Growth and characterization of Y-BA-CU-O high-Tc superconductor thin films

Two types of growth processes of Y-Ba-Cu-O thin films were investigated: three step processes involving post deposition high temperature anneals, and in situ growth processes. Films were deposited by sequential ion beam sputtering from elemental Y, Ba and Cu targets, and characterized by x-ray diffraction, transmission and scanning electron microscopy, energy dispersive x-ray analysis, Rutherford backscattering spectrometry, and low temperature resistivity measurements. In the three step process, multilayers of ~60 Å periodicity were deposited on (001) SrTiO3, annealed in oxygen at 850-900°C, and subsequently at 400-500°C, to obtain the superconducting YBa2Cu3O7-δ phase. The films were epitaxial, predominantly single phase YBa2Cu3O7-δ, with different orientations. The nucleation and growth of Y-Ba-Cu-O films deposited on (001) SrTiO3 by magnetron sputtering from separate Y, BaF2 and Cu sources and grown by a three step process, was investigated by transmission electron microscopy. The in situ growth of YBa2Cu3O7-δ films by sequential ion beam sputtering was investigated. The films were deposited following the stacking sequence of YBa2Cu3O7-δ, with the individual layer thicknesses nominally equal to one monolayer, at temperatures between 550 and 750°C. O2 was supplied during growth. Epitaxial, c-axis oriented YBa2Cu3O7-δ films were obtained on MgO and SrTiO3. The correlations between deposition parameters, and structural and electrical properties were investigated. The films had expanded c-axis lattice parameters. The superconducting transition temperatures decreased with the enlargement of the c-lattice parameter. The deposition temperature was the main parameter controlling the lattice expansion. This was later interpreted in terms of the thermally activated dissociation of O2 at the film surface. We proposed that the expansion of the c-lattice parameter was a consequence of kinetic limitations to the incorporation of oxygen into the films during growth. This led to a consistent description of the results obtained in this work, and results reported in the literature for other in situ growth techniques. The films also presented inhomogeneous lattice distortions along the c-direction, that were larger for films with large lattice parameters. The superconducting transitions were broader for films with large inhomogeneous strains. The microstructure of films grown on several substrates (SrTiO3, MgO, SiO2/Si) under different growth conditions was investigated

Nonequilibrium Partitioning During Rapid Solidification of Si-As Alloys

The velocity dependence of the partition coefficient was measured for rapid solidification of polycrystalline Si-4.5 at% As and Si-9 at% As alloys induced by pulsed laser melting. The results constitute the first test of partitioning models both for the high velocity regime and for non-dilute alloys. The continuous growth model (CGM) of Aziz and Kaplan fits the data well, but with an unusually low diffusive speed of 0.46 m/s. The data show negligible dependence of partitioning on concentration, also consistent with the CGM. The predictions of the Hillert-Sundman model are inconsistent with partitioning results. Using the aperiodic stepwise growth model (ASGM) of Goldman and Aziz, an average over crystallographic orientations with parameters from independent single-crystal experiments is shown to be reasonably consistent with these polycrystalline partitioning results. The results, combined with others, indicate that the CGM without solute drag and its extension to lateral ledge motion, the ASGM, are the only models that fit the data for both solute partioning and kinetic undercooling interface response functions. No current solute drag models can match both partitioning and undercooling measurements.Engineering and Applied Science

Study of interfacial reactions and phase stabilization of mixed Sc, Dy, Hf high-k oxides by attenuated total reflectance infrared spectroscopy

Grazing angle attenuated total reflectance Fourier transform infrared spectroscopy is applied to study ultrathin film Hf⁴⁺, Sc³⁺ and Dy³⁺ oxides, due to its high surface sensitivity. The (multi)metal oxides studied, are of interest as high-k dielectrics. Important properties affecting the permittivity, such as the amorphous or crystalline phase and interfacial reactions, are characterized. Dy₂O₃ is prone to silicate formation on SiO₂/Si substrates, which is expressed in DyScO₃ as well, but suppressed in HfDyOₓ. Sc₂O₃, HfScOₓ and HfO₂ were found to be stable in contact with SiO₂/Si. Deposition of HfO₂ in between Dy₂O₃ or DyScO₃ and SiO₂, prevents silicate formation, showing a buffer-like behavior for the HfO₂. Doping of HfO₂ with Dy or Sc prevents monoclinic phase crystallization. Instead, a cubic phase is obtained, which allows a higher permittivity of the films. The phase remains stable after anneal at high temperature.status: publishe

Strontium niobate high-k dielectrics: Film deposition and material properties

Strontium niobate ultrathin films were processed by water-based chemical solution deposition, an approach that offers environmental benefits. SrNb2O6 and SrNb2O7 show high-k values, which is important for applications such as alternative gate dielectrics. The study of ultrathin films (thickness <30 nm) is crucial, as this is the thickness range for the application envisaged, and as film properties depend strongly on the film thickness. SrNb2O6 had a lower crystallization temperature, less interfacial silicate, lower carbonate content, and higher roughness compared to SrNb2O7. The k values of amorphous films were limited for both compositions (k = 12-14). Crystallization and complete removal of organics or carbonates were accomplished by high-temperature annealing, but increased the roughness and leakage current. For SrNb2O7, interfacial silicates were formed as well. Intermediate calcination steps improved the surface smoothness and increased the k value of SrNb2O6 up to 30. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.status: publishe

Seed layer and multistack approaches to reduce leakage in SrTiO3-based metal-insulator-metal capacitors using TiN bottom electrode

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High-k dielectrics and metal gates for future generation memory devices

The requirements and development of high-k dielectric films for application in storage cells of future generation flash and Dynamic Random Access Memory (DRAM) devices are reviewed. Dielectrics with k-value in the 9-30 range are studied as insulators between charge storage layers and control gates in flash devices. For this application, large band gaps (> 6 eV) and band offsets are required, as well as low trap densities. Materials studied include aluminates and scandates. For DRAM metal-insulator-metal (MIM) capacitors, aggressive scaling of the equivalent oxide thickness (with targets down to 0.3 nm) drives the research towards dielectrics with k-values > 50. Due to the high aspect ratio of MIMCap structures, highly conformal deposition techniques are needed, triggering a substantial effort to develop Atomic Layer Deposition (ALD) processes for the deposition of metal gates and high-k dielectrics. Materials studied include Sr and Ba-based perovskites, with SrTiO3 as one of the most promising candidates, as well as tantalates, titanates and niobates