Tuning the properties of a complex disordered material: Full factorial investigation of PECVD-grown amorphous hydrogenated boron carbide

Abstract

A multiresponse 25 full factorial experiment is performed to investigate the effects of growth conditions (temperature, power, pressure, total flow rate, partial precursor flow rate) on the chemical, mechanical, dielectric, electronic, and charge transport properties of thin-film amorphous hydrogenated boron carbide (a-BxC:Hy) grown by plasma-enhanced chemical vapor deposition (PECVD) from ortho-carborane. The main and interaction effects are determined and discussed, and the relationships between properties are investigated via correlation analysis. The process condition with the strongest influence on growth rate is pressure, followed by partial precursor flow rate, with low pressure and high partial flow rate conditions yielding the highest growth rates. The atomic concentration of hydrogen (at.% H) and density are controlled primarily by temperature and power, with low temperature and power conditions leading to relatively soft, hydrogen-rich, low-density, porous films, and vice versa. The B/C ratio is controlled by temperature, power, pressure, and the power*pressure interaction, and is uncorrelated to hydrogen concentration. Thin-film dielectric and electronic structure properties, including high-frequency dielectric constant (ε1), low-frequency/total dielectric constant (κ), optical band gap (ETauc/E04), and Urbach energy (EU), are correlated strongly with at.% H, and weakly to moderately with B/C ratio. These properties are dominated by the influence of temperature, with a second significant influence from the power*pressure interaction. The interaction of power and pressure leads to two opposite growth regimes—high power and high pressure or low power and low pressure—that can produce a-BxC:Hy films with similar dielectric or electronic structure properties. Charge transport properties also show a correlation with at.% H and B/C, but not with the electronic structure and disorder parameters, which suggests a complicated relationship between the two. The range of properties measured highlights the potential of thin-film a-BxC:Hy for low-κ dielectric and neutron detection applications, and suggests clear pathways for future material property optimization.