Helium Bubble Growth in Silicon: Ripening or Bubble Motion and Coalescence?

Voids have been created in Fz and Cz silicon by He implantation and removal of the gas by thermal annealing at temperatures above 900 K. During annealing not only is the helium released from the silicon but the mean bubble size (and thus the mean void size at higher temperatures) is increased. It is commonly believed that this coarsening occurs through the coalescence of moving bubbles and voids. We have utilized cross-sectional transmission electron microscopy with video recording to directly monitor the behaviour of individual bubbles and voids during in-situ annealing. Plan-view and cross-sectional specimens in a high temperature TEM stage have been used. At temperature as low as 773 K bubbles disappear, appear, grow and shrink. Noticeable bubble/void coarsening begins at around 923 K and continues up to the maximum temperature studied, 1073 K. Our data indicate that bubble and void movement appears to play little part in the coarsening process. Instead bubbles, and at higher temperatures voids, appear to grow by a ripening process consisting of the acquisition of vacancies and submicroscopic helium/vacancy clusters.