Phase formation during reactive molybdenum-silicide formation

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Abstract
Silicide formation due to thermal treatment of thin (5–10 nm) molybdenum films on single-crystal, polycrystalline, and hydrogenated amorphous silicon substrates in the temperature range of 100 to 1000 °C was studied, with an emphasis on the initial interactions. The molybdenum deposition, annealing, and characterization using Raman scattering and Auger electron spectroscopy was carried out in UHV in order to minimize the effects of contaminants. Raman spectroscopy is used to distinguish between tetragonal (t-MoSi2) and hexagonal MoSi2 (h-MoSi2). The Raman spectrum of bulk tetragonal MoSi2 exhibits two prominent lines which are associated with the A1g (325 cm−1) and Eg (440 cm−1) modes. The only silicide phases detected in the thin film experiments were t-MoSi2 and h-MoSi2. While hexagonal MoSi2 does not appear in the bulk phase diagram, it is the first silicide phase formed in thin film reactions at a temperature between 300 and 400 °C. The nucleation temperature of h-MoSi2 was the same for Si〈100〉, Si〈111〉, and amorphous Si. Indirect evidence for disordered intermixing of silicon and molybdenum before nucleation of h-MoSi2 is found. Annealing at approximately 800 °C causes the silicide to transform from the hexagonal phase to the tetragonal phase for all substrates. Contaminants interfere with the formation of h-MoSi2 and also retard the transformation of h-MoSi2 to t-MoSi2. For the thin films considered here, the transformation to t-MoSi2 is accompanied by islanding of the silicide film. A lower interfacial energy between the silicon and silicide for h-MoSi2 has been proposed to explain the nucleation of h-MoSi2 before t-MoSi2.