Search

We report the results of systematic experimental and theoretical studies of structural and dynamical properties of short-period GaN/AlN superlattices. The multilayer structures with the thicknesses of the constituent layers varying from two to several monolayers are grown using the submonolayer digital molecular beam epitaxy technique. In the framework of density functional theory, the lattice dynamics properties of the superlattices are studied. Good agreement between the experiment and theory is found, which made it possible to establish unambiguously a relationship between the features observed in the Raman spectra and the microscopic nature of the acoustic and optical phonon modes. The results obtained enhance the capabilities of Raman spectroscopy as a fast and non-destructive characterization method of short-period GaN/AlN superlattices and can be used to optimize growth process parameters for the fabrication of structurally perfect low-dimensional heterostructures.

In this work a comprehensive study is presented for the analysis of epitaxial graphene layers using Raman spectroscopy. A wide range of graphene types is covered, from defective/polycrystalline single layer graphene to multilayer graphene with low defect density. On this basis the influence of strain type, Fermi level and number of layers on the Raman spectrum of graphene is investigated. A detailed view on the 2 D / G dispersion and the respective slopes of uniaxially and biaxially strained graphene is given and its implications on the asymmetry of the G peak analyzed. A linear dependency of the phonon mode asymmetry on uniaxial strain is presented in addition to the known Fermi level dependence. Additional impacts on the asymmetry are found to be arising from the defect density and transfer doping of adsorbates. The discovered transfer doping mechanism is contrary to pure phonon excitation through excitons and exhibits increasing asymmetry with increasing Fermi level. A new characteristic correlation between the 2 D mode line width and the inverse I(D)/I(G) ratio is introduced that allows the determination of the strain type and layer number and explains the difference between Raman line widths of monolayer graphene on different substrates.

Raman spectroscopy is used to evaluate the structural perfection of epitaxial graphene films before and after deposition of a Ni layer to their surface by magnetron sputtering. Two deposition modes with different gas pressures and deposition times are investigated. It is found that Ni deposition under low pressure combined with long deposition time does not lead to the separation of graphene/Ni film. On the other hand, higher pressure and shorter deposition time results in successful but uncontrollable exfoliation of graphene together with the Ni film. The results obtained will serve as the basis for the optimization of Ni deposition modes, in order to achieve complete exfoliation of the graphene film from the SiC substrate without damaging the graphene layer.

The buffer layer samples grown on the Si-face of the 4H- and 6H-SiC substrates were heated in a hydrogen flow at different temperatures (600–900 °C) and heating times (40–60 min) in order t...

Spectral and nonlinear optical properties of InAs nanowires synthesized via molecular beam epitaxy suspended in isopropanol are studied in this work at room temperature. The absorption spectra with a characteristic maximum at a wavelength of 235 nm, as well as the Raman spectra in a range of 180–280 cm–1, are obtained. The nonlinear transmittance of InAs nanowires with diameters less than the exciton Bohr radius was firstly investigated by a two-photon excitation in the transparent region (λ = 468 nm). The effect of cumulative bleaching, the manifestation of which is associated with the dynamic Stark effect, was discovered.

Systematic studies of the effect of the electron concentration on the Raman spectra of single-layer graphene films have been carried out. The samples were grown by thermal destruction of the Si-face of the 4H-SiC substrate. Analysis of the results led us to the conclusion that for the correct estimation of the electron concentration and strain values in graphene using Raman spectroscopy data it is necessary to take into account the value of the Fermi velocity in the graphene layer. This conclusion is valid for graphene on any other substrate as well, since the Fermi velocity in graphene depends on the dielectric constant of the substrate.

We report the results of systematic Raman spectroscopy studies of AlxGa1 –xN (x ~ 0.75) layers grown using plasma-assisted molecular beam epitaxy at various stoichiometric conditions and growth fluxes. The high-intensity asymmetric low-frequency peak obeying Bose statistics is discovered in Raman spectra of the layers grown by temperature-modulated epitaxy at strongly Ga-enriched conditions. Theoretical model is developed to explain the origin and the high intensity of the observed low-frequency peak, which is attributed to the presence of excessive metallic gallium in AlGaN layers and can be explained by vibrations of gallium clusters with a diameter of ~1 nm. The nature of the low-frequency peak is similar to that of the boson peak in glasses, which occupies the same frequency range in Raman spectra. We demonstrate the capabilities of Raman spectroscopy as an express and non-destructive technique for optimization of growth conditions of AlGaN layers to achieve simultaneously the atomically-smooth droplet-free surface morphology and the high structural quality.

