Crystalline structure of epitaxial CaxMg1−xF2 alloys on Si(100) and (111) substrates

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.