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Vol. 8. Issue 1.
Pages 553-560 (January - March 2019)
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Vol. 8. Issue 1.
Pages 553-560 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.04.014
Open Access
Confirmation of spatial coexistence of magneto-electric coupling in Bi0.7Dy0.3FeO3 thin films integrated with Si/ZnO film for MEMS and memory applications
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Deepak Bhatiaa,
Corresponding author
, Himanshu Sharmab, Ramswaroop S. Meenaa, Vaijayanti R. Palkarc
a Department of Electronics Engineering, Rajasthan Technical University, Kota 324010, India
b Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India
c Department of Electrical Engineering and Centre for Excellence in Nanoelectronics, Indian Institute of Technology Bombay, Mumbai 400076, India
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Abstract

Spatial presence of ferroelectric and magnetic domain structure of the multiferroic dysprosium (Dy)-modified BiFeO3 (Bi0.7Dy0.3FeO3 or BDFO) deposited on ZnO at the macroscopic level is investigated in this paper. BDFO thin film is deposited on Si/ZnO using pulsed laser deposition (PLD) technique. Magnetic properties are observed by saturated magnetic hysteresis at room temperature. Ferroelectric hysteresis loop (P–E) is used to compare the response of magnetic field on ferroelectric properties at room temperature of BDFO and BDFO/ZnO thin films. The changes in ferroelectric hysteresis loops with magnetic field ensures about the magnetoelectric (M–E) coupling in BDFO/ZnO films. A well-saturated ferroelectric hysteresis loop with remarkable improvement in remanent polarization (∼1.72μC/cm2) is observed. The obtained results confirm the coexistence of ferromagnetic and ferroelectric ordering with significant coupling at room temperature. The coupling behavior and magnetic transition are also verified using multimode atomic force microscope by applying bias between sample and MFM tip. The leakage current density is measured in the order of 10−5A/cm2. Integration of BDFO films with ZnO piezoelectric thin film suggests in principle its potential in applications of micro electro mechanical systems (MEMS) as well as in memory devices and in strong history dependent systems.

Keywords:
Multiferroics
Multimode atomic force microscopy
Pulsed laser deposition
MEMS
Full Text
1Introduction

Multiferroics materials show the coexistence of significant ferroelectric and ferromagnetic properties within the same phase at room temperature. This phenomenon gives additional complexity to their physical properties and makes them a promising subject for fabrication of a variety of devices. The integration of multiferroic thin films with semiconductor ZnO is considered, looking to its prime importance in memory devices and gate dielectric. The significant improvement in the piezoelectric properties of this combination has already been reported [1]. It is difficult to obtain a good ferroelectric/semiconductor interface and to realize a practical device. Recently developed magnetoelectric multiferroic “dysprosium doped (Dy)” modified BiFeO3 (Bi0.7Dy0.3FeO3 or BDFO) is a new material which exhibits ferromagnetism, ferroelectricity and piezo-response in bulk as well as in thin films at room temperature [1]. It is reported that Dy doped nanoparticles enhance the magnetic properties of BiFeO3 (BFO) as compared to undoped. Dy substitution affects the parent compound structure of BFO and remarkable enhancement in saturation magnetization (Ms) and remanent magnetization (Mr) is observed [2,3]. Improvement in ferroelectric property and ferromagnetism may be correlated by the structural transformation and grain morphology [4]. More interestingly multiferroic (BDFO) have significant coupling along with ferroelectric and ferromagnetic ordering simultaneously at room temperature [5,6]. However, the existence of magnetoelectric (M–E) coupling between magnetic and electric parameters has been predicted theoretically [7]. Thus, there is acute interest in researchers for taking advantage of the said properties by its implementation in device architectures [8]. Among the known one-dimensional nano materials, zinc oxide (ZnO) is unique because of its semiconductor and piezoelectric properties. It is used in variety of applications in MEMS due to exclusive combinations of piezoelectric, optical and electrical properties. However, ZnO is found to be susceptible material for treatment by temperature, wet etching, acid bases and water due to the stresses produced during the processes and its own piezoelectric properties [9–11]. The thin film of BDFO on Si/ZnO is deposited using the pulse laser deposition (PLD). It exhibits excellent dielectric properties such as high dielectric constant (k=30), low leakage and low interface state density. These properties are used for storage of generated electricity as memory device [12,13].

