Polarization In Thin Films Research Articles

Self-driven ultraviolet photodetectors based on ferroelectric depolarization field and interfacial potential

Self-powered ultraviolet photodetectors have attracted considerable interest for their reduced sizes and high portability which makes them possible for application in a hazardous environment. In this work, Pb0.95La0.05Zr0.54Ti0.46O3 (PLZT) thin films with a wide bandgap of ∼3.72 eV and large remanent polarization of ∼33.9 μC/cm2 were synthesized by low-cost sol-gel method, based on which, highly sensitive self-powered ultraviolet (UV) photodetectors with a Au/PLZT/FTO structure were developed. It’s found that the responsivity of the devices can be modulated by changing the direction of polarization in the PLZT thin film owing to coupling effects of the depolarization electric field with interfacial barriers at both Au/PLZT and PLZT/FTO contacts. The devices with PLZT in downward polarization state show a remarkable responsivity of 0.011 mA/W at zero bias to illumination of 340 nm-light with fast-response rise time of 98 ms and decay time of 96 ms. The present work sheds light on the design of self-driven photodetectors free of the p-n junction on the basis of ferroelectric oxides.

Syntheses and Properties of New Smectic Reactive Mesogens and Their Application in Guest-Host Polarizer

We designed and synthesized new smectic liquid crystalline monomers containing photopolymerizable acrylate groups and a mesogenic core unit composed of two benzene rings, one pyrimidine-based heteroaromatic ring and an acetylene linking group. The structures of monomers were characterized by 1H nuclear magnetic resonance spectroscopy, and their mesomorphic behaviors were investigated by differential scanning calorimetry and polarized optical microscopy. Both of monomers showed an enantiotropic phase transition behavior with exhibiting both nematic phase and highly ordered smectic phase. Both of monomers showed smectic phases during the cooling process in the temperature range of 144.2–94.0 °C and 154.0–85.8 °C, respectively. Neither of these compounds showed a homogeneous alignment even at various temperatures, and therefore, their 50:50 mixture was chosen for the preparation of guest-host mixture because of its lowest crystallization point, wide smectic phase temperature range and superior aligning property. Polymerizable “host-guest” mixture formulated from the host liquid crystalline monomer mixture, dichroic dye and additives was injected into the sandwiched glass substrates having rubbed alignment layers at the isotropic temperature. Subsequent in-situ photopolymerization by UV irradiation at room temperature successfully resulted in a thin film polarizer with good polarizing properties. The fabricated guest-host polarizer showed a dichroic ratio (DR) of 13.1 and a degree of polarization (DOP) of 95.5% with the thickness of 5 µm.

Temperature-Independent Polarization of Ultrathin Phthalocyanine-Based Hybrid Organic/Inorganic Heterojunctions.

The combination of organic and inorganic materials at the nanoscale to form functional hybrid structures is a powerful strategy to develop novel electronic devices. The knowledge on semiconductor thin-film polarization brings direct benefits to the hybrid organic/inorganic electronics, becoming primordial for the development of devices such as electromechanical logic gates, solar cells, miniaturized valves, organic diodes, and molecular supercapacitors, among others. Here, we report on the dielectric polarization of ultrathin organic semiconducting films-ca. 5 nm thick metal phthalocyanine ensembles (viz., CuPc, CoPc, F16CuPc)-employed to build up hybrid metal/oxide/molecule heterojunctions. Such hybrid heterostructures are fully integrated into self-rolled nanomembrane-based capacitors and further investigated by impedance spectroscopy measurements as a function of temperature (from 6 to 300 K). The dielectric polarization of the metal phthalocyanines is found to be thermally activated above a specific threshold temperature, which depends on the molecular structure. Below this threshold, the current leakage across the system is suppressed, thus evidencing intrinsic-like polarization mechanisms. The temperature-independent permittivities of the ultrathin molecular films are found to be strongly dependent on the organic/inorganic hybrid interfaces, while the calculated relaxation times are more likely related to each single-molecule polarization. Beyond the advances in determining the temperature dependence of the permittivity for ultrathin phthalocyanine films integrated within solid-state electronics, our results also support the deterministic design of novel functional devices based on nanoscale hybrid organic/inorganic heterojunctions.

