Si Nanoclusters Research Articles

Effects of Thermal Annealing on the Structural and Optical Properties of Carbon-Implanted SiO2

Amorphous carbon (a-C) nanoclusters were synthesized by the implantation of carbon ions (C-) into thermally grown silicon dioxide film (-500 nm thick) on a Si (100) wafer and processed by high temperature thermal annealing. The carbon ions were implanted with an energy of 70 keV at a fluence of 5 x 10(17) atoms/cm2. The implanted samples were annealed at 1100 degrees C for different time periods in a gas mixture of 96% Ar+4% H2. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and High Resolution Transmission Electron Microscopy (HRTEM) were used to study the structural properties of both the as-implanted and annealed samples. HRTEM reveals the formation of nanostructures in the annealed samples. The Raman spectroscopy also confirms the formation of carbon nano-clusters in the samples annealed for 10 min, 30 min, 60 min and 90 min. No Raman features originating from the carbon-clusters are observed for the sample annealed further to 120 min, indicating a complete loss of implanted carbon from the SiO2 layer. The loss of the implanted carbon in the 120 min annealed sample from the SiO2 layer was also observed in the XPS depth profile measurements. Room temperature photoluminescence (PL) spectroscopy revealed visible emissions from the samples pointing to carbon ion induced defects as the origin of a broad 2.0-2.4 eV band, and the intrinsic defects in SiO2 as the possible origin of the -2.9 eV bands. In low temperature photoluminescence spectra, two sharp and intense photoluminescence lines at -3.31 eV and -3.34 eV appear for the samples annealed for 90 min and 120 min, whereas no such bands are observed in the samples annealed for 10 min, 30 min, and 60 min. The Si nano-clusters forming at the Si-SiO2 interface could be the origin of these intense peaks.

Photoluminescence study of the structural evolution of amorphous and crystalline silicon nanoclusters during the thermal annealing of silicon suboxide films with different stoichiometry

The effect of the stoichiometry of thin silicon suboxide films on the processes of the formation and evolution of silicon nanoclusters during thermal annealing is studied by photoluminescence measurements. The samples are produced by the thermal sputtering of a SiO powder in an oxygen atmosphere, with the subsequent deposition of a 500 nm-thick SiOx layer onto a Si substrate. The morphological properties and size of Si nanoclusters are explored by analyzing the photoluminescence spectra and kinetics. A comparative study of the luminescence properties of thin SiOx layers with different stoichiometric parameters, x = 1.10, 1.29, 1.56, and 1.68, is accomplished for samples annealed at different temperatures in the range 850 to 1200°C. The dependences of the photoluminescence decay time on the annealing temperature, the stoichiometric parameter of the initial silicon suboxide film, and the nanocluster size are studied.

Effect of erbium fluoride doping on the photoluminescence of SiO x films

The photoluminescence of SiOx films deposited on c-Si wafers by the thermal evaporation of SiO in a vacuum and, for the first time, doped with ErF3 by coevaporation is studied. It is shown that, like undoped SiOx films, the unannealed SiOx:ErF3 films passivate the surface of the Si wafers and, thus, increase their edge photoluminescence intensity almost fivefold. A similar increase is observed after annealing of the doped films in air at 750°C. Doping with ErF3 suppresses the photoluminescence of Si nanoclusters, if the films have been subjected to high-temperature annealing (at 750°C). In this case, the PL intensity of the band with a peak at ∼890 nm decreases as well. The ∼890 nm band is observed for the first time and, due to its features, is attributed to transitions in SiOx matrix defects. The experimentally observed effect of ErF3 doping on SiOx film photoluminescence is interpreted. An intense photoluminescence signal from Er3+ ions in the nearinfrared spectral region (the 4I11/2 → 4I15/2 and 4I13/2 → 4I15/2 transitions) is observed in the SiOx:ErF3 films annealed in air at 750°C. This finding shows that 1.54 μm luminescent emitters, which are currently in popular demand, can be produced by a simple low-cost method.

