Epitaxial growth of CaxMg1-xF2 alloy composed of cubic fluorite-type CaF2 and tetragonal rutile-type MgF2, on Si substrates was investigated as a possible method for fabricating lattice-matched ultra-thin dielectric layers. The crystalline structure of 1.2-nm-thick CaxMg1-xF2 alloy layers grown on Si(100) and Si(111) substrates by molecular beam epitaxy was characterized. On both Si(100) and Si(111), the grown CaxMg1-xF2 layers exhibited a cubic structure over a wide range of alloy composition (0.2 <= x <= 0.9). The surface was particularly smooth in the case of x=0.9 on Si(111). It was found that, for both Si(100) and Si(111), the first two monolayers grown were composed of pure MgF2 with a cubic structure. This is partly why the cubic CaxMg1-xF2, alloy was stable on the Si substrates. (c) 2005 Elsevier B.V. All rights reserved.