  The effective mass is one of the main factors for enlarging the Seebeck
coefficient and electronic conductivity of SrTiO3-based thermoelectric
materials [1,2]. The goal of this paper is to clarify, how superlattices can
change the effective mass and other features of the bandstructure. The natural
Ruddlesden-Popper phase (SrTiO3)n(SrO)m with n=2, m=1 the situation changes,
because the TiO6-octahedrons are slightly extended, due to diluted density of
the SrO layer. Another effect is the deformed electron density, which leads to
reduced effective mass perpendicular to the layer, but enlarged parallel to the
plane [3]. The average value of the effective mass over this anisotropy of the
2-dimensional electron gas (2DEG) for pure Ruddlesden-Popper phases is smaller,
but can increase beyond the value of pure Pervoskite for certain doping
elements. In the same way, artificial superlattices (SrTiO3)x/(SrTi1-z(Nb)zO3)y
were examined. When fine nanostructures (n=2, m=3 or n=3, m=2) are present the
effective mass increases, when the structure becomes coarser (n=4, m=1) smaller
values are determined.
