It is generally perceived that for a better thermoelectric material degeneracy of band-extrema near Fermi level is one of many criteria, since it directly influences the figure of merit (ZT). And studies suggest that high symmetric crystal structures, like cubic, hexagonal or tetragonal offer the highest degeneracy, so better ZT.
Dear Bazlul,
the word "degeneration" has a double meaning. We have semiconductors with "degenerated bands" as described above by M. Stutzmann. This enhances the density of states by the degeneration factor. The second meaning is "degenerated semiconductor" what means that the Quasi fermi level lies in the bands. In "normal" nondegenerated semiconductors you find the fermilevel near the donor or acceptor level. The thermoelectric efficiency (ZT) passes through a maximum if the fermi level crosses the band minimum (maximum).The semiconductor is just degenerated. This maximum of ZT enhances with the number of simultaneously crossed band extrema.
Therefore, both degeneration factors effect positively on ZT.
Regards
R. Mitdank
Martin Stutzmann
Dear Professor Stutzmann,
Yes, I read that the typical values of degeneracy of the band extrema (Nv) for TE materials are in the range of 6 to 8, which can be as high as 48, 24, 16, 8 and 4 for cubic, hexagonal, tetragonal, orthorhombic and other remaining space group, respectively.
But, my inquiry is what is degeneracy of the band extrema (Nv), and where does it come from in view of the crystal symmetry and nature of band gap (direct or indirect), and influence the ZT of a TE material? I would appreciate if you shed some light on these issues.
Thanks and regards
Dear Bazlul,
the word "degeneration" has a double meaning. We have semiconductors with "degenerated bands" as described above by M. Stutzmann. This enhances the density of states by the degeneration factor. The second meaning is "degenerated semiconductor" what means that the Quasi fermi level lies in the bands. In "normal" nondegenerated semiconductors you find the fermilevel near the donor or acceptor level. The thermoelectric efficiency (ZT) passes through a maximum if the fermi level crosses the band minimum (maximum).The semiconductor is just degenerated. This maximum of ZT enhances with the number of simultaneously crossed band extrema.
Therefore, both degeneration factors effect positively on ZT.
Regards
R. Mitdank
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