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Effect of Mn doping on the structural and electrical properties of periodic Ba0.9Sr0.1TiO3 multilayers
Applied Surface Science 360 (2016) 666–670
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Effect of Mn doping on the structural and electrical properties of periodic Ba0.9 Sr0.1 TiO3 multilayers Xuekun Hong, Tan Shao, Tao Wang, Yushen Liu, Debao Zhang, Yawei Kuang ∗ , Jinfu Feng College of Physics and Electronic Engineering, Changshu Institute of Technology, No. 99, 3rd South Ring Road, Changshu 215500, China
a r t i c l e
i n f o
Article history: Received 13 July 2015 Received in revised form 6 October 2015 Accepted 4 November 2015 Available online 10 November 2015 Keywords: BST Multilayer Mn-doped Oxygen vacancy
a b s t r a c t Undoped and Mn-doped periodic BST multilayers have been prepared by chemical solution deposition method. Mn2+ tends to substitute for Ti4+ in the BST lattice and introduces more oxygen vacancies, which leads to the reduction of the concentration of electrons and the downward shift of the Fermi level. As a result, improved electrical properties, such as lower frequency dispersion of the dielectric constant, lower dielectric loss and leakage current were found for Mn-doped samples. However, excessive Mn leads to the coexistence of Mn2+ and Mn3+ (or Mn4+ ), which is believed to degenerate the electrical performance of BST multilayers. 1 mol% Mn-doped BST multilayer exhibits the best electrical performance, with leakage current density of 2.1 × 10−5 A/cm2 (at 100 kV/cm), dielectric loss of ∼0.08 (at 10 kHz) and the minimum frequency dispersion of the dielectric constant. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Barium strontium titanate, Ba1−x Srx TiO3 (BST), thin films are prospective candidates for dynamic random-access memories, microwave tunable devices, non-volatile memories and uncooled infrared detectors due to its excellent dielectric, ferroelectric, and pyroelectric properties. Moreover, BST is environmentally friendly and its properties can be tailored for specific applications by controlling the barium to strontium ratio [1–4]. Recently, BST based ferroelectric multilayers are becoming promising materials due to their dramatically improved or unexpected properties compared to their thin-film counterparts. The periodic BST multilayers consisting of dense and porous BST bilayers have been used in fabricating distributed Bragg reflectors (DBRs) and F-P microcavities [5,7]. Furthermore, the performance of such DBRs and microcavities is expected to be dynamically tunable with an externally applied electric field due to the electro-optic effect of BST material. This characteristic offers an additional freedom to manipulate the devices applied to photonic band-gap engineering [8]. In such devices, the BST multilayers are desired to possess a low leakage current. Thus, an obvious change in the refractive index can be achieved by an applied electric field. Previous studies showed that the electrical properties of BST thin films strongly depend on the microstructure, bottom electrode
∗ Corresponding author. Tel.: +86 051252251556. E-mail address: [email protected] (Y. Kuang). http://dx.doi.org/10.1016/j.apsusc.2015.11.039 0169-4332/© 2015 Elsevier B.V. All rights reserved.
