Bi4Ti3O12 Films Research Articles

Long-Range Lattice Matching between (100)/(010) Bismuth-Layered Perovskite Structure and (101) Rutile Structure

ABSTRACTa-/b-axis-oriented epitaxial bismuth layer-structured ferroelectric thin films were epitaxially grown on (101)-oriented oxide with rutile structure. The long-range lattice matching between the ferroelectric layer and the bottom rutile layer, particularly the number of rutile units facing one ferroelectric unit and the surface orientation, were discussed for (100)(010)Bi4Ti3O12//(101)TiO2 structure. Cross sectional transmission electron microscope analysis suggests that seven rutile units lie under one a-/b-axis-oriented Bi4Ti3O12 unit with lower misfit dislocation density comparing to eight rutile units by one Bi4Ti3O12 model. Based on this result, the surface orientation at the interface was simulated to give us an appropriate ion alignment model. The titanium layer in the (101)TiO2 structure is most likely to match with the oxygen layer in the a-/b-axis-oriented Bi4Ti3O12 film.

Properties of chemical solution deposited polycrystalline neodymium-modified Bi4Ti3O12

Thin films of neodymium-modified bismuth titanate Bi3.44Nd0.56Ti3O12 (BNT) were grown on Pt/TiO2/SiO2/Si substrates using chemical solution deposition method. The capacitors made by applying top Au electrodes on BNT films showed significantly improved values of the remanent polarization as compared to that using bismuth titanate Bi4Ti3O12 (BT) films. The 2P(r) value for the BNT capacitors was determined to be equal to 38 muC/cm(2) at an applied voltage of 24 V, whereas, for Bi4Ti3O12 ( BT) capacitors a value of 20 muC/cm(2) was measured at the same applied voltage. The maximum piezoelectric and pyroelectric coefficients of 22 pm/V and 112 muC/m(2) K respectively, were measured for the BNT thin films.

Characterization of Ferroelectric Property of C-Axis- and Non-C-Axis-Oriented Epitaxially Grown Bi2VO5.5 Thin Films

Epitaxial Bi2VO5.5 films with c-axis- and non-c-axis-orientations were grown by metalorganic chemical vapor deposition. (001)-, (114)- and (102)-oriented Bi2VO5.5 films were epitaxialy grown on (100), (110) and (111)SrTiO3 substrates, respectively. Electrical properties of the (001)- and (114)-oriented films on (100)SrRuO3∥(100)SrTiO3 and (110)SrRuO3∥(110)SrTiO3 substrates were compared. The dielectric constant (εr) of the (001)-oriented film was smaller than that of the (114)-oriented one, suggesting that εr along the c-axis is smaller than that along other axes. Leakage current density of the (001)-oriented film was smaller than that of the (114)-oriented one. These results were in good agreement with those of the crystallographically equivalent oriented films of SrBi2Ta2O9 and Bi4Ti3O12. Ferroelectricity and domain structure were observed for both films by Polarization-Electric field measurements and scanning nonlinear dielectric microscopy, respectively. However, the large difference of remanent polarization depending on the film orientation observed for SrBi2Ta2O9 and Bi4Ti3O12 films was not observed for Bi2VO5.5 films.

Interface control of Bi4Ti3O12 film growth on Si(100) by use of an ultrathin silicon oxynitride buffer layer

Electrical properties of Bi4Ti3O12 (BiT) films on Si(100) were improved due to insertion of silicon oxynitride (SiON) buffer layers with thicknesses of 1–2 nm. Capacitance–voltage measurements indicated that the improvement was largely attributable to better Si interface properties rather than to the difference of the BiT film quality. By means of x-ray photoelectron spectroscopy and high-resolution transmission microscopy, the Si interfaces of the specimens with and without the SiON buffer layers were investigated. Consequently, we found that a postannealing treatment at 680 °C inevitably resulted in nonuniform growth of Si oxide layers at the Si interface of the specimen without the SiON buffer layer, and that the layer thickness mounted to approximately 10 nm. In contrast, 1–2-nm-thick SiON buffer layers terminated the growth of the additional oxide layer of less than about 3 nm, and the resulting Si oxide layers were quite uniform.

