SrBi2Ta2O9 Films Research Articles

Effect of oxygen plasma on growth, structure and ferroelectric properties of SrBi2Ta2O9 thin films formed by pulsed laser ablation technique

Growth of SrBi2Ta2O9 (SBT) thin films has been carried out in the presence of O2-plasma created by applying a potential at an auxiliary ring electrode placed near the substrate. Effect of plasma excitation potential and polarity, especially negative polarity, on the formation of a proper SBT phase at 700°C and in modifying crystallite orientation and microstructure of SBT films over (1 1 1) oriented Pt film coated over TiO2/SiO2/Si(1 0 0) substrates has been demonstrated. Preferred c-axis orientation of SBT films changes to (a–b) orientation with decrease in plasma excitation potential from −700 to −350 V and eliminates secondary Bi2Pt phase formation even at 600°C Microstructural study show a 2-dimensional large flat c-oriented crystallites formed at −700 V change to small crystallites in conformity with the changed aspect ratio for crystallites in (a–b) plane parallel to film plane. Spectroscopic ellipsometric results are in agreement with the microstructural data. These affects are attributed to O2-ion bombardment during film growth which reduces nucleation barrier for growth of crystallites in (a–b) plane. O2-plasma sustains the cationic species formed by laser ablation, which along with O 2 + ions, provide necessary activation energy and enhance the oxidation rates required for SBT phase formation even at 700°C. SBT films grown in O2-plasma show enhancement in remnant polarization value from 1.2 to 6.6 μC/cm2 and display ferroelectric properties superior to those formed without plasma. Further O2-plasma eliminates post deposition annealing step for observance of enhanced polarization values. This study shows O2-plasma excitation potential could be exploited as a new process parameter in laser ablation growth of ferroelectric oxide thin films.

Nanoscale Properties of SrBi2Ta2O9 Thin Films

AbstractPiezoresponse scanning force microscopy (PFM) was applied to study the nanoscale mechanism of retention loss in SrBi2Ta2O9 (SBT) thin films. Experiments were conducted by performing local polarization reversal within an individual grain with subsequent imaging of a resulting domain structure at different time intervals. The retention behavior of the films was studied as a function of switching conditions and electrode material. SFM was also used for nanoscale mapping of leakage current sites and investigation of electrical conduction mechanism at these sites. For the first time, the development of dielectric breakdown in SBT films was directly observed at nanoscale.

Direct Comparison of Structural and Electrical Properties of Epitaxial (001)-, (116)-, and (103)-Oriented SrBi2Ta2O9 Thin Films on SrTiO3 and Silicon Substrates

AbstractAnisotropies of the properties of the bismuth-layered perovskite SrBi2Ta2O9 (SBT) have been investigated using epitaxial thin films grown by pulsed laser deposition both on conducting Nb-doped SrTiO3 (STO) single crystal substrates and on Si(100) substrates. It has been found that the three-dimensional epitaxy relationship SBT(001)∥STO(001); SBT [110] ∥STO[100] can be applied to all SBT thin films on STO substrates of (001), (011), and (111) orientations. An about 1.7 times larger remanent polarization was obtained in (103)-oriented SBT films than in that of (116) orientation, while the (001)-oriented SBT films revealed no ferroelectricity along their c-axis. Non-c-axis-oriented SBT films with a well-defined (116) orientation were also grown on silicon substrates for the first time. They were deposited on Si(100) covered with a conducting SrRuO3 (110) bottom electrode on a YSZ(100) buffer layer.

