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Sr2Bi4Ti5O18 bilayer composite thin films
Journal Pre-proof Effect of the thickness of Sr2Bi4Ti5O18 transition layer on the properties of BiFeO3/ Sr2Bi4Ti5O18 bilayer composite thin films Xuefeng Zhao, HuiYing Liu, Xiaoyue Pan, LingXu Wang, Zhe Feng, Xiaodong Guo, Peng Shen, Zhibiao Ma, Fengqing Zhang, Qingbo Tian, Suhua Fan PII:
S0272-8842(20)30056-0
DOI:
https://doi.org/10.1016/j.ceramint.2020.01.055
Reference:
CERI 23987
To appear in:
Ceramics International
Received Date: 17 November 2019 Revised Date:
1 January 2020
Accepted Date: 7 January 2020
Please cite this article as: X. Zhao, H. Liu, X. Pan, L. Wang, Z. Feng, X. Guo, P. Shen, Z. Ma, F. Zhang, Q. Tian, S. Fan, Effect of the thickness of Sr2Bi4Ti5O18 transition layer on the properties of BiFeO3/ Sr2Bi4Ti5O18 bilayer composite thin films, Ceramics International (2020), doi: https://doi.org/10.1016/ j.ceramint.2020.01.055. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Effect of the thickness of Sr2Bi4Ti5O18 transition layer on the properties of BiFeO3/Sr2Bi4Ti5O18 bilayer composite thin films Xuefeng Zhao a, HuiYing Liu a, Xiaoyue Pan a, LingXu Wang a, Zhe Feng a, Xiaodong Guo b, Peng Shen a, Zhibiao Ma a, Fengqing Zhang a, *, Qingbo Tian a, Suhua Fan b a. School of Materials Science and Engineering Shandong Jianzhu University, Jinan 250101, Shandong, China b. Shandong Women’s University, Jinan 250300, Shandong, China
Abstract BiFeO3(BFO)/Sr2Bi4Ti5O18(SBT) bilayer composite thin films were prepared using the sol-gel method. The influence of the SBT transition layer thickness on the properties of BFO/SBT bilayer composite thin films was studied. The results showed that an SBT transition layer with the proper thickness can improve the BFO/SBT bilayer composite film performance. X-ray diffraction (XRD) analysis revealed that a bismuth layered perovskite structure is formed in all of the thin films. The scanning electron microscopy (SEM) result showed that the BFO/80 nmSBT sample exhibits good crystallization, an improved density and a uniform minimum grain size compared with the other samples. The X-ray photoelectron spectroscopy (XPS) result showed that the Fe2+ content and oxygen vacancy concentration are lowest in the BFO/80 nm SBT sample. This sample has good ferroelectric properties: the leakage current density has the lowest value of J=5.21×10-5 A/cm2 among the samples at 350 kV/cm, 2Ec=900 kV/cm, and 2Pr=151 µC/cm2. At a low electric field (E<100 kV/cm), the leakage mechanism of the thin film is ohmic conduction, and the Fowler-Nordheim (F-N) tunneling effect is observed at a high electric field (E>100 kV/cm). The relative dielectric constant (εr) is 121, the dielectric loss (tanδ) is 0.024 at 105 Hz. Keywords: BFO/SBT; transition layer thickness; electric performance; XPS;
*Corresponding author: [email protected]
1. Introduction With the rapid development of information technology, the miniaturization and integration of electronic devices have become increasingly demanding. It is urgent to develop additional that multifunctional materials, as BiFeO3 with an ABO3 perovskite structure is one of the few room-temperature single-phase multiferroic materials[1,2]. BFO ferroelectric materials have a higher ferroelectric Curie temperature and antiferromagnetic Neel temperature than compared with those of other single-phase multiferroic materials[3]. Compared to other ABO3 perovskite structure materials, BFO ferroelectric materials have a large remanent polarization intensity of Pr~100 µC/cm2[4,5]. Therefore, BFO has broad application potential in the fields of microelectronics (ferroelectric memory and ferroelectric capacitor), micromechanics, and integrated optics (electro-optical effect devices)[6,7] and has become a popular research topic. However, i) the BFO thin film produces oxygen vacancies due to the volatilization of elemental Bi during the preparation process; ii) as the variable valence of Fe3+ changes to Fe2+, and the electron transfer between them can form a current, and the change in valence will also produce oxygen vacancy; and iii) an impurity phase (Bi2O3, Bi2Fe4O9, etc.) and other nonintrinsic factors such as porosity can easily form in the preparation process, which leads to a large leakage current. The high leakage current affect the performance of the BFO device and can even lead to device failure. Therefore, the high leakage current considerably restricts the practical application of BFO thin films[8]. At present, the main methods to reduce the BFO thin film leakage current are as follows: i) Excessive elemental Bi in the solution preparation is dissolved to avoid the unbalanced stoichiometric ratio caused by the volatilization of elemental Bi, producing an impurity phase and oxygen vacancy[9]. ii) Doping modification[10-12] is achieved via A site doping, B site doping and AB site codoping. iii) Composite thin films form are used as good insulation materials[13-15]. iv) A thin film with good insulation is introduced as a transition layer between the BFO thin film and the substrate to form a bilayer or a multilayer composite film[16,17]. The formation of
bilayer or multilayer thin films is a nanoscale structural design, which causes a more subtle microscopic change compared with ion-scale doping modification. Compared with composite film formation, the formation of bilayer or multilayer thin films is simpler, so researchers have paid increasing attention to this approach. Wu[18]et al. used radio frequency sputtering to prepare BLFZO/BLFSO bilayer films on SrRuO3 substrates to study the combined effect of the bilayer structure and ion substitution on BFO thin films. The remanent polarization of bilayer thin films,with a value of 2Pr~160.2 µC/cm2, resulted in good fatigue properties at a high pressure and high frequency. Ke[19]et al., using magnetic LSMO as the transition layer in BFO films, obtained BFO/LSMO bilayer thin films with excellent magnetic properties and no impurity phase. It was found that the LSMO transition layer could greatly enhance the magnetic performance of BFO thin films: notably, the saturation magnetization of the bilayer thin films was slightly higher than that of LSMO single-phase films. Wu[20] et al. prepared BGFO/BFMO bilayer thin films by radio frequency sputtering. The results show that 120 nmBGFO/120 nmBFMO exhibits a better hysteresis loop and ferroelectric properties, with a value of 2Pr~147.6 µC/cm2, than those of single-phase films. The introduction of a transition layer can effectively prevent charge transfer between the film and the electrode and inhibit the migration of oxygen vacancies in the film, thus improving the thin films properties. Sr2Bi4Ti5O18 is a material with a bismuth-layered perovskite structure. SBT thin films have a low leakage current, a high dielectric constant and favorable fatigue properties: in addition, there is no evident fatigue phenomenon after 1011 cycles of reversal[21]. The good insulation provided by (Bi2O2)2+ allows SBT to play an effective role as an insulator, and the construction of a bilayer structure can inhibit the migration of the oxygen vacancies in the thin film, thus decreasing the leakage current of the bilayer composite film. There have been no reports on the use of SBT as a transition layer with BFO to construct a bilayer composite structure. Therefore, this study used the sol-gel method to prepare BFO/SBT bilayer composite thin films. The influence of the transition layer thickness on the performance and structure of the
BFO/SBT bilayer composite thin films was studied systematically. The thickness of the optimum transition layer and the related mechanism were determined. 2. Experimental procedure Bismuth nitrate pentahydrate (BiN3O9·5H2O), strontium acetate (C4H6O4Sr·0.5H2O), and tetrabutyl titanate (C16H36O4Ti) were used as raw materials for the SBT thin films. Ethylene glycol (HOCH2CH2OH) and acetylacetone(C5H8O2) were used as solvents and chelating agents. The raw materials of the BFO thin films were iron nitrate [Fe(NO3)3·9H2O] and bismuth nitrate [Bi(NO3)3·5H2O], and the solvents included ethylene glycol (HOCH2CH2OH) and glacial acetic acid (CH3COOH). The chelating agent was acetylacetone (C5H8O2). The solid raw materials in this paper were weighed using a METTLER TOLEDO balance. The precursor solutions of SBT and BFO thin films were prepared according to stoichiometric ratios using the sol-gel method. The excess amount of bismuth salt in the precursor solution was 5% because at high temperatures, bismuth salt is highly volatile. The wet films were obtained by homogenizing the prepared precursor solution on the cleaned Pt substrate via a Kw-4A spin coater. The dry films were obtained by drying the wet film on a stainless steel heating plate at a temperature of 200℃. Then, the dry films were placed in a rapid annealing furnace. The SBT and BFO thin films were first presintered at 350℃. The SBT thin films were prepared after annealing at 750℃, and then the BFO films were annealed at 525℃ to obtain crystalline films. The BFO/SBT bilayer composite films were first prepared by spinning different thicknesses (0, 40, 80 and 120 nm) of the SBT transition layer on Pt substrates and then spinning 770 nm BFO films on the basis of the transition layer. The structure of the bilayer composite films was measured by a D8 ADVANCE X-ray diffraction (XRD) diffractometer (Cu-Kα, λ=0.154178 nm). The surface morphology and the thickness of the thin film cross section were measured using cold field emission scanning electron microscopy (FESEM) (SU8010). The ferroelectric properties of the bilayer composite thin films were tested using a ferroelectric tester (Radiant Precision Workstation). A TH2828S Automatic Component Analyzer instrument was used to measure the dielectric properties of the prepared thin films.
