Gd-substituted bismuth titanate film capacitors having ferroelectric reliability and large non-volatile charges

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1 Physica B 388 (27) Gd-substituted bismuth titanate film capacitors having ferroelectric reliability and large non-volatile charges Uong Chon a,, Hyun M. Jang b, Nam S. Shin c, Jae S. Kim b, Do C. Ahn c, Yun S. Kim d, Kwangsoo No d a Research Institute of Industrial Science and Technology (RIST), P.O. Box 135, Pohang 79-33, Republic of Korea b Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang , Republic of Korea c Pohang Accelerator Laboratory (PAL), Pohang , Republic of Korea d Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejon 35-71, Republic of Korea Received 24 June 25; received in revised form 29 May 26; accepted 29 May 26 Abstract Fatigue-free Gd-modified bismuth titanate (Bi 3.15 Gd.85 Ti 3 O 12 ; BGdT) film capacitors having stable charge-retaining characteristics were grown on Pt/TiO 2 /SiO 2 /Si(1 ) substrates using the method of metal-organic sol decomposition. The BGdT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (2P r ) and the non-volatile charge as compared to those of the Bi 4 x La x Ti 3 O 12 (BLT; x ¼.75) film capacitor, currently renowned as a promising candidate for non-volatile memories. The saturated 2P r value of the BGdT capacitor was 75 mc/cm 2 while it remained essentially constant up to read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor demonstrated excellent charge-retention characteristics with its sensing margin of 52 mc/cm 2 and a strong resistance against the imprinting failure. r 26 Elsevier B.V. All rights reserved. PACS: k; Dy; f; e Keywords: Gadolinium-modified bismuth titanate; Ferroelectric; Fatigue-free; FRAM 1. Introduction Extensive research efforts have been made to the realization of high-density non-volatile ferroelectric random access memories (FRAMs) with the reliability in their performances. The fabrication of a high-density FRAM device with 1T/1C-cell structure is now possible by employing standard complementary metal-oxide-semiconductor (CMOS) processes used in the fabrication of dynamic random access memory devices [1]. However, the reliability in the performances of FRAM devices is not satisfactory at present and is mainly limited by the following three causes: (i) polarization fatigue, (ii) imprinted polarization and (iii) time-dependent charge loss of ferroelectric capacitors. Corresponding author. Tel.: ; fax: address: uongchon@rist.re.kr (U. Chon). The polarization fatigue can be successfully overcome by adopting ferroelectric oxides having layered perovskite structure, as exemplified by strontium bismuth tantalate (SrBi 2 Ta 2 O 9, SBT) [2] and, more recently, by lanthanides(ln)-substituted bismuth titanates (Bi 4 x Ln x Ti 3 O 12, BLnT) [3 8]. The imprinted polarization can be reduced, to a considerable degree, by suppressing the formation of aligned defect-dipoles or trapped charges near the electrode film interface [9]. The time-dependent charge loss, which is especially pronounced in Pb(Zr,Ti)O 3 (PZT)- based capacitors [1,1], can also be suppressed by employing ferroelectric oxides having layered perovskite structure. It was reported that the retained charges of Bi 3.15 Sm.85- Ti 3 O 12 (BSmT) capacitor remained essentially constant up to 1 4 sec at 85 1C after applying a writing pulse, demonstrating a stable charge-retaining ability [7]. In addition to the three issues related to the ferroelectric reliability, the non-volatile charge (P nv ) is another /$ - see front matter r 26 Elsevier B.V. All rights reserved. doi:1.116/j.physb

2 U. Chon et al. / Physica B 388 (27) important material parameter to be considered in the design and integration of a FRAM device. It is defined by the difference between the switching polarization (P sw ) and the non-switching polarization (P ns ) and should be large enough for a memory cell to readily distinguish one logic state from the other. Although there is no clear cut-off value for a desirable P nv, it is rather clear that (i) the memory density, (ii) the reliability in device performances and (iii) the flexibility in the device-integration process can be significantly improved with increasing value of P nv. P nv values of PZT capacitors [1,1] are usually in the vicinity of 4 mc/cm 2. Contrary to these, layered perovskite-based