  To study the effect of stress within the thin amorphous film generated atop
Si irradiated by Ar+, we model the film as a viscoelastic medium into which the
ion beam continually injects biaxial compressive stress. We find that at normal
incidence, the model predicts a steady compressive stress of a magnitude
comparable to experiment. However, linear stability analysis at normal
incidence reveals that this mechanism of stress generation is unconditionally
stabilizing due to a purely kinematic material flow, depending on none of the
material parameters. Thus, despite plausible conjectures in the literature as
to its potential role in pattern formation, we conclude that beam stress at
normal incidence is unlikely to be a source of instability at any energy,
supporting recent theories attributing hexagonal ordered dots to the effects of
composition. In addition, we find that the elastic moduli appear in neither the
steady film stress nor the leading order smoothening, suggesting that the
primary effects of stress can be captured even if elasticity is neglected. This
should greatly simplify future analytical studies of highly nonplanar surface
evolution, in which the beam-injected stress is considered to be an important
effect.