The elemental valence of the bilayer composite thin films was measured via an X-ray photon energy spectrum analyzer. 3. Results and discussion The XRD diffraction patterns of the BFO/SBT bilayer composite thin films are shown in Fig. 1. The investigated thicknesses of the SBT transition layer are 0, 40, 80 and 120 nm. The lattice mismatch of the transition layer and the thin film layer greatly influences the bilayer composite structure, and the lattice mismatch of the SBT and BFO is analyzed first. The formula for of lattice mismatch is as follows[22]: S=
a f − as af
(1)
where S represents the lattice mismatch of the two materials; a f is the lattice constant of the upper thin film; and as is the lattice constant of the substrate. The exact meaning of the lattice constant a in Eq.1 is shown in Table 1 When S<5%, the two materials are completely coherent. When S=5%-25%, the two materials are semicoherent. The two materials are completely inconsistent when S>25%. According to the standard card, the lattice constant a=0.3923 nm of Pt(fcc), the lattice constant a=0.3860 nm of the SBT (JCPDS:14-0276) thin film and the lattice constant a=0.5578 nm of the BFO (JCPDS:86-1518) thin film are identified. According to the formula for lattice mismatch, the lattice mismatch between Pt and the SBT thin film and between Pt and the BFO thin film is 29.67% and 30.80%, respectively. Therefore, the lattice constants of BFO and SBT are different, which leads to lattice mismatch. The lattice mismatch between the BFO and SBT layers causes strain in the thin films[32]. Fig. 1 (a) shows that all samples are well crystallized and that a pure perovskite structure is formed without the formation of an impurity phase. In addition, the diffraction peaks of SBT and BTO are reflected in the figure, which shows that the structures of SBT and BTO are well preserved. Peak (012) shifts to a small angle with the introduction of the SBT transition layer, as clearly shown in Fig. 1 (b), which may be due to i) the lattice mismatch between the transition layer and the thin film layer ii) the stress between the two layers and in the film[23]. In addition, we fitted the lattice
constant of all the samples, as shown in Fig. 1 (c). As the thickness of the transition layer increases, the unit cell volume decreases, which corresponds to Fig. 1 (b). A schematic diagram of the double-layer structure is shown in Fig. 1 (d). Table 1 Fig. 1 The cross sections and surface scanning diagrams of the BFO/SBT bilayer composite films are exhibited in Fig. 2 (a)-(d) and Fig. 3 (a)-(d), respectively. As shown in Fig. 2 (a)-(d), the investigated thicknesses of the SBT transition layer are 0, 40, 80 and 120 nm, respectively. The thickness of the BFO film layer is approximately 770 nm. It can be seen from Fig. 2(b) that the dividing line between SBT and BFO is not clear, which may be because i) When the transition layer is 40nm, it is too thin to be within the instrument test error, ii) the substrate offset occurs in the preparation of the test sample so that the transition layer is covered up. Nano Measurer software was used to calculate the statistics via random sampling of different sample surfaces, and the histogram for the particle size analysis was obtained by Gaussian fitting. The illustration in Fig. 3 shows that the average grain sizes of the four samples are approximately 106.7 nm, 129.8 nm, 79.2 nm and 84.2 nm, respectively. As can be seen from Fig. 3, the pinhole defects on the surface of the BFO/SBT bilayer composite films are obviously smaller than those of the single-layer BFO thin films, and the surface of the BFO/SBT bilayer composite film is smoother and denser. As the thickness of the SBT transition layer increases, the grain size of the BFO thin film first increases and then decreases, and the grain sizes become more uniform. Among the studied thicknesses, when the transition layer is 80 nm thick, the surface of the thin film is the densest, and the grain size of the thin film is the smallest and most uniform. It is concluded that the addition of the transition layer improves the crystallization properties of the BFO/SBT bilayer composite films to a certain extent, which may explain why the grain size of the SBT films is smaller and the films are than those of the composite films[24]. Fig. 2 Fig. 3
To confirm the effect of the change in valence state and the oxygen vacancy on the performance of the bilayer composite films, the X-ray photoelectron spectroscopy (XPS) spectra of the thin films with transition layers of different thicknesses were measured. The XPS fitting diagrams of the O1s orbital before etching of the transition layer samples with different thicknesses are shown in Fig. 4 (a)-(d). Two combined energy values, 529 eV and 531 eV, appear. According to the XPS binding energy comparison table, the binding energy of Bi2O3 is 529.8 eV. Therefore the lower binding energy peak is lattice oxygen[25]. The determination of the binding energy near 531 eV is more complicated. To determine if the binding energy peaks near 531 eV, the XPS energy spectrum of the O1s orbital with an etching time of 16 min is further fitted, as shown in Fig. 5 (a)-(d). From Fig. 4 and Fig. 5, the binding energy near 531 eV only exists on the surface of the sample, meaning that this peak may be the adsorbed oxygen species on the surface. Fig. 4 Fig. 5 The leakage behavior in BFO is mainly caused by the variable valence of Fe3+[26]. Therefore, the Fe2p2/3 spectra of the BFO/SBT bilayer composite films are fitted, as shown in Fig. 6 (a)-(d). The ratios of the relative contents of Fe3+ and Fe2+ in the samples with transition layer thicknesses of 0, 40, 80 and 120 nm are 3.28, 4.57, 4.61 and 4.30, respectively. The results show that the concentration of Fe3+ in the films with a transition layer is higher than that in the films without a transition layer. The relative content of Fe3+ in the BFO/80 nm SBT sample is the highest among the samples. According to the defect equation[27], the concentration of Fe3+ is inversely proportional to the oxygen vacancy concentration. Therefore, an SBT transition layer with the appropriate thickness can reduce the oxygen vacancies in the bilayer composite film to a certain extent. When the thickness of the transition layer thickness is 80 nm, the oxygen vacancy content in the bilayer composite film is relatively low. Fe3++O2-→2Fe2++ VO2− +1/2O2 Fig. 6
(2)
Fig. 7 Fig 7 shows the XPS depth profile of elements of the BFO/80 nmSBT sample, which depicts the relationship between the content of each element in the thin films and the etching depth. Fig.7 shows that four distinct regions exist in the SBT/BFO bilayer composite films, which are i) the surface region, ii) the BFO thin film layer, iii) the SBT transition layer, and iv) the Pt substrate. The possible reasons for the high content of Bi, O, Pt and Sr in the surface layer are as follows: i) Bi is volatile at high temperatures[28], ii) surface adsorbed oxygen is present, iii) platinum is sprayed on the surface before testing the ferroelectric performance, and iv) this may be related to the segregation of some of the Sr in SBT transition layer onto the surface of BFO[29]. The contents of various elements in the BFO thin film region remain stable with increasing in etching time, which indicates the uniform distribution of these various elements in the film. Ti ions in the SBT transition layer were observed on the surface of the SBT/BFO bilayer composite films. Similar phenomena were observed in the Y(Ni0.5Mn0.5)O3/SrTiO3[30] structure by other scholars. Fig. 8 exhibits the hysteresis loop of BFO/SBT bilayer composite thin films with different transition layer thicknesses at 1 kHz. The remanent polarization of the sample increases as the test electric field increases because the oxygen vacancies can generate the formation of composite defect pairs, and at a low electric field, a composite defect pair can inhibit the reversal of the ferroelectric domain. However, at a high electric field, the effect of composite defects on ferroelectric domain inversion decreases, and the ferroelectric domain can be flipped more completely, thus inducing a greater remanent polarization than at a low electric field[31,32]. The remanent polarization 2Pr values of the samples with 0, 40, 80 and 120 nm transition layer thicknesses are 106, 147, 151 and 79 µC/cm2 at 1200 kV/cm, respectively. The coercive fields 2Ec are 1100, 1000, 900 and 1000 kV/cm, respectively. As the transition layer becomes thicker, the 2Pr of the bilayer composite film first increases and then decreases, which indicates that the ferroelectric performance of the composite film can be enhanced by adding an SBT transition layer with the appropriate. Among the samples tested, the BFO/80 nm SBT sample has a smaller
