TiO3 Ferroelectric Thin Films Research Articles

Robust Tunability and Newly Emerged Q Resonance of Ba0.8Sr0.2TiO3-Based Microwave Capacitors under Gamma Irradiations.

The complex resonance of dielectric quality factor Q, combined with a capacitance tunability n higher than 3:1 without any dispersion, was achieved in the voltage-tunable interdigital capacitors (IDCs) based on epitaxial Ba0.8Sr0.2TiO3 ferroelectric thin films across the microwave L (1-2 GHz), S (2-4 GHz), and C (4-8 GHz) bands at room temperature. The resonant Q and n features were driven by the microwave responses of the ferroelectric nanodomains engineered in the films. To promote their application in space radiation environments, the evolutions of Q and n both as functions of frequency f (1-8 GHz) and applied electric field E (0-240 kV/cm) were systematically investigated under a series of gamma-ray irradiations up to 100 kGy. The robust capacitance tunability was accompanied by the emergence of an additional Q resonance at 2.3 GHz in most post-irradiated devices, which is ascribed to extra polar nanoregions of expanded surface lattices associated with oxygen vacancies induced by irradiations.

Multiple factors of regulation for transient negative capacitance in PbZr(1−x)Ti(x)O3 ferroelectric thin films

The negative capacitance (NC) of ferroelectric (FE) materials can effectively break the ‘Boltzmann tyranny’ and drive the continuation scaling of Moore’s law. In this work, to find a novel way for amplifying the transient NC, a series network of external resistors and PbZr(1−x)Ti(x)O3 (PZT) FE capacitors was constructed. Uniform modeling and simulation were performed using Kirchhoff’s current law, electrostatics equations, and Landau–Khalatnikov equations. The derived results revealed that the mismatch of switching rate between free charge and polarization during FE domain switching is responsible for the transient NC generation. Some interesting results were obtained for the regulation of the transient NC by various factors such as the strain between the FE film and substrate, the viscosity coefficient, the ratio of Ti components, the external resistance magnitude, and the operating temperature. This work provides considerable insight into the control of FE transient NC, and offers guidance for obtaining larger and longer transient NC in the widely used PZT thin films.

High-performance self-powered ultraviolet photodetector based on bismuth layered perovskite Bi3.15Nd0.85Ti3O12 ferroelectric thin films

The photovoltaic effect of ferroelectric materials has been widely concerned. With the development of wearable technology and the requirements of device miniaturization, self-powered devices have been studying by more and more researchers. The photovoltaic effect of ferroelectric materials provides a possibility for self-powered photodetectors. Here, we have investigated the photovoltaic effect and the self-powered ultraviolet photodetecting properties in Pt/Bi3.15Nd0.85Ti3O12 (BNT)/Pt heterostructure. The Bi3.15Nd0.85Ti3O12 (BNT) thin films were prepared by sol–gel method on Pt/Ti/SiO2/Si substrate. XRD pattern and P-E loops shows that BNT films are polycrystalline with typical ferroelectric hysteresis. The BNT films shows obvious photovoltaic characteristics, with an open-circuit voltage VOC of 0.67 ± 0.04 V and a short-circuit current density JSC of 2.27 ± 0.08 μA/cm2 at the wavelength of 365 nm of a power of 77.50 mW/cm2. The zero bias transient photocurrent response under light ON and OFF states confirms the photodetecting ability of the heterolayered structure. The responsivity and the detection rate of the self-powered ultraviolet photodetector were calculated to be 0.047 ± 0.003 mA/W and (1.47 ± 0.04) × 109 Jones, respectively. The reversal photocurrent response mechanism of the Pt/BNT/Pt heterostructure has been clarified based on the driving internal electric field for the separation and transport of the photo-excited electron-hole pairs. The JSC dependance on the polarization direction reveals the electrical manipulation nature of the BNT based self-powered photodetectors, providing a novel route for the design of photodetectors based on ferroelectric oxides.