capacitors [7,1] tend to have substantially small values of P nv (6 and 17 mc/cm 2 ) and, thus, P nv needs to be substantially improved. In view of these, the main purpose of the present study is to develop layered perovskite-based capacitors with large non-volatile charges and the reliability in their performances. In selecting appropriate trivalent rare-earth lanthanide for this purpose, we depended on our preliminary study. Our preliminary study indicated that the direction and the magnitude of spontaneous polarization of highly c-axis-oriented bismuth titanate-based films were very susceptible to the substitution of trivalent rare-earth lanthanides such as La, Pr, Sm, and Gd for bismuth. Among these rare-earth lanthanides-substituted bismuth titanate-based films, Gd-substituted bismuth titanate film showed the largest value of non-volatile charge (the value of 2P r ¼ 8 mc/cm 2 ). For this reason, we have chosen gadolinium-modified bismuth titanates (BGdT) as model ferroelectric materials that are suitable to the realization of high-density FRAM devices with the ferroelectric reliability. 2. Experimental details BGdT (Bi 4 x Gd x Ti 3 O 12 with x ¼.85) films were fabricated on Pt(1 )/TiO 2 /SiO 2 /Si(1 ) substrates using the method of metal organic sol decomposition (MOSD). The precursor sol for the coating was prepared by dissolving appropriate amounts of bismuth acetate [Bi(CH 3 COO) 3 ], gadolinium acetate [Gd(CH 3 COO) 3 2H 2 O], and titanium isopropoxide {Ti[(CH 3 ) 2 CHO] 4 } in acetic acid solution at room temperature. The dried amorphous films were crystallized by thermal annealing in an oxygen-rich atmosphere at various temperatures ranging between 4 and 7 1C for 1 h. In order to fabricate capacitors, top Pt electrodes were deposited using an RF magnetron sputter. A typical area of the top electrode was 1 4 cm 2. The ferroelectric and dielectric measurements were performed on the BGdT capacitors using a RT6S ferroelectric tester and a HP4194A impedance analyzer equipped with a micrometer probe station. Structural features of ferroelectric domains were examined by employing piezoresponse-imaging technique which is based on the detection of the local electromechanical vibration of a ferroelectric film [11]. An AC modulation voltage of 3 V (peak-to-peak) at a frequency of 17 khz was applied between the conducting tip and the bottom electrode to observe natural ferroelectric domains without electrical poling. 3. Results and discussion Fig. 1 shows the XRD y 2y scan results of the BGdT films annealed at several indicated temperatures for 1 h. All the XRD patterns could be identified and indexed using the XRD data of the perovskite Bi 4 Ti 3 O 12 (BT) phase complied in the JCPDS card. This indicates that the BGdT film maintains a pseudotetragonal-layered structure similar to the perovskite BT even under extensive modifications by gadolinium. The most prominent feature of the XRD patterns in Fig. 1 is that the film annealed at 7 1C shows a highly c-axis-oriented preferential growth with a minor fraction of (1 1 7) orientation. The degree of the ( 1)-type preferential growth, as estimated using Lotgering s orientation factor [12], is 92% for the BGdT film annealed at 7 1C. In order to find out why the BGdT film showed the c-axis preferential growth on Pt(1 )/TiO 2 /SiO 2 /Si(1 ) substrate, the atomic configurations of Pt (1 ) and BGdT ( 1)-type planes were inspected. There are four kinds of BGdT ( 1)-type planes with different atomic configurations. All these planes match very well with Pt (1 ) plane not only energetically but also crystallographically if the following orientational relationship is satisfied: BGdT ( 1) ll Pt (1 ). Additionally, a polycrystalline film on a heterogeneous surface, in general, should grow along the direction normal to its close-packed plane having the lowest surface energy to minimize the interfacial energy. Since ( 1)-type planes are the close-packed planes in the BGdT film, the interfacial energy can be minimized by the formation of a highly c-axis-oriented film. Thus, the observed c-axis-oriented preferential growth of the BGdT Intensity(A.U.) 3 (2) Bi 3.15 Gd.85 Ti 3 O 12 (4) (6) (8) (1) (117) (12) θ (Degree) (14) Pt(111) (16) Pt(2) (18) 7 C 6 C 5 C 4 C Fig. 1. XRD y 2y scan results of the BGdT films annealed at various indicated temperatures ranging between 4 and 7 1C for 1 h.

