Thermal phonon mechanism of amorphous AlN and thermal transport of thin amorphous layers at the interface

Abstract

AlN has received significant attention as a heat spreader for most modern electronic devices and next-generation applications owing to its excellent thermal conductivity. It is usually deposited and fabricated as composites with many substrates (e.g., Si, Si₃N₄, SiC and Al₂O₃). In particular, AlN films exhibit a high conductivity of >100 W m⁻¹ K⁻¹, even though there is an amorphous layer formation at the interface between AlN and the substrates. However, the fundamental understanding of the thermal properties of the amorphous AlN (a-AlN) layer has not yet been elucidated clearly. In this study, we examined the thermal properties of a-AlN. Thermal resistance (TR) of the a-AlN thin layer (nm) was just a few times higher than that of the crystal AlN (c-AlN), while the TR of the bulk a-AlN was 100× times higher than that of the bulk c-AlN. Phonons of a-AlN mainly consisted of diffusons that facilitated thermal transport well in the short range, and the phonon mean free path of a-AlN was in the range of 1.5–3 nm in the diffuson region. In addition, a composite system of c-AlN/a-AlN/c-AlN with an a-AlN layer of less than 3 nm thickness showed very small thermal transport reduction, which was comparable to the pure c-AlN system, while there was continuously a substantial increase in the TR of the composite system with an a-AlN layer of over 3 nm thickness. In addition, other electronic materials (e.g., Al₂O₃, GaN, SiC and Si₃N₄) with mean free paths within 3 nm can be utilized as good thermal bridges in the amorphous phase at the interface. This means that an amorphous thin layer at the interface of different materials of the hetero phases would serve as a thermal bridge.