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Influence of thermal treatment temperature on high-performance varistors prepared by hot-dipping tin oxide thin films in Nb2O5 powder
Applied Surface Science 443 (2018) 301–310
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Influence of thermal treatment temperature on high-performance varistors prepared by hot-dipping tin oxide thin films in Nb2O5 powder Qi Wang a,b,c, Zhijian Peng a,b,⇑, Yang Wang a,b,c, Xiuli Fu c,⇑ a
School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China School of Science, China University of Geosciences, Beijing 100083, PR China c School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China b
a r t i c l e
i n f o
Article history: Received 18 November 2017 Revised 23 January 2018 Accepted 26 February 2018 Available online 6 March 2018 Keywords: Tin oxide Nb2O5 Hot-dipping Thin film Varistor
a b s t r a c t SnOx-Nb2O5 thin film varistors were prepared by hot-dipping oxygen-deficient tin oxide films in Nb2O5 powder in air, and the influence of hot-dipping temperature (HDT) on the varistor performance of the samples was systematically explored. When the HDT increased from 300 to 700 °C, the nonlinear coefficient of the samples raised first and then dropped down, reaching the maximum of 14.73 at 500 °C, and the breakdown electric field exhibited a similar variation trend, gaining the peak value of 0.0201 V/nm at this temperature. Correspondingly, the leakage current decreased first and then increased with increasing HDT, reaching the minimum of 17.1 mA/cm2 at 500 °C. Besides, it was proposed that a grain-boundary defect barrier model was responsible for the nonlinear behavior of the obtained SnOx-Nb2O5 film varistors. This high-performance thin film varistor with nanoscaled thickness might be much promising in nano-devices or devices working in low voltage. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Varistors are a class of devices used in electronic and electric power systems. If the applied voltage suddenly rises above a certain value, the varistor would switch from a highly insulating into conducting state, absorbing the excessive energy in the circuits [1]. Based on their unique nonlinear current-voltage characteristics and high energy handling capabilities, various varistors are widely used to suppress the transient overvoltage to protect diversified devices and circuits from being destroyed [2]. Among them, ZnObased ceramic varistors have been the most extensively studied and most commonly applied ones, due to their excellent nonlinear characteristics [3]. But ZnO-based ceramic varistors have a number of drawbacks, including the multitude of additives, high breakdown voltage, inhomogeneous multiphase microstructure, and unavoidable performance degradation (aging) during application. Therefore, other varistors such as SrTiO3-, TiO2- and SnO2-based systems, are under extensive investigation so as to pursue more appropriate applications [4].
⇑ Corresponding authors at: School of Science, China University of Geosciences, Beijing 100083, PR China (Z. Peng). School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China (X. Fu). E-mail addresses: [email protected] (Z. Peng), [email protected] (X. Fu). https://doi.org/10.1016/j.apsusc.2018.02.264 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.
In particular, SnO2-based ceramic varistors were first reported in 1995 [5], which were prepared by sintering SnO2 powder doped with a small amount of Co2O3 and Nb2O5, and their well-densified samples had excellent nonlinear properties. Since then, SnO2-based varistors have appealed much attention, which are currently considered as promising succedaneums for ZnO-based varistors in certain applications due to their simpler microstructure, lower additive contents, higher thermal conductivity, preferable mechanical properties and better degradation resistance [6,7]. In literature, many researchers prepared various SnO2-based ceramic varistors by trying divergent dopants into the material system, such as CoO, MnO2, Ta2O5 and Nb2O5 [8,9]. Among them, CoO and MnO2 are usually used to improve the densification of SnO2-based ceramics, and Nb2O5, Ta2O5 and the like are often used to enhance their nonlinear properties, behaving as a varistor forming oxide (VFO). Moreover, Bueno et al. [10] indicated that the nature of varistor behavior for SnO2-based varistors was a grain-boundary phenomenon, where the Schottky potential barrier existed in the depletion layer of adjacent grains. On the other hand, with the development of electronic industry, electric devices have to meet the requirements of miniaturization and integration. But, because traditionally SnO2-based ceramic varistors are prepared by sintering SnO2 powder mixing with additives of several metal oxides [7,11], SnO2-based ceramics are usually suitable for high-voltage varistors, limiting their applications in miniature devices or devices working in low voltage. In
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order to solve such issue, it is necessary to reduce the geometry of SnO2-based ceramic varistors. However, due to the pressing process during preparing ceramic varistors, it is difficult to reduce the size of ceramic devices by a wide margin only through traditional dry-pressing formation and sintering, compelling the scientists to look for other methods, such as tape casting and thin-film/ coating techniques [12–15]. In particular, thin film deposition techniques have become more and more mature, providing feasible ways to fabricate thin film varistors so as to meet the standard of miniaturization and integration. Nowadays, there are many kinds of methods capable of preparing various thin film varistors (especially ZnO-based varistors), such as magnetron sputtering, molecular beam epitaxy, pulsed laser deposition, electron beam evaporation, sol-gel method, chemical vapor deposition, ultrasonic spray pyrolysis and so on [16–21]. However, as far as we know, there were seldom literatures on SnO2-based thin film/coating varistors [22,23], possibly because through traditional thin film deposition techniques, it is difficult to form effective grain boundary by VFO films around SnO2 grains for varistors. Even in the only existed literatures about SnO2 film varistors [22,23], the reported preparation method was very complicated with multi-step complex processes and unfavorable organic and inorganic precursors, and the reported multi-component (Cr,Zn,Nb)SnO2 film varistors would have high leakage current due to the less dense structure of the films, although the nonlinear coefficient was fairly high (over 9). Considering the facts that the nonlinear behavior of SnO2-based varistors is triggered by the effective grain boundary around the SnO2 grains, and Nb2O5 is a well-known VFO for SnO2-based ceramic varistors with high electrical resistivity, melting point (1520 °C) and thermostability, and larger ionic radius of Nb5+ (0.074 nm) than Sn4+ (0.069 nm), it is possible that Nb2O5 would well segregate at tin oxide grain boundary, forming a high resistivity insulation layer, thus resulting in highly nonlinear behavior for the tin oxide based thin film varistors that can be applied in low-voltage devices working under tough conditions. Besides, magnetron sputtering exhibits some advantages for preparing various high-quality metal oxide thin films with high sputtering rate and deposition efficiency, strong adhesion between the film and substrate, easily manipulated film composition via using various sputtering targets, and convenience for mass production in industry due to the large area of homogeneous plasma. Therefore, in this work, we first deposited oxygen-deficient tin oxide thin films onto electrical conducting silicon wafers by radio frequency (RF) magnetron sputtering a sintered SnOn (n < 2) ceramic target, and then tried to construct highly effective Nb2O5 grain boundary layers onto tin oxide grains by hot-dipping the as-deposited films in Nb2O5 powder in air at a designed moderate high temperature. Eventually, high-performance SnOx-Nb2O5 thin film varistors with nanoscaled thickness were fabricated. Specifically, the influence of hotdipping temperature (HDT) on the compositional, microstructural, and electrical properties of the obtained samples were systematically investigated. And a grain-boundary defect barrier model based on Schottky barrier was proposed to elucidate the mechanism of the nonlinear behavior for SnOx-Nb2O5 thin film varistors. 2. Experimental 2.1. Samples preparation A JGP450a RF magnetron sputtering equipment (Sky Technology Development Co. Ltd., Chinese Academy of Sciences, Shenyang, China) was used to deposit the oxygen-deficient tin oxide films onto conducting silicon wafers. For deposition, a sintered tin oxide ceramic disk with composition of SnOn (n < 2) was adopted as the
sputtering target. The sputtering was performed at ambient temperature for 20 min with a pure argon gas flow of 20 sccm, a sputtering pressure of 1.0 Pa and a sputtering power of 100 W. Under such conditions, the as-deposited tin oxide films are oxygendeficient with a composition of SnO1.25 (please check Fig. S1 in the Supplementary Materials). After deposition, the obtained films were placed in a half-covered alumina crucible and buried fully in Nb2O5 powder. Then, the samples were heated in a Muffle furnace at different temperatures for different time in air. For the optimization of hot-dipping time, please check Fig. S2. Afterwards, within the optimized time (60 min), the samples were hot-dipped at 300–700 °C, respectively, and then cooled down naturally to room temperature. Finally, all the samples were ultrasonically cleaned in deionized water for 5 min to remove the residual Nb2O5 powder attached on surface. To measure the electrical properties, the top surface of the films was daubed with silver pulp as electrodes. 2.2. Materials characterization The microstructure of the samples was examined by a fieldemission scanning electron microscope (SEM, Quanta FEG 650, FEI, Hillsboro, Oregon, America). The size of tin oxide grains in the samples was calculated from the SEM images on the sample surface, while the film thickness was measured from the crosssectional SEM images. X-ray photoelectron spectroscopy (XPS, Thermo Fisher, Waltham, Massachusetts, America; nonmonochromated Al Ka radiation, photon energy 1486.7 eV) was adopted to examine the elemental composition and chemical state of the obtained samples, for which the correction of the energy shift was carried out by referencing to C 1s line (284.6 eV) [24]. And the elemental distribution in the samples was examined by an energy dispersive X-ray (EDX) spectroscope attached to the SEM. The phase composition of the samples was identified by X-ray diffraction (XRD, D/max-RB, Japan; Cu Ka radiation, k = 1.5418 Å) through using a grazing incident diffraction mode with an incidence angle of 1° at a continuous scanning speed of 4 °/min. In addition, the electrochemical impedance spectra (EIS) of the samples were recorded by an electrochemical workstation (CHI 660E, Chenghua Instruments, Shanghai, China), and the data were analyzed by the ZSimpWin software. 2.3. Measurements of electrical properties The electric field vs current density (E-J) characteristic curves of the samples were measured by a Keithley 2410 Multimeter (Beaverton, Oregon, America) at room temperature, and the nonlinear coefficient (a) of the samples was calculated according to the following equation:
a¼
1 lg E1 A lg E100 mA
ð1Þ
where E100mA and E1A are the electric fields in accordance with the current densities of 100 mA/cm2 and 1 A/cm2, respectively. The E100mA was defined as the breakdown voltage of the varistors (VB), and the current density at the electric field of 0.75E100mA was regarded as the leakage current of the varistors (IL). The applied voltage per grain boundary (Vgb) could be evaluated by the following equation:
V gb ¼ V B
d D
ð2Þ
where VB is the breakdown voltage of the obtained varistors, d is the mean grain size of tin oxide in the samples, and D is the thickness of the varistors thin film.
