Yttria-stabilized Zirconia Substrates Research Articles

Coherent Epitaxy of HfxZr1-xO2 Thin Films by High-pressure Magnetron Sputtering

Due to remarkable high-k and ferroelectric properties in CMOS devices, the study of crystalline HfxZr1-xO2 (HZO) thin films has attracted tremendous interest recently. However, up to now, the epitaxial growth of HZO films has only been achieved by pulse laser deposition, a technique scarcely utilized in CMOS devices. Therefore, developing appropriate epitaxial methods of HZO films (such as sputtering) is fairly necessary, but a challenge at present. In this work, high-quality single-crystalline HZO films were synthesized by high-pressure magnetron sputtering. The epitaxial growth of HZO films on yttria-stabilized zirconia (YSZ) substrate was demonstrated by a combination of high-resolution X-ray diffraction, atom force microscope, and scanning transmission electron microscope. In addition, good insulating characteristics were obtained by replacing insulating substrates with conductive substrates as electrodes. Our results provide a novel way for the epitaxial growth of the single-crystalline structure of HZO thin films towards the high performance of high-k and ferroelectric devices.

Photocarrier behavior and photocatalytic H2O2 synthesis performance of amorphous/crystalline SrTiO3/BiVO4 layered heterostructures

BiVO4 (BVO), as a visible light responsive semiconductor material, has been widely employed in the field of photocatalysis. However, monoclinic BVO is afflicted by a high recombination rate of photogenerated charge carriers and poor charge transport capability, resulting in low charge utilization efficiency. Constructing heterostructures for interface modification of BVO is an effective strategy to improve photocatalytic performance. Herein, amorphous/crystalline SrTiO3/BiVO4 (STO/BVO) layered film nanoheterostructures have been successfully fabricated on the (001) yttrium stabilized zirconia substrate by magnetron sputtering. Under 440 nm monochromatic light irradiation, the H2O2 production rate of the composite film is increased by 1.2 times compared to pure BVO. The type II heterostructure composed of STO and BVO accomplishes selective interface hole transport from BVO to STO, promoting the separation of space charges. Additionally, the interaction between amorphous/crystalline interfaces effectively suppresses the photocorrosion of BVO and strengthens the stability of the film, which is of great significance for studying charge transport behavior and photocatalytic mechanisms.

Engineering Oxide Epitaxy beyond Substrate Constraint.

Orientation engineering is a crucial aspect of thin film growth, and it is rather challenging to engineer film epitaxy beyond the substrate constraint. Guided by density functional theory calculations, we use SrRuO3 (SRO) as a buffer layer and successfully deposit [111]-oriented CoFe2O4 (CFO) on [001]-, [110]-, and [111]-oriented SrTiO3 (STO) substrates. This enables subsequent growth of [111]-oriented functional oxides, such as PbTiO3 (PTO), overcoming the constraint of the substrate. This strategy is quite general and applicable to lanthanum aluminate and yttria-stabilized zirconia substrates as well. X-ray Φ scans and atomic resolution aberration-corrected scanning transmission electron microscopy (AC-STEM) reveal detailed epitaxial relations in each of the cases, with four variants of [111]-CFO found on [001]-STO and two variants found on [110]-STO, formed to mitigate the large lattice misfit strain between the film and substrate. Our strategy thus provides a general pathway for orientation engineering of oxide epitaxy beyond substrate constraint.

Ni migration on porous yttria-stabilized zirconia and yttria-micropatterned fuel electrodes in solid oxide fuel cells

This study investigates the mechanism of Ni migration and methods for preventing Ni migration by utilizing ceramic obstacles that can block or slow Ni movement during solid oxide fuel cell (SOFC) operation. A patterned thin-Ni model anode on mirror-polished yttria-stabilized zirconia (YSZ) substrate exhibited dynamic movement during operation, as reported previously. In this study, we attempt to elucidate the mechanism of Ni migration that occurs only on the surface of solid-state Ni by blocking or slowing this movement using porous patterns of different materials. Porous micropatterns of 100 % YSZ and 80 wt% yttria (Y2O3) + YSZ composites were fabricated on a YSZ electrolyte, and velocity changes and blocking of Ni were observed. The porous YSZ structure changed the velocity of the Ni. In contrast, no Ni migration occurred in the Y2O3/YSZ composite porous structure, despite migration occurring in the neighboring smooth YSZ area in the same cell. The Ni-migrated areas were separated by the Y2O3/YSZ porous area, indicating that Ni migration was not caused by microscale differences in temperature or supplied gas diffusion, and Ni moved only on the cell surface, not via Ni-containing gases because temperatures and gases cannot form micropatterns.

