Advances in integrated circuits have proved critical to the continued miniaturisation of electronic devices. This has had an impact on several application fields where nano-sized and high-power-density electronic devices play an important role such as thermoelectrics, nano- and optoelectronics, fuel cells and solar cells. Increased demand on compact and multifunctional electronic devices has been accompanied by an increase in heat generated by these devices. Within the EU-funded project HEATPRONANO (Heat propagation and thermal conductivity in nanomaterials for nanoscale energy management), scientists investigated how the surface structure and the properties of phonons – which are the main heat carriers – control the thermal conductivity of ultra-thin membranes. The focus was on membranes of silicon, germanium and metal oxides with varying thicknesses ranging from a few nanometres to hundreds of nanometres. Scientists demonstrated that the thermal conductivity of silicon membranes as thin as 4 nm can be 40 times lower than that of bulk crystalline and is largely controlled by the chemical composition and structure of the surface. Combining state-of-the-art atomistic modelling, new fabrication techniques and advanced measurement approaches, scientists unravelled the role of surface oxidation in determining the scattering of phonons. They found that rough layers of the native oxide limit the mean free path of thermal phonons below 100 nm thickness. Experiments also showed that by removing the native oxide, the thermal conductivity of silicon nanostructures is improved by almost one order of magnitude. These results have important implications for the design of future phononic applications since they define at which scale surface nano-structuring affects thermal phonons most effectively. Based on the experimental and theoretical findings, the team further explored phonon coherence as a function of surface roughness disorder and then explained why thermal conductivity in patterned structures is not affected by phonon coherence in room temperature. Results are important for applications of phononic crystal in radio frequency communications and optomechanics, which both depend on the ability to modify the phonon dispersion relation. Controlled levels of disorder could lead to a new class of disordered phononics in analogy to the already active field of disordered photonics. As electronic devices get smaller, design engineers face new challenges in meeting performance, size, weight and operating temperature requirements. Accordingly, improved thermal management plays an important role in the design of such devices so as to be reliable.
Thermal management, miniaturisation, thermal conductivity, electronic devices, HEATPRONANO