Softening of the optical phonon by reduced interatomic bonding strength without depolarization

Ruyue Cao; Qiao-Lin Yang; Hui-Xiong Deng; Su-Huai Wei; John Robertson; Jun-Wei Luo

doi:10.1038/s41586-024-08099-0

Softening of the optical phonon by reduced interatomic bonding strength without depolarization

Nature. 2024 Oct;634(8036):1080-1085. doi: 10.1038/s41586-024-08099-0. Epub 2024 Oct 30.

Authors

Ruyue Cao^{1

2

3}, Qiao-Lin Yang¹, Hui-Xiong Deng^{4

5}, Su-Huai Wei⁶, John Robertson³, Jun-Wei Luo^{7

8}

Affiliations

¹ State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China.
² Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, China.
³ Department of Engineering, University of Cambridge, Cambridge, UK.
⁴ State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China. [email protected].
⁵ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, China. [email protected].
⁶ Eastern Institute of Technology, Ningbo, China. [email protected].
⁷ State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China. [email protected].
⁸ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, China. [email protected].

PMID: 39478211
DOI: 10.1038/s41586-024-08099-0

Abstract

Softening of the transverse optical (TO) phonon, which could trigger ferroelectric phase transition, can usually be achieved by enhancing the long-range Coulomb interaction over the short-range bonding force¹, for example, by increasing the Born effective charges². However, it suffers from depolarization effects^3,4 as the induced ferroelectricity is suppressed on size reduction of the host materials towards high-density nanoscale electronics. Here, we present an alternative route to drive the TO phonon softening by showing that the abnormal soft TO phonon in rocksalt-structured ultrawide-bandgap BeO (ref. ⁵) is mainly induced by a substantial reduction in the short-range bonding interaction due to the Be-O bond stretching caused by an electron cloud-overlap-induced Coulomb repulsion between two adjacent oxygen ions that are arranged octahedrally around an extremely small Be ion. We further demonstrate the emergence of robust ferroelectricity in strain-induced perovskite BaZrO₃ and ultrathin HfO₂ and ZrO₂ films^6,7 grown epitaxially on lattice-mismatched SiO₂/Si substrate arising from the softening of the TO phonon driven by a reduction in the short-range bonding strength of biaxial strain-induced stretching bonds. These findings shed light on developing a unified theory for ferroelectricity enhancement in ultrathin films free from depolarization fields by tailoring chemical bonds using ionic radius differences, strains, doping and lattice distortions.