Improved Ferroelectric and Fatigue Behavior of Bi0.95Gd0.05FeO3

Oct 27, 2010 - ... and BFMO layers were deposited in situ on SrRuO3-buffered Pt/TiO2/SiO2/Si(100) substrates by off-axis radio frequency magnetic sput...
0 downloads 0 Views 4MB Size
Download PDF
19318

J. Phys. Chem. C 2010, 114, 19318–19321

Improved Ferroelectric and Fatigue Behavior of Bi0.95Gd0.05FeO3/BiFe0.95Mn0.05O3 Bilayered Thin Films Jiagang Wu and John Wang* Department of Materials Science and Engineering, Faculty of Engineering, National UniVersity of Singapore, 117574, Singapore ReceiVed: May 16, 2010; ReVised Manuscript ReceiVed: October 5, 2010

Bi0.95Gd0.05FeO3/BiFe0.95Mn0.05O3 (BGFO/BFMO) bilayered thin films with different thickness ratios of BGFO and BFMO layers were deposited in situ on SrRuO3-buffered Pt/TiO2/SiO2/Si(100) substrates by off-axis radio frequency magnetic sputtering, where a highly (110) orientation was developed. Their leakage currents and dielectric loss decrease dramatically due to the introduction of the highly resistive bottom BFMO layer, which promotes the texture development of the top BGFO layer by refining the grain cluster structure in the bilayered structure. The BGFO/BFMO bilayered thin film with the thickness fraction of ∼1:1 exhibits a larger remanent polarization and improved fatigue endurance as compared to those of other film compositions, where an almost fatigue-free behavior was observed up to 1010 switching cycles. Introduction Deposition of multilayered structures has been attempted for tailoring electrical behavior of ferroelectric thin films, where the coupling and interactions among the constituent layers in the structure can strongly affect the film growth and electrical behavior. Indeed an appropriate combination of the constituent layers of different functionalities can lead to much improved electrical performance.1-6 BiFeO3 (BFO) is a well-known multiferroic and promising as a candidate material for highdensity ferroelectric random access memory and several other technologically demanding applications, owing to its high antiferromagnetic ordering temperatures, which are well above room temperature, together with a giant remanent polarization and a high Curie temperature (Tc ) 1103 K).7-11 However, its high leakage current in thin film form at room temperature is believed to arise from structural defects and oxygen nonstoichiometry caused by the valence fluctuation of Fe and the loss of Bi. This can seriously hinder the potentially feasible applications that have been considered for BFO thin films.12 Several different attempts have thus been made to reduce the leakage current of BFO thin films, for example, by employment of a single crystal substrate,13-15 use of an appropriate buffer layer,13,16-18 construction of a multilayer structure,6,19 and cation substitutions for Bi or/and Fe in BFO lattice.11,17,20-23 Although the site substitution can lead to a decrease in leakage current of BFO thin films,11,17,20-23 the exact choice and amount of doping cations have to be properly considered, which are difficult to control in many cases. On the other hand, the multilayer approach has been shown to be effective in lowering the leakage current of BFO thin films.6,19 Indeed, the leakage current can be reduced dramatically by inserting a highly resistant layer between BFO and substrates, although the resulting electrical behavior can vary from one choice to another of the underlayers.6,24 We have investigated Gd-modified BFO thin films which are shown to exhibit a high leakage current and a large remanent polarization. The Mn4+-doped BFO thin film on the other hand exhibits a low leakage current; however, its ferroelectric * To whom correspondence should be addressed. E-mail: msewangj@ nus.edu.sg.