Characterization of Ferroelectric Property of c-axis and non-c-axis Oriented Epitaxially Grown Bismuth Layer-Structured Ferroelectric Thin Films with Different m-numbers Prepared by MOCVD

AbstractEpitaxial thin films of bismuth layer-structured ferroelectrics (BLSF) with different m-numbers, i.e., Bi2VO5.5 (BVO) (m=1), SrBi2Ta2O9 (SBT) (m=2), and Bi4Ti3O12 (BIT) (m=3), were grown by metalorganic chemical vapor deposition (MOCVD). (00l)-oriented films were deposited on (100)SrTiO3. (114)-oriented BVO, (116)-oriented SBT, and (118)-oriented BIT films were deposited on (110)SrTiO3. Moreover, (102)-oriented BVO, (103)-oriented SBT, and (104)-oriented BIT films were deposited on (111)SrTiO3. On (100), (110), and (111)SrTiO3 substrates, c-axis of the deposited films was tilted about 0°, 45°, and 56°, respectively, against perpendicular to the surface of the substrates irrespective of m-number. This suggests the growth of crystallographic equivalent orientation. The distinctive surface morphology originated to the feature of the film orientation was observed. The dielectric constant and the leakage current of c-axis-oriented film was smaller than that of non-c-axis-oriented one, indicating smaller dielectric constant and leakage current along c-axis than a- or b-axes. A larger ferroelectric anisotropy was ascertained for SBT and BIT films. Furthermore, the evaluated spontaneous polarization along a- and c- axes of BIT from the data of the epitaxially grown BIT films well agreed with the reported one for the single crystal. This suggests the ferroelectric property was not strongly affected by the strain in the films.

The Sol-Gel Chemistry of Ferroelectric SrBi2Ta2O9 Thin Layers

A characterization of sol-gel precursors of SrBi2Ta2O9 (SBT) has been carried out. Each molecular precursor, [SrBi2+xTa2(OCH2CH2OCH3)18]n (1) (x = 0.0, 0.2, and 0.4) and [SrTa2(OEt)(OiPr)11(iPrOH)2] (2) was prepared from the mixtures of Bi(OR)3, Sr(OR)2, and Ta(OR)5 (R = Et, iPr, and CH2CH2OCH3), respectively. 1H and 13C NMR spectroscopy and powder X-ray diffractometry have been used to characterize precursor molecules and their oxides, respectively. Especially, X-ray single crystal studies of complex 2 show that the molecule is made up of three octahedra; the central [SrO6] octahedron is sharing two neighboring edges with the peripheral [TaO6] ones. The SBT film derived from the precursor 1 presents outstanding ferroelectric properties (Pr = 9 μC/cm2; Ec = 0.8 V).

Ferroelectric Properties of (001)- and (106)-Oriented SrBi2Ta2O9 Epitaxial Thin Films

Epitaxially grown SrBi2Ta2O9 (SBT) thin films with (001) and (106) orientations were prepared on La-doped SrTiO3 (001) and (110) substrates, respectively, by coating-pyrolysis process. When the films were annealed in air, their epitaxy was poor and no significant difference was observed in the P-E characteristics for the films that have different orientations. By contrast, the crystallinity and epitaxy of the films increased with decreasing oxygen partial pressure, p(O2), of annealing atmosphere. Using these high quality epitaxial films, we observed a distinct difference in P-E hysteresis curves, which reflects the orientation of the films. After postannealing of these films in O2 to compensate for possible oxygen deficiency, which might have been introduced into the SBT films owing to low p(O2) annealing, the anisotropy of the ferroelectric response was maintained and almost the same P-E loops were obtained.

Metal-organic chemical vapour deposition of ferro-electric SrBi2Ta2O9 thin films

We report the application of the metal – organic chemical vapour deposition (MOCVD) technique to the formation of ferro-electric SrBi2Ta2O9 (SBT). MOCVD is a desired technique for the application of high-density ferro-electric memory because it can produce high quality, conformal, very thin films. There has been much progress in gas delivery methods and in precursors, which greatly affect controllability and reproducibility of the MOCVD process for fabricating ferro-electric thin films. With appropriate precursors and optimized gas delivery conditions, MOCVD can produce high-performance ferro-electric SBT films. When forming SBT thin films, precise control of composition is essential to acquire the electrical properties needed for ferro-electric memory applications. Copyright © 2000 John Wiley & Sons, Ltd.

