Epitaxial Yttria-stabilized Zirconia 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.

Preparation of epitaxial yttria-stabilized zirconia with non-quilibrium t' phase and ferroelastic domain by pulsed laser deposition

The non-equilibrium tetragonal t' phase of yttria-stabilizedzirconia (YSZ) plays a key role in improving the toughness of YSZ ceramics owing to its own ferroelastic domain toughening mechanism. Due to the disadvantages of traditional processes for preparing the YSZ t' phase, which requires high heating temperature, difficult to control defects and inability to regulate microstructure, it is not conducive to the preparation of YSZ by these methods. We report the use of substrate clamping and non-equilibrium growth process of PLD to render stable t' epitaxial YSZ films with multiple domain structure on (0001) Al2O3 substrates. Thin film structure and epitaxial relationship with substrate were subsequently analyzed by XRD, RSM, and HRTEM. It was found that the crystal orientation of the YSZ film changes as a function of oxygen pressure during deposition. When oxygenpressure is larger than 100 mtorr, YSZ films with highly oriented ferroelastic domains are obtained. Based on the symmetry relationship, we know that the film has three in-plane domain structures. The epitaxial relationship between the film and the substrate is (002) [110] YSZ // (0001) [112¯0] Al2O3.

Growth mechanism of epitaxial YSZ on Si by Pulsed Laser Deposition

The epitaxial growth of yttria-stabilized zirconia (YSZ) on silicon with native oxide was investigated in order to gain more insight in the growth mechanism. Specifically, attention was paid to the possibilities to control the chemical interactions between YSZ, silicon and oxygen during initial growth. The sources of oxygen during growth proved to play an important role in the growth process, as shown by individual manipulation of all sources present during Pulsed Laser Deposition. Partial oxidation of the YSZ plasma and sufficient delivery of oxygen to the growing film were necessary to prevent silicide formation and obtain optimal YSZ crystalline qualities. In these conditions, thickness increase of the silicon native oxide before growth just started to occur, while a much faster regrowth of silicon oxide at the YSZ-Si interface occurred during growth. Control of all these contributions to the growth process is necessary to obtain reproducible growth of high quality YSZ.

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.

CoFe2O4/buffer layer ultrathin heterostructures on Si(001)

Epitaxial films of ferromagnetic CoFe2O4 (CFO) were grown by pulsed laser deposition on Si(001) buffered with ultrathin yttria-stabilized zirconia (YSZ) layers in a single process. Reflection high-energy electron diffraction was used to monitor in real time the crystallization of YSZ, allowing the fabrication of epitaxial YSZ buffers with thickness of about 2 nm. CFO films, with thicknesses in the 2–50 nm range were subsequently deposited. The magnetization of the CFO films is close to the bulk value. The ultrathin CFO/YSZ heterostructures have very flat morphology (0.1 nm roughness) and thin interfacial SiOx layer (about 2 nm thick) making them suitable for integration in tunnel (e.g., spin injection) devices.

Epitaxial Growth of Ferromagnetic Iron Silicide Thin Films on Silicon with Yttria-Stabilized Zirconia Buffer Layer

(110)-oriented epitaxial Fe3Si films were grown on (100) and (111) Si substrates with an epitaxial yttria-stabilized zirconia (YSZ) buffer layer by rf-magnetron sputtering. The epitaxial Fe3Si films contained a two-domain structure on the (100) substrate with a YSZ buffer layer, and a three-domain structure on the (111) substrate with a YSZ buffer layer. The coercive forces (Hc) of both epitaxial films were each approximately 40 Oe, and the saturation magnetizations (Ms) were each 850 emu/cm3, which are almost at the same level as those of bulk Fe3Si.

