Ion Sputtering-Induced Formation of Semiconducting and Metallic Nanostructures.

Abstract

Irradiation-induced pattern formation has the potential to become a cost-effective method for rapid fabrication of large-area nanostructures. However, on elemental semiconductor surfaces, ion-induced formation of nanostructures such as nanodots and nanorods (NRs) occurs only in the presence of impurities. Alternatively, the surface stoichiometry of III-V compounds is easily modified by ion irradiation, thereby enabling self-organization of nanostructures via a balance between preferential sputtering and ion-enhanced diffusion. In this thesis, several approaches to focused ion beam (FIB)-induced formation of nanostructures, including Ga droplets, InSb ripples, and InSb NRs, were examined, and new insights into their formation were revealed. To separately examine Ga droplet formation and coarsening, Ga droplets were fabricated by FIB irradiation of GaAs substrates with and without pre-patterned holes. We determined the droplet growth rate and size distribution as a function of FIB energy. The data suggest a droplet formation mechanism that involves Ga precipitation from a Ga-rich layer, followed by droplet coarsening via a combination of diffusion and Ostwald ripening or coalescence via droplet migration. To investigate the formation and evolution of ripples on FIB irradiated InSb surfaces, the influence of the local beam incidence angle (theta effective) by varying the distance between beam spots and/or the dwell time was examined. With increasing theta effective, the surface morphology evolves from pits to ripples to featureless surfaces. Continued irradiation of the rippled surfaces leads to island formation on the ripple crests, followed by NR growth. This ripple-nanorod transition, triggered by preferential sputtering and island-induced-self-shielding, provides a new approach for producing dense arrays of NRs. The formation and evolution of irradiation-induced NRs were examined through a comparison of FIB irradiation of InSb wafers and InSb/GaAs heterostructures. Above a critical ion dose, cone-shaped NRs capped with In islands form on both InSb surfaces. For InSb wafers, the NR base diameter increases with ion energy. In the case of InSb/GaAs heterostructures, as the milled depth approaches the InSb/GaAs interface, the cone-shaped NRs transition to capless NRs with a truncated cone shape. These results suggest a growth mechanism in which both the NR cap and body are supplied by redeposition of sputtered atoms.