Elsevier

Journal of Alloys and Compounds

Volume 803, 30 September 2019, Pages 942-949
Journal of Alloys and Compounds

Orientation dependent leakage current behaviors and ferroelectric polarizations of off-axis sputtered BiFeO3 thin films

https://doi.org/10.1016/j.jallcom.2019.06.343Get rights and content

Highlights

  • Epitaxial BFO thin films with (100), (110) and (111) orientations were grown by off-axis sputtering.

  • The BFO thin films showed quite different electrical properties.

  • Various conduction mechanisms were revealed in these orientation-engineered BFO thin films.

  • A large remnant polarization of Pr ∼78 μC/cm2 were achieved in the BFO(100) thin film.

Abstract

Epitaxial BiFeO3 thin films with (100), (110) and (111) orientations were grown on the SrRuO3-buffered SrTiO3 substrates by using an off-axis magnetron sputtering. Unlike the BiFeO3(110) and BiFeO3(111) thin films that exhibited a single rhombohedral phase structure, a dominant rhombohedral phase accompanying with a small amount of tetragonal phase was identified in the BiFeO3(100) thin film. In particular, the leakage currents and ferroelectric polarizations of sputtered BiFeO3 thin films were focused on and these films showed utterly different current density-electric field (J-E) behaviors whether in the positive or negative electric field. Among the three films, the ferroelectric polarization of the BiFeO3(100) thin film presented a good frequency stability and had the maximum remnant polarization of Pr ∼ 78 μC/cm2 @ 10 kHz, which could be further demonstrated by pulsed polarizations of films. The distinct differences in electrical properties of orientation-engineered BiFeO3 thin films in present case can be attributed to their different crystallographic orientations and microstructures.

Introduction

Multiferroics are a class of exciting multifunctional materials that have two or more versatile properties at the same time. Of particular interest is the perovskite BiFeO3 (BFO), which is the only known single-phased multiferroic oxide to date that shows a coexistence of room temperature ferroelectric, antiferromagnetic and ferroelastic behaviors as it has a high Curie temperature (Tc ∼ 830 °C) and a high Néel temperature (TN ∼ 370 °C) [1]. Since the BFO exhibits a coupling between the ferroelectric and magnetic order parameters, giving rise to a noticeable magnetoelectric effect, and thus providing an opportunity for applications in spintronics and magnetoelectronics [[2], [3], [4]]. Moreover, the excellent electrical properties, such as a large remnant polarization with Pr of ∼100 μC/cm2, as well as an outstanding intrinsic/extrinsic piezoelectric response, have been unfolded in the thin film BFO reported in many experimental and theoretical works [[5], [6], [7], [8]], which can even be comparable to those of the Pb(Zr,Ti)O3 (PZT) systems. All these desirable features allow the BFO thin films to be extensively studied and therefore offer a great potential in lead-free multifuctional devices, such as nonvolatile ferroelectric memories and microelectromechanical systems (MEMS) [2,9].

At present, however, current applications of BFO thin films are still limited because it is difficult to control the films’ electrical qualities. Inevitably, the BFO thin films will produce some impure phases and charge defects, e.g. parasitic or secondary phases [2,10], cation/anion vacancies [11,12], owing to the reduction of bismuth oxide and the fluctuated valences of Fe ions. These growth defects are apt to induce high leakage current in BFO thin film, thus leading to a great deterioration of ferroelectric polarization and some related electrical properties. A series of in-depth studies on leakage current have been conducted to improve the electrical qualities of BFO thin films, and knowledge of their conduction mechanisms is a vital step towards solving the leakage current issues. Currently, some charge transport models have been proposed to reveal the underlying leakage behaviors of BFO thin films. For example, both n-type and p-type interface-limited Schottky emissions were verified as the dominant conduction mechanism in BFO thin films with different defect chemistries [13,14], while bulk-limited space-charge-limited-current and Pool-Frenkel emission, as well as interface-limited Fowler-Nordheim tunneling were proposed to clarify the leakage behaviors of BFO thin films under the influences of temperature and electric field [15,16]. And of course these conduction mechanisms mentioned above have also been demonstrated in the ion-modified BFO thin films and nanocomposite BFO thin films [[17], [18], [19]].

In addition, it is generally known that the performances of orientation-engineered ferroelectric films show a great difference due to an inherent anisotropy, which is of great value in regard to the materials design for microelectronic devices. Thus it is necessary to regulate the film's crystallographic orientation in the view of theoretical and technical interests. Usually the oriented or epitaxial growth of ferroelectric thin film can be achieved by matching the film with the acceptable bottom electrode (or buffer layer) and substrate. Surely the BFO thin films are no exception and large anisotropies in multiferroic behaviors, such as ferroelectric polarization, piezoelectric response and magnetization, have been demonstrated in orientation-engineered BFO thin films as a result of particular polarization vector and magnetic easy plane [20]. Indeed, several different orientations have been reported in BFO thin films and they exhibit rather different ferroelectric, magnetic and piezoelectric characteristics, as well as fatigue behaviors [[20], [21], [22], [23]].

Although there have been some reports on orientation-engineered BFO thin films together with their basic electrical/magnetic properties, there is still limited coverage involving leakage mechanisms and polarization evolutions of BFO thin films with different crystallographic orientations. How the film's microstructures including phase structure and morphology evolve with the orientation and how the resulting microstructures affect electrical properties of films have been rarely reported. In this work, the BFO thin films were epitaxially grown on the SrRuO3-buffered SrTiO3(100), SrTiO3(110), SrTiO3(111) substrates by magnetron sputtering, corresponding to the BFO(100), BFO(110), BFO(111) thin films, respectively. This creates a good opportunity to investigate the crystallographic orientation dependent electrical properties of sputtered BFO thin films, especially focusing on the underlying leakage behaviors and polarization evolutions.

Section snippets

Experimental

In present experiment, an off-axis magnetron sputtering was employed to grow the BFO thin films on SrTiO3 (STO) substrates with (100), (110) and (111) orientations in a multi-target vacuum chamber. An ultra-low base pressure was set at 2.0 × 10−4 Pa and the substrate was heated to 650 °C before growing BFO thin films. The SrRuO3 (SRO) bottom electrode was firstly deposited on the STO substrate in a mixed Ar/O2 atmosphere with a flow ratio of 4:1. SRO is one of the most commonly used conductive

Results and discussions

Fig. 1(a) shows the θ-2θ XRD patterns of BFO thin films grown on the STO(100), STO(110) and STO(111) substrates with a bottom electrode of SRO. All the BFO thin films show an expected epitaxial growth, i.e., producing the BFO(100), BFO(110) and BFO(111) thin films. The corresponding in-plane epitaxial relationships,1 as illustrated in Fig. 1(b), are BFO(100) ǁ STO(100), BFO(110) ǁ STO(110), BFO(111) ǁ

Conclusions

In present study, epitaxial BFO thin films with (100), (110) and (111) orientations were grown on STO substrates using an off-axis sputtering technique. In contrast to the BFO(110) thin film having a small amount of impurities, the BFO(100) and BFO(111) thin films were fully crystallized into pure perovskite phases with well-developed microstructures, leading to a reduced leakage current and a low loss, especially the lowest current density (3.3 × 10−3A/cm2 @ 250 kV/cm) and dielectric loss

Acknowledgements

H. Zhu acknowledges the financial support of higher education science and technology project of Shandong Province (Grant No. J18KA023). H. Zhu, Y. Zhao and Y. Wang acknowledge the financial support of the Fundamental Research Fund of Shandong University (Grant No. 2016JC036).

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