Initial growth of insulating overlayers of NaCl on Ge(100) observed by scanning tunneling microscopy with atomic resolution

The epitaxial growth start of an insulating NaCl film on a Ge(100) surface was directly imaged by scanning tunneling microscopy. Atomic resolution was achieved for islands of the first NaCl double layer with upright-standing NaCl dipoles. The tunneling current is preferentially determined by occupied Ge states extending into the NaCl layer. The results corroborate and extend the earlier proposed ''carpetlike'' growth mode of the NaCl layer over monoatomic Ge steps, even for small NaCl islands submonolayer coverage.