Yttria-stabilized Zirconia Buffer Layers Research Articles

Epitaxial Growth of β-Ga2O3 Thin Films on Si with YSZ Buffer Layer.

We report the epitaxial growth of (2̅01)-oriented β-Ga2O3 thin films on a (001) Si substrate using the pulsed laser deposition technique employing epitaxial yttria-stabilized zirconia (YSZ) buffer layers. Epitaxial β-Ga2O3 thin films possess a biaxial compressive strain on YSZ single-crystal substrates while they exhibit a biaxial tensile strain on YSZ-buffered Si substrates. Post-annealing improves the crystalline quality of β-Ga2O3 thin films. High-resolution X-ray diffraction analyses reveal that the epitaxial (2̅01) β-Ga2O3 thin films on Si have eight in-plane domain variants to accommodate the large difference in the crystal structure between monoclinic β-Ga2O3 and cubic YSZ. The results provide a pathway to integrate epitaxial β-Ga2O3 thin films on a Si gold standard substrate, which will expand the application scope beyond high-power electronics.

Controlled strained layer epitaxial growth of EuTiO3 on buffered silicon

In this work, we show the epitaxial growth of (111)-oriented EuTiO3 thin films on (001)-oriented silicon with an in situ grown yttria-stabilized zirconia (YSZ) buffer layer by pulsed laser deposition. X-ray diffraction measurements revealed a homogeneously strained EuTiO3 thin film with a strain dependency on the laser fluence during the film growth. From magnetization vs temperature measurements, we confirmed that the strained EuTiO3 films have an antiferromagnetic to ferromagnetic transition at 3.7 K, which disappeared for unstrained films. Furthermore, we used electron backscatter diffraction to analyze the columnar growth of EuTiO3 on YSZ, which showed four in-plane orientations.

Long-length YBCO coated conductors for ultra-high field applications: gaining engineering current density via pulsed laser deposition/alternating beam-assisted deposition route

Gaining engineering critical current density (Je) in high-temperature superconducting (HTS) coated conductors based on double-disordered YBCO for operation at high fields is one of the key requirements in upcoming magnet/accelerator projects. Currently, the development of a tape with advanced Je-s is performed based on an alternating beam-assisted deposition–pulsed laser deposition (ABAD–PLD) manufacturing route. An obvious way to improve Je-s is to reduce substrate thickness. A 40% Je increase is expected owing to the thickness reduction from 100 to 50 μm. Nevertheless, the reduction of substrate thickness in case of the applied processing technology that employs relatively thick (2–3 μm) yttria-stabilized zirconia buffer layers leads to the manifestation of considerable strain in the tape resulting in strong tape bowing. Advanced processing routes have been developed to suppress this effect. The highest engineering current density was recorded at well above 1000 A mm−2 under an ultra-high field of 18 T at 4.2 K, B//c. Influence of tape bowing and the impact of longitudinal defects are studied via V–I measurements at 77 K and a moderate magnetic field with intrinsic edge gradients. Potential for further gaining of Je was found employing mechanisms of (i) film nucleation from lateral flows that provide material transfer during the PLD process. Further steps include suppressing the instability of instantaneous temperature via quasi-adiabatic pulsed heating of the growing HTS film via (ii) control of laser plume energy, and (iii) energy release during film condensation (condensation enthalpy). These effects disturb the instantaneous temperature of the growing HTS layer, which exhibits very limited capability of heat transfer. Temperature pulses reaching 30–40 K were evaluated via heat transfer modeling. Stabilization of the level of pulsed temperature during layer growth is anticipated to result in a further increase in Je.

