T. N. Zavaritskaya

Preparation and study of the optical properties of porous graphite

Optical properties of porous graphite samples prepared by electrochemical etching were investigated. It is found that electrochemical etching modifies their Raman spectra and gives rise to photoluminescence. The evolution of Raman spectra at the initial etching stages is studied in detail. A model is proposed explaining the salient features of the observed Raman spectra. It assumes the appearance of graphite nanoparticles and the formation of sp3 bonds between the graphite planes at the nanocrystal boundaries.

Raman and X-ray studies of nanocrystals in porous stain-etched germanium

Abstract Using a combination of stain-etching with subsequent annealing in hydrogen, porous germanium films with a high concentration of germanium nanocrystals (NCs) were prepared for the first time. Structural studies of the films were performed by X-ray reflectometry and high-resolution triple-crystal diffractometry and Raman light spectroscopy methods. From a thorough analysis of the experimental data obtained by both methods, it was revealed that the films consist of germanium NCs with average sizes approximately 8–10 nm in annealed films. Other basic information about thickness, porosity, roughness, and lateral coherence length of the films is also presented.

Raman scattering and hot luminescence spectra of Zn1 − xMnxTe quantum wires

The Raman scattering and luminescence spectra of Zn1 − xMnxTe (0 ≤ x ≤ 0.6) quantum wires have been investigated. The quantum wires have been grown by molecular-beam epitaxy on the (100)GaAs substrate with Au used as a catalyst. The spectrum of optical phonons in ZnMnTe quantum wires varies with a variation in x in accordance with an intermediate (between one- and two-mode) type of transformation. The optical phonon spectrum has been analyzed in terms of the microscopic theory. It has been demonstrated that the experimental data can be brought in accord with the theory by properly modifying the calculated density of phonon states for ZnTe. The spatial confinement has been found to affect the electronic states in Zn1 − xMnxTe quantum wires.

Photoluminescence and exciton resonances over the scattered light in multiphonon spectra of resonant scattering in the CdTe/ZnTe superlattices with narrow quantum wells

Photoluminescence and Raman scattering spectra in CdTe/ZnTe heterostructures and superlattices with narrow quantum wells (4.8–9.2 Å) in a temperature range of 5–300 K have been measured. The temperature dependences of the intensity of exciton luminescence in isolated quantum wells have been studied, and the thermal activation energies associated with the effective barriers for electrons and holes have been determined. In CdTe/ZnTe heterostructures, the binding energies of an exciton with a heavy hole have been determined as functions of the quantum well width. The multiphonon Raman spectra that exhibit distinctive features, such as the weak intensity of nLO phonon lines of ZnTe (n < 8), the absence of their dependence on the number n (n > 2), and the multiple participation in scattering of acoustic LA phonons with large wave vector, have been investigated. The results have been explained based on the concept of the relaxation of hot excitons over the exciton band.

Optical phonons in the bulk and on the surface of ZnO and ZnTe/ZnO nanowires in Raman spectra

Raman spectra of ZnO and core-shell ZnTe/ZnO nanowires have been measured under conditions of nonresonant and resonant excitations by Ar+ and He-Cd lasers. The optical vibration frequencies that are characteristic of the wurtzite structure of ZnO crystals have been determined. The Raman-active longitudinal optical (LO) phonons in all the studied structures have a mixed A1 and E1 symmetry. Surface optical modes with frequencies of 460–470 cm−1 have been found. Surface optical modes of single-layer nanowires have been analyzed using the calculation performed in the previous work by P.M. Chassaing and his colleagues, and those of double-layer nanowires have been analyzed in terms of the expressions obtained in the present work. The size of heterogeneities of the nanowires along their axis has been determined.

Manifestation of the composition inhomogeneity of Zn1 − xMgxTe quantum wires in Raman spectra

The resonant Raman spectra of Zn1 − xMgxTe quantum wires have been investigated. The dependences of the frequencies of longitudinal optical phonons of the ZnTe-like and MgTe-like modes (LO1 and LO2) on the photon energy have been found. The character of these dependences correlates with the variation in the frequencies of optical phonons of the Zn1 − xMgxTe alloys on the composition (x). This gives grounds to assume that the quantum wires are inhomogeneous in the composition. This assumption is also confirmed by the fact that the intensity ratio of the LO2 and LO1 modes in the Raman spectra increases with increasing excitation energy.

Diamond-like and graphite-like carbon states in short-period superlattices

The Raman and luminescence spectra are studied in superlattices consisting of carbon layers separated by thin SiC barrier layers. It is shown experimentally that, upon the avalanche annealing of an initially amorphous superlattice, the carbon layers can crystallize into either a diamond-like or graphite-like structure, depending on the geometrical parameters of the superlattice. A method is proposed for obtaining carbon films with a specified crystal modification within a unified technology.

Multilayer graphene structure of carbon in short-period superlattices

The Raman and photoluminescence spectra of short-period C/SiC superlattices produced by RF magnetron sputtering are investigated. The Raman data indicate that, in 35-period Sitall/Ni/[C/SiC] superlattices with the C and SiC effective thicknesses of 3.5 and 3 Å, respectively, subjected to postgrowth avalanche annealing, the carbon layers assume the structure of multilayer graphene with 3–5 graphene sheets per superlattice period. A method for the fabrication of graphene-like carbon structures on the basis of short-period superlattices grown by RF sputtering is suggested and implemented.

Manifestation of outgoing resonance in stokes and anti-stokes spectra of ZnTe and ZnMgTe quantum wires

The spectra of resonant light scattering by ZnTe quantum wires have been measured at excitation energies of 2.18–2.72 eV. The quantum wires have been grown on Si(100) and GaAs(100) substrates by molecular beam epitaxy. The effect of outgoing resonance with the electron transition energy E0 on the intensity of phonon lines of the Stokes spectrum and on the intensity ratio of the Stokes and anti-Stokes spectral lines has been studied. The energy E0 has been determined in ZnTe and ZnMgTe quantum wires from the edge luminescence spectra.

Electrodynamic properties of nanoporous silicon in the range from terahertz to infrared frequencies

The dielectric response (conductivity and permittivity) spectra of a series of nanoporous silicon samples prepared by anodization of low-resistivity single-crystal silicon are measured, for the first time, using terahertz and IR spectroscopy in the frequency range 7–4000 cm−1 at room temperature. The spectra obtained are analyzed in terms of the effective medium theory with a size-dependent dielectric response function of nanoinclusions and averaged dielectric characteristics of the surrounding medium. The geometric and dielectric characteristics of silicon nanoinclusions are determined. The dielectric properties of inclusions are found to be affected by nanosize effects, namely, carrier scattering at crystallite boundaries and a broadening of the band gap due to quantum confinement. The spectra of the samples prepared by adding iodine to the electrolyte exhibit a resonance at frequencies of 150–300 cm−1. The nature of the resonance can be associated with the presence of chemisorbed iodine on the surface of porous silicon. Possible mechanisms responsible for the changes in broadband conductivity and permittivity spectra of single-crystal silicon upon transformation into a nanoporous structure are discussed.