Multi-band silicon quantum dots embedded in an amorphous matrix of silicon carbide

Silicon quantum dots embedded in an amorphous matrix of silicon carbide were realized by a magnetron co-sputtering process and post-annealing. X-ray photoelectron spectroscopy, glancing x-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy were used to characterize the chemical composition and the microstructural properties. The results show that the sizes and size distribution of silicon quantum dots can be tuned by changing the annealing atmosphere and the atom ratio of silicon and carbon in the matrix. A physicochemical mechanism is proposed to demonstrate this formation process. Photoluminescence measurements indicate a multi-band configuration due to the quantum confinement effect of silicon quantum dots with different sizes. The PL spectra are further widened as a result of the existence of amorphous silicon quantum dots. This multi-band configuration would be extremely advantageous in improving the photoelectric conversion efficiency of photovoltaic solar cells.     

Effect of natural aging on photoluminescence of porous silicon

The influence of natural aging on the intensity and position of the photoluminescence (PL) peak in n-type porous silicon (por-Si) has been studied. Changes in the phase composition and relative content of the amorphous and oxide phases in por-Si during aging were determined by simulating the spectra of Si L _2,3 ultrasoft X-ray emission based on the reference spectra of the corresponding phases.     

Sputter deposition of silicon oxynitride gradient and multilayer coatings

The optical properties of silicon oxynitride films deposited by reactive dc magnetron sputtered films have been investigated. In particular the absorption characteristics of silicon nitride thin films in the visible spectrum and their optical bandgap were analyzed with regard to their composition and deposition properties. It can be shown that there is a significant difference between the absorption in the visible spectrum and the optical bandgap for these layers. The influence of unipolar and bipolar pulse modes on the optical layer properties is presented. The extinction coefficient for silicon nitride single layers could be reduced to a value of 2 x 10(-4) at 500 nm without external heating. There is also the dependence of the absorption of silicon oxynitride layers on the discharge voltage. We present the resulting spectra of rugate and edge filters that consist of these layers and offer lower absorption than single layers.     

A microstructural study of silicon carbide fibres through the use of Raman microscopy

The microstructures of three different silicon carbide (SiC) fibres produced by CVD (chemical vapour deposition) have been examined in detail using Raman microscopy. Raman spectra were mapped out across the entire cross-sections of these silicon carbide fibres using an automated x-y stage with a spatial resolution of 1 m. The Raman maps clearly illustrate the variations in microstructure in such types of silicon carbide fibres. It appears that the SCS-type fibres contain carbon as well as SiC whereas the Sigma 1140+ fibre also contains free silicon. Furthermore, the differences in the detailed structures of the carbon and silicon carbide present in the fibres can also be investigated. Raman microscopy is demonstrated to be a very sensitive technique for characterising the composition and microstructure of CVD silicon carbide fibres prepared using different processing conditions.     

Evidence for a silicon oxycarbide phase in the Nicalon silicon carbide fibre

The Nicalon silicon carbide fibre has been studied by X-ray photoelectron spectroscopy. Elements entering the fibre are carbon, silicon and oxygen. In addition to previously reported chemical entities (silicon carbide, silica and graphitic carbon) evidence is found of the presence of a new supplementary phase which is attributed to an intermediate silicon oxycarbide phase. As this phase is found to participate in very appreciable proportions to the composition of the fibre, some influence on the properties of this fibre can be anticipated.     

Morphology, structure, and composition of polycrystalline CdTe films grown on three-dimensional silicon substrates

Polycrystalline CdTe films have been produced on various substrates (glass, ITO-coated glass, sapphire, and microtextured silicon) by quasi-closed space growth, and their structural perfection and surface morphology have been studied by optical microscopy and scanning electron microscopy. Using an energy dispersive X-ray spectrometer and the scanning electron microscope, we have obtained the X-ray emission spectra of the CdTe films and determined their elemental composition. The morphological features of the films have been investigated and the cadmium-to-tellurium atomic ratio in the films has been determined experimentally.     

Orientation-enhanced growth and optical properties of ZnO nanowires grown on porous silicon substrates

ZnO nanowires have been synthesized on porous silicon substrates with different porosities via the vapour-liquid-solid method. The texture coefficient analysed from the XRD spectra indicates that the nanowires are more highly orientated on the appropriate porosity of porous silicon substrate than on the smooth surface of silicon. The Raman spectrum reveals the high quality of the ZnO nanowires. From the temperature-dependent photoluminescence spectra, we deduced the activation energies of free and bound excitons.     