We present an extensive theoretical and experimental study to identify the effect on the Raman spectrum due to interface interdiffusion between GaN and AlN layers in short-period GaN/AlN superlattices (SLs). The Raman spectra for SLs with sharp interfaces and with different degree of interface diffusion are simulated by ab initio calculations and within the framework of the random-element isodisplacement model. The comparison of the results of theoretical calculations and experimental data obtained on PA MBE and MOVPE grown SLs, showed that the bands related to A1(LO) confined phonons are very sensitive to the degree of interface diffusion. As a result, a correlation between the Raman spectra in the range of A1(LO) confined phonons and the interface quality in SLs is obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy.

The results of studies of the structural and optical properties of two-dimensional GaSe layers grown by molecular-beam epitaxy on GaAs(001) and GaAs(112) substrates using a valve cracking cell for the Se source are reported. The influence of the MBE growth parameters (the substrate temperature, Ga flux intensity, Se/Ga incident flux ratio) on the surface morphology of the layers is studied. By means of transmission electron microscopy, electron diffraction technique, and Raman spectroscopy, it is shown that the structure of the GaSe layers corresponds to the γ-GaSe polytype. From X-ray diffraction analysis, it is established that there exist α-Ga2Se3 inclusions in the GaSe layers grown under conditions of high enrichment of the growth surface with Se.

Multigraphene films have been for the first time obtained on the surface of quartz glass via carbonization of polyimide Langmuir–Blodgett films. The Raman spectra of the films show bands G and D and a broad band at 2300–3200 cm–1, which are characteristic of graphite-like films. The transmission electron microscopy demonstrated that the films are constituted by “stacks” of layers spaced by 0.36 nm. The surface resistance of the 5-nm-thick films was 1.2 ± 0.2 kΩ, and the transmittance was 87% at λ = 550 nm. The transmittance was 97% in the IR part of the spectrum and 70–80% in the UV part.

We investigated the growth of III-N/graphene heterostructures as a single process in a MOVPE reactor. Raman spectra revealed that graphene can be successfully deposited on sapphire substrate by propane pyrolysis if temperature exceeds 1060 °C and hydrogen is used as a carrier gas. GaN epitaxial layers and heterostructures on such graphene using both high-temperature AlN buffer layer and low-temperature GaN nucleation layer was demonstrated. Analysis of surface morphology and X-Ray diffraction curves indicate that GaN quality depends on graphene thickness. Use of copper electroplated Ni-based contact layer combined with thermal shock allows exfoliation of large-area III-N LED structures from sapphire.

Transition metal oxyborates M3(BO3)2 (M = Mn, Co, Ni) belong to a group of materials with the kotoite structure and many of them demonstrate complex magnetic behavior as a result of competing interactions between magnetic ions in nonequivalent crystallographic positions. In this paper, we report results of the detailed infrared and Raman spectroscopic studies of a Co3(BO3)2 single crystal. Most of the phonons are observed and identified. Unusually large phonon shifts, in comparison to isostructural compound Ni3(BO3)2, were registered in the low- and mid-infrared frequency range (100–700 cm−1) of the spectra. The experimental findings are supported by the ab initio simulations of the lattice dynamics, which allow us to propose the normal-mode assignments. We argue that the large phonon shift between cobalt and nickel compounds are associated with changes of force constants induced by the changes of M–O bond lengths within the two types of octahedral MO6 groups. In addition to the first-order Raman scattering, intense second-order Raman scattering was detected due to the resonant enhancement regime. Furthermore, we performed the single crystal XRD experiments which did not show any detected evidences of magneto-structural phase transition previously observed in Ni3(BO3)2.

