Influence of annealing temperatures and time on the photoluminescence properties of Si nanocrystals embedded in SiO2

We report on the effects of annealing conditions on the photoluminescence from Si nanocrystal composites fabricated by implantation of Si ions into a SiO₂ matrix, followed by thermal treatment in a nitrogen atmosphere. The evolution of the photoluminescence under different annealing temperatures (900–1100 °C) and annealing time (0.5 up to 5 h) were systematically studied for the implanted samples. After annealing the spectra presented two photoluminescence bands: one centered at 610 nm and another around 800 nm. Combined with transmission electron microscopy, we conclude that the photoluminescence behavior of the two bands suggests different origins for their emissions. The 610 nm band has its origin related to matrix defects, while the 800 nm band can be explained by a model involving recombination via quantum confinement effects of excitons in the Si nanocrystals and the interfacial states recombination process confined in the interfacial region between nanocrystals and SiO₂ matrix.     

Radiation effect on the photoluminescence properties of Si/SiO2 thin films

Silicon ions were implanted into SiO₂ thin films with various doses and energies. For the films implanted with various ion doses the photoluminescence (PL) intensity of 470 nm firstly increased with the increase of Si ion dose, which is similar to the variation trend of displacement per atom (DPA) number during ion radiation. Further increasing Si ion dose the PL intensity of 470 nm decreased gradually since the neutral oxygen vacancy centers were destroyed. For the samples implanted with different energy the variation trend of PL intensity for 470 nm peak is similar to the result of DPA under different radiation energy according to SRIM2006 simulation. With the increase of radiation energy a new PL peak at 550 nm appeared because of the variation of defect type. Combining with the simulation results and PL spectra the radiation effect on Si/SiO₂ thin films were proposed.     

Influence of UV irradiation and RTA process on optical properties of Si implanted SiO2

Si ion implantation was widely used to synthesize specimens of SiO₂ containing supersaturated Si and subsequent high temperature annealing induces the formation of embedded luminescent Si nanocrystals. In this work, the potentialities of excimer UV-light (172 nm, 7.2 eV) irradiation and rapid thermal annealing (RTA) to enhance the photoluminescence and to achieve low temperature formation of Si nanocrystals have been investigated. The Si ions were introduced at acceleration energy of 180 keV to fluence of 7.5 × 10¹⁶ ions/cm². The implanted samples were subsequently irradiated with an excimer-UV lamp. After the process, the samples were rapidly thermal annealed before furnace annealing (FA). Photoluminescence spectra were measured at various stages at the process. We found that the luminescence intensity is strongly enhanced with excimer-UV irradiation and RTA. Moreover, effective visible photoluminescence which is not observed with a simple FA treatment, is found to be observed even after FA at 900 °C, only for specimens treated with excimer-UV lamp and RTA. Based on our experimental results, we discuss the effects of excimer-UV lamp irradiation and RTA process on Si nanocrystals related photoluminescence.     

Monte Carlo study of ion-induced secondary electron emission from SiO2

Here we describe a recently developed direct Monte Carlo program to study kinetic electron emission from SiO₂ target. The program includes excitation of the target electrons (by projectile ions, recoiling target atoms and fast primary electrons), subsequent transport and escape of these electrons from the target surface. The program can be used to calculate the electron yields, distribution of electron excitation points in the target and other physical parameters of the emitted electrons. In order to demonstrate the capabilities of this program, we report a study on the kinetic electron emission from SiO₂ induced by fast (1-10 keV) rare gas ions. The calculated kinetic electron yield for various ion energies and masses is in good agreement with the predictions of most frequently applied theoretical model. In addition, the effects of projectile energy, mass and impact angle on the depth distribution of electron excitation points and average escape depth of the outgoing electrons were investigated. It is important to mention that the existing experimental techniques are not capable to measure these parameters.     

