Structural stability of Cu nanocrystals in SiO2 exposed to high-energy ion irradiation

Cu nanocrystals (NCs) were synthesized in SiO₂ by ion implantation and thermal annealing. Annealing at two different temperatures of 950 °C and 650 °C yielded two different nanocrystal size distributions with an average diameter of 8.1 and 2.5 nm, respectively. Subsequently the NCs were exposed to 5.0 MeV Sn³⁺ ion irradiation simultaneously with a thin Cu film as a bulk reference. The short-range atomic structure and average NC diameter was measured by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and small angle X-ray scattering (SAXS), respectively. Consistent with the high regeneration rate of bulk elemental metals, no irradiation induced defects were observed for the reference, whereas the small NCs (2.5 nm) were dissolved as Cu monomers in the matrix. The latter was attributed to irradiation-induced mixing of Cu, Si and O based on dynamic binary collision simulations. For the large NCs (8.1 nm) only minor structural changes were observed upon irradiation, consistent with a more bulk-like pre-irradiation structure.     

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.     

The excitation power density effect on the Si nanocrystals photoluminescence

In the present work we have studied the photoluminescence (PL) behavior from Si nanocrystals (NCs) as a function of the excitation power density and annealing time. The NCs were produced in a SiO₂ matrix by Si implantations from room temperature (RT) up to 700 °C, followed by post-annealing in N₂ atmosphere at high temperature. With this aim we have changed the excitation power density (from 2 × 10⁻³ W/cm² up to 15 W/cm²) and the annealing time (from 10 min up to 15 h). The strong PL signal, which at 15 W/cm² is composed by a single-peak structure (650–1000 nm) centered at around 780 nm, expands up to 1200 nm showing a two-peak structure when measured at 20 × 10⁻³ W/cm². The peak structure located at the short wavelength side is kept at 780 nm, while the second peak, starting at around 900 nm, redshifts and increases its intensity with the implantation temperature and annealing time. The effect of the annealing time on the PL spectra behavior measured at low excitation power agrees by the first time with the Si NC growth according to quantum confinement effects.     

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.     

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.     

Heavy-ion-induced luminescence of amorphous SiO2 during nanoparticle formation

Silica glass was implanted with negative 60 keV Cu ions at an ion flux from 5 to 75 μA/cm² up to a fluence of 1 × 10¹⁷ ions/cm² at initial sample temperatures of 300, 573 and 773 K. Spectra of ion-induced photon emission (IIPE) were collected in situ in the range from 250 to 850 nm. Optical absorption spectra of implanted specimens were ex situ measured in the range from 190 to 2500 nm.

IIPE spectra showed a broad band centered around 560 nm (2.2 eV) that was assigned to Cu⁺ solutes. The band appeared at the onset of irradiation, increased in intensity up to a fluence of about 5 × 10¹⁵ ions/cm² and then gradually decreased indicating three stage of the ion beam synthesis of nanoclusters: accumulation of implants, nucleation and growth nanoclusters. The IIPE intensity normalized on the ion flux is independent on the ion flux below 20 μA/cm²at higher fluences. The intensity of the band increased with increasing samples temperature, when optical absorption spectra reveal the increase of Cu nanoparticles size.     

Formation processes of nickel oxide nanoparticles in SiO2 by metal-ion implantation combined with thermal oxidation

Formation processes of nickel oxide (NiO) nanoparticles (NPs) in silica glass (SiO₂) by implantation of 60 keV Ni⁻ ions combined with thermal oxidation are studied using cross-sectional transmission electron microscopy (XTEM) and Rutherford backscattering spectrometry. In as-implanted state, Ni metallic NPs form within the surface layer of 80 nm thick. The mean depth z and the standard deviation (Δz)^21/2 of Ni atom distribution determined by XTEM were 43 and 15 nm, respectively. After the oxidation at 800 °C for 1 h, z and (Δz)^21/2 became 47 nm and 20 nm, respectively, i.e. the distribution was almost the same except a small diffusional shift to the deeper region. Mean volume of Ni- and NiO-NPs after heat treatments at 800 °C were 27 and 43 nm³, respectively. The larger mean volume of NiO-NPs is explained from the fact that NiO-NPs include both Ni and O atoms, i.e. approximately 2N atoms, while Ni-NPs include Ni atoms only, i.e. N atoms. Both the Ni- and NiO-NPs include 2.4 × 10³ Ni atoms per NP in average. These results indicate that NiO-NPs are formed by oxidation of Ni-NPs without pronounced redistribution of Ni atoms.     

