Electron microscopy studies on interfacial solid-state reactions induced by electronic excitation.

We have studied the effects of electron irradiation on Pt/a-SiOx thin films by transmission electron microscopy and electron diffraction. Pt2Si was formed by 75 keV electron irradiation at 298 K and 90 K. Such a low-temperature synthesis of Pt2Si can be attributed to the dissociation of a-SiOx induced by electronic excitation; Si-O bonds dissociate through Auger decay of core-holes generated by electronic excitation, and then, dissociated Si atoms form Pt-Si bonds. The morphology of Pt islands extensively changed during Pt2Si formation, even at 90 K. Coalescence and growth of metallic particles are not due to thermal effects during electron irradiation but to athermal processes accompanied by silicide formation. To maintain the reaction interface between metallic particles and the dissociated Si atoms by electronic excitation, a considerable concomitant morphology change occurs. Similarly, Fe2Si was synthesized by using the same technique. In this way, we have demonstrated a versatile method for selectively forming nanoscale metal silicides in electron-irradiated areas at room temperature. We also propose a new mechanism for the crystallization of amorphous alloys, which is mediated by additional solute atoms produced by electronic excitation. Crystallization of amorphous Pd-Si alloy thin films can be realized by 75 keV electron irradiation at 90 K via the electronic excitation, where both knock-on damage and possible thermal crystallization can be excluded. Supply of dissociated Si to the Pd-Si layer may cause instability of the amorphous phase, which serves as the trigger for the remarkable structural change, i.e. additional solute atom-mediated crystallization.