Oxygen octahedral tilt ordering in (Na1/2Bi1/2)TiO3 ferroelectric thin films

Oxygen octahedra tilt (OOT) transition is the most common type of distortion in inorganic ABO3 compounds with a perovskite crystal structure. The importance of OOT transitions is underlined by accompanying changes in the B-O and A-O bonding environments, which consequently affects the electronic states and hence influences electrical, magnetic, and superconducting properties of many perovskite compounds. In recent years, controlled manipulation of the OOT order in perovskite thin film ferroelectrics has been attempted through heteroepitaxial strain engineering. The current study demonstrates an alternative approach whereby OOT ordering in a 200 nm thick polycrystalline thin film of (Na1/2Bi1/2)TiO3 (NBT) Pb-free ferroelectric is induced by applying electric-field along the 111 octahedral tilt axis, which is furthermore enabled by a strong (111) crystallographic texture normal to the film surface. In situ x-ray diffraction reveals that electric-field-induced OOT ordering proceeds through nucleation and rapid growth of domains with ordered a−a−a− tilting, followed by an increase in the tilt angle within the ordered domains.

Observation of large enhancement in energy-storage properties of lead-free polycrystalline 0.5BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3 ferroelectric thin films

Lead-free 0.5BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3 (BZT–BCT) thin films were deposited on Pt(1 1 1)/TiO2/SiO2/Si substrates by pulsed laser deposition. The optimal ferroelectric response with a high saturation polarization Ps ~ 110 µC cm−2, remnant polarization Pr ~ 32.5 µC cm−2 along with a coercive field of 0.18 MV cm−1 was observed from the P–E hysteresis loops under an applied frequency of 1 kHz. A giant recoverable energy-storage density of 93.52 J cm−3 at an applied electric field ~3.47 MV cm−1 was observed. The optimized BZT–BCT thin films exhibited a high dielectric constant with a low dielectric loss at room temperature like their bulk counterpart (high dielectric constant (Ɛ ~ 121 81)) and low dielectric loss (tan δ ~ 0.01–0.03) properties. OPP-PFM images revealed switchable ferroelectric distinct polarization contrasts on the application of a ±12 V dc voltage on the conductive tip at room temperature. Observed enhanced dielectric, ferroelectric and energy density properties of BZT–BCT thin films are useful for next generation electrical energy storage applications.

Crystallization evolution and ferroelectric behavior of Bi3.25La0.75Ti3O12-based thin films prepared by rf-magnetron sputtering

Ta-doped Bi3.25La0.75Ti3O12 (BLTT) ferroelectric thin films were prepared via rf-magnetron sputtering with subsequent annealing treatments. The crystallization evolution and ferroelectric behavior of BLTT thin films were studied using in situ high temperature X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and piezoresponse force microscopy (PFM). With the increase of annealing temperatures, the thin films exhibited a preferred 〈117〉 crystalline orientation first and then weakly crystallizations to c-axis were obtained at high temperatures. SEM analysis reveals that the grain growth might be performed by melting and combining of particles in surface layer of original grains. PFM phase images reveal that less domain switching could be induced for BLTT films with larger grains.

Zr-substituted Ba0.6Sr0.4TiO3 ferroelectric thin films grown by pulsed laser deposition (PLD) at different laser fluence

ABSTRACTZr-substituted Ba0.6Sr0.4TiO3 ((Ba0.6Sr0.4)(Ti0.8 Zr0.2)O3) - BSZT) thin films are deposited by using Pulsed Laser Deposition method with different fluence at optimized conditions on amorphous fused silica substrates. The X-ray diffraction (XRD) pattern of the films confirms the formation of cubic phase. Raman studies are performed to understand vibrational modes present in the films. The optical band gap of deposited BSZT thin films were calculated from the transmittance measurements. Microwave dielectric properties of the thin films deposited at 2 J/cm2 were measured. These results show BSZT thin films are useful in tunable microwave devices.

