Vertical Interface Induced Dielectric Relaxation in Nanocomposite (BaTiO3)1-x:(Sm2O3)x Thin Films.

Vertical interfaces in vertically aligned nanocomposite thin films have been approved to be an effective method to manipulate functionalities. However, several challenges with regard to the understanding on the physical process underlying the manipulation still remain. In this work, because of the ordered interfaces and large interfacial area, heteroepitaxial (BaTiO3)1-x:(Sm2O3)x thin films have been fabricated and used as a model system to investigate the relationship between vertical interfaces and dielectric properties. Due to a relatively large strain generated at the interfaces, vertical interfaces between BaTiO3 and Sm2O3 are revealed to become the sinks to attract oxygen vacancies. The movement of oxygen vacancies is confined at the interfaces and hampered by the misfit dislocations, which contributed to a relaxation behavior in (BaTiO3)1-x:(Sm2O3)x thin films. This work represents an approach to further understand that how interfaces influence on dielectric properties in oxide thin films.

The emergence of novel phenomena and functionalities at artificially constructed oxide heterostructures has stimulated intense research activities over the past decade12. Among these studies, oxide interfaces are very attractive because the coexistence and interplay between different degrees of freedom (charge, orbit, spin, and lattice) at interfaces can lead to rich physical phenomena, including two-dimensional electron gas (2DEG), superconductivity, colossal magnetoresistance, and multiferroic behavior345678910. For instance, Liu et al. demonstrated that oxygen vacancies (VOs) are the dominant origin of the 2DEG at LaAlO3/SrTiO3 interfaces when the LaAlO3 overlayer is amorphous6. A novel ferromagnetic state was observed at the interface between antiferromagnet BiFeO3 and ferromagnet La0.7Sr0.3MnO3, which is directly attributed to an electronic orbital reconstruction at the interface11.

In addition to these conventional lateral interfaces (parallel to substrate surface), vertical interfaces (perpendicular to substrate surface) in vertically aligned nanocomposite thin films have been introduced and used to create or enhance functionalities of oxide thin films89. Compared to lateral interfaces, vertical interfaces possess impressive advantages, such as reduced clamping effect from substrates, larger interfacial area, strain tunability to larger thickness, and easy interface probing etc12131415. Furthermore, such ordered structures allow for precise tuning of mechanical, electronic, and magnetic properties through vertical strain control, as well as interfacial couplings. For example, Moshnyaga et al. showed colossal magnetoresistance effect has been enhanced in (La0.7Ca0.3MnO3)1-x:(MgO)x thin films through lattice strain8. Zheng et al. reported that magnetoelectric coupling has been realized in (BaTiO3)0.65:(CoFe2O4)0.35 thin films by vertical interfaces coulpings9. Besides, vertical interfaces induced strain state reversion and leakage current reduction have been achieved in (BiFeO3)0.5:(Sm2O3)0.5 thin films1216. And enhanced low field magnetoresistance has been reported in heteroepitaxial (La0.7Sr0.3MnO3)0.5:(ZnO)0.5 via tuning the microstructure and vertical interface density17. It is clear that oxide interfaces are effective to control functionalities of oxide thin films. Most previous reports have focused on exploring magnetism, ferroelectricity, magnetoelectric coupling, and electric transportation6891011. However, the question that arises naturally is whether dielectric properties can be manipulated by oxide interfaces. The work presented here suggests an answer in the affirmative.

Relaxation properties have been approved to be critical for the applications (such as transducers, actuators, and sensors etc.) of dielectric materials18192021. It is highly attractive to manipulate relaxation properties through interfaces, which is also helpful to understand the relationship between oxide interfaces and physical properties. It has been showed that VOs is responsible for dielectric relaxations observed in epitaxial K0.5Na0.5NbO3/La0.67Sr0.33MnO3 and Ba0.7Sr0.3TiO3/Bi1.05La0.05FeO3 heterostructures2223. As a typical dielectric oxide, BaTiO3 has attracted extensive studies because of excellent ferroelectric and dielectric properties. For instance, high Curie temperature, positive transverse piezoelectric coefficient, and low leakage current have been obtained in (BaTiO3)0.5:(Sm2O3)0.5 thin films, which has been revealed to be originated from the strain at the vertical interfaces between BaTiO3 and Sm2O3242526. Considering the ordered interfaces and large interfacial area, (BaTiO3)1-x:(Sm2O3)x can be an unique system for investigating the relationship between the interfaces and dielectric properties. In this work, we present a comparative study on dielectric properties of (BaTiO3)1-x:(Sm2O3)x nanocomposite thin films with compositions of x = 0.5 and 0.62. Due to a relatively large strain generated at the interfaces, vertical interfaces between BaTiO3 (BTO) and Sm2O3 are revealed to become the sinks to attract VOs. The movement of VOs is confined at the interfaces and hampered by misfit dislocations along the interfaces, which results to a dielectric relaxation in the (BTO)1-x:(Sm2O3)x (BTO:Sm2O3) nanocomposite thin films.

