Diamond deposition in acetylene-oxygen flames: nucleation and early growthon molybdenum substrates for different pretreatment procedures

The nucleation phase of diamond is of great importance for its epitaxial gr owth, 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 un clear, and several nucleation concepts have been discussed. In this study, the nucleation and early growth phase of diamond in combustion CVD was inve stigated for molybdenum substrates as a function of surface pretreatment. I n acetylene-oxygen flames at atmospheric pressure, four different pretreatm ent procedures were employed including polishing with Al2O3 (no specific pr etreatment) or additional polishing with diamond paste, graphite or adamant ane. Diamond quality, average crystal size and mechanical stress of the fil ms were analysed as a function of deposition time. Diamond growth was found for all these substrate surface preparations; however, qualitative differe nces were observed in the nucleation kinetics. Upon polishing with diamond paste, the initial nucleation phase is considerably shortened and the stres s of the diamond films decreases monotonically. In the other three cases, d iamond growth is observed after an induction period, while film quality and mechanical stress pass a maximum. The latter observation is thought to ref lect the formation of a coherent film from isolated and unaligned crystals. The results are in accord with diamond nucleation on an intermediate molyb denum carbide layer.