A. I. Efimova

Study of Birefringence in Porous Silicon Layers by IR Fourier Spectroscopy

Birefringence in porous silicon layers prepared with different etching currents on a (110) substrate is studied by IR Fourier spectroscopy. The spectra exhibit beats in the intensity of transmitted and reflected radiation due to the summation of the intensities of the ordinary and extraordinary waves interfering in the porous layer. An analysis of the spectra shows the layers to exhibit properties of a negative uniaxial crystal with the optical axis lying in the layer plane. The difference between the refractive indices of the ordinary and extraordinary waves for a layer with a porosity of 80% reaches 18%. The experimental data are in agreement with the calculations based on the effective-medium approximation, which takes into account the anisotropy of silicon nanocrystal arrangement in a porous layer.

Birefringence and Anisotropic Optical Absorption in Porous Silicon

The refractive indices and the coefficients of optical absorption by free charge carriers and local vibrations in porous silicon (por-Si) films, comprising nanometer-sized silicon residues (nanocrystals) separated by nanometer-sized pores (nanopores) formed in the course of electrochemical etching of the initial single crystal silicon, have been studied by polarization-resolved IR absorption spectroscopy techniques. It is shown that the birefringence observed in por-Si is related to the anisotropic shapes of nanocrystals and nanopores, while the anisotropy (dichroism) of absorption by the local vibrational modes is determined predominantly by the microrelief of the surface of nanocrystals. It is demonstrated that silicon-hydrogen surface bonds in nanocrystals can be restored by means of selective hydrogen thermodesorption with the formation of a considerable number of H-terminated surface Si-Si dimers.

Effect of the initial doping level on changes in the free-carrier concentration in porous silicon during ammonia adsorption

Infrared spectroscopy is used to investigate the effect of ammonia adsorption on the concentration of equilibrium charge carriers in porous-silicon layers with various initial types of dopants at different concentrations. It is found that ammonia adsorption results in an increase in the number of free electrons in n-type samples up to a level exceeding 1018 cm−3. In p-type samples, a nonmonotonic dependence of the charge-carrier concentration on ammonia pressure is observed. The obtained results are accounted for by the appearance of adsorption-induced shallow donor states that, along with the initial-dopant and surface-defect states, specify the charge-carrier type and concentration in the silicon nanocrystals of the porous layer after ammonia adsorption.

Channeling study of the orientational dependence of laser-induced damage in GaAs and GaP

Abstract Channeling was used for the study of laser-induced damage in GaAs and GaP samples with different crystallographic orientations of the surface. The greatest damage was observed for (111) GaAs and (111) GaP samples. Possible mechanisms for the damage production are discussed.

The Optical Properties of Polydisperse Glass Microsphere Ensembles in the Infrared Spectral Region

Semitransparent composites with glass microspheres are promising modern materials. In particular, they contribute to the increase of the efficiency of heat-insulating coatings. An experimental study of the transmission and scattering of infrared radiation (at wavelengths from 2 to 25 μm) by a sodium borosilicate glass microsphere powder is presented. The spectra, angular-scattering patterns, and the degree of polarization of scattered radiation were studied.

Effect of free charge carriers on birefringence and dichroism in anisotropic porous silicon layers

The infrared optical properties of anisotropic mesoporous silicon films containing free charge carriers (holes) are studied experimentally and theoretically. The results of simulation of the optical properties of the produced samples in the effective-medium approximation show a heavy dependence of the birefringence, reflection anisotropy, and dichroism on the concentration of free charge carriers. The unsteady behavior of the differential transmittance recorded for the samples at mutually perpendicular polarization directions of light are attributed to the effect of charge carriers with a concentration on the order of 1019 cm–3. The results of the study suggest that anisotropic silicon nanostructures are promising materials for infrared photonics and terahertz engineering.

Effects of photon enhanced lifetime and form anisotropy in silicon nanowire arrays on efficiency of nonlinear-optical processes

Arrays of silicon nanowires (SiNWs) of about 100 nm in diameter formed by metal-assisted chemical etching of crystalline silicon (c-Si) substrates were studied regarding efficiency of optical interactions in them. Strong scattering in the SiNW arrays results in enhanced photon lifetime in them, which was evidenced by measurement of the cross-correlation function for femtosecond pulses scattered by the SiNW arrays. This effect results in an order of magnitude growth of efficiency such processes as spontaneous Raman scattering and third-harmonic (TH) generation compared to the signals for c-Si. Using (110) oriented c-Si substrate we managed to make well-oriented tilted SiNWs demonstrating strongly anisotropy of the TH and coherent anti-Stokes Raman scattering signals. In particular, efficiencies of these processes differ significantly when incident radiation propagates along or perpendicular to the SINWs. The obtained results could be explained by variations of the local fields in SiNWs and scattering cross...

Optical study of equilibrium charge carriers in mesoporous silicon

Absorption and reflection of IR radiation (500...6000 cm - 1 ) by equilibrium free charge carriers are studied in free standing mesoporous Si (meso-PS) films prepared from heavily boron-doped Si wafers (N p 10 2 0 cm - 3 ). The experimental results are well described by a model based on the Bruggeman effective medium approximation and classical Drude theory with a correction on additional carrier scattering on the Si nanocrystal surface. The concentration of free holes in Si nanocrystals is found to be ∼ 10 1 9 cm - 3 and ∼ 10 1 8 cm for the as-prepared and naturally oxidized films, respectively.

Optical anisotropy of nanostructured silicon films studied by Fourier transform infrared spectroscopy

Electrochemically nanostructured Si films with surface orientation (110) prepared at different current density were investigated by Fourier transform infrared spectroscopy. The spectra exhibit beats of interference fringes arisen from the summation of intensities of ordinary and extraordinary waves which interfere in the film. The investigated films are shown to exhibit properties of a negative uniaxial crystal (no > ne) with optical axis lying in the surface plane along [001] direction. The value of birefringence reaches 18% for nanostructured Si films with porosity of 80%. Experimental data agree with calculations based on the effective media approximation for anisotropically spaced Si nanocrystals.