Switchable ferroelectric diode and photovoltaic effects in polycrystalline BiFeO3 thin films grown on transparent substrates
Introduction
BiFeO3 (BFO) as a well-known room-temperature multiferroic material possesses strong ferroelectricity and G-type anti-ferromagnetism simultaneously with high Curie temperature (TC = 1100 K) and Neel temperature (TN = 640 K) [1, 2]. It has been extensively studied in the past decade due to its interesting physical properties and potential applications in functional devices such as magnetoelectric devices [3], [4], [5], nonvolatile memories [6, 7], solar cells [8, 9]. Especially, the ferroelectric switchable diode and photovoltaic effects reported in BFO crystal or epitaxial thin films have attracted much attention [10–15]. These effects were firstly reported by Choi et al. [10] in a BFO bulk crystal. They observed the diode-like rectifying characteristics and the associated photovoltaic effect, and the forward direction of the diode can be reversed with the polarization switching. Subsequently, the ferroelectric switchable diode and resistive switching behaviors were observed in epitaxial BFO thin films, and the origin mechanisms of polarization modulated Schottky barriers were further proposed [12, 13]. Since then, a lot of studies have been carried out on the switchable ferroelectric diode and photovoltaic effects in epitaxial BFO thin films [9, 11, [16], [17], [18], [19], [20]. In a Pt/BFO/SrRuO3 capacitor structure, the ferroelectric depolarization field in the BFO thin film can drive the electron carriers close to one of the interfaces, which results in a band bending and the corresponding variation of potential barriers at the interfaces. In addition, the depolarization field may also effectively separate the photogenerated electron-hole pairs and leads to a substantial bulk photovoltaic effect, which is distinguished from the conventional photovoltaic effect in p-n junctions where the driving force for charge separation is arising from the built-in field in the very thin depletion layer at the junctions [21]. Thus, much higher open-circuit voltage (VOC) and photovoltaic efficiency can be expected in ferroelectrics due to the bulk effect of charge separation. Besides the depolarization field and interface potential barrier, Yang et al. [16] proposed that the electrostatic potential steps at the ferroelectric domain walls also have a significant effect on the separation of the photo-generated carrier. They observed a distinct photovoltaic response in epitaxial BFO thin films with ordered domain patterns at the different electrode and domain wall configurations. Moreover, photoconductivity [22], photoluminescence [23], and switchable photovoltaic effect [24, 25] of polycrystalline BFO thin films have been reported. However, the switchable diode and photovoltaic effects were observed almost entirely in epitaxial thin films or bulk crystals, and very few studies were performed on BFO thin films with transparent substrates. The studies on BFO thin films using transparent substrates have been reported mainly about the ferroelectricity and optical properties of the films [26, 27]. The ferroelectric switchable diode and the photovoltaic effect have not yet been reported in polycrystalline BFO thin films on transparent substrates.
In this paper, we report the fabrication of a polycrystalline BFO thin film on the transparent ITO/quartz substrate (ITO, Indium Tin Oxide) and the observation of ferroelectric switchable diode and photovoltaic effects in the polycrystalline BFO thin film. The using of transparent substrates might be the requirement for designing photodetectors or solar cells on account of light absorption, and the growth of polycrystalline BFO thin films on commercial transparent substrates is much easier and more economical than epitaxial films. Moreover, the using of polycrystalline thin films may facilitate the integration of BFO with traditional semiconductors [28].
Section snippets
Experimental procedures
The transparent conductive ITO (SnO2: In2O3, 10%:90%) layers and ferroelectric BFO thin films were deposited in sequence on quartz plates by pulsed laser deposition. A XeCl (308 nm) excimer laser was used at an energy density of 1.5 J/cm2 at 4 Hz. The BFO thin films were deposited at 540 °C with the oxygen pressure of 7 Pa. After the growth, the BFO films were annealed in situ for 20 min. Prior to the BFO thin films, the conductive ITO layers serving as the bottom electrodes were deposited at
Results and discussion
XRD patterns of the BFO and ITO thin films are shown in Fig. 1. In addition to the diffraction peak of ITO, there are several peaks all from BFO thin film, which indicates a polycrystalline structure for the BFO thin film. Both the ITO layer and the BFO layer are highly crystallized without impure parasitic phases such as Bi2Fe4O9 and Bi46Fe2O72 reported in other studies [30, 31]. The growth process of pure-phase BFO thin films generally needs strict deposition conditions, especially the oxygen
Conclusions
Polycrystalline BiFeO3 thin films have been grown on transparent substrates using the pulsed laser deposition technique. The polycrystalline BFO thin films show good ferroelectricity with a fully saturated hysteresis loop and a large remanent polarization of 60 μC/cm2. It has been demonstrated that the ferroelectric switchable diode and photovoltaic effect can be observed in the polycrystalline BiFeO3 thin films. Fabrication of polycrystalline thin films is more cost-efficient compared with
CRediT authorship contribution statement
Yong Zhou: Investigation, Resources, Writing - original draft, Writing - review & editing. Can Wang: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Supervision, Project administration, Funding acquisition. Shilu Tian: Investigation, Writing - original draft, Writing - review & editing. Xiaokang Yao: Investigation, Writing - original draft, Writing - review & editing. Chen Ge: Investigation, Writing - original draft, Writing - review & editing. Er-Jia Guo:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research was financially supported by National Key R&D Program of China (No. 2019YFA0308500), the National Basic Research Program of China (Grants Nos. 2017YFA0303604), the National Natural Science Foundation of China (Grants Nos. 11874412, 11674385, and 11721404), and the Youth Innovation Promotion Association of CAS (No. 2018008). The authors would like to thank Dr. Hongbao Yao for help with the SHG measurements.
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