PARADIM Highlight #99—External User Project (2024)
Venkatraman Gopalan, Long-Qing Chen, Susan Trolier-McKinstry, Roman Engel-Herbert (Penn State University), David A. Muller, and Darrell G. Schlom (Cornell University)
Like people, the behavior of materials can change dramatically when put in a tight squeeze. Sometimes materials even get better when squeezed, as is the case for the ferroelectric potassium niobate (KNbO3), a material with naturally built-in electric polarization and the opportunity to serve as an environmentally friendly and safe replacement to current lead-based ferroelectrics.
Figure: a) Thermodynamic phase-field simulations of biaxially compressed KNbO3 on SrTiO3, DyScO3, and GdScO3 marked by red, blue, and violet lines. Blue and violet stars indicate the onset of the tetragonal-to-monoclinic transition temperature observed experimentally. b) Rate of Curie temperature change with strain vs. change in remanent polarization with strain plotted for KNbO3 compared to related materials. c) Cartoon of MBE ≈ “atomic spray painting” in action
As the figure shows, KNbO3 is predicted to be far more sensitive to being squeezed than common ferroelectrics. Aided by PARADIM, researchers from Penn State have been able to put these predictions of “strain tuning” to the test. The group used PARADIM’s signature molecular-beam epitaxy system (MBE) to grow KNbO3 thin films by spraying potassium, niobium, and oxygen atoms on an underlying crystal with the same structural motif, but slightly smaller spacing between its atoms. The achieved strains exceeding 1% are found to stabilize the polarization over a much wider temperature range in agreement with theory.
Strong coupling between polarization (P) and strain (ɛ) in ferroelectric complex oxides offers unique opportunities to dramatically tune their properties. Here colossal strain tuning of ferroelectricity in epitaxial KNbO3 thin films grown by sub-oxide molecular beam epitaxy is demonstrated. While bulk KNbO3 exhibits three ferroelectric transitions and a Curie temperature (Tc) of ≈676 K, phase-field modeling predicts that a biaxial strain of as little as −0.6% pushes its Tc > 975 K, its decomposition temperature in air, and for −1.4% strain, to Tc > 1325 K, its melting point. Furthermore, a strain of −1.5% can stabilize a single phase throughout the entire temperature range of its stability. A combination of temperature-dependent second harmonic generation measurements, synchrotron-based X-ray reciprocal space mapping, ferroelectric measurements, and transmission electron microscopy reveal a single tetragonal phase from 10 K to 975 K, an enhancement of ≈46% in the tetragonal phase remanent polarization (Pr), and a ≈200% enhancement in its optical second harmonic generation coefficients over bulk values. These properties in a lead-free system, but with properties comparable or superior to lead-based systems, make it an attractive candidate for applications ranging from high-temperature ferroelectric memory to cryogenic temperature quantum computing.
Bulk unstrained KNbO3 crystals have many phase transitions, which can be a negative thing for applications. Above 676 K, it loses its ferroelectric polarity; below it it becomes a tetragonal phase; this temperature is called the Curie temperature. There are two more phase transitions, one at 492 K to an orthorhombic phase and another at 223 K below which it is in rhombohedral phase as we cool down. (The names here for the phases refer to how the atoms are arranged geometrically within these crystals.)
This achievement showed that when the KNbO3 films were compressed by a little over half a percent in the plane of the film's growth, something dramatic happened. The Curie temperature is pushed out beyond 975 K where the material decomposes. In other words, the material stays polar up until it falls apart. (Theory predicts that if it had not fallen apart, it would have continued to retain its ferroelectrics up to ungodly temperatures well over 3000K, half the temperature of the surface of the sun!) Furthermore, the other phases disappear as well down to low temperatures with about 1.5% stretch. In other words, only a single phase, namely the tetragonal phase, exists at all temperatures from the lowest to the highest; and the rest are all gone!
