Ferroelectric Thin-film Devices Research Articles

Effects of Al doping concentration and top electrode on the ferroelectricity of Al-doped HfO2 thin films

Al-doped HfO2 (HAO) is regarded as one of the potential HfO2 ferroelectric materials owing to its compatibility with the front end of the line process in integration circuits. In this work, atomic layer deposited (ALD) HAO thin films with different Al-doping concentrations and the corresponding devices with Ti or W top electrodes have been explored. It is found that the HAO film properties and corresponding ferroelectric device performances depend greatly on the doping concentration. In the concentration range of 3.0%–5.0%, 4.5% is the most proper doping concentration to induce the ferroelectricity of the thin film. It is revealed by x-ray diffraction and photoelectron spectroscopy that the oxygen vacancies can be modulated by doping, thereby contributing to the formation of the ferroelectric orthorhombic phase. A high remnant polarization (2Pr) of 36.8 ± 0.7 μC/cm2 under 5 mV/cm sweeping is observed through electrical measurement in the device with a 4.5% HAO and W top electrode. In addition, it was found that the choice of W or Ti top electrodes would affect the leakage and breakdown characteristics of the device. This work reveals the mechanism of Al doping and electrode modulation of HfO2 device performance and promotes the development of HAO ferroelectric thin film devices.

Dielectric properties and ferroelectric resistive switching mechanism in the epitaxial (111) BiFeO3 films

Resistive random access memory is promising for next-generation nonvolatile memory, which has high memory density, low power consumption, and fast read/write speed. However, the resistive switching (RS) mechanism in oxide ferroelectric thin film devices is still unclear. In this paper, Pt/BiFeO3/La0.5Sr0.5CoO3(Pt/BFO/LSCO) heterostructures were deposited on (111)SrTiO3(STO) substrates by magnetron sputtering. X-ray diffraction confirmed the epitaxial relationship between (111)BFO//(111)LSCO//(111)STO. Atomic force microscopy (AFM) showed that (111)BFO had an island structure with uniform grain distribution. The Pt/(111)BFO/(111)LSCO heterostructures exhibit obvious ferroelectricity and RS effects. The results of C-V analysis and I-V fitting indicate that the RS is caused by the space-charge-limited conduction mechanism.

Wake-up and fatigue mechanisms in ferroelectric Hf0.5Zr0.5O2 films with symmetric RuO2 electrodes

The mechanisms leading to wake-up and fatigue in ferroelectric hafnium zirconium oxide thin film devices with symmetric RuO2 electrodes are investigated via polarization, relative permittivity, dielectric nonlinearity, pyroelectric coefficient, and microfocus x-ray diffraction (XRD) measurements. The devices are observed to wake-up for up to 103 bipolar pulsed field cycles, after which fatigue occurs with polarization approaching zero following 108 cycles. Wake-up is accompanied by a decrease in both high-field permittivity and hysteresis loop pinching and an increase in the pyroelectric coefficient, indicating that the wake-up process involves a combination of transformations from the tetragonal to the orthorhombic phase and domain depinning from defect redistribution. Fatigue is observed to coincide with an increase in irreversible domain wall motion and a decrease in pyroelectric coefficient. Finite pyroelectric coefficients are measured on fully fatigued devices, indicating that domain pinning is a strong contributor to fatigue and that fatigued devices contain domain structures that are unable to switch under the fields applied for measurement. Microfocus XRD patterns measured on each device reveal that the phase constitution is qualitatively unaffected by field cycling and resultant polarization fatigue. These data indicate that the wake-up process has contributions from both phase transformations and domain depinning, whereas the fatigue process is driven primarily by domain pinning, and the near-zero measured switchable polarization is actually a poled device with immobile domains. These observations provide insight into the physical changes occurring during field cycling of HfO2-based ferroelectrics while examining a possible oxide electrode material for silicon CMOS device implementation.

