Thermal conductivity of Bi2Se3 from bulk to thin films: theory and experiment

We calculate the lattice-driven in-plane (κ) and out-of-plane (κ ⊥) 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 with existing measurements of κ on bulk samples and with our room-temperature thermoreflectance measurements of κ ⊥ on films of thickness (L) ranging from 18 nm to 191 nm. Theoretical calculations compare favourably with the lattice component of the total thermal conductivity , both for bulk and thin films, after subtraction of the electronic component. The latter has been estimated from the Wiedemann-Franz law in n-doped samples. 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 Bi2 layer defects. The measured thermal conductivity monotonically decreases by reducing L, its value is κ ⊥ ≈ 0.39 ± 0.08 W/m·K for d = 18 nm and κ ⊥ = 0.68 ± 0.14 W/m·K for L = 191 nm. The decrease of room-temperature κ ⊥ in Bi2Se3 thin films as a function of sample thickness is due to 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