Thermal Conductivity Of Bi2se3 From Bulk To Thin Films: Theory And Experiment

We calculate the lattice-driven in-plane (kappa(parallel to)) and out-of-plane (kappa(perpendicular to)) thermal conductivities of Bi2Se3 bulk, and of films of different thicknesses, using the Boltzmann equation with phonon scattering times obtained from anharmonic third order density functional perturbation theory. We compare our results for the lattice component of the thermal conductivity with published data for kappa(parallel to) on bulk samples and with our room-temperature thermoreflectance measurements of kappa(perpendicular to) on films of thickness (L) ranging from 18 nm to 191 nm, where the lattice component has been extracted via the Wiedemann-Franz law. Ab initio theoretical calculations on bulk samples, including an effective model to account for finite sample thickness and defect scattering, compare favorably both for the bulk case (from literature) and thin films (new measurements). In the low-T limit the theoretical in-plane lattice thermal conductivity of bulk Bi2Se3 agrees with previous measurements by assuming the occurrence of intercalated Bi-2 layer defects. The measured thermal conductivity monotonically decreases by reducing L; its value is kappa(perpendicular to) approximate to 0.39 +/- 0.08 W/m K for L = 18 nm and kappa(perpendicular to) = 0.68 +/- 0.14 W/m K for L = 191 nm. We show that the decrease of room-temperature kappa(perpendicular to) in Bi2Se3 thin films as a function of sample thickness can be explained by the incoherent scattering of out-of-plane momentum phonons with the film surface. Our work outlines the crucial role of sample thinning in reducing the out-of-plane thermal conductivity.