L. I. Belogorokhova

Optical characterization of porous silicon embedded with CdSe nanoparticles

Arrays of a few nanometer-size clusters have been realized using a porous silicon (PS) matrix by filling its pores with CdSe. The photoluminescence (PL) peak from the embedded area of the PS samples with different luminescence stabilizes at 1.79 eV. This has been interpreted as due to emission from the CdSe clusters with an average size of about 3–5 nm. Likewise, the PL and Raman scattering spectra of the pure PS area of the samples have been compared with those obtained from the embedded areas. PL spectra were examined as a function of laser irradiation. Finally, to analyze the possibility of the formation of metal/porous semiconductor contacts, cross-section structures have been observed by scanning electron microscopy in the electron-beam-induced current mode.

Mixed vibrational modes of PbTe nanocrystallites

Far-infrared reflectance (20–700 cm−1) and Raman spectra of porous PbTe were studied. The spectral dependences found significantly differ from those observed in the case of bulk single-crystal PbTe. Aside from the transverse (TO) and longitudinal (LO) optical phonon modes, additional vibrational modes were detected in the spectra, whose frequencies are close to those in a bulk material. Their number and spectral positions are in satisfactory agreement with the results of calculations carried out to determine the vibrational mode frequencies for nanocrystallites taking into account boundary conditions at their surface. The macroscopic continuum model allows calculation of the frequencies of the mixed phonon modes in quantum dots by the simultaneous consideration of atomic displacements and the accompanying change in the electrostatic potential. It was found experimentally that the detected modes are mixed (LO-TO), as was predicted by theory. The data of atomic-force microscopy allowed us to estimate the average radius of quantum dots, which form the nanorelief of the porous PbTe layer.

A mechanism of oxygen-induced passivation of porous silicon in the HF: HCl: C2H5OH solutions

The problem of stabilizing the properties of porous silicon films was studied. A thermodynamical analysis of electrochemical processes occurring in the course of anodic Si dissolution is performed. A new description of electrode reaction of silicon interaction with hydrofluoric acid is suggested. It is shown that hydrogen-induced passivation of the Si surface governs the equilibrium dissolution potential of the silicon electrode. Thermodynamic calculations indicate that it is possible to substitute the chemically and thermally unstable surface groups (of the SiHx type) for more stable silicon-oxygen compounds directly in the course of formation of porous Si in electrolytes with the addition of strong hydrohalic acids. The results obtained made it possible to explain thermodynamically the stabilizing effect of an HCl additive in electrolytes used for formation of porous Si on its chemical and physical properties.

Optical properties of porous silicon layers processed with a HF:HCI:C2H5OH electrolyte

The properties of porous silicon samples prepared by adding hydrochloric acid to the usual hydrofluoric-acid electrolyte have been studied. These samples exhibit an intense photoluminescence that does not degrade over time and is unaffected by exposure to intense laser illumination. The peak photoluminescence from these layers of porous silicon occurs at photon energies of 1.85–1.9 eV. The photoluminescence signal from samples prepared in the standard way under the same initial conditions but without HCl in the composition of the electrolyte is two orders of magnitude less intense. Studies of the degradation of these porous silicon samples with time and exposure to various power levels of laser illumination revealed that the samples with the maximum content of HCl in the electrolyte composition emitted photoluminescence that was unaffected by laser illumination. In this work, the infrared spectra of the samples was measured in order to monitor the chemical state of their surfaces. It was found that the abrupt 100-fold increase in the intensity of the photoluminescence signal from samples made according to the method proposed here is associated both with distinctive features of the structure of porous silicon layers and with the presence of a thin crystalline layer of SiO2 on the surface of nanocrystallites.

Optical and photoelectric properties of CdxHg1−xTe solid solutions (0.35<x<0.40)

This analysis is made of the spectral dependences of the position of the fundamental absorption edge, the photoconductivity, and the optical spectra of CdxHg1−xTe (0.35<x<0.40) alloys at temperatures 5–300 K in the wave number region (30–5000) cm−1. The nonmonotonic dependence on temperature of the parameters considered is investigated. The reflection spectra are analyzed mathematically by means of dispersion analysis and the Kramers–Kroning relations. The results obtained are explained by the influence of polaron effects on the processes of charge transport in the crystal. It is conjectured that the polaron configuration changes at the characteristic temperatures 60, 160, and 225 K.