characteristics, dopant and deposition technique. Much experimental work has been carried out on the doping of A or B-sites of BST thin films with Mg2+ , Fe3+ , Cr3+ , Co2+ , La3+ , Ce3+ , etc. It has been reported that 1 and 2 mol% Fe-doped Ba0.65 Sr0.35 TiO3 thin films exhibit improved dielectric loss, tunability, and leakage current characteristics as compared to the undoped thin films [9]. The co-doping technique is also an effective way to optimize the electrical properties of BST [10–12]. Among these, Mn is one of the most popular and widely used dopants. In most literature, Mn ions are believed to substitute for Ti and act as acceptors [13,14]. However, Tkach reported that Mn ions may substitute for the Sr2+ site in SrTiO3 ceramics [15]. The valence state of Mn ions is also found to be easily changed from divalent to trivalent or tetravalent [16]. In this paper, Mn doping was chosen to optimize the performance of periodic BST multilayers. Undoped (BST) and Mn-doped Ba0.9 Sr0.1 TiO3 (BSTM) multilayers were fabricated by a modified chemical solution deposition method. 2. Experimental procedures The Ba0.9 Sr0.1 Ti1−x Mnx O3 (X = 0%, 0.5%, 1%, and 5%) precursor solutions with a concentration of 0.3 M/L were prepared using Ba(CH3 COO)2 , Sr(CH3 COO)2 ·1.5H2 O, Mn(CH3 COO)2 ·4H2 O, Ti(C4 H9 O)4 and Polymer PVP (polyvinylpyrrolidone, K30) as the starting materials. Detailed procedures can be found in Ref. [5]. In order to investigate the resulting electrical properties, an integrated metal-insulator-metal (MIM) structure was used. Approximately 180 nm of conductive oxide LaNiO3 (LNO) films
X. Hong et al. / Applied Surface Science 360 (2016) 666–670
3. Results and discussion 3.1. Structural characterization Fig. 1 shows the XRD patterns of the BST and BSTM multilayers. Except for the peaks from the STO substrate and LNO film, all other peaks belong to the perovskite BST. All the samples are singlephase, randomly oriented polycrystalline. The crystalline qualities were evaluated to be similar from the measured intensities and the values of the full-width-half maximum (FWHM) of the diffraction peaks. The slight shifts of the diffraction peak positionstoward
BST ( 211)
BST ( 111)
BST ( 110) * *L O N
20
30
*
1% Mn
*
0.5% Mn
*
5% Mn
Intensity(a.u.)
were first deposited on the SrTiO3 (STO, 100) substrates by spin-coating, to act as the bottom electrodes for samples. The growth of LNO films had been elucidated elsewhere [6]. Five spin-coating/annealing cycles were performed to obtain a desired thickness. The spinning rates were 3500 revolutions per minute (rpm). All the annealing procedures were carried out in a rapid thermal annealing furnace in atmospheric environment. The wet gel film was first dried at 180 ◦ C for 180 s, and then pre-fired at 380 ◦ C for 300 s to remove residual organics, and finally annealed at 750 ◦ C for 360 s. To form the MIM configuration, circular top platinum electrodes with a diameter of 0.2 mm were added by using a dc sputtering instrument through a shadow mask. The crystal structures of BST multilayers were characterized by X-ray diffraction (XRD) with a Cu K␣ radiation source. Microstructures were analyzed by Transmission electron microscopy (TEM), and the chemical information of element was investigated by X-ray photoelectron spectra (XPS). Electrical properties were measured by using an Agilent E4980A precision LCR meter, Radiant precision materials analyzer and Keithley 6430 sub-femto-amp remote source meter.
667
Undoped
40 2θ (deg.)
50
60
Fig. 1. XRD patterns of the samples.
smaller diffraction angles for BSTM sample (X = 5%) demonstrate an increase in the lattice constant. This can be attributed to the substi˚ is larger than tution of Mn for Ti since the radius of Mn2+ (r = 0.67 A) ˚ and smaller than that of Ba2+ (r = 1.35 A) ˚ or that of Ti4+ (r = 0.605 A) ˚ However, no obvious shifts were found for BSTM Sr2+ (r = 1.13 A). samples (X < 5%) probably due to the low Mn content. Fig. 2 presents the cross-sectional TEM morphologies of BST and BSTM samples. All the samples exhibit a periodic structure consisting of thin dense-BST (or BSTM) and thick porous-BST (or BSTM) layers, which is similar to our previous results [5]. The multilayers are crack-free and uniform in thickness. The thickness of the LNO layer is approximately 180 nm, and the thicknesses of 5 BSTM periods are estimated to be 550 nm for all samples from the TEM results.
Fig. 2. Cross-sectional TEM morphologies of the samples. (a) Undoped; (b) 0.5% Mn-doped; (c) 1% Mn-doped; and (d) 5% Mn-doped.
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X. Hong et al. / Applied Surface Science 360 (2016) 666–670
a
b
Ba3d 5/2 Ba2 Ba1
Ba3d 3/2 Ba2 Ba1
Intensity(a. u.)
5% Mn
Intensity(a. u.)