coercive field and the largest remanent polarization. The possible reasons for these results causes are as follows: i) Because of its low leakage current, SBT acts as an insulator in the bilayer composite film, which improves the film resistance thus improving the remanent polarization[33,34]. ii) The better, denser, and smaller the defect pair is during crystallization of the film, the greater the remanent polarization strength of the sample. iii) XRD analysis shows that the lattice mismatch between the SBT transition layer and the BFO film layer produces stress, which affects the remanent polarization of the bilayer composite film to a certain extent[35]. iv) The oxygen vacancy concentration in the film with a transition layer thickness of 80 nm is low, so the ferroelectric properties are clearly identified. v) As the SBT becomes thicker, the remanent polarization of the bilayer composite films decreases, which may be due to the low remanent polarization of the SBT itself, and the addition of the transition layer "dilutes" the remnant polarization of the whole bilayer composite film. Therefore, with a thicker of the transition layer, this "dilution" effect is also enhanced, leading to a decrease in the remanent polarization of the bilayer composite films[36,37]. vi) As the transition layer thickens, the effective electric field acting on BFO decreases, which results in a decrease in the residual polarization of the bilayer composite films. Fig. 8 The leakage performance (J-E) of the BFO/SBT bilayer composite film with different transition layer thicknesses is shown in Fig. 9. In the test electric field, the leakage current density ultimately increases. The leakage current density first decreases and then increases as the transition layer layer becomes thicker. The leakage current density (5.21×10−5 A/cm2 at 350 kV/cm) of the BFO/80 nm SBT thin film is the smallest among the tested films. The phenomenon above can be explained as follows: i) The (Bi2O2)2+ in the SBT transition layer acts as an efficient insulator between BFO and the Pt substrate; this insulator can block the movement of oxygen vacancies, thus reducing the leakage current density of the bilayer composite film[38]. ii) SBT and BFO have different work functions. A potential barrier may be formed at the interface between them, thus blocking the conduction of carriers and reducing the
leakage current density of bilayer composite films[39]. iii) The oxygen vacancies in thin films with smaller grains are easier to pin, so the antileakage performance of thinner film samples is better than that of thicker films. In addition, the proportion of grain boundaries increases, and the leakage current channel lengthens in the samples with smaller grains, which also reduces the leakage current[40]. Table 1 shows the performances results from this article and the performance data from the literature. There are no previous reports that use SBT as a transition layer with BFO to construct a bilayer composite structure. It can be seen from Table 1 that using SBT as the transition layer can improve the remanent polarization and reduce the leakage current of a film to a certain extent. Table 2
Fig. 9 The study of the leakage mechanism is of great significance to reduce the leakage current. The leakage mechanism is mainly divided into volume-limited conductivity and interface-limited conductivity. At present, the volume-limited conductivity is mainly
ohmic
conduction,
space-charge
limited
conduction
(SCLC)
and
Poole-Frenkel (P-F) emission; the interface-limited conductivity mainly includes Schottky emission and the Fowler-Nordheim (F-N) tunneling effect. By analyzing the diagram of log(J) and log(E) shown in Fig. 10, the leakage current mechanism of the bilayer composite films is determined. The different slopes represent different leakage mechanisms. As shown, when the thickness of the transition layer is 0, 40, 80 and 120 nm at a low electric field (E<100 kV/cm), the α values of the samples are 1.2, 1.1, 1.0 and 1.0, respectively. Therefore, the leakage mechanisms of all the thin film samples under a low electric field are mainly ohmic conduction (α~1)[41].This result could be due to the thermal emission of electrons[42], free electrons and holes. As the electric field increases, the α value of the BFO/80 nm SBT sample increases to 2.0, which suggests that the main leakage mechanism is SCLC(α~2)[44]. However, the SCLC leakage mechanism was not observed in the other samples. The SCLC mechanism is mainly related to the number of injected carriers in the trap. When the number of
injected carriers is large enough, the conductivity of the film increases, and the SCLC leakage mechanism is observed. With a further increase in the test electric field, the α values of the samples with transition layer thicknesses of 0, 40, 80 and 120 nm are increased to 2.8/5.3, 3.1/4.6, 4.7, and 4.0/5.6, respectively, indicating that there are other leakage mechanisms operating in the thin film samples under a high electric field. This divergence may arise because the carriers have filled the trap[23], which will be discussed further. Fig. 10 In addition to ohmic conduction and SCLC conduction mechanisms, P-F emission, Schottky emission and F-N tunneling effect mechanisms may also occur. The mechanism of P-F emission is that the composite defect in the defect center can be excited to the conduction band by thermal excitation under a very strong external electric field. The Schottky barrier is weakened because of the application of an electric field, and some electrons can escape the Schottky barrier, corresponding to the Schottky emission mechanism. The equations of these two leakage mechanisms are as follows[45]:
J S = AT 2e J PF = B E e
-
-
φ−
EI −
q 3 E / 4 π ε0 k K BT q 3 E / πε0 k K BT
(3)
(4)
where K represents the optical dielectric constant, φ, E and EI are the height, electric field intensity and ionization energy of the trap, respectively, and A and B represent two constants. T is the heat transfer temperature (RT), q represents the electron charge, ε0 represents the vacuum dielectric constant and KB represents the Boltzmann constant. As shown in Fig. 11 (a)-(b), the curves of ln(J/T2)-E1/2 and ln(J/E)-E1/2 are drawn according to formulas (3) and (4). The above two formulas show that the K value can be deduced from the value of α. Then the value of α can be seen from Fig. 10 (a)-(b). According to the refractive index n=2.5 of BFO, K=n2=6.25[46]. The K value fitted in Fig. 11 (a)-(b) differs greatly from the theoretical K value; therefore, the leakage
mechanism of the thin film sample does not belong to the above two types under a high electric field. The leakage mechanism of the F-N tunneling effect easily occurs in a high electric field. The formula for the F-N tunneling effect is as follows[47]:
J FN = C E 2e
-
ϕ i3
D2 E
(5)
where φi represents the barrier height and C represents a constant. According to the formula, the diagrams of the ln(J/E2) and 1/E curves are drawn in Fig. 11 (c). To determine whether a result is due to the F-N tunneling effect, the linear relationship with the above-mentioned curve is observed and analyzed. As shown in Fig. 11 (c), all of the ln(J/E2) and 1/E curves have a good linear relationship. Therefore, the F-N tunneling effect is the main leakage mechanism of the thin films at high electric fields. At a low electric field, the BFO/SBT bilayer composite thin films primarily exhibit volume-limited conduction, whereas they primarily exhibit interface-limited conduction at a higher electric field. The results of particular measurements (values of important properties) for all the samples are shown in Table 2. Table 3 Fig. 11 Fig. 12 shows the dielectric constant(εr) and loss(tanδ) curves of the bilayer composite films with frequency. As shown in Fig. 12, the dielectric frequency of all the samples has good stability. The dielectric losses of the BFO/SBT bilayer composite films are 0.049, 0.025, 0.024 and 0.45 at 105 Hz, and the dielectric constants are 90.9, 115, 121 and 123 at 105 Hz, respectively. Among the tested films the BFO/80 nmSBT samples have the lowest dielectric loss, which shows that this film has the fewest charge defects and lower conductivity[47]. As can be seen from Fig. 12, the dielectric constant and the dielectric loss increase with as the thickening of the transition layer becomes thicker. Possible reasons for this result are as follows: i) The sample with the transition layer is denser than the sample without a transition layer. The denser the film is, the greater the dielectric constant is, which is consistent with the test results in Fig. 3. ii) The effective tanδ increases with increasing film
thickness[48,49]. iii) The largest dielectric constant of the specimen with a 120 nm thick SBT transitional layer should be related to the space change contribution of space change to the polarization.[50]. iv) With the increase in thickness, the stress is relaxed as discussed above. This is beneficial to the domain wall motion and will leads to a larger dielectric constant[49]. v) The increase in dielectric loss is mainly due to the increase in oxygen vacancies and leakage current as the transition layer becomes thicker. Fig 12.
