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,...     

The effect of Sn impurity on the optical and structural properties of thin silicon films

The effect of tin impurity on the structure and optical properties of thin-film amorphous silicon is investigated. It is established that tin impurity accelerates crystallization of amorphous silicon. Immediately after deposition of a film onto a substrate at a temperature of ∼300°C, there is a crystalline phase of silicon in samples with tin. High-vacuum annealing at 350–750°C leads to growth of the crystalline phase in films with tin: nanocrystals grow in size from ∼3.0 to 4.5 nm. At the same time, in films without tin, only the degree of the short-range order increases. Silicon film without tin remains amorphous over the entire range of annealing temperatures.     

Structural and optical properties of ZnO films produced by a nonvacuum chemical technique

Zinc-oxide films are grown by a new nonvacuum chemical method: the pyrolysis of zinc acetylacetonate at a temperature of 280–300°C. The structural, phonon, and emission properties of the ZnO films are studied by X-ray diffraction analysis, scanning electron microscopy, Raman measurements, and photoluminescence spectroscopy. The high-intensity (0002) peak recorded in the X-ray diffraction spectra indicate the predominant orientation of crystallites in the (0001) direction in the ZnO films. From analysis of the E ₂ ^high mode in the Raman spectrum of the ZnO films, the elastic strains ? _zz (～3.2 × 10^?3) and the quality of the crystal structure are determined. The characteristics of the pyrolytic ZnO films are compared with the corresponding characteristics of ZnO films grown by molecular-beam epitaxy. As a result, the possibility of growing polycrystalline ZnO films of rather high quality by a practically feasible low-temperature technique is demonstrated.     

Low-temperature anti-Stokes photoluminescence in CdSe/ZnSe nanostructures

Intense anti-Stokes photoluminescence was observed at low temperatures in CdSe/ZnSe nanostructures with separate CdSe inserts in a ZnSe matrix; the nominal thickness of these inserts amounted to 1.5 and 0.6 monolayers. It is shown that the intensity of an anti-Stokes band excited by the photons of energies considerably lower than the band’s peak is quadratic in the excitation power; in the case of resonance excitation, a weaker dependence is obtained. A mechanism behind the excitation of anti-Stokes photoluminescence is suggested on the basis of nonlinear two-step two-photon absorption via the deep states of the defect centers including cation vacancies localized at the barrier-nanoisland interface.     

Effects of the lateral ordering of self-assembled SiGe nanoislands grown on strained Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> buffer layers

Atomic-force microscopy, micro-Raman spectroscopy, and high resolution X-ray diffraction are applied to study the spatial ordering in single layers of SiGe nanoislands grown on a strained Si_{1 − x} Ge _x buffer sublayer. It is shown that, apart from stimulating the spatial ordering of nanoislands, the introduction of a Si_{1 − x} Ge _x sublayer leads to an enhanced role for interdiffusion processes. An unusually high increase in the volume of nanoislands in the process of the epitaxy is related to the anomalously strong diffusion from the buffer sublayer into the islands that is induced by nonuniform fields of elastic strains. The anisotropy of the islands shape and spatial ordering is discussed in terms of the anisotropy of the diffusion processes in spatially nonuniform fields of elastic strains.     

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.     

On the nature of doping effect of methane in ZnO thin films deposited by RF-magnetron sputtering

Journal of Materials Science: Materials in Electronics - A comparative study of the effects of methane and hydrogen as reactive agents on the structural, optical, and electrical properties of ZnO...     

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.     

Possibility of graphene growth by close space sublimation

Carbon films on the Si/SiO₂ substrate are fabricated using modified method of close space sublimation at atmospheric pressure. The film properties have been characterized by micro-Raman and X-ray photoelectron spectroscopy and monochromatic ellipsometry methods. Ellipsometrical measurements demonstrated an increase of the silicon oxide film thickness in the course of manufacturing process. The XPS survey spectra of the as-prepared samples indicate that the main elements in the near-surface region are carbon, silicon, and oxygen. The narrow-scan spectra of C1s, Si2p, O1s regions indicate that silicon and oxygen are mainly in the SiO _x (x ≈ 2) oxide form, whereas the main component of C1s spectrum at 284.4 eV comes from the sp²-hybridized carbon phase. Micro-Raman spectra confirmed the formation of graphene films with the number of layers that depended on the distance between the graphite source and substrate.     

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.