Inclined Substrate Deposition by Evaporation of Magnesium Oxide for Coated Conductors
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Inclined Substrate Deposition by Evaporation of Magnesium Oxide for Coated Conductors
Markus Bauer, Ralf Metzger, Robert Semerad, Paul Berberich, and Helmut Kinder Technische Universität München, Physik Department, D-85747 Garching, Germany
ABSTRACT
Biaxially textured MgO buffer layers were deposited on metal substrates using “inclined substrate deposition” (ISD). The influence of the substrate inclination angle, deposition rate, and film thickness on the texture is shown. Scanning electron microscopy reveals columnar growth. We developed a growth model to explain the texturing. To test this model we have carried out 3D Monte-Carlo simulations. We find that the preferred orientation arises from mutual shadowing of the columns and directional surface diffusion due to their initial momentum. YBa2Cu3O7 (YBCO) films deposited on the ISD buffer layers are highly textured. The ab-planes of the YBCO are tilted with respect to the surface by typically 25° towards the direction of MgO vapor incidence. Therefore, the critical current density jc is anisotropic with up to 8 × 105 MA/cm² in one direction and 4 × 105 MA/cm² in the other. For tape coating the MgO deposition direction can be chosen so that the high jc is along the tape.
INTRODUCTION
Thin films of YBCO must be highly textured in order to have good superconducting properties. One way to achieve this is to grow textured buffer layers on arbitrary polycrystalline substrates by ISD. This was first proposed by Hasegawa et al. [1] using pulsed laser deposition of yttria-stabilized ZrO2 (YSZ). We use evaporation techniques to make the ISD process scalable to large areas and high production rates. The buffer material is MgO. It was already reported that high critical current densities were achieved on MgO ISD buffer layers [2]. This shows that ISD-MgO is a very promising candidate for the successful realization of YBCO coated conductors. In this paper we will give an overview of the experimental results. In order to further optimize the buffer layer deposition it is important to understand the mechanism of texturing. We present a model that can explain the growth of textured ISD films. Monte-Carlo simulations based on this model were also carried out. The results will be compared with the experimental findings.
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EXPERIMENTAL DETAILS
The experimental setup for ISD is shown in figure 1. We used an e-gun with MgO or Mg target as evaporation source. Oxygen was supplied locally at the substrate. The deposition rate, which was measured by a quartz crystal monitor, was varied between 0.1 nm/s and 8 nm/s. The substrate was positioned above the source and inclined by an angle α as shown in figure 1. The substrates were mechanically or electropolished Hastelloy C276 or stainless steel pieces (10 × 10 mm²) with a thickness between 0.05 mm and 0.5 mm. For experiments, where no YBCO was deposited on the ISD buffer layer, thermally oxidized Silicon substrates were used as well. Further details of the deposition setup were described earlier [2]. YBCO was deposited by react
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