Dielectric and ferroelectric properties of c-axis

JOURNAL OF APPLIED PHYSICS 99, 124106 共2006兲

Dielectric and ferroelectric properties of c-axis oriented strontium bismuth tantalate thin films applied transverse electric fields K. Kotani, I. Kawayama, and M. Tonouchia兲 Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan

Y. Hottab兲 and H. Tabata Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

共Received 18 August 2005; accepted 4 April 2006; published online 26 June 2006兲 The in-plane dielectric and ferroelectric properties of c-axis oriented epitaxial strontium bismuth tantalate 共SBT兲 thin films were investigated by interdigital surface electrode measurement. The four types of SBT thin films, which have various Sr/ Bi atomic ratios, were prepared on MgO substrates by pulsed laser deposition. The dielectric properties at kilohertz and megahertz frequencies were studied from 20 to 870 K. Above room temperature, all the films show the phase transition. Sr0.99Bi1.61Ta2O9, Sr0.97Bi2.10Ta2O9, and Sr0.88Bi1.79Ta2O9 thin films show frequency dispersions near phase transition temperature. At low temperatures, dielectric and ferroelectric properties of Sr0.97Bi2.10Ta2O9 and Sr0.83Bi2.08Ta2O9 thin films were investigated. Both dielectric constant and remanent polarization show a tendency to decrease as temperature decreases, and the rapid change, which may indicate a phase transition, was observed in Sr0.97Bi2.10Ta2O9 near 80 K. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2205351兴 I. INTRODUCTION

Strontium bismuth tantalate 共SBT兲 has attracted much attention from the viewpoint of application to ferroelectric random access memory 共FeRAM兲 because of their high fatigue endurance and low coercive fields.1,2 However, compared to the other candidates for the FeRAM such as Pb共Zn, Ti兲O3 or La-doped Bi4Ti3O12, SBT has a drawback of remanent polarization reduction.2,3 In order to enhance the remanent polarization, various efforts have been made and reported that Sr-deficient Bi-excess SBT shows almost double magnitude of the remanent polarization.4,5 In addition, electrical properties of alkaline earth or rare earth substituted SBT have been measured.6–8 These measurements have gradually revealed the fundamental properties of SBT and variation of the properties by ionic substitution. The properties of SBT have been mainly observed using polycrystalline bulk samples or polycrystalline thin films on conductive-layer-coated Si substrates. The measured properties, however, depend largely on the thin film crystallinity such as the orientation. This is because SBT has strong anisotropy, which causes bit-to-bit variability of the memory cells.9,10 It is also known that stoichiometric SBT undergoes phase transition at ⬃300 ° C, and phase transition temperature 共TC兲 is varied by the ionic substitution.7,11,12 In order to investigate the intrinsic properties of SBT, it is important to study the dielectric function using epitaxial films and their temperature dependence. The remanent polarization in 共116兲and 共103兲-oriented epitaxial SrBi2Ta2O9 thin films have been already reported, which would be potential functionality for a兲

Electronic mail: [email protected] Present address: Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan.

b兲

0021-8979/2006/99共12兲/124106/6/$23.00

the application to FeRAM.13,14 The remanent polarization in c-axis oriented SBT thin films has not been observed using conventional capacitor structure.14 In this paper, we prepared the c-axis oriented epitaxial SBT thin films and measured the temperature dependence of in-plane electrical properties between 20 and 870 K by employing interdigital electrodes 共IDEs兲. The dielectric and ferroelectric measurements with IDEs have some advantages in comparison with the conventional capacitor structure. As this method does not require bottom electrodes, it is relatively easy to study epitaxial thin films with good crystallinity on suitable substrates or buffer layers;15,16 the data measured with IDEs largely reflect the properties in a-b plane in which ferroelectric spontaneous polarization of SBT appears. Furthermore, there is few literatures concerning the electrical properties in low temperature region, so our study would lead progress for understanding its fundamental properties. II. EXPERIMENTAL PROCEDURE