Ferroelectric phase transitions induced by a strain gradient

In perovskite oxide ferroelectrics, gradients of lattice strain are known to induce nanoscale topological structures, leading to novel or enhanced functionality. Here, we experimentally detect and theoretically analyze thickness distribution of structural properties in epitaxial ${\mathrm{Pb}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}$ films grown on (001) $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrates. We show that the relaxation of substrate-imposed stress produces a strain gradient, which leads to the formation of distinct ferroelectric phases as a function of distance from the film-substrate interface within the same film. Charge carriers trapped at phase boundaries stabilize the induced phases and manifest themselves under electric field. Crosstalk between the phases, where polarization may rotate in one phase and invert in the other one, opens perspectives for advanced ferroelectric thin film devices.

Resistance switching and conduction mechanism on ferroelectric copolymer thin film device

Resistance switching and conduction mechanism on ferroelectric copolymer thin film device

Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films

AbstractDomain switching pathways fundamentally control performance in ferroelectric thin film devices. In epitaxial bismuth ferrite (BiFeO3) films, the domain morphology is known to influence the multiferroic orders. While both striped and mosaic domains have been observed, the origins of the latter have remained unclear. Here, it is shown that domain morphology is defined by the strain profile across the film–substrate interface. In samples with mosaic domains, X‐ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using scanning transmission electron microscopy finds that within 5 nm of the film–substrate interface, the out‐of‐plane strain shows an anomalous dip while the in‐plane strain is constant. Conversely, if uniform strain is maintained across the interface with zero strain gradient, striped domains are formed. Critically, an ex situ thermal treatment, which eliminates the interfacial strain gradient, converts the domains from mosaic to striped. The antiferromagnetic state of the BiFeO3 is also influenced by the domain structure, whereby the mosaic domains disrupt the long‐range spin cycloid. This work demonstrates that atomic scale tuning of interfacial strain gradients is a powerful route to manipulate the global multiferroic orders in epitaxial films.

Ferroelectric and photoluminescence properties of (Ca, Eu)Bi2Ta2O9 thin films prepared by pulsed laser deposition

Thin films of CaBi2Ta2O9 (CBT) and Eu3+ doped CaBi2Ta2O9 (CEBT) were deposited on platinum-coated silicon substrates via the pulsed laser deposition technique. The microstructural, leakage current, ferroelectric behavior and photoluminescence properties of CBT/CEBT films were systematically studied. The CBT/CEBT films form single phase with a polycrystalline perovskite structure is confirmed by X-ray diffraction, and the CEBT films exhibit better crystallinity when doped with a number of Eu3+ ions. Meantime, the ferroelectric hysteresis loops show that enhanced ferroelectric property with a remnant polarization 2Pr = 8 μC/cm2 are obtained from CEBT film when x = 0.15. Leakage current density curves show that the Eu3+ doping could lead to the increase of leakage current of CBT film. Furthermore, under the excitation of 394 nm and 464 nm light, the thin films exhibit yellow and red emission peaks centered at 592 nm and 613 nm, which attributes to the f–f electronic transition of 5D0 → 7F1 and 5D0 → 7F2 of Eu3+ ions. This study suggests that Eu3+ doped CaBi2Ta2O9 films have a potential application in the new multifunctional photoluminescence ferroelectric thin-film devices.

Effect of Pt3Pb on the permittivity and conductivity of lead zirconate titanate thin films

The evolution and interaction of buried interfaces during the synthesis of lead zirconate titanate (PZT) ferroelectric thin film devices have been previously predicted by density functional theory modeling and observed experimentally via in–situ structural characterization. Moreover, the formation and disappearance of an intermetallic phase (Pt3Pb) during the crystallization of the film has been identified as a key process expected to affect the resulting electrical properties of the devices. To elucidate this effect, a combination of direct current (leakage current measurements) and alternating current (impedance spectroscopy) characterization techniques are used to examine the electrical properties of sol–gel–derived PZT thin films. Films with the intermetallic phase exhibit a high potential barrier at the metal–insulator interface (0.83 eV) similar to the fully crystallized films (0.81 eV) and a lower current density than the films without the intermetallic phase, as shown in the Richardson plot. Impedance measurements also revealed that the conductivity of the films with the intermetallic phase, σ = 1.9 × 10−11 S cm−1, and the real part of the relative permittivity at 10 kHz, εr′=42, are lower than the other films (fully crystallized film: σ = 3.5 × 10−10 S cm−1 εr′=566, amorphous crystallized film: σ = 4.6 × 10−11 S cm−1 εr′=121). It is shown that these electrical characterization methods can serve as non–destructive examination tools to track the evolution of secondary phases during device synthesis and fabrication.

