Low temperature fast-neutron and gamma irradiation of glass fiber-epoxy composite. Part 2: Structure and chemistry

Structure and chemistry of S2-glass fiber-epoxy composite to fast-neutron and gamma irradiation at 4.2 K have been studied. Fast-neutron fluence ranged from 0.9 × 10²² to 1 × 10²³ n/m². At the neutron fluence of 3.1 × 10²² n/m², ordering of the amorphous S2-glass structure intervened. Ordering was followed by partial crystallization at the neutron fluence of 1 × 10²³ n/m². Although the epoxy matrix retained its amorphous structure, reactor irradiation induced crosslink formation and hydrogen evolution. These processes became significant at neutron fluences equal to or higher than 1.8 × 10²² n/m². No evidence has been found for hydrogen bubble or void nucleation at the glass-epoxy interface at the neutron fluence of 3.1 × 10²² n/m². Futhermore, sharp compositional changes measured at the glass-epoxy interface preclude an extended irradiation-induced atom mixing at this neutron fluence.     

Ion beam synthesis and recrystallization of amorphous SiGe/SiC structures

Si_1−xGe_x amorphous layers implanted with different doses of carbon (between 5 × 10¹⁵ and 2 × 10¹⁷ cm⁻² and annealed at 700°C and 900°C have been analyzed by Raman and Infrared spectroscopies, electron microscopy and Auger electron spectroscopy. The obtained data show the synthesis of amorphous SiC by implanting at the highest doses. In these cases, recrystallization only occurs at the highest annealing temperature (900°C). The structure of the synthesized SiC strongly depends on the implantation dose, in addition to the anneal temperature. For the highest dose (2 × 10¹⁷ cm⁻²), crystalline β-SiC is formed. Finally, a strong migration of Ge towards the Si substrate is observed from the region where SiC precipitation occurs.     

Carbon clustering in Si1−xCx formed by ion implantation

Silicon-carbon alloys were formed by multiple energy implantation of C⁺ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 10¹⁶ − 3 × 10¹⁷ ions/cm² and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm⁻¹. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.     

Pumping experiment of water on B and LaB6 films with an electron beam evaporator

The pumping characteristic of water vapor on boron and lanthanum hexaboride films formed with an electron beam evaporator have been investigated in high vacuum between 10⁻⁴ and 10⁻³ Pa. The measured initial maximum pumping speeds of water for the fresh B or LaB₆ films with a deposition amount from 2.3 × 10²¹ to 6.7× 10²¹ molecules/m² separately formed on a substrate are 3.2–4.9 m³/sm², and the saturation values of adsorbed water on these films are 2.1 ×10²⁰−1.3 × 10²¹ H₂O molecules/m².     

Measurements of the erosion of stainless steel, carbon, and SiC by hydrogen bombardment in the energy range of 0.5 – 7.5 keV

Total erosion yields by sputtering and blistering for 1 to 15 keV H₂⁺ bombardment at normal incidence have been measured by weight loss of 304 stainless steel, pyrolytic graphite, carbon fibres, glassy carbon and SiC. The erosion yields are in the range of 3 × 10⁻³ to 2.6 × 10⁻² atoms per incident hydrogen atom. Observation in the scanning electron microscope shows that blisters occur in stainless steel and SiC at doses of 5 × 10¹⁸ particles/cm², but disappear at doses of 5 × 10 particles/cm² . The surface roughening observed depends largely on grain orientation. On carbon no blistering could be found. After bombardment the carbon surfaces are generally more smooth than before.     

