Vladimir Poborchii

Photomelting of selenium at low temperature

We report on a photoinduced phenomenon in solids, namely, photomelting at low temperature. We have found that both trigonal and amorphous selenium can be molten by illumination with light at a temperature of ∼77 K. This phenomenon is pure optical (athermal) and it is associated with light-induced breaking of the interchain (intermolecular) bonds in selenium. The photomelting is important for basic science (as an example of photoinduced phase transition in condensed matter and as a key photoinduced phenomenon in selenium and related materials) and for applications (as a tool for fine manipulation with shape of solids by light at low temperatures).

An in situ Raman study of polarization-dependent photocrystallization in amorphous selenium films

Photocrystallization of amorphous selenium (a-Se) films under illumination by polarized light with 632.8 or 647.1 nm wavelength has been studied by Raman spectroscopy. Preferential orientation of trigonal crystalline, selenium (t-Se) obtained as a result of photocrystallization has been observed, threefold c axis of t-Se being oriented perpendicular to the direction of the polarization of the illuminating light. Although the mechanism of polarization-dependent photocrystallization seems to be optical in origin, an alternative, essentially thermal, mechanism of the polarization-dependent photocrystallization of a-Se is discussed.

A VISIBLE-NEAR INFRARED RANGE PHOTONIC CRYSTAL MADE UP OF SI NANOPILLARS

We studied a two-dimensional square lattice of Si nanopillars (SQLN) perspective for applications in waveguides. Reflection spectra of SQLN with the period of 270 nm were studied for different surrounding media, namely air and water. SQLN is shown to display photonic band gap (PBG) behavior in the visible–infrared spectral range. The energy positions and widths of the experimentally observed reflection bands are found to be in agreement with calculated PBG.

Subwavelength-Resolution Raman Microscopy of Si Structures Using Metal-Particle-Topped AFM Probe

Using depolarization of the 364 nm light scattered by a small particle on the (100)Si surface, one can obtain allowed 520 cm-1 Raman signal from the localized area of Si around the particle, while the ordinary Raman signal is forbidden by the polarization selection rules. We have realized this scheme using Ag-particle-topped quartz atomic force microscope (AFM) probe immersed into glycerol droplet on Si surface and applied to local stress measurement. Lateral resolution in the range of 100 nm was demonstrated, and stress variation in a strained Si film was investigated.

Silver-coated silicon pillar photonic crystals: Enhancement of a photonic band gap

For a two-dimensional lattice of Si pillars it is shown both experimentally and theoretically that a photonic band gap for the light polarized perpendicular to pillars can be strongly enhanced by means of a silver coating of the pillars. A sizable omnidirectional photonic band gap is demonstrated for both square and triangular lattice of silver-coated Si pillars in the near-infrared and visible spectral range.

Observation of the forbidden doublet optical phonon in Raman spectra of strained Si for stress analysis

Using high numerical aperture lenses, we detected doublet optical phonon, forbidden by selection rules, in Raman spectra of Si strained in the (001) plane (bulk Si as well as strained Si device structure grown on SiGe). This allowed us to quantitatively determine stress and its distribution in strained Si with the ∼10% accuracy, assuming symmetric biaxial stress. At the same time, we demonstrate some deviations of the real stress from the assumed model. For better accuracy, one has to consider these deviations as well as a possibility of improvement of available Si phonon deformation potential values.

Photonic-band-gap properties of two-dimensional lattices of Si nanopillars

We studied photonic-band-gap properties of two-dimensional lattices of Si nanopillars by theoretical calculation and measurement of reflection and transmission spectra. We focused on advantages of these photonic crystals compared to other Si photonic crystals, which usually operate in the range of transparency of bulk Si (wavelengths longer than ∼1.1 μm). We showed that the available spectral range for the photonic crystals of Si nanopillars can be extended to the submicron wavelengths, light absorption by Si nanopillars being insignificant. Another important advantage of Si nanopillar lattices is the ability to incorporate luminescent materials into the huge free space of this photonic crystal. We demonstrate the inhibition of spontaneous emission of dye incorporated into the nanopillar lattice.

High-spatial-resolution Raman microscopy of stress in shallow-trench-isolated Si structures

Stress in single and periodic shallow-trench-isolated Si structures was examined by 364nm excitation confocal resonance Raman microscopy, laser penetration being restricted to the near-surface region. Using a 1.3 numerical aperture microobjective lens with a theoretical ∼140nm spatial resolution, the authors show that the configuration with both incident and scattered lights polarized parallel to each other and perpendicular to Si stripes is favorable for stress detection in the middle of the stripes, suppressing contributions from their edges. The stresses located in different areas of the structures were identified and analyzed.

Study of stress in a shallow-trench-isolated Si structure using polarized confocal near-UV Raman microscopy of its cross section

Stress in shallow-trench-isolated structures with 250-nm-wide Si stripes was examined by 364nm excitation confocal Raman microscopy. Using objective lenses with the experimentally measured ∼150nm spatial resolution, we studied polarized Raman spectra from the section perpendicular to the stripes. Detection of two optical phonons enabled us to analyze orientation and intensity of local stress quantitatively. Uniaxial [001] compressive stress was found in the Si substrate under both the trench and the stripe, while the stress in the stripe was uniaxial [110] perpendicular to the stripe.

Fabrication of photonic crystals consisting of Si nanopillars by plasma etching using self-formed masks

We have fabricated 2D photonic crystals (square and triangular lattice) with a photonic band gap in the visible light range by periodically arranging Si nanopillars. The fabrication process uses iron clusters as nuclei for self-formation of etching masks to obtain high-aspect-ratio Si nanopillars. First, arrays of openings (30-40 nm in diameter) were defined in resist layers (PMMA) by electron beam patterning. Then iron was deposited with vacuum evaporation to a nominal thickness of 1-2 nm, resulting in cluster formation in the bottom of the openings. After removing the resist by dipping in acetone for 2 minutes, the substrates were ECR etched with SF/sub 6/. During the etching, reaction products in the plasma condense around the clusters, leading to the self-formation of uniform size etching masks. Then Si nanopillars with a high aspect ratio were fabricated. The iron clusters enable fabrication of the nanopillars with an aspect ratio higher than 20, probably owing to their catalytic properties.