Thermal Transport and Heat Capacity of Suspended Silicon-Nitride Structures : Two Regimes, Two Surprises

Orateur : Barry ZINK (Associate Professor Department of Physics and Astronomy University of Denver)

The study of thermal properties of glasses and other disordered or amorphous solids, despite a long and interest ing history, continues to yield surprises. This talk presents new data in two very different regimes that challenge our current understanding and raise new questions. Our current work at DU focuses on measurements of thermal transport and heat capacity in 500 nm thick micromachined suspended silicon nitride (Si-N) bridges at temperatures from $50$ mK to above room temperature. Near room temperature where the phonon mean free paths are expected to be on the order of nanometers, the measured thermal conductivity of Si-N (for material grown by LPCVD in two different furnaces) deviates somewhat from previously reported measurements. It also shows surprising dependence on surface variation even at high temperatures. Surface scattering is expected to affect thermal transport at low temperature but results presented here provide evidence that it also plays a role from 77-325 K. We explore this physics via measurements of thermal transport in Si-N in cases of both intentional an d unintentional surface variation. We find in all cases that modification of the surface lowers the measured thermal conductance of the Si-N bridge. This is strong evidence that vibrational excitations with long mean free paths carry significant heat even at these high temperatures. Measurements of thermal transport in the very different regime well below 1 K are in some ways more in line with expectations, indicating long mean free paths and dominance of surface scattering. However in this regime our heat capacity measurements reveal yet another surprise.

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