Influence of annealing temperature on structural and optical properties of sol-gel derived Ba0.9Gd0.1TiO3 thin films for optoelectronics

In this study, Ba0.9Gd0.1TiO3 (BGT) ferroelectric thin films have been fabricated on SiO2/Si and fused silica substrates using sol-gel technique. The effect of annealing temperature on the structural, surface morphology and optical properties of BGT thin films has been profoundly studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and ultraviolet–visible spectrophotometry (UV–Vis). XRD patterns verify the existence of the perovskite structure with a tetragonal phase in the films annealed at 700 °C and above. The microstrain and dislocation density are found to decrease as the annealing temperature increases indicating that the film's quality is improved. The inter-correlation between average grain size and surface roughness with the annealing temperature is extracted from the AFM results to conclude that the film morphologies are strongly dependent on annealing temperature. In addition, the transmittance spectra illustrate that all the BGT samples exhibit high transmittance in the wavelength region ranging from 400 nm to 800 nm. The optical band gap of the BGT thin films has been evaluated from the transmittance spectra and it shows a decreasing trend as the annealing temperature increases, which is mainly attributed to the quantum size effect.

Sub-kT/q subthreshold slope p-metal-oxide-semiconductor field-effect transistors with single-grained Pb(Zr,Ti)O3 featuring a highly reliable negative capacitance

A reliable on/off switching with an sub-kT/q subthreshold slope (38 mV/dec at room temperature) is experimentally demonstrated with using selectively nucleated laterally crystallized single-grain Pb(Zr,Ti)O3 (PZT) ferroelectric and ZrTiO4 paraelectric thin-film. The combination of ferroelectric and paraelectric thin-film is enabled to form a negative capacitance (NC) at the weak inversion region. However, the PZT grain-boundary easily degrades the NC properties after switching the on/off more than 108 times. It is found that the polarization of PZT is diminished from the path of grain-boundary. Here, we effectively suppress the degradation of NC MOS-FET which did not showed any fatigue even after 108 on/off switching. At the request of the authors this article is retracted due to duplication of figures and significant overlap with other publications by the authors and because of concerns about the accuracy of the description of the devices and materials from which the reported results were obtained. The authors recognize that these represent serious errors and sincerely apologize for any inconvenience they may have caused. The article is retracted from the scientific record with effect from 17 February 2017.

A miniaturized wireless accelerometer with micromachined piezoelectric sensing element

A miniaturized wireless accelerometer using a piezoelectric micro sensing element is designed, theoretically analyzed, and demonstrated for remote mechanical vibration monitoring. The micro sensing element comprises a micromachined monolithic silicon seismic mass and multiple silicon beams coated with in-plane polarized (Pb, La)(Zr, Ti)O3 (PLZT) ferroelectric thin film. A low profile compact package is obtained for accommodating all the micro sensing element, charge amplifier and signal processing circuits, radio frequency module, and battery. The electrical output linearly proportional to the vibration acceleration is amplified, digitalized and transmitted to a remote base unit through radio frequency communication. The effects of the device package on the dynamic response of the wireless piezoelectric accelerometer including shielding of ambient interference are investigated.

Insight into Metalized Interfaces in Nano Devices by Surface Analytical Techniques.

Connections between metals and heterogeneous solid state materials form buried interfaces. These ubiquitous structures play an essential role in determining the performances of many nano- and microdevices. However, the information about the chemistry, structure, and properties of these real interfaces is intrinsically difficult to extract by traditional techniques. Therefore, approaches to efficiently discovering metalized interfaces are in high demand. Here, we demonstrate the transformation of nanoscale metal/oxide interface problems into surface problems through a novel metal-hydrogenation detaching method. We applied this technique to study the thickness dependence in Pb(Zr,Ti)O3 (PZT) ferroelectric thin films, a long-standing interface problem in a model metal/insulator device, and this allowed comprehensive surface analytical techniques to be adapted. A nonstoichiometric interfacial layer of 4.1 nm thick with low mass density, low permittivity, and weak ferroelectricity was quantified at the Pt/PZT interface and attributed to the preferential diffusions among the compositional elements. Targeted interface engineering by Pb rebalance led to a substantial recovery of ferroelectric properties. Our results therefore pave the way to a better understanding of metallized interface in ferroelectric and dielectric nanodevices. We hope that more useful information about metalized interfaces of other solid materials could, analogously, be accessed by surface analytical techniques.