We report the results of experimental and theoretical studies of phonon modes in GaN/AlN superlattices (SLs) with a period of several atomic layers, grown by submonolayer digital plasma-assisted molecular-beam epitaxy, which have a great potential for use in quantum and stress engineering. Using detailed group-theoretical analysis, the genesis of the SL vibrational modes from the modes of bulk AlN and GaN crystals is established. Ab initio calculations in the framework of the density functional theory, aimed at studying the phonon states, are performed for SLs with both equal and unequal layer thicknesses. The frequencies of the vibrational modes are calculated, and atomic displacement patterns are obtained. Raman spectra are calculated and compared with the experimental ones. The results of the ab initio calculations are in good agreement with the experimental Raman spectra and the results of the group-theoretical analysis. As a result of comprehensive studies, the correlations between the parameters of acoustic and optical phonons and the structure of SLs are obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy. The results obtained can be used to optimize the growth technologies aimed to form structurally perfect short-period GaN/AlN SLs.

The structural, chemical, and electronic properties of epitaxial graphene films grown by thermal decomposition of the Si-face of a semi-insulating 6H-SiC substrate in an argon environment are studied by Raman spectroscopy, atomic-force microscopy, the low-energy electron diffraction method, X-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy and X-ray absorption spectroscopy at the carbon K edge. It is shown that the results of a systematic integrated study make it possible to optimize the growth parameters and develop a reliable technology for the growth of high-quality single-layer graphene films with a small fraction of bilayer graphene inclusions.

The intense absorption of CO2 laser radiation in sapphire is used to separate GaN films from GaN templates on sapphire. Scanning of the sapphire substrate by the laser leads to the thermal dissociation of GaN at the GaN/sapphire interface and to the detachment of GaN films from the sapphire. The threshold density of the laser energy at which n-GaN started to dissociate is 1.6 ± 0.5 J/cm2. The mechanical-stress distribution and the surface morphology of GaN films and sapphire substrates before and after laser lift-off are studied by Raman spectroscopy, atomic-force microscopy, and scanning electron microscopy. A vertical Schottky diode with a forward current density of 100 A/cm2 at a voltage of 2 V and a maximum reverse voltage of 150 V is fabricated on the basis of a 9-μm-thick detached n-GaN film.

For the first time it has been experimentally shown that a powder of detonation nanodiamonds (DND) and a saturated acyclic hydrocarbon, mono- or dibasic alcohol, used as the reaction mixture after treatment at high pressures (5–8 GPa) and high temperatures (1300–1800°C) results in the formation of diamond single crystals up to 15 micron in size. The Raman spectrum indicates that the diamonds have a perfect of crystal structure. It has been suggested that the oriented attachment mechanism is responsible for growth of micrometer-size diamond single crystals out of DND particles with sizes of about 5 nm under these technological conditions.

The results are presented on phonon excitations and the electronic structure of Co2TiO4 inverse spinel in which magnetically ordered cobalt ions Co2+ (3d 7) are in equal amounts in tetrahedral and octahedral sublattices below T C = 56 K. Single crystals are studied using optical reflection and absorption in a wide spectral range, Raman scattering, and dielectric spectroscopy methods. The dynamics of infrared and Raman-active phonons are studied, and the features associated with disordering in tetrahedral sites are detected. The d–d electronic transitions recorded in the regions of 3800 and 6300 cm–1 confirm the coordination features of Co2+ ions. An increase in the permittivity in the temperature region below 130 K is detected.

The results of experimental and theoretical studies of phonon modes in short-period GaN/AlN superlattices grown by PA MBE on the (0001) Al2O3 substrate are reported. In the framework of Density Functional Theory and Elastic Continuum Model the lattice dynamics properties are studied of (GaN)m(AlN)n SLs (m+n≤24), where n and m are the number of monolayers. Good agreement between experiment and theory is found, which made it possible to establish unambiguously a relationship between the features observed in the Raman spectra and the microscopic nature of the acoustic phonon modes. The obtained results open new possibilities for analyzing the structural properties of GaN/AlN SLs by Raman spectroscopy.

