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Effect of excess bismuth on the microstructures and electrical properties of strontium bismuth tantalate (SBT) thin films
Thin Solid Films 375 Ž2000. 215᎐219
Effect of excess bismuth on the microstructures and electrical properties of strontium bismuth tantalate Ž SBT. thin films Aidong Li a,b,U , Di Wua,c , Huiqin Ling a,b , Tao Yua,c , Mu Wang a,c , Xiaobo Yina,c , Zhiguo Liu a,c , Naiben Ming a,c a
National Laboratory of Solid State Microstructures, Nanjing Uni¨ ersity, Nanjing 210093, PR China b Material Science and Engineering Department, Nanjing Uni¨ ersity, Nanjing 210093, PR China c Physics Department, Nanjing Uni¨ ersity, Nanjing 210093, PR China Received 16 June 1999; received in revised form 27 January 2000; accepted 3 February 2000
Abstract Strontium bismuth tantalate ŽSBT. films with excess Bi contents were prepared on PtrTiO 2rSiO 2rSi substrates by a metallorganic decomposition technique. Effect of excess Bi contents on the microstructure and electrical properties were investigated. A predominant layered perovskite structure could be formed when an excess Bi less than 30% was added. For films above 30% excess of Bi, secondary phases occurred. The remnant polarization and dielectric constant decreased with excess Bi content. This was attributed to a smaller grain size and the presence of secondary phases. The leakage current characteristics were also examined. Space charge limited mechanism was observed in SBT films. In summary, 10% excess Bi was found to be the optimum composition with respect to grain size, morphology, and electrical properties. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Ferroelectric properties; Structure properties; Electrical properties and measurements
1. Introduction Ferroelectric thin films have been widely investigated for non-volatile memory applications w1x. Recently, bismuth layered perovskites such as SrBi 2Ta 2 O 9 ŽSBT. have been reported to have excellent ferroelectric properties: negligible fatigue, low switching voltage and good retention characteristics w2,3x. These characteristics are very attractive for non-volatile memory applications. At present, SBT thin films have been successfully synthesized by methods such as metallorganic decom-
U
Corresponding author. Tel.: q86-25-3594689; fax: q86-253300535. E-mail address: [email protected] ŽA. Li..
position ŽMOD. w4,5x, sol-gel w6x, pulsed laser deposition w7,8x, metallorganic chemical vapor deposition w9x, and sputtering techniques w10x. However, the relationships between composition, microstructure, properties, and process integration into capacitors have not been extensively studied in comparison with that for PZTbased ferroelectric capacitors. Especially, the structure, morphology and electrical properties of SBT films are closely related to their chemical composition. Chen et al. w5x investigated the effect of excess bismuth Ž0᎐100 mol%. content on the SBT ferroelectric properties. They found that a 30᎐50-mol% excess Bi content was the optimum composition. Contrary results that the remnant polarization Ž Pr . depended strongly on the SrrTa mole ratio but not on the BirTa mole ratio were also reported w11x. Good electrical properties were obtained with a low Sr content. Therefore, the role of
0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 2 4 0 - 2
216
A. Li et al. r Thin Solid Films 375 (2000) 215᎐219
Bi and Sr in SBT thin film is more than controlling the stoichiometry of the film. In this paper, we report the preparation of SBT thin films with excess Bi contents ranging from 10 to 70% on PtrTiO 2rSiO 2rSi substrates by MOD. The effect of excess Bi contents on the microstructure and electrical properties of SBT thin films is discussed. 