EPITAXIAL-GROWTH AND THERMAL-BEHAVIOR OF PB OVERLAYERS ON CU(311) STUDIED BY HIGH-RESOLUTION LOW-ENERGY ELECTRON-DIFFRACTION

The morphology of ultrathin epitaxial lead overlayers (theta-Pb < 1.5 ML) deposited on Cu(311) is investigated by high-resolution low-energy electron diffraction (HR-LEED). The lead grows in the Stranski-Krastanov mode. Well-ordered overlayers are not formed below theta-Pb = 0.5 ML. While within the coverage range 0.5 < theta-Pb < 0.85 ML the Pb atoms are embedded in the troughs of the Cu(311) surface (row structure), the lead grows with a close-packed structure for theta-Pb > 0.85 ML. During growth and annealing the defect density of these overlayers is studied by analyzing the spot profiles of the LEED pattern (SPA-LEED). Annealing causes not only that the average domain size of the overlayers increases, but for the close-packed layer also the periodic structure changes. In both Pb overlayers, with row and close-packed structure, defects can be induced thermally. The melting temperature for this reversible order-disorder phase transition depends on the structure and the coverage of the Pb overlayer. While for the row structure it is in the range 260-310 K, it does not exceed 400 K for the close-packed overlayer, which is much lower than the melting temperature of the monolayer on Cu(111) (T greater-than-or-equal-to 600 K).