Sr3d 5% Mn
1% Mn
0.5% Mn
3/2
Sr3d
5/2
1% Mn
0.5% Mn
Undoped Undoped
800
795
790 785 780 Binding energy(eV )
775
770
140
138
136 134 132 Binding energy(eV )
130
128
Fig. 3. XPS spectra of (a) Ba 3d and (b) Sr 3d of the samples.
Ti 2p 3/2
Intensity(a. u.)
Ti 2 p 1/2 5% Mn 1% Mn 0.5% Mn Undoped
465
462 459 456 Binding energy(eV)
453
Fig. 5. XPS spectra of Ti 2p of the samples.
O1s
O2
O1
Intensity(a. u.)
Fig. 3(a) and (b) shows the XPS survey spectra for Ba 3d and Sr 3d, respectively. Gaussian fitting (dash line) was used to divide the overlapping peaks. As shown in Fig. 3(a), each of the broad Ba 3d5/2 or Ba 3d3/2 peaks can be separated into two peaks. The fitted peak with lower energy, denoted as Ba1, is assigned to Ba in perovskite structure. The fitted Ba2 peak with higher energy originates from the amorphous structure. The ratio of Ba1 to Ba2 is about 0.2 and the spin–orbit splitting energy is calculated to be about 15.3, which are similar values to the results in previous research [17,18]. As shown in Fig. 3(b), the double-humped Sr 3d peaks are fitted into Sr 3d5/2 and Sr 3d3/2 separated by ∼1.7 eV. For 0.5 and 5 mol% Mndoped samples, peaks marked by a downward arrow, suggest the presence of an amorphous structure. To get further insight of the effect of Mn dopant, the XPS spectra of Mn 2p are shown in Fig. 4. The intensity of the Mn 2p peak increases with increasing X value, indicating the addition of Mn. By fitting the spectrum of BSTM (X = 5%), each Mn 2p1/2 or Mn 2p3/2 is divided into two peaks assigned to Mn2+ and Mn3+ or Mn4+ [21,22]. This indicates that the valence state of Mn ions is easily changed. It has been proposed that Ti3+ can be possibly found in the thin BST film and Mn ions can prevent the reduction of Ti4+ to Ti3+ by neutralizing the electrons. In order to clarify that, Fig. 5 shows the XPS spectra of Ti 2p for all samples. It can be seen that all the peaks are symmetric and the doublet peaks can be assigned as Ti4+ rather than Ti3+ states [19]. Fig. 6 shows the high resolution O1s spectra obtained from all samples. All O1s peaks can be fitted into two peaks denoted as O1 and O2, respectively. The lower energy peak is reported to be absorbed oxygen species and hydroxyl ions, which are related to the presence of oxygen vacancy in BST films. The higher energy peak is assigned to oxygen in the lattice. The ratio
5% Mn 1% Mn
0.5% Mn Undoped
534 2p3/2 2+ Mn
532 530 528 Binding energy(eV)
526
Fig. 6. XPS spectra of O 1s of the samples.
Intensity(a. u.)
2p1/2 3+ Mn
2+ Mn
3+ Mn 5% Mn
of O1 to O2, as summarized in Table 1, is usually used to estimate the relative content of oxygen vacancy in the BST films [18,20]. It was found that the amount of oxygen vacancies increases with the addition of Mn-dopant except for BSTM (X = 5%). The following
1% Mn 0.5% Mn
660
655
650 645 640 Binding energy(eV)
635
Fig. 4. XPS spectra of Mn 2p of the samples.
