Surface matters: Huge reduction of heat conduction observed in flat silicon channels
April 28, 2015
The ability of materials to conduct heat is a concept that we are all familiar with from everyday life. The modern story of thermal transport dates back to 1822 when the French physicist Jean-Baptiste Joseph Fourier published his book “Théorie analytique de la chaleur” (The Analytic Theory of Heat), which became a cornerstone of heat transport. He pointed out that the thermal conductivity, i.e., ratio of the heat flux to the temperature gradient is an intrinsic property of the material itself.
Mainz/Bellaterra/Espoo. The advent of nanotechnology, where the rules of classical physics gradually fail as the dimensions shrink, is challenging Fourier's theory of heat in several ways. A paper published in ACS Nano and written by researchers from the Max Planck Institute for Polymer Research (Germany), the Catalan Institute of Nanoscience and Nanotechnology (ICN2, Spain) and the VTT Technical Research Centre of Finland (Finland) in the framework of the project MERGING (Membrane-based phonon engineering for energy harvesting) describes how the nanometer-scale topology and the chemical composition of the surface control the thermal conductivity of ultrathin silicon membranes.
The results show that the thermal conductivity of silicon membranes thinner than 10 nm is 25 times lower than that of bulk crystalline silicon and is controlled to a large extent by the structure and the chemical composition of their surface. Combining state-of-the-art realistic atomistic modeling, sophisticated fabrication techniques, new measurement approaches and the latest parameter-free modeling, researchers unraveled the role of surface oxidation in determining the scattering of phonons (quantized lattice vibrations), which are the main heat carriers in silicon.
Both experiments and modeling showed that removing the native oxide improves the thermal conductivity of silicon nanostructures by almost a factor of two, while successive partial re-oxidation lowers it again. Large-scale molecular dynamics simulations with up to 1,000,000 atoms allowed the researchers to quantify the relative contributions to the reduction of the thermal conductivity arising from the presence of native SiO2 and from the dimensionality reduction evaluated for a model with perfectly specular surfaces.
The different circles represent the studied surfaces of the Si membranes: crystalline, rough, flat with native SiO2, and rough with native SiO2. The right image shows a representative thermal map of the membranes upon a localized thermal excitation used to measure the thermal conductivity.[less]
The different circles represent the studied surfaces of the Si membranes: crystalline, rough, flat with native SiO2, and rough with native SiO2. The right image shows a representative thermal map of the membranes upon a localized thermal excitation used to measure the thermal conductivity.
Silicon is the material of choice for almost all electronic-related applications, where characteristic dimensions below 10 nm have been reached, e.g. in the newest FET transistors, and heat dissipation control becomes essential for their optimum performance.
“The chemical nature of surfaces, therefore, emerges as a new key parameter for improving the performance of Si-based electronic and thermoelectric nanodevices”, says Dr. Davide Donadio. As a result, this work opens new possibilities for novel thermal experiments and designs directed to manipulate heat at the nanoscales.