A Combined Ion Implantation/Nanosecond Laser Irradiation Approach towards Si Nanostructures Doping

The exploitation of Si nanostructures for electronic and optoelectronic devices depends on their electronic doping. We investigate a methodology for As doping of Si nanostructures taking advantages of ion beam implantation and nanosecond laser irradiation melting dynamics. We illustrate the behaviour of As when it is confined, by the implantation technique, in a SiO2/Si/SiO2multilayer and its spatial redistribution after annealing processes. As accumulation at the Si/SiO2interfaces was observed by Rutherford backscattering spectrometry in agreement with a model that assumes a traps distribution in the Si in the first 2-3 nm above the SiO2/Si interfaces. A concentration of 1014 traps/cm2has been evaluated. This result opens perspectives for As doping of Si nanoclusters embedded in SiO2since a Si nanocluster of radius 1 nm embedded in SiO2should trap 13 As atoms at the interface. In order to promote the As incorporation in the nanoclusters for an effective doping, an approach based on ion implantation and nanosecond laser irradiation was investigated. Si nanoclusters were produced in SiO2layer. After As ion implantation and nanosecond laser irradiation, spectroscopic ellipsometry measurements show nanoclusters optical properties consistent with their effective doping.

Copropagating pump and probe experiments on Si-nc in SiO2 rib waveguides doped with Er: The optical role of non-emitting ions

We present a study that demonstrates the limits for achieving net optical gain in an optimized waveguide where Si nanoclusters in SiO2 codoped with Er3+ are the active material. By cross correlating absorption losses measurements with copropagant pump (λpump = 1.48 µm) and probe (λprobe = 1.54 µm) experiments we reveal that the role of more than 80% of the total Er3+ population present on the material (intended for optical amplification purposes) is to absorb the propagating light, since it is unfeasible to invert it.

Facile metallization of dielectric coatings for plasmonic solar cells

Electroless deposition of Ag nanoparticles on PECVD SiNx:H dielectric layers is a new straightforward chemical bath process well adapted to the realization of plasmonic solar cells. Depending on the stoichiometry, SiNx:H contains a certain number of Si nanoclusters embedded in the silicon nitride matrix, that act as reducing agents for Ag ions in solution and thus allows the deposition of Ag nanoparticles directly on the surface. We investigate the influence of the layer stoichiometry and deposition parameters on the nucleation and growth of Ag nanoparticles, and show how they can be adjusted to control the nanoparticle size and density. We also demonstrate that this facile metallization process can be extended to PECVD SiOx layers, another dielectric coating of interest in photovoltaics.

Laser-induced Coulomb explosion in C and Si nanoclusters: The determining role of pulse duration

We investigate, at the quantum-mechanical level, the interaction of ultrashort laser pulses with small (less than ∼4nm) carbon and silicon nanoclusters. We find that the outer-ionization mechanism—by virtue of which electrons are ejected from the outer shells of the clusters—goes through a peak for very short (fs) pulses, whose position depends exclusively on the duration of the pulse while other parameters, such as laser fluence and cluster size, only affect its intensity. Our results thus establish that Coulomb explosion, an important process for the disintegration of materials under intense laser pulses, is almost entirely determined by the duration of the pulse while other “experimental” parameters have relatively modest effects.

Formation of nano-crystalline Si quantum dots in ZnO thin-films using a ZnO/Si multilayer structure

In this study, we propose to fabricate ZnO thin-films with nano-crystalline Si (nc-Si) quantum dots (QDs) embedded using a ZnO/Si multilayer structure by radio-frequency (RF) magnetron sputtering method. Our results show that a high Si sputtering power (PSi) can assist the formation of self-aggregated Si nano-clusters during deposition, which is helpful for the crystallization of nc-Si QDs and ZnO matrix during annealing. Great crystallinity and highly uniform size of nc-Si QDs are obtained for PSi of 110W. We experimentally demonstrate the formation of nc-Si QDs embedded in crystalline ZnO thin-films, which has a great potential for various electro-optical device applications.

