Structural, electronic, and dynamical properties of amorphous gallium arsenide: a comparison between two topological models

We present a detailed study of the effect of local chemical ordering on the structural, electronic, and dynamical properties of amorphous gallium arsenide. Using the recently-proposed ``activation-relaxation technique'' and empirical potentials, we have constructed two 216-atom tetrahedral continuous random networks with different topological properties, which were further relaxed using tight-binding molecular dynamics. The first network corresponds to the traditional, amorphous, Polk-type, network, randomly decorated with Ga and As atoms. The second is an amorphous structure with a minimum of wrong (homopolar) bonds, and therefore a minimum of odd-membered atomic rings, and thus corresponds to the Connell-Temkin model. By comparing the structural, electronic, and dynamical properties of these two models, we show that the Connell-Temkin network is energetically favored over Polk, but that most properties are little affected by the differences in topology. We conclude that most indirect experimental evidence for the presence (or absence) of wrong bonds is much weaker than previously believed and that only direct structural measurements, i.e., of such quantities as partial radial distribution functions, can provide quantitative information on these defects in a-GaAs.Comment: 10 pages, 7 ps figures with eps

Interstitial-fluoride and substitutional-oxygen charge compensations of Er

A detailed crystal-field analysis, based on the Racah' theory, was carried out for the so-called A, B and G1 isolated charge-compensation centers of Er3+ ion doped in CaF2 crystal. Three sets of crystal-field parameters were obtained by a least-squares fitting of the optical data of Er3+ ion diluted in epitaxial Ca1−xErxF2+x thin film. This theoretical analysis confirms the expected $C_{4\nu}$ site symmetry for the A center and the $C_{3\nu}$ site symmetry for the G1 center. For the B center, however, the site symmetry is not exactly $C_{3\nu}$ in contrast to what is believed

Comparative Theoretical Study of the Optical Properties of Silicon/Gold, Silica/Gold Core/Shell and Gold Spherical Nanoparticles

International audienceThe scattering and absorption efficiencies of light by individual silicon/gold core/shell spherical nanoparticles in air are analysed theoretically in the framework of Lorenz-Mie formalism. We have addressed the influence of particle-diameter and gold-shell thickness on the scattering and absorption efficiencies of such nano-heterostructures. For comparison, we also considered the famous silica/gold core/shell nanoparticle and pure gold nanoparticle. Our simulation clearly shows that the optical response of the illuminated Si/Au core/shell nanoparticle differs markedly from that of the famous SiO2/Au heterostructure which in turn does not show a significant difference with that of the pure gold nanoparticle. This difference is clearly evident for shell thickness to outer particle radius ratio of less than 0.5. It manifests itself essentially by the occurrence of a strong and sharp absorption resonance beyond the wavelength of 600 nm where the silica/gold and the pure gold nanoparticles never absorb. The characteristics of this resonance are found to be sensitive to the particle diameter and the shell thickness. In particular, its spectral position can be adjusted over a wide spectral range from the visible to the mid-IR by varying the particle diameter and/or the shell thickness

Topological and chemical disorder in group-IV amorphous semiconductors

Raman scattering can be used as an efficient probe with regard to the understanding of local structure in highly disordered films. This investigation is particularly fruitful if one takes into account Stokes, anti-Stokes and multiple-order processes. The interpretation is given in terms of the whole density of vibrational states. Some recent results obtained with amorphous Si, SiC and C films are presented. Comparison with other techniques, including numeric modelisations, are discussed

Photoluminescence enhancement of silicon nanocrystals placed in the near field of a silicon nanowire

Semiconductor nanowires have an excellent ability to trap, guide, scatter, or absorb light for specific morphology-dependent resonant optical modes. The electromagnetic field enhancement associated with these modes could be used to modify the luminescence of emitters positioned in the vicinity of the nanowire, in a way similar to plasmonic nanostructures. We show that the photoluminescence of a single plane of silicon nanocrystals in silica, positioned at about 3 nm below the surface, can be enhanced by a factor of 2 to 3 in the presence of a silicon nanowire antenna on the silica surface. This could be the basis of a promising fully complementary metal oxide semiconductor compatible process to improve silicon-based light emitting devices, despite a lower enhancement compared to plasmonic nanostructures. Two-dimensional photoluminescence maps recorded for different polarization configurations (incident electric field parallel or perpendicular to the nanowire axis) exhibit different behaviors and can be related to the electric field intensity distribution in the near-field region of the nanowire, where the active silicon nanocrystal layer is located. © 2013 American Physical Society

Photoluminescence enhancement of a silicon nanocrystal plane positioned in the nearfield of a silicon nanowire

Semiconductor nanowires have an excellent ability to trap, guide, scatter or absorb light thanks to the presence of specific resonant optical modes. The electromagnetic field-enhancement associated to such effects can be used to modify the luminescence of emitters positioned in the vicinity of the nanowire, as to plasmonic nanoantennas. We show that the photoluminescence of silicon nanocrystals embedded in silica can be enhanced by a factor of about 3 in the presence of a silicon nanowire on the silica surface. This could be the basis of a promising CMOS-compatible process to improve silicon-based light emitting devices. 2D maps of the nanocrystal photoluminescence exhibit different behaviors as function of laser polarization and can be related to the electric field intensity distribution in the nanowire near-field, where the Si-NCs layer is located. Preliminary results of nonlinear optics for imaging the local electric field are presented