Comportement des semiconducteurs de structure wurtzite à base d'azote sous implantation d'ions terres rares de moyenne énergie

CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

A comparative structural investigation of GaN implanted with rare earth ions at room temperature and 500 °C

The crystallographic damage induced in GaN by 300 keV rare earth ions implantation has been investigated as a function of the implantation temperature. The defect structure of GaN thin films implanted at 500 °C with Eu or Er ions with fluences ranging between 1 × 1015 and 2 × 1016 at./cm2 has been compared with the case of implantation performed at room temperature (RT). Transmission electron microscopy (TEM) investigation shows that less damage is formed during implantation at the higher temperature: basal stacking faults with a majority of I1 type and prismatic stacking faults have been observed as in GaN implanted at RT, but with a lower density. The nanocrystalline layer observed when GaN was implanted at RT with rare earth ion fluences higher than 3 × 1015 at./cm2, did not form for fluences up to 2 × 1016 at./cm2. Implantation of GaN at 500 °C through an ultrathin AlN cap points out the protective role of this cap against the GaN surface erosion that occurs from 8 × 1015 at./cm2. This method appears as a promising way to reduce the induced damage during the GaN implantation process

A comparative investigation of the damage build-up in GaN and Si during rare earth ion implantation

The medium range implantation of rare earth ions at room temperature in GaN layers leads to the formation of point defect clusters, basal and prismatic stacking faults from the lowest fluence. When a threshold fluence of about 3 × 1015 at/cm2 is reached, a highly disordered ‘nanocrystalline layer' (NL) is observed to form at the surface. This layer is made of a mixture of misoriented nanocrystallites and voids. Beyond this NL, I1, I2 and E basal stacking faults (BSFs) have been identified, as well as in GaN implanted at lower fluences than the threshold. Prismatic stacking faults (PSFs) with Drum atomic configuration connect the I1 BSFs. A similar investigation of the damage in Eu implanted Si shows a completely different behaviour; in this case, from the relatively low fluence 1 × 1014 at/cm2, amorphization starts in patches at the projected range and extends very rapidly towards the surface and the bulk, to form a uniform amorphous layer already at 2 × 1014 at/cm2

Caractérisation par Spectroscopie de Photo-courant de Boîtes Quantiques InAs/InP(113)b et (100)

Nous avons utilisé la spectroscopie par photo-courant (PC) pour mettre en évidence la structure électronique d'îlots quantique InAs sur substrat d'InP(113) et (100) pour une émission contrôlée à 1,55µm. L'étude en fonction de la température a montré une variation de l'ordre de 70meV des premiers niveaux d'énergies de transition pour des températures variant entre 77 et 300K. L'échappement des porteurs induits par absorption s'avère être assisté par les effets tunnel et thermoïonique. Les taux d'échappement ainsi que les énergies d'activation correspondants aux différents niveaux d'énergie ont pu être estimés. L'étude comparative en fonction de l'empilement de plans d'îlots (1, 3 et 6 plans) montre de faibles écarts de variation des énergies dans les structures à 3 et à 6 plans. Des simulations utilisant un modèle simple de type kp à une bande ont permis de calculer les niveaux d'énergie dans les îlots et de confirmer les résultats expérimentaux

Effect of ion implantation energy for the synthesis of Ge nanocrystals in SiN films with HfO₂/SiO₂stack tunnel dielectrics for memory application

<p>Abstract</p> <p>Ge nanocrystals (Ge-NCs) embedded in SiN dielectrics with HfO₂/SiO₂stack tunnel dielectrics were synthesized by utilizing low-energy (&#8804;5 keV) ion implantation method followed by conventional thermal annealing at 800&#176;C, the key variable being Ge⁺ion implantation energy. Two different energies (3 and 5 keV) have been chosen for the evolution of Ge-NCs, which have been found to possess significant changes in structural and chemical properties of the Ge⁺-implanted dielectric films, and well reflected in the charge storage properties of the Al/SiN/Ge-NC + SiN/HfO₂/SiO₂/Si metal-insulator-semiconductor (MIS) memory structures. No Ge-NC was detected with a lower implantation energy of 3 keV at a dose of 1.5 &#215; 10¹⁶cm^-2, whereas a well-defined 2D-array of nearly spherical and well-separated Ge-NCs within the SiN matrix was observed for the higher-energy-implanted (5 keV) sample for the same implanted dose. The MIS memory structures implanted with 5 keV exhibits better charge storage and retention characteristics compared to the low-energy-implanted sample, indicating that the charge storage is predominantly in Ge-NCs in the memory capacitor. A significant memory window of 3.95 V has been observed under the low operating voltage of &#177; 6 V with good retention properties, indicating the feasibility of these stack structures for low operating voltage, non-volatile memory devices.</p

Structure and role of ultrathin AlN layers for improving optical activation of rare earth implanted GaN

International audienceIn this work we carry out characterization of GaN implanted with Tm and Eu ions by scanning and transmission electron microscopy. Structural investigation is supported by optical measurements. We have investigated samples implanted at room temperature with different energies (150–300 keV) and fluences (1–7 × 1015 at/cm2). High temperature annealing was performed at 1200 °C and 1300 °C with the GaN surface protected by a 10nm thick AlN cap grown by MOCVD prior to the implantation. Direct implantation in GaN, results in the formation of a high density of stacking faults in the damaged surface layer and a nanocrystalline surface layer when a critical dose is exceeded. Implanting through the AlN capping layer prevents the formation of the nanocrystalline layer even for high fluences but the stacking faults are still generated. The AlN cap also protects the surface during high temperature annealing

Diving into bacterial dormancy: emergence of osmotically stable wall-less forms in an aquatic environment

Abstract Bacteria can respond to environmental stresses by entering a dormant state, called viable but non-culturable (VBNC) state, in which they no longer grow in routine culture media. VBNC pathogens pose thus a significant risk for human and animal health as they are not detected by standard growth-based techniques and can “wake up” back into a vegetative and virulent state. Although hundreds of species were reported to become VBNC in response to different stresses, the molecular mechanisms governing this phenotypic switch remain largely elusive. Here, we characterized the VBNC state transition process in the Gram-positive pathogen Listeria monocytogenes in response to nutritional deprivation. By combining fluorescence microscopy, cryo-electron tomography and analytical biochemistry, we found that starvation in mineral water drives L. monocytogenes into a VBNC state via a mechanism of cell wall (CW) shedding that generates osmotically stable CW-deficient (CWD) coccoid forms. This phenomenon occurs in multiple L. monocytogenes strains and in other Listeria species, suggesting it may be a stress-adapting process transversal to the Listeria genus. Transcriptomic and gene-targeted approaches revealed the stress response regulator SigB and the autolysin NamA as major moderators of CW loss and VBNC state transition. Finally, we show that this CWD dormant state is transient as VBNC Listeria revert back to a walled, vegetative and virulent state after passage in embryonated eggs. Our findings provide unprecedented detail on the mechanisms governing the transition to a VBNC state, and reveal that dormant CWD bacterial forms can naturally arise in aquatic environments without osmotic stabilization. This may represent an alternative strategy for bacterial survival in oligotrophic conditions, which can potentially generate public health-threatening reservoirs of undetectable pathogens