Light particle irradiation effects in Si nanocrystals

Si nanocrystals, formed by Si ion implantation into SiO₂ layers and subsequent annealing at 1150°C, were irradiated at room temperature either with He⁺ions at energies of 30 or 130 keV, or with 400 keV electrons. Transmission electron microscopy (TEM) and photoluminescence (PL) studies were performed. TEM experiments revealed that the Si nanocrystals were ultimately amorphized (for example at ion doses ∼10¹⁶ He cm⁻²) and could not be recrystallized by annealing up to 775°C. This contrasts with previous results on bulk Si, in which electron- and very light ion-irradiation never led to amorphization. Visible photoluminescence, usually ascribed to quantum-size effects in the Si nanocrystals, was found to decrease and vanish after He⁺ ion doses as low as 3 × 10¹²–3 × 10¹³ He cm⁻² (which produce about 1 displacement per nanocrystal). This PL decrease is due to defect-induced non-radiative recombination centers, possibly situated at the Si nanocrystal/SiO₂ interface, and the pre-irradiation PL is restored by a 600°C anneal.     

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.     

High-speed processing with Cl2 cluster ion beam

Cluster ion beam processes can produce high rate sputtering with low damage compared with monomer ion beam processes. Cl₂ cluster ion beams with different size distributions were generated with controlling the ionization conditions. Size distributions were measured using the time-of-flight (TOF) method. Si substrates and SiO₂ films were irradiated with the Cl₂ cluster ions at acceleration energies of 10–30 keV and the etching ratio of Si/SiO₂ was investigated. The sputtering yield increased with acceleration energy and was a few thousand times higher than that of Ar monomer ions. The sputtering yield of Cl₂ cluster ions was about 4400 atoms/ion at 30 keV acceleration energy. The etching ratio of Si/SiO₂ was above eight at acceleration energies in the range 10–30 keV. Thus, SiO₂ can be used as a mask for irradiation with Cl₂ cluster ion beam, which is an advantage for semiconductor processing. In order to keep high sputtering yield and high etching ratio, the cluster size needs to be sufficiently large and size control is important.     

Anisotropic deformation of polystyrene particles by MeV Au ion irradiation

MeV Au irradiation leads to a shape change of polystyrene (PS) and SiO₂ particles from spherical to ellipsoidal, with an aspect ratio that can be precisely controlled by the ion fluence. Sub-micrometer PS and SiO₂ particles were deposited on copper substrates and irradiated with Au ions at 230 K, using an ion energy and fluence ranging from 2 to 10 MeV and 1 × 10¹⁴ ions/cm² to 1 × 10¹⁵ ions/cm². The mechanisms of anisotropic deformation of PS and SiO₂ particles are different because of their distinct physical and chemical properties. At the start of irradiation, the volume of PS particles decrease, then the aspect ratio increases with fluence, whereas for SiO₂ particles the volume remains constant.     

Genesis capturing the sun: Solar wind irradiation at Lagrange 1

Genesis, a member of NASAs Discovery Mission program, is the world’s first sample return mission since the Apollo program to bring home solar matter in ultra-pure materials. Outside the protection of Earth’s magnetosphere at the Earth-Sun Lagrange 1 point, the deployed sample collectors were directly exposed to solar wind irradiation. The natural process of solar wind ion implantation into a highly pure silicon (Si) bulk composition array collector has been measured by spectroscopic ellipsometry and scanning transmission electron microscopy (STEM). Ellipsometry results show that bulk solar wind ions composed of approximately 95% H⁺, 4% He⁺ and <1% other elements physically altered the first 59-63 nm of crystalline silicon substrate during 852.8 days of solar exposure. STEM analysis confirms that the solar accelerated ions caused significant strain and visible structural defects to the silicon structure forming a 60-75 nm thick irradiation damage region directly below the surface SiO₂ native oxide layer. Monte Carlo simulations of solar wind H, He, C, O, Ne, Mg, Si and Fe ion collisions in the Si collector with fluences calculated from the Genesis and ACE spacecrafts were used to estimate the energy deposited and Si vacancies produced by nuclear stopping in a flight-like Si bulk array collector. The coupled deposited energy model with the flown Genesis Si in situ measurements provides new insight into the basic principles of solar wind diffusion and space weathering of materials outside Earth’s magnetosphere.     