Results

Typical x-ray diffraction (θ – 2θ) patterns for the composite thin films are shown in Fig. 1. Only (00l) diffraction peaks appear in the patterns for both thin films and substrates, suggesting that the BTO and Sm2O3 phases coexist in the composite thin films and are preferentially oriented along the c-axis. According to our previous works24252627, the orientation relationship between thin films and substrates is determined to be (002)BTO||(002)Sm2O3||(002)STO and [200]BTO||[220]Sm2O3||[200]STO. It should be noted that, due to the lattice mismatch between the BTO and Sm2O3 (the lattice constants of bulk BTO and Sm2O3 are 4.03 and 10.93 Å, respectively), misfit dislocations are thus generated for partial strain relaxation, which is confirmed by transmission electron microscopy (TEM) measurements and will be discussed later. Additionally, large residual strains of ~2.3% and ~3.4% have been found in the BTO phase in the composite thin films with compositions of x = 0.5 and 0.62 respectively, which is consistent with the reported results2426.

Figure 1

Comparison for the XRD θ-2θ scans for BTO:Sm2O3 thin films with compositions of (a) x = 0.5 and (b) x = 0.62.

In previous works, we have revealed that the BTO and Sm2O3 phases grow alternatively and spontaneously and form a vertically aligned columnar structure in the BTO:Sm2O3 thin films242526. Fig. 2(a) show high resolution TEM images of the BTO:Sm2O3 thin films with compositions of x = 0.5 and 0.62 respectively, which demonstrate the excellent heteroepitaxial growth of the BTO and Sm2O3 on the STO substrates. Combined with previous results242526, these images indicate that self-assembled Sm2O3 nanocolumns are evenly sized, distributed, and embedded in a BTO matrix. And the diameter of single Sm2O3 nanocolumn is about 10 nm. So, the density of interfaces is estimated to be about 108/m. More than this, a periodic arrangement of misfit dislocations is found along the vertical interfaces, as shown in the corresponding Fourier-filtered images in Fig. 2(b). The density of misfit dislocations along the interfaces is estimated to be about 4.0 × 108/m for x = 0.5, and about 5.0 × 108/m for x = 0.62. Considering the density of interfaces, the areal density of misfit dislocations is estimated to be about 4.0 × 1016/m2 for x = 0.5, and about 5.0 × 1016/m2 for x = 0.62. In other words, the density of misfit dislocations is very high in the BTO:Sm2O3 thin films, which may originate from the large lattice mismatch between the BTO and Sm2O3. Besides, the density of misfit dislocations for x = 0.5 is lower than that for x = 0.62. All these results suggest that self-assembled vertical heteroepitaxial nanostructures of BTO:Sm2O3 are synthesized as expected and can be used as model system to explore the relationship between the vertical interfaces and dielectric properties in oxide thin films.

Figure 2

High-resolution TEM images of BTO:Sm2O3 thin films with (a) x = 0.5 and (c) x = 0.62. Corresponding Fourier-filtered (FFT) images along column boundaries are shown as (b) and (d), respectively. The FFT images are enlarged to show misfit dislocations clearly.

To investigate the vertical interface effects on dielectric behavior, the temperature dependence of the real part of dielectric constant (ε‘) and dielectric loss (tanδ) are measured at the frequency ranging from 1 kHz to 1 MHz by using a structure of Pt/BTO:Sm2O3/Nb-STO (shown as Fig. 3). In general, as the frequency increases, the tanδ ~T curve shifts towards a higher temperature region, indicating a typical characteristic of dielectric relaxation phenomenon. Furthermore, it is obvious that ε‘ gradually increases with increasing temperature (shown as insets of Fig. 3). It should be pointed that, because of a relatively large vertical strain observed in the BTO phase in the composite films (~2.3% and 3.4% for x = 0.5 and 0.62, respectively), the ferroelectric Curie temperature of the composite films may be over 833 K, which is comparable to the previous results242829.

Figure 3

Temperature dependence of tanδ for BTO:Sm2O3 thin films with (a) x = 0.5 and (b) x = 0.62 measured at various frequencies. The insets show temperature dependence of dielectric constant.