Dependence of the Structural and Magnetic Properties on the Growth Sequence in Heterostructures Designed by YbFeO3 and BaFe12O19.

The structure and the chemical composition of individual layers as well as of interfaces belonging to the two heterostructures M1 (BaFe12O19/YbFeO3/YSZ) and M2 (YbFeO3/BaFe12O19/YSZ) grown by pulsed laser deposition on yttria-stabilized zirconia (YSZ) substrates are deeply characterized by using a combination of methods such as high-resolution X-ray diffraction, transmission electron microscopy (TEM), and atomic-resolution scanning TEM with energy-dispersive X-ray spectroscopy. The temperature-dependent magnetic properties demonstrate two distinct heterostructures with different coercivity, anisotropy fields, and first anisotropy constants, which are related to the defect concentrations within the individual layers and to the degree of intermixing at the interface. The heterostructure with the stacking order BaFe12O19/YbFeO3, i.e., M1, exhibits a distinctive interface without any chemical intermixture, while an Fe-rich crystalline phase is observed in M2 both in atomic-resolution EDX maps and in mass density profiles. Additionally, M1 shows high c-axis orientation, which induces a higher anisotropy constant K1 as well as a larger coercivity due to a high number of phase boundaries. Despite the existence of a canted antiferromagnetic/ferromagnetic combination (T < 140 K), both heterostructures M1 and M2 do not reveal any detectable exchange bias at T = 50 K. Additionally, compressive residual strain on the BaM layer is found to be suppressing the ferromagnetism, thus reducing the Curie temperature (Tc) in the case of M1. These findings suggest that M1 (BaFe12O19/YbFeO3/YSZ) is suitable for magnetic storage applications.

Epitaxy‐Driven Ferroelectric/Non‐Ferroelectric Polymorph Selection in an All‐Fluorite System

AbstractFilms of ferroelectric hafnia have hitherto been deposited on electrodes with a non‐fluorite crystal structure. As a result, they are polycrystalline, contain fractions of non‐ferroelectric polymorphs or have poor crystal quality. Here, a strategy that circumvents all these limitations is shown. Seven nanometers‐thick epitaxial Hf0.5Zr0.5O2 (HZO) films are deposited directly on yttria‐stabilized zirconia (YSZ) single‐crystals. The fluorite structure of the whole system enables coherent epitaxy, while the substrate orientation induces polymorph‐selective growth, being the HZO films orthorhombic on YSZ(111) and monoclinic on YSZ(001). Besides, the YSZ substrate can play the role of a buried floating electrode under the appropriate measuring conditions (temperature and frequency) thanks to its thermally‐activated oxygen conductivity. Indeed, out‐of‐plane ferroelectric switching is confirmed in the orthorhombic HZO samples at 185 °C and 0.01 Hz frequency. This original approach avoids the need to deposit conducting bottom layers, allowing high‐quality orthorhombic hafnia to be obtained directly on the substrate and its ferroelectric nature to be studied. Moreover, it constitutes a case of ion‐driven ferroelectric switching, and thus gives support to the recently proposed relationship between ionic conductivity, and ferroelectricity in fluorite systems.

Highly Durable Photoelectrode on the Bendable Yttria-Stabilized Zirconia Substrate for Efficient Photoelectrochemical Water Splitting

Unlike conventional planar photoelectrodes, bendable photoelectrodes expand their applications through mechanical flexibility, making flexible green energy electronics a promising research domain. A 3 mol% yttria-stabilized zirconia (3YSZ) substrate, proving flexibility...

Epitaxial Strain Modulation of Photoelectrochemical Properties of Epitaxial Bismuth Vanadate Photoanodes

