Jagadish Chandra Mahato | Indian Association for the Cultivation of Science (original) (raw)
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Papers by Jagadish Chandra Mahato
Journal of Applied Physics, 2013
We report on the growth of a monolayer thick fractal nanostructures of Ag on flat-top Ag islands,... more 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, D H ¼ 1.75 6 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 (D P ) and perimeter-area (D 0 P ) relationship. Simulations of the DLA process, with varying sticking probability, lead to different cluster morphologies [P. Meakin, Phys. Rev. A 27, 1495]; however, the value of D H is insensitive to this difference in morphology. We suggest that the morphology can be characterized by additional fractal dimension(s) D P and/or D 0 P , besides D H . We also show that within the DLA process D P ¼ D H [C. Amitrano et al., Phys. Rev. A 40, 1713] is only a special case; in general, D P and D H can be unequal. Characterization of fractal morphology is important for fractals in nanoelectronics, as fractal morphology would determine the electron transport behavior. V C 2013 AIP Publishing LLC.
Journal of Applied Physics, 2013
ABSTRACT We report on our observation of negative differential resistance (NDR) in electron tunne... more ABSTRACT 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.
Growth of Ag film on surfaces was examined by Rutherford backscattering spectrometry (RBS), scann... more Growth of Ag film on 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. Physics 978-0-7354-0905-7/$30.00 FIGURE 3. PEEM images: (a) surface decorated with Ag-depleted grain boundaries (FOV 100 μm) (b) a magnified part (FOV 30 μm) of the image in (a).
Epitaxial growth of cobalt islands on nonreactive metal/semiconductor surface has been investigat... more Epitaxial growth of cobalt islands on nonreactive metal/semiconductor surface has been investigated in-situ by scanning tunneling microscopy (STM). When 1.6×10 13 Co atoms/cm 2 are deposited on a pre-deposited Ag (7.5×10 14 atoms/cm 2 ) on Si (111)-(77) surface at room temperature, predominantly Co islands of monatomic or biatomic height grow. When this sample is annealed at 500 O 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 O reconstruction upon annealing. Apparently interaction between Co and Ag prevents the surface reconstruction of the Ag layer.
Surface Science, 2014
Sub-monolayer Co deposition on clean Si(111)-(7 × 7) surfaces has been found to form nanoscale Co... more Sub-monolayer Co deposition on clean Si(111)-(7 × 7) surfaces has been found to form nanoscale CoSi 2 islands with a surrounding trench of one Si bilayer depth and mainly hexagonal shape. The trench surface structure is largely like that of the disordered '1×1' phase of the Si(111)-7×7↔'1×1' phase transition and comprises mostly disordered Si adatoms with small ordered patches of (11 × 11), (9 × 9), c(5 × √5), c(4 × 4) and (2 × 2) structures along with some Co-ring clusters. This disordered '1 × 1' structure within the trench has formed at 600°C, the growth temperature of CoSi 2 in reactive deposition epitaxy, much below the order-disorder phase transition temperature on Si(111)-(7 × 7). The structure around the trench remains (7 × 7). Electronically the trench is semiconducting. The surrounding 7 × 7 structure being metallic, the island-trench structure forms a lateral metal-semiconductor-metal structure.
