Masatoshi Mitsuya

Method of Distinguishing SrBi2Ta2O9 Phase from Fluorite Phase Using X-Ray Diffraction Reciprocal Space Mapping

In the Sr–Bi–Ta–Nb–O system, three crystallographic phases are known to exist: the SrBi2(Ta1-xNbx)O9 (SBTN) perovskite, fluorite and pyrochlore phases. It is considered that the fluorite phase is a low-temperature phase of SBT, which tends to grow in excess bismuth compositions, and the pyrochlore phase tends to grow in bismuth-deficient compositions. In conventional X-ray diffraction (XRD) characterization, the SBTN phase shows strong (115) diffraction around 29 [2θ deg]. Unfortunately, however, the other two phases also show their (111) and (222) diffractions near the same angle when the film is prepared on a platinum-coated silicon substrate. Therefore, the phase identification of the SBTN phase from the other two phases is almost impossible by the conventional technique. We employed XRD reciprocal space mapping to distinguish these phases in the present study. The three crystallographic phases were identified and distinguished from each other. It is ascertained that this technique is effective to identify crystallographic phases especially in the case in which more than two phases show similar diffraction angles.

Local Epitaxial Growth of (103) One-Axis-Oriented SrBi2Ta2O9 Thin Films Prepared at Low Deposition Temperature by Metalorganic Chemical Vapor Deposition and Their Electrical Properties

We compared a directly crystallized SrBi2Ta2O9 (SBT) film with one crystallized by solid-phase reaction from the fluorite phase prepared by metalorganic chemical vapor deposition (MOCVD). The region of the Bi/Ta mole ratio showing large remanent polarization (2Pr) was narrow for the film directly crystallized from the gas phase compared with that crystallized by solid-phase reaction. Moreover, the Bi/Ta mole ratio showing the maximum 2Pr value differed according to the preparation method used. The crystallinity and the orientation of the SBT phase directly crystallized from the gas phase were strongly influenced by those of the substrate; the (103)-oriented SBT grains directly crystallized from the gas phase grew hetero-epitaxially on (111)-oriented Pt grains. As a result, the direct crystallization of the film from the gas phase lowered the crystallization temperature of the SBT phase and resulted in a (103) one-axis-oriented film.

Low Temperature Direct Crystallization of SrBi2(Ta1-xNbx)2O9 Thin Films by Thermal Metalorganic Chemical Vapor Deposition and Their Properties

Directly crystallized SrBi2(Ta1-xNbx)2O9 (SBTN) films were deposited on (111) Pt/Ti/SiO2/Si substrates at 585–670°C by thermal metalorganic chemical vapor deposition (MOCVD). The crystalline SBTN film was directly deposited at 670°C irrespective of the deposition rate, but its leakage current markedly decreased when the deposition rate decreased from 5.0 to 2.1 nm/min. When the deposition rate was below 2.1 nm/min, an SBTN film with large ferroelectricity was deposited even at 585°C, and strong (103)-orientation was ascertained by an X-ray reciprocal space mapping method. This orientation is considered to locally epitaxially occur on a (111)-oriented Pt substrate. Twice the remanent polarization (2Pr) and twice the coercive field (2Ec) of the film deposited at 585°C were 12.2 µC/cm2 and 160 kV/cm, respectively. When the deposition temperature increased, the film became randomly oriented which was in response to the orientation change in the Pt substrate from single (111) to a mixed orientation of (111) and (100) orientations by heating before starting the film deposition. 2Pr and 2Ec of the film deposited at 670°C increased to 23.8 µC/cm2 and 190 kV/cm, respectively.

Preparation and Characterization of SrBi2(Ta1-xNbx)2O9 Thin Films by Metalorganic Chemical Vapor Deposition from Two Organometallic Source Bottles

SrBi2(Ta1-xNbx)2O9 (SBTN) thin films were first prepared on Pt/Ti/SiO2/Si substrates by metalorganic chemical vapor deposition (MOCVD) with high compositional reproducibility. Bi(CH3)3, a mixture of Sr[Ta(OC2H5)6]2 and Sr[Nb(OC2H5)6]2, and O2 gas were used as sources. The Nb/(Ta+Nb) ratio in the film was almost the same as that of the source materials. The film, deposited at 500°C following heat treatment at 800°C for 30 min in O2 atmosphere, consisted of an almost single phase of SBTN. The remanent polarization and the coercive field of the 330 nm-thick film were 8.5 µC/cm2 and 91 kV/cm, respectively. This film showed negligible fatigue after 5×1010 polarization switching cycles.

Quantitative Effects of Preferred Orientation and Impurity Phases on Ferroelectric Properties of SrBi2(Ta1-xNbx)2O9 Thin Films Measured by X-Ray Diffraction Reciprocal Space Mapping

Effects of preferred orientation and impurity phases on the ferroelectric property of SrBi2(Ta1-xNbx)2O9 (SBTN) thin films deposited by pulsed metalorganic chemical vapor deposition (pulsed-MOCVD) were quantitatively characterized by employing the X-ray diffraction reciprocal space mapping (XRD-RSM) technique. The SBTN thin films deposited on Pt/Ti- and Pt/TiO2-coated SiO2/(001) Si substrates were found to have {103} preferred orientation and that deposited on Ir/TiO2-coated SiO2/(001) Si substrate was found to have {207} preferred orientation, although conventional θ–2θ profiles of these films showed similar patterns and showed no specific information on preferred orientation. Moreover, the existence of impurity phases such as fluorite and pyrochlore phases were also clearly distinguished from the SBTN phase, and the relative volume fractions of each phase were estimated. The observed remanent polarizations of these films can be explained by taking account of the orientation of the SBTN phase and the volume fraction of the impurity phases. This shows that the XRD-RSM technique is a useful technique for the quantitative estimation of the ferroelectricity of polycrystalline SBTN film.

