Processus électroniques d'excitation et de relaxation<br />dans les solides diélectriques excités par des impulsions<br />ir et xuv ultracourtes

We studied excitation and relaxation of electrons involved during interaction of visible andVUV femtosecond pulses with dielectrics. The generated population of hot electrons, havingenergy of few eV to few tens of eV above the bottom of the conduction band, is responsible ofphenomena ranging to defect creation to optical breakdown. Owing to two techniques : photoemissionand transient photoconductivity we improve the understanding of the The firstphotoemission experiments deal with dielectrics irradiated by 30 fs IR pulses. The photoemissionspectra measured show a large population of electrons which energy rise up to 40 eV. Weinterpret this result in terms of a new absorption process : direct multiphotons interbranchtransitions. The 2nd type of photoemission experiments are time resolved "pump/probe" investigation.We study the relaxation of electrons excited by a VUV pulses. We used the highorder harmonics (HOH) as light sources. We found surprisingly long decay time in the rangeof ps timescale. Last type of experiments is photoconductivity studies of diamond samples.Using HOH as light source we measure the displacement current induced by excited electronsin the conduction band. Those electrons relax mainly by impact ionisation creating secondaryelectrons. Hence by probing the number of electrons we were able to measure the efficiencyof these relaxation processes. We observe a diminution of this efficiency when the energy ofexcitating photons is above 20 eV. Owing to Monte-Carlo simulation we interpret this resultin terms of band structure effect.Nous avons étudiés l'excitation d'un solide diélectrique par une impulsion laser femtoseconde(fs) intense dans le domaine visible où XUV. Ce type d'irradiation produit des électrons excités avec des énergies initiales qui vont de quelques eV à quelques dizaines d'eV au dessusdu bas de la bande de conduction. La relaxation de ces électrons est à l'origine de nombreuxphénomènes tels que l'ablation laser, le claquage optique ou le transport des électrons "chauds " dans les matériaux à intérêt technologique (SiO2 et diamant). L'objectif de ce travailde thèse est d'étudier de façon directe et de mieux comprendre ces mécanismes de relaxationélectroniques. Deux techniques expérimentales complémentaires, utilisant les impulsions XUVultrabrèves, issues de la génération d'harmoniques d'ordres élevés, ont été mises oeuvre pourmener à bien ces études. Tout d'abord, les expériences de photoémission ont permis de mettreen lumière un nouveau mécanisme d'absorption du rayonnement par les électrons de la bandede conduction : les transitions multiphotoniques interbandes. Nos résultats montrent que ceprocessus est le mécanisme dominant d'échauffement des électrons. Cette conclusion est deplus corroborée par les résultats d'un modèle théorique basé sur la résolution de l'équationde Schrödinger dépendante du temps. D'autre part, des expériences " pompe/sonde " dephotoémission résolue en temps ont eu pour but de sonder la population d'électrons excitéspar une impulsion XUV et de suivre son évolution temporelle sur une échelle de temps fsà ps. Les temps de décroissance mesurés sont de l'ordre de quelques ps pour des électronsde 30 eV. L'interprétation des ces durées de vie longue est problématique. Nous suggéronsun modèle de relaxation en deux étapes, tout d'abord purement électronique et rapide, puisd'interaction avec le réseau plus lente, pour expliquer ces résultats expérimentaux. Le secondtype d'expériences porte sur une spectroscopie de photoconduction sur du diamant. En utilisantles harmoniques d'ordres élevés comme source d'excitation nous avons mesuré le courantde déplacement induit qui permet d'accéder au nombre d'électrons excités en fonction del'énergie des photons incidents. Cette information permet d'étudier l'efficacité de l'ionisationpar impact (collision inélastique électron/électron). Nos résultats peuvent s'interpréter par lastructure particulière du diamant qui comporte une deuxième bande interdite 10 eV au dessusdu bas de la bande de conduction. Des simulations Monte-Carlo permettent de confirmer cetteinterprétation

Relaxation and excitation electronic processes in dielectrics irradiated by ultrafast IR and VUV pulses