Graphene is considered as a promising candidate for manufacturing of sensors due to its extreme sensitivity to molecule absorption. In this work, we show the connection between the electrical and optical properties of epitaxial graphene chips grown on 4H-SiC and intended for the production of protein-based sensors. Using of a complex of techniques, including Raman spectroscopy, atomic force microscopy, Kelvin probe microscopy, study of I-V characteristics and low-frequency noise, it is shown that the character of frequency dependence of the spectral density of voltage fluctuations and its value at a frequency of 1 Hz can be used for classification and selection of graphene chips for their application as sensors. Classification of the graphene chips will allow more efficient development of graphene-based biosensors.

In the context of the dielectric continuum model, a correlation is established between the frequencies of delocalized A(TO) and E(LO) phonons in the Raman spectra of short-period AlN/GaN superlattices and the ratio between the layer thicknesses in the structure. It is shown that elastic strains produced in the materials of the layer during superlattice growth only slightly influence the correlation dependence of the frequencies of A(TO) and E(LO) modes with high intensities in the Raman spectra on the structural parameter defining the ratios between the layer thicknesses. The results of calculations of the phonon frequencies are in good agreement with available experimental data and can be used for spectroscopic diagnostics of AlN/GaN superlattices.

The graphene growth by thermal decomposition of silicon carbide at the temperature of ~1400°C in a high vacuum of ~10–6 Torr has been optimized. By Raman spectroscopy, the mean thickness of obtained graphene (2–4 single layers) has been estimated and the presence of high-quality graphene areas in the samples has been demonstrated. It has been found out that the four-point resistance of graphene increases in the region of its interface with water approximately by 25%. For the graphene–water interface in the transistor geometry, with variation in the gate-to-source voltage, the field effect corresponding to the hole type of charge carries in graphene has been revealed.

The process of formation of graphene layers on a copper substrate is studied as a function of the pressure in the growth chamber. It is shown that the graphene layers form by nucleation and growth of graphene nuclei that later combine into a continuous layer. Growth in the pressure is accompanied by thickening of the synthesized graphene, intensification of compressive stresses, and appearance of structure defects.

The main problem of the epitaxial growth of thick AlN layers on a Si substrate consists in the formation of cracks, which complicates the application of structures of this kind in the fabrication of semiconductor devices. The possibility of obtaining crack-free AlN layers with a thickness exceeding 1 μm and a mirror- smooth surface by hydride vapor-phase epitaxy is demonstrated. The properties of the layers are studied by X-diffraction analysis, optical and scanning electron microscopy, and Raman spectroscopy.

Epitaxial graphene layers are produced with the aid of thermal destruction of the surface of a semi-insulating SiC substrate. Raman spectroscopy and atomic-force microscopy are employed in the study of the film homogeneity. A prototype of the gas sensor based on the films is fabricated. The device is sensitive to the NO2 molecules at a level of 5 ppb (five particles per billion). A possibility of the industrial application of the sensor is discussed.

We report the study of electrical transport in few-layered CVD-graphene located on nanostructured surfaces in view of its potential application as a transparent contact to optoelectronic devices. Two specific surfaces with a different characteristic feature scale are analyzed: semiconductor micropyramids covered with SiO2 layer and opal structures composed of SiO2 nanospheres. Scanning tunneling microscopy (STM) and scanning electron microscopy (SEM), as well as Raman spectroscopy, have been used to determine graphene/substrate surface profile. The graphene transfer on the opal face centered cubic arrangement of spheres with a diameter of 230 nm leads to graphene corrugation (graphene partially reproduces the opal surface profile). This structure results in a reduction by more than 3 times of the graphene sheet conductivity compared to the conductivity of reference graphene located on a planar SiO2 surface but does not affect the contact resistance to graphene. The graphene transfer onto an organized array of micropyramids results in a graphene suspension. Unlike opal, the graphene suspension on pyramids leads to a reduction of both the contact resistance and the sheet resistance of graphene compared to resistance of the reference graphene/flat SiO2 sample. The sample annealing is favorable to improve the contact resistance to CVD-graphene; however, it leads to the increase of its sheet resistance.

Comprehensive study of high-quality monolayer graphene samples grown by thermal destruction of the Si-face of the 4H-SiC substrate was carried out. Analysis of the data obtained by Raman spectroscopy and angle-resolved photoemission spectroscopy suggest the need to use the Fermi velocity in the graphene layer under study to obtain a correct estimate of the electron concentration and strain values using Raman data. This statement is valid not only for graphene on the SiC substrate, but for graphene on any other substrate as well, since the Fermi velocity in graphene depends on the dielectric constant of the substrate.