Raman spectroscopic fingerprint of ferroelectric SrBi2Ta2O9 thin films: A rapid distinction method for fluorite and pyrochlore phases

We present the use of Raman spectroscopy as a rapid and sensitive means for the phase characterization of ferroelectric SrBi2(Ta1-xNbx)2O9 (SBTN) thin films. It is shown that frequency shifts, together with Raman selection rules, are characteristic of layered perovskite, fluorite and pyrochlore structures, and thus the Raman spectra can be used as a fingerprint of the symmetry of the examined film. We also find the enormous potential of Raman spectroscopy to detect and quantify fractions of the fluorite and pyrochlore phases coexistent with the SBTN phase.

Low-Temperature Preparation of SrBi2Ta2O9 Thin Films by Electron Cyclotron Resonance Plasma-Enhanced Metalorganic Chemical Vapor Deposition and Their Electrical Properties

A SrBi2Ta2O9 (SBT) thin film was prepared by electron cyclotron resonance plasma-enhanced metalorganic chemical vapor deposition (ECR-MOCVD). The deposition temperature dependence of the composition of the film was lesser than that of films prepared by conventional thermal MOCVD. An almost single phase of SBT was obtained at 610°C. The crystallinity of this film was higher than that of the film prepared by thermal MOCVD at 500°C and subsequent heat treatment at 800°C. The leakage current density of this film was small, on the order of 10-8 A/cm2 up to 240 kV/cm. Moreover, two fold the remanent polarization and the coercive field at an applied electric field of 400 kV/cm were 14.5 µC/cm2 and 77 kV/cm, respectively. These values were larger than those of the film prepared by thermal MOCVD at 500°C with heat treatment at 800°C.

Low leakage current and good ferroelectric properties of SrBi2(Ta0.7Nb0.3)2O9–Bi3TiTaO9 solid solution thin film

(1−x)SrBi2(Ta0.7Nb0.3)2O9+xBi3TiTaO9 (x=0–0.5) solid-solution (SBTN+BTT) films of low defect contents were directly crystallized on (111)Pt/Ti/SiO2/Si substrates at 650 °C by metalorganic chemical vapor deposition. The deposited films showed a strong (103) orientation. The remanent polarization (Pr) of the directly crystallized SBTN (x=0) was very small. However, the Pr value increased to 7.1 μC/cm2 by adding 30% of BTT (x=0.3) and was almost equal to that of Sr0.8Bi2.2(Ta0.7Nb0.3)2O9(S0.8B2.2TN), which is widely studied for nonvolatile memory applications. The leakage current density of the SBTN+BTT solid solution was on the order of 10−8 A/cm2 for fields up to 200 kV/cm due to its low defect contents character, while that of S0.8B2.2TN was above 10−6 A/cm2 due to the existence of defects in the Sr sites. The solid-solution film showed a fatigue-free character.

Direct Preparation of Crystalline SrBi2(Ta1-xNbx)2O9 Thin Films by Thermal Metalorganic Chemical Vapor Deposition at Low Temperature

Polycrystalline SrBi2(Ta1-xNbx)2O9 (SBTN) thin films with large ferroelectricity were directly prepared on Pt/Ti/SiO2/Si substrates even at 585°C by thermal metalorganic chemical vapor deposition (MOCVD). Thin films mainly consisting of the SBTN phase were obtained even at 585°C. The (103)-oriented film changed to a (001)-oriented one when the deposition temperature increased. The 200-nm-thick film deposited at 585°C had large ferroelectricity, i.e., two times the remanent polarization (2Pr) and two times the coercive field (2Ec) of 12.2 µC/cm2 and 160 kV/cm, respectively. When the deposition temperature was increased to 670°C, the 2Pr and 2Ec values increased to 23.8 µC/cm2 and 190 kV/cm, respectively.

Property Improvement of 75 nm-thick Directly-crystallized SrBi2Ta2O9 Thin Films by Pulse-introduced Metalorganic Chemical Vapor Deposition at Low Temperature

Crystallized SrBi2Ta2O9 (SBT) films were deposited on (111) Ir/TiO2/SiO2/Si substrates at 650°C by metalorganic chemical vapor deposition (MOCVD). Crystallized SBT films from 75 to 200 nm in thickness were directly deposited, but its remanent polarization (Pr) decreased when the film thickness decreased for the film deposited by conventional continuous-MOCVD. This Pr value was increased by 50% by using the source gas pulse-introduction technique (pulse-MOCVD) at 75 nm thicknesses. Moreover, the leakage current was dramatically improved to be on the order of 10-5 A/cm2 up to 600 kV/cm. This film exhibited strong (103) orientation of the crystal axis, while the continuous gas-introduced film showed a mixture of (00l) and (103) orientations.