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DOI: 10.1039/C6RA09326D (Paper) RSC Adv., 2016, 6, 59150-59154

Effect of insertion of low leakage polar layer on leakage current and multiferroic properties of BiFeO3/BaTiO3 multilayer structure†

Savita Sharmaac, Monika Tomarb, Ashok Kumard, Nitin K. Puric and Vinay Gupta*a
aDepartment of Physics and Astrophysics, University of Delhi, Delhi, India. E-mail: drguptavinay@gmail.com; vgupta@physics.du.ac.in
bPhysics Department, Miranda House, University of Delhi, Delhi, India
cDepartment of Applied Physics, Delhi Technological University, Delhi, India
dCSIR-National Physical Laboratory, Dr K. S. Krishnan Marg, Delhi, India

Received 11th April 2016 , Accepted 8th June 2016

First published on 13th June 2016

Abstract

Polycrystalline BiFeO3 is a well established leaky room temperature multiferroic material. The present work focuses on the realization of high end functionalities such as polarization, low leakage current and high magnetization in a sandwich structure of BaTiO3 and BiFeO3. Interestingly, it was found that the BaTiO3 barrier layer effectively reduces the leakage current which in turn significantly improves the ferroelectric and ferromagnetic properties. The increase in the number of BaTiO3 barrier layers stops the charge carriers for long range mobility. X-ray diffraction (XRD) analysis suggests that an increase in the number of BaTiO3 layers significantly increases the stress modulus in the both the constituent layers along c-axis and tetragonality in the BiFeO3 films which helps in improving the multiferroic properties in multilayered structure. The saturation polarization (Ps) and magnetization (Ms) significantly increased from 18 to 99 μC cm−2 and 28 to 95 emu cm−3 respectively with increase in number of barrier layers from 2 to 6. The BiFeO3/BaTiO3 multilayered structure with enhanced multiferroic properties finds potential application in the tunable devices.

Introduction

Multiferroic materials that possess simultaneous existence of more than one ferroic parameter are of great scientific and technological importance due to their magnetoelectric (ME) properties originating from coupling between the ferroelectric and ferromagnetic orders. Among multiferroic oxides, bismuth ferrite (BiFeO3) is the only multiferroic material at room temperature with high Curie (TC ∼ 1103 K) and Neel (TN ∼ 643 K) temperature possessing both ferroelectric and ferromagnetic properties, making it suitable for high temperature applications.1–5 Inspite of the interesting multiferroic features exhibited by BiFeO3, there are some drawbacks involved, like high leakage current and complex magnetic ordering which hinders its applications in electronic devices. Barium titanate (BaTiO3) is a promising perovskite material, well established for its versatile ferroelectric, non-linear optic, piezoelectric and dielectric properties.6–10 However, multiferroic properties in BaTiO3 are not established due to the absence of ferromagnetic orders. It may be seen from the recent research that the properties may be enhanced by material engineering including doping with other metal ions or making multilayered structures.11–17 In the multilayered thin film structures, two different ferroic order materials allow modifications of the properties of parent materials by strain engineering.12 Lorenz et al. showed that BaTiO3–BiFeO3 multilayers show the highest magnetization of 2.3 emu cm−3, due to interface magnetic moments and exchange coupling.12 There have been few reports in recent years on improvement in multiferroic properties in multilayered thin film structures.12,13 Touplet et al. and Yang et al. reported an enhancement in magnetic properties by employing BiFeO3/BaTiO3 multilayer system.18,19 Detailed studies of electrical and magnetic properties of the multilayered thin film structures are required to understand their appropriate device applications. BiFeO3/BaTiO3 multilayers provide with a promising alternative in exploring the full potential of BiFeO3 single layer thin film while maintaining the ferroelectric nature.18,19 The focus is to couple the BiFeO3 film with a less leaky material in multilayer like structure. PLD is advantageous in terms of excellent control over stoichiometric composition of oxide films with complex composition, especially BiFeO3 which contains highly volatile bismuth. As composition is the key factor to determine crystal structure, PLD technique has attractive advantages in controlling the desirable pyrochlore structure of BaTiO3 and BiFeO3 thin films by tuning the substrate temperature, deposition rate and time.20,21 Also PLD offers flexibility to fabricate different multilayers heterostructures with advantage for high melting point oxide materials. The study of layered and composite multiferroic heterostructures is essential for searching new candidate multiferroics. In this study, we report the structural, electrical, ferroelectric and ferromagnetic properties of BiFeO3/BaTiO3 multilayered thin films prepared by PLD technique.