4. Conclusions The thickness effect of the SBT transition layer on the properties of BFO/SBT bilayer composite films was studied in this paper. After detailed analysis, it was found that the properties of the BFO/SBT bilayer composite films can be improved with an optimal SBT transition layer thickness. The XRD and SEM analysis results show that all of the BFO/SBT bilayer composite films grow as a layered perovskite structure without other impurity phases. The BFO/80 nm SBT samples exhibit good crystallization, a uniform minimum grain size, and an improved density compared with the other samples. The XPS results show the lowest concentration of the oxygen vacancies and Fe2+ in the BFO/80 nm SBT samples. The abovementioned samples have good ferroelectric properties: 2Pr=151 µC/cm2, 2Ec= 900 kV/cm, and J=5.21×10-5 A/cm2 at 350 kV/cm. Under a high electric field (E>100 kV/cm), the leakage mechanism is the F-N tunneling effect, which is ohmic conduction at a low electric field (E<100 kV/cm). The relative dielectric constant is εr=121, and the dielectric loss is tanδ= 0.024 at 105 Hz.
Acknowledgments This work was supported by funding from the Research Fund for the Doctoral Program of Shandong Jianzhu University (Grant No. XNBS1626) and the Program of the Housing and Urban-Rural Construction Department of Shandong Province (2019-K7-10).
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Fig.1 XRD pattern of BFO/SBT bilayer composite thin films:(a) 2θ=20°-60° (b) 2θ=31.5°-33°(c)lattice constant (d)structure schematic diagram Fig.2 Cross sections diagram of BFO/SBT bilayer composite films:(a) 0 nm (b) 40 nm (c) 80 nm (d) 120 nm Fig.3 Surface diagram of BFO/SBT bilayer composite films:(a) 0 nm (b) 40 nm (c) 80 nm (d) 120 nm Fig.4 O1s XPS surface spectra of BFO/SBT bilayer composite films:(a) 0 nm (b) 40 nm (c) 80 nm (d) 120 nm Fig.5 O1s XPS spectra of BFO/SBT bilayer composite film after Ar+ ion for 16 min (a) 0nm (b) 40nm (c) 80nm (d) 120nm Fig.6 Fe2p2/3 XPS spectra of BFO/SBT bilayer composite films:(a) 0 nm (b) 40 nm (c) 80 nm (d) 120 nm Fig.7 XPS elemental depth profiling of BFO/80 nmSBT sample bilayer composite film Fig.8 P-E hysteresis loops of BFO/ SBT bilayer composite film (a)0 nm (b)40 nm (c)80 nm (d)120 nm Fig.9 J-E curves of BFO/SBT bilayer composite thin film Fig. 10 logJ-logE curves of BFO/SBT bilayer composite films Fig. 11 (a) Scbottky emission (b) Pool-Frenkel emission (c) Fowler-Nordeim tunneling plotted of bilayer composite thin films Fig. 12 The dielectric constant and dielectric loss of the BFO/SBT bilayer composite film with frequency
Table 1 The exact meaning of the lattice constant a in Eq.1
thickness of transition
0
40
80
120
af
a BFO
a BFO
a BFO
a BFO
as
a Pt
aSBT
aSBT
aSBT
29.67
30.80
30.80
30.80
layer(nm)
S (%)
Table 2 The performances of this article and the literature data contrast table.
2Pr ferroelectric material
flexible substrate
2Ec
Ref. J(A/cm2)
method (µC/cm2)
(kV/cm)
BiCrO3/BiFeO3
Pt/Ti/Si/SiO2
CSD
37
1004
SrTiO3/ BiFeO3
Pt/Ti/Si/SiO2
PLD
96
540~550
BiFeO3/Bi3.25Sm0.75Ti2.98V0.02O12
Pt/Ti/Si/SiO2
PLD
71.8
148
BiFeO3/BaTiO3
SrTiO3
2.88×10-4
[8]
[13] [35]
RF-MS
[51] 6.26×10-6
4.4 (sputtering) Sel-gel
BiCrO3/BiFeO3
Pt/Ti/Si/SiO2
[52] 16.95
57.08
1.37×10-4
(spin coating) BiFeO3/Bi3.5Nd0.5Ti3O12
BiFeO3/Bi0.5(Na0.85K0.15)0.5TiO3
ITO
MOD
70.2
8.38×10-6
[53]
Pt/Ti/Si/SiO2
CSD
21.6
8.76×10-7
[54]
34.8
1.71×10-4
Sel-gel BiFeO3/Bi5Ti3FeO15
Pt/Ti/Si/SiO2
[55]
(spin coating) BiFeO3/CoFe2O4
SrTiO3
BiFeO3/ BaTiO3
Pt/Ti/Si/SiO2
self-assembly
[56]
91.4
RF-MS
[57] 30
(sputtering) Sel-gel BiFeO3/Pb(Zr0.52Ti0.48)O3
Pt/Ti/Si/SiO2
[58] 80
(spin coating) BiFeO3/ Sr2Bi4Ti5O18
Sel-gel Pt/Ti/Si/SiO2
[This 151
(spin coating)
900
5.21×10-5 work]
Table 3 The results of particular measurements (values of important properties) for all samples. thickness of transition layer(nm)
2Pr(µC/cm2)
2Ec(kV/cm)
@ 1200kV/cm 0
106
J(A/cm2)
leakage mechanism
Fe3+:Fe2+
Ohmic conduction
3.28
@ 350kV/cm 1100
5.85×10−4
1000
9.30×10
−5
5.21×10
−5
1.72×10
−3
and F-N tunneling effect 40
147
Ohmic conduction
4.57
and F-N tunneling effect 80
151
900
Ohmic conduction
4.61
and F-N tunneling effect 120
79
1000
Ohmic conduction and F-N tunneling effect
4.30
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
Fig. 12
Declaration of interests ☒ 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.
