Croissance épitaxiale des matériaux semi-conducteurs III-V et IV sur graphène pour des applications optoélectroniques

L’hétéroépitaxie conventionnelle permet la croissance épitaxiale de couches cristallines de haute qualité sur des substrats compatibles. Cependant, en raison des désaccords de maille/thermique, la formation de défauts cristallins tels que les dislocations dans les couches hétéroépitaxiales entrave fortement l’hétérointégration de divers semi-conducteurs et le développement de dispositifs de haute performance. Alternativement, l’épitaxie Van der Waals (VdWE), un nouveau paradigme de croissance épitaxiale permet la croissance des semiconducteurs cristallins sur des matériaux 2D sans les contraintes susmentionnées. Ce nouveau concept d’hétéroépitaxie mettant en œuvre de faibles interactions de type VdW est principalement dicté par les propriétés de la surface des matériaux 2D. Le graphène, combinant la flexibilité mécanique et la faible liaison de surface, s’est avéré être l’un des matériaux 2D les plus populaires pour la croissance épitaxiale VdW. Ce concept a suscité un grand intérêt dans la communauté scientifique, mais sa mise en œuvre est entravée par la compréhension incomplète et immature des processus gouvernant l’étape de nucléation. Ainsi, cette question reste ouverte, car aucune solution viable n’a encore été proposée et il n’y a pas de vision claire sur cette étape fondamentale de la croissance. En effet, la nucléation étant un processus éphémère est très difficile à observer en détail avec les techniques d’analyse conventionnelles post-croissance. En outre, les chercheurs n’ont pas pu fournir de preuves tangibles pour les nombreuses spéculations proposées sur la cinétique des premiers stades de la croissance des semi-conducteurs à 3D sur le graphène (2D). Ce défi a soulevé la question suivante : « Le graphène est-il le substrat ultime pour résoudre les défis fondamentaux de l’hétérointégration des matériaux ayant des désaccords de maille? ». Pour cette raison, les mécanismes gouvernant la nucléation des matériaux sur graphène font l’objet de débats depuis plus d’une décennie. Dans cette thèse, nous avons démontré pour la première fois l’épitaxie VdW de Ge sur monocouche suspendue de graphène (S-SLG) par observation directe à l’intérieur d’un microscope électronique en transmission (TEM). Pour ce faire, la synthèse de couches de graphène de haute qualité a été développée. Le graphène a été synthétisé sur le Ge (100) par CVD en utilisant une voie compatible avec l’intégration verticale avec les semi-conducteurs. Lors de cette étude, nous avons démontré que l’état de surface du substrat joue un rôle crucial dans la qualité du graphène déposé. Ainsi, un traitement de surface efficace, basé sur l’acide bromhydrique (HBr) a été mis au point. En effet, des couches de graphène de haute qualité avec un ratio ID/IG aussi faible que 0,2 ont été obtenues sur substrat de Ge (100). Pour mieux contrôler la nucléation qui est la phase centrale de la croissance cristalline, nous proposons de nouvelles perspectives à partir des observations en temps réel de la croissance in situ de Ge sur graphène en utilisant un TEM à haute résolution (HR). Nous avons étudié les mécanismes clés régissant la nucléation et la croissance du matériau 3D sur la surface du graphène. Alors que la faible énergie de surface du graphène rend difficile la nucléation de Ge sur la surface propre sans défaut, une stratégie en deux étapes a été proposée : Nucléation à basse température (220 °C) et recuit à des températures plus élevées afin d’améliorer la qualité cristalline des cristaux de Ge. À travers les images HRTEM, nous avons déterminé une taille critique de 0,7-1 nm2 permettant la nucléation de Ge sur SLG. Cette taille critique, qui n’a jamais été rapportée avant ce rapport, est cohérente avec celle prédite par la théorie classique de la nucléation. En outre, les données en temps réel ont révélé que, en raison de faibles interactions vdW, les germes Ge peuvent flotter librement à la surface du substrat de graphène, et leuriv coalescence est gouverné par un processus de murissement d’Ostwald extrême rapide. Notre observation majeure, cependant, a été l’implémentation des doubles hétérostructures de Ge/SLG/Ge (3D/2D/3D). Dans les conditions expérimentales employées, nous avons découvert une diffusion verticale (DV) des particules de Ge à travers le réseau hexagonal compact du graphène. Un tel phénomène intrigant ne peut être observé à l’aide des méthodes conventionnelles d’analyse ex situ, qui sont normalement réalisées après la croissance. Ces résultats ont mis en évidence le mécanisme de croissance des semi-conducteurs sur les monocouches de graphène et ont offert une nouvelle voie vers des dispositifs hybrides semiconducteurs/graphène à hautes performances. Finalement, dans l’ambition d’optimiser la croissance de matériaux III-V avec de bonnes propriétés pour la VdWE, une nouvelle approche d’épitaxie hybride a été mise au point et évaluée. Cette technique hybride d’épitaxie, utilisant à la fois des sources solides et gazeuses, est alternative aux techniques standards comme l’épitaxie par faisceaux moléculaires (MBE) et chimiques (CBE). Les résultats expérimentaux de la croissance d’AlInAs et de GaInAs sur substrats d’InP, ont démontré que cette nouvelle approche