BLT Thin Films Research Articles

Effect of Thickness on Ferroelectric Properties of Bi3.25La0.75Ti3O12 Thin Films

The pursuit of low-power/low-voltage operation in devices has prompted a keen interest in the mesoscale effects within ferroelectric thin films. The downsizing of ferroelectrics can significantly influence performance; for instance, the remanent polarization and coercive field are susceptible to alterations based on thickness. In this study, randomly oriented Bi3.25La0.75Ti3O12 thin films were fabricated on Pt/Ti/SiO2/Si substrates using the sol–gel method, and SEM observations revealed rod-like grains in all thin films. The investigation delved into the correlation between dielectric and ferroelectric properties with thin film thickness. The thin film exhibited an increased remanent polarization and a reduced coercive electric field. Additionally, the ferroelectric domain structure was scrutinized through PFM, and the resistor properties of the BLT4 thin film were studied, which shows the potential of BLT thin films in non-volatile memory and memristor.

Effect of grain size on ferroelectric and dielectric properties of Bi3.25La0.75Ti3O12 thin films prepared by rf-magnetron sputtering

[Formula: see text][Formula: see text]Ti3[Formula: see text](BLT) thin films are promising materials used in non-volatile memories. In this work, BLT films were deposited on Pt(111)/Ti/SiO2/Si substrates by rf-magnetron sputtering method followed by annealing treatments. The microstructures of BLT thin films were investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). With the increase in annealing temperature, the grain size increased significantly and the preferred crystalline orientation changed. A well-saturated hysteresis loop with a superior remnant polarization of 15.4 [Formula: see text]C/cm2 was obtained for BLT thin films annealed at 700∘C. The results show that the dielectric constant decreased with the increase in grain sizes.

Structural and electrical study of BLT thin film deposited on different substrates by electron gun evaporation

Purpose The purpose of this study was to deposit Bi4Ti3O12 films by electron gun evaporation technique starting from Bi3.25La0.75Ti3O12 as a target without annealing. The films have been deposited on Si(100), on thin film buffer layer of Pt and glass substrates. X-ray diffraction (XRD) was used to analyze structure of the films, which possesses a good structure with (0010) preferred orientation. Electronic behavior of the samples has been studied. Design/methodology/approach The dependence of both the structure and quality of the BLT thin films on different substrates is studied using XRD. The electrical characteristics were determined using capacitance–voltage (C–V) and current–voltage (I–V) measurements at the frequency of 1 MHz. Optical properties of the grown films deposited on glass substrates were characterized by optical transmittance measurements (UV-Vis). Findings The XRD analysis approved the crystallographer structure of the prepared Bi4Ti3O12 films. The optical properties of deposited film (transmittance and the band gap value) are extracted by UV-Vis spectrum. Originality/value High crystalline quality Bi4Ti3O12 films have been obtained using different substrates at room temperature by means of electron gun deposition. The electrical and ferroelectric properties of thin films were studied using I–V and C–V measurements. The band gap has been found to be about 3.62 eV for the studied film deposited on glass substrate. Electron beam evaporation technique is the most interesting methods, once considering many advantages; such as its stability, reproducibility, high deposition rate and the compositions of the films are controlled.

Fatigue‐Free Aurivillius Phase Ferroelectric Thin Films with Ultrahigh Energy Storage Performance

AbstractDielectric capacitors have become a key enabling technology for electronics and electrical systems. Although great strides have been made in the development of ferroelectric ceramic and thin films for capacitors, much less attention has been given to preventing polarization fatigue, while improving the energy density, of ferroelectrics. Here superior capacitive properties and outstanding stability are reported over 107 charge/discharge cycles and a wide temperature range of −60 to 200 °C of ferroelectric Aurivillius phase Bi3.25La0.75Ti3O12‐BiFeO3 (BLT‐BFO), which represents one of the best capacitive performances recorded for the ferroelectric materials. The modification of BLT thin films with BFO overcomes the constraints of ferroelectric Aurivillius compounds and presents an unprecedented combination of the ideal features including improved polarization, reduced ferroelectric hysteresis, and lowered leakage current for high‐energy‐density capacitors. Given the lead‐free and fatigue‐free nature of this Aurivillius phase ferroelectric, this work unveils a new approach towards high‐performance eco‐friendly ferroelectric materials for electrical energy storage applications.

