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 decade 1,2 . 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 behavior [3][4][5][6][7][8][9][10] . For instance, Liu et al. demonstrated that oxygen vacancies (V O s) are the dominant origin of the 2DEG at LaAlO 3 /SrTiO 3 interfaces when the LaAlO 3 overlayer is amorphous 6 . A novel ferromagnetic state was observed at the interface between antiferromagnet BiFeO 3 and ferromagnet La 0.7 Sr 0.3 MnO 3 , which is directly attributed to an electronic orbital reconstruction at the interface 11 .
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 films 8,9 . 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 etc [12][13][14][15] . 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 12,16 . And enhanced low field magnetoresistance has been reported in heteroepitaxial (La 0.7 Sr 0.3 MnO 3 ) 0.5 :(ZnO) 0.5 via tuning the microstructure and vertical interface density 17 . 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 transportation 6,[8][9][10][11] . 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 materials [18][19][20][21] . 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 V O s is responsible for dielectric relaxations observed in epitaxial K 0.5 Na 0.5 NbO 3 /La 0.67 Sr 0.33 MnO 3 and Ba 0.7 Sr 0.3 TiO 3 /Bi 1.05 La 0.05 FeO 3 heterostructures 22,23 . As a typical dielectric oxide, BaTiO 3 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 (BaTiO 3 ) 0.5 :(Sm 2 O 3 ) 0.5 thin films, which has been revealed to be originated from the strain at the vertical interfaces between BaTiO 3 and Sm 2 O 3 [24][25][26] . Considering the ordered interfaces and large interfacial area, (BaTiO 3 ) 1-x :(Sm 2 O 3 ) 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 (BaTiO 3 ) 1-x :(Sm 2 O 3 ) 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 BaTiO 3 (BTO) and Sm 2 O 3 are revealed to become the sinks to attract V O s. The movement of V O s is confined at the interfaces and hampered by misfit dislocations along the interfaces, which results to a dielectric relaxation in the (BTO) 1-x :(Sm 2 O 3 ) x (BTO:Sm 2 O 3 ) 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 Sm 2 O 3 phases coexist in the composite thin films and are preferentially oriented along the c-axis. According to our previous works [24][25][26][27] , the orientation relationship between thin films and substrates is determined to be (002) BTO ||(002) Sm2O3 ||(002) STO STO . It should be noted that, due to the lattice mismatch between the BTO and Sm 2 O 3 (the lattice constants of bulk BTO and Sm 2 O 3 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 results 24,26 .
In previous works, we have revealed that the BTO and Sm 2 O 3 phases grow alternatively and spontaneously and form a vertically aligned columnar structure in the BTO:Sm 2 O 3 thin films [24][25][26] . Fig. 2  In other words, the density of misfit dislocations is very high in the BTO:Sm 2 O 3 thin films, which may originate from the large lattice mismatch between the BTO and Sm 2 O 3 . 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:Sm 2 O 3 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.
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:Sm 2 O 3 /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 results 24,28,29 . Figure 4 shows the frequency dependent tanδ for the BTO:Sm 2 O 3 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:Sm 2 O 3 thin films in terms of the Arrhenius Law where f 0 is the pre-exponential factor, E a is activation energy required for relaxation process, k B is the Boltzmann constant, and T P is the temperature where the maximum loss tangent occurs. The Arrhenius plots were shown as insets of Fig. 4(a),(b). The values of E a and f 0 were found to be 0. " " as a function of temperature at a series of frequencies were illustrated in Fig. 5(a),(c). As we expected, well-defined  M "(T ) peaks have been found in the whole temperature range. The ′′ M ~ T curve shifts towards higher temperature with increasing frequency, indicating a typical relaxation nature. The Arrhenius plots for Ln (f max ) vs 10 3 /T were also shown in Fig. 5(b),(d). Accordingly, the relaxation parameters of E a and f 0 were deduced to be 0.54 eV and 4.85 × 10 8 Hz for x = 0.5, and 0.59 eV and 1.72 × 10 9 Hz for x = 0.62, respectively. The activation energy obtained from M "(T ) is almost the same as the calculated values from tanδ(T) (see insets of Fig. 4(a),(b)), which further confirms that the fitting results are reasonable. It should be pointed out that, because the relaxation time (τ = 1/f) for M "(T ) and tanδ (T) follow the general rule of τ tanδ > τ M" 30,31 , the pre-exponential factor deduced for M "(T ) is always one order of magnitude larger than that estimated from tanδ (T). Now it is important to investigate the origin of the dielectric relaxation in the BTO:Sm 2 O 3 thin films. As a Pt/BTO:Sm 2 O 3 /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 Sm 2 O 3 phase, and the vertical interfaces. Up to now, as far as