The built-in electrical polarity, which is of great importance for the applications mentioned above, increases by 46%. An optical property called second harmonic generation (which changes the color of light incident on the crystal) is enhanced by 200%. A lot of applications require high temperatures, such as say jet engines, or space launches or energy plants or computing and memory where environments are hot etc. Being able to use ferroelectrics there becomes a challenge. There are other ferroelectrics with high curie temperatures, but a combination of improved properties makes KNbO3 to be a promising candidate if further development of the electrical properties of the material are conducted. The two most important enhancements would be (1) being able to grow on silicon instead of exotic oxide substrates and (2) growing films with lower electrical loss by improving the film growth.
PARADIM’s role was to help these researchers from Penn State realize the material that they have been dreaming about for decades. Professor Gopalan made thin films of this material during his PhD work, so knows just how challenging it can be to grow in the form of thin films. In this work, PARADIM helped Professor Gopalan and his student grow this material by a technique that had never before been used to grow KNbO3, but works well. The method is a new variant of MBE known as suboxide MBE. MBE is the gold-standard preparation method for many other electronic materials with excellent perfection. For KNbO3, conventional MBE would involve a molecular beam of potassium (K), a molecular beam of niobium (Nb), and a molecular beam of oxygen (or some other oxidant). The reason that MBE had not been previously applied to KNbO3 is for two reasons. First, potassium is highly flammable and producing a molecular beam of it is technically challenging due to safety. Second, niobium has a quite low vapor pressure, requiring quite high temperatures to evaporate. PARADIM helped Professor Gopalan and his student overcome both challenges. First PARADIM developed a way to handle potassium safely by mixing it with indium. Unlike pure potassium, which can burst into flames when exposed to air, the potassium+indium mixture is 80% indium and is stable in air. This makes it easy to handle to insert the potassium+indium mixture into the MBE machine. Now, when this mixture is heated in the ultra-high vacuum environment inside of the MBE, only the potassium evaporates from the potassium+indium mixture because the vapor pressure of potassium is 10 billion times higher than that of indium. That was one key trick. The other involved the niobium molecular beam. The prior work of Prof. Zi-Kui Liu at Penn State working with PARADIM researchers (K.M. Adkison, S-L. Shang, B.J. Bocklund, D. Klimm, D.G. Schlom, and Z.K. Liu, “Suitability of Binary Oxides for Molecular-Beam Epitaxy Source Materials: A Comprehensive Thermodynamic Analysis,” APL Materials 8 (2020) 081110), showed through thermodynamic calculations that a lower-temperature alternative to evaporating elemental niobium is to heat up niobium oxide (Nb2O5). When heated in vacuum, Nb2O5 gives off a molecular beam of NbO2 at a sufficiently low temperature making it possible to generate a far more stable Nb-containing molecular beam than with elemental niobium, enabling the thin-film growth team of the PARADIM user and PARADIM PhD trainee to grow KNbO3 for the first time by suboxide MBE. Not only are these the first KNbO3 films grown by any type of MBE, they have the highest structural quality of any KNbO3 thin films ever grown. This advance put PARADIM user Prof. Gopalan in a position to test the theoretical predictions of how a strain imposed by an underlying substrate could improve the properties of the ferroelectric KNbO3. The theory members of his team (Prof. Long-Qing Chen and his student from Penn State) provided a map of how to improve the properties of KNbO3, which in combination with PARADIM’s ability to deposit high-quality films of KNbO3 put the highly collaborative team led by Prof. Gopalan in a unique position to complete this exciting research.
The work was initiated by Prof. V. Gopalan at Penn State University via a successful PARADIM proposal and evolved into an extensive collaboration including members of the PARADIM In-House Team, CHESS, Advanced Photon Source at Argonne National Lab, SLAC, and the Ohio State University among others.