behavior are not ideal. It would therefore be of interest to investigate the bilayered structure consisting of BFO layers with different cation substitutions by taking the advantage of the respective layers and in particular their coupling. The Bi0.95Gd0.05FeO3/BiFe0.95Mn0.05O3 (BGFO/BFMO) bilayered thin films are varied with different thickness ratios of BGFO and BFMO layers, which are deposited in situ on SrRuO3 (SRO)-buffered Pt/TiO2/SiO2/Si(100) substrates by off-axis radio frequency (rf) magnetic sputtering. Their dependences of ferroelectric properties and fatigue behavior on the respective BGFO/BFMO thicknesses are investigated. Experimental Methods To deposit the multilayered structure, a thin SRO buffer layer was first deposited in situ on Pt(111)/Ti/SiO2/Si(100) substrates at the substrate temperature of 680 °C. Presintered Bi1.05Gd0.05FeO3 and Bi1.10Fe0.95Mn0.05O3 ceramic targets were employed to deposit the bilayered thin films on SRO/Pt(111)/Ti/SiO2/ Si(100) substrates by off-axis rf magnetron sputtering at the substrate temperature of 570 °C. The total thickness of each thin film is fixed at ∼240 nm, while the respective layer thicknesses of BGFO and BFMO are varied. The thickness ratios between BGFO and BFMO are controlled at ∼150 nm/90 nm, ∼120 nm/120 nm, and ∼90 nm/150 nm, respectively. These bilayered thin films are defined as 150BGFO/90BFMO, 120BGFO/120BFMO, and 90BGFO/150BFMO, according to the layer thickness, respectively. To deposit the bilayer structures, the sputtering power was fixed at 120 W for both BGFO and BFMO layers. They were deposited with the oxygen (O) to argon (Ar) ratio of 1:4, under a working pressure of 1.2 × 10-2 Torr. Detailed deposition procedure for the SRO buffer layer was described elsewhere.14,16,21 Single-layered BGFO and BFMO thin films on the SRO buffer layer were also deposited on Pt(111)/Ti/SiO2/Si(100) substrates by off-axis rf magnetron sputtering at the same preparation conditions for comparison purposes, which are defined as 240BGFO and 240BFMO, respectively. Circular Au electrodes of 0.2 mm in diameter were sputtered on the film surface by using a shadow mask in order to investigate their electrical behavior.

10.1021/jp104460r  2010 American Chemical Society Published on Web 10/27/2010

Bi0.95Gd0.05FeO3/BiFe0.95Mn0.05O3 Bilayered Thin Films

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19319

Figure 1. (a) XRD patterns of BGFO, BFMO, and bilayered thin films of different BGFO/BFMO thickness ratios. (b) XRD patterns of BGFO/ BFMO bilayered thin films of different thickness ratios in the 2θ angle range of 31-33°, measured at a scanning speed of 0.009°/s.

The phases present in each film and film textures were analyzed by using X-ray diffraction (XRD, Bruker D8 Advanced XRD, Bruker AXS Inc., Madison, WI, Cu KR). Field emission scanning electron microscopy (FE-SEM, Philips, XL30) was employed to study the surface morphologies of thin films. Their leakage behavior was measured by using a Keithley meter (Keithley 6430, Cleveland, OH). An impedance analyzer (Solartron Grain phase Analyzer) was employed to investigate their dielectric properties. Their ferroelectric and fatigue behavior was studied by using a Radiant precise workstation (Radiant Technologies, Medina, NY).

Figure 2. (a) Cross section of the 120BGFO/120BFMO bilayered thin film and surface morphologies of (b) 240BGFO single-layered, (c) 150BGFO/90BFMO bilayered, (d) 120BGFO/120BFMO bilayered, (e) 90BGFO/150BFMO bilayered, and (f) 240BFMO single-layered thin films.

Results and Discussion Figure 1a shows the XRD patterns of BGFO, BFMO, and bilayered thin films of different BGFO/BFMO thickness ratios. All thin films possess a pure single-phase perovskite structure, where secondary phases were not detected. In addition, they all exhibit a highly (110) texture orientation, where only (110) peak was detected in the 2θ angle range investigated. The highly (110) orientation texture will benefit the ferroelectric behavior, as the polarization occurs in (111) direction of BFO. Figure 1b shows the enlarged XRD patterns for the BGFO/BFMO bilayered thin films of different thickness ratios in the 2θ angle range of 31-33°, measured at a scanning speed of 0.009°/s, where the (110) peaks of the BGFO and BFMO layers were resolved by the Lorentzian method. The BGFO and BFMO phases were well retained in the bilayered thin films. The relative (110) peak intensity of the top BGFO layer increases with its increasing thicknesses. In principle, the mean grain size (D) can be determined by the Scherrer equation of D ) ke¨/aˆ cose`, when grain size is small, where k is a constant, e¨ is the X-ray wavelength (1.5405 Å), aˆ is the fullwidth at half-maximum (fwhm) of the diffraction line, and e` is the diffraction angle. There appears a slightly decrease in grain size of the top BGFO layer from 150BGFO/90BFMO to 90BGFO/150BFMO, in the range of 39-45 nm according to our calculation. However, it is commonly known that the Scherrer equation is more suitable for the grain size in the lower nanometer range. The calculated, small increase of grain size is thus not significant. Indeed, we have examined the film texture carefully and confirmed that the discrete grains sizes in the films of different layer combinations are comparable. There is however an apparent variation in grain cluster size among the thin films of different BGFO/ BFMO combinations. Figure 2a shows the cross section of the 120BGFO/120BFMO bilayered thin film, where the film texture is dense, crack-free, and well adhered to the buffer layer and substrate. Parts b-f of Figure 2 show the surface morphologies