Experimental

The multilayered thin films of BiFeO3/BaTiO3 were deposited on Pt/Ti/SiO2/Si and Si substrates by ablating ceramic targets of BiFeO3 and BaTiO3 respectively, using a Nd:YAG pulsed laser (wavelength of 266 nm at 4th harmonic). Prior to multilayer film deposition, Pt metal thin layer of 70 nm thickness was deposited over the passivated Si (SiO2/Si) substrate by E-beam evaporation technique, a buffer layer of Ti (20 nm) was deposited in situ over SiO2/Si substrate prior to Pt deposition to improve the adhesion of Pt on SiO2/Si substrate. Typical PLD deposition conditions are reported elsewhere.22 Total thickness of the multilayered structures was kept fixed at 350 nm with varying number and thickness of individual BaTiO3 and BiFeO3 layers at a substrate temperature of 750 °C. Thickness of (BiFeO3/BaTiO3) multilayered thin films was measured using surface profiler (Dektak 150). The number of layers were varied from two to seven and whilst keeping BaTiO3 as the bottom layer (BiFeO3/BaTiO3/Pt/SiO2/Si, BaTiO3/BiFeO3/BaTiO3/Pt/SiO2/Si, BiFeO3/BaTiO3/BiFeO3/BaTiO3/Pt/SiO2/Si, BaTiO3/BiFeO3/BaTiO3/BiFeO3/BaTiO3/Pt/SiO2/Si, BiFeO3/BaTiO3/BiFeO3/BaTiO3/BiFeO3/BaTiO3/Pt/SiO2/Si, BaTiO3/BiFeO3/BaTiO3/BiFeO3/BaTiO3/BiFeO3/BaTiO3/Pt/SiO2/Si). For the ferroelectric measurements also passivated Si substrate having (100) orientation [SiO2/Si (100)] has been used. The Pt/Ti bilayer has been fabricated on SiO2/Si surface prior to the deposition of BiFeO3/BaTiO3 multilayer system. The ferroelectric measurements have been made in MIM configuration where Pt/Ti deposited on SiO2/Si was considered as bottom electrode. Au circular dots (600 μm diameter, 40 nm thickness) were deposited by thermal evaporation technique through a shadow mask for the top electrodes. The schematic of the MIM capacitor configuration (Au/BiFeO3/BaTiO3/BiFeO3/BaTiO3/BiFeO3/BaTiO3/Pt/SiO2/Si) is shown in Fig. S1 (ESI†). For the magnetic measurements, multilayered thin film samples were prepared on Si (100) substrate under similar deposition conditions. The corresponding magnetic hysteresis loops were measured at room temperature using the vibrating sample magnetometer. Structural characterization of films was carried out using X-ray diffraction (XRD) analysis (Bruker D8). A Radiant Technology Precision ferroelectric workstation was used to measure the room temperature ferroelectric polarization hysteresis of the prepared samples in MIM capacitor configuration at a frequency of 1 kHz and at an applied bias of 10 V.