2. Experimental details Thin films of SrBi 2q xTa 2 O 9 ŽSBT. were prepared on PtrTiO 2rSiO 2rSi substrates using MOD spin coating technique. The bismuth 2-ethylhexanoate or bismuth acetate was dissolved into 2-ethylhexanoates precursor solution to obtain 0.1 molrl solutions with 10᎐70 mol % excess Bi. Films with various thicknesses ranging from 330 to 660 nm were produced by repetition of depositionrthermal treatment process. The spin rate was ; 3000 rev.rmin and the baking temperature was ; 400⬚C. A final annealing was conducted at 750⬚C in flowing oxygen for 60 min. The Pt or Au top electrodes with a diameter of 0.2 mm were sputtered or evaporated on SBT thin films by using shadow masks. The films with top electrodes were post-annealed at 750⬚C in oxygen for 30 min to improve the electrical properties. The film compositions were determined by inductively coupled plasma ŽICP. analyses. Effects of excess Bi contents on the structure and surface morphology were systematically investigated by X-ray diffraction ŽXRD., atomic force microscopy ŽAFM., and scanning electron microscopy ŽSEM.. Ferroelectric properties, leakage current, and dielectric characteristics of SBT film capacitors were measured. 3. Results and discussion The film composition with 10᎐70% excess Bi contents was examined by ICP. The measured values basically correspond to the nominal ones of the precursor solutions. No evident Bi loss is detected after the heat treatment process. The XRD patterns of 330-nm thick SBT films with varied Bi composition using bismuth 2-ethylhexanoate as Bi additives annealed at 750⬚C are illustrated in Fig. 1. Films containing 10% excess Bi consist of a perovskite polycrystalline phase without detectable impurity. When less than 30% excess Bi is added, a main layered perovskite structure can be formed along with an extremely weak unknown peak at approximately 2 s 30⬚. This peak was differently indexed as the Ž107. plane of SBT film by Song et al. w10x, as a Bi 2 Pt phase by Atsuki et al. w11x and as a BTO phase by Seong et al. w9x. Based on large amounts of XRD analyses of SBT films, the unknown peak might belong to a defect-type BTO phase at relatively low excess Bi content. For
Fig. 1. XRD patterns of the SBT films with different excess Bi composition using bismuth 2-ethylhexanoate as Bi additive annealed at 750⬚C.
films with 30᎐50% excess Bi, secondary phases such as stronger Bi 2 Pt or Bi 2 O 3 peaks, as well as layered SBT phase, are observed. This indicates that the interface diffusion between the film and the Pt bottom electrode increases with the amount of excess Bi, which begins to occur as bismuth oxide. When the excess of Bi content is increased to 70%, BTO phase along with Bi 2 O 3 appears instead of Bi 2 Pt. Meanwhile SBT structure becomes distorted due to the evident shift or weakening of SBT peaks, especially for peaks at low angle. According to the values of Ž115., Ž200., and Ž220. ˚ planes, the calculated lattice constants Ž as 5.492 A, ˚ c s 24.779 A ˚ . are significantly different bs 5.505 A, ˚ from that of SBT film with 10% excess Bi Ž as 5.516 A, ˚ c s 24.953 A ˚ .. This implies an extremely bs 5.515 A, severe Bi segregation and a possible diffusion of TiO 2 below the bottom electrode through the Pt layer, leading to the formation of BTO phase. Above all, formation of the bismuth layer structure is independent of the bismuth content between 10 and 25% excess Bi. Exceeding that range, secondary phases such as Bi 2 Pt, Bi 2 O 3 , or BTO occur. This differs from the result reported by Chen et al. that the Bi 2 O 3 secondary phase appeared only when the amount of excess Bi exceeded 50% w5x. The surface morphologies of the SBT films using bismuth 2-ethylhexanoate as Bi additive were examined by SEM, as shown in Fig. 2. The surface morphology is very sensitive to the film composition. The films with 10% excess Bi have dense spherical grains of ; 120 nm. When the excess of Bi content is increased to 50%, the morphology degrades with smaller grains and more pinholes. This result is different from Chen’s and Atsuki’s reports w5,11x, where increasing the excess of Bi facilitated the grain growth. AFM was also used to
A. Li et al. r Thin Solid Films 375 (2000) 215᎐219
217
Fig. 2. SEM images of the SBT films with different excess Bi composition using bismuth 2-ethylhexanoate as Bi additive annealed at 750⬚C.