Figure 3. Leakage current log(J) vs log(E) of 240BFMO singlelayered, 90BGFO/150BFMO bilayered, 120BGFO/120BFMO bilayered, 150BGFO/90BFMO bilayered, and 240BGFO single-layered thin films.

of BGFO, BFMO, and bilayered thin films with different BGFO/ BFMO thickness ratios. The BGFO/BFMO bilayered thin films exhibit obviously different grain morphologies as compared to that of the single-layered BGFO thin film. They exhibit a cluster texture, consisting of variable number of small discrete grains in each cluster. Their grain cluster sizes are refined, compared to that of the single-layered BGFO. This indicates that the bottom BFMO layer has a direct effect on the texture development of the top BGFO layer, where a more uniform texture is resulted, as shown by the apparent refinement in grain cluster size. The observed refinement in grain clusters and film texture will undoubtedly be a contributing parameter toward the decrease in leakage current and enhancement in ferroelectric behavior, as discussed later. Figure 3 plots the leakage current log(J) vs log(E) for BGFO, BFMO, and bilayered thin films with different BGFO/BFMO thickness ratios. The leakage current of BGFO/BFMO bilayered thin films at field above 102 kV/cm is about 2 orders of

19320

J. Phys. Chem. C, Vol. 114, No. 45, 2010

Wu and Wang

Figure 4. (a) Dielectric behavior and (b) the comparison of theoretically calculated and experimental values of the BGFO/BFMO bilayered, 240BGFO single-layered, and 240BFMO single-layered thin films.

magnitude lower than that of the single-layered BGFO thin film. The single-layered BFMO thin film exhibits a low leakage current, and therefore it plays a positive role in reducing the leakage current of the bilayered thin film. As discussed later, the refined grain cluster sizes in parts b-f of Figure 2 are also contributing toward the reduction in leakage current of BGFO/ BFMO bilayered thin films.25 Figure 4a plots the frequency dependence of relative permittivity (εr) and loss tangent (tan δ) of BGFO, BFMO, and bilayered thin films with different BGFO/BFMO thickness ratios. The εr value of BGFO/BFMO bilayered thin films is obviously higher than that of single-layered BGFO thin film, which is accounted for by the higher permittivity of the bottom BFMO layer. The bilayered thin films can be considered as consisting of two capacitors in series, i.e., 1/εBGFO/BFMO ) (dBGFO/ εBGFO + dBFMO/εBFMO)/(dBGFO + dBFMO), where the εBGFO, εBFMO, and εBGFO/BFMO are the relative permittivities of BGFO, BFMO, and bilayered thin films with different BGFO/BFMO thickness ratios, respectively; dBGFO and dBFMO are the thicknesses of BGFO and BFMO layers, respectively. The εr values measured for single-layered BGFO and BFMO at 100 kHz were 59 and 131, respectively. However, the εr value calculated for the BGFO/BFMO bilayered thin films is lower than the measured one (Figure 4b), indicating that the bilayered structure cannot be simply considered as a series connection of the two individual component layers.24 The bilayered BGFO/BFMO thin films also exhibit a much lower tan δ value than that of the single-layered BGFO thin film, where the low tan δ value is attributed to the decrease in the free movable charge density caused by the introduction of the bottom BFMO layer, which is highly resistive,26 as indicated by its low leakage current in Figure 3. As discussed above, the bottom BFMO layer had promoted the texture development of the top BGFO layer, where the grain clusters were refined. Figures 5 plot the P-E hysteresis loops of BGFO, BFMO, and bilayered thin films with different BGFO/BFMO thickness ratios, measured at 3.3 kHz and room temperature. The singlelayered BFMO thin film shows a well established P-E loop, however with a rather low remanent polarization, as shown in Figure 5c. Although the single-layered BGFO thin film deposited on the SRO/Pt/TiO2/SiO2/Si substrate demonstrates a high remanent polarization, a rather undesirable ferroelectric behavior with roundish shape is observed due to the considerably high leakage current, as shown in Figure 5b. It is indeed difficult to obtain well-saturated P-E curves for the Gd-modified BFO thin film at room temperature because of the high leakage current (Figure 3) in association with oxygen vacancies and the valence shift of Fe2+ and Fe3+.12 In contrast, the BGFO/BFMO bilayered thin films exhibit much better P-E loops as compared to that of the single-layered BGFO thin film. Among the three bilayered

Figure 5. P-E loops of (a) BGFO/BFMO bilayered, (b) 240BGFO single-layered, and (c) 240BFMO single-layered thin films.

