Regulating crystal structure and ferroelectricity in Sr doped HfO2 thin films fabricated by metallo-organic decomposition

HfO₂ based binary ferroelectric oxides are promising candidate for nonvolatile memory devices due to their compatibility with the current Si-based technology. In this work, Sr doped HfO₂ (Sr:HfO₂) ferroelectric thin films with Sr concentration from 0% to 10?mol% were prepared on the platinum electrodes by metallo-organic decomposition (MOD). It was demonstrated that uniform Sr:HfO₂ thin films with extremely low roughness can be achieved and crystallized by MOD under a 700?°C annealing process. A wake-up stage was believed more essential for the ferroelectricity of the MOD derived Sr:HfO₂ thin film, since the remnant polarization of 13.3 µC/cm² and high dielectric constant of 30 were obtained after 10⁵ cycling tests. The transformation from monoclinic phase to cubic phase was observed with increasing the Sr concentration and the thickness of the films. X-ray photoelectron spectroscopy analysis confirmed the bonding type of O-Hf-O and O-Sr-O bonds in the film. The microscopic crystal structure of ferroelectric orthorhombic phase was observed by high resolution transmission electronic microscope. The intrinsic ferroelectricity of Sr:HfO₂ film was demonstrated by the hysteresis polarization-voltage loops and distinct current peaks in the current-voltage curve. Stable domain structure and its switching dynamics were monitored by piezoresponse force microscopy, indicating the native polarization of Sr:HfO₂. This work will provide a controllable routine to fabricate ferroelectric HfO₂ based thin films using MOD method.     

Y-doped HfO2 deposited by atomic layer deposition using a cocktail precursor for DRAM capacitor dielectric application

A Y-doped HfO₂ thin film deposited using a cocktail precursor for a DRAM capacitor dielectric application was investigated. It has been difficult to adapt HfO₂, a potential high-dielectric-constant material, deposited by a typical thin-film deposition technique to actual devices owing to its low dielectric constant of approximately 20, resulting from its monoclinic-phase crystal structure. Although several methods have been investigated to increase the dielectric constant by crystal structure transformation to the tetragonal phase, which has a dielectric constant as high as approximately 40, the formation of the monoclinic phase was not successfully suppressed. In this study, the tetragonal-phase formation of HfO₂ thin films was investigated using a cocktail precursor consisting of Y and Hf precursors. The monoclinic formation suppression mechanism in the Y-doped HfO₂ thin film was determined from the physical and chemical analyses results. Moreover, the leakage current change caused by the introduced oxygen vacancy with respect to the Y dopant concentration was investigated. Improved electrical properties of the dielectric constant and leakage current were achieved with Y-doped HfO₂.     

Effect of annealing protection atmosphere on the ferroelectric yttrium doped hafnium oxide thin films

The 10 nm thick yttrium doped hafnium oxide (Y:HfO₂) thin films, prepared by chemical solution deposition which using all-inorganic aqueous salt reagents, were fabricated on Si (100) substrates. The crystalline structure, chemical composition and ferroelectric properties of thin films, annealed in protection atmosphere of Air, Ar and N₂, were examined. Result showed that the crystalline structure and ferroelectric properties of films exhibited a strong annealing protection atmosphere dependence. When compared to annealing protection atmosphere of Air and Ar, the films with the N₂ exhibited lowest m-phase fraction of 19.4%, and the highest oxygen vacancy percentage content of 3.06%, accompanied with the highest relative permittivity of 50.9 and the remanent polarization of 14.6 μC/cm². These excellent ferroelectric properties were correlated with asymmetric orthorhombic phase and the concentration of oxygen vacancy introduced from the nitrogen doping concentration.     

