X-ray grazing incidence investigations of focused ion beam interactions with a Si and GaAs surfaces

We report on the study of a two-dimensional dot lattice structures which was produced on GaAs and Si (001) substrates using a Ga⁺ focused ion beam in normal incidence with a spot size of about 50 nm, an energy of 25 keV and a dose of 10¹⁴ cm^-2. The fabricated 2D-lattice structures consist of dots of almost circular shape with an area of about 2000 nm² and a period of 250x250 nm². The whole implanted area has a size of less than 0.2 mm². We have investigated the interaction of the implanted ions with the host lattice as a function of the implantation conditions using grazing incidence diffraction at the ID10 and ID1 beam lines at the ESRF. The low-dose implantation creates interstitials and vacancies below the surface generating a weak displacement field resulting in a 2D periodical strain field in case of Si substrate. For the GaAs substrate we found a much more complex scattering pattern which depends on the in-plane orientation of the 2D dot lattice with respect to the crystallographic orientation of the crystal. A much stronger scattering contrast can be found if the 2D dot lattice misaligned by 14° degree. A simulation taking the interaction between the implanted ions and the host lattice into account shows an enhanced channelling of the ions into low-index crystallographic directions. Thus the dependence of the profile of implantation damage on the crystalline structure influences the scattering patterns.