ULTRATHIN IONIC FILMS EPITAXIALLY GROWN ON III-V SEMICONDUCTORS STUDIED WITH ATOMIC RESOLUTION

摘要

Ultrathin films (> 0.3 ML) of NaCl and KBr have been grown epitaxially on GaAs (001) and InSb (001) surfaces, respectively. Scanning tunnelling (STM) and non-contact atomic force (NC-AFM) microscopies in ultrahigh vacuum were used to study surface structures generated by growth. It was found that initially islands of monatomic thickness are formed. These islands are often cut along <110> crystallographic directions and the distribution of these islands on the substrate surface is anisotropic, which reflects the anisotropic diffusion of KBr molecules during growth. We argue that the KBr/InSb interface is stabilized by a bond between the halide ion and Am atoms arranged in chains on (4x1) InSb. At 1-1.5 ML, a wetting single-atomic KBr film is formed and material in excess of 1 ML forms rectangular islands with edges oriented along <100> and <010> crystallographic directions. For multilayer KBr coverages, the growth is basically a layer-by-layer type, but due to slow diffusion of KBr molecules down across steps, the (n + 1)th layer starts to grow before the completion of the nth one. As a result, pyramidal structures of rectangular bases are formed on the surface. These rough films can be, with thermal annealing, converted to flat films exposing large (> 0.1μm) atomically flat (001) terraces. Experiments on nanoscale modification of such terraces by electron excitation are also described.