3 192 ARTICLE IN PRESS U. Chon et al. / Physica B 388 (27) film is closely related to its thermodynamic stability on Pt(1 )/Ti/SiO 2 /Si(1 ) substrate. However, The BGdT films, fabricated at low temperatures below 7 1C showed a random orientation. The random orientation of the films is expected to be caused by an incomplete crystallization at low temperature. Enhancement of c-axis-oriented preferential growth in BGdT film with increment of annealing temperature was also reported by other researchers [13,14]. Fig. 2 presents the variations of 2P r and E c (coercive field) of the BGdT capacitor with the applied voltage. The P r E c P r (µc/cm 2 ) E c (kv/cm) Applied Voltage (V) Fig. 2. Variation of 2P r (filled circles) and E c (open circles) of the Pt/BGdT/Pt film capacitor with various applied voltages ranging between 4 and 14 V. Inset (a) represents polarization hysteresis loops measured at various applied voltages. Inset (b) shows relative dielectric permittivity and dissipation factor of the Pt/BGdT/Pt capacitor as a function of frequency. The corresponding film was thermally annealed at 7 1C for 1 h. Fig. 3. AFM images and piezoelectric response of the c-axis-oriented BGdT film: (a) topographic image, (b) phase contrast image of a 1 1 mm scanned area and (c) piezoelectric hysteresis curve of the c-axis-oriented BGdT film.

4 U. Chon et al. / Physica B 388 (27) inset (a) shows hysteresis loops of the capacitor measured at various applied voltages. The film thickness, as estimated using cross-sectional FE-SEM, was 36 nm. As shown in Fig. 2, the capacitor is nearly saturated at an applied voltage of 14 V (i.e., 39 kv/cm), and 2P r value at this applied voltage is 75 mc/cm 2. This value is significantly higher than 2P r of 27 mc/cm 2 for the highly c-axis-oriented Bi 3.25 La.75 Ti 3 O 12 capacitor [6], currently renowned as a fatigue-free ferroelectric capacitor. The substitution of lanthanides for Bi in the layered perovskites structure is known to reduce a structural distortion resulting in a relaxation of internal strain. Raman study on lanthanidessubstituted bismuth titanate has revealed that the A-sitesubstituted lanthanide ions increase TiO 6 symmetry, internal strain relaxation, lower Ti O hybridization, and decreased polarization along the a-axis [15]. This indicates that polarization vector in lanthanides-substituted bismuth titanate is tilted towards the c-axis from the a-axis. Our preliminary studies showed that the polarization tilt was mainly related to the TiO 6 octahedron unit adjacent to the interleaving Bi 2 O 2 layer rather than to the TiO 6 unit of the inner central octahedron layer [8]. Recent reports on the large 2P r values of highly c-axis-oriented BT-based films such as Nd-modified BT [16,17], La-modified BT [15,18] films, Gd-modified BT [19], and Sm-modified BT [2] basically accord with our observation. As shown in the inset (b), the relative dielectric permittivity and the dissipation factor are 468 and.42 at a frequency of 1 MHz, respectively. Although the dielectric permittivity decreases steadily with increasing frequency, there is no sudden change in its value up to 1 MHz. The dielectric loss (tan d) shows little change with increasing frequency up to 1 MHz. All these indicate that the observed P E hysteresis behavior of the BGdT capacitor originates from the ferroelectric polarization switching of bound charges, not from the response of