The work is featured on Penn State News and PHYS.org
S. Hazra, T. Schwaigert, A. Ross, H. Lu, U. Saha, V. Trinquet, B. Akkopru-Akgun, B.Z. Gregory, A. Mangu, S. Sarker, T. Kuznetsova, S. Sarker, X. Li, M.R. Barone, X. Xu, J.W. Freeland, R. Engel-Herbert, A.M. Lindenberg, A. Singer, S. Trolier-McKinstry, D.A. Muller, G.-M. Rignanese, S. Salmani-Rezaie, V.A. Stoica, A. Gruverman, L.-Q. Chen, D.G. Schlom, and V. Gopalan, "Colossal Strain Tuning of Ferroelectric Transitions in KNbO3 Thin Films," Adv. Mater. 36, 2408664 (2024). DOI: 10.1002/adma.202408664
S.H. and T.S. contributed equally to this work. S.H., A.M., J.W.F., A.M.L., V.A.S., and V.G. acknowledge support from the DOE-BES grant DE-SC0012375 for thin film growth efforts (S.H.), optical SHG experiments (S.H., V.G.), x-ray characterization (S.H., A.M., J.W.F., V.A.S., V.G.), and manuscript preparation. This work made use of the synthesis and electron microscopy facilities of the Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), which are supported by the National Science Foundation under Cooperative Agreement No. DMR-2039380. TS, MRB, DAM, and DGS, also acknowledge the support of the National Science Foundation under Cooperative Agreement No. DMR-2039380. A.R., U.S., T.K., R.E.H., L.Q.C., and V.G. acknowledge support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-SC0020145 for the phase-field simulations (A.R., U.S., L.Q.C.) and part of the growth efforts (T.K., R.E.H, and V.G.). A.R. also acknowledges the support of the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE1255832. The phase-field simulations in this work were performed using Bridges-2 at the Pittsburg Supercomputing Center through allocation MAT230041 from the ACCESS program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603 and #2138296.The PFM studies at UNL have been supported by the UNL Grand Challenges catalyst award “Quantum Approaches addressing Global Threats”. B.Z.G. and A.S. acknowledge support for the generation of 3D reciprocal space maps from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-SC0019414. V.T. acknowledges the support from the FRS-FNRS through an FRIA Grant. Computational resources have been provided by the supercomputing facilities of the Université Catholique de Louvain (CISM/UCL) and the Consortium des Équipements de Calcul Intensif en Fédération Wallonie Bruxelles (CÉCI) funded by the Fond de la Recherche Scientifique de Belgique (F.R.S.-FNRS) under convention 2.5020.11 and by the Walloon Region. The present research benefited from computational resources made available on Lucia, the Tier-1 supercomputer of the Walloon Region, infrastructure funded by the Walloon Region under grant agreement No. 1910247. Research conducted at the Center for High-Energy X-ray Science (CHEXS) was supported by the National Science Foundation (BIO, ENG, and MPS Directorates) under award DMR-1829070. X.L and X.X. acknowledge the support of Intel. This work made use of a Helios FIB supported by NSF (grant no. DMR-1539918) and the Cornell Center for Materials Research (CCMR) Shared Facilities, which were supported through the NSF MRSEC Program (grant no. DMR-1719875). The Thermo Fisher Spectra 300 X-CFEG was acquired with support from PARADIM (NSF MIP DMR-2039380) and Cornell University. Part of the Electron microscopy experiment was performed at the Center for Electron Microscopy and Analysis (CEMAS) at The Ohio State University. S.S. and V.G. acknowledge support from National Science Foundation grant number NSF DMR-2210933 for optical SHG characterization. For electrical leakage measurements, the authors acknowledge the Center for 3D Ferroelectric Microelectronics (3DFeM), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, under Award No. DE-SC0021118.
Data availability
All data associated with the use of PARADIM facilities to grow and characterize the new material—some 133 GB are publicly available to fuel future materials discovery at https://doi.org/10.34863/fs5e-s772 facilitated through the PARADIM Data Collective.