630
Table 1 O1/O2 ratio of the BST and BSTM multilayers. Multilayers
O1/O2
BST BSTM (X = 0.5%) BSTM (X = 1%) BSTM (X = 5%)
1.178 1.405 1.522 0.567
X. Hong et al. / Applied Surface Science 360 (2016) 666–670
BSTM (X=5%) BSTM (X=1%) BSTM (X=0.5 %)
20
-20
8
1 1 O2 or O2 2 4
1
1 + Mn2+ + Vo¨ or Vo¨ 2
+ OO or OO 2
→ Mn4+ orMn3+
(1)
(2)
On one hand, Mn doping tends to increase the concentration of oxygen vacancies as shown in Eq. (1). On the other hand, the excessive Mn2+ may changed into Mn3+ or Mn4+ by consuming some oxygen vacancies, which leads to the decrease of oxygen vacancies and the coexistence of Mn2+ and Mn3+ or Mn4+ . It is should be noted that Ba 3d, Sr 3d, Ti 2p, O1s and Mn 2p all undergo a similar downward shift in the binding energies with increasing Mn concentration, indicating a downward shift of the Fermi level and a decrease of the concentration of electrons [12]. 3.2. Electrical properties The frequency dependence of dielectric constant and loss tangent are shown in Fig. 7. It can be observed that the introduction of Mn ions decreases the dielectric constant significantly from 1145 to 460 at 10 kHz. Compared to Ti4+ , Mn2+ with larger radius in the O octahedron is more difficult to shift and thus leads to a smaller polarization. Less frequency dispersion of the dielectric constant was also found for BSTM (X = 1%) multilayer compared to other samples. With increasing Mn content, the dielectric loss decreases first and then increases. BSTM (X = 1%) multilayer shows the lowest dielectric loss. For BSTM (X = 5%) multilayer, an obvious peak of the dielectric loss can be found between 104 and 105 Hz, which is probably caused by MnTi − VO dipoles and further investigations are needed to clarify this hypothesis. The P–E hysteresis loops at room temperatures are shown in Fig. 8. The remnant polarization decreases with Mn doping, which is well matched with the variation of the dielectric constant and can be explained by the following equation: P = ε0 (εr − 1) E
(3)
where P, E, ε0 and εr are the polarization, electric field, permittivity of free space and dynamic dielectric constant, respectively. A small coercive field Ec can also be found for BSTM (X = 1%) multilayer. The P–E loop of BST is distorted and not shown in Fig. 8. Fig. 9 shows the leakage current characteristics of BST and BSTM multilayers with positive bias voltage applied to the top Pt electrodes. As can be seen, the leakage current was appreciably
-50
12 10
8 Mn(5%) Mn(1%)Mn(0.5%)
0 E (kV / cm)
50
100
Fig. 8. P E hysteresis loops at room temperatures of the samples.
Leakage current density(A/cm 2)
BaO (orSrO) + MnO → BaBa (orSrSr ) + Mn Ti + 2Oo + Vo¨
14
10
-100
mechanisms from the viewpoint of defect chemistry may account for this [23]:
16
14 12
-40
Fig. 7. Frequency dependencies of the dielectric constant and the loss tangent (inset) of the samples.
16
2Pr
0
2Ec
P (μC/cm 2)
40
669
0.1 0.01
BST BSTM ( X = 0.5%) BSTM ( X = 1%) BSTM ( X = 5%)
1E-3 1E-4 1E-5 1E-6 1E-7
100
200 E (kV / cm)
300
400
Fig. 9. Leakage current density versus applied electric field of the samples.