Intrinsically Switchable Frequency Reconfigurable Barium Strontium Titanate Resonators and Filters

Multimode transceivers for the next generation of wireless devices require new and innovative frequency-agile microwave circuit designs to reduce their complexity, size, and cost. Intrinsically switchable frequency reconfigurable thin-film bulk acoustic resonators (FBARs) and filters that utilize these resonators are presented here for the very first time. Barium strontium titanate (BST) is a ferroelectric material that exhibits electric-field-induced piezoelectricity, which allows for the design of voltage controlled bulk acoustic wave devices. In this paper, a frequency reconfigurable resonator consisting of two series connected BST-based FBARs and a reconfigurable dual-band filter consisting of two 1.5 stage ladder-type bandpass filters are discussed. Their small size, simple design, and zero static power consumption make multifunctional ferroelectric thin-film devices attractive for use in adaptive and reconfigurable radios.

Closed-form analysis of high-resistivity thin-film electrode in the hybrid varactor structure

Hybrid ferroelectric thin-film varactor device architecture is presented that allows for integrating the features of both structures of coplanar and parallel-plate varactors. In this structure, a high-resistivity oxide thin film is used as dc bias bottom electrode, while the existence of the high-resistivity oxide thin-film electrode is considered as disturbance to the original electric field in the coplanar varactor. Using analytic solution of electromagnetic field, the disturbance caused by high-resistivity thin film in microwave TEM wave is derived. The result shows that this disturbance could be modeled as serial or shunt resistance in the transmission line equivalent circuit and the magnitude of it is proportional to the conductivity and thickness of the conductor thin film. With conductivity and thickness low enough, the conductor film will be “transparent” to microwave field. Using high-resistance dc bias electrode “transparent” to microwave is expected to be able to bring much more flexibility in the developing of microwave ferroelectric tunable devices.

In situ X-ray diffraction of lead zirconate titanate piezoMEMS cantilever during actuation

Synchrotron X-ray diffraction (XRD) was used to probe the electric-field-induced response of a 500nm lead zirconate titanate (52/48, Zr/Ti) (PZT) based piezoelectric microelectromechanical system (piezoMEMS) device. 90° ferroelectric/ferroelastic domain reorientation was observed in a cantilever comprised of a 500nm thick PZT film on a 3μm thick elastic layer composite of SiO2 and Si3N4. Diffraction data from sectors both parallel- and perpendicular-to-field showed the presence of ferroelastic texture, which is typically seen in in situ electric field diffraction studies of bulk tetragonal perovskite ferroelectrics. The fraction of domains reoriented into the field direction was quantified through the intensity changes of the 002 and 200 diffraction profiles. The maximum induced volume fraction calculated from the results was 20%, which is comparable to values seen in previous bulk and thin film ferroelectric diffraction studies. The novelty of the present work is that a fully released ferroelectric thin film device of micron scale dimensions (down to 60,000μm3) was interrogated in situ with an applied electric field using synchrotron XRD. Furthermore, the experiment demonstrates that 90° ferroelectric/ferroelastic domain reorientation can be characterized in samples of such small dimensions.

Giant Resistive Switching via Control of Ferroelectric Charged Domain Walls.

Controlled switching of resistivity in ferroelectric thin films is demonstrated by writing and erasing stable, nanoscale, strongly charged domain walls using an in situ transmission electron microscopy technique. The resistance can be read nondestructively and presents the largest off/on ratio (≈10(5) ) ever reported in room-temperature ferroelectric devices, opening new avenues for engineering ferroelectric thin-film devices.

Imprint Control of BaTiO3 Thin Films via Chemically Induced Surface Polarization Pinning.