Ferroelectric ZrO2 ultrathin films on silicon for metal-ferroelectric-semiconductor capacitors and transistors

Enhanced ferroelectric properties of nanoscale ZrO₂ thin films by an HfO₂ seed layer are demonstrated in metal-ferroelectric-semiconductor (Si) capacitors and transistors prepared with a low thermal budget of 400 °C. The seeding effect of the HfO₂ layer leads to the enhancement of crystallization into the orthorhombic phase and the increase of remnant polarization of the sub-10 nm ZrO₂/HfO₂ bilayer structure. The ferroelectric field-effect transistor with the ZrO₂/HfO₂ bilayer gate stack reveals a large memory window of ~1.2 V and a steep subthreshold swing below 60 mV/decade. As compared with the Hf_0.5Zr_0.5O₂ thin film, superior ferroelectric properties of the ZrO₂/HfO₂ bilayer structure show great potential for ferroelectric memory devices fabricated on Si substrates.     

Improved ferroelectric properties in KBiFe2O5-polymer composite film

Composite films of KBiFe₂O₅ and PVA were prepared with a varied weight percentage of PVA by spin coating method on platinised silicon wafer. The composite films were characterised for dielectric and ferroelectric properties in comparison to pure KBiFe₂O₅thick film. The frequency dependent permittivity measurement indicated a higher contribution of space charge/interfacial polarization in KBiFe₂O₅ thick film than that in the KBiFe₂O₅ -PVA composite films. The ferroelectric properties in composite films were enhanced due to the reduction in leakage current and they showed well saturated polarization hysteresis loops. The remanent polarization value in KBiFe₂O₅ -PVA composite film (Pr?=?3.7?μC/cm²) was found to be 2.5 times higher than that of the pure KBiFe₂O₅ film (Pr?=?1.45?μC/cm²) and with the increase in PVA percentage, the remanent polarization values also increased. The space charge dominated conduction in pure KBiFe₂O₅ became transformed to field assisted ionic conduction in the KBiFe₂O₅ composite films. The polymeric matrix in KBiFe₂O₅ composites helped in reducing the leakage current of pure KBiFe₂O₅ by four orders of magnitude and further reduced with the increase in the PVA content in the composite. The composite films showed fatigue free polarization even after 10⁸ switching cycles whereas the polarization degradation in KBiFe₂O₅ film was 30% around 10⁸switching cycles.     

Effect of Substrate on Structure and Multiferrocity of (La,Mn) CoSubstituted BiFeO3 Thin Films

In this article, we report the substrate effect on ferroelectric and magnetic properties of epitaxial BiFeO₃‐based thin films at room temperature. (La, Mn) cosubstituted BiFeO₃ (BFOLM) thin films were deposited on differently lattice mismatched single‐crystal substrates to manipulate the strain states in the as‐deposited films. All the films with 30‐nm thick CaRuO₃ bottom electrodes exhibited highly epitaxial growth behavior with a slightly monoclinic distorted lattice structure while their strain states are drastically different as confirmed by X‐ray reciprocal space mapping. These films possessed significantly different macroscopic ferroelectric properties with giant remanent polarization of 101 ± 2, 65 ± 2, and 48 ± 2 μC/cm² for the films grown on SrTiO₃, (La, Sr)(Al, Ta)O₃, and LaAlO₃, respectively. It is found that the room‐temperature magnetic properties are also in accordance with their strain state, having a reciprocal relationship with polarization. For example, the enhanced magnetization is associated with the suppressed polarization and vice versa. The stain tunability of multiferroic properties in BFOLM thin films are presumably ascribed to the polarization rotation and oxygen octahedral tilts.     

Effects of yttrium on wettability and interactions between molten superalloy and SiO2-based ceramic core

In order to obtain high-quality superalloy castings, the wettability and interactions between superalloy melts with various Y contents and SiO₂-based ceramic cores were investigated at 1823 K. The results indicated that the wettability and interface reactions were affected by the content of Y in the alloy. For the alloys with Y content less than 0.011 wt%, no Y-oxide was found at the interface, but HfO₂, Al₂O₃ and ZrO₂ phases were formed, and the wetting angle dropped slightly. However, different Y-oxides precipitated at the alloy-ceramic interface for the alloys with Y content more than 0.017 wt%, and the wetting angle dropped sharply. When the content of Y was 0.017 and 0.025 wt%, Al₂O₃, Y₃Al₂(AlO₄)₃, HfO₂ and ZrO₂ phases were formed at the interface. When the content of Y was 0.1 wt%, YAlO₃, Y₃Al₅O₁₂, Y₄Al₂O₉, HfO₂ and ZrO₂ phases were formed. The formation of different reaction products was mainly caused by the change of Y activity (a_Y) in the alloy. The reaction between Y and SiO₂ could improve the wettability of the system apparently.     

