Linear and nonlinear optical properties of low-dimensional structures on vicinal silicon (111) surfaces

Reflectance anisotropy spectroscopy (RAS) and second harmonic generation (SHG) have been used to probe various low-dimensional (LD) structures grown on vicinal Si(111) surfaces under ultra-high vacuum (UHV) conditions. Reflectance anisotropy spectroscopy (RAS) spectra of single domain Si(111)-5×2-Au, Si(557)-5×1-Au, Si(111)-4×1-In and Si(111)-3×1-Ag structures showed a large minima in the region below 3 eV, which show a systematically higher polarizability perpendicular to the adsorbate chains, for all the structures. The detail, however, varies with the adsorbate, the reconstruction and the offcut. The RAS spectra of Si(111)-5×2-Au showed changes with offcut angle in the 1 to 3 eV region, where Au-induced structures appear. The variation was found consistent with different populations of singleand double-chain gold structures formed in the region of the steps. One-dimensional (1D) arrays of nanodots were grown on a single domain (3×1)-Ag surface by depositing 0.5 monolayer (ML) of Ag on the Si(111)-3×1-Ag surface. Reflectance anisotropy spectroscopy (RAS) was used, together with scanning tunneling spectroscopy (STS), to investigate the optical and electronic properties of the nanodots. Scanning tunneling spectroscopy (STS) of the Ag nanodots revealed a bandgap of ~0.6 eV, which is attributed to the opening of a Kubo gap due to confinement of the electron wavefunction. The nanodots showed a very small optical response in the infrared (IR) region which is consistent with their non-metallic character. Larger, anisotropic 3D Ag islands were formed by further deposition of Ag. Scanning tunneling spectroscopy (STS) showed a well-developed metallic character, in agreement with their larger size. Reflectance anisotropy spectroscopy (RAS) measurements extended into the IR region, showed a large positive anisotropy consistent with the electronic properties of the nanostructures measured by STS. The large positive IR anisotropy for the 3D Ag islands is in the region where anisotropic Drude-like intraband transitions begin to become significant, the larger polarizability being aligned along the length of the islands, where the inelastic scattering electron mean free path is expected to be longer. The anisotropy was also found to be influenced by the offcut angle of the substrate, and hence the terrace width. The agreement between the electronic properties, as measured by STS and the IR measurements indicates that infrared RAS should be a useful technique for the non-contact probing of the metallicity of aligned anisotropic nanostructures. iii The Ag nanostructures were then capped with amorphous Si to form a near-IR transparent protecting layer. The samples were stable to exposure to ambient conditions for significant periods. The RAS spectra were compared to model calculations, which support the conclusion that the metallic nanostructures survived the capping, while the (3×1)-Ag areas not covered by the nanostructures are modified by the deposition of Si. In contrast, the quasi-1D structure of Si(111)-4×1-In did not survive the capping by amorphous Si as the optical signature of the (4×1)-In structure was quenched on capping. Optical second harmonic generation (SHG) was used to study the structural symmetry of 3×1-Ag, 3 3 -Ag, Ag nanodots and Ag islands grown on vicinal Si(111). For the Si(111)1 3 -Ag structure, using s-output measurements, it was found that there are tensor components making significant contributions to the SH intensity that are identically zero in the presence of a mirror plane. This is a conclusive proof that the (3×1)-Ag structure does not possess a mirror plane in the [ 2 1 1 ] direction, in agreement with a previous SHG study of this structure grown on a singular surface. These results were contrasted with those from Si(111)3 3  -Ag, where the response is consistent with the presence of a mirror plane. Analysis of SHG studies of Ag nanodots and Ag islands revealed deviation from 3m symmetry, implying interaction of the Ag nanostructures with the underlying (3×1)-Ag chain.