Ferroelectric Bi4Ti3O12 Films on Si(100) with An Ultrathin Buffer Layer of Silicon Oxynitride: A Comparative Study Using X-Ray Photoelectron Spectroscopy

Using pulsed-laser deposition, ferroelectric Bi4Ti3O12 (BiT) films were grown on Si(100) with and without an ultrathin buffer layer of silicon oxynitride (SiON), and the interface states were investigated using X-ray photoelectron spectroscopy. For both as-grown specimens, the additional oxidation of the interface Si was observed, and their thicknesses were almost identical. Due to the postannealing at 700°C in an oxygen ambient, on the other hand, a large difference in the Si oxidation was observed between the two specimens. The BiT films on Si(100) without the SiON buffer layer failed in preventing the significant development of the interfacial Si oxidation. On the contrary, it was clarified that the 1.3-nm-thick SiON buffer layers suppressed the additional oxidation to less than 3.5 nm.

Effects of an ultrathin silicon oxynitride buffer layer on electrical properties of ferroelectric Bi4Ti3O12 thin films on p-Si(100) surfaces

Electrical properties of ferroelectric Bi4Ti3O12 (BiT) films on Si(100) using a 1 nm thick silicon oxynitride (SiON) buffer were investigated. The capacitance–voltage (C–V) characteristics of Au/BiT/SiON/Si(100) exhibited hysteresis loops with a memory window of 2 V due to the ferroelectricity, and did not show large carrier injections. The effects of the SiON buffer were demonstrated in current–voltage characteristics. In the reverse bias region, a leakage current density of the specimen without the SiON buffer was much larger than that of the specimen with the buffer. Apart from these electrical measurements, anomalous features appeared in C–V characteristics of the illuminated specimen, which were likely to be due to the ac response of the optically generated electrons in some trap states at the interface.

Epitaxial structure SrTiO3〈011〉 on Si〈001〉

An epitaxial structure, 〈011〉-oriented SrTiO3 film on Si〈001〉 substrate, is developed by inserting an epitaxial Ce0.12Zr0.88O2 buffer layer. Films are prepared by pulsed-laser deposition and evaluated by x-ray diffraction. Origin of this epitaxial growth is considered as a result of the ionic bonding at the interface of perovskite (SrTiO3) and fluorite (Ce0.12Zr0.88O2) structures. SrTiO3(011) surface of this epitaxial structure leads to a non-c-axis-oriented epitaxial growth of bismuth-layerstructured-ferroelectric Bi4Ti3O12 film on Si〈001〉. Unique surface morphologies and superior electrical properties are presented.

Film thickness dependence of ferroelectric properties of c-axis-oriented epitaxial Bi4Ti3O12 thin films prepared by metalorganic chemical vapor deposition

The film thickness dependence of the ferroelectricity of c-axis-oriented epitaxially grown Bi4Ti3O12 thin films was investigated. The c-axis-oriented Bi4Ti3O12 films with thicknesses from 35 to 400 nm were epitaxially grown on (100)CaRuO3//(100)SrTiO3 substrates by metalorganic chemical vapor deposition. The dielectric constant and the coercive field of the films were almost independent of the film thickness. These values were about 151 and 18.8 kV/cm, respectively. The spontaneous polarization Ps was almost constant at 4.0 μC/cm2 and was independent of the film thickness above 140 nm. This value is almost the same as the reported value for the single crystal along the c axis. However, the Ps value decreased with decreasing film thickness below 100 nm. Moreover, the remanent polarization continuously decreased as the film thickness decreased below 400 nm. As a result, the ferroelectric property of the epitaxially grown Bi4Ti3O12 films also depended on the film thickness and was similar to that of simple perovskite-structured ferroelectric thin films, such as Pb(Zr, Ti)O3 and (Pb, La)(Zr, Ti)O3 thin films.

