We report conformal chemical vapor deposition (CVD) of boron carbide thin films on silicon substrates with 8:1 aspect-ratio morphologies, using triethylboron (TEB, B(C2H5)3) as single source CVD precursor. Step coverage (SC) calculated from the cross-sectional scanning electron microscopy measurements shows that films deposited at ≤450 °C were perfectly conformal (SC = 1). We attribute this to the low reaction probability at low substrate temperatures enabling more gas phase diffusion into the features. The chemical state of the material, determined by X-ray photoelectron spectroscopy, shows as carbide with B-B, B-C, C-B and C-C chemical bonds. Quantitative analysis by time-of-flight elastic recoil detection analysis reveals that films deposited at 450 °C are boron-rich with around 82.5 at.% B, 15.6 at.% C, 1.3 at.% O and 0.6 at.% H, i.e., about B5C. The film density as measured by X-ray reflectometry, varies from 1.9 to 2.28 g/cm3 depending on deposition temperature.