We have recently shown that conformal, i.e., SC = 1, amorphous boron carbide films can be deposited in 8:1 aspect ratio trenches from triethylboron, B(C2H5)3 (TEB), in a hydrogen ambient at 450 °C substrate temperature and 5 kPa total pressure. We noted 450 °C as the upper temperature limit for SC = 1, and that the SC decreased with increasing temperature. On the other hand, film density improved at temperatures above 450 °C, making higher temperature deposition desirable. The molecular mass of TEB is 98 amu, and its gas phase chemistry renders only lighter species. Thus, xenon (Xe), with an atomic mass of 131 amu, is a suitable diffusion additive to test our above hypothesis also at higher temperatures. The process pressure was chosen to be in the transitional flow regime, allowing to find a balance between effective mass transport and desired gas phase collisions between precursor molecules and the diffusion additive.