Specific Features and Nature of the 890 nm Photoluminescence Band Detected in SiO x Films after Low-Temperature Annealing

A band with a peak at 890 nm is detected in the photoluminescence spectra of SiO x (x ≈ 1.3) films deposited by thermal evaporation of SiO and annealed in air at 650–1150°C. The 890-nm band appears after low-temperature (∼650°C) annealing and exhibits a number of features: (i) as the annealing temperature is elevated to 1150°C, the position of the band peak remains unchanged, whereas the intensity increases by two orders of magnitude; (ii) the effects of the annealing atmosphere (air, vacuum) and the excitation wavelength and power density on the intensity of the 890-nm band differ from the corresponding effects on the well-known bands observable in the ranges 600–650 and 700–800 nm; and (iii) the photoluminescence decay is first fast and then much slower, with corresponding lifetimes of ∼9 and ∼70 μs. The observed features are inconsistent with the interpretation of photoluminescence observed in SiO x so far. Specifically, the earlier observed photoluminescence was attributed to transitions between the band and defect states in the matrix and between the states of band tails, transitions inside Si nanoclusters, and intraion transitions in rare-earth impurity ions. Therefore, we consider here the possibility of attributing the 890-nm band to transitions in local centers formed by silicon ions twofold- and/or threefold-coordinated with oxygen; i.e., we attempt to interpret the 890-nm band in the same manner as was done for luminescence in SiO2 glasses and films slightly deficient in oxygen.

Formation of silicon nanoclusters in buried ultra-thin oxide layers

The peculiarities of buried layer formation obtained by co-implantation of O2 ions with the energy of 130 keV and carbon ions within the energy range of 30-50 keV have been investigated. The corresponding ion doses for carbon and oxygen ions were equal to and , respectively. It has been observed that annealing at 1150 °C results in enhanced oxygen diffusion towards the region with a maximum carbon concentration. Analysis of X-ray diffraction patterns with a SIMS depth profiles inherent to annealed samples suggests formation of Si nanoclusters in the region with maximum concentrations of carbon and oxygen vacancies. The intensive luminescence has been observed with the maximum at 600 nm, which could be associated with silicon nano-inclusions in thin stoichiometric SiO 2 16 cm 10 2 − × 2 17 cm 10 8 . 1 − ×

Size distribution dependence of the dielectric function of Si quantum dots described by a modified Maxwell-Garnett formulation

Numerical inversion of effective medium equations used in ellipsometry data analysis are carried out, without using any parameterized dispersion formula, in order to investigate on the influence of size distribution (SD) on the dielectric properties of silicon quantum dots (Si QDs) within a silica matrix over the energy range [0.65--6.5 eV]. To do so the dielectric function (DF) of the whole inhomogeneous layer is properly determined, and then a modified version of the Maxwell-Garnett (MG) formula, which is often used in the ellipsometric modeling of nanoscale Si, is set forth. This formula, which accounts for SD, was formerly introduced by B\'anyai and Koch. We show that a small change in the size dispersion value may induce a sensitive modification in the line shape of the DF. It is also pointed out that if only the filling factor f is accounted for as in the classical MG formula this may lead to the overestimation of the amplitude of the DF of the Si nanoclusters. Furthermore the use of f solely may induce an underevaluation of the optical-gap energy of the Si QDs but also of the broadening and transition energies associated to the ${E}_{1}$-like and ${E}_{2}$-like critical points of crystalline Si. Likewise it is shown that SD does have an impact on the static dielectric constant ${\ensuremath{\varepsilon}}_{0}$ at low frequency, contrary to what has been generally supposed. Indeed it is demonstrated that provided that either quantum confinement or surface polarization is considered, then the parameters describing the size dependence of ${\ensuremath{\varepsilon}}_{0}$ at 0.65 eV are subjected to sensitive changes as the size dispersion \ensuremath{\sigma} increases.

Stimulated Creation of the SOI Structures with Si Nano-Clustersw by Low–Dose SIMOX Technology

The peculiarities of a buried layer formation obtained by a co-implantation of O2 ions with the energy of 130 keV and carbon ions within the energy range of 30-50 keV have been investigated. The corresponding ion doses for carbon and oxygen ions were equal to 2∙1016 cm-2 and 1.8∙1017 cm-2, respectively. It has been observed that annealing at 1150°C results in enhanced oxygen diffusion towards the region with a maximum carbon concentration. Analysis of x-ray diffraction patterns and TEM images confirm formation of Si nanoclusters in the SiO2 buried layer. The intensive luminescence with the maximum at 600 nm has been observed in the synthesized structures.