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Fig. 1. Typical SEM surface images on the surfaces of the as-deposited thin film (a), and the obtained samples by hot-dipping the as-deposited tin oxide films in Nb2O5 powder for 60 min at different temperatures: (b) 300, (c) 400, (d) 500, (e) 600 and (f) 700 °C. The inset of each image displays the grain size distribution in the films. (g) The calculated average grain size of all the obtained samples as a function of HDT.
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Fig. 2. Typical XPS surface analysis results: full spectrum (a), and narrow spectra of O 1s (b), Sn 3d (c) and Nb 3d (d) for the samples prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min in air.
3. Results and discussion 3.1. Microstructure, elemental distribution and phase composition Fig. 1a–f exhibits typical SEM images on the surfaces of the samples prepared by hot-dipping the as-deposited film in Nb2O5 powder in air for 60 min at different temperatures together with the original one. These results could reveal the influence of HDT on morphology evolution of the obtained samples. It could be seen that the surfaces of all the samples were smooth and flat, and the boundaries between the grains were clear, but the grain size in the hot-dipped samples was larger than that in the as-deposited film. Moreover, on the basis of these SEM images, the grain sizes of all the hot-dipped samples prepared at different temperatures were calculated, which are displayed in Fig. 1g as a function of the HDT. For comparison, that of the original, as-deposited film is also presented. As is seen in this figure, the grain size of the hot-dipped samples increased continuously from about 18 to 23 nm with the HDT increased from 300 to 700 °C, and all of them were larger than that of the as-deposited film (approximately 15 nm). During the hot-dipping process, while more O atoms would diffuse into the film at a higher temperature, leading to the oxidation of the asdeposited oxygen-deficient grains into products with a composition closer to stoichiometric tin oxide (SnO2), a higher temperature would also provide more energy for the tin oxide grains to re-form and grow. As a result, the grain size of the hot-dipped samples rose
up gradually with increasing HDT, and it may result in more homogeneous grain size. The elemental composition and chemical state of all the hotdipped samples were examined by XPS. Typical results measured from the surface of the sample hot-dipped at 500 °C are shown in Fig. 2. Compared with the as-deposited, oxygen-deficient tin oxide film with a chemical formula of SnO1.25 (see Fig. S1), all the hot-dipped samples were a composite of Sn, O, and Nb instead. As shown in Fig. 2b, the asymmetric peak of O 1s spectrum could be fitted into three peaks of OI, OII and OIII. According to previous reports, the OI peak at 530.5 ± 0.1 eV should be assigned to the crystal lattice oxygen in SnO2 and/or Nb2O5 [25–27], the OII peak around 531 eV could be attributed to the O 2 state adsorbed at oxygen vacancies due to the chemisorbed oxygen in nonstoichiometric tin oxide [28], and the OIII peak near 531.8 eV might arise due to the presence of water, hydroxyl species or other oxygen-containing species incorporated possibly by physical adsorption in the film [29]. Fig. 2c displays the XPS spectrum of Sn atom, from which two incisive peaks around 486.8 ± 0.2 and 495.2 ± 0.1 eV could be observed. According to Refs. [30,31], the origin of the peak near 486.8 eV could be attributed to Sn 3d5/2, and the one near 495.2 eV is due to the existence of Sn 3d3/2. With regard to the Nb 3d spectrum as shown in Fig. 2d, only one weak peak at 207.4 ± 0.2 eV could be fitted, which was corresponding to Nb 3d5/2 [32], indicating that a small amount of Nb atoms diffused into the tin oxide films through thermal diffusion during
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Fig. 3. EDX mapping results on the fresh surface of typical sample prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min in air: (a) EDX scanning area, and (b–d) mapping spectra of O, Sn and Nb atoms, respectively. (e) The calculated content of Nb atoms in all the hot-dipped samples as a function of the HDT.
the designed hot-dipping process. All these results by XPS surface analysis are consistent with those by the XPS depth analysis (see Fig. S3), indicating that the obtained films have a consistent composition throughout the films. In order to figure out the distribution of Sn, O and Nb atoms in the hot-dipped films, all the obtained samples were first examined by EDX mapping on their fresh surfaces. Typical results of the sample prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min are shown in Fig. 3, in which Fig. 3a shows the scanning area. It is seen from Fig. 3b and c that O and Sn atoms are homogeneously distributed in the scanning area, revealing that tin oxide spread uniformly across the film surface. However, it can be clearly seen from Fig. 3d that, unlike Sn and O atoms, Nb atoms stayed at the boundary of tin oxide grains, corresponding with their positions in the scanning area shown in Fig. 3a, constructing a typical structure of a SnOx-Nb2O5 varistor. Furthermore, the content of Nb atoms could be calculated as a function of HDT on the basis of quantitatively elementary analysis of each sample, and the results are displayed in Fig. 3e. From this figure, it can be seen that the content of Nb atoms in the films increased first and decreased again with the HDT increased from
300 to 700 °C, reaching the maximum of 6.12% for the sample hot-dipped at 500 °C. The content variation of Nb atoms could be explained as follows. Because the Nb2O5 diffusion is a thermal process, so a moderate increase of the hot-dipping temperature is beneficial for its diffusion into the tin oxide film. When the HDT increased from 300 to 500 °C, a higher temperature would provide more energy to Nb2O5, leading to more Nb atoms diffusing into the tin oxide film, staying at the grain boundary. As a result, the measured content of Nb atoms in the films raised gradually from 3.19% to 6.12%. However, as the HDT further increased above 500 °C, tin oxide grains with larger size would form in the films (see Fig. 1g), which would reduce the effective area of the grain boundary, thus reducing the amount of Nb2O5 in the film under the solid solution limit at a specific temperature. Finally, the measured content of Nb atoms fell down to 4.73% again. The distribution of Sn, O and Nb atoms in the hot-dipped films was further examined by EDX mapping on the cross-section of the samples. Fig. 4 exhibits the results on the cross-sectional surface of typical sample prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min in air. As is seen From Fig. 4a, the obtained film had a neat cross-section with a uniform
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Fig. 4. EDX mapping results on the cross-section of typical sample prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min in air: (a) EDX scanning area, and (b–d) EDX mapping spectra of Sn, O and Nb atoms, respectively.