Growth of epitaxial bismuth ruthenate pyrochlore films on yttria-stabilized zirconia (YSZ) and YSZ-buffered Si substrates by metal–organic chemical vapor deposition

Bismuth ruthenate (Bi2Ru2O7) thin films were deposited on (111) yttria-stabilized zirconia (YSZ) single crystals and (111)YSZ//(111)Si substrates by chemical vapor deposition using Bi(CH3)2[2-(CH3)2NCH2C6H4] and Ru(C7H11)(C7H9) as source materials and oxygen as a reactant gas. The film composition and X-ray diffraction ω-2θ scan profiles for thin films deposited using various source flow rate ratios revealed the existence of a process window to obtain stoichiometric Bi2Ru2O7. Within the process window, the kind of substrates did not strongly affect the deposition rates of Bi and Ru or the Bi/Ru ratio of the deposited films. It was also confirmed that the Bi2Ru2O7 thin films grew epitaxially on both substrates within the process window. In particular, epitaxial Bi2Ru2O7 thin films grew on the Si substrate with a YSZ buffer layer. Scanning electron microscopy revealed the formation of continuous dense Bi2Ru2O7 films without any cracks or voids, and a relatively smooth interface between Bi2Ru2O7 and the YSZ buffer layer on Si. The resistivity of the Bi2Ru2O7 films was almost constant (~550 and ~950 μΩcm for the films deposited on (111)YSZ single-crystal substrates and (111)YSZ//(111)Si substrates, respectively), even when the ratio of Bi and Ru source gas flow rates was changed in the process window. In addition, the resistivity of Bi2Ru2O7 films prepared on both kinds of substrates was almost independent of temperature from 10 to 300 K.

Ferroelectricity in epitaxial Y-doped HfO2 thin film integrated on Si substrate

We report on the ferroelectricity of a Y-doped HfO2 thin film epitaxially grown on Si substrate, with an yttria-stabilized zirconia buffer layer pre-deposited on the substrate. Piezoresponse force microscopy results show the ferroelectric domain pattern, implying the existence of ferroelectricity in the epitaxial HfO2 film. The epitaxially stabilized HfO2 film in the form of a metal-ferroelectric-insulator-semiconductor structure exhibits ferroelectric hysteresis with a clear ferroelectric switching current in polarization-voltage measurements. The HfO2 thin film also demonstrates ferroelectric retention comparable to that of current perovskite-based metal-ferroelectric-insulator-semiconductor structures.

High ferroelectric polarization in c-oriented BaTiO3 epitaxial thin films on SrTiO3/Si(001)

The integration of epitaxial BaTiO3 films on silicon, combining c-orientation, surface flatness, and high ferroelectric polarization is of main interest towards its use in memory devices. This combination of properties has been only achieved so far by using yttria-stabilized zirconia buffer layers. Here, the all-perovskite BaTiO3/LaNiO3/SrTiO3 heterostructure is grown monolithically on Si(001). The BaTiO3 films are epitaxial and c-oriented and present low surface roughness and high remnant ferroelectric polarization around 6 μC/cm2. This result paves the way towards the fabrication of lead-free BaTiO3 ferroelectric memories on silicon platforms.

Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates

We investigated ferroelectric characteristics of BiFeO3 (BFO) thin films on SrRuO3 (SRO)/yttria-stabilized zirconia (YSZ)/glass substrates grown by pulsed laser deposition. YSZ buffer layers were employed to grow highly crystallized BFO thin films as well as SRO bottom electrodes on glass substrates. The BFO thin films exhibited good ferroelectric properties with a remanent polarization of 2Pr = 59.6 μC/cm2 and fast switching behavior within about 125 ns. Piezoelectric force microscopy (PFM) study revealed that the BFO thin films have much smaller mosaic ferroelectric domain patterns than epitaxial BFO thin films on Nb:SrTiO3 substrates. Presumably these small domain widths which originated from smaller domain energy give rise to the faster electrical switching behavior in comparison with the epitaxial BFO thin films on Nb:SrTiO3 substrates.

Graphene–Silicon Layered Structures on Single‐Crystalline Ir(111) Thin Films

Single-crystalline transition metal films are ideal playing fields for the epitaxial growth of graphene and graphene-base materials. Graphene-silicon layered structures were successfully constructed on Ir(111) thin film on Si substrate with an yttria-stabilized zirconia buffer layer via intercalation approach. Such hetero-layered structures are compatible with current Si-based microelectronic technique, showing high promise for applications in future micro- and nano-electronic devices.

