A new 30°-rotated (7×7) superstructure formed by codeposition of Cu and Si on Si(111) (7×7)

An unusual (7×7)R30° reconstruction caused by copper and silicon codeposition on Si(111) (7×7) at between 240 and 300°C was observed with high-resolution low-energy electron diffraction (HRLEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy (AES). The HRLEED measurements of the deposited film showed six widened first-order superlattice beams with a separation consistent with that of the Si (7×7) reconstruction, and clearly inconsistent with the well-documented incommensurate “5×5” structure. The (7×7) reconstruction of the initial Si(111) surface, though, was completely absent from the HRLEED pattern of the deposited surface. STM measurements of the deposited film showed a structure with about five to ten exposed layers, terraces on the order of 50 to 100 nm across, and pits at least 2 nm deep and on the order of 10 nm across. The pits likely resulted from Cu decoration of the film defects. The atomic-resolution images showed a loose hexagonal array of superstructures, with observable rotations (17°) with respect to one another and with some clustering of atoms within the superstructure. AES measurements indicated that the surface chemical composition was largely Si, with 9±4% Cu. The wide range of observed in-plane domain orientations was likely the result of thermally activated nucleation within the terraces.