Studies of ferroelectric heterostructure thin films, interfaces, and device-related processes via in situ analytical techniques

The science and technology of ferroelectric thin films has experienced an explosive development during the last ten years. Low-density non-volatile ferroelectric random access memories (NVFRAMs) are now incorporated in commercial products such as “smart cards”, while high permittivity capacitors are incorporated in cellular phones. However, substantial work is still needed to develop materials integration strategies for high-density memories. We have demonstrated that the implementation of complementary in situ characterization techniques is critical to understand film growth and device processes relevant to device development. We are using uniquely integrated time of flight ion scattering and recoil spectroscopy (TOF-ISARS) and spectroscopic ellipsometry (SE) techniques to perform in situ, real-time studies of film growth processes in the high background gas pressure required to growth ferroelectric thin films. TOF-ISARS provides information on surface processes, while SE permits the investigation of buried interfaces as they are being formed. Recent studies on SrBi2Ta2O9 (SBT) and BaxSr1-xTiO3 (BST) film growth and interface processes are discussed. Direct imaging of ferroelectric domains under applied electric fields can provide valuable information to understand domain dynamics in ferroelectric films. We discuss results of piezoresponse scanning force microscopy (SFM) imaging for nanoscale studies of polarization reversal and retention loss in Pb(ZrxTi1-x)O3 (PZT)-based capacitors. Another powerful technique suitable for in situ, real-time characterization of film growth processes and ferroelectric film-based device operation is based on synchrotron X-ray scattering, which is currently being implemented at Argonne National Laboratory.

Investigation of preannealing method for preparation of robust SrBi2Ta2O9 thin films by chemical solution deposition

Bismuth layered-perovskite thin films of SrBi2Ta2O9 (SBT) solid solution were prepared by chemical solution deposition method. In order to form the robust SBT thin films, preannealing methods such as rapid thermal annealing (RTA) and furnace annealing were studied in terms of physical and electrical properties of SBT thin films. SBT thin films preannealed by furnace exhibited better surface morphology and electrical properties than those preannealed by RTA. The crystallization mechanism of SBT thin films by heat treatment was examined by using X-ray diffraction (XRD) analysis and scanning electron micrograph (SEM). For the robust SBT thin films with (P*-P⁁) value of about 20 uC/cm2, leakage current density of less than 10−7 A/cm2, and breakdown voltage of 15 V, the optimal condition of furnace preannealing was found to be 700 °C for 30 min.

Different fatigue behaviors of SrBi2Ta2O9 and Bi3TiTaO9 films: Role of perovskite layers

To investigate the role of the perovskite layers on fatigue behaviors, SrBi2Ta2O9(SBT) and Bi3TiTaO9 (BTT) films were prepared by pulsed laser deposition using 15% Bi-excess bulk targets. The SBT and the BTT films grown at the similar deposition conditions showed similar growth behaviors, electrical properties, and retention characteristics. However, these films showed very different fatigue behaviors. The difference should come from the oxygen stability in the perovskite layer. Our work demonstrates that oxygen stability of the perovskite layers, as well as the self-regulating adjustment of the Bi2O2 layers, should be considered in the search for new candidate materials for nonvolatile ferroelectric memory devices.

Direct deposition of SrBi2Ta2O9 Film on IrO2 electrode using liquid source CVD method

SrBi2Ta2O9 (SBT) thin films were directly deposited on IrO2 electrode using liquid source CVD (LS-CVD) method. Bi deficient region in the SBT films, which existed near the conventional Pt bottom electrode, was successfully eliminated using IrO2 bottom electrode. X-ray diffraction analysis showed our SBT films on IrO2 have good crystalline quality. Satisfactory electric properties (6 μC/cm2 of 2Pr and 10−6 A/cm2 of leakage current) were obtained

Epitaxial bismuth-layer-structured perovskite ferroelectric thin films grown by pulsed laser deposition

