1. magnetic confinement - ENEA - Fusione

116
4. MISCELLANEOUS
4.4 Advanced Superconducting
Materials and Devices
freely rotating sample holder to incline the substrate normal with respect to the
vapour incidence direction of an angle α.
At normal incidence (α=0°), the CeO 2 film showed a fibre texture, with the [111]
direction normal to the substrate and no in-plane texture. On inclining the substrate,
a biaxial textured growth of CeO 2 films is induced (fig. 4.5). The [00l] axis is tilted
relative to the substrate normal n of an angle γ in the opposite direction with respect
to the incidence of the vapour flux. The (200) and (020) poles, at the same χ angle, are
well defined, indicating a high degree of texture. The φ-scan FWHM values decrease
rapidly on increasing α from 15° to 45°, and become almost constant between 45° and
75° (fig. 4.6a). The film thickness has an important influence on the degree of texture:
films more than 1-µm thick show a sharp texture, as can be seen from the φ–scan
FWHM value reported in figure 4.6b.
Qualitatively, the texturing of ISD-CeO 2 film can be explained by considering the
anisotropic growth rate of crystal planes and the anisotropic diffusion along different
crystal directions. In general, films preferentially grow with the fast growing plane
perpendicular to the vapour flux. Typically in fcc materials, such as CeO 2 , the fast
growing plane is the close-packed {111} plane. In the inclined configuration, the same
growth mechanism is present and the {111} planes are the top planes of the column.
These planes are not exactly orthogonal to the vapour incidence direction because of
directional diffusion, which is due to the momentum conservation of the adsorbed
atom parallel to the film surface. Directional diffusion is also responsible for CeO 2
in-plane alignment. The grains that present the crystallographic direction with the
highest diffusion rate aligned with the directional diffusion can grow more than the
other grains, due to a higher mass transport effect. In ISD-CeO 2 films, this direction
coincides with the direction along the {111} planes. The bigger grains mask the
other grains, hence promoting a selection of film orientation; texture improves with
film thickness.
Fig. 4.5 - X-ray (002)
CeO 2 pole figures for
1.5-mm-thick film
deposited at α=55° and
Tsub=200°C in vacuum.
The arrow indicates the γ
angle; the ×, the
deposition direction.
φ-scan FWHM (degrees)
40
30
20
10
10 20 30 40 50 60 70 80
α (degrees)
FWHM
γ
a) b)
90
FWHM 90
40
γ
60
γ (degrees)
φ-scan FWHM (degrees)
30
20
10
0 0,5 1 1,5 2
thickness (µm)
60
γ (degrees)
Fig. 4.6 - φ-scan FWHM
and γ angle values for
CeO 2 film deposited at
Tsub=200°C in vacuum a)
vs. α value (1.5-µm-thick
samples) and b) vs. film
thickness (samples
deposited at α=55°).
4.4.4 MgB 2
film fabrication
Different fabrication techniques were used for MgB 2 films on single crystal
substrates. Two main methods were followed: as grown, performed by pulsed laser
deposition (PLD) and in situ annealing, performed both by PLD and by electron
beam evaporation.
In the as-grown method, the MgB 2 film is deposited directly on the single-crystal
substrate, which is heated at a certain deposition temperature in an inert gas
atmosphere.