Effect of Si implantation on the microstructure of silicon nanocrystals and surrounding SiO2 layer

Si nanocrystals (Si-nc) embedded in a SiO₂ layer have been characterized by means of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). For local Si concentration in excess  8 × 10²¹ Si⁺/cm³, the size of the Si-nc was found to be 3 nm and comparatively homogeneous throughout the whole implanted layer. For local Si concentration in excess of 2.4 × 10²² Si⁺/cm³, the Si-nc diameter ranges from 2 to 12 nm in the sample, the Si-nc in the middle region of the implanted layer being bigger than those near the surface and the bottom of the layer. Also, Si-nc are visible deeper than the implanted depth. Characterization by XPS shows that a large quantity of oxygen was depleted from the first 25 nm in this sample (also visible on TEM image) and most of the SiO₂ bonds have been replaced by Si–O bonds. Experimental and simulation results suggest that a local Si concentration in excess of 3 × 10²¹ Si/cm³ is required for the production of Si-nc.     

Absorption and photoluminescence study of Al2O3 single crystal irradiated with fast neutrons

Colour centers formation in Al₂O₃ by reactor neutrons were investigated by optical measurements (absorption and photoluminescence). The irradiation’s were performed at 40 °C, up to fast neutron (E_n > 1.2 MeV) fluence of 1.4 × 10¹⁸ n cm⁻². After irradiation the coloration of the sample increases with the neutron fluence and absorption band at about 203, 255, 300, 357 and 450 nm appear in the UV-visible spectrum. The evolution of each absorption bands as a function of fluence and annealing temperature is presented and discussed. The results indicate that at higher fluence and above 350 °C the F⁺ center starts to aggregate to F center clusters (F₂, F₂⁺ and ). These aggregates disappear completely above 650 °C whereas the F and F⁺ centers persist even after annealing at 900 °C. It is clear also from the results that the absorption band at 300 nm is due to the contribution of both F₂ center and interstitial ions.     

Ion beam induced desorption from thin films: SiO2 single layers and SiO2/Si multilayers

The occurrence of O₂ molecular loss from the bulk of SiO₂ single layers and SiO₂/Si multilayers as a result of 50 MeV Cu⁹⁺ irradiation has been investigated. This process did not take place with a significant rate, if it occurs at all. Instead both Si and O are removed from the SiO₂ surface region, releasing molecular O₂. If an elemental Si layer is on top in a multilayer, removal of Si and O with an appreciable rate is not observed. The irradiation creates bubbles in the SiO₂/Si multilayers, which contain O₂. The distinct SiO₂ sublayers remain chemically intact. The bubbles deteriorate the depth resolution in elastic recoil detection.     

Enhancement of luminescence from encapsulated Si nanocrystals in SiO2 with rapid thermal anneals

Potentialities of rapid thermal annealing to enhance the photoluminescence emission of Si nanocrystals in SiO₂ have been investigated. Ion implantation was used to synthesize specimens of SiO₂ containing excess Si with different concentrations. Si precipitation to form nanocrystals in implanted samples takes place with a conventional furnace anneal. The photoluminescence intensity and the peak energy of emission from Si nanocrystals depend on implanted ion fluence. Moreover, the luminescence intensity is strongly enhanced with a rapid thermal anneal prior to a conventional furnace anneal. The luminescence intensity, however, decreases with a rapid thermal anneal following a conventional furnace anneal. It is found that the order of heat treatment is an important factor in intensities of the luminescence. Moreover, the luminescence peak energy is found to be dependent, but a little, on thermal history of specimens. Based on our experimental results, we discuss about the mechanism of an enhancement of the photoluminescence, together with the mechanism of photoemission from encapsulated Si nanocrystals produced in a SiO₂ matrix by ion implantation and annealing.     

Investigation of BF2 implants in silicon through SiO2 films Redistribution of fluorine and boron under rapid thermal annealing

Boron difluoride (BF₂⁺) ions implantation through protecting oxide films was investigated to understand the behaviour of fluorine in damaged region under rapid thermal annealing (RTA) as well as in attempt to form shallow p⁺/n junctions. The implants redistribution profiles as a function of annealing temperatures and time have been monitored by the secondary ion mass spectrometry (SIMS). Implantation induced point-defects are detected by means of deep level transient spectroscopy (DLTS) technique. The effects of both fluorine and the RTA ion-implant damage annihilation on the transient enhanced boron diffusion are investigated. In particular, the fluorine segregation behaviour near the post implantation disturbed/crystalline interface, resulting in clustering and void formation, as well as near the initial oxide/crystalline silicon is closely related to used technological data.     

