NER: Enhancement of the Efficiency of Thermoelectric Devices via Engineering of the Electron-Phonon Interaction in Quantum Dot Superlattices

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This proposal was received in response to the National Science and Engineering Initiative, Program Solicitation NSF 01-157, in the NER category. The proposal focuses on a new approache to enhancement of the thermoelectric figure of merit ZT of semiconductor materials via tuning of electron-phonon interaction in multiple arrays of semiconductor quantum dots. Quantum dot arrays are potential candidates for the thermoelectric "electron transmitting - phonon blocking" material. At the same time, in order to achieve a significant ZT improvement and compete with conventional thermoelectrics one has to design and fabricate quantum dot superlattice where carrier transport is facilitated by the formation of carrier mini-bands. Indeed, hopping conductivity typical for lateral carrier transport in random quantum dot arrays has very low carrier mobility while mini-band transport may lead to very high mobility values, particularly if one manages to suppress at least partially the carrier scattering via smart mini-band engineering. The important task addressed in this proposal is theoretical proof-of-concept investigation of requirements for achieving mini-band transport regime and carrier scattering suppression in quantum dot superlattices.Modification of acoustic phonon modes in quantum dot arrays due to spatial confinement and boundary scattering leads to a change in the phonon density of states, a decrease of the phonon group velocity, and corresponding drop of the in-plane lattice thermal conductivity. Acoustic phonon confinement, e.g. modification of phonon dispersion due to nanostructure boundaries, is much less researched phenomenon than carrier confinement effect. At the same time, it can serve as an additional tool to decrease the lattice thermal conductivity value and increase the effectiveness of the thermoelectric device. Thus, another task of this project is a study of the required structure parameters, e.g. dot size, shape, acoustic mismatch, etc., for achieving desirable acoustic phonon transport features such as low group velocity along direction of interest; resonant phonon scattering conditions; decoupling of phonon bath from electrons; etc. New efficient thermoelectric devices based on nanostructured materials may have a tremendous impact on a wide range of energy needs due to their inherent advantages such as high reliability, light weight, compactness, quit operation, and environmental safety.