Thermoelectric phenomena in low-dimensional semiconductor systems

University dissertation from KFS AB

Abstract: This thesis investigates thermoelectric effects, predominantly in low-dimensional semiconductors. Various novel phenomena are predicted, investigated and discussed. The introductory part of the thesis contains a summary in Swedish for the general public as well as a general introduction to thermoelectric effects using both a semi-classical theory valid for diffusive electron transport, as well as the Landauer approach valid for ballistic electron systems. After the introduction, six original papers are presented. In paper 1 and 2 we investigate the thermoelectric current reversal in the presence of potential barriers. The quantum mechanical transmission function in relation to the chemical potential determines the sign of the current. This thermopower anomaly does not violate the thermodynamic law. In paper 3 we predict a substep in the conductance of a quantum wire, due to interaction of the quantum confined electrons and phonons. This substep should be experimentally observable. Paper 4 deals with a quantum ring, not connected to any particle reservoirs. However, electron-phonon interaction is introduced at two points. Using a Boltzmann-equation approach the non-equilibrium electron distribution is calculated. Furthermore, electric and heat currents appear in the ring whenever the two phonon baths are not of equal temperature. This can be developed into a thermionic couple. In paper 5 the zone-folding effect of phonons in a superlattice is studied. The transmission function of phonons is calculated via a transfer matrix approach. Surprisingly, it is concluded that the zone folding effect has negligible effect on thermal conductance. Finally, paper 6 combines quantum mechanical tunneling and thermionic emission into a new cool chip, operable at room temperature. The cooling power and efficiency of such a proposed chip is investigated, and is found very favorable.

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