Scanning tunnelling microscopy was used to study the initial growth stages of CaF2 on Si(001)-(2 x 1) surfaces in situ. At high coverage, the deposited CaF2 molecules nucleate into small hemispherical islands at room temperature, which were of the order of 1 to 3nm in diameter and cover the Si(001) surface homogeneously. Annealing this surface at 520C produces a drastic change in the surface morphology. Due to the increased mobility of the deposited CaF2 molecules, they rearrange into long and narrow islands typically of a few nm width and whose length was often more than four times the width. The long, narrow islands grow three-dimensionally, and their long axes were aligned along [110] or [1¯10] of the substrate Si lattice. Increasing the substrate temperature to about 610C induced a shape transition of the islands from long and narrow to two-dimensional layers. The scanning tunnelling microscopy image showed that the layers consisted of rows aligned with respect to the underlying Si(001) lattice. Considering the coverage of the deposited CaF2 molecules, the row-like layers were not reconstructed Si(001) surfaces induced by CaF2 adsorption, but were layers that were formed initially in the CaF2 hetero-epitaxial growth on a Si(001) surface. scanning tunnelling microscopy images clearly show that the terraces were completely covered with row-like layers at 670C. The formation of the row-like layers could be attributed to the chemical reaction between CaF2 and Si. The perpendicular spacing between adjacent rows varies from 0.8 to 2.4nm. Thus, it was suggested that the inhomogeneous row-like layers will prevent growth of CaF2 films with high crystalline quality on Si(001).

Surface Reconstruction in CaF2/Si(001) Investigated by Scanning Tunnelling Microscopy. T.Sumiya, T.Miura, H.Fujinuma, S.Tanaka: Surface Science, 1997, 376[1-3], 192-204