INVESTIGATION ON STRUCTURAL AND FERROELECTRIC PROPERTIES OF Bi3.25La0.75Ti3O12 THIN FILMS

Lanthanum doped bismuth titanate (Bi3.25La0.75Ti3O12 abbreviated as BLT) has been known as one of typical materials for Bi-layered perovskite structure which possess several unique properties such as good fatigue with metal electrode and stable remanent polarization, hence it has potential applications in ferroelectric random access memory. In this work, the BLT thin films were fabricated on Pt/TiO2/SiO2/Si substrates by using a solution process, and then their features including crystal structure, surface morphology, and electrical properties were characterized by using X-ray diffraction system (XRD), scanning electron microscopy (SEM), and electrical measurement system (Radiant Precision LC 10), respectively. The obtained results point out that the BLT thin film annealed at 725 oC is mostly optimum from a viewpoint of film quality and ferroelectricity. In particular, the optimum BLT thin film having a thickness of 200 nm does not contain any cracks on the sample surface, and the grain size is closed to 400 nm from SEM observation. XRD patterns imply that the BLT thin film had stoichiometric structure with preferred orientations of (117) and (006), when annealed at temperatures higher than 725 oC. In addition, we found the influence of La (0.75) dopping on c-axis-oriented growth of BLT thin films is clear from the structural analysis. The remanent polarization of optimum BLT thin film is approximately 10 μC/cm2, but the ferroelectric hysteresis loops are not saturated at low applied voltages.

Structural and optical properties of Fe doped bismuth titanate thin film deposited by RF sputtering

Achieving wide band gap tunability in complex oxide materials is highly desirable for the development of current materials research and photovoltaic device applications. To tune the band gap of complex oxides, ferroelectric iron doped BLT thin films (BLFT) were fabricated on SrTiO3 substrates by RF sputtering at various growth temperatures. The structural and optical properties were analyzed by X-ray diffraction, scanning electron microscopy, and ultraviolet–visible absorption spectroscopy. As the growth temperature was increased from 500 to 800 °C, the crystallinity of the BLFT thin films was enhanced by the smooth surfaces, whereas the optical band gap was increased from 2.69 to 2.75 eV. On the other hand, the optimized deposition temperature of the BLFT film was 750 °C because of the almost complete crystalline phase. This result was found to be sensitive to the deposition temperature when the BLFT epitaxial thin-films were prepared by RF sputtering for the band-gap narrowing.

Influence of Oxygen to Argon Ratio on the Optical Band Gap of Bi3.25La0.75Fe1Ti2O12 Thin Films Deposited by RF Sputtering.

Achieving wide band gap tunability in ferroelectric perovskite oxides is desirable for the development of photovoltaic device applications and solar cells. To tune the wide band gap of perovskite oxides, ferroelectric iron doped BLT thin films (BLFT) on SrTiO3 substrates were fabricated by RF sputtering with simple control of the oxygen content. The structural and optical properties were analyzed by X-ray diffraction, scanning electron microscopy and ultraviolet-visible absorption spectroscopy. As the oxygen content in the mixed Ar+O2 atmosphere was increased from 0% to 50%, the optical band gap of the thin films were decreased from 2.8 eV to 2.64 eV. The BLFT film deposited at an Ar/O2 ratio of 1/1 exhibited a significantly lower optical band gap than the other samples. This simple sputtering approach to controlling the band gap with a simple method can provide a new candidate tool for manipulating optoelectronics devices.

Effect of Annealing Temperature on the Properties of Sputtered Bi3.25La0.75Ti3O12Thin Films

<TEX>$Bi_{3.25}La_{0.75}Ti_3O_{12}$</TEX>(BLT) thin films were prepared on the Pt(150 nm)/Ti(50 nm)/<TEX>$SiO_2$</TEX>/Si substrate using the rf magnetron sputtering method. The BLT thin films were annealed at temperatures ranging from <TEX>$600^{\circ}C$</TEX> to <TEX>$750^{\circ}C$</TEX> using the rapid thermal annealing. The structure and surface morphology of the thin films were characterized by x-ray diffraction and field emission scanning electron microscopy. The hysteresis loop of the BLT thin films showed that the remanent polarization (2Pr) of the film annealed at <TEX>$700^{\circ}C$</TEX> was 10.92 <TEX>${\mu}C/cm^2$</TEX>. The fatigue characteristic of the BLT thin film annealed at <TEX>$700^{\circ}C$</TEX> was shown change polarization up to <TEX>$1.2{\times}10^9$</TEX> switching cycles. We confirmed the excellent remnant polarization (Pr) and fatigue properties compared with other fabrication methods and suggested a good method for BLT thin films fabrications.

