Enhanced luminescence from Si quantum dots/SiO2 multilayers by hydrogen annealing

Si quantum dots/SiO₂ multilayers were prepared by annealing a-Si:H/SiO₂ stacked structures at 1100 °C . Photo- and electro-luminescence band around 750 nm can be observed from Si QDs/SiO₂ multilayers due to the recombination of electron-hole pairs in Si QDs/SiO₂ interfaces. The electro-luminescence intensity was obviously enhanced after post hydrogen annealing at 400 °C. Electron spin resonance measurements were used to characterize the change of the defect states after hydrogen annealing. It is found that there exists a-centers (g value = 2.006), which is related to the Si dangling bonds in Si QDs in our samples. Hydrogen annealing can significantly reduce non-luminescent a-centers and enhance the electro-luminescence intensity consequently.     

Self-aggregated Si quantum dots in amorphous Si-rich SiC

Amorphous silicon quantum dots (Si-QDs) self-aggregated in silicon-rich silicon carbide are synthesized by growing with plasma-enhanced chemical vapor deposition on (100)-oriented Si substrate. Under the environment of Argon (Ar)-diluted Silane (SiH₄) and pure methane (CH₄), the substrate temperature and RF power are set as 350 °C and 120 W, respectively, to provide the Si-rich SiC with changing fluence ratio (R = [CH₄ ]/[SiH₄] + [CH₄]). By tuning the fluence ratio from 50% to 70%, the composition ratio x of Si-rich Si_1 ? xC_x film is varied from 0.27 to 0.34 as characterized by X-ray photoelectron spectroscopy (XPS), which reveals the component of Si_2p decreasing from 66.3 to 59.5%, and the component of C_1s increasing from 23.9% to 31% to confirm the formation of Si-rich SiC matrix. Annealing of the SiC sample from 650 °C to 1050 °C at 200 °C increment for 30 min induces the very tiny shift on the wavenumber of the crystalline Si (c-Si) related peak due to the precipitation of Si-QDs within the SiC matrix, and the Raman scattering spectra indicate a broadened Raman peak ranging from 410 to 520 cm^? 1 related to the amorphous Si accompanied with the significant enhancement for SiC bond related peak at 980 cm^? 1. From the high resolution transmission electron microscopy images, the critical temperature for Si-QD precipitation is found to be 850 °C. The self-assembly of the crystallized Si-QDs with the size of 3 ± 0.5 nm and the volume density of (3 ± 1) × 10¹⁸ (#/cm³) in Si-rich SiC film with R = 70% are observed after annealing at higher temperature.     

Thermal annealing of a-Si/Au superlattice thin films

The superlattice films, which consist of amorphous silicon (a-Si) and amorphous gold (Au), were prepared by ultra-high vacuum evaporation system. The first layer was grown a-Si with a thickness of 4.2 nm and the second layer was grown Au with a thickness of 0.8 nm. Thermal annealing was performed at 473, 673, and 873 K, respectively. The structural properties of the films were investigated using transmission electron microscope (TEM), X-ray diffraction (XRD), and Raman scattering spectroscopy. The electrical property was assessed by the temperature dependence of electrical conductivity. A crystallization of Si and a forming of Au nanoparticles were observed in all of the annealing films. The crystalline volume fraction reached 70% by annealing time for 15 min. An average diameter of the Au nanoparticles embedded in Si matrix also increased with increasing the annealing temperature. At annealing temperature above 873 K, Au atoms migrated toward the film surface. It was observed that the electrical conductivity changed in several temperatures.     

Characterizations of pulsed laser deposited SiC thin films

Thin films of silicon carbide (SiC) were prepared using pulsed laser deposition (PLD) on Si(1 0 0) substrates at a temperature of 370 °C. Various structural characterizations showed the development of short-range SiC precipitates in the films. These films were annealed isochronally at temperatures of 800 °C, 1000 °C and 1200 °C for 2 h under an inert environment. Thermally induced crystalline ordering of SiC into β-SiC phase was investigated by X-ray diffraction (XRD), Raman spectroscopy and Fourier transforms infrared (FTIR) spectroscopic measurements. In addition to the crystallization of SiC films, high temperature annealing resulted in the dissolution of carbon clusters found in the as-grown films.     

Crystallization of polymer-derived SiC/BN/C composites investigated by TEM

The crystallization behavior of two polymer-derived Si/B/C/N ceramics with similar compositions lying close to the three-phase field BN + SiC + C was investigated by (high-resolution) transmission electron microscopy. The materials were high-temperature mass stable up to T = 2000 °C. During thermolysis at 1050 °C a homogeneous amorphous solid formed. SiC crystallization started at about 1400 °C. Further annealing to higher temperatures up to 2000 °C led to formation of microstructures composed of SiC crystals embedded into a structured BNC_x matrix phase. With increasing temperature, both the size of the crystallites and the ordering of the matrix phase increased.     