Atomic configuration and far-infrared optical spectra of lattice vibrations of ZnTe and CdTe modes in Cd1−xZnxTe (x = 0–0.2)

Lattice reflection spectra of Cd 1-x Zn x Te (x = 0-0.2) alloys have been analyzed using the Kramers-Kroning method, and the function of the crystal dielectric permeability has been restored from the reflection spectra using dispersion analysis with subsequent expansion into Lorenz contours of lattice oscillators. The oscillator frequencies correspond to four CdTe oscillation modes and four ZnTe modes in accordance with the model for the CdZnTe structure consisting of five Cd(4 - n)Zn(n)Te (n = 0-4) basis tetrahedral cells of Cd and Zn cations around one common Te anion. Analysis of the lattice oscillator strengths for compositions x = 0.06-0.2 revealed no deviation from the random Cd and Zn distribution over the cation sublattice. At x ≃ 0.05, the experimental distributions deviate drastically from the theoretically predicted oscillator strengths. For this reason, we treated the data for 0.04 < x < 0.05 under the assumption that the electronic density of the CdTe bonds is shifted toward Te. This phenomenon may initiate the formation of Te-Te centers with a localized hole and violates the cubic symmetry in the vicinity of Te atoms.

Features of the spectral dependences of transmittance of organic semiconductors based on tert-butyl substituted lutetium phthalocyanine molecules

Vibronic properties of organic semiconductors based on tert-butyl substituted phthalocyanine lutetium diphthalocyanine molecules are studied by IR and Raman spectroscopy. It is shown that substitution of several carbon atoms in initial phthalocyanine (Pc) ligands with 13C isotope atoms causes a spectral shift in the main absorption lines attributed to benzene, isoindol, and peripheral C-H groups. A comparison of spectral characteristics showed that the shift can vary from 3 to 1 cm−1.

Optical phonons of circular wires in porous GaP

The reflection spectra of porous gallium phosphide samples are investigated in the far infrared region of wave numbers (10–700 cm−1). In addition of the longitudinal and transverse optical phonon modes corresponding to the bulk material, additional vibrational modes are detected. Their number and spectral position are correctly described by a model of a dispersive dielectric medium under the assumption that porous gallium phosphide is formed by crystallites whose shape is close to cylindrical. It is concluded that such vibrational modes are optical phonons confined by the volume of the quantum wire. The experimental optical reflection spectra are used to obtain estimates of the average diameter of nanocrystallites forming the porous GaP layer.

The investigation of structural perfection of CdxHg1−xTe/CdZnTe epitaxial layers by the Raman scattering method

The distribution and character of variations in the intensity of lattice vibrations that are induced by structural defects of various nature over the layer depth of CdxHg1−xTe/CdZnTe epitaxial structures are investigated. The results are discussed in detail. Experimental results are obtained by the Raman scattering method in a micromode over the cleaved surface of the sample formed normally to the [111] face immediately prior to measurements. Special attention is paid to the possibility of obtaining information on the presence of structural defects in the cation sublattice and tellurium precipitates in CdxHg1−xTe epitaxial layers.

Vibronic states in organic semiconductors based on non-metal naphthalocyanine. Detection of heterocyclic phthalocyanine compounds in a flexible dielectric matrix

The vibronic properties of semiconductor structures based on non-metal naphthalocyanine molecules are studied using IR and Raman spectroscopy methods. New absorption lines in the transmission spectra of such materials are detected and identified. Three transmission lines are observed in the range 2830—3028 cm−1, which characterize carbon-hydrogen bonds of peripheral molecular groups. Their spectral positions are 2959, 2906, and 2866 cm−1. It is detected that the phthalocyanine ring can also exhibit its specific vibronic properties in the Raman spectra at 767, 717, and 679 cm−1. The naphthalocyanine molecule in the organic dielectric matrix of microfibers is described using IR spectroscopy. It is shown that the set of vibrations characterizing the isoindol group, pyrrole ring, naphtha group, and C-H bonds, allows an accurate enough description of the vibronic states of the naphthalocyanine complex in complex heterostructures to be made. The spectral range with fundamental modes, characterizing a naphthalocyanine semiconductor in a heterostructure, is 600–1600 cm−1. A comparison of the compositions of complex systems with a similar heterostructure containing lutetium diphthalocyanine demonstrated few errors.