O3 Films Research Articles

The ferroelectric and piezoelectric properties of (Hf1−x Ce x )O2 films on indium tin oxide/Pt/TiO x /SiO2/(100)Si substrates obtained using a no-heating radio-frequency magnetron sputtering deposition method

The effect of composition and film thickness on the ferroelectric and piezoelectric properties of (Hf1−x Ce x )O2 films deposited without substrate heating was investigated. (Hf1−x Ce x )O2 films with various x values (x = 0.07–0.27) and thickness (150–880 nm) were deposited via RF magnetron sputtering on indium tin oxide (ITO)/Pt/TiO x /SiO2/(100)Si substrates. The crystalline phases of the films were observed by X-ray diffraction. The measurements of electrical properties revealed ferroelectric phases in the x range of 0.11–0.21. The film with x = 0.16 exhibited the maximum remanent polarization (Pr) of 15 μC cm−2, as well as the highest effective piezoelectric coefficient. In addition, the ferroelectric and structural properties remained almost unchanged with increasing film thickness. Therefore, the no-heating deposition of ferroelectric (Hf1−x Ce x )O2 films and their phase stability with respect to thickness were demonstrated in this study. This work provides a pathway for the deposition of ferroelectric (Hf1−x Ce x )O2 films on flexible, wearable sensors.

Epitaxial growth of α-(AlxGa1−x)2O3 by suboxide molecular-beam epitaxy at 1 µm/h

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow α-(AlxGa1−x)2O3 films on (110) sapphire substrates over the 0 < x < 0.95 range of aluminum content. In S-MBE, 99.98% of the gallium-containing molecular beam arrives at the substrate in a preoxidized form as gallium suboxide (Ga2O). This bypasses the rate-limiting step of conventional MBE for the growth of gallium oxide (Ga2O3) from a gallium molecular beam and allows us to grow fully epitaxial α-(AlxGa1−x)2O3 films at growth rates exceeding 1 µm/h and relatively low substrate temperature (Tsub = 605 ± 15 °C). The ability to grow α-(AlxGa1−x)2O3 over the nominally full composition range is confirmed by Vegard’s law applied to the x-ray diffraction data and by optical bandgap measurements with ultraviolet–visible spectroscopy. We show that S-MBE allows straightforward composition control and bandgap selection for α-(AlxGa1−x)2O3 films as the aluminum incorporation x in the film is linear with the relative flux ratio of aluminum to Ga2O. The films are characterized by atomic-force microscopy, x-ray diffraction, and scanning transmission electron microscopy (STEM). These α-(AlxGa1−x)2O3 films grown by S-MBE at record growth rates exhibit a rocking curve full width at half maximum of ≊ 12 arc secs, rms roughness <1 nm, and are fully commensurate for x ≥ 0.5 for 20–50 nm thick films. STEM imaging of the x = 0.78 sample reveals high structural quality and uniform composition. Despite the high structural quality of the films, our attempts at doping with silicon result in highly insulating films.

Low oxidation conditions in pulsed laser deposition enhance polarization without degradation of endurance and retention in Hf0.5Zr0.5O2 films

Interplay between oxygen vacancies and the stabilization of the ferroelectric orthorhombic phase in doped HfO2, as well as the resulting impact on endurance and retention, is far from being well understood. In Hf0.5Zr0.5O2 (HZO) thin films, it is commonly found that high polarization occurs usually at the the expense of robustness upon cycling due to the polarization–endurance dilemma. It has been reported that HZO thin films grown by pulsed laser deposition under the mixed Ar and O2 atmosphere exhibit a high polarization. Here, we show that this strategy enables functional properties tuning, allowing to obtain HZO films with high polarization at low oxidation conditions without degradation of endurance and retention.