Morphology and optical properties of titanium-doped porous silicon carbide layers

The effect of rapid thermal annealing (RTA) in oxygen on the properties of titanium-doped porous silicon carbide (por-SiC) layers has been studied. The data on the surface morphology and the photoluminescence (PL) spectra show that the high-temperature diffusion annealing is accompanied by the formation of titanium silicides, by an increase in the grain size of the material, and by the emergence of pores on the surface of the por-SiC-Ti sample. The results of PL measurements are consistent with data on the phase composition and the surface morphology of the samples. It is established that RTA leads to modification of the titanium-doped por-SiC structure.     

Heterogeneous effect in carbonized porous silicon

The spectra of photoluminescence from carbonized porous silicon were measured upon the anodic polarization of samples in an oxidative electrolyte. A significant increase in the intensity of a blue-green emission band was observed and attributed to a change in parameters of the silicon-silicon carbide heterostructure. In addition, the carbonized porous silicon exhibits effective electroluminescence in an oxidizing electrolyte.     

Low-pressure vapor-phase epitaxy of silicon on porous silicon

Silicon has been deposited by low-pressure vapor-phase epitaxy (LPVPE) on porous silicon at much lower temperatures than in conventional CVD epitaxy. The epitaxy was performed at 823°C and 0.03 mbar using SiCl₂H₂. After preoxidation to stabilize the porous structure the surface of preoxidized porous silicon was chemically cleaned and finally thermally cleaned at temperatures below 860°C just before the epitaxial step. Epitaxial layers with very good crystalline quality as characterized by RBS channeling spectra and TEM cross sections were obtained. The original microstructure of porous silicon was only slightly modified during the epitaxial process. Full oxidation of the buried porous silicon layer was subsequently possible.     

Properties of silicon nitride films prepared by plasma-enhanced chemical vapour deposition of SiH4-N2 mixtures

The refractive indices and IR absorption spectra are measured for silicon nitride films plasma deposited from SiH₄-N₂-H₂ gas mixtures. The composition of the film (N:Si ratio) is derived from the value of the refractive index and the concentration of bonded hydrogen as Si-H and N-H in the film is estimated from the absorption intensities in the IR spectrum. The optimum deposition conditions for giving excellent insulating silicon nitride films are confirmed to be same as the conditions for giving films with stoichoimetric composition and the lowest amount of incorporated hydrogen.     

Porous silicon upon multicrystalline silicon: Structure and photoluminiscence

Ultrathin porous silicon layers have been stain-etched upon multicrystalline silicon (multi-Si) substrates. We studied optical and structural properties of porous silicon by photoluminescence, photo-luminescence excitation, reflection, atomic force microscopy and scanning tunnel microscopy methods. It was observed that the thickness of porous silicon did not exceed 20 nm. The photoluminescence method has shown that photoluminescence spectra of porous silicon of different grains have shown that they differ insignificantly (∼10%) in intensity. It was found that por-Si layers with optimal antireflection characteristics was obtained during etching time 7 min. In the paper the comparison of the reflection characteristics of investigated samples por-Si with industrial antireflection coating is presented.     

Effect of nanoporous silicon coating on silicon solar cell performance

The surface modification of silicon solar cells was used for improvement of photovoltaic characteristics of silicon solar cells. A screen-printed solar cell technology is used to fabricate n⁺-p silicon solar cell. Nanoporous silicon (PS) layer on n⁺-type Si wafers or on the frontal surface of (n⁺-p)Si solar cell was formed by electrochemical etching in HF-containing solution. The surface morphology, porosity, spectra of photoluminescence and reflectance of PS layers were analyzed. The photovoltaic characteristics of two silicon solar cell type with and without PS layer (PS/(n⁺-p)Si and (n⁺-p)Si cell) were measured and compared. The spectra of photosensitivity of cells were measured in the wavelength range of 300-1100 nm. An average reflection of the porous silicon layer, fabricated on a polished silicon surface, is decreased to 4%. A remarkable increment of the conversion efficiency by 20% have been achieved for PS/(n⁺-p)Si solar cell comparing to (n⁺-p)Si solar cell without PS layer. The results, related with improving of the performance of PS/(n⁺-p)Si solar cell, have been attributed to the effective antireflection and the wide-gap window role of nanoporous silicon on the silicon solar cell.     

Hydrogenated microcrystalline silicon films prepared by VHF-PECVD and single junction solar cell

Intrinsic microcrystalline silicon films have been prepared with very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) from silane/hydrogen mixture at 180°C. The effect of silane concentration and discharge power on the growth of silicon films was investigated. Samples were investigated by Fourier transform infrared spectroscopy, Raman scattering and X-ray diffraction. The Raman spectrum shows that the morphological transition from microcrystalline to amorphous occurs under conditions of high silane concentration and low discharge power. X-ray diffraction spectra indicate a preferential growth direction of all microcrystalline silicon films in the (111) plane. In addition, a solar cell with an efficiency of 5.1% has been obtained with the intrinsic microcrystalline layer prepared at 10W.     

