW-HAXPES unraveled the plane termination, and lattice reconstruction along with a substantial FE-type distortion near the last unit cell. These distortions are not the same between the samples. However, on a larger scale, a tetragonal out-of-plane compression up to 1% is observed on the three differently oriented AlOx/KTO systems, and they extend upto 10 nm into the substrate from the interface. On the spectroscopic part, monochromated EELS measurements shown clear fine structure variations at the Ta-O_2,3 edge near the interface, which constitutes the real space imaging of the spectroscopic charge, harboring the 2DEG. RIXS measurements shown possible in gap states, and differential charge localization between the samples. Such a combined study with microscopy and spectroscopy unfolds the more interesting physics occurring at the interface, which is not accessible by many bulk probing techniques.<br/><br/>[1] Liu, Changjiang, et al. Science 371.6530: 716-721 (2021). [2] Ueno, K., et al. Nature nanotechnology 6.7: 408-412 (2011). [3] Chen, Zheng, et al. Physical Review Letters 126.2: 026802 (2021). [4] Vicente-Arche, Luis M., et al. Advanced Materials 33.43: 2102102 (2021). [5] Mallik, S., et al. Nature Communications 13.1 (2022): 4625. [6] Hua, Xiangyu, et al. NPJ Quantum Materials 7.1 (2022): 97.