freely moving charges. Fig. 3(a) and (b) presents the atomic force microscope (AFM) topographic and phase contrast images of the highly c-axis-oriented BGdT film. The measured phase difference between the tip-vibration signal and the low ACmodulation voltage corresponds to the difference in the domain-polarization orientation [11]. Thus, regions with opposite contrast in the c-axis-oriented film indicate the presence of the two anti-parallel ferroelectric domains with a non-zero component of P r along the c-axis. As presented in Fig. 3(c), the piezoelectric hysteresis characteristics were evident in the c-axis-oriented BGdT film capacitor. The observed hysteresis curve clearly demonstrates the ferroelectric behavior and the existence of the c-component polarization in the present BGdT film. As shown in the P E curves measured before and after read/write switching cycles, the fatigue-free characteristics are evident in the BGdT capacitor. The P E curves of Fig. 4(a) were obtained at an applied voltage of 12 V before and after the electrical fatigue test. The applied voltage and frequency for the fatigue test were 76 V and 1 MHz, respectively. This voltage ensures a Retainedcharge (μc/cm 2 ) P (μc/cm 2 ) (+P sw )-(+P ns ) (-P sw )-(-P ns ) Relaxation Time (sec) polarization switching of 72 mc/cm 2 in the capacitor as shown in Fig. 2. However, the fatigue test with higher voltage could not be successfully executed due to damage of the top Pt electrode, caused by heat generated from a repetitive application of high voltage. The values of 2P r and E c before the fatigue test were 68 mc/cm 2 and 136 kv/ cm at an applied voltage of 12 V, respectively. After being subjected to cycles, these were still retained at 67 mc/cm 2 and 133 kv/cm. Besides, the P E curves do not show any noticeable asymmetric behavior resulting in imprint failures, even after being subjected to switching cycles. The charge-retention characteristics of the BGdT capacitor are summarized in Fig. 4(b) by plotting the nonvolatile charge (i.e., difference between 7P sw and 7P ns )as a function of time. As reported previously [7,8], four distinct test-pulse sequences were employed for measuring the charge retention of +P sw,+p ns, P sw,and P ns. The non-volatile charge value, as defined by P nv ¼ (7P sw ) (7P ns ), of the capacitor was 52 mc/cm 2 and remained essentially constant up to 1 5 s after applying a writing pulse, demonstrating an excellent charge-retaining ability of the BGdT capacitor. Compared with the reported P nv E:1cycle E (kv/cm) D:4.5X11 cycles Fig. 4. Summary of ferroelectric reliability of the Pt/BGdT/Pt film capacitor: (a) P E hysteresis loops measured at an applied voltage of 12 V before and after being subjected to read/write switching cycles at a frequency of 1 MHz and (b) retained non-volatile charge of the Pt/ BGdT/Pt capacitor at 85 1C plotted as a function of time after the application of a writing voltage of 12 V. (a) (b)

