Optical-luminescence properties of ZnO and TiO2 nanopowders obtained by pulsed laser reactive ablation

Journal of Materials Science: Materials in Electronics - The ZnO and TiO2 nanopowders have been prepared by means of the pulsed laser reactive ablation of metallic (Zn, Ti) targets. The Structural,...     

Transformation of a SiC/por-SiC/TiO2 structure during rapid thermal annealing

The surface morphology and the Raman and photoluminescence spectra of a SiC/por-SiC/TiO₂ structure before and after rapid thermal annealing are studied. It is shown that rapid thermal annealing brings about the appearance of new bands in the Raman spectrum; these bands are characteristic of carbon compounds. An analysis of the spectra of photoluminescence excited by radiation with an energy lower than that of the band-gap energy in 6H-SiC has shown that the appearance of photoluminescence in porous silicon carbide is related to impurity states, which are formed at the surface due to products of chemical reactions in the course of etching.     

X-ray diffraction analysis and scanning micro-Raman spectroscopy of structural irregularities and strains deep inside the multilayered InGaN/GaN heterostructure

High-resolution X-ray diffraction analysis and scanning confocal Raman spectroscopy are used to study the spatial distribution of strains in the In _x Ga_{1 − x} N/GaN layers and structural quality of these layers in a multilayered light-emitting diode structure produced by metal-organic chemical vapor deposition onto (0001)-oriented sapphire substrates. It is shown that elastic strains almost completely relax at the heterointerface between the thick GaN buffer layer and In _x Ga_{1 − x} N/GaN buffer superlattice. It is established that the GaN layers in the superlattice are in a stretched state, whereas the alloy layers are in a compressed state. In magnitude, the stretching strains in the GaN layers are lower than the compressive strains in the InGaN layers. It is shown that, as compared to the buffer layers, the layers of the superlattice contain a smaller number of dislocations and the distribution of dislocations is more randomly disordered. In micro-Raman studies on scanning through the thickness of the multilayered structure, direct evidence is obtained for the asymmetric gradient distributions of strains and crystal imperfections of the epitaxial nitride layers along the direction of growth. It is shown that the emission intensity of the In _x Ga_{1 − x} N quantum well is considerably (more than 30 times) higher than the emission intensity of the GaN barrier layers, suggesting the high efficiency of trapping of charge carriers by the quantum well.     

Si-rich Al2O3 films grown by RF magnetron sputtering: structural and photoluminescence properties versus annealing treatment

Silicon-rich Al₂O₃ films (Si_x(Al₂O₃)_1−x) were co-sputtered from two separate silicon and alumina targets onto a long silicon oxide substrate. The effects of different annealing treatments on the structure and light emission of the films versus x were investigated by means of spectroscopic ellipsometry, X-ray diffraction, micro-Raman scattering, and micro-photoluminescence (PL) methods. The formation of amorphous Si clusters upon the deposition process was found for the films with x ≥ 0.38. The annealing treatment of the films at 1,050°C to 1,150°C results in formation of Si nanocrystallites (Si-ncs). It was observed that their size depends on the type of this treatment. The conventional annealing at 1,150°C for 30 min of the samples with x = 0.5 to 0.68 leads to the formation of Si-ncs with the mean size of about 14 nm, whereas rapid thermal annealing of similar samples at 1,050°C for 1 min showed the presence of Si-ncs with sizes of about 5 nm. Two main broad PL bands were observed in the 500- to 900-nm spectral range with peak positions at 575 to 600 nm and 700 to 750 nm accompanied by near-infrared tail. The low-temperature measurement revealed that the intensity of the main PL band did not change with cooling contrary to the behavior expected for quantum confined Si-ncs. Based on the analysis of PL spectrum, it is supposed that the near-infrared PL component originates from the exciton recombination in the Si-ncs. However, the most intense emission in the visible spectral range is due to either defects in matrix or electron states at the Si-nc/matrix interface.     

Laser-annealing-induced features of the Raman spectra of quartz/Si and glass/Si structures

The effect of laser radiation on the characteristics of amorphous silicon films on glassy or quartz substrates are studied by Raman spectroscopy. It is established that an increase in the laser-treatment power yields a phase transition from amorphous silicon to nanocrystalline silicon. The variation in the relation between the nanocrystalline and amorphous silicon fractions in the films is described in the context of the critical impact model.     