Surface-adsorbed polar molecules can significantly alter the ferroelectric properties of oxide thin films. Thus, fundamental understanding and controlling the effect of surface adsorbates are crucial for the implementation of ferroelectric thin film devices, such as ferroelectric tunnel junctions. Herein, we report an imprint control of BaTiO3 (BTO) thin films by chemically induced surface polarization pinning in the top few atomic layers of the water-exposed BTO films. Our studies based on synchrotron X-ray scattering and coherent Bragg rod analysis demonstrate that the chemically induced surface polarization is not switchable but reduces the polarization imprint and improves the bistability of ferroelectric phase in BTO tunnel junctions. We conclude that the chemical treatment of ferroelectric thin films with polar molecules may serve as a simple yet powerful strategy to enhance functional properties of ferroelectric tunnel junctions for their practical applications.

Effect of polarization fatigue on the Rayleigh coefficients of ferroelectric lead zirconate titanate thin films: Experimental evidence and implications

The effect of polarization fatigue on the Rayleigh coefficients of ferroelectric lead zirconate titanate (PZT) thin film was systematically investigated. It was found that electrical fatigue strongly affects the Rayleigh behaviour of the PZT film. Both the reversible and irreversible Rayleigh coefficients decrease with increasing the number of switching cycles. This phenomenon is attributed to the growth of an interfacial degraded layer between the electrode and the film during electrical cycling. The methodology used in this work could serve as an alternative way for evaluating the fatigue endurance and degradation in dielectric properties of ferroelectric thin-film devices during applications.

Real-time observation of local strain effects on nonvolatile ferroelectric memory storage mechanisms.

We use in situ transmission electron microscopy to directly observe, at high temporal and spatial resolution, the interaction of ferroelectric domains and dislocation networks within BiFeO3 thin films. The experimental observations are compared with a phase field model constructed to simulate the dynamics of domains in the presence of dislocations and their resulting strain fields. We demonstrate that a global network of misfit dislocations at the film-substrate interface can act as nucleation sites and slow down domain propagation in the vicinity of the dislocations. Networks of individual threading dislocations emanating from the film-electrode interface play a more dramatic role in pinning domain motion. These dislocations may be responsible for the domain behavior in ferroelectric thin-film devices deviating from conventional Kolmogorov-Avrami-Ishibashi dynamics toward a Nucleation Limited Switching model.

Fabrication and Characterization of Ferroelectric Thin Film Ba<sub>x</sub>Sr<sub>1-x</sub>TiO<sub>3 </sub>for Application in Light Intensity Detector

Ferroelectric thin film devices have been demonstrated using BaxSr1-xTiO3 (BST) as a light intensity detector. Various compositions of BST (x=0.7, 0.8 and 0.9) were prepared using Chemical Solution Deposition method. Solution of barium acetate, strontium acetate, titanium (IV) iso-propoxide in a mixture of acetic acid and ethylene glycol was spin-coated onto a silicon substrate. The BST film was annealed at 800°C. The microstructure and the crystalline characterization of BST were studied using X-Ray Diffraction (XRD) pattern with General Structure Analysis System (GSAS) refinement. The film thickness was measured using Scanning Electron Microscopy (SEM). The response of BST film to source of light intensity was investigated with IRC meter and resonator coupling. The results show that the resistance of a thin BST film decrease as the intensity of light increase. On resonator coupling, VOUT shows a slight increases as the intensity of light increases.

Nanoscale domain switchings of Bi3.15Dy0.85Ti3O12 thin film under the simultaneous application of polarizing voltage and loading force