Figure 4 shows the frequency dependent tanδ for the BTO:Sm2O3 thin films measured at different temperatures. The peaks of tanδ shift towards a higher frequency region with increasing temperature, further approving the existence of dielectric relaxation in the composite thin films. In order to explore the physical mechanism of the relaxation process, we calculated the relaxation parameters for BTO:Sm2O3 thin films in terms of the Arrhenius Law

Figure 4

Frequency dependence of tanδ for BTO:Sm2O3 thin films with (a) x = 0.5 and (b) x = 0.62 measured at different temperatures. The insets show the Arrhenius plots of relaxation times. The red straight lines in insets are the linear fitting based on the Arrhenius law.

where is the pre-exponential factor, is activation energy required for relaxation process, is the Boltzmann constant, and is the temperature where the maximum loss tangent occurs. The Arrhenius plots were shown as insets of Fig. 4(a). The values of and were found to be 0.53 eV and 2.17 × 107 Hz for x = 0.5, and 0.61 eV and 2.19 × 108 Hz for x = 0.62, respectively.

To further understand the physical process of the observed dielectric relaxation in the BTO:Sm2O3 thin films, the imaginary () part of electric modulus () given by M "= as a function of temperature at a series of frequencies were illustrated in Fig. 5(a). As we expected, well-defined M "(T ) peaks have been found in the whole temperature range. The  ~ T curve shifts towards higher temperature with increasing frequency, indicating a typical relaxation nature. The Arrhenius plots for Ln (fmax) vs 103/T were also shown in Fig. 5(b). Accordingly, the relaxation parameters of and were deduced to be 0.54 eV and 4.85 × 108 Hz for x = 0.5, and 0.59 eV and 1.72 × 109 Hz for x = 0.62, respectively. The activation energy obtained from is almost the same as the calculated values from tanδ(T) (see insets of Fig. 4(a)), which further confirms that the fitting results are reasonable. It should be pointed out that, because the relaxation time (τ = 1/f) for and tanδ (T) follow the general rule of τtan > τ3031, the pre-exponential factor deduced for is always one order of magnitude larger than that estimated from tanδ (T).

Figure 5

Variation of as a function of temperature for BTO:Sm2O3 thin films with (a) x = 0.5 and (c) x = 0.62 measured at different frequencies. The corresponding Arrhenius plots of the frequency against temperature were shown in (b) and (d), respectively. The solid curves are the best fits to the Arrhenius law.

Now it is important to investigate the origin of the dielectric relaxation in the BTO:Sm2O3 thin films. As a Pt/BTO:Sm2O3/Nb-STO vertical capacitor has been used in the dielectric measurements, the composite thin film can be reviewed as three parts connected in parallel: the BTO phase, the Sm2O3 phase, and the vertical interfaces. Up to now, as far as we know, there are no reports on dielectric relaxation in the Sm2O3. And the activation energy of BTO-based perovskite oxides is 0.88 ~ 1.56 eV32333435, which is obviously higher than those in the present work. To further exclude the influence of the BTO and Sm2O3 phases on dielectric relaxation, the dielectric properties of pure BTO and Sm2O3 thin films were measured (not shown). There is no obvious dielectric relaxation in the pure Sm2O3 thin film. And, dielectric relaxation was observed in the pure BTO film with an E value of 1.08 eV, which is in consistent with the previous results. Therefore, neither the BTO nor the Sm2O3 phase is responsible for the dielectric relaxation observed in the composite films. In other words, the vertical interfaces dominate the relaxation behavior. On the other hand, it is well known that the dielectric loss is closely correlated with the leakage current in oxide thin films. And we have demonstrated that the leakage behavior is dominated by the vertical interfaces in (BTO)0.5:(Sm2O3)0.5 thin films, which further approves that the vertical interfaces are those who resulted to the dielectric relaxation26. It has also been reported that the electrode interfaces related to VOs gradients affect fatigue and dielectric loss in ferroelectric oxides36. However, the vertical interfacial area is about twenty ~ forty times of the electrode interfacial area in the BTO:Sm2O3 thin films. Though the contribution of the electrode interfaces may dominate the dielectric behavior in the pure BTO and Sm2O3 thin films, the dielectric behavior of the BTO:Sm2O3 composite films is totally different from those of the pure thin films. And, as we discussed earlier, the density of misfit dislocations is very high in the composite thin films. Considering all these factors, the contribution of the electrode interfaces to the dielectric relaxation in the BTO:Sm2O3 thin films should be neglected in the present work.

It is well known that VOs have been demonstrated to be intrinsic defects and are often unavoidable in oxide thin films. The relaxation occurring at high temperatures are exclusively related to the VOs. And an activation energy of 0.3 ~ 1.0 eV is the typical value for relaxation behavior caused by VOs, which is verified by many previous reports32373839. According to the values obtained in the BTO:Sm2O3 thin films, the dielectric relaxation in the measured temperature region was proposed to associate with VOs. On the other hand, because of the structural discontinuity as well as the strain, the interfaces have been approved to attract and gather the VOs64041424344. For example, strain-driven accumulation of Vs along the vertical interfaces has been observed in (REBa2Cu3O7-δ)1-x:(BaZrO3)x composite thin films45. More than this, in our previous work, electron energy loss spectroscopy (EELS) measurements revealed that a large concentration of VOs forms at the vertical interfaces in (SrTiO3)0.5:(Sm2O3)0.5 composite thin films due to a large lattice misfit46. Considering a large vertical strain generated at the interfaces in the present work, the vertical interfaces are believed to become the sinks to attract VOs, which is the origin of the dielectric relaxation in the composite thin films26474849.

