M.O. Ruault

Simulation of the α-annealing effect in apatitic structures by He-ion irradiation: Influence of the silicate/phosphate ratio and of the OH−/F− substitution

In fluoroapatite, the α-annealing is a crystal recovery process due to the electronic energy loss of α-particles emitted by radionuclides. This effect, which can be accompanied by a thermal recovery, is accounting for the crystalline state of natural calcium phosphate apatites containing long lifetime actinide isotopes. The aim of the present study is to determine the influence of the silicate–phosphate grouping substitution, on the α-annealing efficiency. In addition, measurements have been performed on hydroxyapatite Ca10(PO4)6(OH)2 and compared with those obtained on fluoroapatite Ca10(PO4)6F2 to evaluate the influence of the OH−/F− substitution on the annealing efficiency. Using a transmission electron microscope (TEM) on line with an ion implanter, the main result obtained in this study is that α-annealing is strongly dependent on the chemical composition of the mineral. Our results show that this effect is decreasing when the SiO4/PO4 ratio increases and when F− is substituted for OH− anions.

Interaction of defects and metals with nanocavities in silicon

Abstract Ion implantation of H or He into silicon, followed by annealing can create a band of nanocavities. Such nanocavities can exhibit a range of interesting and often non-equilibrium interactions with defects and metals during subsequent implantation and annealing. This paper gives an overview of such interactions, concentrating on cavities produced by H-implantation. The evolution of cavities during annealing is briefly treated, followed by illustrations of the very efficient gettering ability of cavities for fast diffusing metals. For low metal concentrations introduced into the near-surface by implantation, the metal atoms decorate the cavity walls during annealing but can be displaced by oxygen under certain conditions. At high metal concentrations, precipitation and second phase (silicide) formation can occur at cavities but silicide formation and dissolution are found to be controlled by the availability or removal of silicon interstitials, leading to non-equilibrium behaviour. When silicon that contains cavities is irradiated with silicon ions, irradiation-induced defects interact with cavities, leading to preferential amorphisation at certain temperatures. Continued irradiation leads to cavity shrinkage during bombardment, which is most efficient when the region around the cavities is amorphised.

Transmission electron microscopy study of damage by ion implantation in gold Evidence for a spike threshold

Abstract Nine different atomic species, from K to Yb, have been implanted into gold at energies ranging from 20 to 150 keV. The nature and depth distribution of the resultant defect clusters were studied by transmission electron microscopy techniques as well as by a modification of the 21/2-dimensions stereo technique developed by Mitchell and Bell (1976). The effects of implanted ion dose and sample purity were determined. The cluster depth distributions are in overall agreement with the damage distributions deduced from the energy-deposition calculations of Winterbon, Sigmund and Sanders (1970). The nature of the defect clusters is found to depend on the mass and energy of the incoming ion, in agreement with our previously reported work. It is suggested that these provide evidence for the decisive influence of the deposited energy density on the nature of visible damage. We conclude that it is possible to distinguish between cascade and ‘spike’ effects, the latter setting in when the average energy per ...

Determination of the defect creation mechanism in the mono-silicated fluoroapatite. Disorder modeling under repository conditions

Abstract Amorphization resulting from α-decay events can strongly reduce the chemical durability of nuclear waste matrices. The creation rate of defects produced by α recoils in mono-silicated fluoroapatite was determined using a transmission electron microscope (TEM) on line with an ion implanter and compared to previous results obtained in fully phosphated fluoroapatite. In both materials, the defect creation is controlled at room temperature, by the amorphization process directly in the cascade. Furthermore, it has been shown previously that in mono-silicated fluoroapatite, the disorder recovery proceeds mainly via α-annealing. Taking into account our already published data and new results on the defect creation in the mono-silicated fluoroapatite, we have modeled the amorphization level evolution versus time under repository conditions. The main conclusion is the following: thanks to α-annealing, mono-silicated fluoroapatite loaded with 244 Cm, will maintain a disorder at a level low enough to prevent the total amorphization of the host lattice during the long-term disposal.