est efficace pour croitre des couches avec d’excellentes propriétés cristallines, optiques et à faible dopage résiduel.Abstract : Conventional heteroepitaxy allows epitaxial growth of highly crystalline layers onto compatible substrates. However, due to the lattice/thermal mismatch, the formation of crystalline defects such as dislocations in heteroepitaxial layers severely impede the heterointegration of various semiconductors with complementary properties and the development of high-performance devices. Alternatively, Van der Waals epitaxy (vdWE), a new paradigm of epitaxial growth can enable the growth of crystalline semiconductors on 2D materials without the aforementioned constraints. This new concept of heteroepitaxy, mediated by weak VdW interactions, is mainly dictated by the surface properties of emerging 2D materials, which also have many novel electrical, optical, thermal, and mechanical properties. Graphene, a 2D material that combines mechanical flexibility and weak surface bonding, is found to be one of the most popular for VdW epitaxial growth. This is currently the subject of intense research in the community, but tangible exploitation of such a compelling phenomenon is impeded by the immature and incomplete understanding of the nucleation and growth processes. Therefore, the question remains open as no viable solution was reached yet and there is no clear picture of the nucleation step. In fact, the nucleation process, which is a fleeting event is very difficult to observe in detail by using conventional ex situ analyses. Besides, researchers could not provide hard evidence for the several speculations involving the kinetics of threedimensional (3D) crystal growth over 2D graphene, at early stages of growth process. This challenge raised the following question: “Is graphene the ultimate substrate to solve fundamental challenges of heterointegration of mismatched materials?” For matters as such, governing mechanisms for nucleation on graphene has been debated for more than a decade now; however, despite the promises of VdW epitaxy, the lack of understanding of basic phenomena is still a major obstacle to the new applications. In this thesis, we demonstrated for the first time, in-situ TEM observation of VdWE of Ge on freestanding single layer graphene suspended graphene monolayers (S-SLG). To do this, the synthesis of high-quality graphene layers was developed. Using a compatible approach for the monolithic integration of 3D semiconductors on 2D materials, graphene was synthesized on germanium (Ge) (100) by CVD. In this study, we demonstrated that the physical and chemical surface state of the substrate play a crucial role in the quality of deposited graphene layers. Thus, an effective surface treatment, based on hydrobromic acid (HBr) has been developed. Highquality graphene layers with an ID/IG ratio as low as 0.2 were obtained on Ge (100). To better control the nucleation that is the central phase of crystalline growth, we offer new perspectives from real-time observations of Ge's in situ growth on graphene using a highresolution TEM. Through meticulous analysis of the collected video data, we investigated the key mechanisms governing the nucleation and the growth of sp3-bonded 3D material on the weakly interacting graphene surfaces. Whereas the low surface energy of the graphene layer prevented the nucleation of Ge over pristine and defect-free graphene, a two-step strategy consists of nucleating at low temperature (220 °C) and annealing at higher temperatures, resulted in growth of highly crystalline quality Ge. In view of the high-resolution TEM images, we determined a critical size of 0.7-1 nm2 enabling the nucleation of Ge on SLG. This critical size, which has never been reported prior to this report, is consistent with the one predicted by the classical nucleation theory. Moreover, the real-time data revealed that, due to weak VdW interactions, the Ge germs can freely float on the surface of graphene substrate,vi making the coarsening process of Ge layer to be dominated by a very fast Ostwald ripening process. Our major finding, however, was achieved by examining the 3D/2D/3D (2D/2D/2D) configuration of the Ge/graphene/Ge double heterostructures. Under the experimental conditions employed in our work, we recorded a vertical diffusion (VD) of Ge particles through the closely packed hexagonal ring of single layers of graphene. Such an intriguing and yet unexplored phenomenon cannot be obtained by means of the conventional ex situ analysis methods, which are normally carried out after the growth of material. These findings highlighted the growth mechanism of semiconductors on graphene monolayers and provided a new path to high-performance semiconductor/graphene hybrid devices. Finally, in the ambition to optimize the growth of III-V materials with good properties for VdWE, a new hybrid epitaxy approach has been proposed and evaluated. This hybrid epitaxy technique, using both solid and gaseous sources, is an alternative approach to standard techniques such as molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE). Experimental results of the growth of AlInAs and GaInAs on InP substrates showed that this new approach is effective in growing layers with excellent crystalline, optical properties, and low residual doping