AlN/GaN superlattices (SL) grown by metalorganic vapor-phase epitaxy with the period of SLs varied from 20 nm to 140 nm, and the thickness of the structures ranged from 0.7 to 1 µm were studied by polarized Raman spectroscopy technique. The peaks within a complex spectral feature of the symmetry A1(TO) observed at about 580 cm−1, were assigned to the interface and quasi-confined phonons. Frequency splitting between the peaks was found monotonously increasing along with the SL period. This dependence was explained using the dielectric continuum model. A method based on this finding was proposed for estimating the period SL using Raman spectroscopy. This method extends the traditional approach based on the studies of folded acoustic phonons in the short-period SLs.

A complete group-theoretical analysis of vibrational modes in ε, β, γ-GaSe polytypes has been performed. Raman spectroscopy data obtained on single crystals of ε-GaSe with a subsequent group-theoretical analysis were used to analyze the Raman spectra low-dimensional GaSe structures. Information was obtained on the structural properties of epitaxial layers and arrays of nanorods of the ε-polytype GaSe grown by molecular beam epitaxy on GaAs (001) substrates.

The structural, chemical, and electronic characteristics of graphene grown by thermal decomposition of a singlecrystal SiC substrate in Ar atmosphere are presented. It is shown that this technology allows the creation of high-quality monolayer graphene films with a small fraction of bilayer graphene inclusions. The performance of graphene on SiC as a gas sensor or a biosensor was tested. The sensitivity of gas sensors to NO2 on the order of 1 ppb and that of biosensors to fluorescein with concentration on the order of 1 ng/mL and to bovine serum albumin-fluorescein conjugate with concentration on the order of 1 ng/mL were determined.

High quality single crystal SiC films were prepared by carbonization of polyimide Langmuir-Blodgett films on Si substrate. The films formed after annealing of the polyimide films at 1000°C, 1100°C, 1200°C were studied by Fourier transform-infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Raman spectroscopy, transmission electon microscopy (TEM), transmission electron diffraction (TED), and scanning electron microscopy (SEM). XRD study and HRTEM cross-section revealed that the crystalline SiC film begins to grow on Si (111) substrate at 1000°C. According to the HRTEM cross-section image five planes in 3C-SiC (111) film are aligned with four Si(111) planes at the SiC/Si interface. It was shown the SiC films (35 nm) grown on Si(111) at 1200°C have mainly cubic 3C-SiC structure with a little presence of hexagonal polytypes. Only 3C-SiC films (30 nm) were formed on Si (100) substrate at the same temperature. It was shown the SiC films (30-35 nm) are able to cover the voids in Si substrate with size up to 10 μm.

Fermi level pinning at the oxidized (110) surfaces of III-As nanowires (GaAs, InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe microscopy, we show that the Fermi level at oxidized cleavage surfaces of ternary Al$_{x}$Ga$_{1-x}$As (0$\le$x$\le$0.45) and Ga$_{x}$In$_{1-x}$As (0$\le$x$\le$1) alloys is pinned at the same position of 4.8$\pm$0.1 eV with regard to the vacuum level. The finding implies a unified mechanism of the Fermi level pinning for such surfaces. Further investigation, performed by Raman scattering and photoluminescence spectroscopy, shows that photooxidation of the Al$_{x}$Ga$_{1-x}$As and Ga$_{x}$In$_{1-x}$As nanowires leads to the accumulation of an excess arsenic on their crystal surfaces which is accompanied by a strong decrease of the band-edge photoluminescence intensity. We conclude that the surface excess arsenic in crystalline or amorphous forms is responsible for the Fermi level pinning at oxidized (110) surfaces of III-As nanowires., Comment: 7 pages, 3 figures

The Raman scattering spectra of nanowire samples in the region of fundamental modes (50–320 cm−1) are studied to analyze the structural peculiarities of GaAs nanowires (nanowhiskers) grown on a Si (111) substrate. Factor analysis of the experimental data array consisting of 55 spectra, each of which contains 300 points, is carried out. It is found that the number of linearly independent contributions in these data arrays amounts to only two spectra. The form of each linearly independent contribution is close to zincblende (ZB) and wurtzite (WZ) spectra. It is established that ZB and WZ phases exist in nanowhiskers. Comparison with the calculated frequencies of the normal vibrations of hexagonal GaAs polytypes makes it possible to assume that the spectra of crystal nanowhiskers are combinations of ZB and WZ spectra with good accuracy and agree with the scheme of phonon branch folding of the ZB Brillouin zone.

