Linear optical properties of Ge nanocrystals in silica

The absorption and extinctionspectra of Genanocrystals in silica formed by ion implantation are studied using photothermal deflection and transmission spectroscopies. It is found that scattering makes a significant contribution to the extinction spectrum, damping the spectral features and resulting in a Rayleigh scattering-likeω⁴ dependence. In contrast, the spectra measured by photothermal deflection clearly show features such as the E1/E1+Δ1 transitions. The Tauc gap is extracted to be ∼0.7±0.1 eV

Direct growth of nickel disilicide nanocrystals in silicon dioxide films

Nickel disilicide (NiS₂)nanocrystals (NCs) have been grown in silicon-rich oxide (SiOₓ)films ion implanted with nickel by annealing at 1100°C. It was found that NiS₂ NCs grew into well-defined single crystalline structures embedded in a SiOₓ matrix and were approximately spherical in shape. The size of NCs can be influenced by limiting either the Ni or excess Si concentration. It was found that the resulting NCs could be produced with diameters in the range from 5to40nm in the SiOₓ layers with excess Si concentrations of 4–8at.% implanted with Ni concentrations of 0.1–10at.%.This work was carried out under Scientists Exchange Program between the Korea Science and Engineering Foundation and the Australian Academy of Science, and supported by the Research Grant from the Kangwon National University

Formation of nickel-based nanocrystal monolayers for nonvolatile memory applications

A simple method for fabricating metal silicide nanocrystal layers with narrow spatial distributions is demonstrated and shown to produce structures suitable for nonvolatile memory applications. The method is based on high-temperature annealing of a sandwich structure comprised of a thin metal (Ni)film sandwiched between two silicon-rich oxide (SiOₓ) layers and has the feature in which the size of the NCs can be controlled by varying the silicon concentrations in the SiOₓ layers or the initial nickelfilm thickness. The typical nanocrystal diameters and densities are 3.6nm and 1.2×10¹²cm⁻², respectively. Capacitance-voltage (C-V) measurements on test structures with these characteristics are shown to have C-V characteristics suitable for nonvolatile memory applications, including a C-V memory window of 11.7V for sweep voltages between −12V and +12.This work was supported by the Korea Research Foundation Grant funded by the Korean Government MOEHRD, KRF-2007-313-C00269 and by the Australian Research Council through its Discovery Grant Program

Rapid, substrate-independent thickness determination of large area graphene layers

Phase-shifting interferometric imaging is shown to be a powerful analytical tool for studying graphene films, providing quantitative analysis of large area samples with an optical thickness resolution of ≤0.05 nm. The technique is readily able to identify single sheets of graphene and to quantitatively distinguish between layers composed of multiple graphene sheets. The thickness resolution of the technique is shown to result from the phase shift produced by a graphene film as incident and reflected light pass through it, rather than from path-length differences produced by surface height variations. This is enhanced by the high refractive index of graphene, estimated in this work to be nG = 2.99 ± 0.18.The authors wish to acknowledge the Australian Research Council for financial support

High-endurance megahertz electrical self-oscillation in Ti/NbO x bilayer structures

Electrical self-oscillation is reported for a Ti/NbOx negative differential resistance device incorporated in a simple electric circuit configuration. Measurements confirm stable operation of the oscillator at source voltages as low as 1.06 V

Threshold switching and electrical self-oscillation in niobium oxide films

Electrical self-sustained oscillations have been observed in a broad range of two-terminal systems and are of interest as possible building blocks for bio-inspired neuromorphic computing. In this work, we experimentally explore voltage-controlled oscillations in NbOx devices with a particular focus on understanding how the frequency and waveform are influenced by circuit parameters. We also introduce a finite element model of the device based on a Joule-heating induced insulator-metal transition. The electroformed device structure is represented by a cylindrical conductive channel (filament) comprised of NbO/NbO2 zones and surrounded by an Nb2O5−x matrix. The model is shown to reproduce the current-controlled negative differential resistance observed in measured current-voltage curves, and is combined with circuit elements to simulate the waveforms and dynamics of an isolated Pearson–Anson oscillator. Such modeling is shown to provide considerable insight into the relationship between the material response and device and circuit characteristics

Hyperfine clock transitions of bismuth donors in silicon detected by spin-dependent recombination

Bismuth donors ion-implanted in 28Si and natSi are studied using magnetic resonance spectroscopy based on spin-dependent recombination. The hyperfine clock transition, at which the linewidth is significantly narrowed, is observed for the bismuth donors. The experimental results are modeled quantitatively by molecular orbital theory for a coupled pair consisting of a bismuth donor and a spin-dependent recombination readout center, including the effect of hyperfine and Zeeman interactions

Self-assembled nanoparticle spirals from two-dimensional compositional banding in thin films

A self-assembly process is reported in which spiral patterns of goldnanoparticles form on siliconsurfaces during the epitaxial crystallization of thin gold-silicon alloy layers. This behavior is observed only for gold concentrations above a critical value and is shown to result from two-dimensional compositional banding of a liquid alloy layer during the crystallization process. The compositional banding consists of alternate gold-rich and silicon-rich alloy bands, which are shown to be a direct consequence of free energy minimization, the band spacing being that which gives the maximum diffusive composition-separation rate. Goldnanoparticles subsequently form by Ostwald ripening on the surface of the gold-rich bands to give rise to the observed spiral patterns.We thank P. Evans and D. Button for MEVVA implantation at Australian Nuclear Science Technology Organization under an AINSE Grant No. AINGRA05155P. We thank S.K. Bhargava at RMIT University for the financial support to D.K.V. to carry out initial stages of this research