High-Mobility Two-Dimensional Electron Gases based on Strain Engineered Ferroelectric SrTiO₃ Thin Films

Two-dimensional electron gases (2DEGs) based on the quantum paraelectric SrTiO₃ display fascinating properties such as large electron mobilities, superconductivity and efficient spin-charge interconversion owing to their Rashba spin-orbit coupling.^1-3 However, such 2DEGs have almost exclusively been generated in SrTiO₃ single crystals, with the few attempts to replace crystals by heteroepitaxial SrTiO₃ thin films leading to low carrier mobilities. This is limiting the potential to integrate SrTiO₃ 2DEGs in future devices as well as the possibility to introduce additional functionalities specific to SrTiO₃ thin films, such as strain-induced ferroelectricity. Here, we use oxide molecular beam epitaxy to grow high quality strain-engineered SrTiO₃ films that are ferroelectric up to 170 K. We then generate a 2DEG by sputtering a thin Al layer and demonstrate an increase in both the low and room temperature mobilities by up to factor of four compared to earlier literature. Furthermore, through Raman spectroscopy and magneto-transport measurements, we show that the ferroelectric character is retained after 2DEG formation. These results thus qualify our samples as ferroelectric 2DEGs up to temperatures well above previous results based on Ca-SrTiO₃ substrates (~30 K)⁴, opening the way towards ferroelectric 2DEGs operating at room temperature.<br/><b>References :</b><br/>1. M. Bibes, J. E. Villegas, and A. Barthelemy, Adv. Phys. 60, 5 (2011).<br/>2. H. Y. Hwang, Y. Iwasa, M. Kawasaki, B. Keimer, N. Nagaosa, and Y. Tokura, Nat. Mater. 11, 103 (2012).<br/>3. S. Varotto, A. Johansson, B. Göbel, L. M. Vicente-Arche1, S. Mallik, J. Bréhin , R. Salazar, F. Bertran, P. Le Fèvre, N. Bergeal, J. Rault, I. Mertig & M. Bibes, Nat. Commun., 13, 6165 (2022).<br/>4. C. W. Rischau, X. Lin, C. P. Grams, D. Finck, S.Harms, J. Engelmayer, T. Lorenz, Y. Gallais, B. Fauqué, J. Hemberger, and K. Behnia, Nat Phys, 13, 643 (2017).