Effect of tin on the irradiation growth of polycrystalline zirconium

Measurements of irradiation growth of polycrystalline Zr-1.5% Sn and Zr-0.1% Sn alloys at 353 K and 553 K have been made following fast neutron irradiation with fluences up to 3.1 × 10²⁵ n/m². At 353 K, growth of Zr-1.5% Sn virtually saturated at a strain of 4.5 × 10⁻⁴ after a fluence of ˜10²⁴ n/m². At this temperature, Zr-0.1% Sn continued to grów until ˜ 2 × 10²⁵ n/m², when the strain levelled off at ˜ 1.2×10⁻³. At 553 K, Zr-1.5% Sn initially grew about twice as fast as the 0.1% Sn alloy, but both eventually reached the same steady state rate of ˜ 2.4 × 10⁻²⁹ m²/n. Comparison of the data for the 1.5% Sn material with those for Zircaloy-2 from earlier work reveals that at 353 K, growth is suppressed by the presence of Sn atoms, which may serve as vacancy traps. However, at 553 K, minor additions and impurities in Zircaloy-2 (such as Fe, Ni, Cr and O) play an important role and cannot be neglected. The growth behaviour of Zr-0.1% Sn is similar to that of pure polycrystalline zirconium, especially at 353 K, indicating that the addition of Sn at this concentration does not strongly influence the growth of zirconium.     

Investigations of lattice location and mobility of carbon in GaAs using the channeling technique

In order to get information about the lattice location and the mobility of ¹²C in GaAs wafers, the channeling technique in combination with nuclear reaction analysis (NRA) is a powerful method. The targets were implanted with ¹²C ions (2.6 × 10¹³−2.6 × 10¹⁵cm⁻²) at energies between 60 and 3000 keV. This corresponds to implantation depths of about 0.1 to 3 μm. Using the nuclear reaction ¹²C(d, p)¹³C a depth distribution of the implanted carbon is obtained. The relationship between the concentration of ¹⁴C in random and along the 100 and 110 axial directions gives information about the substitutional lattice location of carbon within the GaAs crystal. In addition, we measured lattice defect depth distributions with a 1.5 MeV ⁴He⁺ beam before and after thermal annealing at temperatures up to 600°C.     

Damage production in GaAs during MeV ion implantation

The influence of the nuclear and electronic energy loss on the damage production in GaAs has been studied by Se⁺ ion implantation at T_I = 293 K with energies ranging from 2 MeV up to 20 MeV. The ion dose was varied between 5 × 10¹² /cm² and 1 × 10¹⁵ /cm². The damage production was investigated using RBS in channeling regime. Temperature and energy dependent backscattering measurements and TEM investigations were performed to study the kind of defects in more detail. The resulting defect profiles are compared with the depth distribution of the nuclear and electronic energy loss which were simulated by TRIM 87. The results show that the remaining defect concentration strongly decreases with increasing implantation energy even if the same energy density is deposited into nuclear processes. We suppose, that the electronic energy loss increases the defect transformation and annealing during implantation at T_I = 293 K. The defects in the samples implanted with energies greater than 5 MeV are characterized as point defects, point defect clusters and small dislocation loops; the kind of defects are the same over the whole implantation depth and the existence of amorphous zones can be widely excluded.     

XPS and optical studies of Xe-implanted and annealed YSZ single crystals

Xe⁺ ion implantation with 200 keV was completed at room temperature up to a fluence of 1 × 10¹⁷ ion/cm² in yttria-stabilized zirconia (YSZ) single crystals. Optical absorption and X-ray photoelectron spectroscopy (XPS) were used to characterize the changes of optical properties and charge state in the as-implanted and annealed crystals. A broad absorption band centered at 522 or 497 nm was observed in the optical absorption spectra of samples implanted with fluences of 1 × 10¹⁶ ion/cm² and 1 × 10¹⁷ ion/cm², respectively. These two absorption bands both disappeared due to recombination of color centers after annealing at 250 °C. XPS measurements showed two Gaussian components of O_1s spectrum assigned to Zr–O and Y–O, respectively, in YSZ single crystals. After ion implantation, these two peaks merged into a single peak with the increasing etching depth. However, this single peak split into two Gaussian components again after annealing at 250 °C. The concentration of Xe decreased drastically after annealing at 900 °C. And the XPS measurement barely detected the Xe. There was no change in the photoluminescence of YSZ single crystals with a fluence of 1 × 10¹⁷ ion/cm² after annealing up to 900 °C.     

