Formation of Nanostructures on Semiconductor Surfaces using Focused-Ion Beams

Recently, nanostructures have shown significant promise for various applications in electronics, optoelectronics, and photonics. For example, nanometer-sized metallic droplets often form on compound semiconductor surfaces during epitaxial growth, thermal annealing, and/or ion irradiation. Such metal droplets may be used for the growth of quantum dots, via droplet epitaxy, or for nanowires, via the vapor-liquid-solid mode. For both cases, the nanostructure formation mechanisms are not well understood. Therefore, in this work, we have examined the formation of nanostructures during FIB irradiation of GaAs, GaSb, GaN, InAs, InP and InSb surfaces. On these semiconductor surfaces, randomly distributed group III rich droplets are observed to form at a critical dose. Subsequent ion beam irradiation leads to growth, motion, and coalescence of the droplets. On some surfaces, further irradiation leads to droplet motion; on other surfaces, nanowires are directly nucleated. We have examined the droplet formation mechanisms by introducing nucleation sites and effective diffusion lengths. Detailed nanostructure formation mechanisms based upon ion-enhanced surface diffusion and sputtering and/or sublimation will be discussed. In addition, strategies for fabrication of 3D nanocomposites will be presented.