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Abstract
We show that holes and perpendicular mode ripples that are generated at low argon ion beam energies and incidence angles on room temperature silicon targets (C.S. Madi, et al., Phys. Rev. Lett. 101 (2008) 246102; C.S. Madi, et al., J. Phys. Condens. Matter 21 (2009) 224010) are caused by multiple scattering events from the impingement of the primary ion beam on adjacent silicon shields. We show that in a geometry that minimizes these multiple scattering events, only ultra-smooth stable silicon surfaces are for incidence angles <50° from normal. We present a revised topographical phase diagram of 250–1000eV Ar+ ion bombarded silicon surfaces in the linear regime of surface dynamics in the absence of secondary scattering effects. It is characterized only by a diverging wavelength phase transition from parallel mode ripples to a flat stable surface as the incidence angle falls below about 50° from normal incidence, and a crossover to perpendicular mode ripples as the incidence angle crosses above about 80°.
Highlights
Pattern formation resulting from uniform ion irradiation of solid surfaces in the low energy regime where the energy loss is dominated by nuclear collision cascades has been the topic of continued experimental and theoretical investigations
The theory does not reproduce the experimental observation of a constant-wavelength bifurcation from a flat surface to a rippled surface as the incidence angle is further decreased past 20° [3,6]
We show experimentally that the origin of the constant wavelength bifurcation exhibited by silicon at low ion beam incidence angle and energy is a multiple scattering artifact of either scattered argon or recoiled silicon impinging upon the target surface
Summary
Pattern formation resulting from uniform ion irradiation of solid surfaces in the low energy regime (typically 102-104 eV) where the energy loss is dominated by nuclear collision cascades has been the topic of continued experimental and theoretical investigations. Theory, whose predictions are worked out for 250 eV Ar+ impacts on amorphous silicon, reproduces the magnitude of the experimental wavelength as well as the experimental observation of a diverging-wavelength bifurcation - a phase transition from a rippled surface to a flat, stable surface - as the angle from normal incidence is reduced toward ~45° from higher angle. The theory does not reproduce the experimental observation of a constant-wavelength bifurcation from a flat surface to a rippled surface as the incidence angle is further decreased past 20° [3,6]. We show experimentally that the origin of the constant wavelength bifurcation exhibited by silicon at low ion beam incidence angle and energy is a multiple scattering artifact of either scattered argon or recoiled silicon impinging upon the target surface
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