Thin film growth of silver sulphide I: Initial stages of heteroepitaxy on NaCl

Abstract

Thin films of Ag 2S were prepared by co-evaporation from two separate sources on NaCl substrate crystals. The initial growth stages were studied by transmission electron microscopy. Two different modifications were found depending on the deposited amounts of Ag 2S but not on the substrate temperature. In the first stage the Ag 2S islands show a triclinic structure and are fourfold azimuthally oriented to the substrate where their (110) contact plane is parallel to the NaCl (001) cleavage face. With increasing film thickness the newly observed triclinic modification is transformed in the monoclinic modification (acanthite) where the Ag 2S islands change their orientation via (010) to (012). Independent of the phase transformation and the orientation change the Ag 2S islands preserve their fourfold orientation to the substrate. The epitaxial relationships for the three orientations are presented.