Ion Irradiation-Induced Nanoscale Porous Structures in Semiconductors.

Energetic ion beam bombardment of semiconductors often leads to the development of complex nanostructures at or near the material’s surface that self-organize into patterns with well defined dimensions and spatial distributions. These self-organized structures are unique in that their morphologies are dependent upon fundamental properties of the irradiated material, as well as upon the irradiation conditions. Although the formation mechanics behind one- and two-dimensional ion irradiation-induced structures have been well established, the mechanisms behind formation of fully three-dimensional structures are less well understood. In this dissertation, ion irradiation-induced formation of three-dimensional nanoporous structures is studied in four common semiconductors: silicon, germanium, gallium antinomy, and indium antimony. The effects of varying ion energy, number, fluence, and flux, as well as other conditions such as temperature, are studied experimentally via ion irradiation of the materials and subsequent analysis of the microstructure, primarily through electron microscopy techniques. Irradiation conditions are shown to have a direct impact on porous network thickness, depth, and density but very little impact on porous network morphology and irradiation-induced feature size. Using the experimental data, a theoretical analysis of irradiation-induced three-dimensional nanoporous structure formation in semiconductors is presented. Amorphization is found to play a key role in the nucleation and growth of voids that form the porous structures, as materials that become amorphized via ion irradiation form extensive porous networks at much lower ion fluences and lower homologous temperatures than materials that remain crystalline during irradiation. Melting temperature, bond strength, and atomic displacement energy all show distinct correlations to porous network growth. Two case studies are also presented examining changes in material properties of the irradiation-induced nanostructures due to quantum confinement effects, specifically changes in phase stability of nanofibers and increases in photoluminescence of the nanostructured semiconductors. All nanoscale semiconductor porous networks exhibited a distinct increase in photoluminescent intensity over their unirradiated counterparts within some range of excitation wavelengths.Ph.D.Nuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/77803/1/alexpb_1.pd