Chemical and interfacial design in the visible-light-absorbing ferroelectric thin films

Ferroelectric thin films with switchable polarization and anomalous photoelectric effects have received extensive attention recently. However, the improvement of photoelectric performance is accompanied by the weakening of ferroelectricity. Here, both chemical and interlayer design are used to regulate the polarization and optical properties of BiFeO₃-based ferroelectric films. We achieved an improvement in both ferroelectricity and bandgap by chemical composition. The remanent polarization has been enhanced to 73.8 μC/cm² from 0.2 μC/cm², ascribed to the structural transition. The band gap of Eu-BiFeO₃ films has been reduced to 2.23 eV from 2.42 eV due to the unique energy level from Eu 4f, indicating the enhanced visible-light-absorbing capability. We have designed a "sandwich" interfacial structure of homogeneous Eu-BiFeO₃ films. A clever combination between optimal ferroelectricity and narrow band gap with near Eu contents of BFO films would generate an interfacial layer with a homogeneous gradient component, which should favor the switching of ferroelectric domains. The results show that the remanent polarization improved by 17 % to 86.2 μC/cm² while the band gap has also improved. Intriguingly, the short-circuit current density (J_sc) and open circuit (V_oc) of the photovoltaic signal of the optimal films are 89.0 nA and 0.412 V, respectively. This provides a simple and intelligent way to design the ferroelectric-photoelectric thin films and lays the foundation for optical information storage devices.     

Fabrication of high-performance Y2O3 stabilized hafnium dioxide refractories

Y₂O₃ stabilized HfO₂ ceramics with 5 mol% and 8 mol% Y₂O₃, respectively, were successfully fabricated by pressureless sintering at 1600 °C for 1 h. The phase distribution and transformation, microstructure, and relative density were investigated for Y₂O₃ stabilized HfO₂ ceramics. Cubic phase was observed in the Y₂O₃ stabilized hafnium dioxide while pure HfO₂ showed monoclinical phase only. The SEM images of fractured surface indicated two kinds of structures existed in modified HfO₂, the solid solution region and uniform polygon grains, and some holes caused by Kirkendall effect. Refractoriness test showed that high temperature volumetric stability of the material can be effectively enhanced by adding Y₂O₃ into HfO₂.     

Enhanced switchable ferroelectric photovoltaic in BiFeO3 based films through chemical-strain-tuned polarization

Ferroelectric photovoltaic (FE-PV) materials have generated widespread attention due to their unique switchable photovoltaic behavior, but suffering from low photocurrent and remanent polarization. Herein, enhanced ferroelectric polarization and switchable photovoltaic in BiFeO₃ based thin films were achieved by the optimization of Bi content. The compact and uniform films with few defects were obtained by the control of chemical composition. The remanent polarization increased from 3.4 to 73.9 μC cm^?2 showing a qualitative leap. Intriguingly, the control range of photovoltaic signal between two polarization directions of the short-circuit current density (J_SC) and open circuit (V_OC) in present films exhibited an increase of 99.2% and 278.9%, respectively. It is suggested that the ferroelectric polarization was the main driving force for enhancing switchable ferroelectric photovoltaic. Therefore, the present work outstands a simple idea to enhance switchable ferroelectric photovoltaic based on the chemical engineering, providing a promising pathway for the development of photovoltaic devices.     