Yttria-stabilized zirconia buffered silicon to optimize in-plane electrical conductivity of [Ca2CoO3]0.62[CoO2] thin films

The monolithic integration of thermoelectric generators and magnetoresistive functionality on the basis of misfit cobaltate [Ca2CoO3]0.62[CoO2] thin films into silicon technology is a prerequisite for their application in miniaturized electric circuits. Here, we report on [Ca2CoO3]0.62[CoO2] thin films grown by pulsed laser deposition on (001)-silicon with a thin epitaxial yttria-stabilized zirconia (YSZ) buffer layer. X-ray diffraction and cross-sectional high resolution transmission electron microscopy analysis reveal that high quality c-axis oriented heteroepitaxial [Ca2CoO3]0.62[CoO2] films with a 12-fold in-plane rotational symmetry can be grown, which exhibit remarkable lower electrical resistivity compared to those with random in-plane orientation. This result is explained by energetically preferred epitaxial growth directions of the pseudo hexagonal [CoO2] sublayer in monoclinic [Ca2CoO3]0.62[CoO2] onto the cubic (001)-YSZ surface leading to a highly symmetric in-plane mutual orientation of the charge transporting CoO2 sublayer domains.

Ni-based anode-supported Al2O3-doped-Y2O3-stabilized ZrO2 thin electrolyte solid oxide fuel cells with Y2O3-stabilized ZrO2 buffer layer

In order to reduce the sintering temperature of Ni-based anode-supported thin 8 mol% yttria-stabilized zirconia (YSZ) elsectrolyte solid oxide fuel cells (SOFCs), alumina, with a weight percent of 1, 3, 5, and 7, is respectively doped into YSZ as sintering aid. A pure YSZ buffer layer is introduced between the Al2O3-doped-YSZ electrolyte and Ni–YSZ anode, to prevent Al2O3 and NiO from forming non-conductive spinel NiAl2O4. The experimental results show that doping proper amount of Al2O3 doping can reduce the sintering temperature of YSZ, e.g., 1 wt.% doping decreases the temperature from 1673 K to 1573 K. Anode-supported SOFCs are prepared with Al2O3-doped-YSZ electrolytes sintered at different temperatures. Electrochemical characterization of the SOFCs shows that the single cell with 1 wt.% alumina-doped YSZ electrolyte sintered at 1573 K gives the highest output. The effect of alumina doping on sintering behavior and electrical performance of YSZ is discussed in detail.

Texture and morphology developments of Yttria-stabilized zirconia (YSZ) buffer layer for coated conductors by RF sputtering

Yttria-stabilized zirconia (YSZ) films are deposited on CeO2/NiW tapes by RF sputtering for (Gd) BCO-coated conductors. Surface morphology and texture developments are investigated as the O2:Ar ratio and sputtering power increase. Both the grain size and the roughness of YSZ films increase as sputtering power increases. A simple model proposed by Bartelt et al. is used to explain the results. The strain relaxation mechanism plays a major role in the sudden increase in roughness for YSZ film deposited at a sputtering power of 50W. It is observed that low O2:Ar ratio favors the growth of (200) orientation for YSZ film, while high O2:Ar ratio favors the growth of (111) orientation. The preferential orientation (200) of the YSZ film with low O2:Ar ratio arises from template effect of CeO2 buffer layer. The (111) orientation of YSZ film with high O2:Ar ratio is due to thermodynamic mechanism aiming for the lowest surface energy. The grain size and roughness of our optimal YSZ film are 10–15nm and 0.7nm, respectively. The out-of-plane texture of our optimal YSZ film is 4.05°. The in-plane texture is 6.35°. The plan view and cross-section SEM images of YSZ films on CeO2/NiW tapes show a flat, dense and no micro-cracks morphology. Based on these results, (Gd) BCO films are deposited by RF sputtering, achieving self-field Jc of 4.0 MA/cm2 at 77K.

