The epitaxial growth characteristics of alloys composed of CaF2 and MgF2 (CaxMg1-xF2) layers on Si(111) substrates using Molecular beam epitaxy (MBE) were investigated. For MgF2, growth temperatures lower than 400 degrees 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(110)[001]//Si(111)[1 (1) over bar1]. 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(111) substrates over a wide range of composition, i.e. 0.2 <= x <= 0.9. (c) 2005 Elsevier B.V. All rights reserved.