investigate the surface roughness of films in a 10 = 10m area. The roughness of the films with excess Bi below 50% is similar and ; 12 nm. The films with 70% excess Bi exhibit rod-like grains with a larger roughness of ; 18 nm, which is possibly associated with the Bi 2 O 3 and BTO secondary phase formation along with the distorted SBT phase. The hysteresis loops of the SBT films annealed at 750⬚C using bismuth 2-ethylhexanoate as a Bi additive are shown in Fig. 3. Increasing the excess Bi composition to 50%, the Pr decreases correspondingly, which might be related to the small grain size and the presence of secondary phases. Due to the conductive Bi 2 O 3 phase formation, the films with 70% excess Bi show poor hysteresis loop. In addition, the SBT films with varied Bi content obtained by using bismuth acetate as a Bi additive show nearly same microstructure and ferroelectric properties as by using bismuth 2-ethylhexanoate as a Bi additive. Fig. 4 are plots of the dielectric constant r and loss tangent tan␦ of 660-nm thick SBT films annealed at 750⬚C in the range of 10᎐35% excess Bi, using bismuth acetate as the Bi additive, as functions of frequency. For SBT films with various Bi compositions, the dielectric constants decrease gradually with increasing Bi contents. The loss tangents are almost the same and increase slowly when the frequency changes from 100 Hz to 100 kHz. Above 100 kHz, the loss tangents rise. The leakage current density of SBT films with various excess Bi, using bismuth acetate as Bi additive, as a
function of applied voltage is shown in Fig. 5. Typically, the leakage current density JA increases gradually from ; 10y8 to ; 10y7 Arcm2 with the applied voltage until a threshold voltage is reached. Above the threshold voltage Ž2᎐3 V., the JA increases abruptly with voltage. The JA decreases again when a smaller voltage is applied. The voltage-dependent leakage current behavior is similar to that observed by Koiwa et al. in their sol-gel derived SBT films w12x, where the leakage current increased rapidly when 4 V voltage was applied. The SBT films with 35% excess Bi have a higher JA . The secondary phase and the severe deviation from
Fig. 3. Hysteresis loops of the SBT films with different excess Bi composition using bismuth 2-ethylhexanoate as Bi additive after 750⬚C annealing and post-annealing.
218
A. Li et al. r Thin Solid Films 375 (2000) 215᎐219
Fig. 6. LogŽ JA . y LogŽ V . curves for the SBT films with 10 and 15% excess Bi. The solid line is the best linear fitted line.
4. Conclusions
Fig. 4. Plots of Ža. dielectric constant and Žb. loss tangent of the SBT films with different excess Bi compositions annealed at 750⬚C using bismuth acetate as Bi additive as functions of frequency.
the stoichiometry might be responsible for the higher JA . In some of our films, space-charge limited current ŽSCLC. mechanism was observed as described by Scott et al. w13x. LogŽ JA . vs. LogŽ V . curves for the SBT films with 10 and 15% excess Bi content are illustrated in Fig. 6. The solid line is the best linear fitted line. As described by the SCLC theory, a quadratic behavior above the threshold voltage and ohmic behavior below is observed. However, the exponent value above the threshold is slightly larger than 2. This may be due to a continuous distribution of traps w13x. The amount of space charge has an important role with regards to the leakage current. A similar phenomenon was also observed in MOD-derived SBT films on Pt by Watanable et al. w14x. At present, it is unclear what is the origin of the large amount of space charge.
In summary, the effect of excess Bi on the structure, surface morphology and electrical properties of SBT films prepared by MOD were investigated systematically. A predominant layered perovskite structure could be formed with excess Bi less than 30%. When above 30% excess of Bi was used, secondary phases such as Bi 2 Pt, Bi 2 O 3 , and BTO appeared. This indicated that the interface diffusion between SBT film and the Pt bottom electrode becomes severe with the amount of excess Bi. The surface morphology degraded and the grain size decreased with increasing excess Bi composition to 70%. The Pr and dielectric constant decreased with excess Bi content. This was attributed to a smaller grain size and the presence of secondary phases at high Bi content. Space charge limited mechanism was observed in our films. In summary, for the excess Bi concentration tested, 10% excess Bi in the precursor solution was found to be the optimum composition with respect to grain size, morphology, and electrical properties. Acknowledgements This work was sponsored by Motorola’s ESTLMaterials Technology Laboratories and ESTL-China Technology Center. It was also supported by a grant for State Key Program for Basic Research of China. We would like to thank Dr Peir Y. Chu for his great help, useful advice and discussions. References
Fig. 5. Leakage current density JA -Voltage dependence of the SBT films with different excess Bi composition annealed at 750⬚C using bismuth acetate as the Bi additive.
w1x J.F. Scott, C.A. Araujo, Science 246 Ž1989. 1400. w2x I. Koiwa, K. Tani, J. Mita, T. Iwabuchi, Jpn. J. Appl. Phys. 37 Ž1998. 192. w3x T. Chen, T. Li, X. Zhang, S.B. Desu, J. Mater. Res. 12 Ž1997. 1569. w4x K. Amanuma, T. Hase, Y. Migasaka, Appl. Phys. Lett. 66 Ž1995. 221.