The electric and structural characteristics of silicon-doped GaN and Al0.3Ga0.7N layers grown by molecular beam epitaxy (MBE) using silane have been analyzed by the Hall effect, Raman spectroscopy, and high-resolution X-ray diffractometry. It is established that the electron concentration linearly increases up to n = 4 × 1020 cm−3 with an increase in the silane flow rate for GaN:Si, whereas the corresponding dependence for Al0.3Ga0.7N:Si is sublinear and the maximum electron concentration is found to be n = 4 × 1019 cm−3. X-ray measurements of sample macrobending indicate a decrease in biaxial compressive stress with an increase in the electron concentration in both GaN:Si and Al0.3Ga0.7N:Si layers. The parameters of the dislocation structure, estimated from the measured broadenings of X-ray reflections, are analyzed.

Optical and structural properties of thick InGaN layers grown by MOCVD and MBE were studied by photoluminescence, optical transmission and Raman spectroscopies and X-ray diffraction analysis. Optical bandgap, Urbach energy, and full widths at half maximum (FWHM) of photoluminescence and Raman spectra depending on the InGaN alloy composition were determined experimentally. Minimal theoretical linewidth of photoluminescence spectra resulted from random distribution of In and Ga atoms in cation sublattice was calculated.

Photorefractive properties and structural and optical homogeneity of (1) LiNbO3:Cu crystals ([Cu] = 0.015 mas %) grown from a congruent melt, (2) nominally pure stoichiometric crystals grown from a melt with 58.6 mol % of Li2O (LiNbO3stoich), and (3) nominally pure congruent crystals (LiNbO3congr) have been studied using the Raman-spectroscopy method with excitation in the UV, visible, and near-IR ranges; the laser-conoscopy method; and the electron paramagnetic resonance-spectroscopy method. In optically uniaxial LiNbO3 crystals, a weak optical biaxiality has been revealed, which is attributed to an insignificant deformation of the optical indicatrix. This deformation can be caused both by the initial structural inhomogeneity of crystals and by the photorefractive effect. It has been shown that, under the action of light, charge exchange of copper cations Cu2+ → Cu+ takes place in the crystal LiNbO3:Cu ([Cu] = 0.015 mas %). The LiNbO3:Cu crystal exhibits photorefractive properties not only because of the occurrence of intrinsic defects with electrons localized at them, as is the case with the LiNbO3stoich and LiNbO3congr crystals, but also due to the charge exchange of copper cations under the action of the laser radiation.

The structure, energy of formation, and Raman spectra of several polytypes (3C, 2H, 4H, and 8H) of gallium arsenide GaAs have been investigated using quantum mechanical calculations based on the local density functional theory. It has been found that the energy of the formation of hexagonal polytypes increases with an increase in the length of the periodicity and approaches the value corresponding to the ground state, i.e., to the structure of the 3C polytype. It has been shown that the calculated frequencies of normal vibrations of different polytypes are consistent, with good accuracy (±6 cm−1), with the scheme of folding of the phonon branches. In the calculated Raman spectra of the polytypes, there are new lines (forbidden in the spectrum of the 3C polytype) which can serve as characteristic lines of other polytypes. Similar lines can be found in the Raman spectra of GaAs nanowhiskers. This result has opened up new prospects for the application of Raman spectroscopy to the characterization of the structure of these nano-objects.

