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A4: Theory - bulk materials
We present an analysis of the thermoelectric properties of
of $n$-type GeTe and SnTe in relation to the
lead chalcogenides PbTe and PbSe.
We find that the singly degenerate conduction bands
of semiconducting GeTe and SnTe are
highly non-ellipsoidal, even very close to the band edges.
This leads to isoenergy surfaces
with a strongly corrugated shape that is clearly
evident at carrier concentrations well below 0.005 $e$ per formula unit (7 - 9 $\times$ 10$^{19}$cm$^{-3}$ depending on material)
Analysis within Boltzmann theory
suggests that this corrugation may be
favorable for the thermoelectric transport.
Our calculations also indicate that values of the power factor
for these two materials may well exceed
those of PbTe and PbSe.
As a result these materials may exhibit $n$-type
performance exceeding that of the lead chalcogenides.