STRUCTURE DETERMINATION OF THE NISI2(111) SURFACE WITH MEDIUM-ENERGY ION BACKSCATTERING FROM INDIVIDUAL MONOLAYERS

The surface structure of the epitaxial NiSi2/Si(111) system has been d etermined applying new ion scattering methods. By detection of backsca ttered ions with ultrahigh energy resolution the signals from successi ve atomic layers are separated. Angular distributions of the yield of ions mainly backscattered from a single Ni monolayer directly provide the (sub)surface atom coordinates. In addition, analysis of the energy losses in the first atomic layer, which depend on the specific ion tr ajectories, allows an independent structure determination. Using eithe r of the two methods, the NiSi2(111) surface is found to have a bulkli ke topology, i.e. it is terminated by a Si-Ni-Si triple layer. Other s urface structure models, such as termination by a Si double layer, are ruled out. The outermost Ni-Ni and Ni-Si interlayer spacings are foun d to be contracted with respect to their bulk values in the strained s ilicide by 0.05 +/- 0.02 and 0.12 +/- 0.02 angstrom, respectively.