Phase pure and clean MgB2
films have been fabricated on different types of single-crystal substrate, including SiC, sapphire, MgO,
LaAlO3,
SrTiO3, and
yttrium-stabilized ZrO2
(YSZ), via the hybrid physical–chemical vapor deposition (HPCVD) technique. Pure
MgB2
films with different structural textures are obtained by selecting
the substrate and its orientation, such as off-axis epitaxial
MgB2 on MgO(211) or
textured MgB2 on YSZ(110)
substrates. Films show Tc(0)
in the range 37–41 K, and residual resistivity
ρ(42 K) between
0.3 and 5 µΩ cm
as the substrate and film thickness are varied. The crystal structures and film
morphologies are presented and discussed. Further investigations are focused on
c-axis epitaxial
films. Epitaxial MgB2
films with thickness about 300 nm manifest
Tc(0) = 41.2 K and residual
resistivity ρ(42 K) = 0.35 µΩ cm on
SiC(0001) while Tc(0) = 39.5 K and ρ(42 K) = 0.27 µΩ cm
on c-cut sapphire, reflecting that the films are of high quality and in the ‘clean limit’. Low
upper critical field and large anisotropy of the upper critical field
γ comparable to
the results from MgB2
single crystals are also revealed. The smooth surface of the epitaxial films, with
root-mean-square (RMS) roughness less than 5 nm, makes them well qualified
for device applications. Microwave performance measurements are carried out
using a coplanar resonator on a 300 nm thick film on sapphire substrate and no
power dependence of unloaded quality factor in the output power range of
−10–25 dB is found.
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