Semiconductor nanostructures for thermal and thermoelectric applications

2019 edition

Jose Manuel Sojo Gordillo

Thermoelectric devices (TE) enable direct conversion between thermal and electrical energy, thus providing an alternative for power generation (Seebeck effect) to the conventional ways of power production. This type of power generation stands out for its high simplicity, absence of mobile parts, adaptability to a range of temperature differences and capacity for long-term operation for extended periods of time thanks to their intrinsic reliability.

The conversion of thermal energy into usable electrical energy by thermoelectric generation appears as a viable solution not only in terms of energy efficiency but also as an effective feeding solution for the rising field of the Internet of Things (IoT). IoT has started to spread rapidly in the last few years and is expected to gain further recognition as the Next Industrial Revolution. For a successful realization of IoT, they should be smart, autonomous, miniaturized, and they should have wireless communication capabilities. Therefore, thermoelectric generation appears as a logic and cost-effective solution.

The proposal of this project aims to continue the research on nanostructures aimed to serve as thermoelectric generators in microdevices and explore the fabrication of novel ones for potentially new applications.

In order to achieve better TE properties of the NW structures, one proposed path is to tune their geometrical shape and take advantage of quantum phenomena at the nanoscale. Basically, the research would be focused on developing novel CVD conditions at which the NW can growth in complex geometries. These lead to outstanding TE and thermal properties. The most promising geometries are strong saw-teeth shaped NW and longitudinal variation of Si-Ge composition of the NW (and/or both at the same time). These approaches could possibly achieve even lower thermal conductivities than regular straight nanowires without changing noticeably the power factor (PF), enhancing the figure of merit (ZT).

Additionally, as some theoretical studies point out, a particular geometry of nanostructures (cones) could potentially lead to significant differences in the thermal conductance depending on the direction of the heat flow due to differences in the evolution  speed of phononic modes at the two sides of the cone. This could be achieved if controlled aperture angle conic shaped nanostructures could be growth. This nanostructure has a completely novel property, thermal rectification. This new possibility would open the door to work into a promising emerging field of study, the concept of thermal logics, with a huge number of potential applications.