S0272-8842(20)30056-0
DOI:
https://doi.org/10.1016/j.ceramint.2020.01.055
Reference:
CERI 23987
To appear in:
Ceramics International
Received Date: 17 November 2019 Revised Date:
1 January 2020
Accepted Date: 7 January 2020
Please cite this article as: X. Zhao, H. Liu, X. Pan, L. Wang, Z. Feng, X. Guo, P. Shen, Z. Ma, F. Zhang, Q. Tian, S. Fan, Effect of the thickness of Sr2Bi4Ti5O18 transition layer on the properties of BiFeO3/ Sr2Bi4Ti5O18 bilayer composite thin films, Ceramics International (2020), doi: https://doi.org/10.1016/ j.ceramint.2020.01.055. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Effect of the thickness of Sr2Bi4Ti5O18 transition layer on the properties of BiFeO3/Sr2Bi4Ti5O18 bilayer composite thin films Xuefeng Zhao a, HuiYing Liu a, Xiaoyue Pan a, LingXu Wang a, Zhe Feng a, Xiaodong Guo b, Peng Shen a, Zhibiao Ma a, Fengqing Zhang a, *, Qingbo Tian a, Suhua Fan b a. School of Materials Science and Engineering Shandong Jianzhu University, Jinan 250101, Shandong, China b. Shandong Women’s University, Jinan 250300, Shandong, China
Abstract BiFeO3(BFO)/Sr2Bi4Ti5O18(SBT) bilayer composite thin films were prepared using the sol-gel method. The influence of the SBT transition layer thickness on the properties of BFO/SBT bilayer composite thin films was studied. The results showed that an SBT transition layer with the proper thickness can improve the BFO/SBT bilayer composite film performance. X-ray diffraction (XRD) analysis revealed that a bismuth layered perovskite structure is formed in all of the thin films. The scanning electron microscopy (SEM) result showed that the BFO/80 nmSBT sample exhibits good crystallization, an improved density and a uniform minimum grain size compared with the other samples. The X-ray photoelectron spectroscopy (XPS) result showed that the Fe2+ content and oxygen vacancy concentration are lowest in the BFO/80 nm SBT sample. This sample has good ferroelectric properties: the leakage current density has the lowest value of J=5.21×10-5 A/cm2 among the samples at 350 kV/cm, 2Ec=900 kV/cm, and 2Pr=151 µC/cm2. At a low electric field (E<100 kV/cm), the leakage mechanism of the thin film is ohmic conduction, and the Fowler-Nordheim (F-N) tunneling effect is observed at a high electric field (E>100 kV/cm). The relative dielectric constant (εr) is 121, the dielectric loss (tanδ) is 0.024 at 105 Hz. Keywords: BFO/SBT; transition layer thickness; electric performance; XPS;
*Corresponding author: [email protected]
1. Introduction With the rapid development of information technology, the miniaturization and integration of electronic devices have become increasingly demanding. It is urgent to develop additional that multifunctional materials, as BiFeO3 with an ABO3 perovskite structure is one of the few room-temperature single-phase multiferroic materials[1,2]. BFO ferroelectric materials have a higher ferroelectric Curie temperature and antiferromagnetic Neel temperature than compared with those of other single-phase multiferroic materials[3]. Compared to other ABO3 perovskite structure materials, BFO ferroelectric materials have a large remanent polarization intensity of Pr~100 µC/cm2[4,5]. Therefore, BFO has broad application potential in the fields of microelectronics (ferroelectric memory and ferroelectric capacitor), micromechanics, and integrated optics (electro-optical effect devices)[6,7] and has become a popular research topic. However, i) the BFO thin film produces oxygen vacancies due to the volatilization of elemental Bi during the preparation process; ii) as the variable valence of Fe3+ changes to Fe2+, and the electron transfer between them can form a current, and the change in valence will also produce oxygen vacancy; and iii) an impurity phase (Bi2O3, Bi2Fe4O9, etc.) and other nonintrinsic factors such as porosity can easily form in the preparation process, which leads to a large leakage current. The high leakage current affect the performance of the BFO device and can even lead to device failure. Therefore, the high leakage current considerably restricts the practical application of BFO thin films[8]. At present, the main methods to reduce the BFO thin film leakage current are as follows: i) Excessive elemental Bi in the solution preparation is dissolved to avoid the unbalanced stoichiometric ratio caused by the volatilization of elemental Bi, producing an impurity phase and oxygen vacancy[9]. ii) Doping modification[10-12] is achieved via A site doping, B site doping and AB site codoping. iii) Composite thin films form are used as good insulation materials[13-15]. iv) A thin film with good insulation is introduced as a transition layer between the BFO thin film and the substrate to form a bilayer or a multilayer composite film[16,17]. The formation of
bilayer or multilayer thin films is a nanoscale structural design, which causes a more subtle microscopic change compared with ion-scale doping modification. Compared with composite film formation, the formation of bilayer or multilayer thin films is simpler, so researchers have paid increasing attention to this approach. Wu[18]et al. used radio frequency sputtering to prepare BLFZO/BLFSO bilayer films on SrRuO3 substrates to study the combined effect of the bilayer structure and ion substitution on BFO thin films. The remanent polarization of bilayer thin films,with a value of 2Pr~160.2 µC/cm2, resulted in good fatigue properties at a high pressure and high frequency. Ke[19]et al., using magnetic LSMO as the transition layer in BFO films, obtained BFO/LSMO bilayer thin films with excellent magnetic properties and no impurity phase. It was found that the LSMO transition layer could greatly enhance the magnetic performance of BFO thin films: notably, the saturation magnetization of the bilayer thin films was slightly higher than that of LSMO single-phase films. Wu[20] et al. prepared BGFO/BFMO bilayer thin films by radio frequency sputtering. The results show that 120 nmBGFO/120 nmBFMO exhibits a better hysteresis loop and ferroelectric properties, with a value of 2Pr~147.6 µC/cm2, than those of single-phase films. The introduction of a transition layer can effectively prevent charge transfer between the film and the electrode and inhibit the migration of oxygen vacancies in the film, thus improving the thin films properties. Sr2Bi4Ti5O18 is a material with a bismuth-layered perovskite structure. SBT thin films have a low leakage current, a high dielectric constant and favorable fatigue properties: in addition, there is no evident fatigue phenomenon after 1011 cycles of reversal[21]. The good insulation provided by (Bi2O2)2+ allows SBT to play an effective role as an insulator, and the construction of a bilayer structure can inhibit the migration of the oxygen vacancies in the thin film, thus decreasing the leakage current of the bilayer composite film. There have been no reports on the use of SBT as a transition layer with BFO to construct a bilayer composite structure. Therefore, this study used the sol-gel method to prepare BFO/SBT bilayer composite thin films. The influence of the transition layer thickness on the performance and structure of the
BFO/SBT bilayer composite thin films was studied systematically. The thickness of the optimum transition layer and the related mechanism were determined. 2. Experimental procedure Bismuth nitrate pentahydrate (BiN3O9·5H2O), strontium acetate (C4H6O4Sr·0.5H2O), and tetrabutyl titanate (C16H36O4Ti) were used as raw materials for the SBT thin films. Ethylene glycol (HOCH2CH2OH) and acetylacetone(C5H8O2) were used as solvents and chelating agents. The raw materials of the BFO thin films were iron nitrate [Fe(NO3)3·9H2O] and bismuth nitrate [Bi(NO3)3·5H2O], and the solvents included ethylene glycol (HOCH2CH2OH) and glacial acetic acid (CH3COOH). The chelating agent was acetylacetone (C5H8O2). The solid raw materials in this paper were weighed using a METTLER TOLEDO balance. The precursor solutions of SBT and BFO thin films were prepared according to stoichiometric ratios using the sol-gel method. The excess amount of bismuth salt in the precursor solution was 5% because at high temperatures, bismuth salt is highly volatile. The wet films were obtained by homogenizing the prepared precursor solution on the cleaned Pt substrate via a Kw-4A spin coater. The dry films were obtained by drying the wet film on a stainless steel heating plate at a temperature of 200℃. Then, the dry films were placed in a rapid annealing furnace. The SBT and BFO thin films were first presintered at 350℃. The SBT thin films were prepared after annealing at 750℃, and then the BFO films were annealed at 525℃ to obtain crystalline films. The BFO/SBT bilayer composite films were first prepared by spinning different thicknesses (0, 40, 80 and 120 nm) of the SBT transition layer on Pt substrates and then spinning 770 nm BFO films on the basis of the transition layer. The structure of the bilayer composite films was measured by a D8 ADVANCE X-ray diffraction (XRD) diffractometer (Cu-Kα, λ=0.154178 nm). The surface morphology and the thickness of the thin film cross section were measured using cold field emission scanning electron microscopy (FESEM) (SU8010). The ferroelectric properties of the bilayer composite thin films were tested using a ferroelectric tester (Radiant Precision Workstation). A TH2828S Automatic Component Analyzer instrument was used to measure the dielectric properties of the prepared thin films.