A. Li et al. r Thin Solid Films 375 (2000) 215᎐219 w5x T.C. Chen, T.K. Li, X.B. Zhang, S.B. Desu, J. Mater. Res. 12 Ž1997. 1569. w6x T. Ito, M. Ushikubo, S. Yokoyama, H. Matsunaga, T. Atsuki, T. Yonezawa, K. Ogi, Jpn. J. Appl. Phys. 35 Ž1996. 4925. w7x S.B Desu, D.P. Vijay, X. Zhang, B.P. He, Appl. Phys. Lett. 69 Ž1996. 1719. w8x H.M. Yang, S.L. Luo, W.T. Lin, J. Mater. Res. 12 Ž1997. 1145. w9x N.J. Seong, C.H. Yang, W.C. Shin, S.G. Yoon, Appl. Phys. Lett. 72 Ž1998. 1374.
219
w10x T.K. Song, J.K. Lee, H.J. Jung, Appl. Phys. Lett. 71 Ž1996. 3839. w11x T. Atsuki, N. Soyama, T. Yonezawa, T. Yonezawa, K. Ogi, Jpn. J. Appl. Phys. 34 Ž1995. 5096. w12x I. Koiwa, K. Tani, J. Mita, T. Iwabuchi, Jpn. J. Appl. Phys. 37 Ž1998. 192. w13x J.F. Scott, F.M. Ross, C.A. Paz de Araujo, M.C. Scott, M. Huffman, MRS Bull. 21 Ž1996. 33. w14x K. Watanabe, A.J. Hartmann, R.N. Lamb, J.F. Scott, J. Appl. Phys. 84 Ž1998. 2170.
Effect of excess bismuth on the microstructures and electrical properties of strontium bismuth tantalate Ž SBT. thin films Aidong Li a,b,U , Di Wua,c , Huiqin Ling a,b , Tao Yua,c , Mu Wang a,c , Xiaobo Yina,c , Zhiguo Liu a,c , Naiben Ming a,c a
National Laboratory of Solid State Microstructures, Nanjing Uni¨ ersity, Nanjing 210093, PR China b Material Science and Engineering Department, Nanjing Uni¨ ersity, Nanjing 210093, PR China c Physics Department, Nanjing Uni¨ ersity, Nanjing 210093, PR China Received 16 June 1999; received in revised form 27 January 2000; accepted 3 February 2000
Abstract Strontium bismuth tantalate ŽSBT. films with excess Bi contents were prepared on PtrTiO 2rSiO 2rSi substrates by a metallorganic decomposition technique. Effect of excess Bi contents on the microstructure and electrical properties were investigated. A predominant layered perovskite structure could be formed when an excess Bi less than 30% was added. For films above 30% excess of Bi, secondary phases occurred. The remnant polarization and dielectric constant decreased with excess Bi content. This was attributed to a smaller grain size and the presence of secondary phases. The leakage current characteristics were also examined. Space charge limited mechanism was observed in SBT films. In summary, 10% excess Bi was found to be the optimum composition with respect to grain size, morphology, and electrical properties. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Ferroelectric properties; Structure properties; Electrical properties and measurements
1. Introduction Ferroelectric thin films have been widely investigated for non-volatile memory applications w1x. Recently, bismuth layered perovskites such as SrBi 2Ta 2 O 9 ŽSBT. have been reported to have excellent ferroelectric properties: negligible fatigue, low switching voltage and good retention characteristics w2,3x. These characteristics are very attractive for non-volatile memory applications. At present, SBT thin films have been successfully synthesized by methods such as metallorganic decom-
U
Corresponding author. Tel.: q86-25-3594689; fax: q86-253300535. E-mail address: [email protected] ŽA. Li..
