R. Batabyal

Nanodot to nanowire: A strain-driven shape transition in self-organized endotaxial CoSi2 on Si(100)

We report a phenomenon of strain-driven shape transition in the growth of nanoscale self-organized endotaxial CoSi2 islands on Si(100) substrates. Nanodots of CoSi2 grow in the square shape following the four fold symmetry of the Si(100) substrate, up to a critical size of 67 × 67 nm2, where a shape transition takes place. Larger islands grow as nanowires with ever increasing length and the width decreasing to an asymptotic value of ∼25 nm. This produces long nanowires of nearly constant width. The endotaxial nanostructures grow into the Si substrate with a small extension above the surface.

Self-organized one-atom thick fractal nanoclusters via field-induced atomic transport

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 ...

Negative differential resistance in electron tunneling in ultrathin films near the two-dimensional limit

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.

Early stage fractal growth in thin films below the percolation limit

We demonstrate the fractal growth of epitaxial Ag thin films on Si(111) surfaces using scanning tunneling microscopy (STM). The initial stage growth of Ag thin films provides islands of compact shape. These compact-shaped two-dimensional (2D) islands follow the Euclidian dimension 2. As the islands grow they become fractal in nature. The fractal (Hausdorff) dimension of the islands depends on the coverage of the Ag thin films. The mechanism responsible for this fractal nature of the Ag nanostructures varies from diffusion limited aggregation (DLA) to diffusion limited cluster aggregation (DLCA).

Simultaneous growth of sub-nanometer deep vacancy island and epitaxial silicide islands on Si (111)

Novel surface structures in the growth of self-organized CoSi2 epitaxial nanostructures by Co deposition on heated Si (111) surfaces have been investigated with scanning tunneling microscopy at room temperature. At sub-monolayer Co deposition on clean Si (111), Si vacancy islands of one bilayer depth are observed. CoSi2 islands grow within the Si vacancy island as well as on the surface. The number of Si atoms contained in the CoSi2 island grown within a vacancy is ∼ 1.5 times the number of Si atoms depleted due to the vacancy creation. This excess of Si atoms in the island arguably diffuse from beneath the island.

Manifestation of surface and interface properties of Ag overlayer on Si (111)

Ag/Si (111) interface has been investigated using first principles DFT approach, to establish the unique interplay between the structural and electronic properties. The interface induced properties, such as evolution of metal-induced gap states (MIGS), formation of Schottky barrier height (SBH), for this rectifying Ag/Si contact have been reported.

Growth of a-few-atom wide nanowires with different surface reconstructions via desorption of Au on vicinal Si (111) surfaces

We present scanning tunneling microscopy studies of adsorption and desorption of Au on vicinal Si (111) - 7×7 (4° miscut) surfaces. The Au film transforms into percolated structures due to annealing at 400 °C. On annealing at 800 °C, the Au film is found to evaporate completely from the terrace whereas the step edges retain the signature of Au in the form of droplets. This also modifies the atomic structure of the step-bunched facets which show nanowire-like structures with various surface reconstructions like 7×5, 5×3√2 and 5×2 which are Au-induced reconstructions.

Desorption of Ag from Grain Boundaries in Ag Film on Br and H‐Passivated Si(111) Surfaces

Growth of Ag film on Br‐ and H‐passivated Si(111) surfaces was examined by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and photoemission electron microscopy (PEEM) techniques. The phenomenon of thermal grooving was observed after annealing at higher temperatures. Hierarchical desorption of Ag from the grain boundaries produce a fractal structure of Ag‐depleted regions. Hierarchical desorption may be used for nanopatterning of the layer.

Self‐organized Growth of Cobalt Nanostructures on Ag/Si (111)‐7×7 Surfaces

Epitaxial growth of cobalt islands on nonreactive metal/semiconductor surface has been investigated in‐situ by scanning tunneling microscopy (STM). When 1.6×1013 Co atoms/cm2 are deposited on a pre‐deposited Ag (7.5×1014 atoms/cm2) on Si (111)‐(7×7) surface at room temperature, predominantly Co islands of monatomic or biatomic height grow. When this sample is annealed at 500 °C, faceted Co islands of multiatomic height appear, while Ag does not show any particular structure. Without Co deposition the Ag layer undergoes a (3×3) R30° reconstruction upon annealing. Apparently interaction between Co and Ag prevents the surface reconstruction of the Ag layer.