Precipitation of silicon nanoclusters by laser direct-write

The ability to use a laser to direct-write tracks of localized emission enhancement in PECVD-deposited Silicon rich oxide (SRO) films is demonstrated. For this purpose, 400 nm thick SRO films with varying excess Si content were irradiated with loosely focused 355 nm, 12 ps pulses at 80 MHz while being translated at 2mm/s. Mapping of areas irradiated with energies between 4.7 nJ and 5.5 nJ/pulse exhibits regions with the largest emission enhancement. Raman and photoluminescence (PL) measurements suggest precipitation of amorphous and crystalline Si nanoclusters. In the most emissive regions, the PL efficiency of the laser-annealed films was ~70% of that obtained by standard oven-annealing processes. Stress in Si crystals in some areas is identified as leading to quenching of the PL and is hypothesized to be caused by the densification of SRO matrix.

Quantum emission efficiency of nanocrystalline and amorphous Si quantum dots

The paper presents the comparison of emission efficiencies for crystalline Si quantum dots (QDs) and amorphous Si nanoclusters (QDs) embedded in hydrogenated amorphous (a-Si:H) films grown by the hot wire-CVD method (HW-CVD) at the variation of technological parameters. The correlations between the intensities of different PL bands and the volumes of Si nanocrystals (nc-Si:H) and/or an amorphous (a-Si:H) phase have been revealed using X-ray diffraction (XRD) and photoluminescence (PL) methods. These correlations permit to discuss the PL mechanisms in a-Si:H films with embedded nc-Si QDs. The QD parameters of nc-Si:H and a-Si:H QDs have been estimated from PL results and have been compared (for nc-Si QDs) with the parameters obtained by the XRD method. Using PL and XRD results the relations between quantum emission efficiencies for crystalline ( η cr ) and amorphous ( η am ) QDs have been estimated and discussed for all studied QD samples. It is revealed that a-Si:H films prepared by HW-CVD with the variation of wire temperatures are characterized by better passivation of nonradiative recombination centers in comparison with the films prepared at the variation of substrate temperatures or oxygen flows.

Formation of silicon nanocrystals in SiN x film on PET substrates using femtosecond laser pulses

The formation of Si nanoclusters under the action of femtosecond laser pulses in a SiN x film containing excess silicon has been studied. The initial film was grown by plasmachemical deposition at 100°C on a PET substrate. The pulsed crystallization was effected by a Ti-sapphire laser operating at a wavelength of 800 nm and a pulse duration of about 50 fs. According to the Raman spectroscopy data, the pulsed laser annealing stimulated the accumulation of excess silicon in nanoclusters and their crystallization. The proposed approach can be used for the formation of semiconductor nanocrystals in dielectric films on various plastic (polymer) substrates.

Structural properties of nanocomposite SiO2 (Si) films obtained by ion-plasma sputtering and thermal annealing

The nanocomposite SiO2(Si) films containing Si nanoclusters inside insulating SiO2 matrix are promising for many nanoelectronics applications. The ion-plasma sputtering of Si in O2 containing gas mixture and following thermal annealing have been used to form nanocomposite SiO2(Si) films. The structural properties of the obtained films have been studied using several methods. Among them, there were ellipsometry, IR spectroscopy, Raman spectroscopy, and AFM. Transition of SiOx matrix into insulating SiO2 matrix has been revealed by IR spectroscopy. The shift of the transmittance spectra toward high frequency region and the increase in their intensity have been observed. The existence of amorphous and nanocrystalline phases into SiO2(Si) films have been confirmed using Raman spectroscopy. Two material phases on the film surface, namely SiO2 and Si, and surface density of silicon nanoclusters have been determined using AFM. It was shown that the size of silicon nanoclusters and their surface density depend on the level of enrichment with silicon of the initial SiOx film after ion-plasma sputtering and the temperature of subsequent annealing.

