Energy harvesting using thermoelectric devices has been very attractive because they can directly convert heat into electricity and vice versa. The thermoelectric performance is described by a figure-of-merit, ZT=S2σT/k, where S, σ, T and k are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. To improve a ZT value, it is required to increase the Seebeck coefficient and electrical conductivity and to reduce the thermal conductivity. However, this is very challenging because the electrical conductivity and thermal conductivity are coupled together with Wiedeman-Franz law, k=σTL, where L is Lorenz number.
GeTe thermoelectric material has a high carrier concentration due to a Ge vacancy, leading to the high electrical conductivity and low Seebeck coefficient. Therefore, to increase Seebeck coefficient, it is necessary to suppress the carrier concentration by doping element with three valence electrons. GeTe-based thermoelectric materials have a characteristic herringbone structure with an alternating bright and dark contrast, resulting from domains with different polarities caused by cubic-to-rhombohedral phase transformation. A herringbone structure is beneficial to increasing a phonon scattering, leading to reduction of thermal conductivity. Thus, if we can control the herringbone structure in GeTe, the thermoelectric performance will be improved.
In this work, we investigated the microstructure and thermoelectric properties of GeTe-based materials. Especially, the effects of doping elements on the herringbone structure were examined in detail. In this presentation, we will discuss them.