Density and atomic coordination dictate vibrational characteristics and thermal conductivity of amorphous silicon carbide

Silicon carbide coatings and thin films are used for a wide array of applications ranging from thermal barrier coatings to microelectronics. In this paper, we report on the role of mass density and atomic coordination on the fundamental vibrational characteristics and thermal conductivity of amorphous silicon carbide systems through a combination of experiments and systematic atomistic simulations. We use time domain thermoreflectance to show that the thermal conductivity of hydrogenated amorphous silicon carbide can be increased twofold with $\ensuremath{\sim}40%$ increase in the mass density. A simple description of thermal transport applicable to a range of amorphous solids where diffusion of thermal energy is predominantly driven by nonpropagating modes cannot fully describe our experimental measurements. Our molecular dynamics simulations in conjunction with our lattice dynamics calculations shed light on the intrinsic role of atomic coordination in dictating the contributions from both propagating and nonpropagating modes in amorphous silicon carbide structures. More specifically, we find that as the concentration of $s{p}^{3}$ hybridized carbon atoms is increased by up to $10%$ with increasing mass densities, the contribution from propagons can be increased from $\ensuremath{\sim}25%$ to $\ensuremath{\sim}40%$, after which further increments in the mass density and the $s{p}^{3}$ fraction does not lead to higher contributions from propagons. In contrast, contributions from the nonpropagating modes increases monotonically with increasing mass density and $s{p}^{3}$ hybridization. Our results pave a path forward to manipulate the thermal conductivity of amoprhous silicon carbide systems based on varying the atomic coordination.