E. V. Astrova

Electrotunable in-plane one-dimensional photonic structure based on silicon and liquid crystal

The model of the electro-optical effect, due to the reorientation of liquid crystal molecules from a pseudoisotropic to a homeotropic state, in a composite photonic structure with a liquid crystal filler, is elaborated. A composite (110) grooved silicon photonic structure for the middle infrared range was designed and fabricated on a silicon-on-insulator platform. Polarized reflection spectra, demonstrating the electro-optical effect, have been obtained by means of Fourier transform infrared microscopy. The relative shift of the band edge at half intensity in the region of 10μm was found experimentally to be 1.6% compared to 2.2% as predicted by theory.

Process induced deep-level defects in high purity silicon

Deep-level defects appear in silicon upon heat treatment of wafers with surface disordered by mechanical lapping or introducing high concentration impurity in diffusion layer, i.e. in regimes typical of fabrication of high voltage devices. By means of capacitance transient spectroscopy, combined with other methods, it was shown that dominant electron traps with ionization energies of 0.28 and 0.54 eV of double level donor have low recombination activity, but affect the resistivity of high purity Si and play a key role in limiting the p-n junction breakdown voltage . A careful study of the defect parameters showed their similarity to sulphur-related centres in Si.

Silicon photonic crystal filter with ultrawide passband characteristics

We report on what is believed to be the first example of an ultrawide, bandpass filter, based on a high-contrast multicomponent one-dimensional Si photonic crystal (PC). The effect of the disappearance of a limited number of flat stopbands and their replacement with extended passbands is demonstrated over a wide IR range. The passbands obtained exhibit a high transmission of 92% to 96% and a substantial bandwidth of 1800 nm, which is spectrally flat within the passband. The multicomponent PC model suggested can be applied to the design of any micro- or nanostructured semiconductor or dielectric material for application across a wide spectral range.

One-dimensional photonic crystal obtained by vertical anisotropic etching of silicon

The potentialities of vertical anisotropic etching of (110) silicon for the fabrication of one-dimensional photonic crystal with a high refractive index contrast have been studied. It is shown that advances toward the near-IR spectral range are limited by the mechanical strength of thin silicon walls. The device structures obtained consist of 50 trenches, 114 µm deep, with 1.8-µm-thick Si walls (structure period 8 µm). Their reflectance spectra in the wavelength range 2.5–16.5 µm show good agreement with calculation results, although the main photonic band gap at λ≈28±10 µm remained outside the spectral region of measurements.

Variation of the parameters and composition of thin films of porous silicon as a result of oxidation: Ellipsometric studies

The variation of the optical characteristics of thin films of oxidized porous silicon as a function of the preparation regime and subsequent heat treatment is investigated by ellipsometry. It is shown that the refractive index, optical thickness, and extinction coefficient of porous silicon films decrease monotonically, but the film thickness increases as the degree of oxidation of the silicon base layer increases. An analysis of the film thickness as a function of the degree of oxidation shows that it differs very little from the same dependence for the nonporous film. The composition of the films is determined from the measured refractive index at a wavelength λ=632.8 nm by means of curves calculated on the basis of the three-component Bruggeman model of the effective medium for layers with different initial porosities.

Fabrication Technology of Heterojunctions in the Lattice of a 2D Photonic Crystal Based on Macroporous Silicon

Design and fabrication technology of a microcavity structure based on a double heterojunction in macroporous silicon is suggested. The fabrication process of a strip of a 2D photonic crystal constituted by a finite number of lattice periods and the technique for defect formation by local opening of macropores on the substrate side, followed by filling of these macropores with a nematic liquid crystal, are considered.

Numerical methods for calculation of optical properties of layered structures

Three methods, namely 2×2 and 4×4 transfer matrix methods as well as scattering matrix method, for simulation of the transmission and reflection spectra of the layered structures are described in this paper. The advantages of each of these methods for simulation of the optical spectra of one-dimensional photonic crystals are analyzed. The modified 2×2 transfer matrix method is suggested for calculation of the reflection and transmission coefficients of the layered structures in situation when the incident light beam has a cone-like shape.

Strains and crystal lattice defects arising in macroporous silicon under oxidation

The deformation behavior of a macroporous silicon wafer subjected to high-temperature oxidation has been studied, and the basic parameters describing the sample bending and subsequent stress relaxation when oxide is removed are determined. X-ray diffractometry and topography were used to determine the sample bending radius and lattice parameters, and to reveal the areas of dislocation generation. The strain of a silicon lattice in oxidized macroporous Si is about 10−4, and it decreases by an order of magnitude after oxide dissolution. The plastic part of the strain is accompanied by the generation of dislocations in the most strained regions of a structure, i.e., at the interfaces between the porous layer and substrate in the vertical direction and between the central porous region and the pore-free edge in the horizontal plane. The dislocation density is ∼104 cm−2.

Polarized infrared and Raman spectroscopy studies of the liquid crystal E7 alignment in composites based on grooved silicon

Alignment of liquid-crystal filler molecules and the electro-optical effect in composite photonic crystals based on grooved silicon are studied. It is found that the nematic liquid crystal molecules that fill the grooves are predominantly aligned in a planar configuration with respect to the silicon walls. The liquid crystal molecules are realigned homeotropically with respect to the groove walls under the influence of an electric field. The effect detected can be used to adjust the photonic band gap of a one-dimensional photonic crystal.

FTIR and Raman investigation of vertically etched silicon as a 1D photonic crystal

The reflection spectra of composite materials on the base of grooved silicon and grooved silicon infiltrated with nematic liquid crystal (LC) have been calculated using the optimal parameters of a grooved silicon matrix suitable for the infrared range. The grooved silicon structures with different lattice constants (A=16, 12, 8 and 4 mm) have been designed and prepared. An important parameter of these structures is the thickness of the silicon walls (DSi). This has been obtained using simulations of the spectra. This parameter was used for further analysis of the spectra of composite material grooved Si-LC. The experimental reflection is reaching of 65% in maximum (with signal modulation from maximum to minimum up to 55%) for the composite structures with a small number of lattice periods that makes these structures very perspective with a potential applications. The analysis of the polarised infrared spectra of Si structures infiltrated with LC allows one to determine the orientation and the refractive index (NLC) of the liquid crystal. For the samples with a distance between Si walls of 6-10 mm, it was found that the refractive index was NLC=~ 1.5 for polarised light and NLC 1.5 for s-polarised light. This leads to the conclusion on the planar orientation of liquid crystal molecules with respect to the Si walls. For the samples with distance between Si walls less than 3 mm, a homeotropic alignment of liquid crystal molecules has been found. Micro-Raman spectroscopy has been applied for analysis of stress in such Si structures. The maximum stress of about 2 GPa was obtained on the top of Si walls (under Si dioxide layer).