The distorted perovskite BDFO exhibits antiferrromagnetic behavior (TN380°C) and ferroelectric property (Tc810°C) ordering, which ensures BDFO to be the promising candidate for device applications in high temperature range [7,14–16]. Study of temperature effect on magnetic domain structure of multiferroic thin films have been presented by Palkar and Prashanti in [12]. To use such exceptional properties, the study of magnetic domain structure of this material is essential. In this paper, the M–H coupling of BDFO/ZnO combination and magnetic properties are observed by saturated magnetic hysteresis at room temperature. Coupling is also verified by observing magnetic domain pattern with the effect of voltage. A multimode atomic force microscopy (MAFM) is used to establish the coexistence of ferroelectric and magnetic domains in these thin films and to study the images obtained at specific spatial area. The substantial changes are observed in electric polarization (Ps) with variation in applied magnetic field. This ensures that the alignment of ferroelectric domains occurs due to the magnetic field. The zigzag patterns in the thin films are also converted in the striped patterns significantly by the application of externally applied electric field [8]. More interestingly, even if the applied field is turned off, the formed magnetic domain pattern persists in the sample. This result shows a direct imaging evidence of magnetic hysteresis with applied field [17,18]. Ferroelectric hysteresis loop (P–E) is obtained by loop tracer (TF Analyser 2000, aix ACCT Systems GmbH, Germany) in order to determine the response of magnetic field on ferroelectric properties at room temperature of BDFO and BDFO/ZnO thin films [18,19]. This paper reports the characteristics of multiferroic thin film integrated with piezoelectric ZnO, which are directly deposited on p-type low resistivity Si for storage purpose of electricity generated by BDFO/ZnO nanocomposite. The excellent multifunctional properties of BDFO/ZnO films could find for a variety of device applications like sensors, energy scavengers, electricity generators, etc. [1,7].

2Experimental details

Conducting p-type Si substrate with resistivity of (0.0001–0.0005Ωcm) is used for deposition. In order to avoid the dead layer, silicon wafers are cleaned by standard Radio Corporation of America (RCA) method before loading them into dielectric sputter. ZnO thin films are deposited on Si by dielectric sputtering using a ZnO target (99.9%) with a diameter of 2in and thickness of 3mm [20]. During the deposition of ZnO thin films the RF power is 150W, the base pressure is 5×10−5mbar and operating pressure is 2.2×10−2mbar. Thin films are deposited in Ar-atmosphere with a deposition rate of 15nm/min.

On the other hand, partial co-precipitation route is used to prepare powder sample of Bi0.7Dy0.3FeO3 which is described elsewhere [7]. The pellet form is obtained by compacting powder material and pellets are sintered at 800°C for 2h [8]. Thus, obtained pellet is highly dense and used as target for pulsed laser deposition (PLD) method. BDFO thin films of 300nm are deposited using PLD. Complex Pro 201, KrF excimer laser with λ=248nm and energy density [8] of 2J/cm2, is used for deposition of BDFO thin films. During deposition, substrate temperature, target to substrate distance and O2 pressure are 650°C, 5cm and 4.5×10−1mbar, respectively. Thickness of thin film is found to be 300nm using the 22,500 number of pulses with a repetition rate of 10Hz. The deposited film thickness is assessed by profilometer (Ambios, USA). Further, the characterization of deposited films is done by different techniques. X-ray diffraction (XRD) is used to determine the phase purity and crystal structure. However, the surface morphology of the films is accomplished by a multimode atomic force microscope (MAFM) in MFM mode, scanning electron microscopy (SEM) and atomic force microscope (AFM). Magnetization of the thin films, as a function of applied magnetic field at room temperature is measured using a SQUID-VSM (Quantum Design Inc., USA) and ferroelectric hysteresis loop tracer, is used to carry out electric polarization measurements.

3Results and discussion3.1Crystal structure and surface morphology

XRD patterns obtained for ZnO and BDFO/ZnO films are shown in Fig. 1. X-ray from Rigaku (Cu-Kα radiation, λ=1.5405??) is used for structural phase identifications. The BDFO/ZnO films exhibit a pervoskite structure similar to that of pure BFO. It is obvious that Dy substitution has affected the structure of parent compound BFO as reported in literature [2,3,21,22]. All the peaks of the patterns are indexed.

Fig. 1.

X-ray diffraction (XRD) patterns of BDFO/ZnO and ZnO films grown on Si substrate.

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The XRD pattern indicates that the BDFO film deposited on the ZnO films is a single phase and polycrystalline in nature. This is due to the lattice mismatch between ZnO and BDFO. However, the XRD pattern of ZnO thin film indicates that diffraction peak located around 34.422° is very high and just ZnO (002) diffraction peak is observed. So, the deposited ZnO thin film on the Si substrate has a high c-axis preferred orientation, which is essential to achieve a ZnO film with high piezoelectric quality [9,10,20].

To observe the grain morphology of BDFO and ZnO films, scanning electron microscopy (SEM) is done using Raith-150. SEM is also used to find the uniformity and the thickness of BDFO films over ZnO films. Fig. 2 shows the SEM images of ZnO and BDFO/ZnO films at different magnifications and it can be clearly observed that the of BDFO films on ZnO films is polycrystalline in nature with granular structure. The average grain size is of the order of 35–70nm.

Fig. 2.