Resonant Raman scattering and atomic force microscopy of InGaAs/GaAs multilayer nanostructures with quantum dots

The transition from two-dimensional (2D) pseudomorphic growth to the three-dimensional (3D) (nanoisland) growth in In_xGa_1?xAs/GaAs multilayer structures grown by molecular-beam epitaxy was investigated by atomic force microscopy, photoluminescence, and Raman scattering. The nominal In content x in In_xGa_1?xAs was varied from 0.20 to 0.50. The thicknesses of the deposited In_xGa_1?xAs and GaAs layers were 14 and 70 monolayers, respectively. It is shown that, at these thicknesses, the 2D-3D transition occurs at x ≥ 0.27. It is ascertained that the formation of quantum dots (nanoislands) does not follow the classical Stranski-Krastanov mechanism but is significantly modified by the processes of vertical segregation of In atoms and interdiffusion of Ga atoms. As a result, the In_xGa_1?xAs layer can be modeled by a 2D layer with a low In content (x < 0.20), which undergoes a transition into a thin layer containing nanoislands enriched with In (x > 0.60). For multilayer In_xGa_1?xAs structures, lateral alignment of quantum dots into chains oriented along the \([\overline 1 10]\) direction can be implemented and the homogeneity of the sizes of quantum dots can be improved.     

Comparative characteristics of the Raman scattering spectra of graphene films on conductive and semi-insulating 6H-SiC substrates

The raman scattering (RS) spectra of graphene on semi-insulating and conductive 6H-SiC substrates formed by preliminary and additional annealing of silicon carbide at various temperatures are studied. The degree of perfection of the graphene films and sizes of its clusters are estimated. It is shown that the temperature of additional annealing in the case of conductive substrates should be higher than that for semi-insulating substrates to obtain graphene layers with the same structural perfection.     

Polarized Raman spectroscopy and X-ray diffuse scattering in InGaAs/GaAs(100) quantum-dot chains

Using polarized Raman spectroscopy and high resolution X-ray diffraction we have investigated self-organized In_0.45Ga_0.55As quantum-dot chains in InGaAs/GaAs multilayer structures. It is shown that the formation of InGaAs QDs in InGaAs/GaAs multilayered structures is accompanied by a strong improvement in the uniformity of size and shapes of QDs as well as vertical alignment and lateral ordering. At mean densities, extended chains of QDs (up to 5 μm) appear along the $ [1\bar 10] $ direction; however, increased ordering of QDs along the [110] direction could be observed, too. For the first time, InGaAs dot-chains were investigated using polarized Raman scattering. Observation of optical phonons localized in InGaAs QDs and two-dimensional (2D) layers is demonstrated. An obvious anisotropy in the intensity of Raman modes was observed when the electric field vector of the exciting laser beam is parallel or perpendicular to the wire-like axis $ [1\bar 10] $ of dot-chains. This effect may be related to symmetry lowering effects and real anisotropic geometry of the QDs and 2D wetting layers.     

Field emission properties of pointed cathodes based on graphene films on SiC

Electrical properties of low-threshold field emission cathodes produced by growth nanocluster graphene films on the pointed surface of heavily doped n ⁺SiC by sublimation epitaxy have been considered. The quality of the graphene coating has been assessed based on the morphological studies and Raman spectroscopy. Using the volt–ampere characteristics the work function from a pointed cathode with graphene coating was calculated (～ 0.76 eV). Such a low value of the work function is explained on the assumptions that the graphene film has the nanocluster nature and the sources of the field emission are graphene nanoclusters.     

Lateral ordering of quantum dots and wires in the (In,Ga)As/GaAs(100) multilayer structures

The surface morphology and optical properties of the (In,Ga)As/GaAs(100) multilayer structures with self-organized quantum dots and quantum wires, which were grown by molecular-beam epitaxy, are investigated. It is found that the ordered arrangement of quantum dots in the heterointerface plane starts to form during the growth of the first periods of the multilayer structure. As the number of periods increases, quantum dots line up in series and form wires along the \([0\bar 11]\) direction. An increase in the lateral ordering of the structures under consideration correlates with an increase in the optical emission anisotropy governed by relaxation anisotropy of elastic strains and by the shape of nano-objects. A possible mechanism of lateral ordering of quantum dots and wires in multilayer structures, which includes both anisotropy effects of the strain fields and adatom diffusion, as well as the elastic interaction of neighboring quantum dots, is discussed.