Nanocrystallization-induced structural evolution of intergranular amorphous phase in Fe78B13Si9 alloy

The nanocrystallization-induced structural evolution of the intergranular amorphous phase in a Fe₇₈B₁₃Si₉ alloy was investigated by X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM), and positron annihilation spectroscopy. Crystallization occurs at 773 K, where nanocrystallites of α-Fe with an average grain size of a few tens of nanometers are formed in an amorphous matrix. With increasing annealing temperature up to 973 K, the average grain size increases up to ∼80 nm. In the as-prepared sample corresponding to an amorphous precursor, more than 90% of the positrons are localized at vacancy-sized free volumes dominantly surrounded by Fe atoms and other positrons are trapped by microvoids. Along with the appearance of nanocrystallites and their growth due to annealing, the concentration of microvoids is increased in the intergranular amorphous phase.     

Cross-sectional TEM study on Ni-mediated crystallization of amorphous silicon

Structural characteristics of polycrystalline silicon (poly-Si) made by Ni-mediated crystallization of amorphous silicon (a-Si) were investigated by cross-sectional transmission electron transmission (XTEM) according to various a-Si thickness. The Ni area density of ∼10¹⁴ cm⁻² was deposited onto a-Si and it was annealed at 500 °C in the presence of an electric field of 10 V/cm. It is found that NiSi₂ precipitates form at the top and bottom interfaces of a-Si during annealing. After reaching its critical size the crystallization proceeds from the top and bottom interfaces. The growth of needle-like Si crystallites has been seen, showing a migration of NiSi₂ precipitates through the a-Si network. 1700 nm thick a-Si can be crystallized within 30 min which is longer than that (10 min) of 50 nm thick a-Si. However, the quality of 50 nm thick poly-Si is better than that of 300 nm or 1700 nm thick poly-Si.     

Structural and luminescence properties of amorphous SiO2 nanoparticles

We report an experimental study on the photoluminescence band peaked at 2.7 eV (blue band) induced by thermal treatments in nanometric amorphous SiO₂. In particular the emission dependence on the nanometric particles size as a function of their mean diameter from 7 nm up to 40 nm is investigated. We found that the emission amplitude increases on decreasing the particle diameter, showing a strong correlation between the blue band and the nanometric nature of the particles. By Raman spectroscopy measurements it is evidenced that the SiO₂ nanoparticles matrix is significantly affected by the reduction of size. Basing on the shell-like model, these findings are interpreted assuming that the defects responsible for the photoluminescence are localized on a surface shell of the particles and not simply on their surface. In addition it is found that the generation efficiency of these defects depends on the structural properties of the SiO₂ matrix in the surface shell.     

Ultra-thin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

In this work, we present studies of ultra-thin polycrystalline silicon layers (5–100 nm) prepared by the aluminum-induced layer exchange process. Here, a substrate/Al/oxide/amorphous Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system of 577 °C, leading to a layer exchange and the crystallization of the amorphous Si. We have studied the process dynamics and grain growth, as well as structural properties of the obtained polycrystalline Si thin films. Furthermore, we derive a theoretical estimate of the grain density and examine characteristic thermal activation energies of the process. The structural properties have been investigated by Raman spectroscopy. A good crystalline quality down to a layer thickness of 10 nm has been observed.     

Structure and optical properties of light-emitting Si films fabricated by neutral cluster deposition and subsequent high-temperature annealing

As a new development of our previous study on the production of light-emitting amorphous Si (a-Si) films by the neutral cluster deposition (NCD) method, we have fabricated light-emitting Si films with improved emission intensity by the combined methods of NCD and subsequent high-temperature annealing. The structure of these films is best characterized by Si nanocrystals, surrounded by an interfacial a-SiO_x (x < 2) layer, embedded in an a-SiO₂ film. These improved Si films were observed by atomic force microscopy and high-resolution transmission electron microscopy, and analyzed by means of X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence (PL) and Fourier transform infrared-attenuated total reflection measurements. The PL curves of the annealed samples exhibit peaks around 600 nm, at almost the same position as the unannealed samples. Their PL intensities, however, have increased to approximately five times those of the unannealed samples. The source of the luminescence is most likely due to electron-hole recombination in the a-SiO₂/Si interfacial a-SiO_x layer.     