Solar fuels derived from photoelectrochemical water splitting have the potential to significantly contribute to achieving sustainability in our global energy infrastructure. Transition metal oxide (TMO) photoabsorbers are interesting candidates as photoelectrodes due to their excellent bandgap tunability, relative stability and cost-effectiveness. Further improvements in the photoelectrochemical performance of such photoelectrodes would entail strategies beyond the discovery of new compositions and structures [1]. Strain engineering in particular is a powerful tool to alter the material’s electronic structure and the resulting properties. For instance, optoelectronic properties (i.e., light absorption characteristics) [2] and surface catalytic properties [3] could be modulated by lattice-mismatch imposed epitaxial strain. Much less understood, however, is how epitaxial strain modulation affects the band bending and surface electronic structure; these properties dictate the photogenerated carrier separation at the photoelectrode/electrolyte interface, and thus the photoelectrochemical performance.To gain further insight into the impact of epitaxial strain on the photoelectrochemical performance of TMO photoelectrodes, monoclinic bismuth vanadate (BiVO4) was chosen as a material platform for study given its desirable (bandgap ~2.4 eV, favorable band edge positions and relatively long minority diffusion lengths) and undesirable (localized carrier transport, photocorrosion) characteristics [1]. High crystalline quality epitaxial BiVO4 thin films were used as a model system in order to minimize complications arising from grain boundaries and different crystalline orientations inherent in polycrystalline systems. BiVO4 films with an epitaxial indium tin oxide (ITO) conducting layer were deposited on (001)-oriented yttrium-stabilized zirconia (YSZ) single crystalline substrates. YSZ substrates with different dopant concentrations were used to impose different epitaxial strain states on the BiVO4 films. This strategy allowed us to conduct extensive photoelectrochemical measurements (voltammograms, incident photon-to-current efficiency, impedance spectroscopy) and x-ray photoelectron spectroscopic (XPS) measurements as a function of the strain state. Further investigation with varying BiVO4 film thickness was also performed to decouple any thickness-dependent contributions from the epitaxial strain effects. Through strain tensor decomposition analysis, Tauc bandgaps correlates well with deviatoric strains, which in turn seem to suggest the role of volume-preserving lattice distortions. Finally, we will show that the observed trends in photocurrents, incident photon-to-current efficiency spectra, flatband potentials obtained from Mott-Schottky analysis, and surface electronic structure obtained from XPS can be rationalized in terms of lattice-induced distortions on the BiO8 dodecahedra.

Femtosecond Pulsed Laser Irradiation of Zirconia for Embedding Silver Nanoparticles in Surface Nanopores

Femtosecond pulsed laser irradiation was performed to investigate the feasibility and fundamental characteristics of embedding silver nanoparticles onto zirconia ceramic surfaces. By irradiating laser, nanopores were fabricated on the surface of the yttria-stabilized zirconia (YSZ) substrate, and silver nanoparticles were infiltrated and immobilized into the pores using a commercial nano-silver dispersion solution. Numerous nanopores embedded with silver nanoparticles were successfully obtained on the YSZ surface while keeping the grains’ shapes unchanged by controlling laser parameters. Optimizing laser fluence and scanning speed near the ablation threshold made it possible to remove only the excess dispersant that remained on the surface while keeping silver in the pores and without causing machining of the surface of the YSZ substrate. In addition, about 60% embedding in the nanopores was achieved. It was found that the shorter pulse width was suitable to avoid evaporating both dispersant and silver. Cross-sectional observation revealed that the silver nanoparticles were agglomerated to form clumps and were embedded without a gap at the bottom of the pores at a depth of about 600 nm. After laser irradiation, no significant laser-induced phase change was observed in the YSZ substrate, indicating that there was no in-process thermal damage to the bulk. These findings demonstrated the possibility of adding a metal nanoparticle to the zirconia surface by using only a laser process without damaging the properties of the base material during the process. New applications of zirconia, such as the generation of functional surfaces with antibacterial properties, are expected.

Microstructural Evolution and Gas-Tight Properties of Yttria-Stabilized Zirconia/Crofer 22H Stainless Steel Brazed Joints with the Ag-Ge-Si Filler for Use in Solid-Oxide Fuel Cells

In this paper, a novel 95Ag-2.5Ge-2.5Si (in wt %) filler is utilized for brazing yttria-stabilized zirconia (YSZ) electrolytes and commercial Crofer 22H interconnects for solid-oxide fuel cells’ (SOFCs) sealing application. Before brazing, surface metallization is applied on YSZ and Crofer 22H substrates to improve the wetting performance of the filler on YSZ and Crofer 22H substrates. The brazing procedure is performed at 900 °C for 10 min under a high vacuum (~10−6 torr) to prepare sandwiched YSZ/Crofer 22H brazed coupons. The metallization mentioned above can achieve reactive wetting toward YSZ ceramics. A Si/Ti-rich oxide layer and an Fe-Cr-Si alloying phase are formed at the brazed joints’ YSZ/filler and filler/Crofer 22H interfaces. After exposure to air at 750 °C for 100 h, Cu and Si contents suffer from oxidation and form CuO and SiO2, respectively, in the brazed zone and the YSZ/filler interface of the joints. The Fe-Cr-Si alloying phase at the filler/Crofer 22H interface is preserved without apparent oxidation. The pressure-drop test results show that the brazed joints’ gas tightness does not deteriorate significantly after thermal aging, which is attributed to the good interfacial integrity of thermal-aged joints.