Deposition of Bi3.15Nd0.85Ti3O12 ferroelectric thin films on 5-inch diameter Si wafers by a modified pulsed laser deposition method

5-inch diameter Bi3.15Nd0.85Ti3O12 (BNT) ferroelectric thin films were prepared on Si wafers by a modified pulsed laser deposition method, in which two laser scanning rates and 6-inch diameter target were used. The investigation on the distribution of the film thickness reveals that the laser scanning technique used in conjunction with the large-diameter target can improve the uniformity of film thickness. Especially the reasonable setup of the laser scanning rate ablating the fringe area of the large target has a prominent effect on weakening the edge deviation of thickness distribution. Such phenomenon was fundamentally explained by the simulation results on the ablation tracks of the laser beam on target. Through optimizing the laser scanning rates, BNT thin film with uniform thickness was obtained on 5-inch diameter Si wafer and the maximum variation in film thickness was found to be less than ±2.5%. The results of X-ray photoelectron spectroscopy and X-ray diffraction show that the prepared 5-inch BNT thin film has great uniformity in stoichiometry and crystallinity. In addition, the measurements of electrical properties indicated that the prepared BNT thin film exhibits good ferroelectric property and I–V characteristic.

Growth kinetics of induced domains in Ba0.8Sr0.2TiO3 ferroelectric thin films

The possibility of the formation of stable domain states in Ba0.8Sr0.2TiO3 films on the initial surface and in a preliminarily polarized region of the film has been demonstrated using piezoresponse force microscopy. The velocity of lateral motion of a domain wall, coercive field, minimal domain size, and recording time for the domain formation under the applied voltage have been calculated.

Mechanical stress effect on leakage current in Bi3.25La0.75Ti3O12 thin films

Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films were prepared on Pt/Ti/SiO2/Si substrates by metalorganic decomposition method. Mechanical stress effect on leakage current was investigated. The results demonstrate that the leakage current increases with increasing electric field. Under mechanical tensile or compressive stress, the increase trend of the leakage current with electric field shows negligible change in low field region (below 75kV/cm), while that gets weakened in high field region (above 75kV/cm) compared with that at free state (zero stress). At a certain electric field (above 75kV/cm), the leakage current decreases with increasing tensile or compressive stress. The effect of domain reorientation induced by stress combined with the contribution from charge injected into films is found to be reasonable for the explanation of the leakage current in BLT thin films under different stress.

Nonlinear optical absorption tuning in Bi3.15Nd0.85Ti3O12 ferroelectric thin films by thickness

The tunability of nonlinear optical (NLO) absorption in Bi3.15Nd0.85Ti3O12 (BNT) ferroelectric thin films was investigated through the open aperture Z-scan method with femtosecond laser pulses at the wavelength of 800 nm. NLO absorption responses of the BNT films were observed to be highly sensitive to the film thickness. As the film thickness increases from 106.8 to 139.7 nm, the NLO absorption changes from saturable absorption (SA) to reverse saturable absorption (RSA). When the film thickness further increases to 312.9 nm, the RSA effect is enhanced. A band-gap-related competition between the ground-state excitation and the two-photon absorption is responsible for the absorption switching behavior. Such a tunable NLO absorption can widen the photonic application of the BNT thin films.

Influence of deposition temperature on microstructure and electrical properties of modified (Ba, Sr)TiO3 ferroelectric thin films

Modified Ba0.67Sr0.33TiO3 (BST) thin films were deposited on Pt/Ti/SiO2/Si substrates using RF magnetron sputtering. By optimizing the deposition temperature (Td), the dielectric and ferroelectric properties of BST films were improved. The onset of crystallization of the BST films was observed at about 600°C, beyond which there was an increase in grain size with further increasing Td up to 700°C. The better crystallized film deposited at 700°C with a thickness of 670nm shows a polycrystalline and dense structure, a dielectric constant (εr)=953.40 (at 100kHz), and a remanent polarization (2Pr)=23.87μC/cm2. The dielectric tunability at 1MHz increases as Td increases, and reaches the highest value of 55.40% for an applied field of 770kV/cm at the highest Td of 700°C. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electric performance analysis also indicate that the deposition temperature during growth strongly affects the microstructure of BST films. This in turn influences the dielectric, ferroelectric and tunable behavior of BST films.

