Trimethylboron as Single-Source Precursor for Boron−Carbon Thin Film Synthesis by Plasma Chemical Vapor Deposition

Boron−carbon (BxC) thin films are potential neutron converting layers for B-based neutron detectors. However, as common material choices for such detectors do not tolerate temperatures above 500 °C, a low temperature deposition route is required. Here, we study trimethylboron B(CH3)3 (TMB) as a single-source precursor for the deposition of BxC thin films by plasma CVD using Ar plasma. The effect of plasma power, TMB/Ar flow ratio and total pressure, on the film composition, morphology, chemical bonding, and microstructures are investigated. Dense and boron-rich films (B/C = 1.9) are achieved at high TMB flow under a low total pressure and high plasma power, which rendered an approximate substrate temperature of ∼300 °C. Films mainly contain B−C bonds with the presence of B−O and C−C, which is attributed to be the origin of formed amorphous carbon in the films. The high H content (15 ± 5 at. %) is almost independent of deposition parameters and contributed to lower the film density (2.16 g/cm). The plasma compositional analysis shows that the TMB molecule decomposes to mainly atomic H, C2, BH, and CH. A plasma chemical model for the decomposition of TMB with BH and CH as the plausible film depositing species in the plasma is proposed. ■ INTRODUCTION Boron carbide is an interesting material due to its high hardness, good wear resistance and low weight, as well as their high thermal and chemical stability. Boron carbide is also a potentially interesting semiconductor material where the band gap of boron rich boron carbides, with a B/C ratio of 4−50, has been found to increase with the B/C ratio. Furthermore, the isotope B is intended to be applied as basis for solid-state neutron converting material for neutron detectors, given the high neutron absorption cross-section of B for thermal neutrons via either reaction 1 or 2 when a neutron is captured by a B atom: