Epitaxial Growth of Ag2S Film on Cleaved Surface of MgO(001)

Epitaxial films of monoclinic Ag2S with various thicknesses were prepared on cleaved surfaces of MgO(001) by molecular beam epitaxy. The epitaxial relations of the films to the substrates were determined by X-ray diffractometry. For thin films, there are three kinds of crystallites with (012), (-112), and (040) parallel to (001)MgO. These are all equivalent in the high-temperature cubic form of Ag2S, corresponding to {110}. With respect to epitaxy within the substrate surface for the crystallites, the diagonal direction of the body-centered pseudocubic sulfur arrays of Ag2S, [100] and [201], are parallel to [100]MgO, which is due to the coincidence of lattice dimensions of the film and substrate. For thick films, the epitaxial relations are (-112)(001) MgO and [421][010]MgO, or (012)(001)MgO and [4, -2, 1][010]MgO. The epitaxy for thick films is restricted not only by the surface periodicity of the substrate but also by the existence of steps generated during cleaving. The textures of thick films observed by polarized microscopy confirm that the epitaxial growth of the thick films is restricted by the steps.