Abstract:

Mean-free paths of phonons are crucial for understanding heat transport in micro/nano-structured materials like nanowires, superlattices and graphene. However, very little is known about the mean free paths of phonons, even for bulk silicon that is the most common material in semiconductor industry. Ju and Goodson¹ measured the in-plane thermal conductivity of thin silicon films and concluded that the effectivemean free path of silicon at room temperature should be around 300 nm, which is contradictory to the kinetic theory on gray approximation which suggests 41 nm.² More recently, several first principles calculations^{3, 4}  revealed that the mean free paths of phononsin Si vary from 1 nm to 100 mm at room temperature; phonons with mean free paths larger than 5 mm contribute 20% of the total thermal conductivity. These contradictory conclusions render the mean free paths of phonons in silicon an unsolved problem. Unfortunately up till now there are very few experimental techniques that can measure the mean free path of phonons. FDTR method proposedby Koh⁵ and thermal conductivity spectroscopy method proposed by Minnich⁶ are claimed to be able to measure phonon mean free paths but both have their limitations.

This study proposes an experimental approach to study the distributions of phonons’ mean-freepaths in Si at different temperatures.

The file can be downloaded here:

Oral QE report v4.pdf