The simultaneous realization of low thermal conductivity and high thermoelectric power factor in materials has long been the goal for the social use of high-performance thermoelectric modules. Nanostructuring approaches have drawn considerable attention because of the success in reducing thermal conductivity. On the contrary, enhancement of the thermoelectric power factor, namely, the simultaneous increase of the Seebeck coefficient and electrical conductivity, has been difficult. We propose a method for the power factor enhancement by introducing coherent homoepitaxial interfaces with controlled dopant concentration, which enables the quasiballistic transmission of high-energy carriers. The wavenumber of the high-energy carriers is nearly conserved through the interfaces, resulting in simultaneous realization of a high Seebeck coefficient and relatively high electrical mobility. Here, we experimentally demonstrate the dopant-controlled epitaxial interface effect for the thermoelectric power factor enhancement using our "embedded-ZnO nanowire structure" having high-quality nanowire interfaces. This presents the methodology for substantial power factor enhancement by interface carrier scattering.

We develop transparent epitaxial SnO2 films with low thermal conductivity and high carrier mobility by domain engineering using the substrates with low symmetry: intentional control of the domain size and the defect density between crystal domains. The epitaxial SnO2 films on r-Al2O3 (a low symmetry substrate) exhibit a twice higher mobility than the epitaxial SnO2 films on c-Al2O3 (a high symmetry substrate), resulting in twice larger thermoelectric power factor in the SnO2 films on r-Al2O3. This mobility difference is likely attributed to the defect density between crystal domains. Furthermore, both samples exhibit almost the same thermal conductivities (∼5.1 ± 0.4 W m−1 K−1 for SnO2/r-Al2O3 sample and ∼5.5 ± 1.0 W m−1 K−1 for SnO2/c-Al2O3 sample), because their domain sizes are almost the same. The uni-leg type film thermoelectric power generator composed of the domain-engineered SnO2 film generates the maximum power density of ∼54 μW m−2 at the temperature difference of 20 K. This demonstrates that a transparent film thermoelectric power generator based on the domain engineering is promising to run some internet of things sensors in our human society.

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We revealed the most significant parameter determining an areal density of nanowires (NWs) in physical vapor transport. Seed layer characters such as the crystallinity and the surface roughness can affect the NW growth. We controlled the surface roughness and the crystallinity of seed layers both by annealing process and using Ge nanocrystals. With increase of the surface roughness of seed layers, the areal density of NWs increased, which was independent of the crystallinity of seed layers. Therefore, the large surface roughness was considered to be a critical parameter for high areal density of NWs. This study demonstrated that the substrate with well-controlled surface roughness brought control of an areal density of NWs. This well-controlled technique of NW areal density will open for the next generation NW devices.