We report a phenomenon of strain-driven shape transition in the growth of nanoscale self-organized endotaxial CoSi 2 islands on Si (100) substrates. Small square shaped islands as small as 15×15 nm 2 have been observed. Islands grow in the square shape following the four fold symmetry of the Si (100) substrate, up to a critical size of 67× 67 nm 2 . A shape transition takes place at this critical size. Larger islands adopt a rectangular shape with ever increasing length and the width decreasing to an asymptotic value of ~25 nm. This produces long wires of nearly constant width.We have observed nanowire islands with aspect ratios as large as ~ 20:1. The long nanowire heterostructures grow partly above (~ 3 nm) the surface, but mostly into (~17 nm) the Si substrate. These self-organized nanostructures behave as nanoscale Schottky diodes. They may be useful in Si-nanofabrication and find potential application in constructing nano devices.

We report on the growth of a monolayer thick fractal nanostructures of Ag on flat-top Ag islands, grown on Si(111). Upon application of a voltage pulse at an edge of the flat-top Ag island from a scanning tunneling microscope tip, Ag atoms climb from the edge onto the top of the island. These atoms aggregate to form precisely one-atom thick nanostructures of fractal nature. The fractal (Hausdorff) dimension, DH = 1.75 ± 0.05, of this nanostructure has been determined by analyzing the morphology of the growing nanocluster, imaged by scanning tunneling microscopy, following the application of the voltage pulse. This value of the fractal dimension is consistent with the diffusion limited aggregation (DLA) model. We also determined two other fractal dimensions based on perimeter-radius-of-gyration (DP) and perimeter-area (D′P) relationship. Simulations of the DLA process, with varying sticking probability, lead to different cluster morphologies [P. Meakin, Phys. Rev. A 27, 1495 (1983)]; however, the value of DH is insensitive to this difference in morphology. We suggest that the morphology can be characterized by additional fractal dimension(s) DP and/or D′P, besides DH. We also show that within the DLA process DP = DH [C. Amitrano et al., Phys. Rev. A 40, 1713 (1989)] is only a special case; in general, DP and DH can be unequal. Characterization of fractal morphology is important for fractals in nanoelectronics, as fractal morphology would determine the electron transport behavior.

Iron silicide (cubic FeSi2) nanowires have been grown on Si(110) by reactive deposition epitaxy and investigated by scanning tunneling microscopy and scanning/transmission electron microscopy. On an otherwise uniform nanowire, a semi-periodic pattern along the edges of FeSi2 nanowires has been discovered. The origin of such growth patterns has been traced to initial growth of silicide nanodots with a pyramidal Si base at the chevron-like atomic arrangement of a clean reconstructed Si(110) surface. The pyramidal base evolves into a comb-like structure along the edges of the nanowires. This causes the semi-periodic structure of the iron silicide nanowires along their edges.

We report on our observation of negative differential resistance (NDR) in electron tunneling conductance in atomic-scale ultrathin Ag films on Si(111) substrates. NDR was observed by scanning tunneling spectroscopy measurements. The tunneling conductance depends on the electronic local density of states (LDOS) of the sample. We show that the sample bias voltage, at which negative differential resistance and peak negative conductance occur, depends on the film thickness. This can be understood from the variation in the LDOS of the Ag films as a function of film thickness down to the two-dimensional limit of one atomic layer. First principles density functional theory calculations have been used to explain the results.

Ag/Si(111) heterojunction having $25% lattice mismatch can be formed by placing four surface unit cells of Ag(111) on three surface unit cells of Si(111), thereby reducing the effective strain in the Ag film to $0.3% in this coincidence site epitaxial growth. We have carried out first-principles investigation of such Ag/Si(111) interface to establish the interplay between the structural and electronic properties. While the Ag overlayer affects the reconstruction of the Si(111) surface, we find that the geometrical relaxation of the Ag atoms is influenced by the subsurface Si-layer and the concomitant lattice mismatch. The electronic density of states show some oscillations near the Fermi energy, which have been compared with the scanning tunnelling spectroscopic (STS) experimental data. The signature of metal-induced gap states (MIGS) for this metalsemiconductor heterojunction has been established from the evolution of localized gap states in the layer projected density of states. Our DFT estimated work function values for Ag(100), (110) and (111) have an excellent agreement with the available experimental results. Also, the p-type Schottky barrier height (SBH) of this rectifying Ag/Si contact has been calculated from the Kohn-Sham estimates of E F À E v modified by the interface induced dipole.