| Popis: |
It is shown that small dislocation loops of diameter 30 A or less cause field-ion image contrast while wholly inside the tip. During a field-evaporation sequence that brings the loop successively closer to the surface, characteristic changes in the contrast occur which make it possible to distinguish between vacancy-type and interstitial-type loops. For a single photograph, however, unambiguous interpretation of the image contrast observed is not possible for small loops. The contrast effects for small loops are in many cases rather slight distortions of the image rings and, for a single photograph, could occur for other reasons such as tip asymmetry or the presence of impurities. For this reason an entire fieldevaporation sequence is essential. It is pointed out that only sessile loops can be observed in the FIM, since the stresses associated with the electric field will remove glissile loops from the specimen. |
