N. N. Mel’nik

Raman spectra of structures with CdTe-, ZnTe-, and CdSe-based quantum dots and their relation to the fabrication technology

Quantum-dot structures based on the CdTe, ZnTe, and CdSe semiconductors are prepared by molecular-beam epitaxy, colloid chemistry methods, and ball milling, and their Raman spectra are studied. Localized longitudinal phonons are observed in all spectra. The dependence of the localized phonon frequency on the thickness of the ZnTe barrier in CdTe/ZnTe quantum-dot superlattices is used to derive the dispersion relation for longitudinal phonons in ZnTe. The Raman spectra of ensembles of colloidal quantum dots differ from the spectra of the other objects by the absence of tellurium bands and a strong intensity of the longitudinal phonon band of CdTe. It is revealed that the spectra depend on the technology employed to prepare quantum-dot structures.

High-pressure synthesis and characterization of superconducting boron-doped diamond

Abstract We present a scanning tunneling microscopy/spectroscopy (STM/STS) study of synthetic polycrystalline boron–doped diamond in the temperature range 0.5–4.3 K. At 4.3 K the sample–surface was very non–uniform and tunneling I(V) spectra were typical for p–type semiconductors. After cooling below the superconducting transition temperature, we detected and measured the superconducting gap of diamonds. At temperatures around 0.5 K the energy gap was around 0.8 and 1 mV (for two differentsamples).

Raman scattering study of NaNO2-infiltrated opal photonic crystals

We have studied light scattering in synthetic opal crystals infiltrated with ferroelectric sodium nitrite, NaNO2, and have analyzed simple models for the energy band structure of photonic crystals. Expressions have been derived for the group velocity of photons whose energy is close to the photonic band gap. Our results indicate that the infiltration of photonic crystals with NaNO2 markedly increases the Raman scattering intensity.

Anharmonicity of short-wavelength acoustic phonons in silicon at high temperatures

Second-order Raman spectra corresponding to transverse acoustic phonons are studied in detail for crystalline Si over the temperature range 20–620°C. The largest relative softening and anharmonicity at the boundaries of the Brillouin zone were observed for the TA(X) mode. Extrapolation of the TA(X) frequency to high temperatures suggests that the Si lattice should be dynamically unstable at temperatures on the order of a doubled melting temperature. It is found that the main contribution to the softening of the transverse acoustic phonons in silicon comes from the anharmonicity and not from the volume expansion.

Synthesis, structure, and physical properties of boron-doped diamond

Microcrystalline boron-doped diamond powders consisting of octahedrally faceted crystals have been synthesized in the C-H-B system at a pressure of 8 GPa and temperatures above 2000 K. The presence of boron has been shown to reduce the parameters of diamond synthesis compared to the binary system C-H (naphthalene). One possible reason for the reduction in synthesis parameters is the formation of less perfect graphite in the boron system in an intermediate step of diamond synthesis. At B/(C + B) ratios of about 5–10 at % in the C-H-B (naphthalene + boron) system, superconducting diamond microcrystals have been synthesized.

Two-mode nature of the Raman spectrum of lithium niobate crystals

The ordering of the host and impurity cations in nominally pure (with different values of Li/Nbratio) and doped lithium niobate crystals is studied using Raman spectroscopy. It is shown that depending on the composition-controlled ordering of structural units of the cationic sublattice the crystal may exhibit, in the region of bridge stretching vibrations of the oxygen ions, either single-mode or two-mode behavior. The nominally pure lithium niobate crystals show, within the homogeneity region, single-mode behavior, while the crystals doped with divalent or trivalent cations show single-mode behavior at low concentrations of the dopant and two-mode behavior at higher concentrations.

Ion implantation of porous gallium phosphide

The effect of irradiation by Ar ions and thermal annealing on the properties of porous gallium phosphide (por-GaP) obtained by electrolytic methods is investigated. It is shown on the basis of Raman scattering and photoluminescence data that, in contrast with porous silicon, por-GaP does not have high radiation hardness, and that thermal annealing of defects in layers amorphized by ion implantation is impeded by the absence of a good crystal base for solid-state epitaxial recrystallization processes. Data on radiation-induced defect formation and from probing of the material with a rare-earth “luminescence probe” are consistent with a mesoporous structure of the material.

The role of interdiffusion and spatial confinement in the formation of resonant raman spectra of Ge/Si(100) heterostructures with quantum-dot arrays

The phonon modes of self-assembled Ge/Si quantum dots grown by molecular-beam epitaxy in an apparatus integrated with a chamber of the scanning tunneling microscope into a single high-vacuum system are investigated using Raman spectroscopy. It is revealed that the Ge-Ge and Si-Ge vibrational modes are considerably enhanced upon excitation of excitons between the valence band Λ3 and the conduction band Λ1 (the E1 and E1 + Δ1 transitions). This makes it possible to observe the Raman spectrum of very small amounts of germanium, such as one layer of quantum dots with a germanium layer thickness of ≈10 Å. The enhancement of these modes suggests a strong electron-phonon interaction of the vibrational modes with the E1 and E1 + Δ1 excitons in the quantum dot. It is demonstrated that the frequency of the Ge-Ge mode decreases by 10 cm−1 with a decrease in the thickness of the Ge layer from 10 to 6 Å due to the spatial-confinement effect. The optimum thickness of the Ge layer for which the size dispersion of quantum dots is minimum is determined.

Raman spectra and restructurizations in the system of solid solutions NaTa y Nb1−y O3

By recording the Raman spectra, concentrational restructurizations in the system of solid solutions NaTayNb1−yO3 were investigated. The regions of relatively stable existence of solid solutions (y<0.2, y>0.8) and the region of a rather high disordering (0.2<y<0.9) with maximum deformation of the elementary cell at y=0.5 have been revealed. It is found that at y=0.5 a concentrational phase transition is observed, and at y ∼ 0.25 and 0.8 concentrational restructurizations occur. It is shown that the octahedral anions of BO6 are centrosymmetric when y<0.2 and y>0.9, and the system of solid solutions NaTayNb1−yO3 is antiferroelectric when y<0.2. In the range of concentrations 0.2<y< 0.9, the structure of NaTayNb1−yO3 is characterized by the presence of polar clusters.

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.