Results and discussion

Fig. 1(a) shows the XRD (θ–2θ) diffraction patterns of the prepared BiFeO3/BaTiO3 multilayer structure with varying number of layers (from two to seven) deposited on Si substrates. It can be seen from the XRD patterns that all multilayer structures are polycrystalline having pure perovskite structure of BaTiO3 and BiFeO3 without having any impurity and secondary phase formation (Fig. 1(a)). All XRD peaks correspond to both the tetragonal and rhombohedral structures of BaTiO3 and BiFeO3 respectively. The lattice parameters (a and c), c/a distortion ratio along with induced stress along ‘c’ axis in both the BaTiO3 and BiFeO3 thin films in multilayer structure, have been calculated from the XRD data and results are summarized in Table ST 1 (ESI†). The value of stress modulus (%) of BaTiO3 and BiFeO3 was found to be about 1.115% and 3.980% respectively for the BiFeO3/BaTiO3 six layer structure, and is relatively large. The large value of stress modulus present in the BiFeO3 layer increases the lattice distortion and thus, may lead to an improvement in the ferroelectric and multiferroic properties of corresponding BiFeO3/BaTiO3 multilayer structure. The variation in c/a distortion ratio in both BiFeO3 and BaTiO3 thin films in the prepared multilayer structures as a function of number of layers from 2 to 7 is shown in Fig. 1(b). It may be noted from Fig. 1(b) that the c/a ratio for both BaTiO3 and BiFeO3 thin films in multilayer structure (n > 2) shows an oscillating behavior with consecutive deposition of BaTiO3 and BiFeO3 layers (i.e. with increasing number from layers 3 to 7). However, the lattice distortion was found to increase in magnitude continuously with every layer of BiFeO3 thin film in the multilayer structure (i.e. for 4 and 6 layer system). The large lattice distortion observed for multilayer structure having 6 layers of BaTiO3 and BiFeO3 was expected to result in improved ferroelectric property.
image file: c6ra09326d-f1.tif
Fig. 1 (a) XRD pattern of BiFeO3/BaTiO3 multilayer structures having different number of BiFeO3 and BaTiO3 layers. (b) Variation in c/a distortion ratio in both BiFeO3 and BaTiO3 thin films in the prepared BiFeO3/BaTiO3 multilayer structures.

Fig. 2 represents the plots of current density versus applied electric field (J–E) characteristics for the BiFeO3/BaTiO3 multilayer structures recorded at room temperature in MFM configuration as a function of number of layers (BiFeO3 and BaTiO3) from two to seven. It can be seen from Fig. 2 that the J–E curves are symmetrical and the magnitude of leakage current decreases appreciably with increase in number of layers from two to six and thereafter increases in the case of seven layer BiFeO3/BaTiO3 system. It is important to point out that the leakage current reduced by many orders of magnitude in case of multilayer BiFeO3/BaTiO3 structures as compared to that reported for bare BiFeO3 thin film by other workers.20,21 Fig. 2 inset shows the variation in the leakage current density for the BiFeO3/BaTiO3 multilayer structure as a function of number of constituent layers measured at an applied bias of 285 kV cm−1. Leakage current was found to be minimum for the BiFeO3/BaTiO3 multilayer capacitor having 6 layers and was found to be about 3.18 × 10−8 A cm−2 at an applied field of 285 kV cm−1. However, the corresponding value of leakage current in the case of seven layer BiFeO3/BaTiO3 structure was about 5.16 × 10−6 A cm−2.


image file: c6ra09326d-f2.tif
Fig. 2 J–E characteristics of BiFeO3/BaTiO3 multilayer structure having different number of constituent layers. Inset shows variation in leakage current density with number of measured at an applied bias of 285 kV cm−1 for BiFeO3/BaTiO3 layers.