thin films, the 120BGFO/120BFMO bilayered thin film exhibits the best ferroelectric behavior (2Pr ≈ 147.6 µC/cm2 and 2Ec ≈ 687.2 kV/cm), which is much higher than that of the Gd- and Mn-co-doped BFO thin films reported.22 While the BGFO single-layered thin film exhibits a higher polarization than that of the BFMO single-layered thin film, due to the different types of dopant involved, the presence of the bottom BFMO layer has benefitted the texture development of the top BGFO layer in the bilayered structure. As discussed earlier, the single-layered BGFO thin film exhibits a rougher film surface, consisting of relatively large grain clusters. There is no doubt that such coarse grain cluster structure and rough texture are associated with a high defect population, which is detrimental to both leakage and ferroelectric behavior. Indeed, the single-layered BGFO thin film demonstrates a much higher leakage current density than those of the BGFO/BFMO bilayered structures, as shown in Figure 3. The refined film texture of the BGFO/BFMO bilayered structure is therefore largely responsible for the observed enhancement in polarization. Indeed, they show higher polarization than that of either of the BGFO and BFMO constituent layers, suggesting the strong coupling effect between the two in developing a dense and refined film texture. Moreover, a highly (110) oriented film texture is developed for the bilayered thin film, which gives a large projection of polarization from (111) for BFO (polarization direction).14 Figure 6a shows the normalized polarization as a function of the number of switching cycles for both single-layered and bilayered thin films, measured at an electrical field of ∼540 kV/cm and frequency of 200 kHz. There is a quick decrease in polarization value up to 106 switching cycles for the singlelayered BFMO thin film, confirming its poor fatigue resistance. In contrast, the single-layered BGFO thin film exhibits better fatigue endurance than that of BFMO. It is however characterized by a “wake up” phenomenon, where there is an increase in polarization after 106 switching cycles, followed by a fall in polarization with the extended number of switching cycles. By

Bi0.95Gd0.05FeO3/BiFe0.95Mn0.05O3 Bilayered Thin Films

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19321 BiFe0.95Mn0.05O3 thickness ratios, deposited in situ on SrRuO3buffered Pt/TiO2/SiO2/Si(100) substrates by off-axis rf magnetic sputtering. Their ferroelectric and fatigue behavior were tailored by adjusting the thickness ratios between the constituent BGFO and BFMO layers. Among the different BGFO/BFMO thickness ratios investigated in the present work, the 120BGFO/120BFMO bilayered thin film exhibits a large remanent polarization (2Pr ≈ 147.6 µC/cm2 and 2Ec ≈ 687.2 kV/cm), desirable dielectric behavior (εr ≈ 98 and tan δ ≈ 1.62% at 10 kHz), and much improved fatigue behavior. The highly resistive bottom BFMO layer in the bilayered structure is beneficial to the texture development of the top BGFO layer in refining grain clusters and development of (110) film orientation. Acknowledgment. The authors gratefully acknowledge the support of the Singapore Millennium Foundation, the National University of Singapore, and the Science and Engineering Research Council (A*Star, Singapore). References and Notes

Figure 6. (a) Fatigue behavior of BGFO/BFMO bilayered, 240BGFO single-layered, and 240BFMO single-layered thin films. The P-E loops before and after fatigue for (b) 240BGFO single-layered, (c) 150BGFO/ 90BFMO bilayered, (d) 120BGFO/120BFMO bilayered, (e) 90BGFO/ 150BFMO bilayered, and (f) 240BFMO single-layered thin films.

comparison, the 120BGFO/120BFMO bilayered thin film demonstrates the most desirable fatigue behavior, i.e., there is little fatigue degradation upon 1010 switching cycles, and the “wake up” phenomenon is also completely invisible. To further confirm the fatigue endurance of these thin films, their P-E loops were measured before and after fatigue, as shown in parts b-f of Figures 6. The observed changes in polarization support the fatigue measurement. The “wake-up” phenomenon observed for the single-layered BGFO is largely due the poor film texture, where coarse grain clusters were observed. The high defect population is responsible for the “wake up” phenomenon,27,28 where the repeated switching led to an increase in polarization, due to charge accumulations occurring at the defect sites. Because of the improvement in texture development, an almost fatigue-free behavior was observed for the 120BGFO/120BFMO bilayered thin film, as confirmed by the fatigue endurance and P-E loops in parts a and d of Figure 6. As discussed earlier, the presence of the bottom BFMO layer promoted the texture development of the top BGFO layer in the bilayered structure, where there was a reduced defect population. In addition to observed refinment in grain cluster structure, the bottom BFMO buffer layer is able to compensate for the likely bismuth loss during film deposition of the top BGFO layer. In addition, Mn4+ in BFMO, as a donor, can also suppresses the formation of oxygen vacancies, and thereby lowering the concentration of oxygen vacancies.29 Conclusion A highly (110) oriented film texture is developed for the bilayered thin films consisting of different Bi0.95Gd0.05FeO3/