Fabrication of conductive Nb-doped anatase TiO2 thin films by mist chemical vapor deposition using aqueous solutions of water-soluble Ti and Nb compounds

Electrically conductive Nb-doped anatase TiO2 (Ti1−x Nb x O2: TNO) films can be fabricated through mist CVD using aqueous precursor solutions of water-soluble oxo-peroxo-glycolato titanium complex and ammonium niobium oxalate. Post-deposition annealing in vacuum crystallizes the as-deposited amorphous films into a conductive anatase phase, leading to the fabrication of conductive TNO films. Notably, the addition of H2O2 to the precursor solutions enhances both the crystallinity and conductivity of the annealed TNO films. A Ti0.77N0.23O2 film, annealed at 700 °C, exhibits a resistivity of 2.0 × 10−2 Ω cm at ambient temperature. In general, the solution-based fabrication of TiO2 films relies on organic solvents, which are sometimes toxic and explosive. Here, we demonstrate for the first time that conductive TNO films can be prepared from less toxic and nonflammable aqueous solutions. These findings mark a significant advancement towards a more environmentally compatible process for fabricating TNO films with sufficient conductivity.

Anomalous polarization-switching phenomena and noteworthy pyroelectricity in ferroelectric Hf0.5Zr0.5O2 polycrystalline films

Hafnia-based oxide thin films exhibit unconventional robust ferroelectricity at the nanoscale, paving the way for advancements in next-generation nanoelectronics. However, the stability and dynamics of ferroelectric polarization in hafnia-based oxides films, particularly at the nanoscale, warrant further exploration. In this study, we delve into the factors influencing the formation of the ferroelectric phase. Our findings illuminate that the confinement effect of the capped layer enhances the stabilization of the metastable orthorhombic phase, thereby bolstering its ferroelectricity and endurance. Furthermore, we identify an unprecedented polarization-switching behavior in HfO2-based ferroelectric films, characterized by a decrease in the remanent polarization and an increase in the coercive field as the temperature drops. This peculiar phenomenon can be ascribed to the reduction in the long-range interaction of polarization dipoles upon cooling, as evidenced by the dielectric spectrum. The diminished long-range interaction results in the emergence of nanodomains and a re-entrant relaxer behavior, offering novel insights into the origin of ferroelectricity and the stability of ferroelectric polarization in hafnia-based thin films. Given the commendable compatibility of Hf0.5Zr0.5O2 films with Si substrates and a pyroelectric coefficient of −62.1 μC/m2·K, they emerge as a prime candidate for sensor device applications.

In-plane magnetic properties and anisotropy of scandium substituted thulium iron garnet thin films for fluxgate magnetometer

In-plane easily magnetized scandium substituted thulium iron garnet films(Tm3Fe5-xScxO12) with soft magnetic properties have recently attracted attention for fluxgate magnetometer applications. In this paper, thin Tm3Fe5-xScxO12 films were prepared on gadolinium gallium garnet substrates by liquid phase epitaxial(LPE) method. Microstructural properties, composition, magnetic properties and the in-plane anisotropy of the films were discussed. We found that the films turned from tensile stress to compressive stress as the Sc3+ substitution decreased, and the in-plane anisotropy was mainly caused by stress reducing as Sc3+ declined. By analyzing the in-plane magnetic hysteresis loops, the magnetic performance was the best when x = 0.75. Tm3Fe4.25Sc0.75O12 film had the lowest in-plane coercive field(0.006 Oe) and the highest initial magnetic permeability(378), which was more beneficial to realize fluxgate magnetometers.

Enhancing ferroelectric performance in hafnia-based MFIS capacitor through interface passivation and bulk doping

In recent times, there has been a notable surge of interests in hafnia (HfO2)-based ferroelectrics, primarily due to their remarkable ferroelectric properties employed in ultra-thin configurations, alongside their compatibility with the conventional CMOS manufacturing process. In order to harness the full potential of HfO2-based films for high-performance non-volatile memory applications, it is imperative to enhance their ferroelectric characteristics and durability. This study introduces a straightforward approach aimed at augmenting the ferroelectric performance of Hf x Zr1−x O2 (HZO) films deposited on silicon (Si) substrates through the engineering of oxygen vacancies (V O). The results of this endeavor demonstrate a significant enhancement in ferroelectric performance, characterized by a 2Pr value of 47 μC cm−2 and impressive endurance, enduring up to 108 cycles under an 8 MV cm−1 electric field without the need of a wake-up process. This marked improvement can be attributed to a dual-pronged approach, involving the incorporation of an Al2O3 interlayer and the introduction of Al atoms into the HZO film. The Al2O3 interlayer primarily serves to mitigate the presence of oxygen vacancies at the interface, while the introduction of Al dopants elevates the concentration of oxygen vacancies within the bulk material. This modulation of oxygen vacancy concentration proves instrumental in facilitating the formation of a ferroelectric o-III phase within the HZO-based films, thereby further augmenting their ferroelectric performance. This innovative and effective strategy offers an alternative avenue for enhancing the ferroelectric properties of materials characterized by a fluorite crystal structure.