Sputter epitaxy and characterization of manganese-doped indium tin oxide films with different crystallographic orientations

Epitaxial Mn-doped indium tin oxide (ITO) films were deposited on single-crystal yttria stabilized zirconia (YSZ) substrates with (111), (110), and (100) crystal plane orientations using RF magnetron sputtering. The epitaxial relationship between the Mn-doped ITO films and the YSZ substrates was studied using x-ray diffraction (XRD) patterns in the ω–2θ scan mode and XRD pole figures. The Mn-doped ITO films on the YSZ(111) and YSZ(110) substrates exhibited a higher degree of crystallinity than the film on the YSZ(100) substrate as per the x-ray rocking curves. Fluctuations in the crystalline alignment were found to significantly influence the electrical properties of Mn-doped ITO films. Ferromagnetic hysteresis loops were observed at room temperature for all the epitaxial Mn-doped ITO films, irrespective of their crystallographic orientation. The magnetic properties of the epitaxial Mn-doped ITO films suggest that a combination of delocalized charge carrier-mediated interaction and bound magnetic polaron-driven interaction is required to explain the origin of ferromagnetism in these films. The Mn-doped ITO film on the YSZ(111) substrate exhibited the most desirable characteristics in terms of crystallinity, surface smoothness, electrical conductivity, and magnetic properties.

Pulsed laser deposition grown La0.3Ca0.7Fe0.7Cr0.3O3-δ thin films on yttria-stabilized zirconia substrates for fundamental electrochemical energy conversion studies

La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr) perovskites, normally deposited as a powder-based ink to form porous electrode layers, have been shown to be exceptional catalysts for the electrochemical splitting of CO2 to form CO at one electrode and O2 at the other during electrolysis in solid oxide cells, while also being highly active and stable under fuel cell conditions. This work constitutes the first report of thin film deposition of LCFCr via pulsed laser deposition on a 001-oriented yttria-stabilized zirconia substrate, used to simulate the ionically conducting electrolyte in operating cells. The primary goal of this work was to determine fundamental LCFCr properties and performance metrics without complications from morphology effects. Scanning Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy analysis confirmed that oriented, dense, uniform, and smooth films with a thickness of ca. 25 nm and a RMS surface roughness of 0.2 nm are grown at 700 °C from a stoichiometric LCFCr target, while containing all the components of LCFCr in their expected oxidation states. These thin films have allowed the determination of the oxidation states of the redox active species on the electrode surface. Moreover, the preliminary electrochemical response of the LCFCr thin films in an air environment is shown to mimic the trends observed in more common highly porous LCFCr electrodes, thus arguing for a similar reaction mechanism and kinetics per real surface area.

Effect of substrate and growth method on vanadium dioxide thin films by RF magnetron sputtering: Vanadium metal oxidation vs reactive sputtering

Vanadium dioxide (VO2) undergoes a metal–insulator phase transition at ∼70 °C and has attracted substantial interest for potential applications in electronics, including those in neuromorphic computing. The vanadium–oxygen system has a rather complicated phase diagram, and controlling the stoichiometry and the phase of thin films of vanadium oxides is a well-known challenge. We explore the novel combination of two methods of VO2 thin film deposition using off-axis RF magnetron sputtering on (100)- and (111)-oriented yttria-stabilized zirconia (YSZ) substrates: reactive sputtering of vanadium in an oxygen environment and sputtering of vanadium metal followed by oxidation to VO2. Interestingly, the reactive sputtering process on both substrate orientations yields the metastable semiconducting VO2 (B) phase, which is structurally stabilized by the YSZ surface. The metal sputtering and oxidation process on YSZ produces mainly the equilibrium monoclinic (or M1) phase of VO2 that exhibits a metal–insulator transition. Using this method, we obtained thin films of (010)-textured polycrystalline VO2 (M1) that show a metal–insulator transition with an on/off ratio larger than 1000.

