[Invited] Phonon heat conduction in phononic crystal membranes
Résumé
We characterize nanometer-thin suspended silicon membranes in the pristine state or perforated in phononic crystals with periods below 100 nm, by means of scanning thermal microscopy (SThM) and Raman thermometry (RTh). Spatial resolution of SThM is shown to be similar to the one of RTh on these samples, once the air contribution is eliminated. Temperature distributions along the membranes are obtained, but require precise determination of the optical absorption for RTh as the membranes are also photonic objects. We find that the techniques are sensitive to cross-plane and in-plane heat conduction, and that the native oxide on both sides of the membranes cannot be neglected. The perforation depth of the holes is also a key quantity. For instance, the reduction of thermal conductivity in comparison to bulk silicon is 2.5 for a pristine membrane of 60 nm thickness, and limited to 6 for moderately-perforated arrays with pitch equal to thickness. The results are analyzed in light of phonon mean free path computations in complex geometries.