Importance of screening for urinary tract infection in African Black diabetics

Urinary tract infections are common and often latent in diabetic patients. The objective of this study was to determinethe frequency of urinary tract infections and the organisms responsible.This is a descriptive type prospective study conducted in the Endocrinology and Diabetology ward of Donka UniversityHospital. All diabetic patients hospitalised from January 3rd to April 4th 2013 were included in the study.Two hundred and sixty-two diabetics had been screened, 64 (24.42%) had a urinary tract infection 41 women (64%)and 23 men (36%). The average age was 56.48 16.3 years with a range of 20-90 years. Urinary tract infections weremore frequent between the ages of 45 and 74 (67.20%). Sixty patients were married, four were single. Fifty-six patientswere type 2 diabetes against eight type I diabetes. Urine test strips showed 60 cases of pyuria, 36 cases of nitruria, 21cases of proteinuria and three cases of hematuria. Organisms identified were: Escherichia coli in 31 cases (48.43%): 25women and six men; Klebsiella pneumoniae in 14 cases (21.87%): nine women and five men; Staphylococcus aureusin 11 cases (17.18%): six women and five men; and Proteus mirabilis in eight cases (12.51%): two women and six men.The clinical manifestations were: asymptomatic bacteriuria in 57 cases (89.06%); acute cystitis in five cases (7.81%); andacute urethritis in two cases (3.13%).Urinary infection in African diabetics is very common. It requires a systematic screening with urine test strips becauseasymptomatic types are by far the most common. This would allow early treatment and avoid serious complications

Evaluation de l’utilisation du protocole national de prise en charge du paludisme simple des enfants à Matam, Guinée Conakry

L'objectif était d'évaluer l'usage du nouveau protocole national de prise en charge du paludisme simple. Il s'agit d'une étude transversale de type descriptif menée du 01 octobre au 31 Décembre 2013. L'enquête a été faite dans les six services de santé de la commune de Matam et a intéressé tous les patients de 0-15 ans venant en consultation dans ces centres de santé d'une part et les prestataires de soins de ces dits centres d'autre part. Nous avons enregistré 545 patients dont 52,1% de sexe féminin, l'âge moyen était de 5,62±5. Plus de la moitié (60,2%) des prescriptions étaient conformes au protocole mais aucun prestataire ne s'en servait parce qu'inaccessible. Seulement 72% savaient de l'existence de ce protocole. La disponibilité et l'utilisation du protocole national de prise en charge du paludisme simple restent faibles. La formation continue du personnel semble nécessaire.Pan African Medical Journal 2016; 2

Hybrid MBE-CBE Growth and Characterization of Al 0.48 In 0.52 As on InP(100) for avalanche photodiode applications Motivation