Spectroscopic ellipsometry study of nickel induced crystallization of a-Si

The aim of this work is to present a spectroscopic ellipsometry study focused on the annealing time effect on nickel metal induced crystallization of amorphous silicon thin films. For this purpose silicon layers with 80 and 125 nm were used on the top of which a 0.5 nm Ni thick layer was deposited. The ellipsometry simulation using a Bruggemann Effective Medium Approximation shows that films with 80 nm reach a crystalline fraction of 72% after 1 h annealing, appearing to be full crystallized after 2 h. No significant structural improvement is detected for longer annealing times. On the 125 nm samples the crystalline volume fraction after 1 h is only around 7%, requiring 5 h to get a similar crystalline fraction than the one achieved with the thinner film. This means that the time required for full crystallization will be strongly determined by the Si layer thickness. Using a new fitting approach the Ni content within the films was also determined by SE and related to the silicon film thickness.     

Structural and magnetic properties of nanocrystalline particles in an amorphous Fe73.5Nb3CuSi13.5B9 matrix

We report structural and magnetic properties of fine particles embedded in an amorphous magnetic matrix. As-quenched amorphous Fe_73.5Nb₃CuSi_13.5B₉ ribbons (FINEMET) were submitted to the thermal treatments of several times (1 ? t ? 240 min) at 570 °C using a conventional furnace. The analyses of the X-ray diffraction patterns at room temperature reveal that our samples consist of single phase Fe₃Si nanocrystals embedded in a residual amorphous phase. Magnetic measurements show that the saturation moment at T = 450 °C increases as a function of annealing time. This behavior is attributed to an increase of the fraction of nanocrystallites in the residual amorphous phase.     

Surface enhanced Raman scattering in an amorphous matrix for Raman amplification

In order to improve the efficiency of Raman Amplifiers, the Surface Enhanced Raman Scattering (SERS) effect of an amorphous matrix of TiO₂ was studied. First, optimisation of the amorphous layer quality was performed by depositing thin films on glass substrates at different temperatures. Then, thin films of amorphous TiO₂ were deposited on silicon commercial gold SERS substrates (Klarite®) by a dip-coating process. The SERS effect was demonstrated by the great difference of Raman intensities of the amorphous TiO₂ matrix dip-coated on active and inactive parts of Klarite® substrate under 633 nm and 780 nm laser excitations in the tail of the Surface Plasmon Resonance band of gold nanoparticles.     

Optical investigations of germanium nanoclusters – Rich SiO2 layers produced by ion beam synthesis

In this work, refractive index and extinction coefficient spectra of germanium nanoclusters – rich SiO₂ layers have been determined using variable angle spectroscopic ellipsometry (VASE) in the 250–1000 nm range. The samples were produced by Ge⁺ ion implantation into SiO₂ layers on Si substrates and subsequent annealing at temperatures from 700 to 1100 °C. It is known from previous investigations of similar samples that the Ge nanoclusterization process starts already at 800 °C and spherical Ge nanocrystallites 5–8 nm in diameter are observed in the SiO₂ layers after annealing for 1 h at even higher temperatures of 1000–1100 °C. Rutherford backscattering spectrometry (RBS) was employed to measure the Ge atom concentration depth profiles in the studied samples. The RBS results helped us choose realistic models for the VASE analysis which were necessary for a proper interpretation of the VASE data. It has been found that the refraction index value for the SiO₂/Si layer increases after Ge implantation. This effect can be explained by a defect-dependent compaction of ion-bombarded layers. A band’s tail in the extinction coefficient spectra for all the samples is observed which originates from a strong ultraviolet absorption band at 6.8 eV due to a Germanium Oxygen-Deficiency Center (GeODC) and/or a Ge-E’center in SiO₂. The annealing process results in the emergence of weaker extinction coefficient bands in the 400–600 nm region, associated with direct band-to-band transitions in Ge nanostructures. Transformation of these bands, including their blue-shift with the increasing annealing temperature could be explained via a quantum-confinement mechanism, by size and structural changes in Ge nanostructures.     

Characterization of chromium silicide thin layer formed on amorphous silicon films

A detailed investigation of the compositional, optical and electrical properties of a chromium silicide layer grown at room temperature on top of doped amorphous silicon films is presented. The formation of the layer is promoted only when phosphorous atoms are present in the film. The deposition of a very thin n-type doped layer (around 5 nm) on top of a p-type doped film has allowed us to achieve the chromium silicide formation also on p-type material without changing its doping properties. Angle resolved X-ray photoelectron spectroscopy measurements demonstrate the presence of chromium-oxide, chromium silicide and metallic chromium in similar percentages for both p- and n-type doped layers. From the ellipsometric analysis, the refractive index spectra have been extracted, and the layer thickness has been estimated to be 5 nm for both p- and n-type doped layers. From planar conductivity measurements, we have found that the chromium silicide promotes an activation energy reduction from 0.24 eV down to 0.017 eV for the n-type layer and from 0.36 eV down to 0.14 eV for the p-type film.     