The elemental valence of the bilayer composite thin films was measured via an X-ray photon energy spectrum analyzer. 3. Results and discussion The XRD diffraction patterns of the BFO/SBT bilayer composite thin films are shown in Fig. 1. The investigated thicknesses of the SBT transition layer are 0, 40, 80 and 120 nm. The lattice mismatch of the transition layer and the thin film layer greatly influences the bilayer composite structure, and the lattice mismatch of the SBT and BFO is analyzed first. The formula for of lattice mismatch is as follows[22]: S=
a f − as af
(1)
where S represents the lattice mismatch of the two materials; a f is the lattice constant of the upper thin film; and as is the lattice constant of the substrate. The exact meaning of the lattice constant a in Eq.1 is shown in Table 1 When S<5%, the two materials are completely coherent. When S=5%-25%, the two materials are semicoherent. The two materials are completely inconsistent when S>25%. According to the standard card, the lattice constant a=0.3923 nm of Pt(fcc), the lattice constant a=0.3860 nm of the SBT (JCPDS:14-0276) thin film and the lattice constant a=0.5578 nm of the BFO (JCPDS:86-1518) thin film are identified. According to the formula for lattice mismatch, the lattice mismatch between Pt and the SBT thin film and between Pt and the BFO thin film is 29.67% and 30.80%, respectively. Therefore, the lattice constants of BFO and SBT are different, which leads to lattice mismatch. The lattice mismatch between the BFO and SBT layers causes strain in the thin films[32]. Fig. 1 (a) shows that all samples are well crystallized and that a pure perovskite structure is formed without the formation of an impurity phase. In addition, the diffraction peaks of SBT and BTO are reflected in the figure, which shows that the structures of SBT and BTO are well preserved. Peak (012) shifts to a small angle with the introduction of the SBT transition layer, as clearly shown in Fig. 1 (b), which may be due to i) the lattice mismatch between the transition layer and the thin film layer ii) the stress between the two layers and in the film[23]. In addition, we fitted the lattice
constant of all the samples, as shown in Fig. 1 (c). As the thickness of the transition layer increases, the unit cell volume decreases, which corresponds to Fig. 1 (b). A schematic diagram of the double-layer structure is shown in Fig. 1 (d). Table 1 Fig. 1 The cross sections and surface scanning diagrams of the BFO/SBT bilayer composite films are exhibited in Fig. 2 (a)-(d) and Fig. 3 (a)-(d), respectively. As shown in Fig. 2 (a)-(d), the investigated thicknesses of the SBT transition layer are 0, 40, 80 and 120 nm, respectively. The thickness of the BFO film layer is approximately 770 nm. It can be seen from Fig. 2(b) that the dividing line between SBT and BFO is not clear, which may be because i) When the transition layer is 40nm, it is too thin to be within the instrument test error, ii) the substrate offset occurs in the preparation of the test sample so that the transition layer is covered up. Nano Measurer software was used to calculate the statistics via random sampling of different sample surfaces, and the histogram for the particle size analysis was obtained by Gaussian fitting. The illustration in Fig. 3 shows that the average grain sizes of the four samples are approximately 106.7 nm, 129.8 nm, 79.2 nm and 84.2 nm, respectively. As can be seen from Fig. 3, the pinhole defects on the surface of the BFO/SBT bilayer composite films are obviously smaller than those of the single-layer BFO thin films, and the surface of the BFO/SBT bilayer composite film is smoother and denser. As the thickness of the SBT transition layer increases, the grain size of the BFO thin film first increases and then decreases, and the grain sizes become more uniform. Among the studied thicknesses, when the transition layer is 80 nm thick, the surface of the thin film is the densest, and the grain size of the thin film is the smallest and most uniform. It is concluded that the addition of the transition layer improves the crystallization properties of the BFO/SBT bilayer composite films to a certain extent, which may explain why the grain size of the SBT films is smaller and the films are than those of the composite films[24]. Fig. 2 Fig. 3
To confirm the effect of the change in valence state and the oxygen vacancy on the performance of the bilayer composite films, the X-ray photoelectron spectroscopy (XPS) spectra of the thin films with transition layers of different thicknesses were measured. The XPS fitting diagrams of the O1s orbital before etching of the transition layer samples with different thicknesses are shown in Fig. 4 (a)-(d). Two combined energy values, 529 eV and 531 eV, appear. According to the XPS binding energy comparison table, the binding energy of Bi2O3 is 529.8 eV. Therefore the lower binding energy peak is lattice oxygen[25]. The determination of the binding energy near 531 eV is more complicated. To determine if the binding energy peaks near 531 eV, the XPS energy spectrum of the O1s orbital with an etching time of 16 min is further fitted, as shown in Fig. 5 (a)-(d). From Fig. 4 and Fig. 5, the binding energy near 531 eV only exists on the surface of the sample, meaning that this peak may be the adsorbed oxygen species on the surface. Fig. 4 Fig. 5 The leakage behavior in BFO is mainly caused by the variable valence of Fe3+[26]. Therefore, the Fe2p2/3 spectra of the BFO/SBT bilayer composite films are fitted, as shown in Fig. 6 (a)-(d). The ratios of the relative contents of Fe3+ and Fe2+ in the samples with transition layer thicknesses of 0, 40, 80 and 120 nm are 3.28, 4.57, 4.61 and 4.30, respectively. The results show that the concentration of Fe3+ in the films with a transition layer is higher than that in the films without a transition layer. The relative content of Fe3+ in the BFO/80 nm SBT sample is the highest among the samples. According to the defect equation[27], the concentration of Fe3+ is inversely proportional to the oxygen vacancy concentration. Therefore, an SBT transition layer with the appropriate thickness can reduce the oxygen vacancies in the bilayer composite film to a certain extent. When the thickness of the transition layer thickness is 80 nm, the oxygen vacancy content in the bilayer composite film is relatively low. Fe3++O2-→2Fe2++ VO2− +1/2O2 Fig. 6
(2)
Fig. 7 Fig 7 shows the XPS depth profile of elements of the BFO/80 nmSBT sample, which depicts the relationship between the content of each element in the thin films and the etching depth. Fig.7 shows that four distinct regions exist in the SBT/BFO bilayer composite films, which are i) the surface region, ii) the BFO thin film layer, iii) the SBT transition layer, and iv) the Pt substrate. The possible reasons for the high content of Bi, O, Pt and Sr in the surface layer are as follows: i) Bi is volatile at high temperatures[28], ii) surface adsorbed oxygen is present, iii) platinum is sprayed on the surface before testing the ferroelectric performance, and iv) this may be related to the segregation of some of the Sr in SBT transition layer onto the surface of BFO[29]. The contents of various elements in the BFO thin film region remain stable with increasing in etching time, which indicates the uniform distribution of these various elements in the film. Ti ions in the SBT transition layer were observed on the surface of the SBT/BFO bilayer composite films. Similar phenomena were observed in the Y(Ni0.5Mn0.5)O3/SrTiO3[30] structure by other scholars. Fig. 8 exhibits the hysteresis loop of BFO/SBT bilayer composite thin films with different transition layer thicknesses at 1 kHz. The remanent polarization of the sample increases as the test electric field increases because the oxygen vacancies can generate the formation of composite defect pairs, and at a low electric field, a composite defect pair can inhibit the reversal of the ferroelectric domain. However, at a high electric field, the effect of composite defects on ferroelectric domain inversion decreases, and the ferroelectric domain can be flipped more completely, thus inducing a greater remanent polarization than at a low electric field[31,32]. The remanent polarization 2Pr values of the samples with 0, 40, 80 and 120 nm transition layer thicknesses are 106, 147, 151 and 79 µC/cm2 at 1200 kV/cm, respectively. The coercive fields 2Ec are 1100, 1000, 900 and 1000 kV/cm, respectively. As the transition layer becomes thicker, the 2Pr of the bilayer composite film first increases and then decreases, which indicates that the ferroelectric performance of the composite film can be enhanced by adding an SBT transition layer with the appropriate. Among the samples tested, the BFO/80 nm SBT sample has a smaller