Structure and morphology of ion irradiated Au nanocrystals in SiO2

We have investigated the effect of ion irradiation on the structure and morphology of Au nanocrystals (NCs) fabricated by ion beam synthesis in a thin SiO₂ layer on a Si substrate. Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements show a significant drop in the average Au–Au coordination, as well as a loss of medium and long range order with increasing irradiation dose. Small angle X-ray scattering (SAXS) measurements reveal a concomitant reduction in average NC size. These observations are a consequence of structural disorder and collisional mixing induced by the irradiation. The observed reduction in average Au–Au coordination by EXAFS differs significantly from that estimated from the average NC sizes evaluated using SAXS. This behavior can be explained by the dissolution of Au NCs into the SiO₂ matrix. A significant bond-length contraction indicates that part of this material forms small Au clusters (dimers, trimers, etc.) during irradiation that cannot be detected by SAXS. Combining the results from SAXS and EXAFS measurements, we estimate the volume fraction of such clusters.     

Plasma nitridation of thin SiO2 films: AES, ELS and IR study

Auger electron spectroscopy, low-energy electron loss spectroscopy and infrared spectroscopy are used to investigate the nitridation of thin (10–22 nm) thermal SiO₂ in RF soft NH₃ plasma. It is found that plasma action at a substrate temperature of 573 K can completely nitridate the thermal oxide to an oxynitride layer. The layers obtained are macroscopic mixtures of two phases SiO₂ and Si₃N₄, rather than amorphous polymers of Si, O and N.     

Effects of MeV Si ions bombardment on the thermoelectric generator from SiO2/SiO2 + Cu and SiO2/SiO2 + Au nanolayered multilayer films

The defects and disorder in the thin films caused by MeV ions bombardment and the grain boundaries of these nanoscale clusters increase phonon scattering and increase the chance of an inelastic interaction and phonon annihilation. We prepared the thermoelectric generator devices from 100 alternating layers of SiO₂/SiO₂ + Cu multi-nano layered superlattice films at the total thickness of 382 nm and 50 alternating layers of SiO₂/SiO₂ + Au multi-nano layered superlattice films at the total thickness of 147 nm using the physical vapor deposition (PVD). Rutherford Backscattering Spectrometry (RBS) and RUMP simulation have been used to determine the stoichiometry of the elements of SiO₂, Cu and Au in the multilayer films and the thickness of the grown multi-layer films. The 5 MeV Si ions bombardments have been performed using the AAMU-Center for Irradiation of Materials (CIM) Pelletron ion beam accelerator to make quantum (nano) dots and/or quantum (quantum) clusters in the multilayered superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric generator devices before and after Si ion bombardments we have measured Seebeck coefficient, cross-plane electrical conductivity, and thermal conductivity in the cross-plane geometry for different fluences.     

Effect of forming gas anneals on the photoluminescence from nanocrystalline silicon formed by Si implantation into SiO2 matrix

The blue region of the room temperature photoluminescence spectrum from Si nanocrystallites formed in SiO₂ by Si⁺ ion implantation has been observed for the first time after annealing in a forming gas (10% H₂ + 90% N₂) ambient. Thermally grown SiO₂ on Si substrates were implanted with a dose of 2 × 10¹⁷ Si⁺ cm⁻² at energies of 200 keV and 400 keV. For reference purposes, quartz silica was implanted also with the same dose of 200 keV Si⁺ ions. The implanted samples were annealed in nitrogen and forming gas at 900°C for 3 to 180 min. Both the SiO₂ and quartz samples exhibited luminescence at about 380 nm which was weak, but detectable, before annealing. During extended anneals in forming gas, the intensity increased by a factor of about 2 above that recorded after a nitrogen anneal but the peak position was unchanged. The intensity was greater in samples annealed in forming gas which is due to the additional hydrogen. It would seem that this blue luminescence originates from new luminescent centres in the matrix caused by the Si⁺ ion implantation.     

Blue luminescence from poly(methyl methacrylate) modified ZnO and anatase TiO2 nanocrystals prepared using γ radiation

The surface of ZnO and anatase TiO₂ nanocrystals was modified by PMMA through γ radiation. The modified nanocrystals were investigated with photoluminescence (PL) and Fourier transform infrared (FTIR) spectra. A stable blue luminescence peak (420 nm) can be observed for the modified ZnO and anatase TiO₂ nanocrystals. The intensities of luminescence of the modified ZnO and anatase TiO₂ in aqueous solution are very stable, even unchanged after storage time of one month.     