LaFeO3 thin films on Yttria Stabilized Zirconia flexible substrate

Thin films of LaFeO3 for photocatalytic applications were grown by Pulsed Laser Deposition on a flexible polycrystalline Tetragonal Yttria Stabilized Zirconia substrate (T-YSZ). Their structure and morphology were investigated using scanning electron microscopy, transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy (STEM), energy dispersive spectroscopy available in the STEM system, X-ray diffraction, and Raman spectroscopy techniques. The band gap energies and the life time of the charge carrier were deduced from UV-Vis and time resolved fluorescence spectroscopies. The polycrystalline substrate T-YSZ was homogeneously covered by the LaFeO3 films, even at the substrate grain boundaries. The surface of the films presents a nanostructuration with flat ended and tip ended grains. The exposed facets are mainly {100}C. Atomic–scale microstructure investigations revealed twins in LaFeO3 grains, as consequence of the curvature of the T-YSZ substrate. Chemical analyses showed iron and oxygen deficiency in the LaFeO3 layers. Two optical transitions with energies of 1.9 eV and 2.5 eV corresponding to visible light absorption were evidenced, and the average life time of charges carriers was found to be 5 ns.

Smoothing the wrinkle formation and improving dye rejection performance in porous graphene oxide membranes using high surface curvature hollow fiber substrates

Assembly of atomic-scale thin two-dimensional (2D) graphene oxide (GO) nanosheets into a membrane with macroscopic three-dimensional (3D) laminar structure is accompanied by the formation of wrinkles. The size and pattern of wrinkles, which depend on the substrate curvature and the thickness of the 3D laminar structure, adversely affect the separation performance of the membrane. Therefore, smoothing the formed wrinkles is essential to boost the separation performance of GO-based membranes. For the first time, we proposed a new approach to prepare porous hollow fiber (HF) yttria-stabilized zirconia (YSZ) substrates with small-diameter (660–790 μm, outer diameter (OD)) by stretching the nascent HF using gravitational force. Also, using a delayed phase inversion, a plurality of radial micro-channels opening from the interior surface of HFs can be realized. The resultant HF substrates show promising permeation characteristics and good fracture strength owing to their open microstructure and the robustness of YSZ. Small-diameter HF substrates (OD = 660 μm) with higher surface curvature ensured the fabrication of porous GO (PGO) membranes with better dye separation performance. The rejection of methyl red (MR, MW = 269.3 g mol−1) dye molecule for small-diameter HF PGO membranes (OD 660 μm) was 10%–167% higher than that of membranes fabricated using large-diameters (OD 1250–2600 μm). This study shed light on the fabrication of high-performance thin film membranes by tailoring the geometric features of substrates.

Massive enhancement in sensitivity of mixed potential sensors towards methane and natural gas through magnesia stabilized zirconia low ionic conductivity substrate

Monitoring of methane leaks from natural gas (NG) sources during production and transport is vital to combating the threat of greenhouse gases to the global climate. An additive manufacturing route was developed for the printing of a CTE matched, low reactivity and low ionic conductivity magnesia stabilized zirconia (MSZ) substrate for production of a four-electrode (indium tin oxide, lanthanum strontium chromite, and Au sensing electrodes measured against a Pt negative electrode) same-side planar mixed potential electrochemical sensor (MPES) with porous 3 mol% YSZ as electrolyte. The MSZ substrate based MPES achieves a 40 ppm limit of detection for methane in a simulated natural gas mix. Additionally, a 20 times enhanced response in the ITO vs. Pt signal over sensors using yttria-stabilized zirconia (YSZ) substrates at 100 ppm CH4 in simulated natural gas is observed. This research illustrates the importance of a CTE matched, low reactivity, low conductivity substrate for optimal performance of same-side planar MPES devices for NG leak detection while allowing for application of high temperature co-fired ceramic technologies.

Effect of thickness on the superconducting properties of the thin films of FeTe0.55Se0.45 deposited on YSZ substrates using Pulsed laser deposition (PLD) technique

