Scanning Tunneling Microscopy Investigation of Rare Earth Silicide and Alkaline Earth Fluoride Nanostructures on Silicon(001) Surfaces.

摘要

Many low dimensional structures arise from self-assembly when depositing metals on silicon surfaces, including both quantum dots and quantum wires. One class of these objects are rare earth silicide nanowires (RENW) grown on Si(001). In this dissertation, NW thermal stability, control of NW cross section, and associated surface reconstructions are studied by Scanning Tunneling Microscopy (STM). We test thulium and find for the first time that it forms NWs and these NWs are stable against prolonged annealing. We also find that the RENWs nucleate at 2x7 reconstruction domain boundaries. These results pave the way for precise control over NW size, placement, and integration with functional nanostructures and nanodevices;The common thread that links all these studies is the extent to which nanostructures can be controlled by careful growth conditions, not just by substrate temperature and the amount of material deposited, but also by timing of post-deposition annealing, etc. The grown nanostructures are metastable and result from a balance of energetic considerations and kinetics.;Another type of self-assembled NWs on Si(001) are insulating CaF 2 NWs. As an ideal model system for epitaxial growth of an insulator on a semiconductor surface, CaF2 offers unique properties such as simple structure, good lattice match to silicon and congruent evaporation. In this thesis the growth behavior of CaF2 on the Si(001) surface is investigated. At low coverages CaF2 molecules randomly locate on Si(001). Features observed at this stage are explained in terms of dissociated fragments of CaF2 terminating the dangling bonds of Si dimers. Etching is observed after surface is saturated by these features with a 2x1 periodicity. A 2xn phase, grown at 750°C, suggests the dissociation of CaF2, as proved by the simulation of LEED patterns. A c(4x4) phase is observed from 0.5ML to about 1ML with deposition temperature from 600°C to 700°C. At the highest CaF2 deposition coverages studied, a stripe phase and CaF2 NWs are observed by a combination of STM, AFM and SEM. The results provide a significant expansion in the knowledge of CaF2 on Si(001).