Dielectric and ferroelectric properties of Bi2Ti2O7 thin films with (111) orientation

Transparent and crack-free Bi2Ti2O7 thin films with strong (111) orientation were successfully prepared on n-Si(100) by chemical solution deposition (CSD) using bismuth nitrate and titanium butoxide as starting materials. The growth procedure of the Bi2Ti2O7 thin films is discussed in this paper. The surface morphology of the Bi2Ti2O7 film was investigated by using Electric Force Microscope (EFM), and the crystallization of the films was studied by x-ray diffraction (XRD). Bismuth titanate thin film prepared on (100) silicon substrate showed strong (111) orientation. Its dielectric properties and the current-voltage (I-V) characteristics were measured. The dielectric constant of the Bi2Ti2O7 thin films vs. frequency in the temperature range of 100–800°C were studied. The dielectric constant and the dielectric loss for Bi2Ti2O7 are 118 and 0.07 respectively at 100 KHz. For the Bi2Ti2O7 films with 0.4 μrn in thickness annealed at 580°C for 40 minutes, their leakage current density is 4.06×10−7 A/cm2 at an voltage of 15 V. The ferroelectric phase transition has been observed distinctly and the Curie temperature was determined for the Bi7Ti2O7 ceramic films. Capacitance vs. temperature was measured from 27°800°C at 1KHz. 10KHz, 100KHz and 1MHz.

Microstructure and properties of Mn- or Nb-doped Bi4Ti3O12 thin films by chemical solution deposition

Bismuth titanate (Bi4Ti3O12) has a layered perovskite structure and exhibits strong anisotropic physical properties. Orientation control is important for the Bi4Ti3O12 thin films to apply to various electronic devices. In this study, we investigate the effects of Mn- or Nb-doping on the microstructure and properties of Bi4Ti3O12 thin films. Mn-doped or Nb-doped Bi4Ti3O12 thin films were fabricated by chemical solution deposition. The spin-coated films were fired at 800°C for lmin and successively annealed at 700°C for 1 hour in oxygen gas. The surface morphology of the deposited films varied with Mn- or Nb-doping. The surface of the non-doped Bi4Ti3O12 films consisted of plate-like grains. Mn- or Nb-doping suppressed grain growth. Non-doped Bi4Ti3O12 films showed c-axis orientation, while Mn- or Nb-doped films showed mixed c-axis and (117) orientation. Remanent polarization (Pr) and the coercive field (Ec) of the non-doped Bi4Ti3O12 films on Pt/Ti/SiO2/Si were 1.3μC/cm2 and, while 1 mol% Mn-doped Bi4Ti3O12 films exhibited a Pr of 2.2 μC/cm2. Mn-doping decreased leakage current. Nb-doping increased permittivity and remanent polarization, but also increased leakage current.

Ferroelectricity in epitaxial pulsed laser deposited bismuth-layered perovskite thin films of different crystallographic orientations

Epitaxial thin films of various bismuth-layered perovskites SrBi2Ta2O9, Bi4Ti3O12, BaBi4Ti4O15, and Ba2Bi4Ti5O18 were deposited by pulsed laser deposition onto epitaxial conducting LaNiO3 or SrRuO3 electrodes on single crystalline SrTiO3 substrates with different crystallographic orientations or on top of epitaxial buffer layers on (100) silicon. The conductive perovskite electrodes and the epitaxial ferroelectric films are strongly influenced by the nature of the substrate, and bismuth-layered perovskite ferroelectric films with mixed (100), (110)-and (001)-orientations as well as with uniform (001)-, (116)-and (103)-orientations have been obtained. Structure and morphology investigations performed by X-ray diffraction analysis, scanning probe microscopy, and transmission electron microscopy reveal the different epitaxial relationships between films and substrates. A clear correlation of the crystallographic orientation of the epitaxial films with their ferroelectric properties is illustrated by macroscopic and microscopic measurements of epitaxial bismuth-layered perovskite thin films of different crystallographic orientations.

Hetero-epitaxial Growth of (1, 0, m+1) One Axis-oriented Bismuth Layered Structured Ferroelectrics Thin Films Directly Crystallized by MOCVD