reduced in Mn-doped multilayers. Mn doping tends to reduce the concentration of electrons and lowers the leakage current. However, an enhanced local hopping of electrons between Mn2+ and Mn3+ or Mn4+ due to excessive Mn doping may increase the leakage current density and provide a mechanism for the large dielectric loss of the BSTM (X = 5%) sample. The high leakage current of the BST multilayer also accounts for the distortion in its P–E loop. 4. Conclusions In summary, BST and BSTM multilayer samples with periodic configurations were prepared by using a modified chemical solution deposition method. The electrical properties can be optimized by moderate Mn doping. For low Mn content, Mn doping tends to increase the concentration of oxygen vacancies and decrease that of electrons, which provides a mechanism for the reduction of leakage current and dielectric loss. However, the coexistence of Mn ions with different valence states due to excessive Mn doping may provide a local hopping channel for electrons and in turn, increases the leakage current and the dielectric loss. Compared to Ti4+ , the larger Mn2+ in the O octahedron also leads to a smaller polarization. Our findings indicate that Mn doping is an effective method to modify the electrical properties of periodic BST multilayers. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grants no. 61106126) and the Jiangsu Qing Lan Project is acknowledged.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of Mn doping on the structural and electrical properties of periodic Ba0.9 Sr0.1 TiO3 multilayers Xuekun Hong, Tan Shao, Tao Wang, Yushen Liu, Debao Zhang, Yawei Kuang ∗ , Jinfu Feng College of Physics and Electronic Engineering, Changshu Institute of Technology, No. 99, 3rd South Ring Road, Changshu 215500, China
a r t i c l e
i n f o
Article history: Received 13 July 2015 Received in revised form 6 October 2015 Accepted 4 November 2015 Available online 10 November 2015 Keywords: BST Multilayer Mn-doped Oxygen vacancy
a b s t r a c t Undoped and Mn-doped periodic BST multilayers have been prepared by chemical solution deposition method. Mn2+ tends to substitute for Ti4+ in the BST lattice and introduces more oxygen vacancies, which leads to the reduction of the concentration of electrons and the downward shift of the Fermi level. As a result, improved electrical properties, such as lower frequency dispersion of the dielectric constant, lower dielectric loss and leakage current were found for Mn-doped samples. However, excessive Mn leads to the coexistence of Mn2+ and Mn3+ (or Mn4+ ), which is believed to degenerate the electrical performance of BST multilayers. 1 mol% Mn-doped BST multilayer exhibits the best electrical performance, with leakage current density of 2.1 × 10−5 A/cm2 (at 100 kV/cm), dielectric loss of ∼0.08 (at 10 kHz) and the minimum frequency dispersion of the dielectric constant. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Barium strontium titanate, Ba1−x Srx TiO3 (BST), thin films are prospective candidates for dynamic random-access memories, microwave tunable devices, non-volatile memories and uncooled infrared detectors due to its excellent dielectric, ferroelectric, and pyroelectric properties. Moreover, BST is environmentally friendly and its properties can be tailored for specific applications by controlling the barium to strontium ratio [1–4]. Recently, BST based ferroelectric multilayers are becoming promising materials due to their dramatically improved or unexpected properties compared to their thin-film counterparts. The periodic BST multilayers consisting of dense and porous BST bilayers have been used in fabricating distributed Bragg reflectors (DBRs) and F-P microcavities [5,7]. Furthermore, the performance of such DBRs and microcavities is expected to be dynamically tunable with an externally applied electric field due to the electro-optic effect of BST material. This characteristic offers an additional freedom to manipulate the devices applied to photonic band-gap engineering [8]. In such devices, the BST multilayers are desired to possess a low leakage current. Thus, an obvious change in the refractive index can be achieved by an applied electric field. Previous studies showed that the electrical properties of BST thin films strongly depend on the microstructure, bottom electrode
∗ Corresponding author. Tel.: +86 051252251556. E-mail address: [email protected] (Y. Kuang). http://dx.doi.org/10.1016/j.apsusc.2015.11.039 0169-4332/© 2015 Elsevier B.V. All rights reserved.
