Nobuyasu Naruse

The simultaneous increase of Seebeck coefficient and electrical conductivity is vital for high thermoelectric performance. With Fermi level upshift in the non-parabolic band, the effective mass increases, bringing relatively high Seebeck coefficient in addition to higher electrical conductivity. However, it is difficult to introduce many carriers for a drastic Fermi level upshift because of the solubility limit of dopants. Here, we introduce many native donor-type point defects in ZnO films by growth orientation control, influencing strain and crystallinity. This introduction not only raises the Fermi level but also enhances band non-parabolicity via hybridization with defect levels, resulting in a higher effective mass. This led to a simultaneous realization of high electrical conductivity and Seebeck coefficient, which is a non-parabolic band effect. The ZnO films with this effect exhibited a thermoelectric power factor of 11.8 μW cm^-1 K^-2 at 300 K, which is the highest among all the transparent thermoelectric thin film materials ever reported. This method provides a new approach to increase the thermoelectric power factor.

We develop compressively-strained epitaxial Ge1−xSnx films with large x (>0.1) on Si substrates. Therein, large x and compressive biaxial strain cause a band convergence by movement of Γ valley down to L valleys near the Fermi level, which leads to high Seebeck coefficient while increasing alloy phonon scattering rate related to a low thermal conductivity. Furthermore, a higher electron mobility is also realized due to the reduction of the conductivity effective mass coming from the large contribution of small effective mass of Γ valley. Further increase in x of Ge1−xSnx films decreases thermal conductivities. As a result, the doped compressively-strained epitaxial Ge0.88Sn0.12 films on Si substrates exhibit two times higher thermoelectric power factor (∼15 μW cm−1 K−2) at room temperature than the previously-reported Ge1−xSnx films at almost the same thermal conductivity. This highlights that the application of compressive in-plane strain in Ge1−xSnx films with large x is a promising approach for enhancing a thermoelectric performance.

The atomic force microscopy statistical results indicate that the heights of the synthesised dimers and trimers are approximately 0.37 nm and 0.50 nm, respectively.

This study demonstrates a simultaneous realization of ultralow thermal conductivity and high thermoelectric power factor in epitaxial GeTe thin films/Si substrates by a combination of the interface introduction by domain engineering and the suppression of Ge vacancy generation by point defect control. We formed epitaxial Te-poor GeTe thin films having low-angle grain boundaries with a misorientation angle close to 0° or twin interfaces with a misorientation angle close to 180°. The control of interfaces and point defects gave rise to ultralow lattice thermal conductivity of ∼0.7 ± 0.2 W m^-1 K^-1. This value was the same in the order of magnitude as the theoretical minimum lattice thermal conductivity of ∼0.5 W m^-1 K^-1 calculated by the Cahill-Pohl model. At the same time, the GeTe thin films exhibited a high thermoelectric power factor because of the suppression of Ge vacancy generation and a small contribution of grain boundary carrier scattering. The outstanding combined technique of domain engineering and point defect control can be a great approach for developing high-performance thermoelectric films.

A transparent thermoelectric material requires not only high thermoelectric performance but also high optical transmittance. However, in transparent nanostructured thermoelectric materials, the nanostructure interface brings the trade-off relationship between thermal conductivity and optical transmittance. We propose an approach for the simultaneous control of thermal conductivity and optical transmittance in the epitaxial nanostructured films, where carriers can be smoothly transported. This is realized by the interface design based on the three strategies: (1) a large atomic mass difference at the heterointerface for low thermal conductivity; (2) heterointerface with almost the same refractive index and flat surface for high optical transmittance; and (3) epitaxial heterointerface for smooth carrier transport. We formed epitaxial ZnO/SnO₂ multilayer films based on this design guideline. The multilayer films exhibit lower thermal conductivity and higher optical transmittance than an ever reported transparent nanostructured thermoelectric material. These results highlight that this design is promising to realize high-performance transparent nanostructured thermoelectric materials.

Vanadium dioxide (VO₂) material, known for changing physical properties due to metal-insulator transition (MIT) near room temperature, has been reported to undergo a phase change depending on the strain. This fact can be a significant problem for nanoscale devices in VO₂, where the strain field covers a large area fraction, spatially non-uniform, and the amount of strain can vary during the MIT process. Direct measurement of the strain field distribution during MIT is expected to establish a methodology for material phase identification. We have demonstrated the effectiveness of geometric phase analysis (GPA), high-resolution transmission electron microscopy techniques, and transmission electron diffraction (TED). The GPA images show that the nanoregions of interest are under tensile strain conditions of less than 0.4 % as well as a compressive strain of about 0.7 % (Rutile phase VO₂[100] direction), indicating that the origin of the newly emerged TED spots in MIT contains a triclinic phase. This study provides a substantial understanding of the strain-temperature phase diagram and strain engineering strategies for effective phase management of nanoscale VO₂. Graphical abstract