(Invited) Tuning of Heat Transport across Thin Films of Polycristalline AlN via Multiscale Structural Defects

Thermal conductivity of the elemental components of device-like structures is of fundamental importance for enhancing the thermoelectrical efficiency. However, the accurate description and control of their thermal properties remain a difficult task due to multiscale defects, and the anisotropic and heterogeneous structures usually present in thin films fabricated by vapor deposition techniques. Experimental thermal characterization, accurate theoretical description and controlled modulation of the thermal properties of these materials are therefore desirable.

To accurately asset the thermal role of structural features which are present in mono- and nanocrystalline thin films, we are studying the size-dependent reduction of their effective thermal conductivity by optical and electrothermal techniques. We propose a physical and quantitative description of the reduction of the effective thermal conductivity with film thickness based on an electron microscope study, thickness-dependent 3ω measurements and a theoretical model. The latter takes into account the distribution of the grain geometry and considers the films as a serial assembly of three layers, composed of parallelepiped grains. These structural zones are namely: the near-interface, transition, and columnar region.  We show that the resistances due to the interface with the substrate and to the near-interface and transition regions dominate the thermal conduction as the film thickness is scaled down; while for thick films, the effects of the increased mean free paths and intrinsic resistance resulting from the columnar region predominate. These transitions in the thermal transport mechanisms appear due to the relative size of each domain with respect to the total film thickness. In addition, optical techniques have been used to perform thermal measurements on epitaxially grown films. The main objective of these optical measurements is to identify differences in heat transport mechanisms between mono- and nanocrystalline systems, which are related to multiscale defects, such as dislocations and grain boundaries. Furthermore, recent studies of the thermal conductivity using Raman thermometry of III-V/Si structures will be reported, where the relative dimensions of opening in Si prior to the growth of the III-V layers has a marked impact in the value of the thermal conductivity, being larger for larger opening.