position ŽMOD. w4,5x, sol-gel w6x, pulsed laser deposition w7,8x, metallorganic chemical vapor deposition w9x, and sputtering techniques w10x. However, the relationships between composition, microstructure, properties, and process integration into capacitors have not been extensively studied in comparison with that for PZTbased ferroelectric capacitors. Especially, the structure, morphology and electrical properties of SBT films are closely related to their chemical composition. Chen et al. w5x investigated the effect of excess bismuth Ž0᎐100 mol%. content on the SBT ferroelectric properties. They found that a 30᎐50-mol% excess Bi content was the optimum composition. Contrary results that the remnant polarization Ž Pr . depended strongly on the SrrTa mole ratio but not on the BirTa mole ratio were also reported w11x. Good electrical properties were obtained with a low Sr content. Therefore, the role of
0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 2 4 0 - 2
216
A. Li et al. r Thin Solid Films 375 (2000) 215᎐219
Bi and Sr in SBT thin film is more than controlling the stoichiometry of the film. In this paper, we report the preparation of SBT thin films with excess Bi contents ranging from 10 to 70% on PtrTiO 2rSiO 2rSi substrates by MOD. The effect of excess Bi contents on the microstructure and electrical properties of SBT thin films is discussed. 2. Experimental details Thin films of SrBi 2q xTa 2 O 9 ŽSBT. were prepared on PtrTiO 2rSiO 2rSi substrates using MOD spin coating technique. The bismuth 2-ethylhexanoate or bismuth acetate was dissolved into 2-ethylhexanoates precursor solution to obtain 0.1 molrl solutions with 10᎐70 mol % excess Bi. Films with various thicknesses ranging from 330 to 660 nm were produced by repetition of depositionrthermal treatment process. The spin rate was ; 3000 rev.rmin and the baking temperature was ; 400⬚C. A final annealing was conducted at 750⬚C in flowing oxygen for 60 min. The Pt or Au top electrodes with a diameter of 0.2 mm were sputtered or evaporated on SBT thin films by using shadow masks. The films with top electrodes were post-annealed at 750⬚C in oxygen for 30 min to improve the electrical properties. The film compositions were determined by inductively coupled plasma ŽICP. analyses. Effects of excess Bi contents on the structure and surface morphology were systematically investigated by X-ray diffraction ŽXRD., atomic force microscopy ŽAFM., and scanning electron microscopy ŽSEM.. Ferroelectric properties, leakage current, and dielectric characteristics of SBT film capacitors were measured. 3. Results and discussion The film composition with 10᎐70% excess Bi contents was examined by ICP. The measured values basically correspond to the nominal ones of the precursor solutions. No evident Bi loss is detected after the heat treatment process. The XRD patterns of 330-nm thick SBT films with varied Bi composition using bismuth 2-ethylhexanoate as Bi additives annealed at 750⬚C are illustrated in Fig. 1. Films containing 10% excess Bi consist of a perovskite polycrystalline phase without detectable impurity. When less than 30% excess Bi is added, a main layered perovskite structure can be formed along with an extremely weak unknown peak at approximately 2 s 30⬚. This peak was differently indexed as the Ž107. plane of SBT film by Song et al. w10x, as a Bi 2 Pt phase by Atsuki et al. w11x and as a BTO phase by Seong et al. w9x. Based on large amounts of XRD analyses of SBT films, the unknown peak might belong to a defect-type BTO phase at relatively low excess Bi content. For
Fig. 1. XRD patterns of the SBT films with different excess Bi composition using bismuth 2-ethylhexanoate as Bi additive annealed at 750⬚C.
