Growth Kinetics of Magnesium Silicide Formation Via Reaction of Mg Gas and Si Substrates

Abstract

The thermal, electrical, optical, and chemical properties of magnesium silicide (Mg2Si) have led to consideration of this semiconducting material for use in thermoelectric, optical, sensor, corrosion mitigation, and other applications. The relative non-toxicity and earth-abundant nature of the constituent elements of this silicide are also attractive characteristics. One approach for generating patterned magnesium silicide structures for use in devices is the direct conformal reaction of patterned silicon structures with magnesium gas. The kinetic mechanism controlling such conformal gas/solid reaction will be discussed in this presentation. The rate of thickening of magnesium silicide films, via the reaction of magnesium gas with planar silicon single crystal substrates, has been examined via measurements of mass change (∆m/A) and film thickness (∆X from SEM analyses of reacted cross-sections) as a function of time at 600oC. To avoid oxidation during such reaction, experiments were conducted within metal ampoules that had been carefully sealed in an argon atmosphere. Upon heating of the sealed ampoules to 600oC within an argon atmosphere tube furnace, magnesium gas (generated from solid magnesium within the ampoule) migrated to, and underwent reaction with, the planar silicon substrate. Measurements of ∆m/A vs. time and ∆X vs. time were both consistent with a parabolic rate law for the thickening of the Mg2Si layer, with the rate constants from both types of measurements in good agreement. Variations in the magnesium gas diffusion distance, and in the orientation of the starting silicon single crystal, were found to have negligible influence on the reaction kinetics. Such kinetic data were consistent with Mg2Si formation controlled by solid-state diffusion through the Mg2Si product layer. Inert marker experiments (using MgO particles) were used to determine the dominant interface at which new Mg2Si formed, and to evaluate the associated diffusing species controlling such formation. Kinetic analyses from this solid/gas reaction study will be compared to prior work on the formation of Mg2Si via solid-state reaction of magnesium with silicon. Figure 1