Leveraging epitaxy to control thermal transport

Tahereh Majdi (1), Souvik Pal (2), Rakesh Sahu (2), Ishwar K. Puri (2)

Gaseous and liquid precursors introduced above a crystalline substrate may nucleate and grow to form a thin film. If the thin film grows into one or more crystallographic orientations - influenced by the substrate - the thin film is called epitaxial. A film and substrate composed of the same elements, homoepitaxy, have their lattice parameters perfectly matched and so do not exhibit interfacial-bond straining. In heteroepitaxy, i.e. when the film and substrate materials differ, the lattice parameters are necessarily mismatched. The thin film lattice will either strain to accommodate its crystallographic differences with the substrate, or dislocation defects form at the interface which relaxes any strain in the film 1. Strain changes the interatomic spacing; consequently, the electronic and phonon dispersion curves change. The objective of this study is to utilize strain induced by heteroepitaxy as a passive mechanism to tune the thermal properties of a thin film. For this, we simulate the physical vapor deposition of ZnTe with a lattice parameter of a_film = 0.615 nm on a CdTe substrate with a_substrate = 0.652 nm resulting in a lattice mismatch of 100(a_substrate-a_film)/a_film. The resulting thin films are single crystalline and develop in-plane tensile strain and out-of-plane compressive strain. The out-of-plane thermal conductivity of the strained heteroepitaxial film is measured using non-equilibrium molecular dynamic simulations and is compared to an unstrained homoepitaxial ZnTe thin film. The ability to passively change thermal properties of a material, as illustrated here, is important for thermal devices such as thermoelectrics or thermal diodes.

1. Ohring, M. Materials science of thin films - Deposition & structure. (Academic Press, 2002).