The aim of this paper is the study of porous Si prepared by preferential anodic dissolution in concentrated HF acid solutions. Porous silicon layers exhibited extremely efficient luminescence in the 700-900 nm range at room temperature. Basic characteristics of this luminescence strongly suggest the intrinsic origin of the process, directly related to quantum confinement. The additional transmission-electron-microscopy and electron-diffraction studies - were performed to support hypothesis that luminescence originates from silicon nanostructures.
We describe a method for the calculation of the optical properties of a two-dimensional array of non-overlapping metallic particles (approximated by spheres) adsorbed on a dielectric slab. The interest in such systems arises to a large degree from their possible use as coatings, e.g. for solar energy absorbers and similar technological purposes. The formalism is an extension of the methods which have been developed in relation to electron scattering by two-dimensional atomic layers and takes fully into account multiple scattering of light between the particles of the overlayer and between the overlayer and the substrate. Scattering of light by multilayers or by an infinite crystal of non-overlapping spheres can be dealt with by a straightforward extension of the theory as in the theory of low-energy electron diffraction. Our calculations show that the usual approximation of replacement of the metallic particles by effective dipoles fails when the size of the particles or the concentration of particles increases beyond a limit and that l-pole contribution in interparticle scattering beyond the dipolar (l=1) one introduces new structure in the absorbance versus frequency curve. The reflection and absorption of light as a function of frequency is obtained numerically for selected examples. We consider in particular the variation of these quantities with concentration coverage. We examine also the effect of disorder.
The optical constants (the refractive index n, the absorption index k and the absorption coefficient α) of Bi_{2}Te_{3} thin films were determined in the wave-lenght range of 2.5 to 10 µm. The shape of the absorption edge in Bi_{2}Te_{3} thin films has been determined from transmittance and reflectance measurements. The edge is of the form expected for direct transition corresponding to E_{g} = 0.21 eV. The optical constants were used to determine the high frequency dielectric constant ε_{0} = 58, the optical conductivity σ =σ_{1} + σ_{2} as well as the volume and surface energy loss functions. All these parameters were used to get some information about the intraband and interband transitions.
The optical constants of vacuum deposited CuInSe_{2} thin films of different thicknesses (60-135 nm) were determined in the photon energy from 1.03 to 3.1 eV. It was found that both the refractive index n and the absorption index k are independent of the film thickness. The analysis of the experimental points of the refractive index revealed the existence of normal dispersion and fits Sellmeier dispersion formula for single oscillator model. Using the previous model the optical dielectric constant as well as the oscillator energy and dispersion parameter have been calculated. CuInSe_{2} is found to be a direct gap semiconductor with a gap energy of 1.03 eV. At energies well above the absorption edge, the absorption behaviour can be explained by the existence of a forbidden direct transition with the same direct energy gap and an indirect one with energy gap of 0.85 eV.
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