To understand the mechanism of the observed relaxation behavior, a model has been proposed and shown in Fig. 6. In this system, as shown in Fig. 6(a), Sm2O3 are nanocolumns embedded in a BTO matrix and vertical sandwich capacitors with a configuration of Pt/BTO:Sm2O3/Nb-STO have been used to investigate the dielectric properties. Electric field is applied parallel to the interfaces between Sm2O3 and BTO (shown as Fig. 6(b)). On the other hand, VOs have been attracted at the interfaces and can be viewed as ions with positive charges5051. With the assistance of an electric field, VOs can move along the vertical interfaces in the direction of electric field. However, the long range movement of VOs will be hampered by the misfit dislocations observed in the vertical interfaces, which results to the dielectric relaxation of the BTO:Sm2O3 thin films. In addition, with the increasing density of misfit dislocations, the values of the activation energy varied from 0.53 to 0.61 eV, further confirming the above mechanism.

Figure 6

(a) Schematic diagram of Pt/BTO:Sm2O3/Nb-STO vertical sandwich capacitors. (b) The expanded view of the dashed part in the schematic diagram to show the interfaces between Sm2O3 nanocolumns and BTO matrix. E and blue lines represent the electric field and the pathway of movement of VOs, respectively.

The outcome of our above analysis shows that self-assembled vertically aligned nanocomposite thin films have three unique features: ordered vertical interfaces, large interfacial area, and VOs gathered at the vertical interfaces264546474849. With the assistance of an electric field, VOs can only move up and down along the vertical interfaces, which means that the transportation of VOs has been effectively confined. Meanwhile, misfit dislocations formed along the vertical interfaces can be used to manipulate the dynamics of VOs. These unique features are very helpful to investigate the transportation mechanism of VOs, and to enhance ion conductivity in oxides, which play a key role in determining the performance of energy conversion and storage devices, such as thin films solid oxide fuel cells, photocatalysts, and batteries525354.

Discussion

In summary, epitaxial (BTO)1-x:(Sm2O3)x vertically aligned nanocomposite thin films with compositions of x = 0.5 and 0.62 have been fabricated by pulsed laser deposition, which were used as model system to investigate the relationship between the microstructure, the interfaces, and the dielectric behavior. The structural discontinuity and a relatively large residual strain attract the accumulation of VOs at the vertical interfaces between the BTO and Sm2O3. With the assistance of an electric field, the movement of VOs has been confined along the interfaces and been hampered by the misfit dislocations, which results to an interface-induced relaxation behavior. The present work has broad implications for the understanding of the correlation between the interfaces and physical properties, for the manipulating or optimizing of functionalities in the nanocomposite oxide thin films, and for the utilization of dielectric materials in high-temperature applications. More than this, the unique characteristics of vertically aligned nanocompsite thin films present potential applications in energy conversion and storage devices.

Methods

Epitaxial (BTO)1-x:(Sm2O3)x thin films with compositions of x = 0.5 and 0.62 were deposited on (001) oriented SrTiO3 (STO) and Nb-doped SrTiO3 (Nb-STO) substrates by pulsed laser deposition (PLD) with a KrF excimer laser (Lambda Physik, λ = 248 nm). A laser fluence of ~2 J/cm2 with a repetition rate of 3 Hz were focused onto composite targets with different molar ratios. An optimized substrate temperature of 720 oC and oxygen pressure of 25 Pa were used during depositions. Immediately following depositions, films were annealed in situ for one hour at a temperature of 450 oC and an oxygen pressure of 0.8 atm. X-ray diffraction (XRD, Rigaku K/Max) and transmission electron microscopy (TEM, FEI Tecnai F20 analytical microscope) were used to investigate the microstructure of thin films. The thickness of thin films was measured by cross-sectional TEM.

For electrical measurements, vertical sandwich capacitors with a configuration of Pt/BTO:Sm2O3/Nb-STO were fabricated, where thin films with a thickness of ~200 nm were used and Pt top electrodes with an area of 8 × 10−4 cm2 were deposited by sputtering. The dielectric properties were investigated using an Agilent 4294 A Impedance Analyzer. The measurements were performed at selected temperatures in a Linkam Scientific Instruments HFS600E-PB4 system.

Additional Information

How to cite this article: Li, W. et al. Vertical Interface Induced Dielectric Relaxation in Nanocomposite (BaTiO3)1-x:(Sm2O3)x Thin Films. Sci. Rep. 5, 11335; doi: 10.1038/srep11335 (2015).