Three-dimensional silicon-based nanostructures in opal matrix: Preparation and properties
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Three-dimensional silicon-based nanostructures in opal matrix: Preparation and properties A.B. Pevtsov, V.G. Golubev, V.A. Kosobukin, D.A. Kurdyukov, and A.V. Medvedev Ioffe Physico-Technical Institute, RAS, 194021 St. Petersburg, Russia ABSTRACT Three-dimensional opal-silicon composites with both direct (a variable extent of filling of opal voids with silicon) and inverted structures have been synthesized. A structural analysis of these fabricated systems is performed. Reflectance spectra from the (111) surface of the composites are measured within the spectral range 400-900 nm. Observed spectral features are interpreted as a manifestation of the [111] direction photonic band gap that is tunable in position and width in the visible and near-infrared spectral ranges. INTRODUCTION In the recent years the intensive research has been done on photonic crystals. Photonic crystals are periodic structures in which the dielectric constant is modulated with a period comparable to wavelength of light [1]. The Bragg diffraction of electromagnetic eigenmodes at the edge of the Brillouin zone of these structures leads to an appearance of the photonic band gap in their electromagnetic spectrum [2,3]. If there is a complete photonic band gap, the light propagation in any direction within the spectral range coinciding with the photonic band gap is inhibited. This is crucial for potential applications of the photonic crystals such as optical communication [4,5], lasers [6] and quantum computers [7]. Artificial opals are considered to be promising materials with photonic crystal properties due to their rather simple preparation and possibility of wide-range variation of their optical properties [8-12]. Artificial opals have a structure with the fcc lattice formed by close-packed amorphous silica spheres with diameters 150 to 1000 nm [13,14]. Voids forming between the spheres constitute up to 26% of total volume (when the spheres are tangent). This allows changing the optical contrast of material η = ε v ε s [15] via the introduction of various fillings into the voids (where εs and εv are, respectively, volume dielectric constants in and out of the space occupied by the silica spheres in opal). According to a theoretical estimation [15] the complete photonic band gap may arise at η ≥ 2.8. In bare opals where η is significantly smaller realization of the complete photonic band gap seems to be out of reach. Therefore, materials with high dielectric constant are to be used as fillings. In particular, one can use semiconductors such as InP, Si, Ge (hereafter the composites obtained via the opal void infilling are called “direct opals”). Even greater increase of η is possible by inverting an opal structure where inverting implies removing of material of silica spheres from an original (direct) opal-semiconductor composite [14,15]. As a result one gets a three-dimensional semiconductor lattice, which constitutes 26% of total volume and is placed into a matrix of air spheres, which amounts to the remaining 74% of volume. Among other semicond
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