Photoluminescence dynamics of organic molecule-passivated Si nanoclusters

The size-selected phenyl-passivated Si nanoclusters with the mean diameters of 1.5 and 1.3 nm have been prepared in the solution route. The intense PL observed in Si nanoclusters, in which the band gap energies increase up to approximately 4 eV, originates from electron-hole pair recombination. From time-resolved PL, two kinds of exponential PL decay components with fast and slow lifetime were observed. The identical decays with slow lifetime will be derived from the surface states of Si nanoclusters passivated by phenyl molecules. The size-dependent zero-phonon assisted optical transitions by quantum size effects occur at a high rate of sub-nanosecond timescale.

Structural and photoluminescence properties of Si nanoclusters obtained by ion implantation into Si 3N 4 films

In the present paper we report structural and photoluminescence (PL) results from samples obtained by Si implantation into stoichiometric silicon nitride (Si 3N 4) films. The Si excess was introduced in the matrix by 170 keV Si implantation performed at different temperatures with a fluence of Φ=1×10 17 Si/cm 2. The annealing temperature was varied between 350 and 900 °C in order to form the Si precipitates. PL measurements, with a 488 nm Ar laser as an excitation source, show two superimposed broad PL bands centered around 760 and 900 nm. The maximum PL yield is achieved for the samples annealed at 475 °C. Transmission electron microscopy (TEM) measurements show the formation of amorphous nanoclusters and their evolution with the annealing temperature.

Electronic states on silicon surface after deposition and annealing of SiO x films

The spectrum of the photoconductivity induced by the polarization field of charges at surface states and traps in the film bulk has been analyzed to determine the energy band diagram at the c-Si-SiOx interface and the changes in the electronic states after the film annealing. It is found that the energy bands are bent at the Si-SiOx interface and the Si surface is enriched in electrons. In equilibrium the photocurrent peak at 1.1 eV is due to the band-to-band transitions in the silicon part of the interface. Annealing shifts the peak to higher energies; this shift increases with an increase in the annealing temperature from 650 to 1000°C. This effect is accompanied by a decrease in the photocurrent at ≤1.1 eV and weakening of the band-edge photoluminescence near the Si surface. The changes revealed are explained by the formation of an oxide layer with Si nanoclusters at the Si-SiOx interface upon annealing. This process is caused by oxygen diffusion from the SiOx film, which occurs mainly via defects on the Si wafer surface. The photoconductivity spectrum of the samples charged by short-term application of a negative potential to silicon exhibits electronic transitions in the SiOx film, both from the matrix electronic states and from the states of the defects and Si nanoclusters in the film.

Influence of manganese on mechanical properties, irradiation susceptibility and microstructure of ferritic steels, alloys and welds

The ferritic steels used world wide for nuclear pressure vessels and irradiated structures contain manganese but knowledge of the role of manganese is mainly confined to the unirradiated condition. Information from the mid 1950s to 2009 on the influence of manganese on unirradiated and irradiated properties is analysed. In unirradiated material manganese raises ambient strength, improves notched impact toughness, benefits high temperature strength, promotes dynamic strain aging and enhances creep resistance. However, by combining with carbon and by increasing phosphorus diffusion, manganese increases intergranular embrittlement after thermal aging. Neutron irradiated steels are deleteriously affected by manganese. First, the dose coefficients of irradiation hardening and embrittlement increase linearly with manganese, particularly in steels with uncombined nitrogen, and the morphology of irradiation induced interstitial loops is altered. Second, manganese occurs in irradiation induced Cu rich precipitates, altering their morphology and the associated hardening. Third, manganese becomes incorporated into irradiation enhanced Cu–Ni–Mn precipitates and Mn–Ni–Si nanoclusters that increase hardening and embrittlement at high doses. Finally, manganese segregates at grain boundaries under irradiation. Overall, the unirradiated benefits are outweighed by deleterious irradiation effects. These factors are important in supporting continued reactor operations, extending lifetimes of existing nuclear vessels and structures, improving steel specifications for advanced reactors and developing steels for nuclear fusion applications.