Effects of interlayer roughness on deposition rate and morphology of aerosol-deposited Al2O3 thick films

The influence of the surface roughness of Al₂O₃ interlayers on the growth of Al₂O₃ thick films fabricated by an aerosol deposition (AD) process was investigated as an approach to improving the plasma resistance of the films. The Al₂O₃ interlayer was fabricated by a plasma electrolytic oxide (PEO) method. This method is capable of fabricating films on the entire surface area of 3-dimensional substrates, whereas the AD process has difficulties with depositing films on complex shapes, such as on edges and corners, and inside holes. To prevent degradation of the plasma resistance with increasing working time, the thickness of the Al₂O₃ interlayer was increased by the PEO method. The surface roughness of the Al₂O₃ interlayer was increased linearly by increasing the thickness of the Al₂O₃ interlayer. On Al₂O₃ interlayers with surface roughness values of more than 1.5 μm (R_a), Al₂O₃ films were not grown by the AD process. To investigate the effect of the surface roughness of the Al₂O₃ interlayer on the growth of Al₂O₃ films on the Al₂O₃ interlayer, we attempted to deposit Al₂O₃ films on an Al₂O₃ interlayer whose surface roughness was decreased from 1.5 μm to 0.8 μm by polishing. As a result, an Al₂O₃ film of 2.0 μm in thickness was grown by the AD process. These study results support the conclusion that controlling of the surface roughness is the most important factor in aerosol-deposited film growth.     

Effect of repeated composite sol infiltrations on the dielectric and piezoelectric properties of a Bi0.5(Na0.82K0.18)0.5TiO3 lead free thick film

Bi_0.5(Na_0.82K_0.18)_0.5TiO₃ lead free thick films have been produced using a combination of screen printing and subsequent infiltration of corresponding composite sol. Their structure, dielectric, ferroelectric and piezoelectric properties were investigated with variation in the number of composite sol infiltrations and the nanopowder loading in composite sol. Dielectric constant, remanent polarization, and piezoelectric coefficient have been shown to increase with increasing numbers of composite sol infiltration. Dielectric and ferroelectric properties of the thick films are found to be strongly dependent on the powder concentration of composite sols. The resulting 40 μm thick films infiltrated with 1.5 g/ml composite sols have maximum relative permittivity of 569 (at 10 kHz), remanent polarization of 21.3 μC/cm², coercive field of 80 kV/cm, and longitudinal effective piezoelectric coefficient d_33eff of 109 pm/V. The performance of these lead free piezoelectric thick films is comparable to the corresponding bulk ceramics.     

Low-temperature joining of SiC ceramics with Y2O3-Al2O3 interlayer by SiO2-based liquid phase extrusion strategy

In order to reduce the joining temperature of SiC ceramics by glass-ceramic joining, some oxides were usually introduced into to Y₂O₃–Al₂O₃ for reducing the eutectic temperature. However, the joints might have poor high-temperature resistance due to the low melting point of the joining layer. In the present work, based on novel SiO₂-based liquid phase extrusion strategy, joining of SiC ceramics with Y₂O₃–Al₂O₃ interlayer was carried out by using Y₂O₃–Al₂O₃–SiO₂ as the filler through spark plasma sintering (SPS). The SiO₂-free interlayer of Y₂O₃–Al₂O₃ was used for comparison. It was found that SiC joints using Y₂O₃–Al₂O₃ could be only joined at a high temperature of 1800 °C, and the thickness of the interlayer was about 20 μm. The shear strength of the joint obtained at 1800 °C was 89.62 ± 4.67 MPa and the failure located in the SiC matrix. By contrast, reliable joining of SiC ceramics could be finished at as low as 1550 °C by extrusion of SiO₂-containing liquid phase when using Y₂O₃–Al₂O₃–SiO₂ as the interlayer, alongside the interlayer thickness of only several microns. The joint strengths after joining at 1550 °C was 84.90 ± 3.48 MPa and the failure located in matrix position. The joining mechanism was discussed by combining the detailed microstructure analysis and phase diagram.     

