Specific heat and thermal conductivity of low-stress amorphous Si-N membranes

We present values of the specific heat and thermal conductivity from 3-300 K of low-stress amorphous silicon-nitride thin-films determined from measurements using a membrane-based microcalorimeter. The thermal conductivity has a temperature dependence often seen in amorphous solids, but the magnitude is large, with the expected plateau occurring at significantly higher temperatures than seen in other amorphous systems. Specific heat measurements show that the expected ‘peak’ in the vibrational spectrum also occurs at relatively high temperatures. The estimated phonon mean-free-path at 300 K is ≈5 Å, comparable to the inter-atomic spacing, as seen in other amorphous solids. Below ≈ 20 K the mean free path is comparable to or exceeds the thickness of the membrane, indicating that surface scattering dominates the thermal transport. This surface scattering is found to be either specular or diffuse, depending on details of the membrane processing, which affects both the thermal conductivity and specific heat below 10 K.