Thin film epitaxy and near bulk semiconductor to metal transition in VO2/NiO/YSZ/Si(001) heterostructures

Abstract

Scalable synthesis of graphene on single crystal Ir(111) films

We have investigated single crystal Ir(111) films grown heteroepitaxially on Si(111) wafers with yttria-stabilized zirconia (YSZ) buffer layers as possible substrates for an up-scalable synthesis of graphene. Graphene was grown by chemical vapor deposition (CVD) of ethylene. As surface analytical techniques we have used scanning tunneling microscopy (STM), low-energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The mosaic spread of the metal films was below 0.2° similar to or even below that of standard Ir bulk single crystals, and the films were basically twin-free. The film surfaces could be improved by annealing so that they attained the perfection of bulk single crystals. Depending on the CVD conditions a lattice-aligned graphene layer or a film consisting of different rotational domains were obtained. STM data of the non-rotated phase and of the phases rotated by 14° and 19° were acquired. The quality of the graphene was comparable to graphene grown on bulk Ir(111) single crystals.

Epitaxial growth of thin films and nanodots of ZnO on Si(111) by pulsed laser deposition

Epitaxial (0001) ZnO thin films were grown on (111) Si substrates buffered with intermediate epitaxial (111) yttria-stabilized zirconia (YSZ) layers by pulsed laser deposition (PLD). X-ray diffraction and transmission electron microscopy characterizations revealed that the YSZ buffer layers enabled the epitaxial growth of structurally high quality ZnO films and an atomically sharp ZnO/YSZ interface, proving to be an effective epitaxial template. The epitaxial orientation relationships were revealed as follows: (0001) ZnO||(111) YSZ||(111) Si and [1¯21¯0] ZnO||[1¯10] YSZ||[1¯10] Si. Room temperature photoluminescence spectrum of the ZnO films showed the excitonic ultraviolet emission with few green emissions relevant to oxygen vacancies in the film. Furthermore, we fabricated ZnO nanostructures on the same (111) YSZ||(111) Si substrates by simply manipulating PLD conditions for the epitaxial film growth. The size control of the ZnO nanodots was realized by varying the number of laser pulses. A blueshift behavior induced by quantum confinement was observed, as the nanodot size decreases.

The role of deposition conditions on the structure and magnetic properties of SrTi1-xFexO3 films

SrTi1-xFexO3 thin films were grown on Si substrates with a double buffer layer of ceria and yttria-stabilized zirconia at various temperatures, oxygen pressures and Fe contents. Films with in-plane compressive strain and an oxygen deficiency have a strong magnetic anisotropy with an out-of-plane easy axis, whereas films deposited at low temperature and in an oxygen atmosphere have no ferromagnetism at room temperature, which is attributed to the lack of lattice strain.

Grain size of nanocrystalline YSZ buffer layers fabricated by ion beam deposition

Amorphous or nanocrystalline thin films are capable to be efficient diffusion barrier layers for YBCO coated conductors of ion beam assisted deposition route. Nanocrystalline Yttria-Stabilized Zirconia (YSZ) buffer layers were fabricated by the ion beam deposition. Kr and Ar gases were utilized for the dual Kaufman type sputtering ion sources sequently. Both of the ion beams were fixed at 200mA, while the ion energy was in the range of 450–2000eV. The thickness of the YSZ buffer layers was several hundreds of nanometers. The grain size of YSZ thin films, which varied from 2 to 20nm, was calculated by Scherrer Formulation based on the X-ray diffraction measurement results. The grain size always decreased at first and then increased when the ion energy was increased. In the cases Kr gas was utilized and Ar gas was utilized, the grain size reached its minimum value at the ion energy of about 1800eV and 1000eV, respectively. Such phenomenon was discussed using the thermodynamic theory of thin film nucleation. Deposition rate and substrate temperature were the two chief variables of the critical nuclei concentration and grain size.