films with 30᎐50% excess Bi, secondary phases such as stronger Bi 2 Pt or Bi 2 O 3 peaks, as well as layered SBT phase, are observed. This indicates that the interface diffusion between the film and the Pt bottom electrode increases with the amount of excess Bi, which begins to occur as bismuth oxide. When the excess of Bi content is increased to 70%, BTO phase along with Bi 2 O 3 appears instead of Bi 2 Pt. Meanwhile SBT structure becomes distorted due to the evident shift or weakening of SBT peaks, especially for peaks at low angle. According to the values of Ž115., Ž200., and Ž220. ˚ planes, the calculated lattice constants Ž as 5.492 A, ˚ c s 24.779 A ˚ . are significantly different bs 5.505 A, ˚ from that of SBT film with 10% excess Bi Ž as 5.516 A, ˚ c s 24.953 A ˚ .. This implies an extremely bs 5.515 A, severe Bi segregation and a possible diffusion of TiO 2 below the bottom electrode through the Pt layer, leading to the formation of BTO phase. Above all, formation of the bismuth layer structure is independent of the bismuth content between 10 and 25% excess Bi. Exceeding that range, secondary phases such as Bi 2 Pt, Bi 2 O 3 , or BTO occur. This differs from the result reported by Chen et al. that the Bi 2 O 3 secondary phase appeared only when the amount of excess Bi exceeded 50% w5x. The surface morphologies of the SBT films using bismuth 2-ethylhexanoate as Bi additive were examined by SEM, as shown in Fig. 2. The surface morphology is very sensitive to the film composition. The films with 10% excess Bi have dense spherical grains of ; 120 nm. When the excess of Bi content is increased to 50%, the morphology degrades with smaller grains and more pinholes. This result is different from Chen’s and Atsuki’s reports w5,11x, where increasing the excess of Bi facilitated the grain growth. AFM was also used to
A. Li et al. r Thin Solid Films 375 (2000) 215᎐219
217
Fig. 2. SEM images of the SBT films with different excess Bi composition using bismuth 2-ethylhexanoate as Bi additive annealed at 750⬚C.
investigate the surface roughness of films in a 10 = 10m area. The roughness of the films with excess Bi below 50% is similar and ; 12 nm. The films with 70% excess Bi exhibit rod-like grains with a larger roughness of ; 18 nm, which is possibly associated with the Bi 2 O 3 and BTO secondary phase formation along with the distorted SBT phase. The hysteresis loops of the SBT films annealed at 750⬚C using bismuth 2-ethylhexanoate as a Bi additive are shown in Fig. 3. Increasing the excess Bi composition to 50%, the Pr decreases correspondingly, which might be related to the small grain size and the presence of secondary phases. Due to the conductive Bi 2 O 3 phase formation, the films with 70% excess Bi show poor hysteresis loop. In addition, the SBT films with varied Bi content obtained by using bismuth acetate as a Bi additive show nearly same microstructure and ferroelectric properties as by using bismuth 2-ethylhexanoate as a Bi additive. Fig. 4 are plots of the dielectric constant r and loss tangent tan␦ of 660-nm thick SBT films annealed at 750⬚C in the range of 10᎐35% excess Bi, using bismuth acetate as the Bi additive, as functions of frequency. For SBT films with various Bi compositions, the dielectric constants decrease gradually with increasing Bi contents. The loss tangents are almost the same and increase slowly when the frequency changes from 100 Hz to 100 kHz. Above 100 kHz, the loss tangents rise. The leakage current density of SBT films with various excess Bi, using bismuth acetate as Bi additive, as a
function of applied voltage is shown in Fig. 5. Typically, the leakage current density JA increases gradually from ; 10y8 to ; 10y7 Arcm2 with the applied voltage until a threshold voltage is reached. Above the threshold voltage Ž2᎐3 V., the JA increases abruptly with voltage. The JA decreases again when a smaller voltage is applied. The voltage-dependent leakage current behavior is similar to that observed by Koiwa et al. in their sol-gel derived SBT films w12x, where the leakage current increased rapidly when 4 V voltage was applied. The SBT films with 35% excess Bi have a higher JA . The secondary phase and the severe deviation from
Fig. 3. Hysteresis loops of the SBT films with different excess Bi composition using bismuth 2-ethylhexanoate as Bi additive after 750⬚C annealing and post-annealing.
218
A. Li et al. r Thin Solid Films 375 (2000) 215᎐219
Fig. 6. LogŽ JA . y LogŽ V . curves for the SBT films with 10 and 15% excess Bi. The solid line is the best linear fitted line.