Optical property of silicon quantum dots embedded in silicon nitride by thermal annealing

We present the effects on the thermal annealing of silicon quantum dots (Si QDs) embedded in silicon nitride. The improved photoluminescence (PL) intensities and the red-shifted PL spectra were obtained with annealing treatment in the range of 700 to 1000 °C. The shifts of PL spectra were attributed to the increase in the size of Si QDs. The improvement of the PL intensities was also attributed to the reduction of point defects at Si QD/silicon nitride interface and in the silicon nitride due to hydrogen passivation effects.     

R.F. reactive sputter deposition of hydrogenated amorphous silicon carbide films

The preparation of hydrogenated amorphous silicon carbide films by r.f. reactive sputtering of a silicon target in Ar-CH₄ gas mixtures with and without an r.f. bias on the substrates was studied. Starting with a pure silicon target and increasing monotonically the CH₄ percentage from 0% to about 10%, films with 1 ⩾ x ⩾ 0 were obtained at decreasing deposition rates. After sputtering for some hours in methane-rich gas mixtures, carbon atoms were incorporated into the silicon target surface, probably as a result of atomic peening, and nearly stoichiometric SiC films were prepared by sputtering of such a target in pure argon. The different mechanisms of film formation, deposition rate, composition, hardness, friction coefficient and stresses in the films as functions of the partial pressure of methane and the value of the r.f. bias were investigated. The IR spectra offilms with different carbon contents were analysed. The greatest hardness was found for nearly stoichiometric SiC films deposited with a bias.     

High accuracy method for Auger analysis of the composition of uncleaned silicon oxide surface

A new procedure is described for preparing standard spectra of the inverse self-convolution of the density of states for silicon oxide required for performing non-destructive quantitative Auger analysis of SiOx. Research results are used for determining the composition of silicon oxide formed at the surface of quantum threads in porous silicon with high-temperature annealing in a hydrogen atmosphere and during water etching.Translated from Izmeritelnaya Tekhnika, No. 10, pp. 57–60, October, 2004.     

Oxidation behavior of micro- and nanostructured silicon powders

We have studied the effect of preparation procedure, crystallite shape and size, and oxidation conditions on the oxidation behavior of silicon micro- and nanopowders. The oxidation process is shown to occur in two steps, with a transition at a certain thickness of the oxidized layer, which depends on the oxidation time, particle shape, and particle size. The composition and physicochemical properties of the oxide films have been determined in relation to the shape and size of the silicon particles. The results indicate that the silicon micro- and nanopowders differ in oxidation mechanism from single-crystal silicon.     

Oxygen distribution in silicon nitride powders

The oxygen content of ten different silicon nitride powders was determined by bulk chemical analysis and surface-sensitive X-ray photoemission spectroscopy (XPS). In silicon nitride powders prepared from silicon and silica by nitridation and carbothermal reduction in a nitrogen atmosphere, respectively, only a minor part of the total oxygen content of 0.9 to 2.5 wt% was found in a surface layer of less than 1 nm thick, whereas an appreciable amount can be attributed to oxygen dissolved in the bulk. Powders made by silicon diimide decomposition, however, are characterized by a higher oxygen concentration at the particle surface relative to the bulk, which may be further reduced by chemical treatment. The surface layer composition corresponds to an intermediate state between silica and silicon oxynitride.     

KrF excimer laser crystallization of silicon thin films

A KrF excimer laser operating at 249 nm has been employed to crystallize silicon thin films deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) and by RF magnetron sputtering on Corning glass and SiO₂. All films display a substantial improvement in crystallinity after ELC with the optimum laser fluence for as-deposited ECR films being higher than for sputtered films. This is probably related to the presence of Si-H_x bonds in the former. A pronounced bimodality in the Raman spectra of some amorphous, as-deposited ECR samples has been observed after laser crystallization where, in addition to the peak at 520cm^-1, a strong peak at 509cm^-1 is also present. Such behavior has not been reported previously to our knowledge in ELC silicon films. Interestingly, the XRD spectra of these samples do not exhibit any peaks suggesting the films are composed of nano-grain material. The dehydrogenation of ECR films by ELC has been demonstrated to be substantial, the hydrogen content typically decreasing from ~30 at % in an as-deposited film to ~10 at % after a single low fluence laser shot. Raman spectroscopy has shown that the film bonding changes from predominantly Si-H₂ to Si-H after ELC. Electrical resistivity measurements of phosphorus-doped films show a controllable and repeatable change with laser fluence. The results in this paper show that it is possible to crystallize and controllably change the electrical characteristics of ECR PECVD produced silicon thin films by ELC.