Study of magnetoimpedance effect of Co-based amorphous ribbons after current annealing at various kinds of ambient pressure

The physical properties of magnetic amorphous alloys can be improved by using different annealing processes. In this paper, for annealing purposes, different driving currents were flowed through Co-based magnetic amorphous ribbons at different ambient air pressures between 5 × 10^?5 mbar and 10³ mbar. The magnetoimpedance effect in the annealed samples was studied at a frequency interval between 250 kHz and 10 MHz. Magnetic properties and microstructures of samples were investigated by means of alternative gradient force magnetometers and X-ray diffraction. Regarding the giant magnetoimpedance (GMI) effect, annealing at different ambient pressures with the same current can lead to various responses. For annealing at higher pressures such as in air, higher current is necessary for crystallization of samples which results in the development of greater transverse magnetic anisotropy.     

Silicon light emissions from boron implant-induced defect engineering

We studied the electroluminescence of boron-implanted p–n junction silicon light emitting-diodes (Si LEDs) engineered with the implant-induced extended defects of different types. By varying the post implant annealing conditions to tune the extended defects and by using plan-view transmission electron microscopy to identify them, we found that {1 1 3} defects along Si〈1 1 0〉 are the ones that result in strong silicon light emission of the p–n junction Si LEDs other than {1 1 1} perfect prismatic and {1 1 1} faulted Frank dislocation loops. The electroluminescence peak intensity at about 1.1 eV of {1 1 3} defect-engineered Si LEDs is about twenty-five times higher than that of dislocation defect-engineered Si LEDs.     

Light-induced annealing of hole trap states: A new aspect of light-induced changes in hydrogenated amorphous silicon

Details of light-induced annealing of hole trap state in undoped hydrogenated amorphous silicon (a-Si:H) have been studied; it has been found that prolonged illumination significantly reduces the density of hole trap states in the energy range deeper than 0.5 eV, and subsequent thermal annealing increases the density of hole trap states and restored the sample to the initial state before the illumination. We can speculate, from the experimental results and discussion in this work, that defect conversion processes are taking place during the long exposure to light; Si dangling bonds are generated from the precursors or latent sites which manifested as hole trap states located between 0.5 and 0.7 eV from the top of the valence band.     

Multilayer planar structures prepared from chalcogenide thin films of As–Se and Ge–Se systems and polymer thin films using thermal evaporation and spin-coating techniques

We report the preparation of multilayers based on polyamide–imide polymer and As–Se or Ge–Se chalcogenide thin films. Chalcogenide films of As–Se and Ge–Se systems were deposited using a thermal evaporation method periodically alternating with spin-coated Polyamide–imide films. Fifteen layers of PAI + As–Se system and nineteen layers of PAI + Ge–Se system were coated. Optical properties of prepared multilayers have been established using UV–vis–NIR and Ellipsometric spectroscopy. Both, PAI + As–Se and PAI + Ge–Se multilayer systems, exhibited the high-reflection bands centered around 830 nm and 1350 nm, respectively. The shift of the band position of PAI + Ge–Se multilayers to lower energies was caused by higher thickness of Ge–Se films. The bandwidth of reflection band of 8 PAI + 7 As–Se multilayer was ～90 nm while bandwidth of PAI + Ge–Se system decreased to ～70 nm because Ge–Se films have 0.1 lower refractive index against As–Se films. Design of 1D-photonic crystals based on alternating chalcogenide and polymer films is a new opportunity for application of chalcogenide thin films as optical materials for near-infrared region.     

Optical properties of microcrystalline 3C–SiC:H films measured by resonant photothermal bending spectroscopy

Optical absorption coefficient spectra of hydrogenated microcrystalline cubic silicon carbide (μc-3C–SiC:H) films prepared by Hot-Wire CVD method have been estimated for the first time by resonant photothermal bending spectroscopy (resonant-PBS). The optical bandgap energy and its temperature coefficient of μc-3C–SiC:H film is found to be about 2.2 eV and 2.3 × 10⁻⁴ eV K⁻¹, respectively. The absorption coefficient spectra of localized states, which are related to grain boundaries, do not change by exposure of air and thermal annealing. The localized state of μc-3C–SiC:H has different properties for impurity incorporation compared with that of hydrogenated microcrystalline silicon (μc-Si:H) film.