The growth mechanism and structure of thin films is to a large extent determined
by the substrate used.
Furthermore, the choice of the substrate material is
governed by the application involved.
For instance, for YBCO thin films used
in microwave applications, (001) LaA103 or (001) MgO substrate with its lower
dielectric constant is preferred over (001) SrTiO3 substrates,
which have a high dielectric constant.
However, twinning is commonly observed in LaA103 crystals.
It has been found that (100) is an active twin plane in LaA103 [73].
This twinning results in a rough crystal surface, strain and non-isotropic microwave properties
for (001) LaA103 wafers. To overcome the twinning problem, a new twin-free
substrate, Lao.gSro.7Alo.65Tao.3509 (LSAT), has been developed, which is a solid
solution of LaA103 and Sr2A1TaO6 [74, 75].
The dielectric constant and thermal
expansion coefficient of LSAT are comparable to those of LaA103 as seen from
Table 5.1.