The nanoscale domain switchings of Bi3.15Dy0.85Ti3O12 thin film annealed at 700 °C were observed by piezoresponse force microscopy under the different DC polarizing voltages, loading forces, and simultaneously both of them. Then, the formation mechanisms of domain switchings were analyzed from the perspective of dynamics of ferroelectric nanodomain and equilibrium state free energy of ferroelectric nanodomain. First, the 180° domain switchings were observed under the single polarizing voltages ±12 V. However, there are small amounts of anti-parallel domains. Second, the 90° domain switching was detected under the different single loading forces (150 and 300 nN). Finally, both 90° and 180° domain switchings were obtained under the simultaneously applied fields of polarizing voltage (+10 V) and loading forces (100 and 200 nN). The formation mechanisms of typical electric-generated nanoscale domain switchings were analyzed by forward domain-growth mechanism and grain deformation, and the abnormal electric-generated domain switching is interpreted by grain boundary effect and built-in electric field. Furthermore, we adopted the domain switching criterion from the perspective of equilibrium state free energy of ferroelectric nanodomain to uniformly explain the mechanisms of domain switchings induced by different external fields. It is necessary to understand the evolution mechanism of nanoscale domain switching for ferroelectric thin film devices.

Bright up-conversion green photoluminescence in Ho3+-Yb3+ co-doped Bi4Ti3O12 ferroelectric thin films

Ho3+-Yb3+ co-doped bismuth titanate ferroelectric thin films were prepared by a chemical solution deposition method on fused silica substrates and their up-conversion luminescence characteristics excited by a 980 nm diode laser were investigated. The two emission bands centered at 546 and 656 nm in the emission spectra can be assigned to 5F4 + 5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+ ions, respectively. A bright green emission was observed even when the laser pumping power was relatively low. The dependence of the emission intensity on the pumping power indicated that the up-conversion emission in the thin films was a two-photon process. The up-conversion emission mechanism is discussed in detail. This study suggests that Ho3+ and Yb3+ co-doped Bi4Ti3O12 thin films can be applied to the fabrication of new multifunctional photoluminescence ferroelectric thin-film devices.

Strong upconversion luminescence properties of Yb3+ and Er3+ codoped Bi4Ti3O12 ferroelectric thin films

Strong upconversion green luminescence was observed in Er3+ and Yb3+ codoped Bi4Ti3O12 ferroelectric thin films. The thin films were prepared on fused silica and Pt/TiO2/SiO2/Si substrates by chemical solution deposition method. There are two strong green emission bands centered at 524 and 545 nm and a weak red emission band centered at 667 nm in the upconversion luminescence spectra pumped by 980 nm at room temperature. They correspond to the radiative relaxation of Er3+ from H211/2, S43/2, and F49/2 levels to the ground level I415/2, respectively. Bi3.65Yb0.3Er0.05Ti3O12 thin films exhibit higher green emission intensity by a factor of about 30 compared with Bi3.95Er0.05Ti3O12 thin films. The upconversion emission mechanism of the thin films has been discussed. In addition, Bi3.65Yb0.3Er0.05Ti3O12 ferroelectric thin films on Pt/TiO2/SiO2/Si substrates also show well-defined hysteresis loops with a remnant polarization (2Pr) of 28.2 μC/cm2. This study opens the possibility of constructing new integrated photoluminescent ferroelectric thin film devices.

Structural defects and local chemistry across ferroelectric–electrode interfaces in epitaxial heterostructures

We present a detailed investigation of the chemistry at the growth interface between the bottom electrode and ferroelectric film in (001)-oriented epitaxial ferroelectric thin-film heterostructures. Three different ferroelectric systems, namely PbZr0.2Ti0.8O3, PbZr0.52Ti0.48O3, and BaTiO3 deposited on SrRuO3/SrTiO3, were investigated to compare and contrast the role of lattice mismatch versus the volatility of the deposited cation species. A combination of transmission electron microscopy-based imaging and spectroscopy reveals distinct correlations among the ferroelectric thin-film composition, the deposition process, and chemical gradients observed across the ferroelectric–electrode interface. Sr diffusion from the electrode into the ferroelectric film was found to be dominant in PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 thin films. Conversely, Pb diffusion was found to be prevalent in PbZr0.52Ti0.48O3/SrRuO3/SrTiO3 thin films. The BaTiO3/SrRuO3/SrTiO3 heterostructure was found to have atomically sharp interfaces with no signature of any interdiffusion. We show that controlling the volatility of the cation species is as crucial as lattice mismatch in the fabrication of defect-free ferroelectric thin-film devices.