International audienceIn this work, we demonstrate the epitaxial growth of high quality, low strain and low background doping of Al0.48In0.52As at 500°C on Fe-doped semi-insulating InP(100) substrate by using hybrid MBE-CBE technique. The precursors that were used are: solid aluminum, solid indium, TriMethylIndium (TMIn) and thermally cracked arsine. Using Nomarski, we observed smooth surfaces for the as grown layers. High-Resolution X-ray Diffraction (HR-XRD) in the vicinity of the (004) reflexion shows a lattice mismatch in the range -137 to 127ppm. The carrier density of undoped layers, obtained by Hall measurement at room temperature, is as low as 3E+15 cm-3 which is three orders of magnitude lower than the identical layers grown by organometallics sources. Photoluminescence (PL) for Al0.48In0.52As at low temperature (LT) shows a good optical quality. The quality and purity of the alloys grown here are compatible with high performance APD for optical communication

Hybrid MBE-CBE Growth and Characterization of undoped In 0,53 Ga 0,47 As on Fe-InP(001) for avalanche photodiodes (APDs)

International audienc

Evaluation of antimycobacterial activity of medicinal plants used by Malian traditional medicine practitioners to treat tuberculosis

Global Tuberculosis (TB) control is facing major challenges such as occurrence of multidrug-resistant (MDR) and extensively drug-resistant tuberculosis (XDR). The current TB drugs are getting less effective and associated with side effects limiting their use, especially with MDR and XDR infected patients. In Mali, many medicinal plants are used against various diseases including bacterial infections. The study aimed at studying the antimycobacterial activities of 60 extracts from 22 Malian medicinal. The antibacterial activity against Mycobacterium tuberculosis H37Rv was assessed employing micro-broth dilution method. Out of 60 extracts evaluated, eleven from nine different plants were found to be active against H37Rv strain. The minimal inhibitory concentrations (MICs) ranked from 125 μg/mL to 1250 μg/mL. The most active extracts (125 μg/mL) were represented by ethanolic extract of Saba senegalensis and Vitellaria paradoxa leaves, dichloromethane extract of Cola cordifolia leaves, Strychnos spinosa and Ximenia Americana roots. Ethanolic extract of Zizyphus mauritiana, Guiera senegalensis and methanolic extract of Anthocleista djalonensis also prevented the growth of H37Rv at 250 μg/mL. The results suggest that Saba senegalensis, Vitellaria paradoxa, Cola cordifolia, Strychnos spinosa and Ximenia Americana could be potential sources of antimycobacterial molecule

Assessment of a health facility based active case finding system for Ebola virus disease in Mbandaka, Democratic Republic of the Congo, June-July 2018.

BACKGROUND: The ninth outbreak of Ebola Virus Disease (EVD) in the Democratic Republic of the Congo occurred in Équateur Province from 8 May-24 July 2018. A system of health facility (HF)-based active case finding (ACF) was implemented in Mbandaka, a regional capital with four confirmed EVD cases, following completion of contact tracing. The goal of this HF-based ACF system was to look for undetected EVD cases among patients that visited HFs beginning one week prior to the system's implementation. METHODS: From 23 June - 24 July 2018, ACF teams visited HFs in Mbandaka and reviewed all medical records as far back as 17 June for any consultations meeting the suspected EVD case definition. The teams then assessed whether to validate these as suspected EVD cases based on factors such as recovery, epidemiological links, and their clinical judgement. ACF teams also assessed HFs' awareness of EVD symptoms and the process for alerting suspected cases. We calculated descriptive statistics regarding the characteristics of reviewed consultations, alert cases, and visited HFs. We also used univariate and multivariate random effects logistic regression models to evaluate the impact of repeated ACF visits to the same HF on the staff's awareness of EVD. RESULTS: ACF teams reviewed 37,746 consultations, of which 690 met the definition of a suspected case of EVD. Two were validated as suspected EVD cases and transferred to the Ebola Treatment Unit for testing; both tested negative. Repeated ACF visits to the same HF were significantly associated with improved EVD awareness (p < 0.001) in univariate and multivariate analyses. CONCLUSION: HF-based ACF during EVD outbreaks may improve EVD awareness and reveal many individuals meeting the suspected case definition. However, many who meet this definition may not have EVD, depending on the population size covered by ACF and amount of ongoing EVD transmission. Given the burdensome procedure of testing suspected EVD cases, future HF-based ACF systems would benefit from improved clarity on which patients require further testing