thickness of roughly 370 nm, combining closely with the substrate silicon wafer. From Fig. 4b and c, it can be seen that both Sn and O atoms spread uniformly in the scanning area (see Fig. 4a), indicating that tin oxide distributed homogeneously along the cross-section of the film. From Fig. 4d, it can be seen that Nb atoms diffused throughout the whole film along thickness without any gradient, which is in accordance with the data by XPS depth analysis presented in Fig. S3, but mainly distributed at the boundary of tin oxide grains (also see Fig. 4a), which is consistent with the EDX results on the film surface. In summary, according to the results of XPS and EDX, it could be deduced that Nb atoms diffused into tin oxide grains in the films during the hot-dipping, staying at the grain boundary. XRD was adopted to identify the phase composition of the samples fabricated by hot-dipping the as-deposited tin oxide films in Nb2O5 powder at different temperatures. The results are shown in Fig. 5a in comparison with that of the as-deposited film. As is seen, all the hot-dipped samples presented obvious diffraction peaks at 22.6°, 33.9° and 51.8°, corresponding to (1 1 0), (1 0 1) and (2 1 1) crystal planes of the rutile structure tin oxide phase (JCPDS card no. 41-1445), respectively. And the samples prepared at higher temperatures might exhibit more weak peaks in accordance with the crystalline planes of (1 1 2), (3 2 0), (2 0 0) and (2 2 2) of tin oxide. However, from the as-deposited film, merely (1 1 0) and (1 0 1) peaks could be detected, indicating its relatively poor crystallinity. Therefore, it can be deduced that the present hot-dipping could promote the continuous growth of tin oxide grains as compared with those in the as-deposited films, thus improving their crystallinity. The lattice constant of tin oxide grains in the samples could be calculated based on the recorded XRD results. Because the tin oxide grain in the present films is of rutile structure, the lattice constants a and b of the tin oxide grains are identical. Thus, only the lattice constants a and c of the samples could be presented. As is seen from Fig. 5b, the lattice constants a and c of all the
samples prepared at different HDTs were smaller than those of stoichiometric tin oxide (JCPDS card no. 41-1445, a = 4.738 and c = 3.187 Å), indicating a certain degree of lattice shrinkage mainly due to the existence of oxygen vacancy (VO) in the lattices although other defects like interstitial tin (Sni), tin vacancy (VSn) and interstitial oxygen (Oi) may also appear in oxygen-deficient tin oxide films [24,33]. Moreover, the calculated lattice volume of tin oxide grains in the samples is shown in Fig. 5c as a function of HDT. As is seen from this figure, when the HDT increased from 300 to 700 °C, the lattice volumes for all the obtained samples were smaller than that of the stoichiometric tin oxide (71.54 Å3), presenting a slight fluctuation around 70 Å3, which is consistent with the variation trends of lattice constants a and c as shown in Fig. 5b. Considering the larger radius of Nb5+ ions than that of Sn4+ ions and different contents of Nb atoms in the obtained films, the slight fluctuations of lattice volume with HDT imply that Nb5+ ions did not enter into the lattice of tin oxide grains. Instead, they would, most possibly, stay at the grain boundary. In addition, due to the high melting point of Nb2O5 (1520 °C), Nb atoms in the films would stay in a form of Nb2O5 in the obtained samples. However, no any additional phase could be detected excepting tin oxide single phase from all the hot-dipped samples by grazing incident XRD technique, possibly because the content of Nb2O5 diffused into the samples was too low, less than the detection limit of XRD. In a word, combining with the results of XPS, EDX and XRD, it can be concluded that only a small amount of Nb2O5 diffused into the tin oxide films during the hot-dipping process, staying at the grain boundary as an insulation layer. 3.2. Varistor properties and electrical conducting mechanism The recorded E-J characteristic curves of the obtained samples by hot-dipping the as-deposited oxygen-deficient tin oxide films in Nb2O5 powder in air at various temperatures for 60 min are displayed in Fig. 6a. Based on the E-J curves, the corresponding
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Fig. 5. (a) XRD patterns for the samples prepared by hot-dipping tin oxide films in Nb2O5 powder at various temperatures. (b) Lattice constants and (c) lattice volume of tin oxide grains in the samples as a function of HDT. For comparison, those of the as-deposited tin oxide film are also presented.
varistor parameters of the obtained samples were calculated and the results are shown in Fig. 6b and c as a function of HDT. It is seen from Fig. 6b that, with increasing HDT, the nonlinear coefficient raised first and then fell down, reaching the maximum of 14.73 when the HDT was 500 °C. Correspondingly, the leakage current of the obtained varistor decreased first and then rose up with increasing HDT, acquiring the minimum of 17.1 mA/cm2 at 500 °C. It is known that, the highly nonlinear characteristics of SnO2-based varistors could be explained by the tunneling of electrons across the grain-boundary barrier [1,2]. Therefore, the variation of nonlinear coefficient of the obtained film varistors in this work could be attributed to two factors: the completeness or effectiveness of Nb2O5 insulation layer at grain boundary and the oxidation level of tin oxide grains. When the HDT increased from 300 to 500 °C, the content of Nb atoms in the films increased from 3.19% to 6.12% (see Fig. 2e), revealing that more Nb atoms would be detained at the grain boundary, which would cause more complete Nb2O5 layer between the adjacent grains, resulting in more effective highresistance insulating grain boundary. On the other hand, the asdeposited film was oxygen-deficient with a composition of SnO1.25 (see Fig. S1), which would be oxidized during the hot-dipping process. This process would result in tin oxide film with a composition closer to stoichiometric SnO2, increasing the grain resistance, finally degrading the nonlinear performance of the obtained varistors. Combining these two contradict factors, when the HDT increased up to 500 °C, the film varistor exhibits the highest value of nonlinear coefficient, for which the enhanced completeness of Nb2O5 highresistant insulation layer plays a dominant role. However, if the HDT increased above 500 °C, the content of Nb atoms decreased again (see Fig. 3e), and thus the completeness of the Nb2O5 insula-
tion layer was degraded. Meanwhile, the oxidized level of the tin oxide grains became even higher, resulting in tin oxide films with an even closer composition to SnO2. Both factors would cause the decrease of the corresponding nonlinear coefficient of the obtained varistors. Correspondingly, as is well known, the thermionic emission and/or tunneling current are the major transport mechanisms for the leakage current [34,35]. A larger nonlinear coefficient of the varistors is corresponding to a higher tunneling current, leading to a lower leakage current. However, a smaller nonlinear coefficient would generate a relatively higher thermal stimulated current, which would result in a higher leakage current of the varistors. Therefore, the leakage current of the present varistor samples exhibited an opposite trend with their nonlinear coefficient as the HDT increased. Furthermore, as is seen from Fig. 6c, when the HDT increased from 300 to 700 °C, the breakdown electric field of the obtained varistors (E100mA) increased first and then decreased, obtaining the peak value of 0.0201 V/nm at 500 °C. Meanwhile, the voltage per grain boundary exhibits slight variation around 0.37 eV with increasing HDT in this work. Considering that the grain size of the obtained samples presented a monotonous increase with increasing HDT, which normally results in decreasing E100mA, the increase of E100mA with increasing HDT for the present varistors should be ascribed to the content of Nb atoms in films. When the HDT was 500 °C, the obtained sample presented the maximum content of Nb atoms (see Fig. 3e), corresponding to the most complete Nb2O5 insulation layer at the grain boundary between the adjacent tin oxides, thus leading to the highest E100mA. In summary, the varistor prepared at 500 °C exhibits the maximum nonlinear coefficient of 14.73, minimum leakage current of
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Fig. 6. (a) E-J characteristic curves of the varistors prepared by hot-dipping tin oxide thin films in Nb2O5 powder at different temperatures. (b) The calculated nonlinear coefficient (a) and leakage current (IL) of the obtained film varistors as a function of HDT. (c) Breakdown electric field (E100mA) and voltage per grain boundary (Vgb) of the varistors as a function of HDT.
17.1 mA/cm2, and a breakdown electric field of 0.0201 V/nm under the optimized conditions in this work. In order to elucidate the electrical conducting mechanism of the obtained varistors, all the samples were first examined by EIS. Fig. 7a presents the complex impedance spectra of the samples prepared at different HDTs in the range of 300–700 °C. For details, those of the samples prepared at 300 and 400 °C are displayed with larger magnification as an inset of this figure. Moreover, the
corresponding equivalent circuit is also shown as an inset in Fig. 7a. From this figure, it can be seen that all the spectra could be fitted into semicircles with different radii, which increased first and then decreased as the HDT increased from 300 to 700 °C. According to the complex impedance spectra shown in Fig. 7a, the grain resistance (Rg) and grain boundary resistance (Rgb) of each sample could be calculated and the results are displayed in Fig. 7b. As is seen from this figure, the grain resistance rose up
Fig. 7. (a) Complex impedance spectra, and (b) grain resistance (Rg) and grain boundary resistance (Rgb) of the SnOx-Nb2O5 film varistors fabricated at different temperatures for 60 min in air. The insets in (a) are the equivalent circuit and the magnified impedance spectra of the samples prepared at the HDT of 300 and 400 °C, respectively.