Iridium oxide top electrodes for piezo- and pyroelectric performance enhancements in lead zirconate titanate thin-film devices

Iridium oxide films of various morphologies are produced by manipulating the reactive gas flow rate in a reactive sputtering process. This study explores the benefits of using planar and nanostructured IrOx top electrodes for Pb(Zr0.52Ti0.48)O3 (PZT) devices employed in piezoelectric and pyroelectric applications in comparison with conventional platinum electrodes. For the first time, the Young’s modulus of a planar IrOx thin film was directly measured to be 300 ± 15 GPa. The piezoelectric performance of devices with planar IrOx showed up to 245% more strain compared to those capped with a Pt electrode. Nanostructured, 2D IrOx platelet networks are observed for films deposited with high oxygen flow rates. The compatibility with various cleanroom processes, such as liftoff, ultrasonic cleaning, and deposition on etched surfaces, is tested for the IrOx nanostructured layers. These films are shown to have low optical reflectivity while retaining good PZT thin-film capacitor polarization and dielectric permittivity. An improved pyroelectric energy harvesting figure of merit by a factor of 2.5× was observed for the IrOx compared to Pt films on the same PZT material.

Mechanically induced ferroelectric switching in BaTiO3 thin films

The ability to reverse or switch the polarization of a ferroelectric thin film through a mechanical force under an atomic force microscopy (AFM) tip offers the exciting possibility of a voltage-free control of ferroelectricity. One of the important metrics for characterizing such a switching process is the critical force Fc required to reverse a polarization. However, the experimentally measured values of Fc display a large uncertainty and vary significantly even for the same ferroelectric film. Here, using BaTiO3 thin films as a model system, we systematically evaluate Fc using a combination of AFM-based experiments and phase-field simulations. In particular, we study the influence of the AFM tip radius, misfit strain, and film thickness on Fc as well as the interplay between the flexoelectric and piezoelectric effects. This work provides a deeper understanding on the mechanism and control of mechanically induced ferroelectric switching and thus guidance for exploring potential ferroelectric-based nanodevices based on mechanical switching.

TiO2/airgap-based polarizing beam splitter: design, simulation, and fabrication based on MEMS technologies

In this paper, we introduce a new thin-film polarizing beam splitter (PBS) based on air as a layer with a low refractive index (L) and titanium dioxide (TiO2) as a material with a high refractive index (H). A cube-type PBS presents an active area of approximately 1.0 × 1.0 mm2, and the airgap layer in conjunction with TiO2 thin-films provides an extremely high refractive index contrast (1.2). The PBS cube is projected onto a borosilicate glass substrate and the multiple layers obtained through TFCalcTM simulations, expressed in multiples of a quarter-wavelength optical thickness, are 1.6H L 1.1H 1.5L 1.1H L 1.6H. The projected structure ensures the separation of the polarizing states of light (transmission of the p-polarization and reflection of the s-polarization) from the visible to near infrared spectral range, over a bandwidth higher than 170 nm. This is the minimal working bandwidth obtained for visible range (centered at 550 nm), considering a polarization degree higher than 0.95. The projected PBS is fabricated based on conventional microelectromechanical systems (MEMS) technologies, namely, thin-film deposition, annealing, and etching processes. Finally, the fabricated PBS was experimentally characterized, with the measured transmittance of the fabricated structure reaching close to the expected TFCalcTM simulation result. This characterization is the final validation of the structure since, theoretically, reflection is the complement of transmission in a dielectric thin-film multilayer.

A Strategy for Improving the Quality of Ghost Imaging

A new method is proposed to improve the signal-to-noise ratio (SNR) of regions of interest (ROIs) in a ghost imaging (GI) system with uneven speckle illumination. The imaging results in a GI system can be distorted when there is an uneven distribution of light. In this study, three thin-film polarizers are used to create illumination patterns in uneven light intensity distribution. In particular, the polarizer set is loaded on the object arm only, that is, the original uniformly distributed light field is still acquired by the reference arm. This small change in the light path eliminates the distortion caused by uneven illumination while increasing the SNR of the ROI. This strategy has been confirmed in principle and through simulation and experiments.