Positron annihilation on the surfaces of SiO2 films thermally grown on single crystal of Cz-Si

Two-detector coincidence system and mono-energetic slow positron beam has been applied to measure the Doppler broadening spectra for single crystals of SiO₂, SiO₂ films with different thickness thermally grown on single crystal of Cz-Si, and single crystal of Si without oxide film. Oxygen is recognized as a peak at about 11.85 × 10⁻³m₀c on the ratio curves. The S parameters decrease with the increase of positron implantation energy for the single crystal of SiO₂ and Si without oxide film. However, for the thermally grown SiO₂-Si sample, the S parameters in near surface of the sample increase with positron implantation energy. It is due to the formation of silicon oxide at the surface, which lead to lower S value. S and W parameters vary with positron implantation depth indicate that the SiO₂-Si system consist of a surface layer, a SiO₂ layer, a SiO₂-Si interface layer and a semi-infinite Si substrate.     

Scattering of light molecules from thin Al2O3 films

Molecular oxygen and hydrogen ions were scattered at grazing incidence from various thin Al₂O₃ films. The energy of incident particles was varied from 390 to 1000 eV. For scattered positive oxygen ions, negative ion fractions of up to 17% were recorded. For scattered positive hydrogen ions, the negative ion fractions reached up to 2%. These findings qualify thin films of Al₂O₃ as possible candidates for use as charge state conversion surfaces in neutral particle sensing instruments, which will work in space.     

Glass-ceramic nuclear waste forms obtained from SiO2-Al2O3-CaO-ZrO2-TiO2 glasses containing lanthanides (Ce, Nd, Eu, Gd, Yb) and actinides (Th): study of internal crystallization

Glass-ceramic waste forms such as zirconolite (nominally CaZrTi₂O₇) based ones can be envisaged as good candidates for minor actinides or Pu immobilization. Such materials, in which the actinides (or lanthanides used as actinide surrogates) would be preferentially incorporated into zirconolite crystals homogeneously dispersed in a durable glassy matrix, can be prepared by controlled crystallization (nucleation + crystal growth) of parent glasses belonging to the SiO₂-Al₂O₃-CaO-ZrO₂-TiO₂ system. In this work we present the effects of the nature of the minor actinide surrogate (Ce, Nd, Eu, Gd, Yb, Th) on the structure, the microstructure and the composition of the zirconolite crystals formed in the bulk of the glass-ceramics. The amount of lanthanides and thorium incorporated into zirconolite crystals is discussed in relation with the capacity of the glass to accommodate these elements and of the crystals to incorporate them in the calcium and zirconium sites of their structure.     

Er site in Er + Au-implanted SiO2: Effect of annealing in reducing atmosphere

The Er site in Er + Au-implanted silica has been investigated by x-ray absorption spectroscopy, in particular after annealing in reducing atmosphere (H₂(4%):N₂(95%)) at temperature ranging from 100 to 800 °C. The EXAFS analysis shows that Er ions are surrounded by a first shell of O atoms, while the absence of signal from further coordination shell indicates a disordered site. The Er-O distance is lower than that of the Er₂O₃; it is suggested that the annealing in reducing atmosphere leads to a significant reduction of the first shell coordination number. Correspondingly, in the XANES region of the spectrum, it is observed a decrease in the white line intensity for annealing temperature higher than 400 °C; similar annealing treatments in inert atmosphere did not result in significant changes of the near-edge region of the X-ray absorption spectrum. These results enlighten that the annealing procedure, normally used to tailor the size distribution of the metal clusters present in the matrix and/or to modify the matrix structure, can also have an effect on the site of the Er ions, and possibly on the rare-earth optical properties.     

A dilatometric study of the solubility of U4O9 in UO2

Phase relationships in the system UO₂-O₉ were studied using a dilatometei in which the O/U ratio of a UU_2+x specimen could be both controlled and measured. Phase boundary temperatures were indicated by changes in expansion or contraction rate during heating or cooling, respectively. The solubility of U₄O₉ in UO₂ agreed with the results of previous workers using other techniques. The dependence of solubility on temperature is complex, and appears to be influenced by a high-temperature phase transition in U₄0₉.