Ion Beam Irradiation Preparation And Optical Properties Of Self-Organized Nanostructures On Silicon Surfaces

Ion beam irradiation can produce self-organized ordered nanostructures on a variety of semiconductor surfaces. This method attracts a lot of attentions and is regarded to have a promising application in the field of nanofabrication due to its simplicity in instruments and technological operation and good controllability in nanostructure morphology.In this thesis, the effect of metal impurity on ion-induced nanostructuring is studied. The morphology evolution of Si (100) surface under 1200 eV Ar+ ion bombardment with an ion flux of 200?A cm-2 in the presence or absence of Fe impurity incorporation is presented. The formation of ordered nanodot patterns is observed only when the stationary Fe areal density in the surface is above the threshold value of 8×1014 cm-2. At Fe concentrations below this threshold smoothing dominates and pattern formation is inhibited.On the basis of this experimental work, a continuum theoretical model including the metal impurity effect is developed to describe the evolution of surface morphology and composition during metal incorporated ion bombardment of Si. The model integrates curvature-dependent erosion, ion-enhanced viscous flow, ballistic smoothing, preferential sputtering and surface stress induced mass current as the relevant surface processes, in which preferential sputtering and surface stress induced mass current are caused by impurity incorporation. Surface stress induced mass current together with curvature-dependent erosion overcomes ballistic smoothing, leading to nanodot arrays formation. Preferential sputtering, cooperating with the morphological instability, accomplishes the in-phase modulation in composition. The simulation based on this model qualitatively reproduces Fe motivated ion patterning and the surface evolution of prepatterned Si (100) during postsputtering.We also studied the formation of cone arrays on Si surfaces under high flux ion bombardment with metal impurity incorporation and the optical properties of the cone arrays. Self-organized cone arrays with an average height of?350 nm and an average inter-distance of?250 nm are fabricated on Si (100) by means of Ar+ ion bombardment at normal incidence with ion energy of 1.5 keV and current density of 1000?A cm-2. For each cone, the apex bears metal-enriched and polycrystalline. The rest is nearly free of metal atoms and shows good crystallinity with the same crystallographic orientation as the substrate. The nano-structured Si surface appears black as seen by the naked eye. The measured reflectance of the surface is less than 11%over the wavelength range from 350 to 2000 nm as compared to that of>30% for the polished Si. An enhancement of more than 25% in absorption is observed in this region. These special optical properties find these ion-induced cone arrays a black Si application. Incorporation of metal atoms such as Fe is found to be mandatory for the formation of the cone arrays during ion bombardment.The morphology evolution of a dot-patterned Si (100) surface under small flux (15?A cm-2) ion bombardment is studied. It was found that both the average dot size and the surface roughness decline steadily against sputtering time, which cannot be explained by the usually adopted Bradley-Harper model. Since the Si surface remains partly crystalline during small flux ion bombardment instead of being completely amorphous for the high flux case, we refined the Bradley-Harper model by including the Ehrlich-Schwoebel effect. The numerical simulation based on this new model is qualitatively in agreement with the experiment.