Superior ferroelectric photovoltaic properties in Fe -modified (Pb,La) (Zr,Ti)O3 thin film by improving the remnant polarization and reducing the band gap

In order to develop ferroelectric photovoltaic devices with high power conversion efficiency, ferroelectric materials must have simultaneously large remnant polarization and narrow band gap so as to efficiently separate photo-generated carriers and absorb more sunlight. Based on this idea, in this report, we introduce Fe3+ into Pb0·93La0·07(Zr0·6Ti0.4)0.9825O3 ferroelectric thin film to increase the remnant polarization and decrease the band gap of the thin film. In doing so, we prepare Fe3+ doping Pb0·93La0·07(Zr0·6Ti0.4)0.9825O3 thin-film based photovoltaic devices. The experimental results indicate that with increasing the Fe3+ amount, the remnant polarization of the film first improves to the maximum value of 50 μC/cm2 at the 4.8 mol% Fe3+ content and then reduces gradually, while the band gap continuously decreases. In addition, at a negative poling voltage, the device exhibits larger short-circuit current and open-circuit voltage in comparison with those obtained at the positive poling voltage, which is attributed to the depolarization electric field originating from the remnant polarization of ferroelectric thin films in the same direction as the built-in electric field caused by the Schottky barrier. In this report, the most superior photovoltaic performances with the open-circuit voltage of as large as −0.55 V and short-circuit current of as high as 0.4 μA/cm2 are obtained in the device with 4.8 mol% Fe3+ amount and at −5 V poling voltage. This is on account of the improved sunlight absorbing properties and photo-generated carriers separation ability of the device. This work provides a novel idea for designing and preparing ferroelectric photovoltaic devices with high power conversion efficiency.

Variational Phase Field Formulations of Polarization and Phase Transition in Ferroelectric Thin Films

The electric field plays an important role in ferroelectric phase transition. There have been numerous phase field formulations attempting to account for electrostatic interactions subject to different boundary conditions. In this paper, we develop new variational forms of the phase field electrostatic energy and the relaxation dynamics of the polarization vector that involves a hybrid representation in both real and Fourier variables. The new formulations avoid ambiguities appearing in earlier studies and lead to much more effective ways to perform variational studies and numerical simulations. Computations of phase transition and polarization switching in a single domain by applying the new formulations are provided as illustrative examples.

The ultrathin limit of improper ferroelectricity

The secondary nature of polarization in improper ferroelectrics promotes functional properties beyond those of conventional ferroelectrics. In technologically relevant ultrathin films, however, the improper ferroelectric behavior remains largely unexplored. Here, we probe the emergence of the coupled improper polarization and primary distortive order parameter in thin films of hexagonal YMnO3. Combining state-of-the-art in situ characterization techniques separately addressing the improper ferroelectric state and its distortive driving force, we reveal a pronounced thickness dependence of the improper polarization, which we show to originate from the strong modification of the primary order at epitaxial interfaces. Nanoscale confinement effects on the primary order parameter reduce the temperature of the phase transition, which we exploit to visualize its order-disorder character with atomic resolution. Our results advance the understanding of the evolution of improper ferroelectricity within the confinement of ultrathin films, which is essential for their successful implementation in nanoscale applications.