Raman scattering with excitation in the ultraviolet, visible, and near infrared and electron paramagnetic resonance are used to study the photorefractive properties of nominally pure, stoichiometric (R = Li/Nb = 1) single crystals of lithium niobate grown from a melt with 58.6 mol% Li2O (LiNbO3stoich), nominally pure crystals with a congruent composition (LiNbO3congr, R = 0.946), and Cu-doped crystals with a congruent composition (LiNbO3:Cu, [Cu] = 0.015 mass%). Optical irradiation is found to cause charge exchange of copper cations, Ca2+ → Cu, in the LiNbO3:Cu ([Cu] = 0.015 mass%) crystals. Thus, crystalline LiNbO3:Cu ([Cu] = 0.015 mass%) manifests photorefractive properties as a result of charge exchange of copper ions during laser irradiation, in addition to the contribution from intrinsic defects with localized electrons previously observed in LiNbO3stoich and LiNbO3congr.

We report on the first observation of folded acoustic phonons in the Raman spectra of short-period AlN/GaN superlattices (SLs). The values of longitudinal and transverse sound velocities along the [0001] direction of SL were estimated. It is shown that the SL period can be evaluated from Raman data with a good accuracy. Ab initio calculations of the zone-center phonons in (AlN)n(GaN)n SLs with the number of monolayer equal to n = 2,4,6 were performed. The frequencies of phonon modes were calculated and the mode displacement patterns have been established. The theoretical Raman spectra were simulated and compared with experimental ones. A strong sensitivity of A1(LO) phonons confined within the AlN layer to its thickness was revealed both experimentally and in calculations. This finding can be used for quantitative characterization of short-period AlN/GaN SLs.

Nanodiamonds with the 'diamond' 1332.5 cm(-1) Raman line as narrow as 1.8 cm(-1) have been produced by reactive ion etching in oxygen plasma of heteroepitaxial diamond particles grown by microwave plasma enhanced chemical vapour deposition (MWPECVD) on silicon. After the etching, a doublet is recorded in the zero-phonon line photoluminescence spectra of an ensemble of silicon-vacancy (SiV) centres at 10 K. Each line of the doublet is split into two lines corresponding to the optical transitions between the split excited and ground energy levels of the SiV centres. These Raman and photoluminescent features have been observed previously only in low-strain homoepitaxial diamond films and single-crystal diamond.

Correlations between frequencies of the Raman-active A(TO) and E(LO) phonons and the layer thickness ratio in the AlN/GaN superlattices are determined. Theoretical frequency-structure relations are derived by using the dielectric continuum model. It is found that the elastic strains which inevitably arising in the layers upon SL growth affect frequencies of the most Raman-intense polar modes insignificantly. The obtained frequency-structure relations agree well with available experimental data. These results confirm efficiency of the Raman spectroscopy for a fast nondestructive structure characterization.

Raman spectra of GaAs nanowhiskers that are grown on different substrates and differ from one another by the content of the sphalerite and wurtzite phases have been investigated. Special attention has been focused on the manifestation of structural features in the scattering spectra of nanowhiskers. It has been established that the nanowhiskers are characterized both by random inclusions of wurtzite layers in the sphalerite structure and by the continuous growth in the wurtzite phase. The interpretation of the scattering spectrum agrees with the concept of summation of the dispersion curves of the sphalerite structure upon transition to the wurtzite structure, which leads to a transformation of zone-boundary modes at the L point of the Brillouin zone into zone-center modes of the wurtzite structure and, as a consequence, to the appearance of a number of new fundamental modes of different symmetries. An analysis of the Raman spectra has revealed the formation of the hexagonal 4H polytype in narrow layers of nanowhiskers due to a random packing of hexagonal layers. The coexistence of the sphalerite and wurtzite phases in GaAs nanowhiskers completely correlates with the photoluminescence spectra measured for the same samples.