AbstractBismuth layered structured ferroelectrics (BLSF) thin films with different number of octahedron number (m-number) were prepared by MOCVD and directly crystallized on the substrates. Directly-crystallized SrBi2Ta2O9 (SBT) (m=2) films on a (111) Pt/Ti/SiO2/Si substrate were ascertained to have a strong (103) one-axis orientation by the X-ray reciprocal space mapping and to be hetero-epitaxially grown on the (111) Pt grains by the TEM observation. Moreover, directly crystallized Bi2VO5.5 (m=1) and Bi4Ti3O12 (m=3) films deposited on the same substrate showed (102) and (104) one-axis preferred orientations, respectively. These orientations are basically the equal ones with SBT (103) orientation because the tilting angle of c-axis from the substrate surface is also about 55°. Therefore, the direct crystallization is one of the important key techniques for orientation control of BLSF films. Moreover, the directly crystallized SBT film deposited on a (111) Ir/TiOx/SiO2/Si substrate at 570 °C by ECR-MOCVD exhibited (103) one-axis orientation, which also originated from the local epitaxial growth on (111)-oriented Ir grains. The remanent polarization (2Pr), and the coercive field (Ec) of this film were 16.1 μC/cm2 and 83 kV/cm at an applied electric field of 360kV/cm, respectively. This Pr value is about 88% of the expected value of (103)-oriented SBT film from both the Pr values of the (116) and (001)-oriented epitaxial films and detailed crystal analysis.

Preparation and Characterization of a- and b-Axis-Oriented Epitaxially Grown Bi4Ti3O12-Based Thin Films on Rutile-Type Oxides

Abstracta- and b-axis-oriented epitaxial Bi4Ti3O12 films were prepared on (101)TiO2 substrates (rutile) at 600°C by metalorganic chemical vapor deposition (MOCVD). Subsequently, the same films were similarly prepared on (101)-oriented conductive materials with the same rutile structure and similar lattice parameters as TiO2, such as RuO2 and IrO2. Their perfect epitaxial growth was confirmed by several X-ray diffraction measurements. RuO2 and IrO2 were deposited on not only structurally equivalent (101)RuO2//(101)TiO2 and (101)IrO2//(101)TiO2 structures, but were also successfully deposited on the corundum (012)Al2O3 and (110)Al2O3 single crystals by the MOCVD and rf-sputtering method, respectively. A well-saturated P-E hysteresis with a remanent polarization above 20 μC/cm2 was observed for a- and b-axis-oriented Bi4Ti3O12-based materials, (Bi4−xNdx)(Ti3−yVy)O12 (BNTV), that were epitaxially grown on rutile-rutile and rutile-corundum stacking structures. These results should enable broader application of the bismuth layer-structured ferroelectrics.

Ferroelectric property of epitaxial Bi4Ti3O12 films prepared by metalorganic chemical vapor deposition

The orientation dependence of the ferroelectricity of epitaxially grown Bi4Ti3O12 thin films was investigated. The (001)-, (118)-, and (104)-oriented Bi4Ti3O12 films were epitaxially grown on (100)cCaRuO3//(100)SrTiO3, (110)cSrRuO3//(110)SrTiO3, and (111)cSrRuO3//(111)SrTiO3 substrates, respectively, by metalorganic chemical vapor deposition. Ferroelectric property with different magnitude was observed for (001)- and (118)-oriented films but for (104)-oriented film due to its large leakage current. The remanent polarization and the coercive field were 1.5 mC/cm2 and 15 kV/cm, 16.5 νC/cm2 and 132 kV/cm for the (001)- and (118)-oriented thin films, respectively. The spontaneous polarization (PS) was 4.0 νC/cm2 and 27.0 νC/cm2 for (001)- and (118)-oriented films, respectively. This was different from the result of SrBi2Ta2O9 in that the ferroelectricity was not observed for (001)-oriented one, and was in good agreement with the estimation from the crystal structure. The estimated PS values along the c and a axes of Bi4Ti3O12 were 4.0 and 48.4 νC/cm2, respectively, and agreed well with the reported values for the single crystal. Furthermore, both films showed good fatigue endurance after 7.8 × 1010 switching cycles measured with 500 kHz rectangular pulses.