physica status solidi (b), 2013
ABSTRACT Ag/Si(111) heterojunction having ∼25% lattice mismatch can be formed by placing four sur... more ABSTRACT 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 sub-surface 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 metal–semiconductor 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 EF − Ev modified by the interface induced dipole.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2011
Parallel stripes of nanostructures on an n-type Si substrate have been fabricated by implanting 3... more Parallel stripes of nanostructures on an n-type Si substrate have been fabricated by implanting 30 keV Ga + ions from a focused ion beam (FIB) source at three different fluences: 1 Â 10 15 , 2 Â 10 15 and 5 Â 10 15 ions/cm 2 . Two sets of implantation were carried out. In one case, during implantation the substrate was held at room temperature and in the other case at 400°C. Photoemission electron microscopy (PEEM) measurements were carried out on these samples. The implanted parallel stripes, each with a nominal dimension of 4000 Â 100 nm 2 , appear as bright regions in the PEEM image. Line scans of the intensities from PEEM images were recorded along and across these stripes. Intensity profile at the edges of a line scan is broader for the implantation carried out at 400°C compared to room temperature. From the analysis of this intensity profile lateral diffusion coefficient of Ga in silicon was estimated assuming that the PEEM intensity is proportional to Ga concentration. The diffusion coefficient at 400°C has been estimated to be $10 À15 m 2 /s. No significant dependence of diffusion coefficient on ion fluence was observed in the fluence range investigated here. Radiation enhanced diffusion has been discussed in the light of the associated defect distribution due to lateral straggling of the implanted ions.
ACS Applied Materials & Interfaces, 2013
Roughening in the electronic growth of Ag films on Si(111)-(7×7) surfaces for a film thickness ra... more Roughening in the electronic growth of Ag films on Si(111)-(7×7) surfaces for a film thickness ranging from 1 to 30 monolayers is reported. Ag films exhibit the growth of flat-top plateaus of preferential heights due quantum electronic effect. We have observed roughening of the film growth due to instability with linear diffusion characterized by the ln(θ) 1/2 dependence of the local surface slope, where θ is the Ag coverage. The roughening of the surface morphology has been characterized by scaling exponents α, β and 1/z, which are determined using scanning tunneling microscopy. Increased value of α = 0.67 ± 0.04 at the early stage of the electronic growth with two atomic layer height flat-top isolated Ag mounds to 0.77 ± 0.06 at the later stage of the growth when isolated mounds coalesce and form percolated structures maintaining preferential heights of an even number of atomic layers in the Ag mounds indicates the instability in the electronic growth. As a result, interface width W increases as a power law of coverage (θ), W ∼ θ β , with growth exponent β = 0.33 ± 0.03, and lateral correlation length ξ grows as ξ ∼ θ 1/z with 1/z = 0.27 ± 0.05.
Electronic density of states (DOS) at Fermi level has been investigated in ultrathin Ag films gro... more Electronic density of states (DOS) at Fermi level has been investigated in ultrathin Ag films grown on Si(111)-(7×7) down to the two-dimensional limit of a single atomic layer. Measurement of DOS at Fermi level by scanning tunneling spectroscopy shows an approximate (1-γ/d) dependence, where γ is a constant and d is the film thickness. The results are explained in the light of an analytical theory as well as our density functional theory (DFT) calculations. DFT results also show that in the proximity of the interface the DOS values of the film and the substrate are mutually affected by each other.
Thin Solid Films, 2013
The morphology and the size distribution of self-organized cobalt silicide nanostructures, grown ... more The morphology and the size distribution of self-organized cobalt silicide nanostructures, grown on Si substrates with controlled defects, have been investigated. An initial defect structure on the Si (111) surface is produced by quenching the substrate from just below the 7×7 ⇌ '1×1' (disordered) phase transition temperature. This has produced predominantly the Si (111)-(7×7) reconstructed structure along with some disordered regions and defect lines on the substrate surface. The disordered regions contain randomly placed Si adatom ring clusters or a lattice gas of ring clusters and small patches of √7×√7 R 19°structure. These substrates have been preannealed for different durations before 0.5 monolayer Co deposition on them for forming CoSi 2 by reactive deposition epitaxy. With increasing duration of substrate annealing and consequent reduction of defect density and surface roughness, a change of island morphology, a transition from bimodal to monomodal size distribution and an increase of average island size have been observed. Reduction of surface defects via substrate preannealing appears to lead to the growth of homogeneous nanostructures.
We present scanning tunneling microscopy studies of adsorption and desorption of Au on vicinal Si... more 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¥ DQG î ZKLFK DUH $X-induced reconstructions.
Applied Physics Letters, 2014
Applied Physics Letters, 2012
ABSTRACT We demonstrate the fractal growth of epitaxial Ag thin films on Si(111) surfaces using s... more ABSTRACT 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).
