BIT Films Research Articles

Enhancement of Remanent Polarization of BIT-based Thin Films by Ti-site Substitution using Ions with Higher Charge Valences

ABSTRACTBismuth titanate (Bi4Ti3O12; BIT) -based ferroelectric thin films fabricated from a view point of “the site-engineering technique” have been expected to improve the fatal disadvantage of a ferroelectric BIT film, i.e., its low spontaneous polarization; here, Bi- and Ti-site ions in the BIT crystal are cosubstituted by lanthanoid ions and cations with a higher charge valence, respectively, In the present study, we have mainly focused on Ti-site substitution of bismuth titanate (Bi4Ti3O12; BIT)-based thin films using some ions with higher charge valences (V5+, Nb5+, Ta 5+ and W6+; in this study) to enhance the ferroelectric properties of those materials. The BIT-based films with various chemical compositions were fabricated on a (111)Pt/Ti/SiO2/(100)Si substrate by a chemical solution deposition method.Ti-site substitution of BIT films by the higher-valent ions, Bi3.99(Ti2.97V0.03)O12, Bi3.99(Ti2.97Nb0.03)O12, Bi3.99(Ti2.97Ta0.03)O12 and Bi3.98(Ti2.97W0.03)O12, reduced the leakage current density of BIT films from ∼ 10-6 down to ∼ 10-7 A/cm2 at an applied field of 50 kV/cm, while the substitution by the same-valent cation, e.g., Bi4.00(Ti2.97Zr0.03)O12, did not affect the behavior of leakage current. Whereas polarization (P) - electrical field (E) hysteresis loops of non-substituted and Zr-substituted BIT films were distorted due to the leakage current, non-distorted P-E loops were obtained at V5+-, Nb5+-, Ta5+- and W6+-substituted BIT films.Also, Ti-site substitution was effective for improving the ferroelectric properties in lanthanoid-substituted BIT films. In the case of La3+-substituted BIT film (BLT), remanent polarization (Pr) of V5+- and W6+-substituted BLT films, (Bi3.24La0.75)(Ti2.97V0.03)O12 and (Bi3.23La0.75)(Ti2.97W0.03)O12 (13 and 12 μC/cm2, respectively), were larger than those of Zr4+- and non-substituted BLT films, (Bi3.25La0.75)(Ti2.97Zr0.03)O12 and (Bi3.25La0.75)(Ti3.00)O12 (8 and 9 μC/cm2, respectively), while those films had similar coercive field (Ec) of approximately 120 kV/cm. Also in the case of Nd3+-substituted BIT film (BNT), Pr and Ec values of V5+-substituted BNT film, (Bi3.24Nd0.75)(Ti2.98V0.02)O12, were 37 μC/cm2 and 119 kV/cm, respectively, which were comparable with those of conventional Pb-based ferroelectrics such as lead zirconate titanate, Pb(Zr,Ti)O3. We concluded that enhancement of the Pr value was achieved by the charge compensation of oxygen vacancies in BIT-based ferroelectrics using higher-valent cations than Ti4+ ion whereas no obvious differences were found in the crystal orientation and or microstructure of these films.

Growth of Epitaxial Site-Engineered Bi4Ti3O12-Basded Thin Films by Mocvdand Their Characterization

ABSTRACTThin films of BIT, La-substituted BIT (BLT) and La- and V-cosubstituted BIT(BLTV) were epitaxially grown on SrRuO3//SrTiO3 substrates at 850°C by metalorganic chemical vapor deposition (MOCVD), and their electrical properties were systematically compared. All films on (100), (110) and (111)-oriented substrates were epitaxially grown with (001)-, (104)-/(014)-and (118) –preferred orientations, respectively. The leakage current density of the BLTV film was almost the same with that of the BLT film, but was smaller than that of BIT film, suggesting that the La substitution contributed to the decrease of the leakage current density especially in pseudoperovskite layer. Spontaneous polarization of the BLTV film was estimated to be almost the same with the BLT film but was smaller that that of the BIT film. This is explained by the decrease of Tc with the La substitution, while V did not contribute to the change of the Curie temperature (Tc ). On the other hand, the coercive field (Ec) value of the BLTV was smaller than that of the BIT and the BLT films. As a result, La substitution contributed to the decrease of the leakage current density together with the decrease of the spontaneous polarization due to the decrease of the Tc. On the other hand, V substitution contributes to the decrease of the defects that suppress the domain motion and increases the Ec value. Therefore, each substitution of La and V plays different roles and this contribution is remarkable for the films deposited at lower temperature.

Preparation and characterization of Bi 4Ti 3O 12–SrBi 4Ti 4O 15 ferroelectric thin films by pulsed laser deposition

Ferroelectric thin films of Bi 4Ti 3O 12 (BiT) 1− x –SrBi 4Ti 4O 15 (SBTi) x solid solutions have been prepared by pulsed laser deposition (PLD) to obtain a large remnant polarization ( P r ) in bisumuth layer-structured ferroelectrics (BLSFs). The BiT–SBTi ( x=0.5) films prepared on Pt/Ti/SiO 2/Si substrate at 600°C include both the crystalline structures of BiT and SBTi. Surface of the BiT–SBTi film was flat, the grain sizes are from 100 to 150 nm, and the average surface roughness of about 18 nm was much lower than those of BiT (40 nm) and SBTi (30 nm). Polarization at zero electric field was 9.3 μC/cm 2 which was clearly larger than those of BiT and SBTi films prepared at the same PLD conditions, and the coercive field was 60 kV/cm. The dielectric constant at room temperature was about 250 for a measuring frequency of 1 MHz, which is larger than those of other BLSFs.

