Date of Award

2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science

Abstract

Ion beam erosion of solid surfaces is known to produce a variety of surface morpholo- gies, such as pits, mounds or crests. Very often self-organized patterns composed of highly correlated arrays of dots or ripples at sub-micrometer and nanometer length scale could be obtained. Ion beam erosion patterning have demonstrated the poten- tial to tailor related surface properties for optoelectronic and spintronic applications, such as modulated photoemission induced by quantum con¯nement of nanodots and magnetic anisotropy induced by nanoripples. On the other hand, one considerable practical importance and e®ect of ion beam erosion is that of surface smoothing of nanometer features, during etching or ¯lm deposition coincident with energetic species. In my dissertation, systematic investigations of ripple formation and smooth- ing during low energy Ar+ ion erosion of sapphire surfaces using synchrotron grazing incidence small angle x-ray scattering and atomic force microscopy are performed. It is found in the pattern formation that the wavelength of ripples can be varied over a remarkably wide range by changing the ion incidence angle. The ion induced viscous °ow smoothing mechanism explains the general trends of the ripple wavelength at low temperature and incidence angles larger than 30±. The behavior at high temper- atures suggests relaxation by surface di®usion. However, strong smoothing is inferred from the observed ripple wavelength near normal incidence, which is not consistent with either surface di®usion or viscous °ow relaxation. Furthermore, a real-time x- ray scattering experiment is presented showing that ion smoothing of a pre-patterned surface near normal incidence is consistent with the e®ect of a collision-induced lat- eral current. Quantitative agreement is obtained using ion-collision simulations to compute the magnitude of the surface current. The results lead to predictions for the surface morphology phase diagram as a function of ion beam energy and incidence angle that substantially agree with experimental observations. The ion-induced lat- eral current smoothing model is applicable to many surfaces that become amorphous but maintain the stoichiometry of bulk materials during ion bombardment.

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