Si nanocrystals formation by a new ion implantation device

Metallic and non-metallic ion beams can be used to modify the properties of wafer surfaces if accelerated at moderate energies. We developed a new “implantation machine” able to generate ions and to accelerate them up to 80 kV. The ion generation is achieved by a laser-plasma source which creates plasma in expansion. The device consists of a KrF excimer laser and a generating vacuum chamber made of stainless steel. The laser energy was 45 mJ/pulse with a power density of 2.25 × 10⁸ W/cm². The target was kept to positive voltage to accelerate the produced ions. The ion dose was estimated by a fast polarised Faraday cup. This machine was utilised to try synthesizing silicon nanocrystals in SiO₂ matrix. Preliminary results of Si nanocrystals formation and the glancing-angle X-ray diffraction analyses are reported.     

Nanometer-thick SGOI structures produced by Ge+ ion implantation of SiO2 films and subsequent hydrogen transfer of Si layers

Nanometer-thick silicon-germanium-on-insulator (SGOI) structures have been produced by the implantation of Ge⁺ ions into thermally grown SiO₂ layer and subsequent hydrogen transfer of silicon film on the Ge⁺ ion implanted substrate. The intermediate nanometer-thick Ge layer has been formed as a result of the germanium atom segregation at the Si/SiO₂ bonding interface during annealing at temperatures 800–1100 ^оС. From a thermodynamic analysis of Si/Ge/SiO₂ system, it has been suggested that the growth of the epitaxial Ge layer is provided by the formation of a molten layer at the Si/SiO₂ interface due to the Ge accumulation. The effect of germanium on the hole mobility in modulation-doped heterostructures grown over the 3–20 nm thick SGOI layers was studied. An increase in the Hall hole mobility in SGOI-based structures by a factor of 3–5 was obtained in comparison with that in respective Ge-free SOI structures.     

Endotaxial growth of InSb nanocrystals at the bonding interface of the In+ and Sb+ ion implanted SOI structure

Growth of InSb nanocrystals at the Si/SiO₂ bonding interface of silicon-on-insulator (SOI) structures has been studied as a function of the annealing temperature. SOI structures with the ion implanted regions above and below the bonding interface were produced as a result of the hydrogen transfer of the Sb⁺ ion implanted silicon layer from first silicon substrate to the In⁺ ion implanted SiO₂ layer thermally-grown on the second silicon substrate. Rutherford backscattering spectrometry and high-resolution transmission electron microscopy (XTEM) were used to study the properties of the prepared structures. Up-hill diffusion of In and Sb atoms from the implantation regions toward the bonding interface as well as subsequent interface-mediated growth of InSb nanocrystals were observed as the annealing temperature achieved 1100 °C. The strain minimizing orientations of the Si and InSb lattice heteropairs were obtained from XTEM analysis of the grown nanocrystals.     

Stopping of 0.3-1.2 MeV/u protons and alpha particles in Si

The stopping cross sections ε(E) of silicon for protons and alpha particles have been measured over the velocity range 0.3-1.2 MeV/u from a Si//SiO₂//Si (SIMOX) target using the Rutherford backscattering spectrometry (RBS) with special emphasis put on experimental aspects. A detection geometry coupling simultaneously two solid-state Si detectors placed at 165° and 150° relative to each side of the incident beam direction was used to measure the energies of the scattered ions and determine their energy losses within the stopping medium. In this way, the basic energy parameter, E_x, at the Si/SiO₂ interface for a given incident energy E₀ is the same for ions backscattered in the two directions off both the Si and O target elements, and systematic uncertainties in the ε(E) data mainly originating from the target thickness are significantly minimized. A powerful computer code has been elaborated for extracting the relevant ε(E) experimental data and the associated overall uncertainty that amounts to less than 3%. The measured ε(E) data sets were found to be in fair agreement with Paul’s compilation and with values calculated by the SRIM 06 computer code. In the case of ⁴He⁺ ions, experimental data for the γ effective charge parameter have been deduced by scaling the measured stopping cross sections to those of protons crossing the same target with the same velocity, and compared to the predictions of the SRIM 06 computer code. It is found that the γ-parameter values generated by the latter code slightly deviate from experiment over the velocity region around the stopping cross section maximum where strong charge exchanges usually occur.     

