Thermal Characterization of Heterojunction Interfaces and Nanoporous Si Thin Films

Abstract

Thermoelectric (TE) applications serve as an important piece of puzzle in solving the emerging energy demands and challenges in this world. The continuous improvement of TE performance requires collaborative efforts from multiple disciplines including condensed matter physics, nanotechnology, solid-state chemistry, electrical engineering, mechanical engineering, and metrology. In this dissertation, nanotechnology is combined with thermal science, a branch of mechanical engineering, to explore the role of phonon engineering in TE materials.For TE applications, a low phonon thermal conductivity is preferred for higher TE conversion efficiency. In this work, phonon transport is manipulated with interfaces, nanoporosities and point defects. By studying high-quality Si/Ge interfaces formed by film-wafer bonding, the thermal interfacial resistance is measured for the first time for such kinds of interfaces. The importance of Si/Ge atom interdiffusion and interface oxidation on thermal transport is revealed. Thermal conductivity (𝑘) reduction with nanoporous structures is largely restricted by the minimum achievable feature size in the nanofabrication. A highly efficient nanoporous configuration (i.e., offset nanoslots) is proposed to achieve a smaller 𝑘 value, without changing the porosity and feature size. As an example of multi-length scale phonon transport suppression, a further 𝑘 reduction can be achieved with Ga+ ion implantation using a focused ion beam,. The solid-gas heat transfer characteristic of the nanoporous thin film is also studied experimentally. The result is further correlated with the modified two-layer heat transfer model.

The phonon transport engineering methods studied here could be applied to various TE materials to improve their TE performance. Besides, the findings could also provide guidelines for advanced thermal regulations such as cooling of power electronic devices and heat guides.