Effects of assisting and sputtering ion current on ion beam assisted deposition textured yttria stabilized zirconia buffer layers of coated conductors

Biaxially textured yttria stabilized zirconia (0 0 1) thin films were fabricated on untextured hastelloy substrates by ion beam assisted deposition method. The effects of assisting beam current density J a and sputtering beam current density J s on the textures of the films were studied. The results indicate that as J a or J s increase, both the out-of-plane and the in-plane textures are improved initially, and then degrade. The results can be attributed to anisotropic damage and selective sputtering effect of assisting ions. At the same ion-to-atom arrival ratio r, which is reflected with J a / J s value, lower deposition rate can enhance the biaxial texture.

Epitaxial growth of YSZ buffer layer for YBa 2Cu 3O 7− δ coated conductor by reel-to-reel pulsed laser deposition

Yttria-stabilized zirconia (YSZ) buffer layers were deposited on CeO 2 buffered biaxially textured Ni–W substrate by reel-to-reel pulsed laser deposition (PLD) for the application of YBa 2Cu 3O 7− δ (YBCO) coated conductor and the influence of substrate temperature and laser energy on their crystallinity and microstructure were studied. YSZ thin films were prepared with substrate temperature ranging from 600 to 800 °C and laser energy ranging from 120 to 350 mJ. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to investigate how thin film structure and surface morphology depend on these parameters. It was found that the YSZ films grown at substrate temperature below 600 °C or laser energy above 300 mJ showed amorphous phase, the (0 0 1) preferred orientation and the crystallinity of the YSZ films were improved with increasing the temperature, but the surface roughness increased simultaneously, the SEM images of YSZ films on CeO 2/NiW tapes showed surface morphologies without micro-cracks. Based on these results, we developed the epitaxial PLD-YSZ buffer layer process at the tape transfer speed of 3–4 m/h by the reel-to-reel system for 100 m class long YBCO tapes.

Multiferroic Properties of Highly c-Oriented BiFeO[sub 3] Thin Films on Glass Substrates

BiFeO 3 (BFO) thin films were deposited on SrRuO 3 (SRO)/yttria-stabilized zirconia (YSZ)/glass substrates by pulsed laser deposition. To grow highly a- and (110)-oriented SRO bottom electrodes, we used YSZ buffer layers between SRO bottom electrodes and glass substrates. On the highly a- and (110)-oriented SRO bottom electrodes, we obtained highly c- and (011)-oriented BFO thin films in which the structures of BFO thin films were confirmed by X-ray diffraction experiments. The highly c-oriented BFO thin film exhibited a higher ferroelectric polarization (about 40 μC/cm 2 ) than the highly (011)-oriented BFO thin film. The highly c-oriented BFO thin film also showed a ferromagnetic hysteresis loop while the highly (011)-oriented BFO thin film exhibited a superparamagnetic hysteresis loop.

Substrate surface treatment and YSZ buffer layers by IBAD method for coated conductors

In this work, an Ion Beam Assisted Deposition (IBAD) system was utilized to fabricate Yttria-Stabilized Zirconia (YSZ) template films for coated conductors. The surface of the Hastelloy C276 substrate was modified by rolling and electropolishing. The effect of the electropolishing parameters of the substrate on the texture of the YSZ buffer layers was studied. The electropolishing current and time were optimized for short samples of 1 cm×1 cm square shape as 1 A and 60 s, respectively. And the relationship between the roughness of the substrate surface and the texture of the YSZ layer is discussed. Reel-to-reel metal tape moving apparatus was installed and used to produce meter-long buffer layer for coated conductors. The YSZ template film was deposited by IBAD method on meter-long Hastelloy tape with tape shifting speed of 15–20 m/h, and the thickness of the buffer layer was up to about 1.7 μm. The Hastelloy substrate surface was measured by Atomic Force Microscope. The thickness of the YSZ films over length was measured by Thermal Field Emission Scan Electronic Microscopy. X Ray Diffraction Ω-scan and ϕ-scan measurements were performed in order to examine the out-of-plane and in-plane texture of the YSZ buffer layers, respectively.