Ferroelectric SrTiO₃ Induced by Energetic Ion Irradiation

SrTiO₃(STO) is a very important electric material and bulk STO has a centrosymmetric cubic structure at room temperature, and there is a cubic to tetragonal structural transition at 105 K [1]. The dielectric constant of SrTiO3 deviates from the classical Curie–Weiss law at low temperature, and it increases rapidly (up to 1.8 × 10⁴ at 1.4 K) as the temperature is reduced [2]. The low temperature electric behavior of STO approaches a ferroelectric phase transition; however, bulk STO is still paraelectric to the lowest temperature as a result of quantum fluctuations [3]. The ‘quantum paraelectrics’ or ‘incipient ferroelectronics’ of STO and other perovskite oxides have been a topic of considerable interest during the past few decades [3]. The ferroelectric transition of STO at low temperature can be induced by strains from lattice mismatch for thin films, chemical or isotopic substitution, electric field, pressure and controlled grain size. Due to important applications in electronic devices, the coupling between strain and ferroelectricity in STO has been intensively studied, and the relevant work is well illustrated in a recent review article [4]. By controlling the in-plane lattice strain, the critical temperature <i>T</i>c of ferroelectric transition in thin film SrTiO3 or superlattice can be profoundly enhanced [5,7,8], even to room temperature [6]. However, the biaxial strains in thin films depend on the choice of substrate and film thickness, which turns out to be challenging for the synthesis of uniformly strained films because of the undesirable relaxation that occur when the thickness of a sample exceeds the critical values.<br/>In this work [9,10], with energetic ion irradiation, bulk STO turnes form paraelectric to ferroelectric at room temperature due to the irradiation-induced strain. The strain profile in bulk STO is determined by symmetric X-ray diffraction and simulation. The irradiation induced atomic-level defects are characterized with high resolution TEM. The ferroelectric properties of the damaged zone is measured directly with microelectronic techniques. All the results suggest that the out-of-plane strains induced by ion bombardment make a local phase transformation of bulk STO from cubic to tetragonal and result in ferroelectricity at room temperature.<br/><br/>References<br/>[1] Rimai L and Demars G A 1962 Electron paramagnetic resonance of trivalent gadolinium ions in strontium and barium titanates <i>Phys. Rev. </i>127 702–10<br/>[2] Weaver H E 1959 Dielectric properties of single crystals of SrTiO3 at low temperatures <i>J. Phys. Chem. Solids </i>11 274–77<br/>[3] Müller K A and Burkard H 1979 SrTiO₃: an intrinsic quantum paraelectric below 4 K <i>Phys. Rev. </i>B 19 3593–602<br/>[4] Pai Y, Tylan-Tyler A, Irvin P and Levy J 2018 Physics of SrTiO₃–based heterostructures and nanostructures: a review <i>Rep. Prog. Phys. </i>81 036503<br/>[5] Uwe H and Sakudo T 1976 Stress-induced ferroelectricity and soft phonon mode in SrTiO₃ <i>Phys. Rev. </i>B 13 271<br/>[6] Haeni J H <i>et al </i>2004 Room-temperature ferroelectricity in strained SrTiO3 <i>Nature </i>430 758–61<br/>[7] Jang H W <i>et al </i>2010 Ferroelectricity in strain-free SrTiO3 thin films <i>Phys. Rev. Lett. </i>104 197601<br/>[8] Sirenko A A, Bernhard C, Golnik A, Clark A M, Hao J, Si W and Xi X X 2000 Soft-mode hardening in SrTiO3 thin films <i>Nature </i>404 373–6<br/>[9] Zhang FX, Xue H, Keum JK, Boulle A, Zhang Y and Weber WJ 2020, <i>J. Phys.: Condens Matter </i>32<br/>[10] S. Nan, F.X. Zhang et al 2023, (not published)