Leed studies of the first stages of deposition and melting of lead on low index faces of copper

Abstract

The layers formed in the first stages of vacuum deposition of lead on (100), (111) and (110) faces of copper have been examined by LEED. Diffraction patterns characteristic of monoatomic epitaxial films have been observed. Certain of them are interpreted in terms of dense lead layers and, in particular, pseudo-(100) and pseudo-(111) planes. No evidence is found for mixed Cu-Pb layers that could be ascribed to surface alloying. A detailed study has been made of the temperature dependence of the diffracted intensities, I( T). The I( T) plots exhibit departures from the behaviour predicted by Debye-Waller theory in several distinct ways. For the Cu(100) face a comparison is made between I( T) plots for the clean surface and those obtained after deposition of lead. For all three substrate orientations evidence is found for the melting of lead overlayers at a temperature which depends on the orientation and which in one case is below the bulk melting point of lead by as much as 100°C. The diffraction spots characterizing the copper substrate are still visible in the presence of surface molten lead; this observation throws doubt on certain results purporting to show that surface melting of clean metals does not occur before bulk melting. For long evaporations bulk lead crystallites were formed; their epitaxial relations are specified.