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constantly from 4150 to 5090 X with the HDT increased from 300 to 700 °C, which could be attributed to the oxidation of tin oxide grains as discussed in the last section. Because the chemical composition of the as-deposited tin oxide film was SnO1.25, which is far from stoichiometric SnO2 due to a lot of defects existed in tin oxide lattice, thus it has lower electrical resistance [36]. After hotdipping in air with increasing HDT, the oxygen-deficient tin oxides would be oxidized, resulting in products with a composition closer and closer to SnO2. As a result, the grain resistance increased. However, the grain boundary resistance increased first and then decreased with increasing HDT, reaching the maximum of 3.05 106 X at 500 °C. During hot-dipping, when the temperature increased from 300 to 500 °C, more and more Nb atoms would diffuse into the tin oxide films (see Fig. 3e), staying at the grain boundary, improving the completeness of the Nb2O5 grain boundary layer, finally increasing the grain boundary resistance due to the high resistance of Nb2O5. But if the HDT rose up above 500 °C, the content of Nb atoms in the tin oxide films would be reduced (see Fig. 3e), due to the reduced effective area of the grain boundary caused by the increased size of tin oxide grains with increasing HDT (see the detailed discussion in last section). Resultantly, the completeness of the Nb2O5 grain boundary layer was degraded, finally decreasing the grain boundary resistance again. Combining with the nonlinear coefficient of the obtained film varistors, the varistor properties were closely correlated to the variations of the grain resistance and grain boundary resistance. Combining the aforementioned results, a grain-boundary defect barrier model for the present SnOx-Nb2O5 film varistors could be schematically depicted in Fig. 8, which displays the defects in the tin oxide film introduced during film deposition by magnetron sputtering and Nb5+ diffusion through hot-dipping. As is seen from this figure, the barrier was constructed by depletion layer and interface layer, which consist of positively charged donors and negatively charged acceptors, respectively. In the depletion layer, one of the intrinsic defects introduced into the films during the film deposition through magnetron sputtering is interstitial tin (Sni) at the distorted region of SnO2 lattices. The defect Sni would readily escape from the original site to become an interstitial tin ion (Sn4+ i ), simultaneously leaving a tin vacancy ion (V4 Sn ) at its original site in the interface layer. This conversion could be expressed by Eq. (3) [37]. Another intrinsic defect in the tin oxide lattices is oxygen vacancies (VO). The defect VO x would combine with V4 Sn , generating VO. This reaction could be expressed by Eq. (4) [38]. The generated VxO could then be further converted into V+O and/or V2+ O , staying at the depletion layer as positively charged donors. In addition, the Nb2O5 diffusing into the tin oxide grain boundary through hot-dipping would be transformed
Fig. 8. The grain-boundary defect barrier model for the present SnOx-Nb2O5 thin film varistors.
into Nb+Sn based on Eq. (5) at the depletion layer region, which would also contribute to the formation of potential barrier at grain boundary. 4þ Sni ! V 4 Sn þ Sni
ð3Þ
x x V 4 Sn þ 2V O ! V Sn þ 2V O
ð4Þ
SnO2
þ
x 2Nb2 O5 ! 4NbSn þ V 4 Sn þ 10OO
ð5Þ
Moreover, the positively charged donors would be balanced by the negatively charged acceptors distributing in the interface layer (see Fig. 8). Among them, because the hot-dipping was carried out in air during the preparation of the present SnOx-Nb2O5 film varistors, the adsorbed O2 on the samples would capture electrons to generate Oxad according to Eq. (6), and the produced Oxad could further react with negatively charged defects, generating negatively 2 charged acceptors of O ad and/or Oad through Eqs. (7) and (8), respectively [34,38].
1 x O2 ! Oad 2
ð6Þ
x 2 þ V 4 2Oad Sn ! 2Oad þ V Sn
ð7Þ
2 x V 4 Sn þ 4Oad ! 4Oad þ V Sn
ð8Þ (V4 Sn )
Besides, the tin vacancies ion is also essential for the formation of interface layer at the grain boundary as a negatively 2 4 charged acceptor. Thus, O ad, Oad together with VSn would form the negatively charged interface layer. 4+ In summary, the positively charged donors (V+O, V2+ and O , Sni + TaSn) in the depletion layer, together with the negatively charged 2 acceptors (V4 Sn , Oad and Oad ) in the interface layer would form a double barrier potential between the two adjacent tin oxide grains in the film varistor. And the electric transport occurred by tunneling across potential barriers at the grain boundary is responsible for the nonlinear characteristics of a varistor [37,39].
4. Conclusions A series of SnOx-Nb2O5 thin film varistors were fabricated by hot-dipping oxygen-deficient tin oxide film in Nb2O5 powder in air at different temperatures. The effects of HDT on the elemental composition and distribution, phase composition, microstructure, as well as electrical conducting and mechanism of samples were investigated. Nb2O5 would diffuse into tin oxide film during hotdipping, staying at the grain boundary and behaving as a typical VFO due to the formation of high-resistant Nb2O5 insulation layer. When the HDT increased from 300 to 700 °C, the nonlinear coefficient of the obtained varistors rose up first and then dropped down, reaching the maximum of 14.73 when the HDT was 500 °C. Correspondingly, the leakage current decreased first and then increased, acquiring the minimum of 17.1 mA/cm2 at 500 °C. The breakdown electric field of the obtained varistors exhibited the same variation as the nonlinear coefficient with increasing HDT, reaching the peak value of 0.0201 V/nm at 500 °C. In addition, a grain-boundary barrier defect model for the present SnOx-Nb2O5 thin film varistors was proposed, which could illustrate their electrical conducting and mechanism. Considering the high-performance and simple preparation method, the present nanoscaled SnOx-Nb2O5 thin film varistors would be promising in the application of electrical devices working in low voltage.