Effect of substrate temperature on antiphase boundaries and spin polarization of thin Fe3O4 film on Si (100)

Polycrystalline Fe3O4 films with the thickness of 27 nm were grown at different temperature (Ts) on Si (100) substrate by Laser-Molecular Beam Epitaxy to study the influence of density of antiphase boundaries (APBs) and the spin polarization of Fe3O4. Films prepared with Ts in the range of 500 °C - 650 °C were proved to be of good crystallization, preferred orientation, and desirable values of saturation magnetization (a little smaller one for film prepared at Ts = 500 °C). The lowest density of APBs was verified by the measured bulk like electron-phonon (s-p) coupling constant for film prepared at Ts = 550 °C, along with the highest value of spin polarization and the best crystalline quality as the most obvious inflection in the curves of lnρ vs. 1000/T around Verwey transition. This optimum value of Ts can be interpreted by the APBs thermally activated migration process with an activation energy. Films prepared with Ts = 500 °C, 600 °C, and 650 °C still have a reasonable value of the s-p coupling constant compared to recently reported results, while larger magnetoresistance accompanied by the superparamagnetic behavior was observed for films prepared at lower Ts (Ts = 400 °C and 450 °C).

Excellent thermal stability of large polarization in (Bi0.5Na0.5)TiO3-BaTiO3 thin films induced by defect dipole

This work reports the large saturation polarization (Pm) of ~80 µC/cm2 and remnant polarization (Pr) of ~60 µC/cm2 in Mn-doped (Bi0.5Na0.5)TiO3-BaTiO3 (BNBTMn) thin films at elevated temperatures, which are substantially higher than the best values of the bulk counterparts, i.e. Pm (~40 µC/cm2) and Pr (~30 µC/cm2). The positive-up-negative-down (PUND) data suggested the contribution of switchable ferroelectric Pr is less than 50% in the overall Pr obtained in P-E loops at 300 K and keeps decreasing when heating. Thus, the observed large and thermally stable macroscopic polarizations in P-E loops is mainly attributed to the contribution of conformed local defect dipoles, namely MnTi′-Vo•• (oxygen vacancy), under a strong electric field. The evolution of Raman spectra with temperature further confirm the presence of symmetry breaking associated with local polar in our BNBTMn thin films can be remained up to 750 K, much higher than previous reported Curie temperature. This work will shed light on the development of high-temperature nano-scale ferroelectric memories and other related devices with eco-friendly materials.

Electronic ferroelectricity and magnetoelectric effect in phase-separated magnetic oxides

We consider phase separated states in magnetic oxides (MO) thin films. We show that these states have a non-zero electric polarization. Moreover, the polarization is intimately related to a spatial distribution of magnetization in the film. Polarized states with opposite polarization and opposite magnetic configuration are degenerate. An external electric field removes the degeneracy and allows to switch between the two states. So, one can control electric polarization and magnetic configuration of the phase separated MO thin film with the external electric field.

Design of a novel TiO2/airgap-based polarizing micro beam splitter cube

Polarizing beam splitters are key elements widely used in different optical instruments. This paper introduces the design and simulation of a novel thin-film multilayer polarizing micro beam splitter based on airgap layers (n = 1.002828 at 400 nm). The negligible absorption coefficient of the air over a wide spectral region (κ ≈ 0 from about 200 nm and higher) satisfies the conditions of a perfect low refractive index material (L). Moreover, using titanium dioxide (TiO2) as high refractive index material (H), a very high refractive index contrast is obtained. The micro beam splitter optical structure consists in a 7 optimized multilayer of TiO2 and air, providing a refractive contrast higher than 1.2. The polarizing beam splitter cube is projected in a borosilicate glass substrate (BK7) and the optical multilayer obtained, expressed in multiples of the quarter wavelength optical thickness – QWOT, is 1.6H L 1.1H 1.5L 1.1H L 1.6H. This optical structure ensures the transmission of p- polarization and the reflection of s- polarization, from visible to NIR spectral range, over a bandwidth higher than 170 nm. Additionally, the designed polarizing beam splitter can be fabricated using standard microtechnology fabrication processes.

