We report on the effect of high temperature annealing on the reflection spectra of synthetic opals. The analysis of conditions for simultaneous diffraction on the (111) planes parallel and inclined to the sample surface has shown that both annealed and unannealed opals are compressed along the growth [111] axis and the shape of the SiO2 balls forming the opals’ close packed structure can be described as spheroidal. The structure parameters were evaluated from the analysis of the angular dependences of the peak positions in the Bragg reflection spectra of unfilled and glycerol-filled samples. The major effect of annealing is due to the sintering (interpenetration) of the structural elements of opals. The maximum temperature of 1050 °C leads to a 10-fold increase in the degree of spheroid sintering. As a result, the interspheroid spacing decreases by over 10%, while the filling factor increases from 0.75 to 0.96 together with the effective dielectric constant of the opal as a whole (from 1.74 to 2.08). Sintering takes place not only between spheroids, but also inside spheroids between the α-SiO2 nanoparticles constituting them. This results in a noticeable (by ∼7%) increase in the dielectric constant of opal spheroids.
Wide bandgap GaN based semiconductor structures are highly attractive because of their great potential for development of stable optoelectronic devices in the visible-ultraviolet region [1]. Up to now main efforts of researchers have been directed towards the use of properties of tzhe electron subsystem in GaN. The main goal of our work is to create a material where optical and electronic properties of the nitride semiconductor could be combined with unique features of photonic crystals (PhC) based on artificial opals. The novel properties of such materials open new possibilities to mould and control emission and propagation of light in visible optoelectronic devices [2]. The present work is aimed at synthesizing GaN PhC, investigating their optical properties and effect of a photonic band gap (PBG) on the Bragg diffraction of light.To create GaN based 3D PhC we used highly ordered synthetic polydomain opals as a 3D template. The opals have the fcc structure formed by closed-packed a À SiO 2 spheres of 245 nm in diameter. The sublattices of interconnected opal voids were impregnated with Ga 2 O 3 precursor and the samples were annealed in an atmosphere of nitrogen hydrides. XRD, TEM, SEM, and Raman measurements confirmed the structural perfection of GaN in the opal-GaN composites [3,4]. Then, the silica template was selectively removed by etching off the a À SiO 2 skeleton with HF. As a result we have prepared so-called inverse opal-GaN which is in fact 3D PhC entirely consisting of GaN (Fig. 1).When characterizing the samples fabricated main attention was paid to measuring reflection spectra. In what follows we will present results obtained for s-polarization of diffracted light. To measure the reflection spectra from a domain the 8-fold magnified image of the sample was focused on the entrance slit of a spectrometer.Investigation of reflection spectra is one of the most simple and direct methods to probe the band structure of PhC [5]. Due to the periodic modulation of the dielectric constant photonic crystals reveal photonic band structures and, in particular, forbidden energy gaps for light (stop bands) appear in the energy spectrum. Light waves with energies within these stop bands are Bragg reflected and cannot propagate inside the crystal. As a result, in reflection spectra maxima arise in the energy regions of the stop bands. Figure 2a shows reflection spectra of the initial phys. stat. sol. (b) 231, No. 1, R7-R9 (2002)
We have measured temperature dependent impedance spectra of LuFe2O4 multiferroic polycrystalline samples in the temperature range of 100–400 K, in a wide frequency range of measuring AC voltage: from 20 Hz to 120 MHz. In spectra measured at 30–70 MHz and at 200–260 К, we observed anomalous positive temperature coefficient of resistance. We explain the anomaly on the basis of the generalized barrier model.
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