we know, there are no reports on dielectric relaxation in the Sm 2 O 3 . And the activation energy of BTO-based perovskite oxides is 0.88 ~ 1.56 eV [32][33][34][35] , which is obviously higher than those in the present work. To further exclude the influence of the BTO and Sm 2 O 3 phases on dielectric relaxation, the dielectric properties of pure BTO and Sm 2 O 3 thin films were measured (not shown). There is no obvious dielectric relaxation in the pure Sm 2 O 3 thin film. And, dielectric relaxation was observed in the pure BTO film with an E a value of 1.08 eV, which is in consistent with the previous results. Therefore, neither the BTO nor the Sm 2 O 3 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 :(Sm 2 O 3 ) 0.5 thin films, which further approves that the vertical interfaces are those who resulted to the dielectric relaxation 26 . It has also been reported that the electrode interfaces related to V O s gradients affect fatigue and dielectric loss in ferroelectric oxides 36 . However, the vertical interfacial area is about twenty ~ forty times of the electrode interfacial area in the BTO:Sm 2 O 3 thin films. Though the contribution of the electrode interfaces may dominate the dielectric behavior in the pure BTO and Sm 2 O 3 thin films, the dielectric behavior of the BTO:Sm 2 O 3 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:Sm 2 O 3 thin films should be neglected in the present work.
It is well known that V O s 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 V O s. And an activation energy of 0.3 ~ 1.0 eV is the typical value for relaxation behavior caused by V O s, which is verified by many previous reports 32, [37][38][39] . According to the values obtained in the BTO:Sm 2 O 3 thin films, the dielectric relaxation in the measured temperature region was proposed to associate with V O s. On the other hand, because of the structural discontinuity as well as the strain, the interfaces have been approved to attract and gather the V O s 6,40-44 . For example, strain-driven accumulation of V O s along the vertical interfaces has been observed in (REBa 2 Cu 3 O 7-δ ) 1-x :(BaZrO 3 ) x composite thin films 45 . More than this, in our previous work, electron energy loss spectroscopy (EELS) measurements revealed that a large concentration of V O s forms at the vertical interfaces in (SrTiO 3 ) 0.5 :(Sm 2 O 3 ) 0.5 composite thin films due to a large lattice misfit 46 . Considering a large vertical strain generated at the interfaces in the present work, the vertical interfaces are believed to become the sinks to attract V O s, which is the origin of the dielectric relaxation in the composite thin films 26,[47][48][49] .
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), Sm 2 O 3 are nanocolumns embedded in a BTO matrix and vertical sandwich capacitors with a configuration of Pt/BTO:Sm 2 O 3 /Nb-STO have been used to investigate the dielectric properties. Electric field is applied parallel to the interfaces between Sm 2 O 3 and BTO (shown as Fig. 6(b)). On the other hand, V O s have been attracted at the interfaces and can be viewed as ions with positive charges 50,51 . With the assistance of an electric field, V O s can move along the vertical interfaces in the direction of electric field. However, the long range movement of V O s will be hampered by the misfit dislocations observed in the vertical interfaces, which results to the dielectric relaxation of the BTO:Sm 2 O 3 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.
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 V O s gathered at the vertical interfaces 26,[45][46][47][48][49] . With the assistance of an electric field, V O s can only move up and down along the vertical interfaces, which means that the transportation of V O s has been effectively confined. Meanwhile, misfit dislocations formed along the vertical interfaces can be used to manipulate the dynamics of V O s. These unique features are very helpful to investigate the transportation mechanism of V O s, 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 batteries 52-54 .

Discussion
In summary, epitaxial (BTO) 1-x :(Sm 2 O 3 ) 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 V O s at the vertical interfaces between the BTO and Sm 2 O 3 . With the assistance of an electric field, the movement of V O s 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 :(Sm 2 O 3 ) x thin films with compositions of x = 0.5 and 0.62 were deposited on (001) oriented SrTiO 3 (STO) and Nb-doped SrTiO 3 (Nb-STO) substrates by pulsed laser deposition (PLD) with a KrF excimer laser (Lambda Physik, λ = 248 nm). A laser fluence of ~2 J/cm 2 with a repetition rate of 3 Hz were focused onto composite targets with different molar ratios. An optimized substrate temperature of 720 o C 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 o C 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:Sm 2 O 3 / 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 cm 2 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.