Skewed electronic band structure induced by electric polarization in ferroelectric BaTiO3

Norihiro Oshime,Mitsuru Itoh,Yosuke Hamasaki,Sou Yasuhara,Shintaro Yasui,Pierre-Eymeric Janolin,Eiji Ikenaga,Jun Kano,Tatsuo Fujii,Satoshi Hinokuma,Jean-Michel Kiat,Naoshi Ikeda,Akira Yasui,Takayoshi Yokoya,Hitoshi Osawa

doi:10.1038/s41598-020-67651-w

Abstract

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

Highlights

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs)
Gradual band structures can be described by the effect of electric polarization on ferroelectric materials, the common gradual band phenomenon has been discussed as an interfacial effect in a pn junction, which consists of non-polar semiconductors such as Si and G aAs13
Our results show that electronic core levels and valence band shift to a higher energy in accordance with the orientation of the electrical polarization in ferroelectric B aTiO3 (BTO) thin films

Summary

Introduction

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Electronic structures modulated by electric polarization yield a so-called ferroelectric band skewing (FEBS) structure. Our results show that electronic core levels and valence band shift to a higher energy in accordance with the orientation of the electrical polarization in ferroelectric B aTiO3 (BTO) thin films. In the valence band and core-level atomic orbitals, the binding energy of covalent states shifts more than that of non-covalent states.

Results

Conclusion