SEM image of ZnO layer at 200nm. Inset shows the image at 100nm, (b) SEM image of BDFO/ZnO layer at 200nm. Inset shows the image at 100nm.

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Further, Figs. 3 and 4 show atomic force microscope (AFM) images of BDFO thin film and BDFO/ZnO thin film. A multimode atomic force microscope (Nanoscope IV, from Digital Instruments) has been used in this study and it is configured with MFM tip to provide the details about magnetic domain structure. The MFM images are acquired on a specific spatial area of the thin film samples, with maximal lateral scan of 1μm to obtain spatial resolution better than 50nm. Additionally, a tapping cantilever with a cobalt-coated tip is used to obtain the magnetic force microscopy (MFM) images. A strong permanent magnet has been used to magnetize the tip of the cantilever before installing it on the AFM head. In order to investigate any correlation between the sample morphology and the local order parameter, tapping-mode topographic images are obtained during the main scan [18,21]. The tip is raised above the sample surface during the interleave mode, which allow the imaging but relatively weaker with long-term magnetic interactions when minimizing the topography influence [13]. The topological (AFM) images of BDFO/ZnO and BDFO films respectively with scan size 500nm×500nm, 1μm×1μm and color scale at 50nm are shown in Figs. 3 and 4. As seen from the topography images, the domain size is bigger than six to ten times with the conventional grain size on the linear scale. MFM measurement is used to investigate the effect of applied bias on magnetic domain patterns of the sample. Figs. 5 and 6 include the MFM images of BDFO and BDFO/ZnO films, respectively with scan size 500nm×500nm, 1μm×1μm and scan height of 30nm.

Fig. 3.

AFM (topography) images of BDFO thin film of (a) 500nm×500nm, (b) 1μm×1μm scan size and scan height of 30nm.

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Fig. 4.

AFM (topography) images of BDFO/ZnO thin film of (a) 500nm×500nm, (b) 1μm×1μm scan size and scan height of 30nm.

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Fig. 5.

MFM images of BDFO thin films of 300nm thickness for scan area (a) 500nm×500nm, (b) 1μm×1μm.

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Fig. 6.

MFM images of BDFO/ZnO thin films of 300nm thickness for scan area (a) 500nm×500nm, (b) 1μm×1μm.

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The obtained magnetic image displays a highly irregular domain patterns, moving side to side. When no electric field is applied on the sample, zigzag domain structure is observed in grown films. However, more elongated stripe domains are seen when the sample is biased with the low field. This stripe structure is fairly evident even at a higher bias value. The topography and magnetic domain images show unambiguously the ferroelectric writing and the existence of remanent polarization. This reflects the presence of magnetic stripe domains in the BDFO and BDFO/ZnO thin films. In comparison to BDFO film, BDFO/ZnO thin films show weak domain stripes. Quite unusually, even if the electric field is switched off these stripe patterns remain. This clearly exhibits a biased induced magnetic hysteresis. It is also determined that this effect is independent to change in sign of the applied field. Leakage current of the BDFO/ZnO composite at room temperature is measured by sweeping bias voltages as shown in Fig. 7. The leakage current of this device is found in the order of 10−5A, which is slightly greater than the BDFO [6–8], due to an increase in the transmission probability in semiconductor ZnO or by the large area (∼180μm diameter) of top electrodes.

Fig. 7.

Leakage current density–voltage curve of BDFO/ZnO thin films.

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3.2Magnetic properties

The variations of magnetization with magnetic field (M–H curve) at room temperature for BDFO and BDFO/ZnO films grown on silicon substrate are shown in Fig. 8. The inset of Fig. 8(a) and (b) highlights the coercivity of corresponding thin films. It is observed that the Dy enhances the magnetization of Bi0.7Dy0.3FeO3 in comparison to that of BFO [3,12,23–25]. However, the saturation magnetic field is found to be small (0.15T) as compared to that of the BFO. The small field requirement to bring saturation indicates that magnetic domains could easily be polarized at low fields. This property could be advantageous for fabrication in memory devices. Further, an enhanced magnetization (1emu/cc) and enhanced saturation magnetic field (0.3T) is observed in BDFO/ZnO combination at room temperature in comparison to BDFO (0.2emu/cc). The presence of magnetic anisotropy in the films may be result of numerous possible reasons, for example, texturing and magneto crystalline structure of the material, grain size (crystallinity) and stress appears in growth of the film, etc. [7,25–29]. Whereas, magnetocrystalline anisotropy is independent of the shape and grain size and it is intrinsic property of a magnetic material [17]. The value of saturation magnetization at different magnetic fields is depends on the sample crystallographic orientation in the magnetic field.

Fig. 8.

Magnetization with variation in applied magnetic field (M–H) curves of (a) BDFO/ZnO, (b) BDFO thin film.