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.     

Depth-profiling of implanted 28Si by (α,α) and (α,p0) reactions

Silicon nanocrystals enclosed in thin films (Si quantum dots or Si QDs) are regarded to be the cornerstone of future developments in new memory, photovoltaic and optoelectronic products. One way to synthesize these Si QDs is ion implantation in SiO₂ layers followed by thermal annealing post-treatment.Depth-profiling of these implanted Si ions can be performed by reactions induced by α-particles on ²⁸Si. Indeed, for high incident energy, nuclear levels of ³²S and ³¹P can be reached, and cross-sections for (α,α) and (α,p₀) reactions are more intense. This can help to increase the signal for surface silicon, and therefore make distinguishing more easy between implanted Si and Si coming from the SiO₂, even for low fluences.In this work, (α,α) and (α,p₀) reactions are applied to study depth distributions of 70 keV ²⁸Si⁺ ions implanted in 200 nm SiO₂ layers with fluences of 1 × 10¹⁷ and 2 × 10¹⁷ cm^?2. Analysis is performed above E_R = 3864 keV to take advantage of resonances in both (α,α) and (α,p₀) cross-sections. We show how (α,p₀) reactions can complement results provided by resonant backscattering measurements in this complex case.     

Dependence of ion-induced Pd-siliclde formation on nuclear energy deposition density

Pd₂Si formation at the Pd-Si interface induced by irradiation with ions having a wide range of nuclear energy deposition density has been investigated. It is found that the thickness of the suicide layer formed by irradiation is proportional to the ion fluence for irradiation with ions having low energy-deposition densities, while it is proportional to the square root of the fluence for irradiation with ions having energy-deposition densities. The results indicate that Pd₂Si formation is reaction limited when the energy-deposition density at the interface is low and is diffusion limited when it is high. The results are compared with the phenomenological theory developed by Horino et al. and it is shown that such a dependence of the limiting processes on the energy deposition density is induced when the diffusion is thermally activated while the reaction at the interface is radiation-enhanced.     

Low temperature formation of luminescent Si nanocrystals with combined process of excimer UV-light irradiation and RTA

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 achieve low temperature (below 1000 °C) formation of luminescent Si nanocrystals in SiO₂ have been investigated. The Si ions were introduced at acceleration energy of 180 keV to fluences of 7.5 × 10¹⁶ and 1.5 × 10¹⁷ ions/cm². The implanted samples were subsequently irradiated with an excimer-UV lamp for 2 h. After the process, the samples were rapidly thermal annealed at 1050 °C for 5 min before furnace annealing (FA) at 900 °C. Photoluminescence spectra were measured at various stages at the process. Effective visible photoluminescence is found to be observed even after FA at 900 °C, only for specimens treated with excimer-UV lamp and RTA, prior to a low temperature FA process. Based on our experimental results, we discuss the mechanism for the initial formation process of the luminescent Si nanocrystals in SiO₂, together with the effects with excimer lamp irradiation and RTA process on the luminescence.     

Surface oxidation of Si assisted by irradiation with large gas cluster ion beam in an oxygen atmosphere

Surface oxidation of Si assisted by Ar cluster impact with a current density of a few μA/cm² under O₂ atmosphere was investigated. Thin-film formation by cluster ion beam presents a number of advantages such as atomic-scale surface smoothing, high density and precise stoichiometry. We used an Ar cluster ion beam with 20 keV and a mean size of 1000 atoms per cluster and measured the emission yields of Si⁺ and SiO⁺ after Ar cluster ion irradiation in O₂ atmosphere using a quadrupole mass spectrometer to investigate the dependence of Si surface oxidation on oxygen partial pressure. It was found that the Si surface was oxidized by Ar cluster ion irradiation in O₂ atmosphere.     