YSZ buffer layers and YBCO superconducting tapes with enhanced biaxial alignment and properties

Commercial applications of YBa 2Cu 3O 7 (YBCO) superconducting cables require viable and scalable manufacturing processes. We have investigated the evolution of the biaxial alignment of yttria-stabilized zirconia (YSZ) buffer layers with increasing film thickness (50–900 nm) and report on a method of fabricating highly aligned YBCO tapes using a thin epitaxial YSZ buffer layer as template. The method employs magnetron and ion beam assisted deposition (IBAD) techniques followed by epitaxial growth to produce the buffer architectures IBAD-YSZ and epi-YSZ/IBAD-YSZ onto optically polished hastelloy metal substrates. Subsequent in situ deposition of YBCO films is used to determine the biaxial alignment at the surface of the buffer architecture, and to show that 100–200 nm thick epi-YSZ layers suffice to yield YBCO tapes that have enhanced biaxial alignment (Δ φ=9–10°) and high critical current densities: J c (77 K)=(1–2)×10 6 A cm −2 and J c (5 K,1 T)=8×10 6 A cm −2. Atomic force microscopy of the surface microstructure of the YSZ buffer layers and YBCO films reveals some grain coarsening in the epi-YSZ layers compared to the IBAD-YSZ layers while the YBCO tapes show significant outgrowths (∼200 nm) and large grains (800–1200 nm) that are similar to high- J c YBCO films grown on single crystal MgO(1 0 0) substrates.

Growth of epitaxial YSZ films on LiNbO 3 substrates

We report on the epitaxial growth and characterization of yttria-stabilized zirconia (YSZ) films on X-cut LiNbO 3 single crystals. Epitaxial (100) YSZ films were synthesized by the on-axis RF magnetron sputtering technique. Extensive analyses of the structure and microstructure of films with different thickness reveals the existence of an Li-deficient phase, LiNb 3O 8, at the interface between the substrate and the film. The origin of the presence of this interface is discussed, as well as its consequences on the structural and morphologic properties of the epitaxial YSZ film.

Epitaxial yttria-stabilized zirconia on biaxially-textured (001) Ni for YBCO coated conductor

Epitaxial YSZ film was deposited directly on biaxially-textured (001) Ni tapes using pulsed-laser deposition. Nucleation of the YSZ film on Ni is achieved without the intentional removal of the native oxide from the Ni (001) surface. The epitaxial YSZ layer grown on the Ni (001) surface was used as a single buffer layer for a high temperature superconducting coated conductor architecture, yielding YBa 2Cu 3O 7 films with critical current density of 1 MA/cm 2 at 77 K. This architecture eliminates the necessity for a multilayer buffer architecture, as high J c superconducting films are achieved with no intermediate buffer layer between the (001) YSZ and the biaxially textured metal.

Nucleation of epitaxial yttria-stabilized zirconia on biaxially textured (001) Ni for deposited conductors

The nucleation of (001)-oriented yttria-stabilized zirconia (YSZ) directly on the (001) Ni surface is realized via nucleation on an oxygen-terminated nickel surface using pulsed-laser deposition. Under conditions where the nickel surface is either oxygen free or substantially covered with NiO, a mixed orientation of YSZ occurs. The epitaxial YSZ layer grown on a biaxially textured Ni(001) surface was used as a single buffer layer for a high temperature superconducting coated conductor architecture, yielding superconducting YBa2Cu3O7 films with high critical current densities, Jc. This architecture eliminates the necessity for a multilayer buffer architecture, since high Jc superconducting films are achieved with no intermediate buffer layer between the (001) YSZ and the biaxially textured metal.

Structural properties of epitaxial YSZ and doped CeO2 films on different substrates prepared by liquid sources MOCVD (LSMOCVD)

With LSMOCVD it is possible to deposit reproducibly oxide films without need for high vacuum systems. On in-plane aligned oxide and metallic substrates epitaxial film growth takes place. Film morphology, interface layers and surface roughness can be controlled by choice of deposition parameters like temperature, gas phase composition and mass flow. The layers were characterized by SEM, AFM and texture analysis. Yttria-stabilized zirconia (YSZ), ceria and ceria doped with gadolinium are prepared and the dependence of film properties on deposition parameters is investigated. The suitability of these films for the use of buffer layers in YBCO-deposition is discussed.

