Interaction entre les défauts et une interface cristal/amorphe dans le silicium amorphe, étude en microscopie électronique en transmission

L'endommagement induit par implantation de xénon dans le silicium a été étudié par microscopie électronique à transmission (MET). Les implantations réalisées à 350C, à une énergie de 250keV et pour des fluences supérieures à >1x1016 Xe ions/cm2 conduisent à la formation d'une couche amorphe enterrée. Les observations effectuées sur les échantillons recuits montrent la présence d'une rangée de grandes cavités allongées dans la direction perpendiculaire à l'interface. Ceci suggère que, lors de la recristallisation du silicium, le déplacement simultané des deux interfaces entraîne le déplacement du gaz jusqu'à son confinement dans de larges bulles. Afin de mieux appréhender les mécanismes qui conduisent au mouvement des bulles, de l'hélium à faible dose a été implanté dans du silicium préalablement amorphisé par implantation de Li à basse température. L'implantation d'hélium dans le silicium amorphe conduit à la formation de bulles de forme irrégulière. Ce résultat diffère du silicium cristallin où des bulles sphériques sont obtenues pour des conditions d'implantation identiques. Les expériences réalisées in situ dans le MET montrent clairement d'une part que les bulles sont poussées par l'interface, et d'autre part la nucléation de micromacles. Il a été mis en évidence au cours de cette étude que les bulles sont mobiles à plus basse température dans le silicium amorphe que dans la phase cristalline. Lors de la recristallisation, les bulles se trouvent alors confinées dans le matériel amorphe, ce qui résulte en leur coalescence et à la formation de larges bulles une fois que les deux fronts de recristallisation se sont rejoints. De plus, il a été établi que la formation de micromacles dans la région recristallisée est liée à un excès de défauts de type interstitiels dans la zone amorphe. Ce résultat est contraire aux modèles de la littérature qui suggèrent que les micromacles se forment soit sur des plans {111} soit sur les bulles.Transmission electron microscopy (TEM) has been used to investigate the damage produced following high temperature (350 C) Xe implantation into [100] Si at fluencies (>1x1016 Xe ions/cm2) and energy (250keV) which produce a buried amorphous layer; and the defect structures produced following thermal anneals of 400 C, 600 C or 800 C for 30 minutes. Analysis of these samples yielded results which suggested that the Xe gas contained within the amorphous layers was swept by the amorphous/crystalline interfaces during solid phase epitaxial re-crystallisation (SPEG) into large bubbles elongated along a direction perpendicular to the interfaces. In order to further investigate this sweeping effect, buried amorphous layers were produced in Si by implanting Li at liquid nitrogen temperature and post implanting the layers with 1x1016 He ions/cm2. Contrary to the spherical bubbles produced under similar conditions in crystalline Si, irregular shaped bubbles were formed in the amorphous layer. Results from in-situ TEM studies showed that these bubbles are mobile at temperatures lower than expected in crystalline Si. Thus, upon reaching the moving interfaces between amorphous and crystalline Si, the bubbles are forced back into the amorphous material which ultimately results in coalescence of the gas into larger bubbles once the two interfaces combine. In addition, microtwins have been shown to form in regions of the re-crystallising layer where there exists an excess of interstitial-type defects. This is contrary to previous microtwin nucleation models which suggested that microtwins are either formed on [111] planes or on bubbles.POITIERS-BU Sciences (861942102) / SudocSudocFranceUnited KingdomFRG

Damage accumulation in neon implanted silicon

Damage accumulation in neon-implanted silicon with fluences ranging from 5x1014 to 5x1016 Ne cm-² has been studied in detail. As-implanted and annealed samples were investigated by Rutherford backscattering spectrometry under channeling conditions and by transmission electron microscopy in order to quantify and characterize the lattice damage. Wavelength dispersive spectrometry was used to obtain the relative neon content stored in the matrix. Implantation at room temperature leads to the amorphization of the silicon while a high density of nanosized bubbles is observed all along the ion distribution, forming a uniform and continuous layer for implantation temperatures higher than 250°C. Clusters of interstitial defects are also present in the deeper part of the layer corresponding to the end of range of ions. After annealing, the samples implanted at temperatures below 250°C present a polycrystalline structure with blisters at the surface while in the other samples coarsening of bubbles occurs and nanocavities are formed together with extended defects identified as {311} defects. The results are discussed in comparison to the case of helium-implanted silicon and in the light of radiation-enhanced diffusion

Characteristics of 698 patients with dissociative seizures: a UK multicenter study

Objective We aimed to characterize the demographics of adults with dissociative (nonepileptic) seizures, placing emphasis on distribution of age at onset, male:female ratio, levels of deprivation, and dissociative seizure semiology. Methods We collected demographic and clinical data from 698 adults with dissociative seizures recruited to the screening phase of the CODES (Cognitive Behavioural Therapy vs Standardised Medical Care for Adults With Dissociative Non‐Epileptic Seizures) trial from 27 neurology/specialist epilepsy clinics in the UK. We described the cohort in terms of age, age at onset of dissociative seizures, duration of seizure disorder, level of socioeconomic deprivation, and other social and clinical demographic characteristics and their associations. Results In what is, to date, the largest study of adults with dissociative seizures, the overall modal age at dissociative seizure onset was 19 years; median age at onset was 28 years. Although 74% of the sample was female, importantly the male:female ratio varied with age at onset, with 77% of female but only 59% of male participants developing dissociative seizures by the age of 40 years. The frequency of self‐reported previous epilepsy was 27%; nearly half of these epilepsy diagnoses were retrospectively considered erroneous by clinicians. Patients with predominantly hyperkinetic dissociative seizures had a shorter disorder duration prior to diagnosis in this study than patients with hypokinetic seizures (P < .001); dissociative seizure type was not associated with gender. Predominantly hyperkinetic seizures were most commonly seen in patients with symptom onset in their late teens. Thirty percent of the sample reported taking antiepileptic drugs; this was more common in men. More than 50% of the sample lived in areas characterized by the highest levels of deprivation, and more than two‐thirds were unemployed. Significance Females with dissociative seizures were more common at all ages, whereas the proportion of males increased with age at onset. This disorder was associated with socioeconomic deprivation. Those with hypokinetic dissociative seizures may be at risk for delayed diagnosis and treatment