Enhanced Cathode Performance: The Heterostructure Construction of LiCoO2@Co3O4@Li6.4La3Zr1.4Ta0.6O12.

In this study, the heterostructure cathode material LiCoO2@Co3O4@Li6.4La3Zr1.4Ta0.6O12 was prepared by coating Li6.4La3Zr1.4Ta0.6O12 on the surface of LiCoO2 through a one-step solid-phase synthesis. The morphology, structure, electrical state, and elemental contents of both pristine and modified materials were assessed through a range of characterization techniques. Theoretical calculations revealed that the LCO@LLZTO material possessed a reduced diffusion barrier compared to LiCoO2, thereby facilitating the movement of Li ions. Electrochemical tests indicated that the capacity retention rate of the modified cathode composites stood at 70.43% following 300 cycles at a 2C rate. This high rate occurred because the Li6.4La3Zr1.4Ta0.6O12 film on the surface enhanced the migration of Li+, and the spinel phase of Co3O4 had better interfacial stability to alleviate the generation of microcracks by inhibiting the phase change from the layered phase to the rock-salt phase, which considerably improved the electrochemical properties.

Large room temperature magnetoelectric response in quasi 1–2 nanocomposite films on mica substrate

AbstractA novel quasi 1–2 type magnetoelectric (ME) nanocomposite films composted of one‐dimensional CoFe2O4 nanofibers and two‐dimensional PbZr0.2Ti0.8O3 films on flexible mica substrate have been designed and fabricated. The ME coupling effects have been greatly enhanced due to the release of the clamping effect and the high strain transfer efficiency between the large aspect ratio of the nanofibers and ferroelectric thin films. The release of the clamping effect was confirmed by reducing the thickness of the mica substrate, in which the piezoelectric coefficient and ME coupling coefficient of the nanocomposite films have been greatly boosted by the decrease in the mica thickness. A large direct ME coupling coefficient of 630 mV/(cm Oe) and a converse ME coupling coefficient of 1.43 × 10−8 s/m have been achieved in the quasi 1–2 nanocomposite film with a substrate thickness of 25 µm. Furthermore, the stain‐mediated ME coupling was revealed by magnetic field/electric field‐induced local ferroelectric/magnetic domain switching dynamics.

The effect of surface-step-terrace on energy storage density and working temperature of BaZr0.2Ti0.8O3 epitaxial films

Dielectric film capacitors have been widely used in various electronic devices due to the high energy storage density and high charge–discharge speed. The energy storage performances are highly determined by the strain status of the films. Herein, we improved the energy storage performances of epitaxial BaZr0.2Ti0.8O3 film through surface-step-terrace in the vicinal substrate. It is found that the terraces in the vicinal substrate create an additional strain parallel to the terraces and a compressive strain along the out-of-plane direction. The strains result in the increase in the energy storage density of the BZT film up to 66.9 J/cm3 with an energy storage efficiency of 73.2% at room temperature, which is 2.2 times as high as that of the films on regular substrates. Moreover, the strains also lead to the reduction of the hysteresis loss and increase in the up-limited working temperature from 125 to 225 °C with an energy storage density of 52.35 J/cm3 and an efficiency of 70.25%, exhibiting excellent thermal stability. Our work demonstrates that the strains generated by vicinal substrates can improve the energy storage performance greatly and provide a method to improve the energy storage performances of dielectric thin film capacitors.