SBT thin films were prepared by pulsed laser deposition 共PLD兲 technique. Four types of SBT ceramic targets, whose Sr/ Bi compositions before calcination process were 1.0/ 2.0, 1.0/ 2.2, 0.9/ 2.3, and 0.8/ 2.4, were prepared by solid state reaction method. SrCO3共99.9% 兲, Bi2O3共99.99% 兲, and Ta2O5共99.99% 兲 were used as starting materials and weighted according to the compositions. The powders were ball milled and calcined at 800 ° C for 6 h. The calcined powders were mixed again and pressed into pellets, and finally sintered at 1100 ° C for 4 h. The targets had a density of 90%–95% compared with theoretical values. Each SBT target mounted on rotating target holder was ablated by the KrF laser operating at a repetition rate of 2 Hz with laser energy density of 2.0 J / cm2 in vacuum chamber. Polished single crystal MgO 共100兲 substrate was placed in a

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TABLE I. Compositions of compounds and thin films, the lattice constant of c axis and dielectric constant at room temperature, and the phase transition temperature for SBT thin films. The compositions are expressed as Sr/ Bi/ Ta atomic ratio.

SBT-a SBT-b SBT-c SBT-d

Sr/ Bi

Sr/ Bi/ Ta atomic ratio

c 共Å兲

⑀r

T M 共°C兲

1.0/ 2.0 1.0/ 2.2 0.9/ 2.3 0.8/ 2.4

0.99/ 1.61/ 2.00 0.97/ 2.10/ 2.00 0.88/ 1.79/ 2.00 0.83/ 2.08/ 2.00

24.98 24.99 25.04 25.06

200 218 252 159

200 300 320 420

rotating substrate holder that is heated by radiation heater. The distance between the target and the substrate was ⬃4.0 cm and the angle between the laser target and the target substrate was fixed as ⬃45°. The O2 pressure of 30 Pa was maintained in the chamber during the film deposition. After deposition, the chamber was flooded with oxygen until the pressure became 50 kPa and the film was annealed for 3 min and then cooled down to room temperature. These four samples, whose Sr/ Bi compositions were 1.0/ 2.0, 1.0/ 2.2, 0.9/ 2.3, and 0.8/ 2.4, are named as SBT-a, SBT-b, SBT-c, and SBT-d, respectively. The phase structures of the films were analyzed by x-ray diffraction 共XRD兲 ␪-2␪ scan and ␾ scan using Cu K␣ radiation. Film compositions were characterized using a wavelength-dispersive fluorescent x-ray spectroscopy. The film thickness of each sample is in the range between 270 and 310 nm. In order to measure the dielectric properties, a 5-nm-thick Ti adhesion layer and a 100-nm-thick Pt layer were sputtered on SBT thin films and patterned to form the interdigital electrode structure by the standard lift-off techniques. Both finger width and finger gap are 5 ␮m. The length of each electrode is 2 mm and the number of the electrodes is 41. Electrical field has been primarily applied to the 关110兴 direction of SBT. The details of the fabrication process were reported elsewhere.16 To measure the temperature dependence of capacitance from room temperature to ⬃600 ° C, the SBT thin film samples were mounted on a ceramic heater. The interdigital electrodes fabricated on the thin films were directly probed by tungsten needles connected to Agilent 4294A impedance analyzer by low loss coaxial cables. The temperature of the sample was raised at a rate of 10 ° C / min, and the capacitance of the interdigital electrodes on SBT thin film was measured by impedance analyzer from room temperature to 600 ° C with an oscillation level of 0.5 V. The dielectric constant was calculated from the capacitance, electrode geometry, film thickness, and dielectric constant of the MgO substrates.17 The temperature dependence of dielectric constant of SBT-b and SBT-d below room temperature has been observed, separately above room temperature. The SBT thin films were fixed on a cold head of a closed-cycle He refrigerator and the capacitance has been measured in cooling process. The electrical measurements were performed in vacuum chamber. The dielectric constant was calculated in the same way above room temperature. In addition, ferroelectric hysteresis loop was measured with an FCE-1 mea-

surement system 共Toyo Technica兲 at 1 kHz. The magnitude of electrical field is calculated on the assumption that electrical field was uniformly applied parallel to the surface of the film.18 III. RESULTS AND DISCUSSION A. Characterization of the films