Sol-Gel Derived Bismuth Titanate Thin Films with c-Axis Orientation

Bismuth titanate (Bi4Ti3O12), a member of the layered perovskite family, has a unique set of ferroelectric properties, which include a high remanent polarization, low coercive field, and high Curie temperature, that make it a possible candidate for data storage applications. For this investigation, bismuth titanate, or BiT, films were fabricated via sol-gel method to examine the effect of processing on phase development and orientation. Solutions were deposited onto platinized silicon, and then heat treated for one hour at temperatures ranging from 550°C to 700°C in 100% O2. It was found that c-axis orientated BiT films could be formed at temperatures as low as 550°C by using bismuth oxide template layers, while films without bismuth oxide templating possessed a random orientation over the same temperature range.

Preparation of PZT, PLZT and Bi 4Ti 3O 12 thin films from oxide precursors

This work describes a method for preparation of ferroelectric thin films based on a pre-calcination of oxides, to be used as precursor material for a solution preparation. In order to show the viability of the proposed method PZT, PLZT and Bi 4Ti 3O 12 thin films were prepared. The results were analyzed by X-ray diffraction, scanning electron microscopy and Rutherford backscattering spectroscopy. PZT film shows a coexistence of tetragonal (lattice constant a=4·02 Å and c=4·16 Å) and rhombohedral ( a=4·13 Å) phases. For PLZT, lattice constant a was estimated for pseudocubic phase to be found 4·12 Å. The lattice constant a, b and c to Bi 4Ti 3O 12 film (orthorhombic symmetry) obtained were 5·58, 5·53 and 33·67 Å, respectively. The films obtained shown good quality, homogeneity and the desired stoichiometry. Estimated thickness for one layer was approximately 1000 and 1300 Å for BIT and PZT films, respectively.

Epitaxial Bi 4Ti 3O 12 thin film growth using Bi self-limiting function

Epitaxial Bi 4Ti 3O 12 (BIT) thin films are grown by molecular beam epitaxy using the Bi self-limiting function. On appropriate growth conditions, Bi and Ti atoms are supplied with a large Bi/Ti supply ratio. The deficiency of Bi composition in the growing film is prevented by this large ratio, and surplus Bi atoms which do not contribute to BIT growth are evaporated from the surface. BIT films are characterized by reflection high-energy electron diffraction, atomic force microscopy, and X-ray diffraction. The films grow in a two-dimensional layer-by-layer manner whose layer unit is the 1.6 nm thick BIT molecular layer. As a natural consequence of the method, the thickness of BIT films is determined simply by the total amount of Ti atom flux.

Deposition and processing of bismuth titanate thin films for direct wafer bonding

Bi 4Ti 3O 12 (BiT) thin films were deposited by chemical solution deposition (CSD) on different substrates including 3 inch Si and Pt/Si wafers. Annealing and chemical mechanical polishing (CMP) were studied in order to obtain thin films with a surface morphology suitable for direct wafer bonding. The influence of pH value of the polishing slurry, downward force and time of the CMP process on the film surface morphology were studied. Using BiT films deposited on Si and annealed at 550 °C, parameters for a CMP process which allowed to obtain reproducibly flat surfaces suitable for direct wafer bonding were established.

Preparation of Bismuth Titanate Films by Electron Cyclotron Resonance Plasma Sputtering-Chemical Vapor Deposition

Bismuth titanate (Bi 4 Ti 3 O 12 : BIT) thin films were prepared on the Pt courted MgO(100) substrate by electron cyclotron resonance plasma sputtering-chemical vapor deposition (ECR plasma sputtering-CVD). Bi 2 O 3 was used as a sputtering target and tetra-isopropoxy-titanium [Ti(i-C 3 H 7 O) 4 ] as a CVD source. The composition of films was controlled by changing RF power (P RF ) of Bi 2 O 3 target and Ti source temperature (T Ti ). The stoichiometric BIT film was prepared under the condition of P RF =500W, T Ti =63°C, deposition temperature of 650°C and deposition rate of 14 nm/min. Epitaxial relationships between the BIT film and the substrate were determined MgO(100)/Pt(100)/BIT(001) and MgO //Pt //BIT . The remanent polarization and coercive field measured by a Sawyer and Tower bridge circuit at 50 Hz were 1.12 μC/cm 2 and 46 kV/cm, respectively.

Metallorganic Cheamcal Vapor Deposition (Mocvd) of Titanium-Based Ferroelectric Thin Films

AbstractUsing hot-walled metallorganic chemical vapor deposition (MOCVD), thin fihns of lead zirconate titanate (PZT), lead titanate (PbTiO3 or PT) and bismuth titanate (Bi4Ti3O12 or BiT) were successfully prepared. For each material, titanium ethoxide (Ti(C2H5O)4) was used as the precursor for the titanium source, while lead bis-tetramethylheptadione (Pb(thd)2), zirconium tetrakistetramethylheptadione (Zr(thd)4) and triphenyl bismuth (Bi(C6H5)3) were used as sources for lead, zirconium and bismuth, respectively. Dense, specular and highly transparent films were obtained for all three materials. Deposition conditions are given for each of the materials as well as the properties of the resulting films as determined by XRD, SEM and UV-VIS-NIR spectrophotometry. Ferroelectric properties are also given for the PZT and BiT films; for PZT (%Zr = 41; %Ti = 9) annealed at 600 °C, the spontaneous polarization, Ps, was 23 μC/cm2 and the coercive field, Ec, was 65 kV/cm; for BiT annealed at 550 °C, the spontaneous polarization, Ps, was 27 μC/cm2 and the coercive field, Ec, was 240 kV/cm.