Thin films of bismuth-layer-structured perovskites such as SrBi2Ta2O9 (SBT) and BaBi4Ti4O15 (BBiT) with preferred orientations have been grown by pulsed laser deposition (PLD) on epitaxial conducting LaNiO3 (LNO) electrodes on single crystalline (100) SrTiO3 (STO) or on top of epitaxial buffer layers on (100) silicon. X-ray diffraction (XRD) analysis and cross-section electron microscopy reveal that the films consist of c-axis oriented regions and mixed a-axis and c-axis oriented regions. The regions with mixed a-axis and c-axis orientation show high surface roughness due to the rectangular crystallites protruding out of the surface, whereas the c-axis oriented regions show a smooth surface morphology. In the mixed a-axis and c-axis oriented regions, the SBT and BBiT films exhibit saturated ferroelectric hysteresis loops with a remnant polarization Pr of about 2 μC/cm2 and coercive fields Ec of about 20 kV/cm for SBT and 60 kV/cm for BBiT. No polarization fatigue was observed up to 108 cycles neither for SBT nor for BBiT films. The regions having c-axis orientation with a smooth surface morphology in contrast exhibit a linear P-E curve. The results show that the ferroelectric properties of a planar capacitor consisting of ferroelectric Bi-layer-structured perovskites depend on the crystalline orientation of the film.

Effects of growth conditions and RF plasma on crystalline and electrical properties of SrBi2Ta2O9 thin films grown by liquid delivery MOCVD using a double alcoholate

SrBi2Ta2O9 (SBT) thin films were prepared by liquid delivery MOCVD using a double alcoholate, Sr[Ta(OC2H5)6]2. In order to crystallize the deposited films, annealing in low pressure plasma produced by RF power was conducted as well as furnace annealing in O2 atmosphere. It was found that the Sr/Ta and Bi/Ta ratios in the deposited-films approached to the stoichiometric ratio as both deposition temperature and reactor pressure were lowered. An SBT thin film which was deposited at 400°C under 1 Torr and subsequently annealed in O2 atmosphere at 750°C for 30 min showed relatively good ferroelectricity. Remanent polarization (Pr) and coercive field (Ec) of the film were 2.0 μC/cm2 and 45 kV/cm, respectively. It was also ascertained that SBT thin films were crystallized at 650°C by annealing in oxygen plasma and showed ferroelectricity. Pr and EC of the SBT film which was annealed at 650°C in oxygen plasma with RF power of 150 W and pressure of 5 Torr were 1.5 μC/cm2 and 45 kV/cm, respectively.

Electrical properties of (001)- and (116)-oriented epitaxial SrBi2Ta2O9 thin films prepared by metalorganic chemical vapor deposition

(001)- and (116)-oriented epitaxial SrBi2Ta2O9 (SBT) thin films were deposited on (100)SrRuO3∥(100)SrTiO3 substrates at 750 °C and (110)SrRuO3∥(110)SrTiO3 substrates at 820 °C by metalorganic chemical vapor deposition, respectively. The remanent polarization and the coercive field of the 200-nm-thick (116)-oriented SBT films normal to the substrate were 11.4 μC/cm2 and 80 kV/cm, respectively. The dielectric constant of this film was 140 at 1 kHz. On the other hand, the 200-nm-thick (001)-oriented SrBi2Ta2O9 films normal to the substrate showed no ferroelectricity and the dielectric constant was 70 at 1 kHz.

Scaling effect on statistical behavior of switching parameters of ferroelectric capacitors

Scanning force microscopy (SFM) has been used to study nanoscale variations in switching parameters in layered perovskite films of SrBi2Ta2O9 and to investigate the effect of capacitor scaling on the standard deviation of the capacitors’ integral polarization signal. Ferroelectric poling and SFM piezoresponse imaging were performed in a number of regions on the film surface sized 2×2, 1×1, 0.5×0.5, and 0.3×0.3 μm2 with subsequent statistical analysis of the obtained data. It has been found that variations of the polarization signal can be approximated by the normal distribution function. The standard deviation increases with the decrease in the capacitor size, suggesting a stronger effect of grain misalignment in smaller capacitors. The obtained results imply that reliable high-density ferroelectric memories cannot be realized unless a certain capacitor size/grain size ratio is maintained.

