Strain and grain size effects on thermal transport in highly-oriented nanocrystalline bismuth antimony telluride thin films

We investigated the effects of strain and grain size on the thermal transport of highly-oriented nanocrystalline bismuth antimony telluride thin films using both experimental studies and modeling. The fabricated thin films had preferred crystal orientation along the c-axis, average grain sizes of 30 < d < 100 nm, and strain of −0.8% < ε < −1.4% in the c-axis direction, whereas the strain in the a-b-axis direction was constant at 1.7%. The thermal conductivities were measured to be 0.32 < κ < 0.52 W/m/K using a 3ω method at room temperature. To gain insight into the thermal transport in the strained nanocrystalline thin films, we estimated the lattice thermal conductivities from the measured thermal conductivities. We then calculated the lattice thermal conductivity using a simplified phonon transport model that accounts for the strain effect based on the Christoffel equation and uses Lennard-Jones potentials, and the grain size effect based on the full distribution of mean free paths. The theoretical calculations were in good agreement with our experimental results, and we conclude that the decrease of the lattice thermal conductivity of nanocrystalline thin films can be mainly attributed to the nano-size effect rather than the strain effect.