Effects of Sputtering Conditions on Microstructure and Ferroelectric Properties of RF-Sputtered Bi&lt;sub&gt;3.25&lt;/sub&gt;La&lt;sub&gt;0.75&lt;/sub&gt;Ti&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt; Thin Films

The Bi3.25La0.75O12 films were prepared on the Pt/TiO2/SiO2/Si substrate using RF-magnetron sputtering method. The effects of sputtering conditions, including sputtering pressure, oxygen partial pressure and substrate temperature on microstructure, ferroelectric properties were investigated. When deposited at low substrate temperature (room temperature), the BLT thin film exhibits preferentially c-axis oriented. With the increasing of substrate temperature, the (117) diffraction peak become stronger, indicating the changing of the microstructure in the BLT films. The BLT film samples deposited at 4.5-6Pa exhibit desired bismuth-layered structure, with no any preferred orientation. However, when BLT films were deposited above 5 Pa, some weak pyrochlore diffraction peaks appear in the X-ray diffraction pattern. The Pr is a maximum for the BLT film deposited at 5 Pa with O2/Ar ratio 1/5 and decreases with both decreasing and increasing deposition pressure.

EFFECT OF ANNEALING TREATMENT ON FERROELECTRIC AND ELECTRICAL CHARACTERISTICS OF Bi4-XLaXTi3O12 THIN FILMS ON ITO/GLASS SUBSTRATE

In this study, the effects of annealing temperatures on microstructure and growth properties of Bi 3.9 La 0.1 Ti 3 O 12 (BLT) thin films on ITO substrate under conventional furnace annealing processing as a function of annealing temperatures were developed. The ferroelectric and physical properties of BLT thin films were investigated. The maximum capacitance and minimum leakage current density of BLT thin films under conventional furnace annealing processing were 4.5 nF and 10-6 A/cm2, respectively. In addition, the X-ray diffraction (XRD) patterns and grain size of SEM morphology exhibited the growth and nucleation properties of BLT thin films increased with the increase of conventional furnace annealing temperatures. The results demonstrated the correlation between the electrical, physical properties and growth and nucleation features of BLT thin films.

Fabrication of Lead-Free (Na0.5K0.5)NbO3–BaZrO3–(Bi0.5Li0.5)TiO3 Ferroelectric Thin Films on (111)Pt/Ti/SiO2/(100)Si Substrate by Pulsed Laser Deposition

0.92(Na0.5K0.5)NbO3–0.06BaZrO3–0.02(Bi0.5Li0.5)TiO3 (NKN–BZ–BLT) thin films were fabricated on a (111)Pt/Ti/SiO2/(100)Si substrate by pulsed laser deposition (PLD). The quality of the NKN–BZ–BLT thin films was improved by introducing an NKN–BZ–BLT buffer layer, which was deposited at a high substrate temperature of 850 °C. An X-ray diffraction pattern (XRD) showed that the 100pt preferential-oriented NKN–BZ–BLT thin films [on the basis of a pseudotetragonal perovskite (pt) structure] were obtained without and with buffer layers. From the rocking curves, the crystallinity of the film was slightly improved by introducing the buffer layer. SEM images showed that the grain size was increased and cracks were eliminated by introducing the buffer layer. The dielectric constant of the films was comparable to that of NKN–BZ–BLT ceramics. The P–E hysteresis loops showed that the remanent polarization and spontaneous polarization of the films increased with the increase in the thickness of the buffer layer. The NKN–BZ–BLT film with a 0.8-µm-thick buffer layer exhibited a remanent polarization of 6.7 µC/cm2 and a spontaneous polarization of 28.0 µC/cm2. This improvement of the ferroelectric properties would be due to the relaxation of the tensile stress in the presence of the buffer layer.

Fabrication and Characterization of Bismuth Lanthanum Titanate (Bi3.25La0.75Ti3O12) Thin Films for FeRAM Devices

Thin films of Lanthanum doped Bismuth Titanate (Bi3.25La0.75Ti3O12) (BLT) have been prepared and characterized for their applications in ferroelectric random access memory (FeRAM) devices. The films have been characterized via X-ray diffraction (XRD), atomic force microscopy (AFM), capacitance-voltage (C-V), ferroelectric (P-E) and current-voltage (J-V) characteristics. The effect of thickness on these characteristics has also been investigated. Thickness variation shows significant effect on the morphology and ferroelectric properties of the films. The nature of the J-V characteristics was observed to be independent of the thickness of the films. Three distinct regions, i.e., ohmic, trap filled limited and Child's law were explicitly observed in J-V characteristics. The J-V behavior of BLT thin films was found to follow the Lampert's theory of space charge limited conduction in an insulator with traps.

