Thermal Conduction in Silicon Thin Films [Poster]

Event Date: April 7, 2015
Authors: Yuqiang Zeng and Amy Marconnet
Journal: MRS Spring Meeting
MRS Spring Meeting, San Francisco, CA, 2015.

Phonon transport in silicon is a topic of great practical and fundamental interest. Several measurement structures using silicon-on-insulator wafers have been developed to measure the thermal conductivity of silicon thin films. However, in most of these structures, amorphous oxide layers are in parallel with the silicon film and thus heat is conducted in both crystalline silicon layer and amorphous oxide layers. These oxide layers include intentionally deposited electrical insulation layers and unwanted residual buried-oxide layers. Relatively large uncertainty in quantifying the silicon thermal conductivity results due to (a) the uncertainty in thermal conductivity of these oxide layers and (b) poor understanding of interface thermal resistances. The uncertainty in thermal conductivity measurements makes characterization of the size effect in these thin silicon layers challenging.

To better understand phonon transport in pure silicon thin film, we develop a nano-fabricated suspended structure to measure the thermal conductivity. As opposed to most other techniques, our measurement device has a limited oxide layer. Specifically, oxide is deposited under the resistive thermometers, but not across the whole silicon layer. Comparing results with and without the oxide layer sheds light on the impact of silicon-silica interface. In addition, although the thermal conductivity of silicon films with thickness 10-1000 nm have been measured previously, there is a lot of variation in these data, which is partly due to the different measurement techniques and samples used by each group. A series of experimental data of silicon thin films across a range of dimensions (20-1000 nm) measured with the same fabrication and experimental technique excludes these effects.

Experimental data indicates a large reduction in the thermal conductivity of silicon films due to phonon-boundary scattering. Solutions of Boltzmann transport equation are often used to evaluate these effects in particular for thicknesses less than tens of nanometer. But this method cannot be used to investigate the impact of the amorphous oxide layer on phonon transport in the silicon layer. In this work, we apply a variance-reduced Monte Carlo (VRMC) method to study phonon transport in silicon-silica layers. The silicon-silica interface is modelled as a diffusely reflecting boundary, which is a reasonable initial approximation as the microscopic details of phonon scattering at interfaces is poorly understood.
In summary, we develop a suspended structure to measure the thermal conductivity of silicon thin films across a range of dimensions, while using a VRMC model to study how boundary scattering and the silicon-silica interface impact phonon transport. Combining the results of the VRMC model and the thermal conduction experiments allows detailed investigation of the phonon physics and thermal transport in thin silicon films.