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Acknowledgments The authors would like to thank the financial support for this work from the National Natural Science Foundation of China (grant nos. 11674035 and 61274015), and the Fundamental Research Funds for the Central Universities (China). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.02.264. References [1] D.R. Clarke, Varistor ceramics, J. Am. Ceram. Soc. 82 (1999) 485–502. [2] P.R. Bueno, J.A. Varela, E. Longo, SnO2, ZnO and related polycrystalline compound semiconductors: an overview and review on the voltagedependent resistance (non-ohmic) feature, J. Eur. Ceram. Soc. 28 (2008) 505– 529. [3] J.J. Lin, S.T. Li, J.Q. He, L. Zhang, W.F. Liu, J.Y. Li, Zinc interstitial as a universal microscopic origin for the electrical degradation of ZnO-based varistors under the combined DC and temperature condition, J. Eur. Ceram. Soc. 37 (2017) 3535–3540. [4] J. Fayat, M.S. Castro, Defect profile and microstructural development in SnO2based varistors, J. Eur. Ceram. Soc. 23 (2003) 1585–1591. [5] S.A. Pianaro, P.R. Bueno, E. Longo, J.A. Varela, A new SnO2-based varistor system, J. Mater. Sci. Lett. 14 (1995) 692–694. [6] Q.Y. Wei, J.L. He, J. Hu, Dependence of residual voltage ratio behavior of SnO2based varistors on Nb2O5 addition, Sci. China – Technol. Sci. 54 (2011) 1415– 1418. [7] M. Maleki Shahraki, M.A. Bahrevar, S.M.S. Mirghafourian, A.B. Glot, Novel SnO2, ceramic surge absorbers for low voltage applications, Mater. Lett. 145 (2015) 355–358. [8] G. Brankovic´, Z. Brankovic´, J.A. Varela, Nonlinear properties and stability of SnO2 varistors prepared by evaporation and decomposition of suspensions, J. Eur. Ceram. Soc. 25 (2005) 3011–3015. [9] F.M. Filho, A.Z. Simões, A. Ries, I.P. Silva, L. Perazolli, E. Longo, J.A. Varela, Influence of Ta2O5 on the electrical properties of ZnO- and CoO-doped SnO2 varistors, Ceram. Int. 30 (2004) 2277–2281. [10] P.R. Bueno, S.A. Pianaro, E.C. Pereira, L.O.S. Bulhes, E. Longo, J.A. Varela, Investigation of the electrical properties of SnO2 varistor system using impedance spectroscopy, J. Appl. Phys. 84 (1998) 3700–3705. [11] A.B. Glot, R. Bulpett, A.I. Ivon, P.M. Gallegos-Acevedo, Electrical properties of SnO2 ceramics for low voltage varistors, Physica B 457 (2015) 108–112. [12] L.Y. Wang, G.Y. Tang, Z.K. Xu, Preparation and electrical properties of multilayer ZnO varistors with water-based tape casting, Ceram. Int. 35 (2009) 487–492. [13] L.Y. Wang, G.Y. Tang, Z.K. Xu, L.Y. Wang, G.Y. Tang, Z.K. Xu, Comparison of water-based and solvent-based tape casting for preparing multilayer ZnO varistors, J. Am. Ceram. Soc. 91 (2008) 3742–3745. [14] H. Lu, Y.L. Wang, X. Lin, Structures, varistor properties, and electrical stability of ZnO thin films, Mater. Lett. 63 (2009) 2321–2323. [15] D. Xu, K. He, R.H. Yu, Y. Tong, J.P. Qi, X.J. Sun, Y.T. Yang, H.X. Xu, H.M. Yuan, J. Ma, Microstructure and electrical properties of praseodymium oxide doped Bi2O3 based ZnO varistor films, Mater. Tech. 30 (2015) A24–A28. [16] S.M. Jejurikar, A.G. Banpurkar, A.V. Limaye, S.K. Date, S.I. Patil, K.P. Adhi, P. Misra, L.M. Kukreja, R. Bathe, Structural, morphological, and electrical characterization of heteroepitaxial ZnO thin films deposited on Si (100) by pulsed laser deposition: effect of annealing (800 °C) in air, J. Appl. Phys. 99 (2006) 14907. [17] S. Anjan, A. Kiran, A.P. Niranjana, M.K. Kevin, Sol-gel preparation of Zn-V-Mn-O thin films for low voltage varistor applications, Ceram. Int. 43 (2017) 9616– 9621.
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Full Length Article
Influence of thermal treatment temperature on high-performance varistors prepared by hot-dipping tin oxide thin films in Nb2O5 powder Qi Wang a,b,c, Zhijian Peng a,b,⇑, Yang Wang a,b,c, Xiuli Fu c,⇑ a
School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China School of Science, China University of Geosciences, Beijing 100083, PR China c School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China b
a r t i c l e
i n f o
Article history: Received 18 November 2017 Revised 23 January 2018 Accepted 26 February 2018 Available online 6 March 2018 Keywords: Tin oxide Nb2O5 Hot-dipping Thin film Varistor
a b s t r a c t SnOx-Nb2O5 thin film varistors were prepared by hot-dipping oxygen-deficient tin oxide films in Nb2O5 powder in air, and the influence of hot-dipping temperature (HDT) on the varistor performance of the samples was systematically explored. When the HDT increased from 300 to 700 °C, the nonlinear coefficient of the samples raised first and then dropped down, reaching the maximum of 14.73 at 500 °C, and the breakdown electric field exhibited a similar variation trend, gaining the peak value of 0.0201 V/nm at this temperature. Correspondingly, the leakage current decreased first and then increased with increasing HDT, reaching the minimum of 17.1 mA/cm2 at 500 °C. Besides, it was proposed that a grain-boundary defect barrier model was responsible for the nonlinear behavior of the obtained SnOx-Nb2O5 film varistors. This high-performance thin film varistor with nanoscaled thickness might be much promising in nano-devices or devices working in low voltage. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Varistors are a class of devices used in electronic and electric power systems. If the applied voltage suddenly rises above a certain value, the varistor would switch from a highly insulating into conducting state, absorbing the excessive energy in the circuits [1]. Based on their unique nonlinear current-voltage characteristics and high energy handling capabilities, various varistors are widely used to suppress the transient overvoltage to protect diversified devices and circuits from being destroyed [2]. Among them, ZnObased ceramic varistors have been the most extensively studied and most commonly applied ones, due to their excellent nonlinear characteristics [3]. But ZnO-based ceramic varistors have a number of drawbacks, including the multitude of additives, high breakdown voltage, inhomogeneous multiphase microstructure, and unavoidable performance degradation (aging) during application. Therefore, other varistors such as SrTiO3-, TiO2- and SnO2-based systems, are under extensive investigation so as to pursue more appropriate applications [4].
⇑ Corresponding authors at: School of Science, China University of Geosciences, Beijing 100083, PR China (Z. Peng). School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China (X. Fu). E-mail addresses: [email protected] (Z. Peng), [email protected] (X. Fu). https://doi.org/10.1016/j.apsusc.2018.02.264 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.
In particular, SnO2-based ceramic varistors were first reported in 1995 [5], which were prepared by sintering SnO2 powder doped with a small amount of Co2O3 and Nb2O5, and their well-densified samples had excellent nonlinear properties. Since then, SnO2-based varistors have appealed much attention, which are currently considered as promising succedaneums for ZnO-based varistors in certain applications due to their simpler microstructure, lower additive contents, higher thermal conductivity, preferable mechanical properties and better degradation resistance [6,7]. In literature, many researchers prepared various SnO2-based ceramic varistors by trying divergent dopants into the material system, such as CoO, MnO2, Ta2O5 and Nb2O5 [8,9]. Among them, CoO and MnO2 are usually used to improve the densification of SnO2-based ceramics, and Nb2O5, Ta2O5 and the like are often used to enhance their nonlinear properties, behaving as a varistor forming oxide (VFO). Moreover, Bueno et al. [10] indicated that the nature of varistor behavior for SnO2-based varistors was a grain-boundary phenomenon, where the Schottky potential barrier existed in the depletion layer of adjacent grains. On the other hand, with the development of electronic industry, electric devices have to meet the requirements of miniaturization and integration. But, because traditionally SnO2-based ceramic varistors are prepared by sintering SnO2 powder mixing with additives of several metal oxides [7,11], SnO2-based ceramics are usually suitable for high-voltage varistors, limiting their applications in miniature devices or devices working in low voltage. In
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order to solve such issue, it is necessary to reduce the geometry of SnO2-based ceramic varistors. However, due to the pressing process during preparing ceramic varistors, it is difficult to reduce the size of ceramic devices by a wide margin only through traditional dry-pressing formation and sintering, compelling the scientists to look for other methods, such as tape casting and thin-film/ coating techniques [12–15]. In particular, thin film deposition techniques have become more and more mature, providing feasible ways to fabricate thin film varistors so as to meet the standard of miniaturization and integration. Nowadays, there are many kinds of methods capable of preparing various thin film varistors (especially ZnO-based varistors), such as magnetron sputtering, molecular beam epitaxy, pulsed laser deposition, electron beam evaporation, sol-gel method, chemical vapor deposition, ultrasonic spray pyrolysis and so on [16–21]. However, as far as we know, there were seldom literatures on SnO2-based thin film/coating varistors [22,23], possibly because through traditional thin film deposition techniques, it is difficult to form effective grain boundary by VFO films around SnO2 grains for varistors. Even in the only existed literatures about SnO2 film varistors [22,23], the reported preparation method was very complicated with multi-step complex processes and unfavorable organic and inorganic precursors, and the reported multi-component (Cr,Zn,Nb)SnO2 film varistors would have high leakage