Chemical vapour deposition of molybdenum carbides: aspects of phase stability

Thin films of different molybdenum carbides (δ-MoC_1−x, γ′-MoC_1−x and Mo₂C) have been deposited from a gas mixture of MoCl₅/H₂/C₂H₄ at 800°C by CVD. The H₂ content in the vapour has a strong influence on the phase composition and microstructure. Typically, high H₂ contents lead to the formation of nanocrystalline δ-MoC_1−x films while coarse-grained γ′-MoC_1−x is formed with an H₂-free gas mixture. This phase has previously only been synthesized by carburization of Mo in a CO atmosphere and it has therefore been considered as an oxycarbide phase stabilized by the presence of oxygen in the lattice. Our results, however, show that γ′-MoC_1−x films containing only trace amounts of oxygen can be deposited by CVD. Stability calculations using a FP-LMTO method confirmed that the γ′-MoC_1−x phase is stabilized by oxygen but that the difference in energy between e.g. δ-MoC_0.75 and oxygen-free γ′-MoC_0.75 is small enough to allow the synthesis of the latter phase in the absence of kinetic constraints. Annealing experiments of metastable δ-MoC_1−x and γ′-MoC_1−x films showed two different reaction products suggesting that kinetic effects play an important role in the decomposition of these phases.