Impact of HfO2 Dielectric Layer Placement in Hf0.5Zr0.5O2‐Based Ferroelectric Tunnel Junctions for Neuromorphic Applications

AbstractThe use of Hf0.5Zr0.5O2 (HZO) films within hafnia‐based ferroelectric tunnel junctions (FTJ) presents a promising avenue for next‐generation non‐volatile memory devices. HZO exhibits excellent ferroelectric properties, ultra‐thinness, low power consumption, nondestructive readout, and compatibility with silicon devices. In this study, Mo/HZO/n+ Si devices are investigated, incorporating a 1 nm HfO2 dielectric layer at the top and bottom of the HZO ferroelectric layer. Comparing the FTJ device configurations, it is observed that the metal‐ferroelectric‐dielectric‐semiconductor (MFIS) outperforms the metal‐dielectric‐ferroelectric‐semiconductor (MIFS) in terms of ferroelectricity, displaying a high 2Pr value of ≈69 µC cm−2. Additionally, MFIS exhibits lower leakage current, higher tunneling electro‐resistance ratio, and a thin dead layer during short pulse switching, as confirmed through DC double sweeping of I−V characteristics. The modified half‐bias scheme demonstrates a maximum array size of 191 for MFIS, showcasing its superior performance over MIFS. Synaptic characteristics, including potentiation, depression, paired‐pulse facilitation, spike‐rate‐dependent plasticity, and excitatory postsynaptic current, are measured using MFIS, highlighting its outstanding ferroelectric properties. As a physical reservoir, the FTJ device implements 16 states of 4 bits in reservoir computing. Finally, pattern recognition using a deep learning neural network achieves high accuracy with using the Modified National Institute of Standards and Technology dataset.

Intrinsic origin of room temperature ferromagnetic ordering in (In0.97-xCoxMg0.03)2O3 thin films

The (In0.97-xCoxMg0.03)2O3 films with different Co concentration are prepared using RF magnetron sputtering. The XRD and XPS measurements show that the films are highly oriented and both Co and Mg elements present valence state of +2. The EXAFS spectra show that only substitutional Co ions exist in the film with x = 0.02. However, the films with higher Co concentration (x > 0.02) form separate phases of substitutional Co and metallic Co clusters. All the films exhibit n-type conductivity and the carrier concentration firstly decreases and then increases with Co doping. The transport mechanisms are dominated using Mott variable range hopping at low temperature and Hard bandgap hopping at high temperature. The Co dopant can effectively alter the MR characters, which undergoes a clear transition from negative MR to positive MR. All the films exhibit room temperature ferromagnetic hysteresis loop. Clear increase of ferromagnetic hysteresis loop and saturation magnetic moment are observed due to the incorporation of Co ions. The ferromagnetism is ascribed to oxygen vacancies-induced bound magnetic polarizations. The relative percentage of substitutional Co ions decrease with Co doping, which leads to the decreasing of number of bound magnetic polarons induced by oxygen vacancies, and then weaken the Ms.

Ferroelectricity in Ce0.2-HfO2 films around 500 nm in thickness

CeO2-HfO2 solid solution thin films (Hf1-xCexO2) were deposited on Pt(111)/TiO2/SiO2/Si(100) substrates using the chemical solution deposition method. This study investigates the influence of CeO2 content and annealing temperature on the structure and ferroelectric properties of Hf1-xCexO2 films. Ferroelectric behavior is demonstrated in polycrystalline Hf0.80Ce0.20O2 films with thicknesses ranging from 163 to 524 nm. And the structure of the films is analyzed using glancing incidence X-ray diffraction. The comprehensive results indicate that Hf0.80Ce0.20O2 films annealed at 850 °C exhibit excellent ferroelectricity. Square hysteresis loops associated with the ferroelectric orthorhombic phase are observed, even in the 524-nm-thick film. The remnant polarization (Pr) and coercive field (Ec) range from 16 to 18 μC/cm2 and 1100–1250 kV/cm, respectively, under a maximum applied electric field of 2 MV/cm for all Hf0.80Ce0.20O2 films. Furthermore, the film presents thickness-insensitive characteristic. The current work paves the new way to design high-performance thick HfO2-based ferroelectric films.

Fault-tolerant structures against breakdown in heterogeneous ferroelectric PbZr0.52Ti0.48O3 thin films based on adaptive self-repairing effects

The design of fault-tolerant structures against the breakdown of heterogeneous ferroelectric films is essential to enhance the breakdown strength. Based on the self-repairing theory, heterogeneous ferroelectric PbZr0.52Ti0.48O3 (PZT) film capacitors with a fault-tolerant structures against breakdown were synthesized by utilizing dielectric/electrolytic dual characteristics of the amorphous aluminum oxide film (AmAO). The PZT/AmAO composite films with Au top electrodes exhibited significant weak breakdown at low voltages, whereas the breakdown in the composite films with Al top electrodes have been repaired and no longer occurred. Compared to PZT(100 nm)/AmAO(120 nm) composite films with Au top electrodes, breakdown strength of PZT(100 nm)/AmAO(120 nm) composite films with Al top electrodes improves from 278 MV/m to 477 MV/m, energy storage density increased from 14.3 J/cm3 to 22.5 J/cm3, since the heterogeneous interface and structural defects have been repaired by adaptive anodizing of Al electrode under high electric field. Cross-sectional SEM images have confirmed the formation of fresh aluminium oxide films (AAO) at the interfaces of AmAO/Al and PZT/AmAO during the application of an electric field. Defects have been effectively repaired, resulting in an increase in breakdown strength. This study establishes a theoretical and technical foundation for the design of fault-tolerant structures against breakdown in heterogeneous ferroelectric films.

Magnetic field tuning of photoelectric and photoluminescence effects in BiFe0.9Co0.1O3 thin film

The reported BiFe0.9Co0.1O3 film presents an interesting magnetic field tunning effect on the photoelectric properties and photoluminescence spectra. The change rate of the photocurrent up to 123.6% was achieved when applying a 400 Oe magnetic field to the film, which is attributed to the spin scattering of photoelectrons in the film. The experimental result of the magnetic field tuning photoluminescence spectra indicates that the magnetic field inhibits the radiative transition of the photogenerated carriers in the thin film and therefore reduces the intensity of photoluminescence. This research provides a basis for the research and application of devices in the field of multifunctional photoelectric materials.

Electric Field Switching of Magnon Spin Current in a Compensated Ferrimagnet.

Manipulation of directional magnon propagation, known as magnon spin current, is essential for developing magnonic devices featuring nonvolatile functionalities and ultralow power consumption. Magnon spin current can usually be modulated by magnetic field or current-induced spin torques. However, these approaches may lead to energy dissipation due to Joule heating. Electric-field switching of magnon spin current without charge current is highly preferred but challenging to realize. By integrating magnonic and piezoelectric materials, the manipulation of the magnon spin current generated by the spin Seebeck effect in the ferrimagnetic insulator Gd3Fe5O12 (GdIG) film on a piezoelectric substrate is demonstrated. Reversible electric-field switching of magnon polarization without applied charge current is observed. Through strain-mediated magnetoelectric coupling, the electric field induces the magnetic compensation transition between two magnetic states of the GdIG, resulting in its magnetization reversal and the simultaneous switching of magnon spin current. This work establishes a prototype material platform that paves the way for developing magnon logic devices characterized by all electric field reading and writing and reveals the underlying physics principles of their functions.

Self-powered solar blind ultraviolet photodetector based on amorphous (In0.23Ga0.77)2O3/bixbyite (In0.67Ga0.33)2O3 heterojunction