coercive field and the largest remanent polarization. The possible reasons for these results causes are as follows: i) Because of its low leakage current, SBT acts as an insulator in the bilayer composite film, which improves the film resistance thus improving the remanent polarization[33,34]. ii) The better, denser, and smaller the defect pair is during crystallization of the film, the greater the remanent polarization strength of the sample. iii) XRD analysis shows that the lattice mismatch between the SBT transition layer and the BFO film layer produces stress, which affects the remanent polarization of the bilayer composite film to a certain extent[35]. iv) The oxygen vacancy concentration in the film with a transition layer thickness of 80 nm is low, so the ferroelectric properties are clearly identified. v) As the SBT becomes thicker, the remanent polarization of the bilayer composite films decreases, which may be due to the low remanent polarization of the SBT itself, and the addition of the transition layer "dilutes" the remnant polarization of the whole bilayer composite film. Therefore, with a thicker of the transition layer, this "dilution" effect is also enhanced, leading to a decrease in the remanent polarization of the bilayer composite films[36,37]. vi) As the transition layer thickens, the effective electric field acting on BFO decreases, which results in a decrease in the residual polarization of the bilayer composite films. Fig. 8 The leakage performance (J-E) of the BFO/SBT bilayer composite film with different transition layer thicknesses is shown in Fig. 9. In the test electric field, the leakage current density ultimately increases. The leakage current density first decreases and then increases as the transition layer layer becomes thicker. The leakage current density (5.21×10−5 A/cm2 at 350 kV/cm) of the BFO/80 nm SBT thin film is the smallest among the tested films. The phenomenon above can be explained as follows: i) The (Bi2O2)2+ in the SBT transition layer acts as an efficient insulator between BFO and the Pt substrate; this insulator can block the movement of oxygen vacancies, thus reducing the leakage current density of the bilayer composite film[38]. ii) SBT and BFO have different work functions. A potential barrier may be formed at the interface between them, thus blocking the conduction of carriers and reducing the
leakage current density of bilayer composite films[39]. iii) The oxygen vacancies in thin films with smaller grains are easier to pin, so the antileakage performance of thinner film samples is better than that of thicker films. In addition, the proportion of grain boundaries increases, and the leakage current channel lengthens in the samples with smaller grains, which also reduces the leakage current[40]. Table 1 shows the performances results from this article and the performance data from the literature. There are no previous reports that use SBT as a transition layer with BFO to construct a bilayer composite structure. It can be seen from Table 1 that using SBT as the transition layer can improve the remanent polarization and reduce the leakage current of a film to a certain extent. Table 2
Fig. 9 The study of the leakage mechanism is of great significance to reduce the leakage current. The leakage mechanism is mainly divided into volume-limited conductivity and interface-limited conductivity. At present, the volume-limited conductivity is mainly
ohmic
conduction,
space-charge
limited
conduction
(SCLC)
and
Poole-Frenkel (P-F) emission; the interface-limited conductivity mainly includes Schottky emission and the Fowler-Nordheim (F-N) tunneling effect. By analyzing the diagram of log(J) and log(E) shown in Fig. 10, the leakage current mechanism of the bilayer composite films is determined. The different slopes represent different leakage mechanisms. As shown, when the thickness of the transition layer is 0, 40, 80 and 120 nm at a low electric field (E<100 kV/cm), the α values of the samples are 1.2, 1.1, 1.0 and 1.0, respectively. Therefore, the leakage mechanisms of all the thin film samples under a low electric field are mainly ohmic conduction (α~1)[41].This result could be due to the thermal emission of electrons[42], free electrons and holes. As the electric field increases, the α value of the BFO/80 nm SBT sample increases to 2.0, which suggests that the main leakage mechanism is SCLC(α~2)[44]. However, the SCLC leakage mechanism was not observed in the other samples. The SCLC mechanism is mainly related to the number of injected carriers in the trap. When the number of
injected carriers is large enough, the conductivity of the film increases, and the SCLC leakage mechanism is observed. With a further increase in the test electric field, the α values of the samples with transition layer thicknesses of 0, 40, 80 and 120 nm are increased to 2.8/5.3, 3.1/4.6, 4.7, and 4.0/5.6, respectively, indicating that there are other leakage mechanisms operating in the thin film samples under a high electric field. This divergence may arise because the carriers have filled the trap[23], which will be discussed further. Fig. 10 In addition to ohmic conduction and SCLC conduction mechanisms, P-F emission, Schottky emission and F-N tunneling effect mechanisms may also occur. The mechanism of P-F emission is that the composite defect in the defect center can be excited to the conduction band by thermal excitation under a very strong external electric field. The Schottky barrier is weakened because of the application of an electric field, and some electrons can escape the Schottky barrier, corresponding to the Schottky emission mechanism. The equations of these two leakage mechanisms are as follows[45]:
J S = AT 2e J PF = B E e
-
-
φ−
EI −
q 3 E / 4 π ε0 k K BT q 3 E / πε0 k K BT
(3)
(4)
where K represents the optical dielectric constant, φ, E and EI are the height, electric field intensity and ionization energy of the trap, respectively, and A and B represent two constants. T is the heat transfer temperature (RT), q represents the electron charge, ε0 represents the vacuum dielectric constant and KB represents the Boltzmann constant. As shown in Fig. 11 (a)-(b), the curves of ln(J/T2)-E1/2 and ln(J/E)-E1/2 are drawn according to formulas (3) and (4). The above two formulas show that the K value can be deduced from the value of α. Then the value of α can be seen from Fig. 10 (a)-(b). According to the refractive index n=2.5 of BFO, K=n2=6.25[46]. The K value fitted in Fig. 11 (a)-(b) differs greatly from the theoretical K value; therefore, the leakage
mechanism of the thin film sample does not belong to the above two types under a high electric field. The leakage mechanism of the F-N tunneling effect easily occurs in a high electric field. The formula for the F-N tunneling effect is as follows[47]:
J FN = C E 2e
-
ϕ i3
D2 E
(5)
where φi represents the barrier height and C represents a constant. According to the formula, the diagrams of the ln(J/E2) and 1/E curves are drawn in Fig. 11 (c). To determine whether a result is due to the F-N tunneling effect, the linear relationship with the above-mentioned curve is observed and analyzed. As shown in Fig. 11 (c), all of the ln(J/E2) and 1/E curves have a good linear relationship. Therefore, the F-N tunneling effect is the main leakage mechanism of the thin films at high electric fields. At a low electric field, the BFO/SBT bilayer composite thin films primarily exhibit volume-limited conduction, whereas they primarily exhibit interface-limited conduction at a higher electric field. The results of particular measurements (values of important properties) for all the samples are shown in Table 2. Table 3 Fig. 11 Fig. 12 shows the dielectric constant(εr) and loss(tanδ) curves of the bilayer composite films with frequency. As shown in Fig. 12, the dielectric frequency of all the samples has good stability. The dielectric losses of the BFO/SBT bilayer composite films are 0.049, 0.025, 0.024 and 0.45 at 105 Hz, and the dielectric constants are 90.9, 115, 121 and 123 at 105 Hz, respectively. Among the tested films the BFO/80 nmSBT samples have the lowest dielectric loss, which shows that this film has the fewest charge defects and lower conductivity[47]. As can be seen from Fig. 12, the dielectric constant and the dielectric loss increase with as the thickening of the transition layer becomes thicker. Possible reasons for this result are as follows: i) The sample with the transition layer is denser than the sample without a transition layer. The denser the film is, the greater the dielectric constant is, which is consistent with the test results in Fig. 3. ii) The effective tanδ increases with increasing film
thickness[48,49]. iii) The largest dielectric constant of the specimen with a 120 nm thick SBT transitional layer should be related to the space change contribution of space change to the polarization.[50]. iv) With the increase in thickness, the stress is relaxed as discussed above. This is beneficial to the domain wall motion and will leads to a larger dielectric constant[49]. v) The increase in dielectric loss is mainly due to the increase in oxygen vacancies and leakage current as the transition layer becomes thicker. Fig 12.
