Defect Engineering in Complex Oxide Thin Films

Defect engineering in crystals opens new possibilities for discovering novel phenomena and fine-tuning beneficial properties. For perovskite transition metal oxides, however, controlling and understanding the role of the defects have been mainly limited to the oxygen vacancies. In this presentation, we report efficient defect engineering in complex perovskite oxide thin films.

In the first part of my talk, I will report point defect engineering using Pulsed Laser Epitaxy (PLE). In particular, we observed the selective formation of the different types of elemental vacancies and their individual roles in determining the physical properties of perovskite SrTiO₃ homotepitaxial thin films. Among various parameters of PLE, we found that laser energy, oxygen partial pressure (P(O₂)), and oxygen flow rate play distinct roles in influencing both oxygen and cation stoichiometry. In particular, laser energy was mainly responsible for changing the cation stoichiometry, while oxygen flow rate selectively tuned the oxygen vacancy concentration in SrTiO₃ thin films. Moreover, it was found that oxygen vacancy was mainly responsible for the electronic insulator-to-metal transition, but on the contrary to the common belief, did not influence the Sr vacancy induced cubic-to-tetragonal structural transition. The control of multiple phase transitions in complex oxides exploiting selective vacancy engineering provided by PLE opens an unprecedented opportunity for tailoring and understanding the materials properties.

In the second part, I will discuss the role of epitaxial strain as inducing an ordered defect crystal structure. In particular, we show that the ferromagnetic ordering observed in LaCoO₃ thin films is related to a lattice modulation due to the epitaxial strain, which results in a locally-ordered atomic structure. An unconventional strain relaxation behavior identified by strip-like lattice modulation pattern was responsible for the non-zero spin ground state [1,2]. We note that the unconventional strain relaxation did not involve any uncontrolled misfit dislocations or other defects. Our study provides a novel route to tailoring the atomic structure and magnetic properties of functional oxide heterostructures by strain, for various applications. In particular, the catalytic activity of LaCoO₃ thin films under different strain state has been studied to show the close correlation between the crystal structure and the chemical activity [3].

Choi先生は、ペロブスカイト酸化物の薄膜・人工超格子を使った2次元電子ガスや熱電材料の研究をされており、優れた研究成果を数多く出されております。下記に参考論文を列挙しますので、興味を持たれた方は是非本講演会に足をお運びください。

• “Resonant tunnelling in a quantum oxide superlattice” W. S. Choi et al., Nat. Commun. 6, 7424 (2015).
• “Thermopower enhancement by fractional layer control in 2D oxide superlattices” W. S. Choi et al., Adv. Mater. 26, 6701 (2014).
• “Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge” H. Jeen, W. S. Choi et al., Nat. Mater. 12, 1057 (2013).
• “Atomic Layer Engineering of Perovskite Oxides for Chemically Sharp Heterointerfaces” W. S. Choi et al., Adv. Mater. 24, 6423 (2012).
• “Strain-Induced Spin States in Atomically Ordered Cobaltites” W. S. Choi et al., Nano Lett. 12, 4966 (2012).
• “Wide bandgap tunability in complex transition metal oxides by site-specific substitution” W. S. Choi et al., Nat. Commun. 3, 689 (2012).

なお、参加者人数を把握するため、参加予定の方は、 katase@es.hokudai.ac.jp までご連絡頂けると幸いです。大変お手数をお掛けしますが、よろしくお願い致します。