(1) Neaton, J. B.; Rabe, K. M. Appl. Phys. Lett. 2003, 82, 1586–1588. (2) Zhou, Z. H.; Xue, J. M.; Li, W. Z.; Wang, J.; Zhu, H.; Miao, J. M. J. Appl. Phys. 2004, 96, 5706–5711. (3) Dawber, M.; Lichtensteiger, C.; Cantoni, M.; Veithen, M.; Ghosez, P.; Johnston, K.; Rabe, K. M.; Triscone, J. M. Phys. ReV. Lett. 2005, 95, 177601. (4) Lee, H. N.; Christen, H. M.; Chisholm, M. F.; Rouleau, C. M.; Lowndes, D. H. Nature 2005, 433, 395–399. (5) Wu, J. G.; Xiao, D. Q.; Zhu, J. G.; Zhu, J. L.; Tan, J. Z.; Zhang, Q. L. Appl. Phys. Lett. 2007, 90, 082902. (6) Wu, J. G.; Kang, G. Q.; Liu, H. J.; Wang, J. Appl. Phys. Lett. 2009, 94, 172906. (7) Yun, K. Y.; Ricinschi, D.; Kanashima, T.; Noda, M.; Okuyama, M. Jpn. J. Appl. Phys. 2004, 43, L647–L648. (8) Eerenstein, W.; Mathur, N. D.; Scott, J. F. Nature 2006, 442, 759– 765. (9) Ramesh, R.; Spaldin, N. Nat. Mater. 2007, 6, 21–29. (10) Catalan, G.; Scott, J. F. AdV. Mater. 2009, 21, 2463–2466. (11) Martin, L. W.; Chu, Y. H.; Ramesh, R. Mater. Sci. Eng. 2010, 68, 89–133. (12) Qi, X. D.; Dho, J.; Tomov, R.; Blamire, M. G.; MacManus-Driscoll, J. L. Appl. Phys. Lett. 2005, 86, 062903. (13) Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Science 2003, 299, 1719–1722. (14) Wu, J. G.; Wang, J. J. Appl. Phys. 2009, 106, 104111. (15) Be´a, H.; Bibes, M.; Fusil, S.; Bouzehouane, K.; Jacquet, E.; Rode, K.; Bencok, P.; Barthe´le´my, A. Phys. ReV. B. 2006, 74, 020101(R). (16) Wu, J. G.; Wang, J. Acta. Mater. 2010, 58, 1688–1697. (17) Lee, H.; Liang, C. S.; Wu, J. M. Electrochem. Solid State Lett. 2005, 8, F55–F57. (18) Wu, J. G.; Wang, J. J. Appl. Phys. 2010, 107, 034103. (19) Zhao, H. Y.; Kimura, H.; Cheng, Z. X.; Wang, X. L.; Nishida, T. Appl. Phys. Lett. 2009, 95, 232904. (20) Lee, D.; Kim, M. G.; Ryu, S.; Jang, H. M.; Lee, S. G. Appl. Phys. Lett. 2005, 86, 222903. (21) Wu, J. G.; Wang, J. J. Appl. Phys. 2009, 106, 054115. (22) Lahmar, A.; Habouti, S.; Solterbeck, C. H.; Dietze, M.; Es-Souni, M. J. Appl. Phys. 2010, 107, 024104. (23) Yan, F.; Lai, M. O.; Lu, L.; Zhu, T. J. J. Phys. Chem. C 2010, 114, 6994–6998. (24) Wu, J. G.; Kang, G. Q.; Wang, J. Appl. Phys. Lett. 2009, 95, 192901. (25) Hu, G. D.; Cheng, X.; Wu, W. B.; Yang, C. H. Appl. Phys. Lett. 2007, 91, 232909. (26) Cheng, Z. X.; Wang, X. L.; Dou, S. X.; Kimura, H.; Ozawa, K. Phys. ReV. B 2008, 77, 092101. (27) Sim, C. H.; Soon, H. P.; Zhou, Z. H.; Wang, J. Appl. Phys. Lett. 2006, 89, 122905. (28) Wu, J. G.; Wang, J. J. Appl. Phys. 2009, 105, 124107. (29) Dawber, M.; Scott, J. F. Appl. Phys. Lett. 2000, 76, 1060–1062.

JP104460R