characteristics, dopant and deposition technique. Much experimental work has been carried out on the doping of A or B-sites of BST thin films with Mg2+ , Fe3+ , Cr3+ , Co2+ , La3+ , Ce3+ , etc. It has been reported that 1 and 2 mol% Fe-doped Ba0.65 Sr0.35 TiO3 thin films exhibit improved dielectric loss, tunability, and leakage current characteristics as compared to the undoped thin films [9]. The co-doping technique is also an effective way to optimize the electrical properties of BST [10–12]. Among these, Mn is one of the most popular and widely used dopants. In most literature, Mn ions are believed to substitute for Ti and act as acceptors [13,14]. However, Tkach reported that Mn ions may substitute for the Sr2+ site in SrTiO3 ceramics [15]. The valence state of Mn ions is also found to be easily changed from divalent to trivalent or tetravalent [16]. In this paper, Mn doping was chosen to optimize the performance of periodic BST multilayers. Undoped (BST) and Mn-doped Ba0.9 Sr0.1 TiO3 (BSTM) multilayers were fabricated by a modified chemical solution deposition method. 2. Experimental procedures The Ba0.9 Sr0.1 Ti1−x Mnx O3 (X = 0%, 0.5%, 1%, and 5%) precursor solutions with a concentration of 0.3 M/L were prepared using Ba(CH3 COO)2 , Sr(CH3 COO)2 ·1.5H2 O, Mn(CH3 COO)2 ·4H2 O, Ti(C4 H9 O)4 and Polymer PVP (polyvinylpyrrolidone, K30) as the starting materials. Detailed procedures can be found in Ref. [5]. In order to investigate the resulting electrical properties, an integrated metal-insulator-metal (MIM) structure was used. Approximately 180 nm of conductive oxide LaNiO3 (LNO) films
X. Hong et al. / Applied Surface Science 360 (2016) 666–670
3. Results and discussion 3.1. Structural characterization Fig. 1 shows the XRD patterns of the BST and BSTM multilayers. Except for the peaks from the STO substrate and LNO film, all other peaks belong to the perovskite BST. All the samples are singlephase, randomly oriented polycrystalline. The crystalline qualities were evaluated to be similar from the measured intensities and the values of the full-width-half maximum (FWHM) of the diffraction peaks. The slight shifts of the diffraction peak positionstoward
BST ( 211)
BST ( 111)
BST ( 110) * *L O N
20
30
*
1% Mn
*
0.5% Mn
*
5% Mn
Intensity(a.u.)
were first deposited on the SrTiO3 (STO, 100) substrates by spin-coating, to act as the bottom electrodes for samples. The growth of LNO films had been elucidated elsewhere [6]. Five spin-coating/annealing cycles were performed to obtain a desired thickness. The spinning rates were 3500 revolutions per minute (rpm). All the annealing procedures were carried out in a rapid thermal annealing furnace in atmospheric environment. The wet gel film was first dried at 180 ◦ C for 180 s, and then pre-fired at 380 ◦ C for 300 s to remove residual organics, and finally annealed at 750 ◦ C for 360 s. To form the MIM configuration, circular top platinum electrodes with a diameter of 0.2 mm were added by using a dc sputtering instrument through a shadow mask. The crystal structures of BST multilayers were characterized by X-ray diffraction (XRD) with a Cu K␣ radiation source. Microstructures were analyzed by Transmission electron microscopy (TEM), and the chemical information of element was investigated by X-ray photoelectron spectra (XPS). Electrical properties were measured by using an Agilent E4980A precision LCR meter, Radiant precision materials analyzer and Keithley 6430 sub-femto-amp remote source meter.
667
Undoped
40 2θ (deg.)
50
60
Fig. 1. XRD patterns of the samples.
smaller diffraction angles for BSTM sample (X = 5%) demonstrate an increase in the lattice constant. This can be attributed to the substi˚ is larger than tution of Mn for Ti since the radius of Mn2+ (r = 0.67 A) ˚ and smaller than that of Ba2+ (r = 1.35 A) ˚ or that of Ti4+ (r = 0.605 A) ˚ However, no obvious shifts were found for BSTM Sr2+ (r = 1.13 A). samples (X < 5%) probably due to the low Mn content. Fig. 2 presents the cross-sectional TEM morphologies of BST and BSTM samples. All the samples exhibit a periodic structure consisting of thin dense-BST (or BSTM) and thick porous-BST (or BSTM) layers, which is similar to our previous results [5]. The multilayers are crack-free and uniform in thickness. The thickness of the LNO layer is approximately 180 nm, and the thicknesses of 5 BSTM periods are estimated to be 550 nm for all samples from the TEM results.
Fig. 2. Cross-sectional TEM morphologies of the samples. (a) Undoped; (b) 0.5% Mn-doped; (c) 1% Mn-doped; and (d) 5% Mn-doped.
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X. Hong et al. / Applied Surface Science 360 (2016) 666–670
a
b
Ba3d 5/2 Ba2 Ba1
Ba3d 3/2 Ba2 Ba1
Intensity(a. u.)
5% Mn
Intensity(a. u.)