current due to the less dense structure of the films, although the nonlinear coefficient was fairly high (over 9). Considering the facts that the nonlinear behavior of SnO2-based varistors is triggered by the effective grain boundary around the SnO2 grains, and Nb2O5 is a well-known VFO for SnO2-based ceramic varistors with high electrical resistivity, melting point (1520 °C) and thermostability, and larger ionic radius of Nb5+ (0.074 nm) than Sn4+ (0.069 nm), it is possible that Nb2O5 would well segregate at tin oxide grain boundary, forming a high resistivity insulation layer, thus resulting in highly nonlinear behavior for the tin oxide based thin film varistors that can be applied in low-voltage devices working under tough conditions. Besides, magnetron sputtering exhibits some advantages for preparing various high-quality metal oxide thin films with high sputtering rate and deposition efficiency, strong adhesion between the film and substrate, easily manipulated film composition via using various sputtering targets, and convenience for mass production in industry due to the large area of homogeneous plasma. Therefore, in this work, we first deposited oxygen-deficient tin oxide thin films onto electrical conducting silicon wafers by radio frequency (RF) magnetron sputtering a sintered SnOn (n < 2) ceramic target, and then tried to construct highly effective Nb2O5 grain boundary layers onto tin oxide grains by hot-dipping the as-deposited films in Nb2O5 powder in air at a designed moderate high temperature. Eventually, high-performance SnOx-Nb2O5 thin film varistors with nanoscaled thickness were fabricated. Specifically, the influence of hotdipping temperature (HDT) on the compositional, microstructural, and electrical properties of the obtained samples were systematically investigated. And a grain-boundary defect barrier model based on Schottky barrier was proposed to elucidate the mechanism of the nonlinear behavior for SnOx-Nb2O5 thin film varistors. 2. Experimental 2.1. Samples preparation A JGP450a RF magnetron sputtering equipment (Sky Technology Development Co. Ltd., Chinese Academy of Sciences, Shenyang, China) was used to deposit the oxygen-deficient tin oxide films onto conducting silicon wafers. For deposition, a sintered tin oxide ceramic disk with composition of SnOn (n < 2) was adopted as the
sputtering target. The sputtering was performed at ambient temperature for 20 min with a pure argon gas flow of 20 sccm, a sputtering pressure of 1.0 Pa and a sputtering power of 100 W. Under such conditions, the as-deposited tin oxide films are oxygendeficient with a composition of SnO1.25 (please check Fig. S1 in the Supplementary Materials). After deposition, the obtained films were placed in a half-covered alumina crucible and buried fully in Nb2O5 powder. Then, the samples were heated in a Muffle furnace at different temperatures for different time in air. For the optimization of hot-dipping time, please check Fig. S2. Afterwards, within the optimized time (60 min), the samples were hot-dipped at 300–700 °C, respectively, and then cooled down naturally to room temperature. Finally, all the samples were ultrasonically cleaned in deionized water for 5 min to remove the residual Nb2O5 powder attached on surface. To measure the electrical properties, the top surface of the films was daubed with silver pulp as electrodes. 2.2. Materials characterization The microstructure of the samples was examined by a fieldemission scanning electron microscope (SEM, Quanta FEG 650, FEI, Hillsboro, Oregon, America). The size of tin oxide grains in the samples was calculated from the SEM images on the sample surface, while the film thickness was measured from the crosssectional SEM images. X-ray photoelectron spectroscopy (XPS, Thermo Fisher, Waltham, Massachusetts, America; nonmonochromated Al Ka radiation, photon energy 1486.7 eV) was adopted to examine the elemental composition and chemical state of the obtained samples, for which the correction of the energy shift was carried out by referencing to C 1s line (284.6 eV) [24]. And the elemental distribution in the samples was examined by an energy dispersive X-ray (EDX) spectroscope attached to the SEM. The phase composition of the samples was identified by X-ray diffraction (XRD, D/max-RB, Japan; Cu Ka radiation, k = 1.5418 Å) through using a grazing incident diffraction mode with an incidence angle of 1° at a continuous scanning speed of 4 °/min. In addition, the electrochemical impedance spectra (EIS) of the samples were recorded by an electrochemical workstation (CHI 660E, Chenghua Instruments, Shanghai, China), and the data were analyzed by the ZSimpWin software. 2.3. Measurements of electrical properties The electric field vs current density (E-J) characteristic curves of the samples were measured by a Keithley 2410 Multimeter (Beaverton, Oregon, America) at room temperature, and the nonlinear coefficient (a) of the samples was calculated according to the following equation:
a¼
1 lg E1 A lg E100 mA
ð1Þ
where E100mA and E1A are the electric fields in accordance with the current densities of 100 mA/cm2 and 1 A/cm2, respectively. The E100mA was defined as the breakdown voltage of the varistors (VB), and the current density at the electric field of 0.75E100mA was regarded as the leakage current of the varistors (IL). The applied voltage per grain boundary (Vgb) could be evaluated by the following equation:
V gb ¼ V B
d D
ð2Þ
where VB is the breakdown voltage of the obtained varistors, d is the mean grain size of tin oxide in the samples, and D is the thickness of the varistors thin film.
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Fig. 1. Typical SEM surface images on the surfaces of the as-deposited thin film (a), and the obtained samples by hot-dipping the as-deposited tin oxide films in Nb2O5 powder for 60 min at different temperatures: (b) 300, (c) 400, (d) 500, (e) 600 and (f) 700 °C. The inset of each image displays the grain size distribution in the films. (g) The calculated average grain size of all the obtained samples as a function of HDT.
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Fig. 2. Typical XPS surface analysis results: full spectrum (a), and narrow spectra of O 1s (b), Sn 3d (c) and Nb 3d (d) for the samples prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min in air.
3. Results and discussion 3.1. Microstructure, elemental distribution and phase composition Fig. 1a–f exhibits typical SEM images on the surfaces of the samples prepared by hot-dipping the as-deposited film in Nb2O5 powder in air for 60 min at different temperatures together with the original one. These results could reveal the influence of HDT on morphology evolution of the obtained samples. It could be seen that the surfaces of all the samples were smooth and flat, and the boundaries between the grains were clear, but the grain size in the hot-dipped samples was larger than that in the as-deposited film. Moreover, on the basis of these SEM images, the grain sizes of all the hot-dipped samples prepared at different temperatures were calculated, which are displayed in Fig. 1g as a function of the HDT. For comparison, that of the original, as-deposited film is also presented. As is seen in this figure, the grain size of the hot-dipped samples increased continuously from about 18 to 23 nm with the HDT increased from 300 to 700 °C, and all of them were larger than that of the as-deposited film (approximately 15 nm). During the hot-dipping process, while more O atoms would diffuse into the film at a higher temperature, leading to the oxidation of the asdeposited oxygen-deficient grains into products with a composition closer to stoichiometric tin oxide (SnO2), a higher temperature would also provide more energy for the tin oxide grains to re-form and grow. As a result, the grain size of the hot-dipped samples rose
up gradually with increasing HDT, and it may result in more homogeneous grain size. The elemental composition and chemical state of all the hotdipped samples were examined by XPS. Typical results measured from the surface of the sample hot-dipped at 500 °C are shown in Fig. 2. Compared with the as-deposited, oxygen-deficient tin oxide film with a chemical formula of SnO1.25 (see Fig. S1), all the hot-dipped samples were a composite of Sn, O, and Nb instead. As shown in Fig. 2b, the asymmetric peak of O 1s spectrum could be fitted into three peaks of OI, OII and OIII. According to previous reports, the OI peak at 530.5 ± 0.1 eV should be assigned to the crystal lattice oxygen in SnO2 and/or Nb2O5 [25–27], the OII peak around 531 eV could be attributed to the O 2 state adsorbed at oxygen vacancies due to the chemisorbed oxygen in nonstoichiometric tin oxide [28], and the OIII peak near 531.8 eV might arise due to the presence of water, hydroxyl species or other oxygen-containing species incorporated possibly by physical adsorption in the film [29]. Fig. 2c displays the XPS spectrum of Sn atom, from which two incisive peaks around 486.8 ± 0.2 and 495.2 ± 0.1 eV could be observed. According to Refs. [30,31], the origin of the peak near 486.8 eV could be attributed to Sn 3d5/2, and the one near 495.2 eV is due to the existence of Sn 3d3/2. With regard to the Nb 3d spectrum as shown in Fig. 2d, only one weak peak at 207.4 ± 0.2 eV could be fitted, which was corresponding to Nb 3d5/2 [32], indicating that a small amount of Nb atoms diffused into the tin oxide films through thermal diffusion during
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Fig. 3. EDX mapping results on the fresh surface of typical sample prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min in air: (a) EDX scanning area, and (b–d) mapping spectra of O, Sn and Nb atoms, respectively. (e) The calculated content of Nb atoms in all the hot-dipped samples as a function of the HDT.
