Nanoobjects microscopy

Helium complexes behaviour near the free surface in tungsten
Starchenko I.V., Sadanov E.V.
National Science Center “Kharkiv Institute of Physics and Technology”, Akademichna, 1, Kharkiv-61108, Ukraine.
E-mail: igor.starchenko@gmail.com

It is known that implanted helium can form mobile clusters from 6-7 atoms and is able to displace lattice atoms into interstitial positions by creating helium-vacancies complexes. In this work, the study on the free surface influence on this process was performed using the field ion microscopy technique. Irradiation with the energy of 5 keV was performed in the microscope chamber. Later the specimens were subjected to field evaporation. The self-interstitial atom emergence from the volume was registered every new evaporation act after the irradiation. There was a time delay (1 – 20 s) between the evaporation of the material layer and the SIA emergence to the surface. The dependence of the emerging interstitial atom number as a function of the removed layer thickness was found (Fig. 1). The analysis showed that the discovered effects are connected with the influence of the free surface on the process of helium-induced lattice atom displacement. The main stimulating factor for the displacement process is the action of image forces, which decreases as the distance to the material surface increases. Considering that the maximum value of the image forces is directly near the surface, one can expect that the number of helium atoms needed to lattice atom displacement is the lower the closes the surface is. Taking into account that both curves on Fig.1 tend to saturate, it was possible to calculate the value of the image forces action depth – 25 Å [1].

1.	Starchenko I.V., Sadanov E.V. Features of helium complexes behaviour near the free surface in tungsten // PAST.-2019.-2.-P. 3-6.