Ferroelectric polarization and fatigue characterization in bismuth-based Aurivillius thin films at lower voltage

One of significant issue in ferroelectrics is so called “size effect”, that is ferroelectric properties may disappearance at a finite critical thickness, especially those films prepared by solution process. Here, the reliable polarization fatigue resistance and retention performance of chemical solution deposited Aurivillius family thin films Bi6Fe2Ti3O18 with thickness down to 90 nm and at low driving voltage are reported. Intrinsic ferroelectricity is insight and verified by PUND measurement, and further research demonstrate that it can compared with other thicker films. In addition, the voltage-dependence fatigues feature in Bi6Fe2Ti3O18 films have been demonstrated. Considerable remanent polarization 18.9 μC/cm2 and lower operating voltage 6 V make it not only as a promising ferroelectric material in memory application; it also endows non-vacuum solution process as a good alternative for ultrathin ferroelectric films.

Stable ferroelectric properties of Hf0.5Zr0.5O2 thin films within a broad working temperature range

The ferroelectric properties of 20 nm Hf0.5Zr0.5O2 (HZO) thin films has been investigated in a wide temperature range from 100 K to 450 K. The remnant polarization of HZO thin films decreases slightly from 24.6 μC cm−2 (100 K) to 17.9 μC cm−2 (450 K), indicating a robust temperature stability. The capacitors also exhibit excellent endurance properties up to 109 cycles at 100 K and 300 K, and their endurance cycles slightly degrades to 108 at an elevated temperature of 450 K. The results show that HZO ferroelectric thin films have great potential for future emerging memory applications in various harsh temperature environments.

Effect of a Substrate on the Magnetoelectric Effect in Rare-Earth-Doped Bismuth Iron Garnet

The mechanism of relaxation of the electric polarization in thin films of rare-earth-doped bismuth iron garnet on glass and gallium gadolinium garnet substrates is determined in magnetic fields of 0 and 12 kOe in the temperature range of 80–380 K. The change in the sign of the residual electric polarization after switching off the electric field and the magnetic-field-induced shift of the hysteresis loop in the applied magnetic field are found. Linear and quadratic magnetoelectric effects with the tensor components depending on the substrate type are observed. The linear magnetoelectric effect is related to the spin–orbit coupling of electrons at the film–substrate interface, whereas the quadratic one is determined by the exchange–striction mechanism.

Decoupling Polarization, Crystal Tilt and Symmetry in Epitaxially-Strained Ferroelectric Thin Films Using 4D-STEM

Decoupling Polarization, Crystal Tilt and Symmetry in Epitaxially-Strained Ferroelectric Thin Films Using 4D-STEM

Novel BiAlO3 dielectric thin films with high energy density

Annealing temperature is a key element affecting the electrical properties and microstructure of dielectric thin films. In this case, BiAlO3 (BA) thin films were spin-coated on the Pt/Ti/SiO2/Si(100) substrates by sol-gel method and annealed at the temperature ranging from 450 °C to 550 °C. Then the influences of annealing temperatures on the microstructure, dielectric breakdown, and ferroelectric polarization of BiAlO3 thin films were studied. A optimal recoverable energy storage density (Wreco) of 31.1 J/cm3 and efficiency of 82.2% were achieved for the BA thin films annealed at 500 °C due to the ultrahigh dielectric breakdown strength (BDS) of 3535.7 kV/cm. This study offers a new paradigm for developing high-performance lead-free dielectric materials, which can largely extend the energy storage application of Bi-base perovskite materials.

Highly polarized absorption and emission from polymer-stabilized smectic guest-host systems

ABSTRACTThe optical properties of luminescent molecules dissolved in liquid crystals have led to their proposed use in luminescent thin-film polarizers. These molecules are typically required to fulfil a range of criteria related to their absorption and emission properties, degree of alignment and stability; however, concurrently satisfying these requirements has proven a barrier in their practical use. We obtained highly polarized absorption and emission luminescent thin-film polarizer using reactive liquid crystalline monomers and highly dichroic luminescent dyes, both of which have polymerizable end groups. In situ photopolymerization of the liquid crystalline mixtures in the highly ordered SmB phase resulted in the formation of cross-linked polymeric networks in which the anisotropic absorption and emission of the film were fixed. The as-obtained product exhibited a high dichroic ratio (DR = 30) with a large fluorescence quantum yield (ϕF = 0.77). The device is both cheap and easy to fabricate and has the potential to be used in practical electro-optic applications.