HiTc Superconductors: example of Thin Film Textures

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HiTc Superconductors example of Thin Film
Textures

• 27-750, Spring 2005
• A. D. (Tony) Rollett

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High Temp. Superconductors

• Performance the main criterion for performance
  for High critical temperature superconductors is
  current density, JC. The second criterion is
  length of conductor over which a minimum level of
  conductivity can be guaranteed. In other words,
  high JC is useless unless it can be obtained over
  the entire length of the required wire.

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Manufacturing Strategies

• There have been two main strategies for making
  long lengths of superconducting tape/wire.
• Enclose the (brittle) oxide superconductor in a
  ductile metallic sheath, e.g. BSSCO on silver.
• Deposit thin layers of the superconductor, e.g.
  YBCO on a Ni substrate, in a way that ensures
  epitaxy, i.e. alignment of the oxide crystals
  with the substrate crystals.

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IBAD

• IBAD Ion Beam Assisted Deposition
• The principle is to use a highly (cube-)oriented
  metal substrate and deposit buffer layers,
  followed by superconductor layers.
• The buffer layers protect the superconductor
  (generally YBCO) from the metal (typically Ni)
  because the metal poisons the superconductor.

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Epitaxial Deposition

• Several layers are used in between the metal tape
  and the final superconductor layer. Typical
  layer thicknesses are small fractions of 1 µm.

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YBCO

• Two forms of anisotropy apply in YBCO the
  conduction property is essentially confined to
  the a-b plane (i.e. 001, left hand chart). Also,
  any boundary above a few degrees in
  misorientation blocks the current (right hand
  chart).
• Bi-axial alignment is therefore critical.

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YBCO on Substrate

• A YBCO crystal is not far from cubic (tetragonal
  but with c/a 1). Therefore it can deposit
  epitaxially on a (single crystal) substrate with
  either the c-axis perpendicular to the film
  plane, or parallel to it.

c?
c?
a?
a?
LaAlO3
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YBCO on various substrates
c?
a?
YBCO is nearly cubic, so it can deposit with
either the c-axis or the a-axis parallel to the
substrate surface
Note the different positions of the (102) peak
for the c? a ? orientations
Theoreticalpole figuresfor c? a ?
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YBCO (123) on various substrates
c?
a?
Various epitaxialrelationshipsapparent fromthe
pole figures
More a? grainsMore c? grains
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High Resolution Scan ?a 0.5, ?b 0.2
Azimuth, b
Tilta
High resolution scans permit detailed analysis of
peak shape, e.g. measurement of
full-width-half-maximum (FWHM).
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Dependence of film orientation on deposition
temperature
Ref Heidelbach, F., H.-R. Wenk, R. E.
Muenchausen, R. E. Foltyn, N. Nogar and A. D.
Rollett (1996), Textures of laser ablated thin
films of YBa2Cu3O7-d as a function of
deposition temperature. J. Mater. Res., 7,
549-557.
Impact superconduction occurs in the
c-planetherefore c? epitaxy is highly
advantageous tothe electrical properties of the
film.
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Summary

• By choosing an appropriate substrate (one with
  similar lattice parameter), a high Tc
  superconductor can be induced to deposit
  epitaxially, i.e. with all its crystals aligned
  with the crystal(s) of the substrate.
• A well researched approach is to start with a
  highly aligned (cube textured) metal tape (e.g.
  Ni) and deposit various buffer layers with a
  final layer of the superconductor. With careful
  control of the deposition conditions, epitaxy is
  preserved throughout the various layers, allowing
  a highly aligned, long superconductor tape to be
  manufactured.
• Remaining research development challenge
  currently, the mosaic spread in the substrate is
  about 1. It would be very useful to bring the
  spread down and decrease the magnitude of the
  misorientation at grain boundaries in the
  superconductor.

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Notes epitaxial films

• If epitaxy of any kind occurs between a film and
  its substrate, the (inevitable) difference in
  lattice parameter(s) will lead to residual
  stresses. Differences in thermal expansion will
  reinforce this.
• Residual stresses broaden diffraction peaks and
  may distort the unit cell (and lower the crystal
  symmetry), particularly if a high degree of
  epitaxy exists.
• Mosaic spread, or dispersion in orientation is
  always of interest. In epitaxial films, one may
  often assume a Gaussian distribution about an
  ideal component and measure the standard
  deviation or full-width-half-maximum (FWHM).