The compositions of the four SBT thin films obtained by the wavelength-dispersive fluorescent x-ray spectroscopy are listed in Table I. Compared to the atomic composition before calcination, Bi in thin films were defected because of the high volatility of Bi. Especially SBT-c was Bi deficient. It may be due to the mass density of ceramic target. In fact, the density of SBT-c target was slightly less than the other three targets. It is known that the atomic compositions in films are affected not only by the fabrication condition but also by the target density. The XRD ␪-2␪ patterns of SBT thin films are shown in Fig. 1. Every patterns show the peaks corresponding to MgO 共200兲, SBT 共001兲, and an unknown phase which is marked as

FIG. 1. XRD ␪-2␪ patterns of 共a兲 SBT-a, 共b兲 SBT-b, 共c兲 SBT-c, and 共d兲 SBT-d thin films.


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FIG. 2. XRD ␾ scan image of the SBT 共115兲 reflections of SBT-b.

ⴱ in this figure. These unknown peaks might be assigned to SBT 共200兲 or SBT 共202兲.18 In case that SBT is not stoichiometric, however, it is well known that the secondary phases, such as fluorite or pyrochlore, are easily grown in the SBT films. Because the peaks corresponding to such secondary phases appear at almost the same angle as that from SBT in XRD ␪-2␪ patterns,19–21 it is very difficult to identify the peaks by conventional ␪-2␪ scan. The evaluated lattice constant of the c axis is listed in Table I. The lattice constants for SBT-a and that of SBT-b, which have almost the same Sr composition, are almost the same values. This result shows that the lattice constant of the c axis is not affected by a small amount of Bi deficiency. It is likely that the Bi layer is formed and makes the length between pseudoperovskite blocks constant even if Bi is not sufficient. In contrast, the length of the c axis tends to increase as the Sr ratio is decreased. The lattice constant for SBT-c is larger than that for SBT-b, moreover, that for SBT-d is larger than that for SBT-c. The tendency is opposite for that of the polycrystalline bulk samples.8 This variation of

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the lattice constant probably attributes to the lattice distortion resulting from lattice mismatch between SBT and MgO substrates and/or the difference of the thermal expansion of SBT thin films from that of MgO substrates. Further investigation is needed to ascertain the cause of lattice stretch. The in-plane orientations of the SBT thin films were determined using the SBT 共115兲 reflections. Figure 2 shows the ␾ scan image of c-axis oriented SBT-b. The modulation of peaks occurring every 90° indicates that there is no inplane misorientation in the film. Thus, the samples are c-axis oriented epitaxial thin films. As the crystalline structure of SBT is orthorhombic at room temperature, the ␾ scan image does not show the fourfold symmetry by nature. In the SBT films, a and b axis, whose lattice constants are very close,22 would be randomly mixed. Therefore, the ␾ scan image shows as if fourfold symmetry. B. Dielectric properties above room temperature

Figure 3 shows the temperature dependence of the dielectric constant of 共a兲 SBT-a, 共b兲 SBT-b, 共c兲 SBT-c, and 共d兲 SBT-d thin films at frequencies of 10 k, 100 k, and 1 MHz above room temperature. In the measured frequency range, SBT-b shows the obvious frequency dispersion above ⬃400 ° C, whose dielectric constant at lower frequency shows the higher value. The frequency dispersion at high temperatures is probably due to the ionic conduction resulting from the defects of oxygen or the other factors. The dielectric constants at room temperature are summarized in Table I. The measured values of the dielectric constant are between 159 and 252, which are larger than those of c-axis oriented SBT thin films in a capacitor structure 共␧ ⬃ 50兲.14 This indicates that the dielectric constant in a-b plane is several times larger than that along the c axis. In addition, it is interesting to note that the dielectric constant of

FIG. 3. Temperature dependence of dielectric constant above room temperature for 共a兲 SBT-a, 共b兲 SBT-b, 共c兲 SBT-c, and 共d兲 SBT-d at 10 kHz 共square兲, 100 kHz 共circle兲, and 1 MHz 共triangle兲.