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3.3Ferroelectric properties

The main objective of the experiment is to study the effect of electric field on the P–E curve, saturation and remanent polarization. A ferroelectric hysteresis loop tracer at a frequency 100Hz is used to carry out electric polarization measurements in order to determine the ferroelectric properties of BDFO/ZnO thin films by the effect of electric field. In order to measure the polarization, gold pads are used as the top electrode while platinum served as the bottom electrode.

Fig. 9 shows the ferroelectric hysteresis loops of BDFO/ZnO films, which are obtained at different values of applied dc magnetic field. The measurement is started by applying low intensity (∼9V) electric field on the sample, then keeping the electric field constant, the magnetic field is increased in steps as shown in Fig. 9. It is found that ferroelectric loops are improved as magnetic field increases. Saturation polarization increases gradually and reaches saturation for the magnetic field 0.022T. The ratio of remanent polarization (Pr) and saturation polarization (Ps) is calculated with increment in magnetic field value shown in Fig. 10. It is observed that the ratio for BDFO/ZnO film is slightly lower than BDFO film [30–36]. This exhibits comparative weak magnetic hysteresis property of BDFO/ZnO films with presence of M–E coupling.

Fig. 9.

Magnetic field induced ferroelectric polarization (P–E) curves obtained for BDFO/ZnO thin films.

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Fig. 10.

Remanent polarization (Pr) and saturation polarization (Ps) ratio with variation in magnetic field of BDFO/ZnO thin films.

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Ferroelectric hysteresis loop (P–E) measurements are obtained at room temperature for thin films grown on Si are shown in Fig. 11. To begin the measurements, a minimal required voltage is applied initially. There is a significant improvement in ferroelectric hysteresis loops with the increasing electric field. A ferroelectric hysteresis loop having polarization (Ps) and remanent polarization (Pr) of the order of ∼5.4μC/cm2 and ∼1.59μC/cm2, respectively, is observed with good amount of saturation for BDFO film [37–39]. The obtained values are in line with the reported values by Prashanti [6]. The BDFO/ZnO composite also shows well-saturated ferroelectric characteristics. However, the measured value of polarization (Ps) is slightly lesser (∼3.7μC/cm2) but significant improvement is observed in remanent polarization (Pr) which is of the order of ∼1.72μC/cm2. The coercive field (Ec) of 28kV/cm is observed, for maximum applied electric field of 50kV/cm. In BFO it is usually acknowledged that Ec is about 100–110kV/cm. This experiment of ferroelectric measurements suggests the much smaller values of Ec and Pr.

Fig. 11.

Polarization with variation in electric field (P–E) curves obtained for Bi0.7Dy0.3FeO3 (BDFO) and BDFO–ZnO thin films grown on Si substrate at room temperature.

(0.18MB).

The electric field required, for the alignment of ferroelectric domain depends on the presence of impurities in the sample, intrinsic properties of the material, defects in grain boundaries, grain size, orientation and stresses [21]. The co-existence of ferroelectric and magnetic properties is observed in BDFO–ZnO films deposited on Si substrate, as shown in Fig. 9. The presence of M–E coupling provides the greater flexibility in device design.

The leakage current voltage characteristics shown in Fig. 7 confirms that the current developed due to applied bias does not generate the effect of electric-field-induced magnetic hysteresis in the magnetic domain structure. The order of the leakage current (10−5A/cm2) is almost the same as the reported BDFO/Si [34,40]. However, BDFO/ZnO/Si combination provides additional advantage of enhanced magnetic, electric and piezoelectric properties. Due to reduced design complexity, the additional insulation layer is not required.

4Conclusion

The BDFO/ZnO films are directly deposited on Si substrate exhibit coexistence of ferromagnetic and ferroelectric ordering with significant M–E coupling at room temperature. The observation of coupling is demonstrated by the effect of magnetic poling on well-saturated ferroelectric loops. The measured value of remanent polarization Pr is consistent with the previously observed value by Prashanti [7,8]. There is saturation in magnetic hysteresis loop and ferroelectric loop at room temperature and magnetization is also achieved at low magnetic field. The coupling behavior is confirmed by the observed changes in magnetic domain with the application of electric field. The tendency in variation of magnetic as well as electric properties could be attributed to stress induced during the growth process. The leakage current density is measured in the order of 10−5A/cm2. These results suggest that the excellent multifunctional properties of BDFO/ZnO films/composite may be useful for the realization of devices in novel applications such as MEMS devices, memory, sensors, electricity generators, and strong history dependent devices, etc.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgement

The authors wish to acknowledge the Department of Information Technology, Government of India, through the Centre of Excellence in Nanoelectronics, IIT Bombay for the grant (No. 08DIT006). Also, the authors are grateful to the Department of Physics & Department of Material Science, IIT Bombay and Rajasthan Technical University, Kota for providing experimental facilities.

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