Sr3d 5% Mn
1% Mn
0.5% Mn
3/2
Sr3d
5/2
1% Mn
0.5% Mn
Undoped Undoped
800
795
790 785 780 Binding energy(eV )
775
770
140
138
136 134 132 Binding energy(eV )
130
128
Fig. 3. XPS spectra of (a) Ba 3d and (b) Sr 3d of the samples.
Ti 2p 3/2
Intensity(a. u.)
Ti 2 p 1/2 5% Mn 1% Mn 0.5% Mn Undoped
465
462 459 456 Binding energy(eV)
453
Fig. 5. XPS spectra of Ti 2p of the samples.
O1s
O2
O1
Intensity(a. u.)
Fig. 3(a) and (b) shows the XPS survey spectra for Ba 3d and Sr 3d, respectively. Gaussian fitting (dash line) was used to divide the overlapping peaks. As shown in Fig. 3(a), each of the broad Ba 3d5/2 or Ba 3d3/2 peaks can be separated into two peaks. The fitted peak with lower energy, denoted as Ba1, is assigned to Ba in perovskite structure. The fitted Ba2 peak with higher energy originates from the amorphous structure. The ratio of Ba1 to Ba2 is about 0.2 and the spin–orbit splitting energy is calculated to be about 15.3, which are similar values to the results in previous research [17,18]. As shown in Fig. 3(b), the double-humped Sr 3d peaks are fitted into Sr 3d5/2 and Sr 3d3/2 separated by ∼1.7 eV. For 0.5 and 5 mol% Mndoped samples, peaks marked by a downward arrow, suggest the presence of an amorphous structure. To get further insight of the effect of Mn dopant, the XPS spectra of Mn 2p are shown in Fig. 4. The intensity of the Mn 2p peak increases with increasing X value, indicating the addition of Mn. By fitting the spectrum of BSTM (X = 5%), each Mn 2p1/2 or Mn 2p3/2 is divided into two peaks assigned to Mn2+ and Mn3+ or Mn4+ [21,22]. This indicates that the valence state of Mn ions is easily changed. It has been proposed that Ti3+ can be possibly found in the thin BST film and Mn ions can prevent the reduction of Ti4+ to Ti3+ by neutralizing the electrons. In order to clarify that, Fig. 5 shows the XPS spectra of Ti 2p for all samples. It can be seen that all the peaks are symmetric and the doublet peaks can be assigned as Ti4+ rather than Ti3+ states [19]. Fig. 6 shows the high resolution O1s spectra obtained from all samples. All O1s peaks can be fitted into two peaks denoted as O1 and O2, respectively. The lower energy peak is reported to be absorbed oxygen species and hydroxyl ions, which are related to the presence of oxygen vacancy in BST films. The higher energy peak is assigned to oxygen in the lattice. The ratio
5% Mn 1% Mn
0.5% Mn Undoped
534 2p3/2 2+ Mn
532 530 528 Binding energy(eV)
526
Fig. 6. XPS spectra of O 1s of the samples.
Intensity(a. u.)
2p1/2 3+ Mn
2+ Mn
3+ Mn 5% Mn
of O1 to O2, as summarized in Table 1, is usually used to estimate the relative content of oxygen vacancy in the BST films [18,20]. It was found that the amount of oxygen vacancies increases with the addition of Mn-dopant except for BSTM (X = 5%). The following
1% Mn 0.5% Mn
660
655
650 645 640 Binding energy(eV)
635
Fig. 4. XPS spectra of Mn 2p of the samples.