Self-powered solar-blind ultraviolet photodetectors are considered for potential applications in secure communication and space detection. However, high-quality p-type wide bandgap semiconductors are nonexistent due to the self-compensation effect, which makes the design of p-n homojunction photodetectors a challenging proposition to date. In this work, a self-powered solar-blind ultraviolet photodetector is fabricated and discussed, based on a novel heterojunction of (InxGa1−x)2O3 ternary alloy films with two different compositions, which has a flexible design and can be easily fabricated for different applications. The heterojunction consists of an amorphous (In0.23Ga0.77)2O3 on the top of a bixbyite (In0.67Ga0.33)2O3 film prepared by radio frequency magnetron sputtering. The amorphous (In0.23Ga0.77)2O3/bixbyite (In0.67Ga0.33)2O3 heterojunction photodetector exhibits a responsivity of 5.78 mA/W, a detectivity of 1.69 × 1011 cm Hz1/2 W−1, and a high solar-blind UV (248 nm)/visible light (450 nm) rejection ratio of 1.39 × 103 at zero bias, suggesting decent spectral selectivity and high performance. The responsivity and peak wavelength of this photodetector can be tuned by the film thickness of the amorphous (In0.23Ga0.77)2O3. This work provides a new design for self-powered solar-blind UV detectors based on ternary alloy heterojunctions.

Superflexible and Stretchable Ferroelectric Memory on a Biocompatible Platform

AbstractNext‐generation flexible electronics for healthcare applications require biocompatible flexible non‐volatile memory for data storage. Ultra‐thin ferroelectric hafnium oxide films offer great potential for flexible memories due to their potential flexibility and perfect compatibility with modern technologies. This study presents ultra‐flexible and stretchable memory devices based on 10‐nm‐thick Hf0.5Zr0.5O2 film fabricated by an innovative technology involving encapsulation of the devices in a biocompatible organic package. They exhibit high memory functionality (remanent polarization of 27 µC cm−2) and withstand extreme mechanical conditions, including folding in half, multiple bending up to 150 000 bending cycles as well as tension with loads up to 1.5 kg. Further, flexible devices are employed as a platform to elucidate the fundamental role of mechanical stress in ferroelectricity of hafnia both experimentally and theoretically. Direct in situ experiment demonstrates that in‐plane tension causes changes in spontaneous polarization, coercive voltage, permittivity, and conductivity. First‐principle calculations explain the role of mechanical stress in ferroelectric and dielectric properties of hafnia. In applications, this work establishes a foundation for the implementation of biocompatible, high‐performance flexible ferroelectric memory, and in the field of ferroelectric materials fundamentals, it provides insight into the critical role of the residual mechanical stress that is inevitably present in thin films.

Impacts to FeRAM Design Arising From Interfacial Dielectric Layers and Wake-Up Modulation in Ferroelectric Hafnium Zirconium Oxide.

As ferroelectric hafnium zirconium oxide (HZO) becomes more widely utilized in ferroelectric microelectronics, integration impacts of intentional and nonintentional dielectric interfaces and their effects upon the ferroelectric film wake-up (WU) and circuit parameters become important to understand. In this work, the effect of the addition of a linear dielectric aluminum oxide, Al2O3, below a ferroelectric Hf0.58Zr0.42O2 film in a capacitor structure for FeRAM applications with niobium nitride (NbN) electrodes was measured. Depolarization fields resulting from the linear dielectric is observed to induce a reduction of the remanent polarization of the ferroelectric. Addition of the aluminum oxide also impacts the WU of the HZO with respect to the cycling voltage applied. Intricately linked to the design of a FeRAM 1C/1T cell, the metal-ferroelectric-insulator-metal (MFIM) devices are observed to significantly shift charge related to the read states based on aluminum oxide thickness and WU cycling voltage. A 33% reduction in the separation of read states are measured, which complicates how a memory cell is designed and illustrates the importance of clean interfaces in devices.

Improvement in the polarization properties in thin ferroelectric Hf0.5Zr0.5O2 films by two-step flash lamp annealing

In this study, we systematically studied the polarization properties of thin ferroelectric Hf0.5Zr0.5O2 (HZO) films annealed by flash lamp annealing (FLA). We have recently proposed a unique annealing method, two-step FLA, which features high temperature annealing with a minimal increase in thermal budget. Using this technique, we observed improvements in the polarization properties in 5 nm HZO films. These were an increase in the remanent polarization (2Pr) to 24.2 μC cm−2 and better durability compared with other more conventional annealing techniques. In addition, we confirmed that there was no clear degradation in 2Pr under thermal stress. Two-step FLA is one of the effective ways to obtain ferroelectricity with high values of 2Pr and good durability in HZO films thinner than 5 nm.