Effect of zirconium doping on ferroelectric properties and leakage‐current mechanism in Bi3.25La0.75Ti3O12 (BLT) thin films

AbstractLeakage‐current mechanism in BLT films after Zr doping was studied as a function of Zr doping. The ferroelectric properties improved with doping. With appropriate Zr‐doping concentration, such as x = 0.025, the remnant polarization (2Pr) increases by 36% and saturates afterwards. Different mechanisms for leakage‐current conduction were fitted with the observed current density–voltage (J–V) plots, and interplay between the electrode and bulk‐limited conduction was sought. Depending upon the field, the importance of a particular charge‐conduction mechanism (under the coexistance of multiple mechanisms) was studied. Three different regions, i.e., Ohmic, trap‐filled limited, Child's law were explicitly observed in J–V characteristics. Trap density was found to vary with Zr‐doping concentration. The measured VTFL was observed to reduce with increase in the temperature. The J–V behavior of BLT and Zr‐doped BLT thin films were found to follow Lampert's theory of space‐charge‐limited conduction in an insulator with traps. The observed increase in low‐field insulation was correlated with the space‐charge conduction taking into account the modification of dielectric constant with the Zr doping. Like in BLT thin films, the charge transport in Zr‐doped BLT thin films is well controlled by the space‐charge‐limited mechanism.

Ferroelectric Properties of (Na0.5K0.5)NbO3–BaZrO3–(Bi0.5Li0.5)TiO3 Thin Films Deposited on Pt/(001)MgO Substrate by Pulsed Laser Deposition

0.92(Na0.5K0.5)NbO3–0.06BaZrO3–0.02(Bi0.5Li0.5)TiO3 (NKN–BZ–BLT) thin films with a thickness of about 2.0 µm were fabricated on a Pt/(001)MgO substrate by pulsed laser deposition (PLD). The X-ray diffraction pattern (XRD) showed that the NKN–BZ–BLT thin films that were oriented in both the 001pt and the 101pt directions [determined by pseudo-tetragonal perovskite (pt)] were grown on the Pt/(001)MgO substrate. A rocking curve measurement revealed that the fluctuations of the crystalline orientation of the 001pt- and 101pt-oriented NKN–BZ–BLT grains are very small. The reciprocal space map and pole figure showed that the 001pt-oriented NKN–BZ–BLT grains epitaxially grew on the Pt/(001)MgO substrate. On the other hand, the 010pt axis (existing in the in-plane) of the 101pt-oriented NKN–BZ–BLT grains rotated by 45° for the 100 axis (existing in the in-plane) of the (001)MgO substrate. The dielectric constant εr and the dielectric loss tan δ of the NKN–BZ–BLT thin films were 682 and 0.076 at 1 kHz, respectively. The P–E hysteresis loops of the NKN–BZ–BLT thin films showed clear ferroelectricity. The remanent polarization Pr and coercive electric field Ec were 24.5 µC/cm2 and 29.3 kV/cm, respectively. The NKN–BZ–BLT thin films exhibited larger Pr values than NKN–BZ–BLT ceramics.

Effect of Bi2O3 seed layer on crystalline orientation and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films prepared by rf-magnetron sputtering method

Bi 3.25 La 0.75 Ti 3 O 12 (BLT) thin films with Bi2O3 seed layers were prepared on Pt(111)/Ti/SiO2/Si substrate by rf-magnetron sputtering method. The effect of annealing procedure, thickness, and position of Bi2O3 seed layer on the structural and electrical properties of BLT thin film was investigated. When using a two-step annealing procedure, the x-ray diffraction pattern indicated that BLT thin film with a Bi2O3 seed layer showed c-axis preference orientation. Compared with Pt/BLT/Bi2O3/Pt structure film, the Pt/Bi2O3/BLT/Bi2O3/Pt structure film showed higher oriented to c direction. The ferroelectric property of BLT thin film improved remarkably with the incorporation of Bi2O3 seed layer. Under the applied voltage of 14 V, the remnant polarization and coercive voltage of the Bi2O3/BLT/Bi2O3 capacitor with 18-nm-thick Bi2O3 seed layer were 45.2 μC/cm2 and 9.2 V, respectively. Moreover, the Pt/Bi2O3/BLT/Bi2O3/Pt film with 18-nm-thick Bi2O3 seed layers exhibited good fatigue endurance even after 6×109 switching cycles, which was attributed to the double-sided Bi2O3 layer. They decreased the grain boundary density by enhancing the grain size of BLT film and enhanced the c-axis preferential orientation. Compared with BLT film deposited directly on Pt, double-sided Bi2O3 layer would greatly enhance the leakage current resistance of the BLT film.