630
Table 1 O1/O2 ratio of the BST and BSTM multilayers. Multilayers
O1/O2
BST BSTM (X = 0.5%) BSTM (X = 1%) BSTM (X = 5%)
1.178 1.405 1.522 0.567
X. Hong et al. / Applied Surface Science 360 (2016) 666–670
BSTM (X=5%) BSTM (X=1%) BSTM (X=0.5 %)
20
-20
8
1 1 O2 or O2 2 4
1
1 + Mn2+ + Vo¨ or Vo¨ 2
+ OO or OO 2
→ Mn4+ orMn3+
(1)
(2)
On one hand, Mn doping tends to increase the concentration of oxygen vacancies as shown in Eq. (1). On the other hand, the excessive Mn2+ may changed into Mn3+ or Mn4+ by consuming some oxygen vacancies, which leads to the decrease of oxygen vacancies and the coexistence of Mn2+ and Mn3+ or Mn4+ . It is should be noted that Ba 3d, Sr 3d, Ti 2p, O1s and Mn 2p all undergo a similar downward shift in the binding energies with increasing Mn concentration, indicating a downward shift of the Fermi level and a decrease of the concentration of electrons [12]. 3.2. Electrical properties The frequency dependence of dielectric constant and loss tangent are shown in Fig. 7. It can be observed that the introduction of Mn ions decreases the dielectric constant significantly from 1145 to 460 at 10 kHz. Compared to Ti4+ , Mn2+ with larger radius in the O octahedron is more difficult to shift and thus leads to a smaller polarization. Less frequency dispersion of the dielectric constant was also found for BSTM (X = 1%) multilayer compared to other samples. With increasing Mn content, the dielectric loss decreases first and then increases. BSTM (X = 1%) multilayer shows the lowest dielectric loss. For BSTM (X = 5%) multilayer, an obvious peak of the dielectric loss can be found between 104 and 105 Hz, which is probably caused by MnTi − VO dipoles and further investigations are needed to clarify this hypothesis. The P–E hysteresis loops at room temperatures are shown in Fig. 8. The remnant polarization decreases with Mn doping, which is well matched with the variation of the dielectric constant and can be explained by the following equation: P = ε0 (εr − 1) E
(3)
where P, E, ε0 and εr are the polarization, electric field, permittivity of free space and dynamic dielectric constant, respectively. A small coercive field Ec can also be found for BSTM (X = 1%) multilayer. The P–E loop of BST is distorted and not shown in Fig. 8. Fig. 9 shows the leakage current characteristics of BST and BSTM multilayers with positive bias voltage applied to the top Pt electrodes. As can be seen, the leakage current was appreciably
-50
12 10
8 Mn(5%) Mn(1%)Mn(0.5%)
0 E (kV / cm)
50
100
Fig. 8. P E hysteresis loops at room temperatures of the samples.
Leakage current density(A/cm 2)
BaO (orSrO) + MnO → BaBa (orSrSr ) + Mn Ti + 2Oo + Vo¨
14
10
-100
mechanisms from the viewpoint of defect chemistry may account for this [23]:
16
14 12
-40
Fig. 7. Frequency dependencies of the dielectric constant and the loss tangent (inset) of the samples.
16
2Pr
0
2Ec
P (μC/cm 2)
40
669
0.1 0.01
BST BSTM ( X = 0.5%) BSTM ( X = 1%) BSTM ( X = 5%)
1E-3 1E-4 1E-5 1E-6 1E-7
100
200 E (kV / cm)
300
400
Fig. 9. Leakage current density versus applied electric field of the samples.
reduced in Mn-doped multilayers. Mn doping tends to reduce the concentration of electrons and lowers the leakage current. However, an enhanced local hopping of electrons between Mn2+ and Mn3+ or Mn4+ due to excessive Mn doping may increase the leakage current density and provide a mechanism for the large dielectric loss of the BSTM (X = 5%) sample. The high leakage current of the BST multilayer also accounts for the distortion in its P–E loop. 4. Conclusions In summary, BST and BSTM multilayer samples with periodic configurations were prepared by using a modified chemical solution deposition method. The electrical properties can be optimized by moderate Mn doping. For low Mn content, Mn doping tends to increase the concentration of oxygen vacancies and decrease that of electrons, which provides a mechanism for the reduction of leakage current and dielectric loss. However, the coexistence of Mn ions with different valence states due to excessive Mn doping may provide a local hopping channel for electrons and in turn, increases the leakage current and the dielectric loss. Compared to Ti4+ , the larger Mn2+ in the O octahedron also leads to a smaller polarization. Our findings indicate that Mn doping is an effective method to modify the electrical properties of periodic BST multilayers. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grants no. 61106126) and the Jiangsu Qing Lan Project is acknowledged.
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