K. A. Gonchar

Effects of light localization in photoluminescence and Raman scattering in silicon nanostructures

The effect of light localization in photoluminescence (PL) and Raman scattering (RS) in silicon nanowires with diameter of 100 nm was investigated. The optical excitation was done by CW radiation of a YAG:Nd laser at 1.064 μm. The PL an RS intensities were found to increase strongly for the samples with Si nanowires in comparison with corresponding values of c-Si substrate. The effect is explained by an increase of the lifetime of photons in silicon nanowire structures.

Structural and photoluminescent properties of nanowires formed by the metal-assisted chemical etching of monocrystalline silicon with different doping level

Silicon-nanowire layers grown by the metal-assisted chemical etching of (100)-oriented p-type monocrystalline silicon substrates with a resistivity of 10 and 0.001 Ω · cm are studied by electron microscopy, Raman scattering, and photoluminescence measurements. It is established that nanowires grown on lightly doped substrates are structurally nonporous and formed as crystalline cores covered by nanocrystals 3–5 nm in dimensions. Nanowires grown on heavily doped substrates are structurally porous and contain both small nanocrystals and coarser crystallites with equilibrium charge carriers that influence interband radiative recombination. It is found that the photoluminescence intensity of nanowires in the spectral range 1.3–2.0 eV depends on the presence of molecular oxygen.

Dependence of the efficiency of Raman scattering in silicon nanowire arrays on the excitation wavelength

The features of Raman scattering in layers of silicon nanowires from 50 to 350 nm in diameter, obtained by the chemical etching of crystalline silicon (c-Si) wafers with preliminarily deposited silver nanoparticles in fluoric acid solutions are studied. c-Si wafers with various crystallographic orientations and doping levels are used, which is conditioned by the different sizes and degrees of ordering of the formed nanostructures. It is found that the radiation of the Raman scattering of samples is depolarized, and its efficiency depends strongly on the excitation wavelength. Upon excitation by light with a wavelength of 1064 nm, the ratio of Raman-scattering intensities of silicon nanowire samples and c-Si is 2 to 5; as the wavelength decreases, this ratio increases for structures with larger silicon-nanowire diameters and higher degrees of ordering and decreases for less ordered structures. The results obtained are explained by the effect of partial light localization in silicon nanowire arrays.

Enhancement of photoluminescence and raman scattering in one-dimensional photonic crystals based on porous silicon

In porous-silicon-based multilayered structures that exhibit the properties of one-dimensional photonic crystals, an increase in the photoluminescence and Raman scattering intensities is observed upon optical excitation at the wavelength 1.064 μm. When the excitation wavelength falls within the edge of the photonic band gap of the structures, a multiple increase (by a factor larger than 400) in the efficiency of Raman scattering is detected. The effect is attributed to partial localization of excitation light and, correspondingly, to the much longer time of interaction of light with the material in the structures.

Optical Properties of Nanowire Structures Produced by the Metal-Assisted Chemical Etching of Lightly Doped Silicon Crystal Wafers

Layers of Si nanowires produced by the metal-assisted chemical etching of (100)-oriented single-crystal p-Si wafers with a resistivity of 1–20 Ω · cm are studied by reflectance spectroscopy, Raman spectros-copy, and photoluminescence measurements. The nanowire diameters are 20–200 nm. The wafers are supplied by three manufacturing companies and distinguished by their different lifetimes of photoexcited charge carriers. It is established that the Raman intensity for nanowires longer than 1 μm is 3–5 times higher than that for the substrates. The interband photoluminescence intensity of nanowires at the wavelength 1.12 μm is substantially higher than that of the substrates and reaches a maximum for samples with the longest bulk lifetime, suggesting a low nonradiative recombination rate at the nanowire surfaces.

Investigation of halloysite nanotubes with deposited silver nanoparticles by methods of optical spectroscopy

Halloysite nanotube composites covered by silver nanoparticles with the average diameters of 5 nm and 9 nm have been studied by methods of optical spectroscopy of reflectance/transmittance and Raman spectroscopy. It has been established that silver significantly increases the light absorption by nanocomposites in the range of 300 to 700 nm with a maximum near 400 nm, especially for the samples with the nanoparticle size of 9 nm, which is explained by plasmonic effects. The optical absorption increases also in the long-wavelength spectral range, which seems to be due to the localized electronic states in an alumosilicate halloysite matrix after deposition of nanoparticles. Raman spectra of nanocomposites reveal intense scattering peaks at the local phonons, whose intensities are maxima for the samples with the silver nanoparticle sizes of 9 nm, which can be caused by plasmonic enhancement of the light scattering efficiency. The results show the ability to use halloysite nanotube nanocomposites in photonics and biomedicine.

Electrochemical Nanostructuring of Semiconductors by Capillary-Cell Method

Wafers of silicon and compound semiconductors are nanostructured by using electrochemical or chemical etching (stain etching) in etching cell with electrolyte kept by capillary forces. Atomic force microscopy, infrared spectroscopy and Raman scattering methods reveale nanoporous and nanocrystalline structure of the treated surfaces. The formed porous semiconductors demonstrate efficient photoluminescence, which is controlled by etching parameters, i.e. current density, electrolyte content, etc. These results indicate good prospects of the employed capillary-cell method for preparing nanostructured porous materials with desired structure and optical properties.

ELECTROCHEMICAL NANOSTRUCTURING OF SEMICONDUCTOR WAFERS BY CAPILLARY-FORCE-ASSISTED METHOD

Wafers of crystalline silicon (c-Si) and compound semiconductors (GaP, GaAs) were nanostructured by using the electrochemical etching in specially designed cells with two or more electrodes spaced at 100–500 μm distances, which allowed keeping the electrolyte due to capillary forces. Investigations by means of atomic force microscopy and optical spectroscopy revealed nanoporous and nanocrystalline structure of the prepared samples. The employed capillary-force-assisted method is promising for preparation of thin layers of nanostructured semiconductors with desired optical properties having advantages of cost saving, quickness and flexibility in the electrical contact arrangements versus conventional electrochemical etching methods.