Cryogenic-Monochromated STEM-EELS Analysis of Symmetry Breaking and Local Ferroelectricity in SrTiO₃ Based-Heterostructures

The growth of oxide-based electronics is in a good pace, and the apt material knowledge obtained by advanced characterization techniques leverages the complexity in designing devices with intriguing properties. Knowledge of the atomic level arrangements in a material system enables us to make atomic level manipulations, thereby tweaking the electronics. This is made possible by one of the most advanced characterization techniques such as the Scanning Transmission Electron Microscopy (STEM). In this, in addition to obtaining a high-resolution image with good atomic phase contrast, one can have spectroscopic information representative of their electronic states through electron energy loss spectroscopy (EELS). Combining this with controllable external stimuli (Temperature, Photons, etc.) one can study a vast range of electronic and structural states in a chosen material. Some prototypical examples include temperature dependent metal-to-insulator transitions [1], photoinduced superconducting transitions [2], etc.<br/>Here we will be focusing on SrTiO₃ (STO) based heterostructures exhibiting incipient ferroelectricity as a result of broken symmetry near the interface [3]. Some examples are the Al/STO [4], and Ca-doped STO [5], and even NdNiO₃/STO system that have a metal to insulator transition range spanning 150-200K [6]. Understanding the electronic origins of it is of paramount importance, and so is studying its variation with external stimuli. STO is an intriguing system that undergoes a cubic to tetragonal antiferrodistortive phase transition below 105K, and undergoes another transition to become a quantum paraelectric below 37K [7,8]. The influence of these transitions on the interface properties are of significant interest, and a spectro-microscopic study spanning the range of these transitions is much appreciated. In this regard, we carry out RT and cryogenic (~110K) STEM-EELS measurements in a monochromated Nion CHROMATEM microscope equipped with a hybrid pixel direct electron detector Medipix3 [9].<br/>Our experiments in an NdNiO₂/STO demonstrated significant fine structure variations at the Ti-L_2,3 edge, on going from RT to ~110K. It has been previously reported that the cubic to tetragonal antiferrodistortive transition in STO occurs inhomogeneously, that the surface unit cells begin this transition at around ~150K, which possibly spans to the whole STO by ~105K [10]. Hence, the fine structure variations we see at around ~110K near the interface could be representative of this transition, thereby altering the interface properties. Characterizing such variations is enabled by high-energy resolution monochromated EELS, in combination with a cryogenic system. The resolution significantly overpasses previous studies reported in this direction, where, in addition a real space mapping of the fine structure variation couldn’t be done [11, 12]. An investigation in this direction paves the way to better understanding the interesting phenomena such as superconductivity and ferromagnetism that emerges at these interfaces.<br/><br/>[1] Preziosi, Daniele, et al. Nano letters 18.4 (2018): 2226-2232. [2] Yang, Zhen, et al. Small (2023): 2304146. [3] Haeni, J. H., et al. Nature 430.7001 (2004): 758-761.[4] Rödel, Tobias Chris, et al. Advanced Materials 28.10 (2016): 1976-1980. [5] Bréhin, Julien, et al. Physical Review Materials 4.4 (2020): 041002. [6] Palina, Natalia, et al. Nanoscale 9.18 (2017): 6094-6102. [7] Cowley, R. A. Physical Review 134.4A (1964): A981. [8] Müller, K. Alex, and H. Burkard. Physical Review B 19.7 (1979): 3593. [9] Tencé, Marcel, et al. Microscopy and Microanalysis 26.S2 (2020): 1940-1942. [10] Salman, Z., et al. Physical Review B 83.22 (2011): 224112. [11] Haruta, Mitsutaka, et al. Applied Physics Letters 119.23 (2021). [12] Rui, Xue, and Robert F. Klie. Applied Physics Letters 114.23 (2019).