Journal of Applied Physics, 2013
We report on the growth of a monolayer thick fractal nanostructures of Ag on flat-top Ag islands,... more 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, D H ¼ 1.75 6 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 (D P ) and perimeter-area (D 0 P ) relationship. Simulations of the DLA process, with varying sticking probability, lead to different cluster morphologies [P. Meakin, Phys. Rev. A 27, 1495]; however, the value of D H is insensitive to this difference in morphology. We suggest that the morphology can be characterized by additional fractal dimension(s) D P and/or D 0 P , besides D H . We also show that within the DLA process D P ¼ D H [C. Amitrano et al., Phys. Rev. A 40, 1713] is only a special case; in general, D P and D H can be unequal. Characterization of fractal morphology is important for fractals in nanoelectronics, as fractal morphology would determine the electron transport behavior. V C 2013 AIP Publishing LLC.
Journal of Applied Physics, 2013
ABSTRACT We report on our observation of negative differential resistance (NDR) in electron tunne... more ABSTRACT 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.
Growth of Ag film on surfaces was examined by Rutherford backscattering spectrometry (RBS), scann... more Growth of Ag film on 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. Physics 978-0-7354-0905-7/$30.00 FIGURE 3. PEEM images: (a) surface decorated with Ag-depleted grain boundaries (FOV 100 μm) (b) a magnified part (FOV 30 μm) of the image in (a).
Epitaxial growth of cobalt islands on nonreactive metal/semiconductor surface has been investigat... more Epitaxial growth of cobalt islands on nonreactive metal/semiconductor surface has been investigated in-situ by scanning tunneling microscopy (STM). When 1.6×10 13 Co atoms/cm 2 are deposited on a pre-deposited Ag (7.5×10 14 atoms/cm 2 ) on Si (111)-(77) surface at room temperature, predominantly Co islands of monatomic or biatomic height grow. When this sample is annealed at 500 O 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 O reconstruction upon annealing. Apparently interaction between Co and Ag prevents the surface reconstruction of the Ag layer.
Surface Science, 2014
Sub-monolayer Co deposition on clean Si(111)-(7 × 7) surfaces has been found to form nanoscale Co... more Sub-monolayer Co deposition on clean Si(111)-(7 × 7) surfaces has been found to form nanoscale CoSi 2 islands with a surrounding trench of one Si bilayer depth and mainly hexagonal shape. The trench surface structure is largely like that of the disordered '1×1' phase of the Si(111)-7×7↔'1×1' phase transition and comprises mostly disordered Si adatoms with small ordered patches of (11 × 11), (9 × 9), c(5 × √5), c(4 × 4) and (2 × 2) structures along with some Co-ring clusters. This disordered '1 × 1' structure within the trench has formed at 600°C, the growth temperature of CoSi 2 in reactive deposition epitaxy, much below the order-disorder phase transition temperature on Si(111)-(7 × 7). The structure around the trench remains (7 × 7). Electronically the trench is semiconducting. The surrounding 7 × 7 structure being metallic, the island-trench structure forms a lateral metal-semiconductor-metal structure.
physica status solidi (b), 2013
ABSTRACT Ag/Si(111) heterojunction having ∼25% lattice mismatch can be formed by placing four sur... more ABSTRACT 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 sub-surface 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 metal–semiconductor 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 EF − Ev modified by the interface induced dipole.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2011
Parallel stripes of nanostructures on an n-type Si substrate have been fabricated by implanting 3... more Parallel stripes of nanostructures on an n-type Si substrate have been fabricated by implanting 30 keV Ga + ions from a focused ion beam (FIB) source at three different fluences: 1 Â 10 15 , 2 Â 10 15 and 5 Â 10 15 ions/cm 2 . Two sets of implantation were carried out. In one case, during implantation the substrate was held at room temperature and in the other case at 400°C. Photoemission electron microscopy (PEEM) measurements were carried out on these samples. The implanted parallel stripes, each with a nominal dimension of 4000 Â 100 nm 2 , appear as bright regions in the PEEM image. Line scans of the intensities from PEEM images were recorded along and across these stripes. Intensity profile at the edges of a line scan is broader for the implantation carried out at 400°C compared to room temperature. From the analysis of this intensity profile lateral diffusion coefficient of Ga in silicon was estimated assuming that the PEEM intensity is proportional to Ga concentration. The diffusion coefficient at 400°C has been estimated to be $10 À15 m 2 /s. No significant dependence of diffusion coefficient on ion fluence was observed in the fluence range investigated here. Radiation enhanced diffusion has been discussed in the light of the associated defect distribution due to lateral straggling of the implanted ions.