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SBT-b is larger than that of SBT-d. Some reports show the same result, but no interpretation is given.23 We have suggested in other articles that SBT-b value not only displacive ferroelectric components but also have orientational ones.24,25 The orientational polarization would lead the difference in dielectric constant of the two samples. In Fig. 3, the dielectric constant of every sample increases as temperature increases from room temperature and shows the peaks. It indicates that the prepared SBT thin films undergo the ferroelectric phase transition. The peak values of the dielectric constant, however, are much smaller than those of the typical ferroelectric materials such as BaTiO3 共BTO兲 or PbTiO3 共PTO兲, whose dielectric constant at ferroelectric phase transition temperature is about 10 000 or more.26,27 The polycrystalline SBT ceramics and thin films also have small values.23 The small values are presumably related to the difference in the types of phase transition. Onodera et al. reported that ferroelectric SBT changes to antiferroelectric structure at phase transition temperature,28 whereas BTO and PTO undergo the ferroelectric-paraelectric phase transition. We examined the phase transition temperature of each sample. The temperature T M , at which dielectric constant is maximum, is listed in Table I. The T M of the SBT-b and SBT-d are in agreement with the previously reported values of the SBT ceramics.7 Due to the deficient of Bi, the T M of SBT-a and SBT-c are lower than that of reported values. Around the dielectric peaks, all the samples except for SBT-d exhibit frequency dispersion of dielectric constant, and the T M of these samples depend on the frequency. Similar phenomena have been observed in relaxor materials or order-disorder ferroelectric materials. One possible reason for frequency dependence of T M is ionic substitution as in typical relaxor materials. For example, BaBi2Ta2O9 共BBT兲, which has also a layered perovskite structure, is known as a relaxorlike material, and BBT shows the frequency dispersion of dielectric constant.6,29 The substitution of Ba ions for Bi site in Bi layer of BBT has been indicated by neutron diffraction analyses.6 Although there is no report about ionic substitution in SBT crystals, the frequency dependence of TM suggests the displacement of Sr and Bi ions in Bi layer. Another possible reason is the order-disorder phase transition. Some reports suggest that the phase transition of SrBi2Ta2O9 is a crossover between displacive and order-disorder type.30 As SBT-a, SBT-b, and SBT-c might have order-disorder ferroelectric components, the frequency dispersion of TM could be observed.

J. Appl. Phys. 99, 124106 共2006兲

FIG. 4. Temperature dependence of dielectric constant in low temperature region for 共a兲 SBT-b and 共b兲 SBT-d at 10 kHz 共square兲, 100 kHz 共circle兲, and 1 MHz 共triangle兲.

decrease as temperature decreases, which is typical behavior in other dielectric materials but slightly different from that in polycrystalline SBT thin films. 30 The dielectric constants of SBT-b and SBT-d at 100 kHz are 150 and 119, respectively, at 50 K. The dielectric constant of SBT-d monotonically decreases, whereas a bump was observed in SBT-b at ⬃80 K. It may imply a presence of phase transition. In previous reports, however, the results of internal friction measurement, far-infrared spectra, or dielectric measurement do not give any information on phase transition at low temperatures.30–32 In our case, SBT-b may have the ionic substitution in the crystal, and the atomic substituted structure would cause the in-plane phase transition. D. Ferroelectric properties below room temperature

C. Dielectric properties below room temperature

SBT-b and SBT-d thin films were chosen because they had relatively little Bi deficit and their dielectric behavior was largely different above room temperature. The dielectric functions of SBT-b and SBT-d thin films were examined at low temperatures. Figure 4 shows the temperature dependence of dielectric constant of SBT-b and SBT-d thin films below room temperature at frequencies of 10 kHz, 100 kHz, and 1 MHz. While the values of the dielectric constant depend on the frequency, the behaviors are almost the same at any measured frequency. Both the dielectric constants slowly

