Depth resolved lattice-charge coupling in epitaxial BiFeO3 thin film.

Hyeon Jun Lee,Yunseok Kim,Su Yong Lee,Sung Su Lee,Owoong Kwon,Hu Young Jeong,Do Young Noh,Ji Young Jo,Jeong Hun Kwak,Young-Min Kim,Albina Y Borisevich

doi:10.1038/srep38724

Abstract

For epitaxial films, a critical thickness (tc) can create a phenomenological interface between a strained bottom layer and a relaxed top layer. Here, we present an experimental report of how the tc in BiFeO3 thin films acts as a boundary to determine the crystalline phase, ferroelectricity, and piezoelectricity in 60 nm thick BiFeO3/SrRuO3/SrTiO3 substrate. We found larger Fe cation displacement of the relaxed layer than that of strained layer. In the time-resolved X-ray microdiffraction analyses, the piezoelectric response of the BiFeO3 film was resolved into a strained layer with an extremely low piezoelectric coefficient of 2.4 pm/V and a relaxed layer with a piezoelectric coefficient of 32 pm/V. The difference in the Fe displacements between the strained and relaxed layers is in good agreement with the differences in the piezoelectric coefficient due to the electromechanical coupling.

Highlights

For epitaxial films, a critical thickness can create a phenomenological interface between a strained bottom layer and a relaxed top layer
Ferroelectric thin films are suitable for use as a model system in which the effect of the thickness-dependent lattice parameters is critical for the representative functionalities, such as the remnant polarization and piezoelectricity
We report on the individual structural responses arising from the strained and relaxed layers of a ferroelectric BiFeO3 epitaxial thin film grown on SrRuO3/SrTiO3, including structural evolution, ferroelectricity, and piezoelectric responses

Summary

Introduction

A critical thickness (tc) can create a phenomenological interface between a strained bottom layer and a relaxed top layer. Experimental approaches for observing the ferroelectric response of strained layers to date have been focused on reducing the ferroelectric thin film thickness so that it is close to tc [refs 8–13], because conventional ferroelectric thin film analysis tools, such as piezoresponse force microscopy[14] and laser scanning vibrometers[15], only provide an average response that arises from both the highly strained and the relaxed layers. We report on the individual structural responses arising from the strained and relaxed layers of a ferroelectric BiFeO3 epitaxial thin film grown on SrRuO3/SrTiO3, including structural evolution, ferroelectricity, and piezoelectric responses. These were quantitatively investigated using scanning transmission electron www.nature.com/scientificreports/. We were able to differentiate the piezoelectric strain of the strained layer from that of the relaxed layer, which differed by one order of magnitude

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Conclusion