Probing thermal phonon mean free path using phononic crystal nanostructures

We demonstrate an experimental method to obtain information of thermal phonon mean free path (MFP) spectra in thin films using hole based two dimensional phononic crystal nanostructures. The characteristic length of the system can be swept by changing the radius of circular holes of the phononic crystals. Then, thermal conductivities of single-crystalline Si, amorphous SiGe, and poly-SiGe thin films (150 nm thick) were measured for phononic crystals with different characteristic length between 20 nm and 200 nm. We observed different characteristic length dependence of the thermal conductivity among these three systems and the trend can be explained by the thermal phonon MFP spectra of the materials.