Effects of Oxygen Partial Pressure and Laser Energy Density on the Heteroepitaxial Growth of YSZ on Si(001) by Pulsed Laser Deposition.

Yttria-stabilized zirconia (YSZ) was deposited on hydrogen-terminated Si(001) by pulsed laser deposition (PLD) under various oxygen pressures (about 2-2×10-4Pa) and laser energy densities (0.87-1.09J/cm2) at 800°C. The effects of O2 pressure and laser energy density were found to be important to the formation of an epitaxial oxide thin film. With decreasing laser energy density, the O2 pressure required to grow epitaxial YSZ films was shifted toward the lower region. Therefore, epitaxial growth of YSZ films could be achieved at a very low pressure of 2×10-4Pa compared with the known O2 pressure region used for epitaxial growth. The roughness of surfaces was very low, within 0.18-0.26nm.

Microstructure of electron-beam-evaporated epitaxial yttria-stabilized zirconia/CeO 2 bilayers on biaxially textured Ni tape

Transmission and scanning electron microscopy, atomic force microscopy, X-ray pole figure analysis and Auger electron spectroscopy were used to characterize the microstructure and surface topography of epitaxial yttria-stabilized zirconia (YSZ) and CeO 2 thin films deposited by electron beam evaporation on rolling-assisted biaxially textured Ni substrates (RABiTS™). The as-deposited YSZ layer is composed of highly crystallographically aligned, slab-shaped columnar grains with sharply defined, rectangular cross sections and average dimensions of 10 nm by 50 nm by the film thickness. The faces of the YSZ slabs lie on the {110} planes that contain the surface normal. Their caps are roof-shaped with a peak-to-valley height of about 10 nm and a RMS roughness, measured by atomic force microscopy, of 1.3 nm. The resultant surface morphology is rough, but shows a regular, cross-hatched pattern on the length scale of about 10 nm. The length scale and crystallographic directionality of the YSZ microstructure is retained when YBa 2Cu 3O 7− δ is pulsed laser deposited on it, but the YSZ columns appear to have sintered into a less angular, more distinctly porous microstructure. The CeO 2 layer also is columnar, but appears to be denser, with a flatter, less directional surface topography. Auger sputtering-depth profiling experiments revealed that the compositions of both films are constant through the film thickness and that interdiffusion along the surface normal is not extensive.

Initial Stage and Growth Process of Ceria, Yttria-Stabilized-Zirconia and Ceria-Zirconia Mixture Thin Films on Si(100) Surfaces

Initial stages and growth processes of ceria ( CeO2), yttria-stabilized-zirconia (YSZ) and ceria-zirconia mixture ( Ce·ZrO2) thin films on Si(100) surfaces were studied in conjunction with O2 pressure. We showed that (110)-oriented films of CeO2, YSZ and Ce·ZrO2 grew obeying the arrangements of Si atoms in (2×1) or (1×2) structures on a Si(100) surface under about 5×10-7 Torr O2 gas at about 900° C. On the other hand, under 1×10-5 Torr O2 gas, (111)-oriented CeO2 polycrystal films grew because of formation of Ce2O3 and Si O2 layers at the interface between CeO2 and Si(100). Furthermore, the (100)-epitaxial YSZ and Ce·ZrO2 films grew, reducing the SiO2 layer on Si(100) under 1×10-5 Torr O2 gas. We attempted to explain these processes in terms of standard formation enthalpies of CeO2, Ce2O3, ZrO2 and SiO2.