Radiation damage and annealing behaviour of Ge-implanted SiC

In recent years, single-crystal SiC has become an important electronic material due to its excellent physical and chemical properties. The present paper reports a study of the defect reduction and recrysallisation during annealing of Ge⁺-implanted 6H-SiC. Implants have been performed at 200 keV with doses of 1 × 10¹⁴ and 1 × 10¹⁵ cm⁻². Furnace annealing has been carried out at temperatures of 500, 950 and 1500°C. Three analytical techniques including Rutherford backscattering spectrometry in conjunction with channelling (RBS/C), positron annihilation spectroscopy (PAS) and cross-sectional transmission electron microscopy (XTEM) have been employed for sample characterisation. It has been shown that damage removal is more complicated than in ion-implanted Si. The recrystallisation of amorphised SiC layers has been found to be unsatisfactory for temperatures up to 1500°C. The use of ion-beam-induced epitaxial crystallisation (IBIEC) has been more successful as lattice regrowth, although still imperfect, has been observed to occur at a temperature as low as 500°C.     

Time scales of transient enhanced diffusion: free and clustered interstitials

Transient enhanced diffusion (TED) and electrical activation after nonamorphizing Si implantations into lightly B-doped Si multilayers shows two distinct timescales, each related to a different class of interstitial defect. At 700°C, ultrafast TED occurs within the first 15 s with a B diffusivity enhancement of > 2 × 10⁵. Immobile clustered B is present at low concentration levels after the ultrafast transient and persists for an extended period ( 10²–10³ s). The later phase of TED exhibits a near-constant diffusivity enhancement of ≈ 1 × 10⁴, consistent with interstitial injection controlled by dissolving {113} interstitial clusters. The relative contributions of the ultrafast and regular TED regimes to the final diffusive broadening of the B profile depends on the proportion of interstitials that escape capture by {113} clusters growing within the implant damage region upon annealing. Our results explain the ultrafast TED recently observed after medium-dose B implantation. In that case there are enough B atoms to trap a large proportion of interstitials in Si---B clusters, and the remaining interstitials contribute to TED without passing through an intermediate {113} defect stage. The data on the ultrafast TED pulse allows us to extract lower limits for the diffusivities of the Si interstitial (D_I > 2 × 10⁻¹⁰ cm²s⁻¹) and the B interstitial(cy) defect (D_Bi > 2 × 10⁻¹³ cm²s⁻¹) at 700°C.     

Low temperature epitaxial regrowth of mercury implanted sapphire

Hg ions were implanted into sapphire at room temperature and 80 keV energy to a fluence of 1 × 10¹⁵ Hg⁺ / cm². This fluence was enough to produce an amorphous surface layer. The annealing behaviour was studied combining RBS/channeling and hyperfine interaction techniques. Surprisingly, the RBS/channeling results show there is an epitaxial regrowth of the damaged layer after annealing at 800°C for 20 min. Although some of the implanted Hg segregates to the surface during the epitaxial regrowth, a significant fraction is incorporated into regular sites along the c-axis. The hyperfine interactions results, obtained after implantation of a dose of 5 × 10¹² Hg⁺ / cm², show that a small fraction of Hg is probably bound to oxygen. This result is in agreement with the RBS/channeling measurements which also show that the system formed after annealing is stable even at high temperatures.     

Carbon ion-implanted optical waveguides in Nd:YLiF4 crystal: Refractive index profiles and thermal stability

We report on the optical planar waveguides in Nd:YLiF₄ laser crystals fabricated by 6.0 MeV C³⁺ ion implantation at doses of 1 × 10¹⁵ or 2.5 × 10¹⁵ ions/cm², respectively. The refractive index profiles, which are reconstructed according to the measured dark mode spectroscopy, show that the ordinary index had a positive change in the surface region, forming non-leaky waveguide structures. The extraordinary index is with a typical barrier-shaped distribution, which may be mainly due to the nuclear energy deposition of the incident ions into the substrate. In order to investigate the thermal stability of the waveguides, the samples are annealed at temperature of 200–300 °C in air. The results show that waveguide produced by higher-dose carbon implantation remains relatively stable with post-irradiation annealing treatment at 200 °C in air.     

