Lateral straggling and its influence on lateral diffusion in implantation with a focused ion beam

Parallel stripes of nanostructures on an n-type Si substrate have been fabricated by implanting 30 keV Ga+ ions from a focused ion beam (FIB) source at three different fluences: 1 × 1015, 2 × 1015 and 5 × 1015 ions/cm2. Two sets of implantation were carried out. In one case, during implantation the substrate was held at room temperature and in the other case at 400 °C. Photoemission electron microscopy (PEEM) measurements were carried out on these samples. The implanted parallel stripes, each with a nominal dimension of 4000 × 100 nm2, appear as bright regions in the PEEM image. Line scans of the intensities from PEEM images were recorded along and across these stripes. Intensity profile at the edges of a line scan is broader for the implantation carried out at 400 °C compared to room temperature. From the analysis of this intensity profile lateral diffusion coefficient of Ga in silicon was estimated assuming that the PEEM intensity is proportional to Ga concentration. The diffusion coefficient at 400 °C has been estimated to be ∼10−15 m2/s. No significant dependence of diffusion coefficient on ion fluence was observed in the fluence range investigated here. Radiation enhanced diffusion has been discussed in the light of the associated defect distribution due to lateral straggling of the implanted ions.