5 194 ARTICLE IN PRESS U. Chon et al. / Physica B 388 (27) value of a highly c-axis-oriented SBT capacitor (6 mc/cm 2 ) [1] and that of a Bi 3.25 La.75 Ti 3 O 12 capacitor (15 mc/cm 2 ) [21], this is a remarkable improvement. In addition to this, j P sw j was essentially the same as +P sw throughout the relaxation (retaining) time, and the same trend was also observed for j P ns j and +P ns. This indicates that the BGdT capacitor has a strong resistance against the imprinting failure. The imprinting stress usually causes both trapping and aggregation of electrical charges at electrode ferroelectric interfaces. The trapped and aggregated charges are responsible for internal field build-up and screen the spontaneous polarization internally [22]. The trapping and aggregation of the charges at the interface proceed with long-range diffusion of the charges, driven by imprinting voltage [23]. In the c-axis-oriented layered ferroelectrics such as BGdT film, the long-range diffusion of it is not possible because the interleaving (Bi 2 O 2 ) 2+ layers and/or perovskite slabs act as natural barriers for the diffusion of it. This prohibits the build-up of internal field resulting in the imprint failure. 4. Conclusions In conclusion, a highly c-axis-oriented Bi 3.15 Gd.85- Ti 3 O 12 (BGdT) film capacitor having the ferroelectric reliability was successfully grown on a Pt/TiO 2 /SiO 2 / Si(1 ) substrate. The Pt/BGdT/Pt capacitor showed well-saturated P E curves with the saturated remanent polarization (2P r ) of 75mC/cm 2. More importantly, the capacitor did not show any significant fatigue up to switching cycles at a frequency of 1 MHz. In addition to these, the capacitor demonstrated excellent charge-retention characteristics with a resistance against the imprinting failure. Acknowledgments References [1] K.-M. Lee, H.-G. An, J.-K. Lee, Y.-T. Lee, S.-W. Lee, S.-H. Joo, S.- D. Nam, K.-S. Park, M.-S. Lee, S.-O. Park, H.-K. Kang, J.-T. Moon, Jpn. J. Appl. Phys. 4 (21) [2] R. Dat, J.K. Lee, O. Auciello, A.I. Kingon, Appl. Phys. Lett. 67 (1995) 572. [3] B.H. Park, B.S. Kang, S.D. Bu, T.W. Noh, J. Lee, W. Jo, Nature (London) 41 (1999) 682. [4] D. Wu, A.D. Li, T. Zhu, Z.G. Liu, N.B. Ming, J. Appl. Phys. 88 (2) [5] D. Wu, A.D. Li, N.B. Ming, Appl. Phys. Lett. 84 (24) 455. [6] U. Chon, G.-C. Yi, H.M. Jang, Appl. Phys. Lett. 78 (21) 658. [7] U. Chon, K.-B. Kim, H.M. Jang, G.-C. Yi, Appl. Phys. Lett. 79 (21) [8] U. Chon, H.M. Jang, M.G. Kim, C.H. Chang, Phys. Rev. Lett. 89 (22) [9] G.E. Pike, W.L. Warren, D. Dimos, B.A. Tuttle, R. Ramesh, J. Lee, V.G. Keramidas, J.T. Evans Jr., Appl. Phys. Lett. 66 (1995) 484. [1] J.J. Lee, C.L. Thio, S.B. Desu, Phys. Stat. Sol. (a) 151 (1995) 17. [11] S. Hong, J. Woo, H. Shin, J.U. Jeon, Y.E. Pak, E.L. Colla, N. Setter, E. Kim, K. No, J. Appl. Phys. 89 (21) [12] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113. [13] J. Zhai, H. Chen, Appl. Phys. Lett. 82 (23) 442. [14] Y.-M. Sun, Y.-C. Chen, J.-Y. Gan, J.-C. Hwang, Appl. Phys. Lett. 81 (22) [15] C.Y. Yau, R. Palan, K. Tran, R.C. Buchanan, Appl. Phys. Lett. 86 (25) [16] X.L. Zhong, J.B. Wang, X.J. Zheng, Y.C. Zhou, G.W. Yang, Appl. Phys. Lett. 85 (24) [17] X.S. Gao, J.M. Xue, J. Wang, J. Appl. Phys. 97 (25) [18] A.Z. Simo es, A. Ries, F.M. Filho, C.S. Riccardi, J.A. Varela, E. Longo, Appl. Phys. Lett. 85 (24) [19] S.S. Kim, J.C. Bae, W.-J. Kim, J. Cryst. Growth 274 (25) 394. [2] M. Chen, Z.L. Kiu, Y. Wang, C.C. Wang, X.S. Yang, K.L. Yao, Physica B 352 (25) 61. [21] W.S. Yang, N.K. Kim, S.J. Yeom, S.Y. Kweon, J.S. Roh, Jpn. J. Appl. Phys. 41 (22) 727. [22] J.J. Lee, C.L. Thio, S.B. Desu, Phys. Stat. Sol. (a) 151 (1995) 171. [23] G.L. Yuan, J.-M. Liu, Y.P. Wang, D. Wu, S.T. Zhang, Q.Y. Shao, Z.G. Liu, Appl. Phys. Lett. 84 (24) One of the authors (H. M. Jang) acknowledges the financial support from the Ministry of Education, Korea through the BK 21 Program.