Ferroelectric Bi4Ti3O12 thin films on Pt-coated silicon by halide chemical vapor deposition

A halide chemical vapor deposition technique has been developed to grow highly c-axis oriented submicron Bi4Ti3O12 films onto Pt(200 nm)/Ti(20 nm)/SiO2(1200 nm)/Si(100) substrates. BiI3, TiI4, and oxygen were used as precursors. The total gas pressure was found to be the critical processing parameter to grow films with good crystalline and dielectric properties. The 300 nm thick Bi4Ti3O12 films fabricated at 700 °C and total gas pressure of 3.8 Torr exhibit the best dielectric performance: dielectric constant around 200 and loss tan δ∼0.018 at 100 kHz, remnant polarization of 5.3 μC/cm2, induced polarization of 14.9 μC/cm2 at 560 kV/cm, coercive field of 150 kV/cm, electrical tunability of 51% at 350 kV/cm, resistivity of 2×109 Ω cm, and leakage current as low as 3×10−5 A/cm2 at 100 kV/cm. The effect of weak reduction of the remnant polarization and coercive field has been observed in the temperature range from 300 to 77 K and ascribed to the in-plane orientation of the polar a axis in fabricated Bi4Ti3O12 films.

Orientation Control of Metalorganic Chemical Vapor Deposition-Bi4Ti3O12 Thin Film by Sequential Source Gas Supply Method

C-axis-oriented Bi4Ti3O12 thin films were prepared on (111)Pt/Ti/SiO2/Si substrates by metalorganic chemical vapor deposition (MOCVD) using a sequential source gas supply method taking account of the crystal structure of Bi4Ti3O12 along the c-axis. The X-ray diffraction intensities of Bi4Ti3O12(006) and Bi4Ti3O12(117) increased and decreased, respectively, by the sequential source gas supply method and the conventional continuous source gas supply method. The sequential source gas supply method started from Bi double pulses as effective for the preparation of the c-axis-oriented Bi4Ti3O12 film, and the supply corresponding to a half unit cell per sequence showed high c-axis orientation.

Epitaxial Growth of Bi4Ti3O12/CeO2/Ce0.12Zr0.88O2 and Bi4Ti3O12/SrTiO3/Ce0.12Zr0.88O2 Thin Films on Si and Its Application to Metal-Ferroelectric-Insulator-Semiconductor Diodes

Insulating buffer layers, CeO2/Ce0.12Zr0.88O2 and SrTiO3/Ce0.12Zr0.88O2 films, are grown on Si(001) substrates by the pulsed laser deposition technique, and ferroelectric Bi4Ti3O12 films are grown on these buffer layers by the molecular beam epitaxy technique. X-ray diffraction data show that the c-axes of the Bi4Ti3O12 films grown on CeO2/Ce0.12Zr0.88O2 and SrTiO3/Ce0.12Zr0.88O2 buffer layers are normal and inclined 45° to the buffer layer surface, respectively. Memory windows appearing in capacitance and voltage characteristics of diode structures with Pt top electrodes are 1.2 V for a Pt/Bi4Ti3O12/CeO2/Ce0.12Zr0.88O2/Si diode, and 3.5 V for a Pt/Bi4Ti3O12/SrTiO3/Ce0.12Zr0.88O2/Si diode. This difference in magnitude may be due to the different growth orientation and strongly anisotropic polarization of Bi4Ti3O12.

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.

New low temperature preparation of ferroelectric Bi4Ti3O12 thin films by MOCVD method

Pt/Bi4Ti3O12(001)/Bi2SiO5(100)/Si(100) structures have been fabricated by the metal-organic chemical vapor deposition (MOCVD) at 500°C. Bi2SiO5 film is used as a buffer layer to grow ferroelectric Bi4Ti3O12 films because of its relatively high dielectric constant (εr=30). The memory window has a C-V characteristic of about 0.8 V, and the retention time estimated by the zero-bias capacitance for the Pt/100-nm-Bi4Ti3O12/30-nm-Bi2SiO5/Si/Al structure is more than 11 days.

Growth Style of Bi4Ti3O12 Thin Films on CeO2/Ce0.12Zr0.88O2 Buffered Si Substrates

Epitaxial Bi4Ti3O12 thin films are grown on CeO2/Ce0.12Zr0.88O2 buffered Si(100) substrates. The buffer consists of 40 Å Ce0.12Zr0.88O2 and 80 Å CeO2 layers which are grown on Si(100) substrates by an e-beam evaporation method. Bi4Ti3O12 films are grown by the molecular beam epitaxy method using the Bi self-limiting function. Epitaxial growth of these layers on Si is confirmed by X-ray diffraction analyses. Growth style of Bi4Ti3O12 films is examined in detail by observations of reflection high-energy electron diffraction and atomic force microscopy, and the growth mechanism is discussed.