The influence of irradiation and subsequent annealing on Si nanocrystals formed in SiO2 layers

Luminescent Si nanocrystals formed in SiO2 layers were irradiated with electrons and He+ ions with energies of 400 and 25–130 keV, respectively. The effects of irradiation and subsequent annealing at 600–1000°C were studied by the methods of photoluminescence and electron microscopy. After irradiation with low doses (∼1 displacement per nanocrystal), it was found that photoluminescence of nanocrystals was quenched but the number of them increased simultaneously. After irradiation with high doses (∼103 displacements per nanocrystal), amorphization was observed, which is not characteristic of bulk Si. The observed phenomena are explained in terms of the generation of point defects and their trapping by Si-SiO2 interfaces. Photoluminescence of nanocrystals is recovered at annealing temperatures below 800°C; however, an annealing temperature of about 1000°C is required to crystallize the precipitates. An enhancement of photoluminescence observed after annealing is explained by the fact that the intensities of photoluminescence originated from initial nanocrystals and from nanocrystals formed as a result irradiation are summed.

Ion channeling study of P implantation damage in CdTe

Abstract Phosphorus implantation was performed on single-crystal samples of [111] CdTe at incident energies of 50, 100 and 200 keV, with fluences up to 4×1016 P cm 2 . All implants were done at room temperature with current densities below 0.2 μA/cm2. In order to investigate the damage produced, Rutherford backscattering of 4He particles in channeling condition was done in situ at several energies. The dependence of the dechanneling upon the incident 4He beam energy allowed us to characterize the nature of the implantation induced defects. We describe the evolution of the damage resulting from P implantation in CdTe as a function of fluence and implantation energy. In order to understand the nature of the damage, 100 keV P implantations were also monitored with in situ transmission electron muscopy (TEM).

Determination of the defect creation mechanism in fluoroapatite

Abstract Most of strategies to immobilize actinides are concerned with their incorporation in dedicated solid matrices for a long disposal-time. Chemical durability of these matrices is a major problem. Radiation damages due to α-decay events can strongly reduced it. The defect creation mechanism has been determined in fluoroapatite using a transmission electron microscope (TEM) on line with an ion implanter. At room temperature, the defect creation is controlled by amorphization directly in the cascade. From this result and published data on the defect annealing in this material, the disorder evolution as a function of time has been established. An application concerning the incorporation of 244 Cm is considered.

In situ study of ion beam induced Si crystallization from a silicide interface

Abstract By first growing NiSi 2 precipitates in amorphous Si(a-Si) and then irradiating with a 150 keV Si beam, we have studied ion beam induced epitaxial crystallization (IBIEC) of Si initiated at the a-Si/NiSi 2 precipitate interface. We confirm our previous results regarding the existence of two, totally different mechanisms above and below 450°C. Interface roughening, leading to practically isotropic growth, was shown to prevail in the lower temperature range. We present a detailed analysis of the growth process at 500°C, via in situ transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) which demonstrates that Si crystallization is then Ni silicide-mediated, leading to planar growth.

A side-entry liquid He cooled stage for the Philips EM400 electron microscope (ion implantation application)

The authors describe a liquid He cooled stage for the side entry goniometer of the Philips EM400 transmission electron microscope (TEM). A stable temperature of 10K is reached on the specimen. The resolution is 15 AA (1.5 mm) at about 15K. Calculations taking into account electron and ion beam heating are detailed.

Adaptation of an ion implanter on a 100 kV electron microscope for in situ irradiation experiments

An ion implanter with high mass resolution has been connected to a conventional electron microscope. Preliminary results on the evolution of defect clusters created by ions of 10-50 keV in Ni are given. The resolution achieved is better than 1.5 nm for irradiation temperatures above 90K, using conventional holders. In the temperature range 20-100K irradiation experiments are performed using a liquid-helium-cooled holder built in the laboratory. The resolution is then about 3 nm.