Er3+-doped Na0.5Bi0.5TiO3 ferroelectric thin films with enhanced electrical properties and strong green up-conversion luminescence

Ferroelectric materials with up-conversion luminescence (UCL) properties have potential optoelectrical applications for display and sensing, etc. Here, we demonstrate strong green UCL and enhanced electrical properties in Er3+-doped Na0.5Bi0.5TiO3 thin films. The thin films are prepared via using a modified chemical solution deposition method. These thin films are phase-pure and crystallized in perovskite structure. The largest remnant polarization (P r) and highest dielectric constant (ε) are obtained from Na0.5Bi0.49Er0.01TiO3 thin films, and their values are 22 μC/cm2 and 1166, respectively. Meanwhile, strong green UCL at 525 and 548 nm is observed in Er3+-doped thin films. They are attributed to 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions. These thin films have potentials in optoelectrical device applications.

Temperature dependence of ferroelectric properties and the activation energy of polarization reversal in (Pr,Mn)‐codoped BiFeO3 thin films

We applied Vopsaroiu's model to (Bi,Pr)(Fe,Mn)O3 (BPFM) and Pb(Zr,Ti)O3 (PZT) ferroelectric thin films fabricated by chemical solution deposition. The temperature dependences of the saturation polarization and the coercive field were measured in a low‐temperature region from 100 to 200 K. The saturation polarizations of BPFM thin films decreased on decreasing the measurement temperature due to the polarization pinning effect, while that of PZT thin film was almost unchanged over the temperature region. The coercive fields of all the thin films were increased linearly on decreasing the measurement temperature. The activation energies for polarization reversal in as‐grown BPFM, postannealed BPFM, and PZT thin films were 1.18, 1.25, and 0.95 eV, respectively. These results indicate that BPFM thin films have large activation energies for polarization reversal compared with PZT thin films. In addition, the postannealed BPFM thin film has a larger activation energy than the as‐grown BPFM thin film.

Large femtosecond third-order optical nonlinearity of Bi3.15Nd0.85Ti3O12 ferroelectric thin films

Both the linear and nonlinear optical properties of Bi3.15Nd0.85Ti3O12 (BNT0.85) ferroelectric thin films deposited on quartz substrates were investigated. The fundamental optical constants were determined as a function of light wavelength by optical transmittance measurements. By performing single-beam Z-scan experiments with femtosecond laser pulses at a wavelength of 800 nm, the two-photon absorption (TPA) coefficient β and third-order nonlinear refraction index γ were measured to be 1.15 × 102 cm/GW and −8.15 × 10−3 cm2/GW, respectively. The large TPA is attributed to an indirect transition process via the intermediate energy levels and the large refractive nonlinearity is the result of the electronic polarization and ferroelectric polarization arisen from the femtosecond midinfrared radiation. The results indicate that the BNT0.85 thin film is a promising candidate for applications in nonlinear photonic devices.

Spectroscopic ellipsometry study of 0.5BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3 ferroelectric thin films

0.5BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3 thin films have been deposited on Pt/Ti/SiO2/Si substrates by a modified sol–gel technique. The samples are well-crystallized perovskite ferroelectrics at room temperature, with good surface topography. Optical properties of these films in the 280–850nm wavelength range have been investigated by spectroscopic ellipsometry at room temperature. The measured ellipsometry parameters are well fitted by Tauc–Lorentz oscillator dispersion formula. As the wavelength increases, the refractive index and extinction coefficient rise in the beginning and then descend, with a peak at about 300nm. The calculated absorption coefficient demonstrates that as-grown films have selective absorption, with a broad transparent range, and possess a big energy gap (4.18eV).