UVM Theses and Dissertations
Format:
Online
Author:
Zhou, Hua
Dept./Program:
Materials Science Program
Year:
2007
Degree:
PhD
Abstract:
Ion beam erosion of solid surfaces is known to produce a variety of surface morphologies, 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 potential to tailor related surface properties for optoelectronic and spintronic applications, such as modulated photoemission induced by quantum confinement of nanodots and magnetic anisotropy induced by nanoripples. On the other hand, one considerable practical importance and effect of ion beam erosion is that of surface smoothing of nanometer features, during etching or film deposition coincident with energetic species. In my dissertation, systematic investigations of ripple formation and smoothing 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 flow smoothing mechanism explains the general trends of the ripple wavelength at, low temperature and incidence angles larger than 30°.
The behavior at high temperatures suggests relaxation by surface diffusion. However, strong smoothing is inferred from the observed ripple wavelength near normal incidence, which is not consistent with either surface diffusion or viscous flow relaxation. Furthermore, a real-time xray scattering experiment is presented showing that ion smoothing of a pre-patterned surface near normal incidence is consistent with the effect of a collision-induced lateral 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 lateral current smoothing model is applicable to many surfaces that become amorphous but maintain the stoichiometry of bulk materials during ion bombardment.
The behavior at high temperatures suggests relaxation by surface diffusion. However, strong smoothing is inferred from the observed ripple wavelength near normal incidence, which is not consistent with either surface diffusion or viscous flow relaxation. Furthermore, a real-time xray scattering experiment is presented showing that ion smoothing of a pre-patterned surface near normal incidence is consistent with the effect of a collision-induced lateral 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 lateral current smoothing model is applicable to many surfaces that become amorphous but maintain the stoichiometry of bulk materials during ion bombardment.