Plastic deformation of several covalently-bound materials has been studied during ion irradiation. In all of these materials, namely crystalline and amorphous silicon, crystalline and amorphous Si0.9Ge0.1, and amorphous SiO2, the damage created by the ion beam causes density changes in the irradiated region which eventually saturate with ion dose. In the crystalline materials, the density changes were accompanied by a transformation to the amorphous phase. Superimposed on the density changes is plastic deformation which occurs during irradiation of both crystalline and amorphous materials to relieve stresses in the irradiated region. A wafer curvature measurement technique has been developed which allows the contributions from density changes and plastic deformation to be distinguished and the stress dependence of the plastic deformation to be determined.
In all of the amorphous materials, the plastic deformation is Newtonian viscous shear flow, which is characteristic of solids where deformation is governed by the diffusive motion of point defects. The radiation-enhanced shear viscosity per ion was flux-independent, revealing that flow occurs rapidly, probably within the localized damaged regions created by each ion. This viscosity does not depend strongly on the material. In fact, similar viscosities were obtained during measurements of radiation-enhanced plastic deformation of crystalline covalent samples and polycrystalline aluminum films.