Observation of suppressed diffuson and propagon thermal conductivity of hydrogenated amorphous silicon films

Hydrogenated amorphous silicon (a-Si:H) has drawn keen interest as a thin-film semiconductor and superb passivation layer in high-efficiency silicon solar cells due to its low cost, low processing temperature, high compatibility with substrates, and scalable manufacturing. Although the impact of hydrogenation on the structural, optical, and electronic properties of a-Si:H has been extensively studied, the underlying physics of its impact on the thermal properties is still unclear. Here, we synthesize a-Si:H films with well-controlled hydrogen concentrations using plasma-enhanced chemical vapor deposition and systematically study the thermal conductivity of these a-Si:H films using time-domain thermoreflectance. We find that the reduction of thermal conductivity of a-Si:H films is attributed to the suppression of diffuson and propagon contributions as the hydrogen concentration increases. At the maximum hydrogen concentration of 25.4 atomic percentage, the contributions from diffusons and propagons to the thermal conductivity are decreased by 40% (from 1.10 to 0.67 W m^-1 K^-1) and 64% (from 0.61 to 0.22 W m^-1 K^-1), respectively. Such a significant reduction in the thermal conductivity of a-Si:H originates from the hydrogen induced material softening, the decrease in density, and phonon-defect scattering. The results of this work provide fundamental insights into the thermal transport properties of a-Si:H thin films, which is beneficial for the design and optimization of amorphous silicon-based technologies including photovoltaics, large-area electronics, and thermoelectric devices.