Mechanism of elongation of gold or silver nanoparticles in silica by irradiation with swift heavy ions

It has been reported that elongated Au nanoparticles oriented parallel to one another can be synthesized in SiO₂ by ion irradiation. Our aim was to elucidate the mechanism of this elongation. We prepared Au and Ag nanoparticles with a diameter of 20 nm in an SiO₂ matrix. It was found that Au nanoparticles showed greater elongated with a higher flux of ion beam and with thicker SiO₂ films. In contrast, Ag nanoparticles split into two or more shorter nanorods aligned end to end in the direction parallel to the ion beam. These experimental results are discussed in the framework of a thermal spike model of Au and Ag nanorods embedded in SiO₂. The lattice temperature exceeds the melting temperatures of SiO₂, Au and Ag for 100 ns after one 110 MeV Br¹⁰⁺ ion has passed through the middle of an Au or Ag nanorod.     

Effects of ion irradiation on the structural transformation of sol-gel derived TEOS/MTES thin films

Ion beam processing of organic/inorganic thin films has been shown to be an effective means in converting polymeric films into their final ceramic-like state. In this study, hybrid sol-gel derived thin films based on TEOS (tetraethylorthosilicate) Si(OC₂H₅)₄ and MTES (methyltriethoxysilane) CH₃Si(OC₂H₅)₃ were prepared and deposited on Si substrates by spin coating. After the films were allowed to air dry, they were heat treated at 300 °C for 10 min. Ion irradiation was performed at room temperature using 125 keV H⁺ and 250 keV N²⁺ ions with fluences ranging from 1 × 10¹⁴ to 5 × 10¹⁶ ions/cm². FT-IR and Raman spectroscopies were used to quantify the chemical structural transformations which occurred including the evolution of the organic components, the cross-linking of silica clusters, and the clustering of carbon.     

Determination of migration of ion-implanted helium in silica by proton backscattering spectrometry

Understanding the processes caused by ion implantation of light ions in dielectric materials such as silica is important for developing the diagnostic systems used in fusion and fission environments. Recently, it has been shown that ion-implanted helium is able to escape from SiO₂ films. To study this process in details, helium was implanted into the central part of a buried SiO₂ island up to a fluence of 4 × 10¹⁷ He/cm². The implanted helium could be detected in the SiO₂ island, if the oxide was insulated properly from the vacuum. The shape of the helium depth distributions was far from SRIM simulation because helium distributed in the whole 1 μm thick oxide layer. After the ion implantation, helium was observed only on the implanted spot. After nine months the implanted helium filled out the whole oxide island as it was expected from the high diffusivity.     

Nanopatterning of silica with mask-assisted ion implantation

In the present paper we combined ion implantation and nanosphere lithography to regularly dope, by a mask-assisted process, a SiO₂ substrate with rare earth ions (Er) by ion implantation and to fabricate by sputtering a plasmonic 2D periodic array of Au nanostructures on the silica surface spatially coupled to the implanted Er³⁺ ions. The aim of this work is to study how Er³⁺ emission at 1.5 μm can be affected by the interaction with a plasmonic nanostructure. In particular we have found a variation of the radiative lifetime of the Er³⁺ emission and a change from single exponential to bi-exponential of the luminescence intensity decay.     

Ion irradiation effects on surface mechanical behavior and shrinkage of hybrid sol-gel derived silicate thin films

A study of the effects of ion irradiation on the surface mechanical behavior and shrinkage of organic/inorganic modified silicate thin films was performed. The films were synthesized by sol-gel processing from tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES) precursors and spin-coated onto Si substrates. The sol viscosity and the spin velocity were adjusted so that the films produced had a final thickness ranging from 580 to 710 nm after heat treatment. The ion species and incident energies used were selected such that the projected ion range was greater than the film thickness, resulting in fully irradiated films. After heat treatment at 300 °C for 10 min, the films were irradiated with 125 keV H⁺, 250 keV N²⁺ and 2 MeV Cu⁺ ions with fluences ranging from 1 × 10¹⁴ to 1 × 10¹⁶ ions/cm². Both hardness and reduced elastic modulus were seen to exhibit a monotonic increase with fluence for all three ion species. Also, H loss was found to increase monotonically with increase in fluence, while the film thickness was found to decrease with increase in fluence.     

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.