Bulk-like intrinsic phonon thermal conductivity of micrometer thick AlN films.

Aluminum nitride has garnered much attention due to the intrinsically high thermal conductivity. However, engineering thin films of AlN with these high thermal conductivities can be challenging due to vacancies and defects that can form during synthesis. Prior reports of the highest thermal conductivity for AlN films (Hexatech) is ~270 W m-1 K-1 at room temperature. In this work, we report on the cross plane thermal conductivity of ultrahigh purity single crystal AlN films with different thicknesses (~3 to 22 µm) via time-domain thermoreflectance (TDTR) and steady-state thermoreflectance (SSTR) from 80 to 500 K. At room temperature, we report a thermal conductivity of ~320 ± 42 W m-1 K-1, surpassing the values of prior measurements on AlN thin films, and one of the highest cross-plane thermal conductivities of any material for films with equivalent thicknesses, surpassed only by diamond. By conducting first-principles calculations, we show that the thermal conductivity measurements on our thin films in the 250 - 500 K temperature range agree well with the predicted values for bulk thermal conductivity of pure single crystal AlN. Thus, our results demonstrate the viability of high-quality AlN films as promising candidates for high thermal conductivity layers in high-power microelectronic devices. Our results also provide insight into the intrinsic thermal conductivity of thin films, and the nature of phonon-boundary scattering in single crystal epitaxially-grown AlN thin films. The measured thermal conductivities in high-quality AlN thin films are found to be constant, and similar to bulk AlN, regardless of heat penetration depth, film thickness or laser spot size, even when these characteristic length scales are less than the mean free paths of a considerable portion of thermal phonons. Collectively, our data suggest that the intrinsic thermal conductivity of thin films with thicknesses less than thermal phonon mean free paths is the same as bulk as so long as the thermal conductivity of the film is sampled independent of the film/substrate interface.