4. Conclusions The thickness effect of the SBT transition layer on the properties of BFO/SBT bilayer composite films was studied in this paper. After detailed analysis, it was found that the properties of the BFO/SBT bilayer composite films can be improved with an optimal SBT transition layer thickness. The XRD and SEM analysis results show that all of the BFO/SBT bilayer composite films grow as a layered perovskite structure without other impurity phases. The BFO/80 nm SBT samples exhibit good crystallization, a uniform minimum grain size, and an improved density compared with the other samples. The XPS results show the lowest concentration of the oxygen vacancies and Fe2+ in the BFO/80 nm SBT samples. The abovementioned samples have good ferroelectric properties: 2Pr=151 µC/cm2, 2Ec= 900 kV/cm, and J=5.21×10-5 A/cm2 at 350 kV/cm. Under a high electric field (E>100 kV/cm), the leakage mechanism is the F-N tunneling effect, which is ohmic conduction at a low electric field (E<100 kV/cm). The relative dielectric constant is εr=121, and the dielectric loss is tanδ= 0.024 at 105 Hz.
Acknowledgments This work was supported by funding from the Research Fund for the Doctoral Program of Shandong Jianzhu University (Grant No. XNBS1626) and the Program of the Housing and Urban-Rural Construction Department of Shandong Province (2019-K7-10).
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Fig.1 XRD pattern of BFO/SBT bilayer composite thin films:(a) 2θ=20°-60° (b) 2θ=31.5°-33°(c)lattice constant (d)structure schematic diagram Fig.2 Cross sections diagram of BFO/SBT bilayer composite films:(a) 0 nm (b) 40 nm (c) 80 nm (d) 120 nm Fig.3 Surface diagram of BFO/SBT bilayer composite films:(a) 0 nm (b) 40 nm (c) 80 nm (d) 120 nm Fig.4 O1s XPS surface spectra of BFO/SBT bilayer composite films:(a) 0 nm (b) 40 nm (c) 80 nm (d) 120 nm Fig.5 O1s XPS spectra of BFO/SBT bilayer composite film after Ar+ ion for 16 min (a) 0nm (b) 40nm (c) 80nm (d) 120nm Fig.6 Fe2p2/3 XPS spectra of BFO/SBT bilayer composite films:(a) 0 nm (b) 40 nm (c) 80 nm (d) 120 nm Fig.7 XPS elemental depth profiling of BFO/80 nmSBT sample bilayer composite film Fig.8 P-E hysteresis loops of BFO/ SBT bilayer composite film (a)0 nm (b)40 nm (c)80 nm (d)120 nm Fig.9 J-E curves of BFO/SBT bilayer composite thin film Fig. 10 logJ-logE curves of BFO/SBT bilayer composite films Fig. 11 (a) Scbottky emission (b) Pool-Frenkel emission (c) Fowler-Nordeim tunneling plotted of bilayer composite thin films Fig. 12 The dielectric constant and dielectric loss of the BFO/SBT bilayer composite film with frequency
Table 1 The exact meaning of the lattice constant a in Eq.1
thickness of transition
0
40
80
120
af
a BFO
a BFO
a BFO
a BFO
as
a Pt
aSBT
aSBT
aSBT
29.67
30.80
30.80
30.80
layer(nm)
S (%)
Table 2 The performances of this article and the literature data contrast table.
2Pr ferroelectric material
flexible substrate
2Ec
Ref. J(A/cm2)
method (µC/cm2)
(kV/cm)
BiCrO3/BiFeO3
Pt/Ti/Si/SiO2
CSD
37
1004
SrTiO3/ BiFeO3
Pt/Ti/Si/SiO2
PLD
96
540~550
BiFeO3/Bi3.25Sm0.75Ti2.98V0.02O12
Pt/Ti/Si/SiO2
PLD
71.8
148
BiFeO3/BaTiO3
SrTiO3
2.88×10-4
[8]
[13] [35]
RF-MS
[51] 6.26×10-6
4.4 (sputtering) Sel-gel
BiCrO3/BiFeO3
Pt/Ti/Si/SiO2
[52] 16.95
57.08
1.37×10-4
(spin coating) BiFeO3/Bi3.5Nd0.5Ti3O12
BiFeO3/Bi0.5(Na0.85K0.15)0.5TiO3
ITO
MOD
70.2
8.38×10-6
[53]
Pt/Ti/Si/SiO2
CSD
21.6
8.76×10-7
[54]
34.8
1.71×10-4
Sel-gel BiFeO3/Bi5Ti3FeO15
Pt/Ti/Si/SiO2
[55]
(spin coating) BiFeO3/CoFe2O4
SrTiO3
BiFeO3/ BaTiO3
Pt/Ti/Si/SiO2
self-assembly
[56]
91.4
RF-MS
[57] 30
(sputtering) Sel-gel BiFeO3/Pb(Zr0.52Ti0.48)O3
Pt/Ti/Si/SiO2
[58] 80
(spin coating) BiFeO3/ Sr2Bi4Ti5O18
Sel-gel Pt/Ti/Si/SiO2
[This 151
(spin coating)
900
5.21×10-5 work]
Table 3 The results of particular measurements (values of important properties) for all samples. thickness of transition layer(nm)
2Pr(µC/cm2)
2Ec(kV/cm)
@ 1200kV/cm 0
106
J(A/cm2)
leakage mechanism
Fe3+:Fe2+
Ohmic conduction
3.28
@ 350kV/cm 1100
5.85×10−4
1000
9.30×10
−5
5.21×10
−5
1.72×10
−3
and F-N tunneling effect 40
147
Ohmic conduction
4.57
and F-N tunneling effect 80
151
900
Ohmic conduction
4.61
and F-N tunneling effect 120
79
1000
Ohmic conduction and F-N tunneling effect
4.30
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
Fig. 12
Declaration of interests ☒ 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.