Interface-independent sound speed and thermal conductivity of atomic-layer-deposition-grown amorphous $\mathrm{AlN}\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ multilayers with varying oxygen composition

Dielectric amorphous multilayers (AMLs) play a critical role in a wide array of technologies such as optical coatings, nanoelectronics, energy harvesting, and recovery devices. However, despite their wide applications, a robust understanding of the effect of the interplay between chemical and structural disorder on the thermal properties of AMLs is still lacking. Therefore, in this paper, we experimentally and numerically investigate the effects of composition and interface density on the sound speed and thermal conductivity of a series of amorphous aluminum nitride and aluminum oxide multilayers grown via plasma-enhanced atomic layer deposition. To systematically change the composition, the oxygen content of the AMLs is proportionally varied with interface density during growth. We find that the longitudinal sound speed of these AMLs is dictated by the oxygen content instead of the number of interfaces. The thermal conductivity, in contrast, is dictated by both interface density and oxygen content. The interfaces act to decrease the thermal conductivity, whereas the oxygen content increases the thermal conductivity. Due to the competing influence of the interfaces and oxygen content, the thermal conductivity of the AMLs remains nearly constant as a function of interface density. Our study provides crucial insights into the effect of the interplay of composition and interfaces on the sound speed and thermal conductivity of AMLs.

Figure 1

TEM images of the (a) 22- and (b) 2.4-nm-period AMLs. Relative at. $\%$ of oxygen (O) and nitrogen (N) in the (c) 22-, (d) 5-, and (e) 2.4-nm-period multilayers quantified from STEM-EELS line scans. (f) Average film composition as a function of interface density of the AMLs.

Figure 2

(a) Picosecond acoustic response of TDTR measurements for a-AlN, a-${\mathrm{Al}}_2{\mathrm{O}}_3$, and $\mathrm{AlN}\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ multilayer films. (b) and (c) represent longitudinal sound speed of the amorphous films as a function of interface density and oxygen content, respectively. The dashed line in (c) represents the longitudinal sound speed predicted via the rule of mixtures based on the composition of the AMLs.

Figure 3

TDTR-measured cross-plane thermal conductivity of a-AlN, a-${\mathrm{Al}}_2{\mathrm{O}}_3$, and $\mathrm{AlN}\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ multilayer films as a function of (a) interface density and (b) oxygen content. The dashed line in (b) represents the average thermal conductivity of a-AlN and a-${\mathrm{Al}}_2{\mathrm{O}}_3$ control films.

Figure 4

Schematics of the computational domains for an amorphous Si/Ge multilayer corresponding to three cases: (a) sharp interface, (b) intermixed interface, and (c) higher Si content. (d) Thermal conductivity as a function of interface density for the domains presented in (a)–(c). The error bars associated with these thermal conductivity values are $\sim 10\%$.