# Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultra-low thermal conductivity

## TL;DR

This paper reveals that native oxide layers on alloyed silicon membranes induce local resonances that significantly reduce phonon group velocities and mean free paths, leading to ultra-low thermal conductivity, especially at low temperatures.

## Contribution

It uncovers a new phonon interaction mechanism via native oxide layers that enables ultralow thermal conductivity in alloyed silicon membranes, a novel approach for thermal management.

## Key findings

- Native oxide layers induce local phonon resonances.
- Alloying with 5% germanium reduces thermal conductivity by 100 times.
- Resonance mechanism is effective even at low temperatures.

## Abstract

A detailed understanding of the relation between microscopic structure and phonon propagation at the nan oscale is essential to design materials with desired phononic and thermal properties.Here we uncover a new mechanism of phonon interaction in surface oxidized membranes, i.e., native oxide layers interact with phonons in ultra-thin silicon membranes through local resonances. The local resonances reduce the low frequency phonon group velocities and shorten their mean free path. This effect opens up a new strategy for ultralow thermal conductivity design as it complements the scattering mechanism which scatters higher frequency modes effectively. The combination of native oxide layer and alloying with germanium in concentration as small as 5% reduces the thermal conductivity of silicon membranes to 100 time lower than the bulk. In addition, the resonance mechanism produced by native oxide surface layers is particularly effective for thermal condutivity reduction even at very low temperatures, at which only low frequency modes are populated.