Growth characteristics of ultra-thin epitaxial CaxMg1−xF2 alloys on Si(1 1 1) substrates

The epitaxial growth characteristics of alloys composed of CaF2 and MgF2 (CaxMg1−xF2) layers on Si(1 1 1) substrates using molecular beam epitaxy (MBE) were investigated. For MgF2, growth temperatures lower than 400 °C were found to be necessary in order to suppress chemical reactions with the Si substrate. It was found that the epitaxial relationship of tetragonal rutile-type MgF2 was MgF2(1 1 0)[0 0 1]//Si(1 1 1)[11¯0]. By adding a small amount of MgF2 to the CaF2, very smooth surfaces for 1.2-nm-thick CaxMg1−xF2 layers were obtained over a wide range of compositions, i.e. 0.7⩽x⩽0.9. Pinholes, which were always generated in pure CaF2 layers, were eliminated in these alloy layers. Furthermore, CaxMg1−xF2 alloy layers were found to grow epitaxially with a cubic fluorite type of crystalline structure on Si(1 1 1) substrates over a wide range of composition, i.e. 0.2⩽x⩽0.9.