the designed hot-dipping process. All these results by XPS surface analysis are consistent with those by the XPS depth analysis (see Fig. S3), indicating that the obtained films have a consistent composition throughout the films. In order to figure out the distribution of Sn, O and Nb atoms in the hot-dipped films, all the obtained samples were first examined by EDX mapping on their fresh surfaces. Typical results of the sample prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min are shown in Fig. 3, in which Fig. 3a shows the scanning area. It is seen from Fig. 3b and c that O and Sn atoms are homogeneously distributed in the scanning area, revealing that tin oxide spread uniformly across the film surface. However, it can be clearly seen from Fig. 3d that, unlike Sn and O atoms, Nb atoms stayed at the boundary of tin oxide grains, corresponding with their positions in the scanning area shown in Fig. 3a, constructing a typical structure of a SnOx-Nb2O5 varistor. Furthermore, the content of Nb atoms could be calculated as a function of HDT on the basis of quantitatively elementary analysis of each sample, and the results are displayed in Fig. 3e. From this figure, it can be seen that the content of Nb atoms in the films increased first and decreased again with the HDT increased from
300 to 700 °C, reaching the maximum of 6.12% for the sample hot-dipped at 500 °C. The content variation of Nb atoms could be explained as follows. Because the Nb2O5 diffusion is a thermal process, so a moderate increase of the hot-dipping temperature is beneficial for its diffusion into the tin oxide film. When the HDT increased from 300 to 500 °C, a higher temperature would provide more energy to Nb2O5, leading to more Nb atoms diffusing into the tin oxide film, staying at the grain boundary. As a result, the measured content of Nb atoms in the films raised gradually from 3.19% to 6.12%. However, as the HDT further increased above 500 °C, tin oxide grains with larger size would form in the films (see Fig. 1g), which would reduce the effective area of the grain boundary, thus reducing the amount of Nb2O5 in the film under the solid solution limit at a specific temperature. Finally, the measured content of Nb atoms fell down to 4.73% again. The distribution of Sn, O and Nb atoms in the hot-dipped films was further examined by EDX mapping on the cross-section of the samples. Fig. 4 exhibits the results on the cross-sectional surface of typical sample prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min in air. As is seen From Fig. 4a, the obtained film had a neat cross-section with a uniform
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Fig. 4. EDX mapping results on the cross-section of typical sample prepared by hot-dipping the as-deposited tin oxide film in Nb2O5 powder at 500 °C for 60 min in air: (a) EDX scanning area, and (b–d) EDX mapping spectra of Sn, O and Nb atoms, respectively.
thickness of roughly 370 nm, combining closely with the substrate silicon wafer. From Fig. 4b and c, it can be seen that both Sn and O atoms spread uniformly in the scanning area (see Fig. 4a), indicating that tin oxide distributed homogeneously along the cross-section of the film. From Fig. 4d, it can be seen that Nb atoms diffused throughout the whole film along thickness without any gradient, which is in accordance with the data by XPS depth analysis presented in Fig. S3, but mainly distributed at the boundary of tin oxide grains (also see Fig. 4a), which is consistent with the EDX results on the film surface. In summary, according to the results of XPS and EDX, it could be deduced that Nb atoms diffused into tin oxide grains in the films during the hot-dipping, staying at the grain boundary. XRD was adopted to identify the phase composition of the samples fabricated by hot-dipping the as-deposited tin oxide films in Nb2O5 powder at different temperatures. The results are shown in Fig. 5a in comparison with that of the as-deposited film. As is seen, all the hot-dipped samples presented obvious diffraction peaks at 22.6°, 33.9° and 51.8°, corresponding to (1 1 0), (1 0 1) and (2 1 1) crystal planes of the rutile structure tin oxide phase (JCPDS card no. 41-1445), respectively. And the samples prepared at higher temperatures might exhibit more weak peaks in accordance with the crystalline planes of (1 1 2), (3 2 0), (2 0 0) and (2 2 2) of tin oxide. However, from the as-deposited film, merely (1 1 0) and (1 0 1) peaks could be detected, indicating its relatively poor crystallinity. Therefore, it can be deduced that the present hot-dipping could promote the continuous growth of tin oxide grains as compared with those in the as-deposited films, thus improving their crystallinity. The lattice constant of tin oxide grains in the samples could be calculated based on the recorded XRD results. Because the tin oxide grain in the present films is of rutile structure, the lattice constants a and b of the tin oxide grains are identical. Thus, only the lattice constants a and c of the samples could be presented. As is seen from Fig. 5b, the lattice constants a and c of all the
samples prepared at different HDTs were smaller than those of stoichiometric tin oxide (JCPDS card no. 41-1445, a = 4.738 and c = 3.187 Å), indicating a certain degree of lattice shrinkage mainly due to the existence of oxygen vacancy (VO) in the lattices although other defects like interstitial tin (Sni), tin vacancy (VSn) and interstitial oxygen (Oi) may also appear in oxygen-deficient tin oxide films [24,33]. Moreover, the calculated lattice volume of tin oxide grains in the samples is shown in Fig. 5c as a function of HDT. As is seen from this figure, when the HDT increased from 300 to 700 °C, the lattice volumes for all the obtained samples were smaller than that of the stoichiometric tin oxide (71.54 Å3), presenting a slight fluctuation around 70 Å3, which is consistent with the variation trends of lattice constants a and c as shown in Fig. 5b. Considering the larger radius of Nb5+ ions than that of Sn4+ ions and different contents of Nb atoms in the obtained films, the slight fluctuations of lattice volume with HDT imply that Nb5+ ions did not enter into the lattice of tin oxide grains. Instead, they would, most possibly, stay at the grain boundary. In addition, due to the high melting point of Nb2O5 (1520 °C), Nb atoms in the films would stay in a form of Nb2O5 in the obtained samples. However, no any additional phase could be detected excepting tin oxide single phase from all the hot-dipped samples by grazing incident XRD technique, possibly because the content of Nb2O5 diffused into the samples was too low, less than the detection limit of XRD. In a word, combining with the results of XPS, EDX and XRD, it can be concluded that only a small amount of Nb2O5 diffused into the tin oxide films during the hot-dipping process, staying at the grain boundary as an insulation layer. 3.2. Varistor properties and electrical conducting mechanism The recorded E-J characteristic curves of the obtained samples by hot-dipping the as-deposited oxygen-deficient tin oxide films in Nb2O5 powder in air at various temperatures for 60 min are displayed in Fig. 6a. Based on the E-J curves, the corresponding
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307
Fig. 5. (a) XRD patterns for the samples prepared by hot-dipping tin oxide films in Nb2O5 powder at various temperatures. (b) Lattice constants and (c) lattice volume of tin oxide grains in the samples as a function of HDT. For comparison, those of the as-deposited tin oxide film are also presented.
varistor parameters of the obtained samples were calculated and the results are shown in Fig. 6b and c as a function of HDT. It is seen from Fig. 6b that, with increasing HDT, the nonlinear coefficient raised first and then fell down, reaching the maximum of 14.73 when the HDT was 500 °C. Correspondingly, the leakage current of the obtained varistor decreased first and then rose up with increasing HDT, acquiring the minimum of 17.1 mA/cm2 at 500 °C. It is known that, the highly nonlinear characteristics of SnO2-based varistors could be explained by the tunneling of electrons across the grain-boundary barrier [1,2]. Therefore, the variation of nonlinear coefficient of the obtained film varistors in this work could be attributed to two factors: the completeness or effectiveness of Nb2O5 insulation layer at grain boundary and the oxidation level of tin oxide grains. When the HDT increased from 300 to 500 °C, the content of Nb atoms in the films increased from 3.19% to 6.12% (see Fig. 2e), revealing that more Nb atoms would be detained at the grain boundary, which would cause more complete Nb2O5 layer between the adjacent grains, resulting in more effective highresistance insulating grain boundary. On the other hand, the asdeposited film was oxygen-deficient with a composition of SnO1.25 (see Fig. S1), which would be oxidized during the hot-dipping process. This process would result in tin oxide film with a composition closer to stoichiometric SnO2, increasing the grain resistance, finally degrading the nonlinear performance of the obtained varistors. Combining these two contradict factors, when the HDT increased up to 500 °C, the film varistor exhibits the highest value of nonlinear coefficient, for which the enhanced completeness of Nb2O5 highresistant insulation layer plays a dominant role. However, if the HDT increased above 500 °C, the content of Nb atoms decreased again (see Fig. 3e), and thus the completeness of the Nb2O5 insula-
tion layer was degraded. Meanwhile, the oxidized level of the tin oxide grains became even higher, resulting in tin oxide films with an even closer composition to SnO2. Both factors would cause the decrease of the corresponding nonlinear coefficient of the obtained varistors. Correspondingly, as is well known, the thermionic emission and/or tunneling current are the major transport mechanisms for the leakage current [34,35]. A larger nonlinear coefficient of the varistors is corresponding to a higher tunneling current, leading to a lower leakage current. However, a smaller nonlinear coefficient would generate a relatively higher thermal stimulated current, which would result in a higher leakage current of the varistors. Therefore, the leakage current of the present varistor samples exhibited an opposite trend with their nonlinear coefficient as the HDT increased. Furthermore, as is seen from Fig. 6c, when the HDT increased from 300 to 700 °C, the breakdown electric field of the obtained varistors (E100mA) increased first and then decreased, obtaining the peak value of 0.0201 V/nm at 500 °C. Meanwhile, the voltage per grain boundary exhibits slight variation around 0.37 eV with increasing HDT in this work. Considering that the grain size of the obtained samples presented a monotonous increase with increasing HDT, which normally results in decreasing E100mA, the increase of E100mA with increasing HDT for the present varistors should be ascribed to the content of Nb atoms in films. When the HDT was 500 °C, the obtained sample presented the maximum content of Nb atoms (see Fig. 3e), corresponding to the most complete Nb2O5 insulation layer at the grain boundary between the adjacent tin oxides, thus leading to the highest E100mA. In summary, the varistor prepared at 500 °C exhibits the maximum nonlinear coefficient of 14.73, minimum leakage current of
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Fig. 6. (a) E-J characteristic curves of the varistors prepared by hot-dipping tin oxide thin films in Nb2O5 powder at different temperatures. (b) The calculated nonlinear coefficient (a) and leakage current (IL) of the obtained film varistors as a function of HDT. (c) Breakdown electric field (E100mA) and voltage per grain boundary (Vgb) of the varistors as a function of HDT.
