Thermoelectric energy harvesters that convert any source of heat into electricity, are gaining attention due to the rapid increase of power needs of the internet of things. At a given temperature the efficiency of TE devices is determined by a figure of merit (ZT). In order to achieve a high ZT it is necessary to decrease thermal conductivity while maintaining a high power factor (electrical conductivity times the square of Seebeck coefficient) to attain the phonon-glass-electron-crystal regime. Si/Ge superlattices (SL) have been extensively investigated as a TE material for over 30 years. Thermal transport in such SLs is significantly diminished due to phonon scattering at interfaces . Introduction of interstitial defects is a viable approach to introduce additional scattering mechanism to further reduce thermal conductivity and thus improve ZT in SLs . However, understanding of electronic transport in SLs with interstitial defects is critical for development of TE devices with high efficiency. In this work we investigate the effect of interstitial defects on electronic transport in Si/Ge SL by employing first principle DFT calculations in conjunction with semi-classical Boltzmann transport theory. Interstitial defects introduce additional energy levels and strain in the system. To understand the effect of additional levels we investigate electronic transport in bulk silicon with 1.56% of commonly occurred interstitial defects: Ge, C, Si and Li placed in different symmetry locations of the lattice. Interstitials lead to the formation of additional deep and/or shallow energy levels depending on both the guest species type and the symmetry location. Upon comparison of the electronic transport coefficients of all different silicon-interstitial systems we observe that 1.56% of Ge interstitial defects placed in hexagonal sites provide the best improvement of ZT by a factor of 17 with reference to the bulk value. In a parallel study of ideal Si/Ge SLs of varying periods, we demonstrate that the electronic transport properties can be tuned by applying external strain. At higher carrier concentrations positive strain (tension) in the in-plane direction of the SL leads to significant improvement of the Seebeck coefficient. Finally, we introduce interstitial defects in Si/Ge SL to determine how additional energy levels and defect-induced strain can be used to tailor electron transport in superlattices.
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