The log–log plots of J–E curves for all the multilayer structure having different numbers of layers are shown in Fig. 3. The log[thin space (1/6-em)]J versus log[thin space (1/6-em)]E plot was found to be linear for all prepared multilayer structures (Fig. 3). With increasing number of stacking layers of BiFeO3 and BaTiO3 thin films in the multilayer structures, the leakage current measured at an applied bias decreases continuously (Fig. 3). The value of slope of the linear plots shown in Fig. 3 decreases from 2 to 1.82 with increase in the number of layers from 2 to 6 layers and further increases to 2.20 for seven layer structure, indicating the trap charge assisted conduction mechanism in the BiFeO3/BaTiO3 multilayer structures. Such linear variation in log[thin space (1/6-em)]J versus log[thin space (1/6-em)]E is compatible with the Simmons' modification of the Schottky equation.23 The usual Schottky equation assumes that the electron mean free path is longer as compared to the Schottky barrier width. In the case of discrete trap charge assisted conduction in these systems, current density follows the equation:23

 
image file: c6ra09326d-t1.tif(1)
where, E is applied bias field, εr is the dielectric constant of sample, ε0 is the dielectric constant of free space, μp is carrier mobility and θ is the ratio of total density of induced free carriers to trapped carriers. If the internal built-in field is dominated by space charge carriers (either from free or trapped carriers), the current density JL should follow power-law dependence on electric field as J ∝ En, where n is the exponent. In Fig. 3 the values of the slopes (n) obtained from fitted parameters of multilayer BiFeO3/BaTiO3 samples were found to be 2.10 for two layer system and about 1.82 for three to six layer structures, which indicates that current conduction process in multilayer samples was closer to discrete trap carriers.


image file: c6ra09326d-f3.tif
Fig. 3 Log(J) versus log(E) plots for the BiFeO3/BaTiO3 multilayer structures having different numbers of stacking layers.

From the J–E studies, it is identified that the six layer BiFeO3/BaTiO3 system gives the best electrical properties, and hence was chosen for further study. The capacitance–voltage (C–V) loop for six layer BiFeO3/BaTiO3 system (as given in Fig. S2†) showed characteristic butterfly loop with two capacitance peaks for both positive and negative field characterizing spontaneous polarization switching indicating good ferroelectric property.

Fig. 4(a) shows the room temperature PE–hysteresis curves obtained for the BiFeO3/BaTiO3 multilayer structure at 100 kHz. Well defined and saturated PE hysteresis loops were obtained for all prepared BiFeO3/BaTiO3 multilayer structure (Fig. 4(a)). It can be seen that the ferroelectric properties of the BiFeO3/BaTiO3 multilayer structures show enhancement as the number of layers are increased from 2 to 6 while it degrades for the seven layer system. Fig. 4(b) shows the variation in remnant polarization (Pr) and saturation polarization (Ps) with number of layers in the BiFeO3/BaTiO3 multilayer system. Both the Pr as well as Ps increased with increase in the number of alternating layers, having a maximum value for the six layer BiFeO3/BaTiO3 system and decreases on further increasing the number of layers to seven. Maximum remnant polarization (Pr) of about 71.88 μC cm−2 and saturation polarization (Ps) of about 99.80 μC cm−2 has been obtained for the six-layer BiFeO3/BaTiO3 system. This enhancement in ferroelectricity obtained in the multilayer structures can be related to interfacial stress generated in the system due to the presence of BaTiO3 layer. Since stress was increasing with increase in number of layers, the ferroelectric property was improved (Fig. 4(a)). The observed degradation in ferroelectric property for 7 layer system may be due to the formation of composite structure of BiFeO3 and BaTiO3 instead of a layer structure with sharp interfaces. Both layers were expected to inter-diffuse because of decrease in thickness of each layer of BiFeO3 and BaTiO3 (∼50 nm) in the multilayer system having 7 layers for maintaining the total thickness of 350 nm. The obtained results on P–E loops were in accordance with the electrical and dielectric studies of the multilayer structures. The obtained values of Pr, Ps and Ec for all the multilayer structures are summarized in Table ST 2.†


image file: c6ra09326d-f4.tif
Fig. 4 (a) Variation in room temperature ferroelectric polarization with applied electric field (P–E) for the BiFeO3/BaTiO3 multilayer structures having different number of stacking layers. (b) Variation in remnant polarization (Pr) and saturation polarization (Ps) with number of layers in the BiFeO3/BaTiO3 multilayer structures.