The influence of doping on the structural and optical characteristics of Ge2Sb2Te amorphous thin films has been determined with micro-Raman scattering spectroscopy and a variable incident angle spectroscopic ellipsometer (VASE). The measured Raman spectra revealed the existence of two overlapping bands B2 near 130 cm-1 and C1 near 150 cm-1. The additional small features in the responses were revealed in the range less than 100 cm-1. Suggest that micro-Raman scattering spectroscopy can be used to study phase change materials through local structural information. The distinct changes in refractive index and extinction coefficient of Ge2Sb2Te amorphous films due to doping were revealed in the visible range. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on co-doping of high-quality AlN single crystals by group II Beryllium acceptors by means of diffusion from the vapour phase at a temperature of 1850 °C. We discovered that Be is characterized by the high diffusion length, allowing one to produce Be co-doping of sub-mm-thick AlN wafers. We show that Be diffusion led to the quenching of the visible (VIS) 450 nm (2.75 eV) and deep ultraviolet (UV) 265 nm (4.7 eV) optical absorption bands with simultaneous induction of the absorption band peaked at 248 nm (5 eV). By means of electron paramagnetic resonance (EPR), we also found that the presence of Be impurities compensated the donor type paramagnetic centers. Correlation of the EPR data with the optical absorption allowed us to conclude that Be produced in the AlN via diffusion acted predominantly as an acceptor, inducing the shift of the Fermi level to the lower part of the AlN bandgap. This shift of the Fermi level results in recharging of the deep level defects in the AlN bandgap, which explains the observed quenching of the VIS and UV absorption bands.

The composition of the products of destruction of a poly(styrene) film at the edge and inside the electric breakdown channel has been investigated using Raman spectroscopy. It has been demonstrated that, apart from the compounds with double carbon-carbon bonds, the breakdown initiates the formation of aromatic cycles having two to six aromatic rings, graphite nanocrystals with a linear size not above ten aromatic rings, and, finally, graphite single crystals. This suggests that the temperature in the breakdown channel exceeds 1800 K and that the current density in it is ∼106 A/cm2.

Graphene films were prepared by sublimation epitaxy of SiC under vacuum. The films were studied by Auger electron spectroscopy, reflection high-energy electron diffraction, atomic force microscopy, and Raman spectroscopy. It is concluded that the films prepared are polycrystalline and consist of graphene single crystals about 50 nm in size.

It is shown that a study of the dependence of impurity-related resonant first-order Raman scattering on the frequency of excitation light makes it possible to observe the dispersion of polar optical and acoustic branches of vibrational spectrum in hexagonal InN within a wide range of wave vectors. It is established that the wave vectors of excited phonons are uniquely related to the energy of excitation photon. Frequencies of longitudinal optical phonons E1(LO) and A1(LO) in hexagonal InN were measured in the range of excitation-photon energies from 2.81 to 1.17 eV and the frequencies of longitudinal acoustic phonons were measured in the range 2.81–1.83 eV of excitation-photon energies. The obtained dependences made it possible to extrapolate the dispersion of phonons A1(LO) and E1(LO) to as far as the point Γ in the Brillouin zone and estimate the center-band energies of these phonons (these energies have not been uniquely determined so far).

The 3C-SiC layers grown on the 15R-SiC substrates by sublimation epitaxy in vacuum are studied. Using X-ray topography and Raman spectroscopy, it is shown that the obtained layers are of a rather high structural quality. By the data of the Raman spectroscopy and capacitance-voltage measurements, it is established that the electron concentration in the 3C-SiC layer is (4–6) × 1018 cm−3.

Raman and SIMS studies of Mg-doped InN films with a Mg content from NMg = 3.3 × 1019 to 5.5 × 1021 cm–3 are reported. Lattice dynamics of hexagonal InN with substitutional impurities and vacancies has been investigated theoretically and calculated Raman spectra were compared with experimental ones. It is concluded that Raman spectroscopy is a good tool for quantitative characterization of Mg-doped InN. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of AlxGa1–xN (x ∼ 0.0-1.0) alloy layers by hydride vapor pahse epitaxy (HVPE) is reported. Crack-free undoped AlGaN layers from 0.1 to 2 μm thick were grown on 2″ SiC and 2″and 4″ sapphire substrates. For AlxGa1–xN (x ∼ 0.7-0.8) layers grown on sapphire, high-resolution symmetric/asymmetric X-ray diffraction (XRD) measurements resulted in ω-scan rocking curve widths ranging from 250 to 650 arcsec and from 1400 to 1900 arcsec, for the (00.2) and (10.2) reflections, respectively. Minimum XRD-estimated screw and edge dislocation densities in AlGaN layers grown on sapphire were