Electric field dependence of ferroelectric stability in BiFeO3 thin films co-doped with Er and Mn

Bi_0.9Er_0.1Fe_1−xMn_xO₃ (BEFM_xO, x = 0.00–0.03) films are synthesized by a sol–gel technique. The BEFO film exhibits a conduction mechanism based on electron tunneling. The high applied electric field causes dissociation of the defect complex, and the resulting oxygen vacancies contribute to fake polarization. Consequently, the BEFO film has poor polarization stability at high applied electric fields. Coexistence of two phases (with space groups R3c:H and R3m:R) and reduced concentrations of oxygen vacancies and Fe²⁺ in BEFM_xO are achieved by co-doping with Er and Mn. The presence of bulk-based conduction in the BEFM_xO films then leads to ferroelectric domain switching contributing to the real polarization and to excellent ferroelectric stability. In addition, the BEFM_0.02O film shows a typical symmetrical butterfly curve, the highest remnant polarization of ~109 μC/cm², and the highest switching current of ~1.66 mA. It also has the smallest oxygen vacancy concentration and thus the smallest amount of defect complex, which means that there are fewer pinning effects on ferroelectric domains and therefore excellent ferroelectric stability. This excellent ferroelectric stability makes the BEFM_xO films obtain good stability and reliability in the application of ferroelectric memory devices.     

Ferroelectric yttrium doped hafnium oxide films from all-inorganic aqueous precursor solution

We report a unique aqueous solution deposition method to prepare yttrium doped hafnium oxide (Y:HfO₂) thin films using all-inorganic reagents. The composition and chemical bonding features of the films were investigated using X-ray photoelectron spectroscopy. The Y:HfO₂ film was integrated into metal-insulator-semiconductor (MIS) structure capacitors for electrical measurements. A transition of the polarization behavior from apparent ferroelectric-type to linear dielectric-type was observed for films with thickness increasing from 25?nm to 80?nm, which is correlated to the dominant crystal structure change from high-symmetry phase to monoclinic phase evidenced by grazing incidence X-ray diffraction analysis.     

Stress Modulation and Ferroelectric Properties of Nanograined PbTiO3 Thick Films on the Different Substrates Fabricated by Aerosol Deposition

Nanograined PbTiO₃ (PT) thick films were deposited on Si, yttria‐stabilized zirconia (YSZ), and Ni substrates using an aerosol deposition (AD) method at room temperature. The AD PT thick films on each different substrate were annealed at 500°C and 700°C for 1 h to increase the crystallinity. The stresses in the PT film were modulated by controlling the difference in the coefficient of thermal expansion (CTE) between the films and substrates during the thermal annealing process. The morphology of the AD PT films was examined from the polycrystalline dense structure (thickness ~8 μm), and the changes in the crystallographic phase, in‐plane stresses, and ferroelectric properties in annealed films were investigated. In‐plane stress analysis showed that the PT films annealed at 500°C and 700°C on each substrate exhibited compressive stress. Owing to the effects of compressive stress in the PT film, the film showed less tetragonality (c/a ratio) and enhanced ferroelectric behaviors. The change in the polarization–electric field (P–E) hysteresis loop of the PT films was explained by the stress induced from CTE mismatch between the films and substrates.     

Thickness dependent growth and ferroelectric/dielectric properties of phase-pure 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 thin film derived from a modified sol-gel process

Annealing parameter and thickness are two significant factors affecting microstructure and electrical performance of sol-gel derived 0.65Pb(Mg_1/3Nb_2/3)O₃?0.35PbTiO₃ (0.65PMN-0.35PT) thin film. In this paper, various durations are firstly selected for the investigations on annealing parameter of 0.65PMN-0.35PT thin film. Enhanced insulating and ferroelectric properties can be obtained for the film annealed for 1 min due to its phase-pure and homogeneous perovskite structure. Based on this, a series of 0.65PMN-0.35PT thin films with various thicknesses by modifying deposition layer are synthesized annealed for 1 min and the effects of thickness on crystalline, insulating, ferroelectric and dielectric properties are characterized. It reveals that thickness-dependent behavior can be noticed for 0.65PMN-0.35PT thin film with the results that the 8-layered film possesses a relative large remanent polarization (P_r) of 23.34 μC/cm², and reduced leakage current density of 10^?9 A/cm² with low dissipation factor (tanδ) of 0.03 can be achieved for the 14-layered film.     