Fig. 5(a) shows the room temperature MH–hysteresis curves for the BiFeO3/BaTiO3 multilayer structures with increasing the number of layers from 2 to 7. Well defined magnetic hysteresis loops were observed for all the multilayer structures. A continuous enhancement in remnant magnetization (Mr) and saturation magnetization (Ms) with number of BiFeO3/BaTiO3 layers upto six layers in the multilayer BiFeO3/BaTiO3 structure (Fig. 5(a)). However, in the case of seven layer BiFeO3/BaTiO3 structure the magnetization (both Mr and Ms) decreased. Fig. 5(b) shows the variation in remnant magnetization (Mr) and saturation magnetization (Ms) with number of layers in BiFeO3/BaTiO3 systems. Six layer BiFeO3/BaTiO3 system shows the maximum Mr and Ms values. The value of saturation magnetization (94.88 emu cm−2) was found to be maximum for six-layered BiFeO3/BaTiO3 system and decreased for the seven layer BiFeO3/BaTiO3 system to 42.36 emu cm−2. The enhanced magnetization obtained upto six layer system was due to intervening BaTiO3 layer which created constraint resulting into the interfacial stress.


image file: c6ra09326d-f5.tif
Fig. 5 (a) Variation in room temperature magnetization with applied magnetic field (M–H) for the BiFeO3/BaTiO3 multilayer structure having different number of stacking layers. (b) Variation in remnant magnetization (Mr) and saturation magnetization (Ms) with number of layers in BiFeO3/BaTiO3 multilayer system.

However, no continuous trend can be seen in the coercive field for the series of samples with increase in number of BiFeO3/BaTiO3 layers. The values of saturation magnetization and remnant magnetization of all the structures are included in Table ST 2† for comparison. The results on magnetic study are in accordance with the structural, electrical and ferroelectric studies where six layer BiFeO3/BaTiO3 system gives the best results and degradation in the properties for a seven layer system may be related to the non-occurrence of a clean interface between the two constituent layers.

The enhanced multiferroic properties of the BiFeO3/BaTiO3 multilayer structures is attributed to the coupling between electric and magnetic orders besides the stress induced in the multilayer system. It is clearly evident from above results that the properties of BiFeO3/BaTiO3 multilayer structures corresponding to 6 layers are optimum for the realization of BiFeO3 based devices.

Conclusion

In the present work, multilayer BiFeO3/BaTiO3 structure having two to seven alternating layers were deposited on different substrates by PLD technique. X-ray diffraction study reveals the formation of pure phase and polycrystalline perovskite structures of BaTiO3 and BiFeO3 thin films. Stress modulus and c/a ratio were found to increase with increase in number of steady layers upto six and then decrease for the seven layer structure. This was attributed to the formation of composite structure for 7 layer system in comparison to other multilayers having neat interface. Ferroelectric and ferromagnetic (multiferroic) properties of the multilayer BiFeO3/BaTiO3 thin films enhanced with the increase in number of alternating BiFeO3/BaTiO3 layers upto six layers. The leakage current density of the multilayer thin films also showed the same pattern having minimum value of 3.18 × 10−8 A cm−2 for the six layered structure. The typical butterfly shaped C–V curve was obtained for six layer BiFeO3/BaTiO3 structure indicating its good ferroelectric property. Thus, we may conclude that the BiFeO3/BaTiO3 multilayer structures showed the much enhanced multiferroic properties than pure BiFeO3 thin film making it best suited for multifunctional device applications.

Acknowledgements

Authors are thankful to Department of Science and Technology (DST) and University of Delhi for the financial support. One of the authors (SS) is thankful to the Delhi Technological University (DTU) for the teaching assistantship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09326d
This journal is © The Royal Society of Chemistry 2016
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