Reaction pathways in the oxidation and pesting of molybdenum disilicide MoSi2 studied with scanning tunneling microscopy and spectroscopy

Abstract

The oxidation and pesting of Molybdenum Disilicide is a critical limiter in its technical use. The materials disintegration during the pesting reaction in cracks and at grain boundaries prevents direct observation of the process. A model system consisting of silicide crystallites on a Si(001) surface was developed to study the progression of the oxidation reaction in-situ with Scanning Tunneling Microscopy and Spectroscopy. Deposition and annealing of Mo thin films is used to synthesize crystallites in a size range between 10 and 100 nm, and atomic resolution is achieved on several facets. The MoSi2 is oxidized in the temperature range 400–800°C bracketing the pesting regime and beyond, combined with molecular oxygen exposure from 2 to 100 Langmuir. The oxidation process can be subdivided in three distinct regimes defined by their respective oxidation conditions: regime 1 (incubation) where only subtle changes in the electronic structure of the crystallites occurs, to regime 2 with a rapid increase in the crystallite density due to the volume gain of the pesting reaction transforming silicide to oxide, and regime 3 where a silica layer starts to form. The incubation time is attributed to oxygen dissolution and/or formation of a ternary Mo-Si-O phase, which is followed by the release of MoOx molecules. The diffusion of MoOx triggers secondary nucleation events, and contributes to the accelerated oxidation of the surface. Increasing the surface temperature > 650°C promotes desorption and sublimation over diffusion and supports silica formation. This study unraveled the intermediate reaction steps of silicide oxidation, and specifically underscores the critical role of reactant diffusion in the rapid progression of the pesting reaction.