Tuning the length/width aspect ratio of epitaxial unidirectional silicide nanowires on Si(110)-16 × 2 surfaces

The reactive deposition epitaxy growth of self-organized cobalt silicide nanowires (NWs) on clean Si(110) surfaces has been investigated by in situ scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) as well as by scanning electron microscopy (SEM). Half a monolayer of cobalt was deposited on the Si(110) surfaces at ∼600 °C substrate temperature. Following cobalt deposition, the substrates have been annealed for different durations. Cobalt forms aligned cobalt disilicide nanowires upon reaction with the silicon substrate, following the twofold substrate symmetry. With increasing duration of annealing, the NWs have been found to grow with larger aspect ratio (length/width), eventually producing narrower NWs. These self-organized unidirectional NWs of sub-hundred nanometer width and ∼4–7 nm height produce a Schottky barrier with the silicon substrate and are expected to find applications in nanoelectronic devices.

High density nonmagnetic cobalt in thin films

Recently high density (HD) nonmagnetic cobalt has been discovered in a nanoscale cobalt thin film, grown on Si(111) single crystal. This form of cobalt is not only nonmagnetic but also superconducting. These promising results have encouraged further investigations of the growth of the nonmagnetic (NM) phase of cobalt. In the original investigation, the cobalt film had a natural cobalt oxide at the top. We have investigated whether the growth of HD NM cobalt layers in the thin film depends on (i) a capping layer on the cobalt film, (ii) the thickness of the cobalt film and (iii) the nature of the substrate on which the cobalt film is grown. The results of such investigations indicate that for cobalt films capped with a thin gold layer, and for various film thicknesses, HD NM cobalt layers are formed. However, instead of a Si substrate, when the cobalt films are grown on oxide substrates, such as silicon oxide or cobalt oxide, HD NM cobalt layers are not formed. The difference is attributed to the nature-crystalline or amorphous-of the substrate.

Unidirectional endotaxial cobalt di-silicide nanowires on Si(110) substrates

Self-organized growth of well-ordered endotaxial silicide nanowires (NWs) on clean Si(110) surfaces has been investigated by in situ scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). Co deposition on clean Si(110) reconstructed surfaces at ∼600 °C produces unidirectional CoSi₂ NWs by reaction of cobalt with the hot silicon substrate. STM investigations reveal four major types of distinct NWs, all growing along the [-110] in-plane direction except one type growing along the in-plane [-113] direction. There are also some nanodots. The cross-sectional TEM measurements show that the unidirectional NWs are of two types-flat-top and ridged. The NWs grow not only on the substrate but also into the substrate. CoSi₂ in flat top NWs are in the same crystallographic orientation as the substrate Si and the buried interfaces between CoSi₂ and Si are A-type. In the ridged NWs CoSi₂ and Si are in different crystallographic orientations and the interfaces are B-type. The ridged NWs are in general wider and grow deeper into the substrate.

Evidence of Formation of Superdense Nonmagnetic Cobalt

Because of the presence of 3d transition metals in the Earth’s core, magnetism of these materials in their dense phases has been a topic of great interest. Theory predicts a dense face-centred-cubic phase of cobalt, which would be nonmagnetic. However, this dense nonmagnetic cobalt has not yet been observed. Recent investigations in thin film polycrystalline materials have shown the formation of compressive stress, which can increase the density of materials. We have discovered the existence of ultrathin superdense nonmagnetic cobalt layers in a polycrystalline cobalt thin film. The densities of these layers are about 1.2–1.4 times the normal density of Co. This has been revealed by X-ray reflectometry experiments, and corroborated by polarized neutron reflectometry (PNR) experiments. Transmission electron microscopy provides further evidence. The magnetic depth profile, obtained by PNR, shows that the superdense Co layers near the top of the film and at the film-substrate interface are nonmagnetic. The major part of the Co film has the usual density and magnetic moment. These results indicate the possibility of existence of nonmagnetic Co in the earth’s core under high pressure.

Roughening in electronic growth of Ag on Si(111)-(7×7) surfaces

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.

Ascertaining the nanocluster formation within an ion-irradiated Pt/Ni/C multi-trilayer with X-ray absorption spectroscopy

In this work nanoclusters formed in a Pt/Ni/C multi-trilayer by the ion-irradiated method of synthesis are characterized. In particular, an attempt to understand the role of interfaces in the synthesis is made. With this objective, ion-irradiation-induced structural changes in a Pt/Ni/C multi-trilayer using X-ray absorption spectroscopy (at the Ni K-edge) in conjunction with the X-ray standing-wave technique are investigated. The XANES analysis identifies chemical binding at pristine Ni/C and Ni/Pt interfaces, in contrast with physical adsorption at the Pt/C interface. The chemical nature of the interfaces determines their relative stability with respect to irradiation and controls the extent of metallic diffusion. The most interesting structural change, upon irradiation, is the disruption of the Pt/C interface and subsequent migration of Pt atoms towards pre-diffused Ni atoms within the C layer, leading to the formation of Ni-centered Ni-Pt bimetallic nanoclusters (with Ni:Pt = 60:40). These clusters are highly disordered beyond their nearest neighbor and find wide-scale applications as, for example, magnetic devices etc. The implications of these findings on the design goals are discussed.

Synthesis and Characterization of Mn-Doped ZnO Nanocrystals

We report the synthesis and characterization of several sizes of Mn-doped ZnO nanocrystals, both in the free-standing and the capped particle forms. The sizes of these nanocrystals could be controlled by capping them with polyvinylpyrollidone under different synthesis conditions and were estimated by X-ray diffraction and transmission electron microscopy. The absorption properties of PVP-capped Mn-doped ZnO exhibit an interesting variation of the band gap with the concentration of Mn. Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals.

Microstructures of a Pt/Ni/C multilayer system: an X-ray reflectivity and ion scattering study

Microstructural characterization of a synthetic periodic and graded Pt/Ni/C multilayer system by X-ray reflectivity and ion scattering techniques is presented. The experimental reflectivity data are fitted with a theoretical multi-trilayer model with graded periodicity which increases from substrate to film surface along the surface normal direction. The increase in periodicity is found to be due to a linear increase in C-layer thickness from the bottom to the top, with a change of slope nearly at the middle of the multilayer stack. The thicknesses of Pt and Ni layers, the variation of C-layer thickness with depth, interface roughness of Pt/Ni, Ni/C, C/Pt interfaces are determined from the analysis of the reflectivity data. Rutherford backscattering spectrometry measurements were also made on the same sample. Simulated Rutherford back scattering spectrometry data using the parameters obtained from the analysis of the X-ray reflectivity data agree well with the measured Rutherford backscattering spectrum.

Probing atomic migration in nanostructured multilayers: application of X-ray standing wave fields

X-ray standing wave fields, excited in periodic nanostructured multilayers during Bragg diffraction, have been used to probe atomic migration in multilayers. Ion beam induced migration of Fe impurity atoms from the C layers to the Pt layers in a Pt(Fe)/C(Fe) multilayer, where each layer is about 2 nm thick, has been detected. With a depth resolution better than 0.2 nm of this technique, the direction of Fe migration (here outward) and the change of Fe concentration in C (also Pt) layers have been determined. The results of such measurements are important for understanding the properties of multilayers, for example, the evolution of ferromagnetism in the present example [Dev, Microelectron. Eng. 83, 1721 (2006)10.1016/j.mee.2006.01.230].