Parasites and invasions: changes in gastrointestinal helminth assemblages in invasive and native rodents in Senegal

Understanding why some exotic species become widespread and abundant in their colonised range is a fundamental issue that still needs to be addressed. Among many hypotheses, newly established host populations may benefit from a parasite loss ("enemy release" hypothesis) through impoverishment of their original parasite communities or reduced infection levels. Moreover, the fitness of competing native hosts may be negatively affected by the acquisition of exotic taxa from invaders ("parasite spillover") and/or by an increased transmission risk of native parasites due to their amplification by invaders ("parasite spillback"). We focused on gastrointestinal helminth communities to determine whether these predictions could explain the ongoing invasion success of the commensal house mouse (Mus musculus domesticus) and black rat (Rattus rattus), as well as the associated decrease in native Mastomys spp., in Senegal. For both invasive species, our results were consistent with the predictions of the enemy release hypothesis. A decrease in overall gastrointestinal helminth prevalence and infracommunity species richness was observed along the invasion gradients as well as lower specific prevalence/abundance (Aspiculuris tetraptera in Mus musculus domesticus, Hymenolepis diminuta in Rattus rattus) on the invasion fronts. Conversely, we did not find strong evidence of GIH spillover or spillback in invasion fronts, where native and invasive rodents co-occurred. Further experimental research is needed to determine whether and how the loss of gastrointestinal helminths and reduced infection levels along invasion routes may result in any advantageous effects on invader fitness and competitive advantage

Subsequent mortality in survivors of Ebola virus disease in Guinea: a nationwide retrospective cohort study.

BACKGROUND: A record number of people survived Ebola virus infection in the 2013-16 outbreak in west Africa, and the number of survivors has increased after subsequent outbreaks. A range of post-Ebola sequelae have been reported in survivors, but little is known about subsequent mortality. We aimed to investigate subsequent mortality among people discharged from Ebola treatment units. METHODS: From Dec 8, 2015, Surveillance Active en ceinture, the Guinean national survivors' monitoring programme, attempted to contact and follow-up all survivors of Ebola virus disease who were discharged from Ebola treatment units. Survivors were followed up until Sept 30, 2016, and deaths up to this timepoint were recorded. Verbal autopsies were done to gain information about survivors of Ebola virus disease who subsequently died from their closest family members. We calculated the age-standardised mortality ratio compared with the general Guinean population, and assessed risk factors for mortality using survival analysis and a Cox proportional hazards regression model. FINDINGS: Of the 1270 survivors of Ebola virus disease who were discharged from Ebola treatment units in Guinea, information was retrieved for 1130 (89%). Compared with the general Guinean population, survivors of Ebola virus disease had a more than five-times increased risk of mortality up to Dec 31, 2015 (age-standardised mortality ratio 5·2 [95% CI 4·0-6·8]), a mean of 1 year of follow-up after discharge. Thereafter (ie, from Jan 1-Sept 30, 2016), mortality did not differ between survivors of Ebola virus disease and the general population. (0·6 [95% CI 0·2-1·4]). Overall, 59 deaths were reported, and the cause of death was tentatively attributed to renal failure in 37 cases, mostly on the basis of reported anuria. Longer stays (ie, equal to or longer than the median stay) in Ebola treatment units were associated with an increased risk of late death compared with shorter stays (adjusted hazard ratio 2·62 [95% CI 1·43-4·79]). INTERPRETATION: Mortality was high in people who recovered from Ebola virus disease and were discharged from Ebola treatment units in Guinea. The finding that survivors who were hospitalised for longer during primary infection had an increased risk of death, could help to guide current and future survivors' programmes and in the prioritisation of funds in resource-constrained settings. The role of renal failure in late deaths after recovery from Ebola virus disease should be investigated. FUNDING: WHO, International Medical Corps, and the Guinean Red Cross