4. Conclusions
Fig. 4. Plots of Ža. dielectric constant and Žb. loss tangent of the SBT films with different excess Bi compositions annealed at 750⬚C using bismuth acetate as Bi additive as functions of frequency.
the stoichiometry might be responsible for the higher JA . In some of our films, space-charge limited current ŽSCLC. mechanism was observed as described by Scott et al. w13x. LogŽ JA . vs. LogŽ V . curves for the SBT films with 10 and 15% excess Bi content are illustrated in Fig. 6. The solid line is the best linear fitted line. As described by the SCLC theory, a quadratic behavior above the threshold voltage and ohmic behavior below is observed. However, the exponent value above the threshold is slightly larger than 2. This may be due to a continuous distribution of traps w13x. The amount of space charge has an important role with regards to the leakage current. A similar phenomenon was also observed in MOD-derived SBT films on Pt by Watanable et al. w14x. At present, it is unclear what is the origin of the large amount of space charge.
In summary, the effect of excess Bi on the structure, surface morphology and electrical properties of SBT films prepared by MOD were investigated systematically. A predominant layered perovskite structure could be formed with excess Bi less than 30%. When above 30% excess of Bi was used, secondary phases such as Bi 2 Pt, Bi 2 O 3 , and BTO appeared. This indicated that the interface diffusion between SBT film and the Pt bottom electrode becomes severe with the amount of excess Bi. The surface morphology degraded and the grain size decreased with increasing excess Bi composition to 70%. The Pr and dielectric constant decreased with excess Bi content. This was attributed to a smaller grain size and the presence of secondary phases at high Bi content. Space charge limited mechanism was observed in our films. In summary, for the excess Bi concentration tested, 10% excess Bi in the precursor solution was found to be the optimum composition with respect to grain size, morphology, and electrical properties. Acknowledgements This work was sponsored by Motorola’s ESTLMaterials Technology Laboratories and ESTL-China Technology Center. It was also supported by a grant for State Key Program for Basic Research of China. We would like to thank Dr Peir Y. Chu for his great help, useful advice and discussions. References
Fig. 5. Leakage current density JA -Voltage dependence of the SBT films with different excess Bi composition annealed at 750⬚C using bismuth acetate as the Bi additive.
w1x J.F. Scott, C.A. Araujo, Science 246 Ž1989. 1400. w2x I. Koiwa, K. Tani, J. Mita, T. Iwabuchi, Jpn. J. Appl. Phys. 37 Ž1998. 192. w3x T. Chen, T. Li, X. Zhang, S.B. Desu, J. Mater. Res. 12 Ž1997. 1569. w4x K. Amanuma, T. Hase, Y. Migasaka, Appl. Phys. Lett. 66 Ž1995. 221.
A. Li et al. r Thin Solid Films 375 (2000) 215᎐219 w5x T.C. Chen, T.K. Li, X.B. Zhang, S.B. Desu, J. Mater. Res. 12 Ž1997. 1569. w6x T. Ito, M. Ushikubo, S. Yokoyama, H. Matsunaga, T. Atsuki, T. Yonezawa, K. Ogi, Jpn. J. Appl. Phys. 35 Ž1996. 4925. w7x S.B Desu, D.P. Vijay, X. Zhang, B.P. He, Appl. Phys. Lett. 69 Ž1996. 1719. w8x H.M. Yang, S.L. Luo, W.T. Lin, J. Mater. Res. 12 Ž1997. 1145. w9x N.J. Seong, C.H. Yang, W.C. Shin, S.G. Yoon, Appl. Phys. Lett. 72 Ž1998. 1374.
219
w10x T.K. Song, J.K. Lee, H.J. Jung, Appl. Phys. Lett. 71 Ž1996. 3839. w11x T. Atsuki, N. Soyama, T. Yonezawa, T. Yonezawa, K. Ogi, Jpn. J. Appl. Phys. 34 Ž1995. 5096. w12x I. Koiwa, K. Tani, J. Mita, T. Iwabuchi, Jpn. J. Appl. Phys. 37 Ž1998. 192. w13x J.F. Scott, F.M. Ross, C.A. Paz de Araujo, M.C. Scott, M. Huffman, MRS Bull. 21 Ž1996. 33. w14x K. Watanabe, A.J. Hartmann, R.N. Lamb, J.F. Scott, J. Appl. Phys. 84 Ž1998. 2170.