Study of radiation-induced degradation of RPV steels and model alloys by positron annihilation and Mössbauer spectroscopy

The influence of different microstructural processes on the degradation due to radiation embrittlement has studied by positron annihilation and Mössbauer spectroscopy. The materials studied consisted of WWER-440 base (15Kh2MFA) and weld (10KhMFT) RPV steels which were neutron-irradiated at fluence levels of 0.78 × 10²⁴ m⁻², 1.47 × 10²⁴ m⁻² and 2.54 × 10²⁴ m⁻²; WWER-1000 base (15Kh2NMFAA) and weld (12Kh2N2MAA) irradiated at a fluence level 1.12 × 10²⁴ m⁻²; three different model alloys implanted with protons at two dose levels (up to 0.026 dpa), finally the base metal of WWER-1000 (15Kh2NMFAA) was thermally treated with the intention to simulate the P-segregation process. It has been shown possible to correlate the values of parameters obtained by such techniques and data of mechanical testing (ductile-to-brittle transition temperature and upper shelf energy).     

EXAFS studies of thermal annealing effects on the local environment of erbium implanted in LiNbO3

X-ray absorption spectroscopy (XAS) has been applied to the study of thermal annealing effects on the local environment of erbium ions implanted at 300 keV (2 × 10¹⁶ ions/cm²) in LiNbO₃. The radial distribution functions change dramatically as a function of temperature and duration of annealing in air. The best simulations of the shells surrounding Er have been obtained with the Nb and Li site positions for Er after annealing at 500°C for 20 h and 1100°C for 100 h, respectively. Moreover, a shared site (Li, Nb) has to be considered after high temperature annealing for 20 h.     

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.     

Investigation of the radiation hardness on semiconductor devices using the ion micro-beam

Variation of the ion beam induced charge (IBIC) pulse heights due to ion irradiation was investigated on a Si pn diode and a 6H-SiC Schottky diode using a 2 Mev He⁺ micro-beam. Each diode was irradiated with a focused 2 MeV He⁺ micro-beam to a fluence in the range of 1×10⁹–1×10¹³ ions/cm². Charge pulse heights were analyzed as a function of the irradiation fluence. After a 2 MeV ion irradiation to the Si pn junction diode, the IBIC pulse height decreased by 15% at 9.2×10¹² ions/cm². For the SiC Schottky diode, with a fluence of 6.5×10¹² ions/cm², the IBIC pulse height decreased by 49%. Our results show that the IBIC method is applicable to evaluate irradiation damage of Si and SiC devices and has revealed differences in the radiation hardness of devices dependent on both structural and material.     

Photoluminescence of Au formed in 12CaO · 7Al2O3 single crystal by Au-implantation

Au⁺ ion implantation with fluences from 1 × 10¹⁴ to 3 × 10¹⁶ cm⁻² into 12CaO · 7Al₂O₃ (C12A7) single crystals was carried out at a sample temperature of 600 °C. The implanted sample with the fluence of 1 × 10¹⁵ cm⁻² exhibited photoluminescence (PL) bands peaking at 3.1 and 2.3 eV at 150 K when excited by He–Cd laser (325 nm). This was the first observation of PL from C12A7. These two PL bands are possibly due to intra-ionic transitions of an Au⁻ ion having the electronic configuration of 6s², judged from their similarities to those reported on Au⁻ ions in alkali halides. However, when the concentration of the implanted Au ions exceeded the theoretical maximum value of anions encaged in C12A7 (2.3 × 10²¹ cm⁻³), surface plasmon absorption appeared in the optical absorption spectrum, suggesting Au colloids were formed at such high fluences. These observations indicate that negative gold ions are formed in the cages of C12A7 by the Au⁺ implantation if an appropriate fluence is chosen.     

Ion implantation doping and electrical conductivity enhancement of C60 films

Pristine C₆₀ films sublimed onto sheet mica were implanted with 20 keV K⁺ ions and I⁺ ions at doses of 1.0 × 10¹⁶/cm², 3.0 × 10¹⁶/cm² and 5.0 × 10¹⁶/cm², and with 20 keV Ar⁺ ions at a dose of 5.0 × 10¹⁶/cm². The distributions of dopants were studied using Rutherford backscattering spectrometry (RBS). The temperature dependence of sheet resistivity of the films was investigated applying a four-probe system. It was proposed that the conductivity enhancement of K⁺ implanted C₆₀ films was due to the implanted ions in the films, while for I⁺ implanted C₆₀ films, both implanted I⁺ ions and irradiation effects of the ions contributed to the enhancement of conductivity.     

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.