Heteroepitaxial growth of yttria-stabilized zirconia film on oxidized silicon by reactive sputtering

Heteroepitaxial yttria-stabilized zirconia (YSZ) film is difficult to grow on Si by reactive sputtering with Ar+O 2 gas, since the bare Si surface is easily oxidized by plasma radiation. In order to overcome this difficulty, the substrate was subjected to weak oxidation and subsequent deposition of a very thin metallic Zr 1-x Y x film prior to deposition of the YSZ film. It was found that the crystalline quality of the YSZ film depended strongly on the thickness of the metallic film and the Si oxide layer. We observed the initial growth of the epitaxial YSZ film on Si by using reflection high energy electron diffraction (RHEED) and it was found that a very thin YSZ film prepared by metallic film deposition was grown epitaxially on Si but its surface was relatively rough.

Bi-epitaxial weak links based on YBa2Cu3O7- delta on NdGaO3

Thin epitaxial yttria-stabilized zirconia films were used as seed layers to form bi-epitaxial 45 degrees crystallographic boundaries in YBa2Cu3O7- delta films on (110) NdGaO3 substrates. The crystallographic alignment of the seed layer on the substrate and three different in-plane orientations (0 degrees , 9 degrees and 45 degrees ) of c-axis oriented YBa2Cu3O7- delta films on the (100) plane of yttria-stabilized zirconia were analysed. The microwave and magnetic field responses of bi-epitaxial-based Josephson junctions were registered.

YBa2Cu3O7 thin films grown on sapphire with epitaxial yttria-stabilized zirconia buffer layers

Epitaxial yttria-stabilized zirconia (YSZ) buffer layers were grown successfully on sapphire (112̄0) substrates by using rf magnetron sputtering method. The films were cubic in structure with their (100) orientation normal to the substrate surface. YBa2Cu3O7 thin films were deposited on the YSZ/sapphire substrates by the in situ dc magnetron sputtering method. X-ray diffraction analysis showed they were highly c-axis oriented with the zero resistance temperature TCO=92 K and critical current density Jc=1.6×106 A/cm2 at 77 K.

YBa2Cu3O7−x to Si interconnection for hybrid superconductor/ semiconductor integrationa)

We have successfully made low resistance contacts between high-quality films of YBa2Cu3O7−x (YBCO) and single-crystal Si substrates through Ag-Au interconnections. The YBCO films were deposited by laser ablation on an epitaxial yttria-stabilized zirconia buffer layer on Si and had zero-resistance critical temperatures of 83–85 K after patterning into lines. Specific contact resistivities (resistance-area products) of the YBCO to Si interconnection, limited by the Au to Si interface, of 10−6 Ω cm2 were achieved on heavily doped Si after deposition and patterning of the YBCO film. This demonstrates the use of high-temperature superconductors as a wiring layer compatible with conventional Si metal-oxide-semiconductor processing.

Epitaxial yttria-stabilized zirconia on (11̄02)sapphire for YBa2Cu3O7−δ thin films

Epitaxial yttria-stabilized zirconia (YSZ) films were deposited on (11̄02) sapphire by pulsed laser deposition. The films are formed in a cubic phase with the a axis normal to the substrate surface. Ion beam (2.8 MeV He++) channeling measurements show that the YSZ films are highly crystalline with a channeling minimum yield of 8%. The epitaxial relationship between the film and substrate is further confirmed by a cross-section transmission electron microscopy study. Epitaxial YBa2Cu3O7−δ thin films deposited on YSZ/sapphire have Tc and Jc of up to 89 K and 1×106 A/cm2 at 77 K, respectively.

Epitaxial yttria-stabilized zirconia on hydrogen-terminated Si by pulsed laser deposition

Epitaxial yttria-stabilized zirconia films were grown on Si (100) and Si (111) by pulsed laser deposition. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield of 5.3%. A necessary predeposition process is removal of native silicon oxide from the Si prior to film growth. This is done outside the deposition chamber at 23 °C using a wet-chemical hydrogen-termination procedure. Epitaxial YBa2Cu3O7−δ films have been grown on these films.