Raman Study of Screw Dislocations in Epitaxial (001) BiFeO₃ Film
Yen-Chin Huang1, Yi-De Liou1*, Yu-You Chiou1, Yi-Chun Chen1, Yen-Lin Huang2, Ying-Hao Chu2
1Department of Physics, National Cheng Kung University, Tainan, Taiwan
2Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan
* presenting author:YI-DE LIOU, email:leave710584@gmail.com
Defects in materials play important roles in the engineering for electronic applications. Defects often degrade or generate new functionalities of material devices owing to unique properties around the defects, such as formation of ferromagnetism, hindrance of ferroelectric domain wall motion, or high conductivity. If we could effectively control these defects, it will provide opportunities for applications of novel functional devices. In this research, we studied the screw dislocations in the epitaxial (001) BiFeO₃ (BFO) film on SrTiO₃ (STO) substrate. From the results of piezoresponse force microscopy (PFM), there were spiral surfaces at the region of screw dislocations, and the in-plane ferroelectric polarization revolved around the center of the dislocation. By contrast, normal (001) BFO epitaxial films had [110] in-plane polarizations. According to previous references, the orientation of ferroelectric polarization is closely related to relative positions between oxygen octahedron and Bi3+ cations in the pseudo-cubic BFO, so the distortion of pseudo-cubic crystal structures will affect the ferroelectricity. We used polarized micro-Raman spectroscopy which was sensitive to crystal bondings to study differences between screw dislocations and non-screw regions. In Z(XX)Z̅ and Z(XY)Z̅ scattering configurations, the amounts and the resonant frequencies of Raman phonon modes are almost the same in these two areas, but relative intensities are varied. The phonon mode at 375 cm-1 related to the modulation of Fe-O-Fe bond angle had higher intensity at screw dislocation in Z(XX)Z̅. These results showed these two regions had similar symmetry, and the rotated in-plane polarizations came from the oxygen octahedral rotation. We also measured Raman spectra by rotating the polarization direction of the excited laser to compare the crystal symmetry between screw dislocations and non-screw regions.

Keywords: BiFeO3 (BFO), Screw Dislocation, Ferroelectricity, Micro-Raman Spectroscopy, Piezoresponse Force Microscopy (PFM)