Dip-coated La 2Ti 2O 7 as a buffer layer for growth of Bi 3.25La 0.75Ti 3O 12 films with enhanced (0 1 1) orientation

Thin-films of La 2Ti 2O 7 were obtained by dip-coating process using a precursor salt in nitric acid solution. The effects of solution concentration, withdrawal speed, post-annealing duration and temperature were investigated both on grain size and orientation of the La 2Ti 2O 7 thin layers. In addition, a target with the required stoichiometry for PVD deposition of La-substituted Bi 4Ti 3O 12 (BLT) was successfully sintered by spark plasma sintering (SPS) at 750 °C. Finally (0 1 1)-oriented BLT ferroelectric films have been grown by RF sputtering on (1 1 0)-oriented La 2Ti 2O 7 polycrystalline thin-film. A preferential orientation of BLT thin films has been obtained after annealing at a temperature lower than 650 °C.

Polishing damages to electrical properties of BLT thin-film capacitors fabricated by damascene process

Polishing pressure in the damascene process for BLT thin films improves the removal rate and surface roughness; however, the electrical properties of BLT capacitors fabricated by the damascene process with high pressure worsened. Therefore, low pressure was suitable for BLT capacitors fabricated by the damascene process when considering the electrical properties.

Structure-related optical properties of Bi 4 − xLa xTi 3O 12 thin films grown on Pt/Ti/SiO 2/Si substrate

Polycrystalline Bi 4 − x La x Ti 3O 12 (BLT, x = 0, 0.5, 0.75, and 1.0) thin films have been grown on Pt/Ti/SiO 2/Si substrate by metalorganic decomposition method at 650 °C. The studies of X-ray diffraction patterns and atomic force microscopy images indicate that the crystallization of BLT films was affected by the La substituting concentration. The refractive index and extinction coefficient of BLT thin films were determined by fitting the infrared spectroscopic ellipsometric data using a classical dielectric function formula. In the wavelength range of 2.5–8.0 μm, as the La concentration increases, the refractive index decreases. The refractive index of BLT thin films in the wavelength range of 400–1100 nm, derived from the reflectance spectra, decreases with increasing La concentration. The La concentration dependence of optical constants for BLT films was investigated.

Preparation and photocatalytic property of pyrochlore Bi 2Ti 2O 7 and (Bi, La) 2Ti 2O 7 films

Dopant-free Bi 2Ti 2O 7 thin films with pyrochlore structure and La-doped bismuth titanate thin films have been fabricated by means of chemical solution decomposition, and characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy and UV–Vis spectrophotometry in this study. Their photocatalytic activities have been evaluated by photodegrading methyl orange solution, and the optimum processing parameters for the highest photocatalytic activity have been found. Moreover, the effects of La-doping on the phase transformation and the photocalalytic activity have been studied. It has been deduced from the experiment result that substituted La 3+ ions can act as a stabilizer of Aurivillus phase BLT and a grain–growth inhibitor in BLT thin films.

Improvement of dielectric properties of BLT thin films deposited by magnetron sputtering

Well crystallized BLT thin films were deposited by RF magnetron sputtering using a target of Aurivillius phase Bi3.25La0.75Ti3O12(BLT0,75), elaborated in our institute. RF sputtering experiments were performed at room temperature with an argon/oxygen mixture, in a pressure range of 20-50 mTorr. Optimisation of the plasma parameters, namely deposition pressure, RF power and oxygen content in the gas phase, allows obtaining BLT films with a chemical composition close to Bi3.25La0.75Ti3O12. After ex-situ annealing under oxygen atmosphere at 650°C, BLT films deposited on Pt/TiO2/SiO2/Si (multilayer) substrates exhibit well defined rod-like grains morphology. A two step deposition process appeared to be necessary in order to reach satisfying dielectric properties. The effect of the plasma parameters on the chemical composition and electrical properties are presented and discussed.