We report the successful fabrication of thin films of thickness ∼26 nm, 59 nm, 76 nm, and 158 nm of the composition FeTe0.55Se0.45 on the (100)-oriented Yttria-stabilized zirconia (YSZ) substrate using the pulsed laser deposition (PLD) technique. The effect of thickness on the structure and superconducting properties of the thin films has been investigated. X-ray diffraction study reveals the growth of thin films along the c-axis. The temperature-dependent resistivity measurements confirm superconductivity in all the fabricated thin films, and the highest ∼ 16.2 K has been observed for the thin film of thickness ∼59 nm, which is higher than the corresponding value of the polycrystalline target sample. The superconducting parameters such as upper critical magnetic field coherence length thermally activated energy (TAE) U0, and vortex phase state have been measured by magnetotransport measurements near in the different applied magnetic fields up to 12 T. The TAE values determined from the conventional and modified thermally activated flux flow (TAFF) models are discussed and compared. A crossover from 2-dimensional (2D) vortex to 3-dimensional (3D) vortex behaviour has been observed. The vortex phase diagrams are used to observe the transition from the vortex liquid to the vortex glass state in the grown thin films. Moreover, magnetoresistance (MR) measurements show non-saturating linear magnetoresistance (LMR) in the high field region, indicating the presence of the possible topological state in the grown thin films. The values of magnetic field dependent transport critical current density, have been evaluated from the current-voltage (I-V) curves. Furthermore, the normalised pinning force density has been used to characterize the pinning processes using the Dew Hughes’ scaling rule which reveals that the grown thin films contain δl-surface pinning centers.

YSZ-based NO2 Mixed Potentiometric Sensor with Three-Dimensional Ordered Porous Structure

This study presents the preparation of a nitrogen dioxide (NO2) gas sensor based on yttrium-stabilized zirconia (YSZ) using a hard-template method and evaporation self-assembly technology. The sensing electrode employed is nickel oxide, and the sensor exhibits an efficient three-phase boundary. The study systematically investigates the effects of changing the three-phase reaction boundary on the gas-sensing performance of the sensor. A three-dimensional (3D) ordered macroporous (3DOM) structure was deposited on a YSZ substrate using the hard template method to obtain an efficient three-phase boundary. At 450 °C and 100 ppm NO2, the response value of the sensor with a 3D-ordered porous structure was 38.71 mV, which is 1.91 times the response value observed for the untreated YSZ-based sensor (20.3 mV), and the sensitivity significantly improved. In addition, a sensor with a 3D-ordered structure exhibits good 2 selectivity, and long-term stability. The improvement in the sensitivity of the sensor can be attributed to an increase in the area of the three-phase reaction boundary and an increase in the number of active sites in the electrochemical reaction.

Effects of processing parameters on the morphologies of complex sesquioxide thin films

Controlling and predicting the morphology of lanthanide sesquioxides in thin film form is vital to their use in current applications. In the present study, single and codeposited Sm2O3, Er2O3, and Lu2O3 thin films were grown on yttria-stabilized zirconia (8%) substrates by radio frequency magnetron sputtering at room temperature and 500 °C. The effect of two different substrate temperatures and altering the oxide cation on the structural and morphological properties of the films was analyzed. The thin films were characterized by profilometry, scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The single-component Lu2O3 and Sm2O3 films obtained were of the cubic phase, and the Er2O3 was a mix of cubic and monoclinic phases. It was observed for both the Er2O3 and Lu2O3 films that increasing the substrate temperature to 500 °C resulted in larger grained polycrystalline films. In contrast, large grained polycrystalline films were obtained at both room temperature and 500 °C for Sm2O3 and uneven granularity increased as temperature increased. Codeposition of Lu2O3 and Sm2O3, and Lu2O3 and Er2O3 resulted in a cubic bixbyite phase (the C phase of the lanthanide sesquioxide) solid solution. It was observed that the structure and morphology of the films can be controlled by manipulating deposition parameters. Both substrate temperature and altering the oxide cation contributed to changes in crystallinity and grain structure, which can modify the chemical and physical properties of the films for their applications.

Nonlinear Optical Properties in an Epitaxial YbFe2O4 Film Probed by Second Harmonic and Terahertz Generation.

An epitaxial film of YbFe2O4, a candidate for oxide electronic ferroelectrics, was fabricated on yttrium-stabilized zirconia (YSZ) substrate by magnetron sputtering technique. For the film, second harmonic generation (SHG), and a terahertz radiation signal were observed at room temperature, confirming a polar structure of the film. The azimuth angle dependence of SHG shows four leaves-like profiles and is almost identical to that in a bulk single crystal. Based on tensor analyses of the SHG profiles, we could reveal the polarization structure and the relationship between the film structure of YbFe2O4 and the crystal axes of the YSZ substrate. The observed terahertz pulse showed anisotropic polarization dependence consistent with the SHG measurement, and the intensity of the emitted terahertz pulse reached about 9.2% of that emitted from ZnTe, a typical nonlinear crystal, implying that YbFe2O4 can be applied as a terahertz wave generator in which the direction of the electric field can be easily switched.