17.1 mA/cm2, and a breakdown electric field of 0.0201 V/nm under the optimized conditions in this work. In order to elucidate the electrical conducting mechanism of the obtained varistors, all the samples were first examined by EIS. Fig. 7a presents the complex impedance spectra of the samples prepared at different HDTs in the range of 300–700 °C. For details, those of the samples prepared at 300 and 400 °C are displayed with larger magnification as an inset of this figure. Moreover, the
corresponding equivalent circuit is also shown as an inset in Fig. 7a. From this figure, it can be seen that all the spectra could be fitted into semicircles with different radii, which increased first and then decreased as the HDT increased from 300 to 700 °C. According to the complex impedance spectra shown in Fig. 7a, the grain resistance (Rg) and grain boundary resistance (Rgb) of each sample could be calculated and the results are displayed in Fig. 7b. As is seen from this figure, the grain resistance rose up
Fig. 7. (a) Complex impedance spectra, and (b) grain resistance (Rg) and grain boundary resistance (Rgb) of the SnOx-Nb2O5 film varistors fabricated at different temperatures for 60 min in air. The insets in (a) are the equivalent circuit and the magnified impedance spectra of the samples prepared at the HDT of 300 and 400 °C, respectively.
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constantly from 4150 to 5090 X with the HDT increased from 300 to 700 °C, which could be attributed to the oxidation of tin oxide grains as discussed in the last section. Because the chemical composition of the as-deposited tin oxide film was SnO1.25, which is far from stoichiometric SnO2 due to a lot of defects existed in tin oxide lattice, thus it has lower electrical resistance [36]. After hotdipping in air with increasing HDT, the oxygen-deficient tin oxides would be oxidized, resulting in products with a composition closer and closer to SnO2. As a result, the grain resistance increased. However, the grain boundary resistance increased first and then decreased with increasing HDT, reaching the maximum of 3.05 106 X at 500 °C. During hot-dipping, when the temperature increased from 300 to 500 °C, more and more Nb atoms would diffuse into the tin oxide films (see Fig. 3e), staying at the grain boundary, improving the completeness of the Nb2O5 grain boundary layer, finally increasing the grain boundary resistance due to the high resistance of Nb2O5. But if the HDT rose up above 500 °C, the content of Nb atoms in the tin oxide films would be reduced (see Fig. 3e), due to the reduced effective area of the grain boundary caused by the increased size of tin oxide grains with increasing HDT (see the detailed discussion in last section). Resultantly, the completeness of the Nb2O5 grain boundary layer was degraded, finally decreasing the grain boundary resistance again. Combining with the nonlinear coefficient of the obtained film varistors, the varistor properties were closely correlated to the variations of the grain resistance and grain boundary resistance. Combining the aforementioned results, a grain-boundary defect barrier model for the present SnOx-Nb2O5 film varistors could be schematically depicted in Fig. 8, which displays the defects in the tin oxide film introduced during film deposition by magnetron sputtering and Nb5+ diffusion through hot-dipping. As is seen from this figure, the barrier was constructed by depletion layer and interface layer, which consist of positively charged donors and negatively charged acceptors, respectively. In the depletion layer, one of the intrinsic defects introduced into the films during the film deposition through magnetron sputtering is interstitial tin (Sni) at the distorted region of SnO2 lattices. The defect Sni would readily escape from the original site to become an interstitial tin ion (Sn4+ i ), simultaneously leaving a tin vacancy ion (V4 Sn ) at its original site in the interface layer. This conversion could be expressed by Eq. (3) [37]. Another intrinsic defect in the tin oxide lattices is oxygen vacancies (VO). The defect VO x would combine with V4 Sn , generating VO. This reaction could be expressed by Eq. (4) [38]. The generated VxO could then be further converted into V+O and/or V2+ O , staying at the depletion layer as positively charged donors. In addition, the Nb2O5 diffusing into the tin oxide grain boundary through hot-dipping would be transformed
Fig. 8. The grain-boundary defect barrier model for the present SnOx-Nb2O5 thin film varistors.
into Nb+Sn based on Eq. (5) at the depletion layer region, which would also contribute to the formation of potential barrier at grain boundary. 4þ Sni ! V 4 Sn þ Sni
ð3Þ
x x V 4 Sn þ 2V O ! V Sn þ 2V O
ð4Þ
SnO2
þ
x 2Nb2 O5 ! 4NbSn þ V 4 Sn þ 10OO
ð5Þ
Moreover, the positively charged donors would be balanced by the negatively charged acceptors distributing in the interface layer (see Fig. 8). Among them, because the hot-dipping was carried out in air during the preparation of the present SnOx-Nb2O5 film varistors, the adsorbed O2 on the samples would capture electrons to generate Oxad according to Eq. (6), and the produced Oxad could further react with negatively charged defects, generating negatively 2 charged acceptors of O ad and/or Oad through Eqs. (7) and (8), respectively [34,38].
1 x O2 ! Oad 2
ð6Þ
x 2 þ V 4 2Oad Sn ! 2Oad þ V Sn
ð7Þ
2 x V 4 Sn þ 4Oad ! 4Oad þ V Sn
ð8Þ (V4 Sn )
Besides, the tin vacancies ion is also essential for the formation of interface layer at the grain boundary as a negatively 2 4 charged acceptor. Thus, O ad, Oad together with VSn would form the negatively charged interface layer. 4+ In summary, the positively charged donors (V+O, V2+ and O , Sni + TaSn) in the depletion layer, together with the negatively charged 2 acceptors (V4 Sn , Oad and Oad ) in the interface layer would form a double barrier potential between the two adjacent tin oxide grains in the film varistor. And the electric transport occurred by tunneling across potential barriers at the grain boundary is responsible for the nonlinear characteristics of a varistor [37,39].
4. Conclusions A series of SnOx-Nb2O5 thin film varistors were fabricated by hot-dipping oxygen-deficient tin oxide film in Nb2O5 powder in air at different temperatures. The effects of HDT on the elemental composition and distribution, phase composition, microstructure, as well as electrical conducting and mechanism of samples were investigated. Nb2O5 would diffuse into tin oxide film during hotdipping, staying at the grain boundary and behaving as a typical VFO due to the formation of high-resistant Nb2O5 insulation layer. When the HDT increased from 300 to 700 °C, the nonlinear coefficient of the obtained varistors rose up first and then dropped down, reaching the maximum of 14.73 when the HDT was 500 °C. Correspondingly, the leakage current decreased first and then increased, acquiring the minimum of 17.1 mA/cm2 at 500 °C. The breakdown electric field of the obtained varistors exhibited the same variation as the nonlinear coefficient with increasing HDT, reaching the peak value of 0.0201 V/nm at 500 °C. In addition, a grain-boundary barrier defect model for the present SnOx-Nb2O5 thin film varistors was proposed, which could illustrate their electrical conducting and mechanism. Considering the high-performance and simple preparation method, the present nanoscaled SnOx-Nb2O5 thin film varistors would be promising in the application of electrical devices working in low voltage.
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