Scanning tunnelling microscopy of MgO ultrathin films on Ag(001)

The morphology of ultrathin epitaxial MgO layers reactively grown on Ag~001! was investigated by usingscanning tunnelling microscopy. In the initial deposition stage Ag atoms are partially removed from thesubstrate and form extended monoatomic islands leaving vacancy islands in the substrate itself. On individualsubstrate terraces Ag is thereafter found at three atomic levels. For submonolayer deposition MgO condensatesin form of small islands of monoatomic height, located on the original substrate, on the protruding Ag islandsand on the vacancy islands as well. The largest Ag~001! fractional coverage by monoatomic MgO islands is70%. A limited amount of MgO bilayers or trilayers has also been detected ~about 1% fractional coverage!. Atthe nominal deposition of 1 ML flat, squared and connected MgO domains of about 10 nm in width form, withedges along the @110# directions. The actual substrate fractional coverage is about 85% and the occurrence ofbilayers and multilayers becomes significant ~about 30 and 5% fractional coverage, respectively!, resulting inthe formation of three-dimensional pyramidal MgO islands.