Ge nanocrystals embedded in SiO2 in MOS based radiation sensors

In this work, the effects of gamma radiation on the Raman spectra of Ge nanocrystals embedded in SiO₂ have been investigated. SiO₂ films containing nanoparticles of Ge were grown using the r.f.-magnetron sputtering technique. Formation of Ge nanocrystals was observed after high temperature annealing in an inert atmosphere and confirmed by Raman measurements. The intensity of the Raman signal originating from Ge nanocrystals was found to decrease with increasing gamma radiation. The study also includes the gamma radiation effects on MOS structure with Ge nanocrystals embedded in SiO₂. The gamma radiation effects from 500 up to 4000 Gray were investigated. Capacitance-voltage measurements were performed and analyzed. Oxide traps and interface trap charges were calculated. Results show that MOS structure with Ge nanocrystals embedded in SiO₂ is a good candidate to be used in radiation sensors, especially at high radiation doses.     

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.     

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.     

退火温度对掺铒硅1.54μm光致发光的影响

利用金属蒸气真空弧（MEVVA）离子源交稀土金属Er离子注入单晶硅中，经快速退火制备出掺铒硅发光薄膜。RBS分析表明掺铒硅的浓度接近10at%,即可达10^21cm^-3量级。XRD分析薄膜物相结构发现，退火温度升高将导致Er偏析。通过RHEED和AFM显微分析可得，退火温度影响辐照损伤的恢复程度，Si固相外延再结晶和显微形貌。这些结构变化将影响掺铒硅发光薄膜1.54μm光致发光。     

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.     

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.     

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.     

Radiation damage induced by 5 keV Si ion implantation in strained-Si/Si0.8Ge0.2

The damage distributions induced by ultra low energy ion implantation (5 keV Si⁺) in both strained-Si/Si_0.8Ge_0.2 and normal Si are measured using high-resolution RBS/channeling with a depth resolution better than 1 nm. Ion implantation was performed at room temperature over the fluence range from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². Our HRBS results show that the radiation damage induced in the strained Si is slightly larger than that in the normal Si at fluences from 1 × 10¹⁴ to 4 × 10¹⁴ ions/cm² while the amorphous width is almost the same in both strained and normal Si.     

Structural studies of Ge nanocrystals embedded in SiO2 matrix

Germanium nanoparticles embedded in SiO₂ matrix were prepared by atom beam sputtering on a p-type Si substrate. The as-deposited films were annealed at temperatures of 973 and 1073 K under Ar + H₂ atmosphere. The as-deposited and annealed films were characterized by Raman, X-ray diffraction and Fourier transform infrared spectroscopy (FTIR). Rutherford backscattering spectrometry was used to quantify the concentration of Ge in the SiO₂ matrix of the composite thin films. The formation of Ge nanoparticles were observed from the enhanced intensity of the Ge mode in the Raman spectra as a function of annealing, the appearance of Ge(3 1 1) peaks in the X-ray diffraction data and the Ge vibrational mode in the FTIR spectra. We have irradiated the films using 100 MeV Au⁸⁺ ions with a fluence of 1 × 10¹³ ions/cm² and subsequently studied them by Raman and FTIR. The results are compared with the ones obtained by annealing.     

Photoluminescence from neodymium silicide thin films formed by MEVVA ion source

Neodymium silicides were synthesized by Nd ion implanted into Si substrates with the aid of a metal vapor vacuum are(MEVVA)ion source.The blender of Nd5Si4 and NdSi2 was formed in a neodymium-implanted silicon thin film during the as-implanted state,but there was only single neodymium silicide compound in the post-annealed state,and the phase changed from NdSi2 to Nd5Si4 with increasing annealing temperature.The blue-violet luminescence excited by ultra-viloet was observed at the room temperature(RT),and the intensity of photoluminescence(PL)increased with increasing the neodymium ion fluence,Moreover,the photoluminescence was closely dependent on the temperature of rapid thermal annealing(RTA),A mechanism of photoluminescence was discussed.     

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.     

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.