Phonon polarization conversion at boundaries suppresses the thin film thermal conductivity of semiconductors with large group velocity ratio

In semiconductor thin films, phonon boundary scattering dominates the resistance to in-plane heat flow. These processes are elastic and can be specular or diffuse. The problem of whether a phonon, with a given wavelength, reflects specularly or diffusely at a boundary with a given roughness has been studied in the past [Phys. Rev. X, 8 (4), 041004, 2018]. Elastic scattering also permits conversion of phonons from one polarization to another with the same frequency (e.g., transverse to longitudinal and vice-versa), but its effect on the thin film thermal conductivity (κ) is still an open question. Here, we report a hybrid first-principles approach to solve the linearized Peierls-Boltzmann equation, directly in the thin film semiconductor geometry, using a Monte Carlo technique using first-principles phonon properties as input, to study the effect of phonon polarization conversion on the κ of semiconductor thin films. We find that polarization conversion at boundaries strongly suppresses the κ of thin films of materials with large velocity differences between the transverse (TA) and longitudinal (LA) acoustic phonons, such as in indium phosphide, driven by the preferential conversion of LA to TA phonons at the film boundaries. Our work helps identify pathways to design a tunable thermal phonon polarizer.