Enhanced photovoltaic properties of PbTiO3-based ferroelectric thin films prepared by a sol-gel process

PbTiO₃ (PTO), Pb(Mn_0.1Ti_0.9)O₃ (PMTO), Pb(Sr_0.1Ti_0.9)O₃ (PSTO), and Pb(Zr_0.1Ti_0.9)O₃ (PZTO) were prepared on an indium tin oxide (ITO)/glass substrate by a sol-gel method. PTO, PMTO, PSTO, and PZTO films exhibited energy band gaps of 3.55 eV, 3.63 eV, 3.59 eV, and 3.66 eV, respectively. All these films generated high photocurrents due to high shift currents, because carrier migration channels were successfully introduced by a lattice mismatch between the films and ITO substrates. The PMTO thin film exhibited the best ferroelectric and photovoltaic properties, with a photovoltage of 0.74 V, a photocurrent density of 70 μA/cm², and a fill factor of 43.34%, which confirms that shift current and ferroelectric polarization are two main factors that affect the ferroelectric photovoltaic properties. The PSTO, PZTO, and PTO thin films displayed space-charge-limited current (SCLC) when the electric field strength was below 10 kV/cm, and these three films broke down when the electric field strength was above 10 kV/cm. Analysis of the shift current mechanism confirmed that the breakdown of the PZTO and PSTO thin films resulted from Pool Frenkel emission current. The PMTO thin film displayed SCLC in the test range, which indicates that doping with Mn could inhibit defect formation in ferroelectric thin films.     

Effects of annealing temperature on the microstructure,ferroelectric and dielectric properties of W-doped Na0.5Bi0.5TiO3 thin films

The effects of annealing temperature on the structure, morphology, ferroelectric and dielectric properties of Na_0.5Bi_0.5Ti_0.99W_0.01O_3+δ (NBTW) thin films are reported in detail. The films are deposited on indium tin oxide/glass substrates by a sol-gel method and the annealing temperature adopted is in the range of 560–620 °C. All the films can be well crystallized into phase-pure perovskite structures and show smooth surfaces without any cracks. Particularly, the NBTW thin film annealed at 600 °C exhibits a relatively large remanent polarization (P_r) of 20 μC/cm² measured at 750 kV/cm. Additionally, it shows a high dielectric constant of 608 and a low dielectric loss of 0.094 as well as a large dielectric tunability of 62%, making NBTW thin film ideal in the room-temperature tunable device applications.     

Low-temperature sintering and electrical properties of Ba0.68Sr0.32TiO3 thick films

Ba_0.68Sr_0.32TiO₃ (BST) thick films were prepared by screen printing on a flexible fluorophlogopite substrate. In order to realise the co-firing of the BST film with a silver electrode at a lower temperature, the BST precursor was used as a solvent for the screen-printing slurry and the cold sintering technique was used to pretreat the film. The sintering temperature of BST thick films prepared by conventional sintering process was higher than 1200 °C. When sintered at 950 °C, the thick films exhibited a high porosity. The density of the thick films was significantly improved after pretreatment with the cold sintering process (CSP). After the cold-sintered thick films were sintered at 950 °C for 30 min and then fired with a silver electrode, the samples exhibited a relative dielectric constant of 773 (at 25 °C and 10 kHz), a dielectric loss of 0.025, a remanent polarization of 5.3 μC/cm², and a coercive field strength of 38.1 kV/cm. Therefore, the low-temperature co-firing of BST thick films with a silver electrode was successfully realised.