Single atom sputtering events: direct observation of near-surface depleted zones in ion-irradiated tungsten

Abstract

The three-dimensional spatial arrangement of vacancies contained in depleted zones (DZs), of ion-irradiated tungsten specimens, was determined with atomic resolution by the field-ion microscope (FIM) technique. These DZs were detected in the near-surface region of specimens which had been irradiated in situ at less than or equal to15 K with 20 keV W/sup +/, 30 keV W/sup +/, Kr/sup +/, Cu/sup +/, or Ar/sup +/ ions. The values of the ion dose employed were small (less than or equal to 10/sup 13/ ions cm/sup -2/); therefore, each DZ analyzed was the result of the impact of a single projectile ion. At the irradiation temperature (less than or equal to 15/sup 0/K) both the self-intersitital atoms and vacancies were immobile, so that the primary state of radiation damage was preserved. The following properties of each DZ were determined: the total number of vacancies; the number of vacancies in the near-surface region; the spatial extent - that is, the dimensions required to determine a volume; the average vacancy concentration; the average vacancy concentration associated with the non-surface region; the first-nearest neighbor cluster distribution for the vacancies in the near-surface region; the radial distribution function of all the vacancies; the distribution of vacancies as a function of depth normal to the irradiated surface; and the sputtering yield. Most of the above properties of the near-surface DZs had similar values to those of the DZs detected in the bulk of the FIM specimens. The total number of vacancies detected in the near-surface region wasapproximately consistent with theoretical estimates of the average sputtering yield. The sputtering yield of individual DZs exhibited significant fluctuations from the measured average sputtering yield.