The surfaces of Mo bicrystals studied by low-energy ion scattering

Abstract

Molybdenum bicrystals having two crystallographic orientations at the surface were studied by low-energy ion scattering (LEIS). Since the surfaces are part of the same specimen, their history and treatments are identical and thus provide an ideal possibility to compare segregation and annealing processes and to verify the quantification of LEIS. The crystallographic orientation of the seeds, the grown bicrystals and the bicrystalline samples was checked by means of X-ray Laue patterns as well as LEED patterns. The misorientation angles were about 1–2°. Because of the high surface sensitivity of LEIS, the scattered ion signals for the Mo(1 1 0)/Mo(1 0 0) bicrystal grains should reflect the atomic densities of the outermost atomic layers of their surfaces. The atomic density of the (1 0 0) surface is found to be 76% of that of the (1 1 0) surface. The difference with the theoretical value for the outer surface (71%) is ascribed to a small contribution of the second atomic layer for the open (1 0 0) surface. For the faces of a Mo(1 1 0)/Mo(1 1 0) bicrystal identical signals are found. At higher ion doses the bombardment leads to (partial) amorphization of the surface. It is found that the annealing of this layer starts already at 700 K for the (1 1 0), but at 1300 K for the (1 0 0) surface. A strong carbon segregation is already found at 1100 K for Mo(1 0 0), while there is still no segregation for the (1 1 0) surface. The results are explained in terms of differences in surface free energies, atomic mobilities and crystal structure.