Diamond deposition in acetylene–oxygen flames: nucleation and early growth on molybdenum substrates for different pretreatment procedures

The nucleation phase of diamond is of great importance for its epitaxial growth, and a detailed understanding of this process is therefore desired for many applications. It is known that in chemical vapour deposition (CVD) of diamond films, the pretreatment of the substrate surface may influence the initial growth period. Reasons for this observation are, however, often unclear, and several nucleation concepts have been discussed. In this study, the nucleation and early growth phase of diamond in combustion CVD was investigated for molybdenum substrates as a function of surface pretreatment. In acetylene–oxygen flames at atmospheric pressure, four different pretreatment procedures were employed including polishing with Al2O3 (no specific pretreatment) or additional polishing with diamond paste, graphite or adamantane. Diamond quality, average crystal size and mechanical stress of the films were analysed as a function of deposition time. Diamond growth was found for all these substrate surface preparations; however, qualitative differences were observed in the nucleation kinetics. Upon polishing with diamond paste, the initial nucleation phase is considerably shortened and the stress of the diamond films decreases monotonically. In the other three cases, diamond growth is observed after an induction period, while film quality and mechanical stress pass a maximum. The latter observation is thought to reflect the formation of a coherent film from isolated and unaligned crystals. The results are in accord with diamond nucleation on an intermediate molybdenum carbide layer.