ACS Applied Materials & Interfaces, 2013
Roughening in the electronic growth of Ag films on Si(111)-(7×7) surfaces for a film thickness ra... more Roughening in the electronic growth of Ag films on Si(111)-(7×7) surfaces for a film thickness ranging from 1 to 30 monolayers is reported. Ag films exhibit the growth of flat-top plateaus of preferential heights due quantum electronic effect. We have observed roughening of the film growth due to instability with linear diffusion characterized by the ln(θ) 1/2 dependence of the local surface slope, where θ is the Ag coverage. The roughening of the surface morphology has been characterized by scaling exponents α, β and 1/z, which are determined using scanning tunneling microscopy. Increased value of α = 0.67 ± 0.04 at the early stage of the electronic growth with two atomic layer height flat-top isolated Ag mounds to 0.77 ± 0.06 at the later stage of the growth when isolated mounds coalesce and form percolated structures maintaining preferential heights of an even number of atomic layers in the Ag mounds indicates the instability in the electronic growth. As a result, interface width W increases as a power law of coverage (θ), W ∼ θ β , with growth exponent β = 0.33 ± 0.03, and lateral correlation length ξ grows as ξ ∼ θ 1/z with 1/z = 0.27 ± 0.05.
Electronic density of states (DOS) at Fermi level has been investigated in ultrathin Ag films gro... more Electronic density of states (DOS) at Fermi level has been investigated in ultrathin Ag films grown on Si(111)-(7×7) down to the two-dimensional limit of a single atomic layer. Measurement of DOS at Fermi level by scanning tunneling spectroscopy shows an approximate (1-γ/d) dependence, where γ is a constant and d is the film thickness. The results are explained in the light of an analytical theory as well as our density functional theory (DFT) calculations. DFT results also show that in the proximity of the interface the DOS values of the film and the substrate are mutually affected by each other.
Thin Solid Films, 2013
The morphology and the size distribution of self-organized cobalt silicide nanostructures, grown ... more The morphology and the size distribution of self-organized cobalt silicide nanostructures, grown on Si substrates with controlled defects, have been investigated. An initial defect structure on the Si (111) surface is produced by quenching the substrate from just below the 7×7 ⇌ '1×1' (disordered) phase transition temperature. This has produced predominantly the Si (111)-(7×7) reconstructed structure along with some disordered regions and defect lines on the substrate surface. The disordered regions contain randomly placed Si adatom ring clusters or a lattice gas of ring clusters and small patches of √7×√7 R 19°structure. These substrates have been preannealed for different durations before 0.5 monolayer Co deposition on them for forming CoSi 2 by reactive deposition epitaxy. With increasing duration of substrate annealing and consequent reduction of defect density and surface roughness, a change of island morphology, a transition from bimodal to monomodal size distribution and an increase of average island size have been observed. Reduction of surface defects via substrate preannealing appears to lead to the growth of homogeneous nanostructures.
We present scanning tunneling microscopy studies of adsorption and desorption of Au on vicinal Si... more 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¥ DQG î ZKLFK DUH $X-induced reconstructions.
Applied Physics Letters, 2014
Applied Physics Letters, 2012
ABSTRACT We demonstrate the fractal growth of epitaxial Ag thin films on Si(111) surfaces using s... more ABSTRACT 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).