We also examined the ferroelectricity of SBT-b and SBT-d thin films below room temperature. Figures 5 show the polarizations of the SBT-b and SBT-d thin films as a function of electrical field at various temperature points. Both samples show the hysteresis loop, which means that they have ferroelectricity. At 300 K, twice the remanent polarization 共2Pr兲 of the SBT-b and SBT-d are 1.9 and 16.8 ␮C / cm2, respectively. The 2Pr of SBT-d is comparable to that of Sr0.8Bi2.2Ta2O9 polycrystalline bulk samples,8 and it is larger than that of a - / b-axis oriented epitaxial Sr0.8Bi2.2Ta2O9 thin films.33 From these results, even the c -axis oriented SBT-d thin films can


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FIG. 5. P-E hysteresis loops of 共a兲 SBT-b at 100 and 300 K, and 共b兲 SBT-d at 100, 200, and 300 K.

fulfill its memory function by employing planar-type electrodes, which might be potential properties for lateral-type memory device. On the other hand, 2Pr of SBT-b is much less than those of polycrystalline stoichiometric SBT thin films.1,2,4,5 This is probably because the applied electrical field is not large enough to reverse the polarization, or because the Pr of SBT itself is decreased by lattice distortion or defects in the thin film. The shapes of hysteresis loop depend on temperature, which means that ferroelectric properties of both films have temperature dependence. The remanent polarization was smaller at lower temperatures. We estimated the 2Pr for SBT-b and SBT-d thin films at low temperatures, as plotted in Fig. 6. With a decrease in temperatures from 300 to 250 K, the 2Pr of SBT-b thin film shows its increase, and then it shifts to decrease as temperature decreases below 250 K. As well as the temperature dependence of dielectric constant shown in Fig. 4, a bump has seen at ⬃80 K. The remanent polarization of SBT-d shows its slight reduction from room temperature to ⬃200 K, and then it was largely decreased, which is the similar result reported by Yang et al. 30 The temperature dependence of 2Pr contradicts the thermodynamical theory as discussed by Yuan et al.,34 in which remanent polarization should increase as temperature decreases. However, in case that the applied electrical field is not high enough to saturate the polarization, the observed remanent polarization becomes smaller as temperature decreases due to the difficulty of polarization switching at

FIG. 6. The magnitude of 2Pr of 共a兲 SBT-b and 共b兲 SBT-d as a function of temperature.

lower temperatures. Our results are applicable to that case. Evidently, the coercive field is increased as temperature decreases, as shown in Fig. 5. The increase of the coercive field presumably makes difficult to reverse the polarization and the smaller remanent polarization was observed at lower temperatures. IV. SUMMARY

The c -axis oriented epitaxial SBT thin films with different Sr/ Bi compositions were prepared on MgO substrates by PLD method. IDEs were formed on the surface of the films, and the temperature dependence of dielectric constant of the thin films was investigated in wide temperature range between 20 and 870 K. Above room temperature, every four samples show the dielectric peak. This indicates that all the samples undergo the ferroelectric phase transition. Three thin films except for SBT-d show the frequency dispersion of the dielectric constant near phase transition temperature. The result probably suggests the ionic substitution in Bi layer of SBT or the presence of orientational polarization in SBT crystals. At low temperatures, the dielectric constant of SBT-b and SBT-d was slightly decreased as temperature decreased. It is noted that the dielectric constant of SBT-b shows a bump at ⬃80 K, which may indicate a phase transition. The remanent polarizations of the films were measured below room temperature, and abrupt changes were also observed in SBT-b at ⬃80 K. The reduction of remanent polarizations probably attributes to the enhancement of coercive fields in lower temperature.


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ACKNOWLEDGMENTS

The authors are grateful to Dr. M. Misra for his collaboration and valuable discussion. The authors also appreciate Dr. K. Okamoto’s help in the process of the electrodes. This work was supported by the Strategic Information and Communications Research and Development Promotion Fund of Ministry of Internal Affairs and Communications 共MIC兲. A part of this work was also supported by “Nanotechnology Support Project” of the Ministry of Education, Culture, Sports, Science and Technology 共MEXT兲, Japan. 1

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