Ex situ growth of the c-axis preferred oriented SrBi2Ta2O9 thin films on Pt/Ti/SiO2/Si substrates

SrBi2Ta2O9 (SBT) thin films were deposited on (111) oriented Pt bottom electrodes using the rf magnetron sputtering method and then postannealed at 650–800 °C for 30 min in different oxygen atmospheres. The effect of Bi content on the c-axis preferred oriented growth was confirmed by the control of the Bi2O3 loss during the postannealing. With increasing Bi content in SBT thin films, the degree of the c-axis preferred orientation was enhanced. In addition, a bimodal grain size distribution due to the Sr deficiency in the SBT film was observed. It is suggested that the c-axis preferred oriented growth on Pt(111) bottom electrodes can be attributed to growth controlled by surface energy minimization.

Effect of Deposition Temperature and Composition on the Microstructure and Electrical Property of SrBi2Ta2O9 Thin Films Prepared by Metalorganic Chemical Vapor Deposition

SrBi2Ta2O9 (SBT) thin films were prepared by conventional thermal metalorganic chemical vapor deposition (MOCVD) and subsequent heat treatment. The SBT film deposited at 500°C had a smoother surface and better step coverage than that deposited at 750°C. The degree of step coverage deposited at 500°C was 0.82. An almost single phase of SBT was obtained for the film with a Bi/Ta mole ratio of 1.0 by heat treatment at 750°C for 30 min in O2 atmosphere after MOCVD deposition at 500°C. 2Pr and EC at an applied electric field of 620 kV/cm were 12.2 µC/cm2 and 87 kV/cm, respectively, when the film was deposited at 500°C followed by heat treatment at 800°C for 30 min in O2 atmosphere, and its Bi/Ta ratio was 1.2.

Inter-diffusion studies of SrBi2Ta2O9 film prepared on platinized wafer by pulsed laser ablation

Ferroelectric SrBi 2 Ta 2 O 9 (SBT) thin films were prepared on platinized wafer-Pt/TiO x /SiO 2 /Si-by pulsed laser ablation deposition. Phase formation of SBT film was achieved during the deposition process itself, so the post-annealing that is often used by other groups is not necessary. The SBT film was well-crystallized, showing a dominant (115) peak at a deposition temperature as low as 580°C, and exhibited good ferroelectric properties-the remanent polarization and coercive electrical field are 7.1 μC cm -2 (2Pr) and 30 kV cm -1 , respectively, at an applied electrical field of 80 kV cm -1 . Secondary ion mass spectrometry depth profiling and cross-sectional transmission electron microscopy were performed to study the inter-diffusion at the SBT/Pt bottom electrode interface. A sharp SBT/Pt interface was observed at a deposition temperature of 580°C, but a higher deposition temperature (680°C) resulted in significant inter-diffusion at the SBT/Pt interface. The formation of a secondary pyrochlore phase was observed due to Ti diffusion into SBT film at a high deposition temperature of 680°C.

Degradation behavior in the remnant polarization of SrBi2Ta2O9 thin films by hydrogen annealing and its recovery by postannealing

The degradation behavior in the remnant polarization (Pr) of sol–gel-derived SrBi2Ta2O9 (SBT) thin films, with Pt top and bottom electrodes, by annealing under a 5% H2/95% N2 atmosphere is investigated. The hydrogen annealing is performed at temperatures ranging from 250 to 480 °C. By annealing, the Pr drops to almost zero for all temperatures. Postannealing at temperatures higher than 700 °C under an air atmosphere recovers the Pr. Interestingly, the recovery is most ineffective for the sample annealed at the Curie temperature of the SBT films. A phenomenological model that explains this anomalous recovery behavior is presented.

Film structure and ferroelectric properties of in situ grown SrBi 2 Ta 2 O 9 films

Ferroelectric SrBi2Ta2O9 (SBT) films were grown by pulsed-laser deposition (PLD) at different substrate temperatures and fluences. A correlation between film structure and ferroelectric properties is established. The dielectric function e′ of thin SBT films shows a Curie–Weiss behavior well below the peak temperature Tmax and relaxor-like behavior in the vicinity of Tmax. Domain walls have a strong influence on the dielectric and ferroelectric properties and on the polarization fatigue of SBT films below 100 °C. The formation of ferroelectric phases is favored at lower substrate temperatures by incorporating Bi2O3 template layers into the structure.