| AVS 54th International Symposium | |
| Nanometer-scale Science and Technology | Wednesday Sessions |
| Session NS2+EM-WeA |
| Session: | Nanoscale Devices and Nanowires II |
| Presenter: | J.H. Wu, University of Michigan |
| Authors: | J.H. Wu, University of Michigan W. Ye, University of Michigan T. Sun, University of Michigan H.-Y. Chen, University of Michigan K. Thornton, University of Michigan R.S. Goldman, University of Michigan |
| Correspondent: | Click to Email |
Arrays of metallic nanodroplets are of interest for a broad range of applications including magnetic memory arrays, plasmonic waveguides, nanowire growth seeds, and negative index of refraction materials. Although nanometer-sized metallic droplets often form on compound semiconductor surfaces during epitaxial growth, thermal annealing, and/or ion irradiation, the mechanisms of their formation are not well understood. In this work, we are examining the formation and motion of metallic droplets during ion-irradiation of a variety of semiconductor surfaces. We use real-time imaging in a dual-beam focused-ion-beam system followed by quantitative analysis of the instantaneous positions, sizes, and velocities of the droplets in each movie frame. On GaAs and GaSb surfaces, randomly distributed nearly pure liquid-like Ga droplets are observed to form above a critical dose. Subsequent ion beam irradiation results in growth, motion, and coalescence of the droplets. Since droplets are not observed on Si surfaces prepared under similar conditions, the droplet formation is likely associated with the preferential sputtering of group V elements and liberation of Ga from the substrate as it is milled. Under ion beam irradiation, Ga droplet motion is observed, possibly due to Marangoni motion, which is usually driven by a surface tension gradient. Since the Ga droplets are essentially liquid spheres, the weak atomic bonds and droplet shape lead to enhanced sputtering in comparison with the surrounding substrate. The enhanced sputtering at liquid droplets leads to both thermal and surface tension gradients between the droplets and the substrate, thus providing the driving force for droplet motion. Interestingly, a higher droplet velocity is observed on GaSb than on GaAs surfaces, suggesting that droplet motion is dependent on the energetics of the Ga-substrate interface. On GaAs surfaces, most droplets move in a direction opposite to the ion beam scanning direction, presumably due to the FIB-induced thermal gradient on the surface. In addition, the droplet velocity is size-dependent, with higher velocities for larger droplets, suggesting the thermal/surface tension gradients increase with droplet size. The velocity is apparently correlated with the rate of droplet size change, suggesting that a mass transfer/exchange process occurs during droplet motion. This phenomenon is less significant for droplets that have merged with other droplets.