Nous avons étudiés l'excitation d'un solide diélectrique par une impulsion laser femtoseconde(fs) intense dans le domaine visible où XUV. Ce type d'irradiation produit des électrons excités avec des énergies initiales qui vont de quelques eV à quelques dizaines d'eV au dessusdu bas de la bande de conduction. La relaxation de ces électrons est à l'origine de nombreuxphénomènes tels que l'ablation laser, le claquage optique ou le transport des électrons "chauds " dans les matériaux à intérêt technologique (SiO2 et diamant). L'objectif de ce travailde thèse est d'étudier de façon directe et de mieux comprendre ces mécanismes de relaxationélectroniques. Deux techniques expérimentales complémentaires, utilisant les impulsions XUVultrabrèves, issues de la génération d'harmoniques d'ordres élevés, ont été mises oeuvre pourmener à bien ces études. Tout d'abord, les expériences de photoémission ont permis de mettreen lumière un nouveau mécanisme d'absorption du rayonnement par les électrons de la bandede conduction : les transitions multiphotoniques interbandes. Nos résultats montrent que ceprocessus est le mécanisme dominant d'échauffement des électrons. Cette conclusion est deplus corroborée par les résultats d'un modèle théorique basé sur la résolution de l'équationde Schrödinger dépendante du temps. D'autre part, des expériences " pompe/sonde " dephotoémission résolue en temps ont eu pour but de sonder la population d'électrons excitéspar une impulsion XUV et de suivre son évolution temporelle sur une échelle de temps fsà ps. Les temps de décroissance mesurés sont de l'ordre de quelques ps pour des électronsde 30 eV. L'interprétation des ces durées de vie longue est problématique. Nous suggéronsun modèle de relaxation en deux étapes, tout d'abord purement électronique et rapide, puisd'interaction avec le réseau plus lente, pour expliquer ces résultats expérimentaux. Le secondtype d'expériences porte sur une spectroscopie de photoconduction sur du diamant. En utilisantles harmoniques d'ordres élevés comme source d'excitation nous avons mesuré le courantde déplacement induit qui permet d'accéder au nombre d'électrons excités en fonction del'énergie des photons incidents. Cette information permet d'étudier l'efficacité de l'ionisationpar impact (collision inélastique électron/électron). Nos résultats peuvent s'interpréter par lastructure particulière du diamant qui comporte une deuxième bande interdite 10 eV au dessusdu bas de la bande de conduction. Des simulations Monte-Carlo permettent de confirmer cetteinterprétation.We studied excitation and relaxation of electrons involved during interaction of visible andVUV femtosecond pulses with dielectrics. The generated population of hot electrons, havingenergy of few eV to few tens of eV above the bottom of the conduction band, is responsible ofphenomena ranging to defect creation to optical breakdown. Owing to two techniques : photoemissionand transient photoconductivity we improve the understanding of the The firstphotoemission experiments deal with dielectrics irradiated by 30 fs IR pulses. The photoemissionspectra measured show a large population of electrons which energy rise up to 40 eV. Weinterpret this result in terms of a new absorption process : direct multiphotons interbranchtransitions. The 2nd type of photoemission experiments are time resolved "pump/probe" investigation.We study the relaxation of electrons excited by a VUV pulses. We used the highorder harmonics (HOH) as light sources. We found surprisingly long decay time in the rangeof ps timescale. Last type of experiments is photoconductivity studies of diamond samples.Using HOH as light source we measure the displacement current induced by excited electronsin the conduction band. Those electrons relax mainly by impact ionisation creating secondaryelectrons. Hence by probing the number of electrons we were able to measure the efficiencyof these relaxation processes. We observe a diminution of this efficiency when the energy ofexcitating photons is above 20 eV. Owing to Monte-Carlo simulation we interpret this resultin terms of band structure effect

Atomistic Insight on the Threshold Switching Mechanism in Innovative Amorphous Chalcogenide Thin Films Used in Advanced OTS Selector Devices

9:15 AM - EP07.04.03 Atomistic Insight on the Threshold Switching Mechanism in Innovative Amorphous Chalcogenide Thin Films Used in Advanced OTS Selector Devices Pierre Noe1,Anthonin Verdy1,Jean-Yves Raty2,Francesco d'Acapito3,Gabriele Navarro1,Françoise Hippert4,Jérôme Gaudin5,Mathieu Bernard1 Université Grenoble-Alpes, CEA-LETI1,FNRS-Liège University2,CNR-IOM-OGG c/o ESRF3,LNCMI (CNRS, Université Grenoble Alpes, UPS, INSA)4,Centre Lasers Intenses et Applications5 Hide Abstract Chalcogenide materials exhibit a unique portfolio of properties which has led to their wide use for non-volatile memory applications such as optical data storage or more recently Phase-Change Random Access Memory [1]. Chalcogenide glasses (CGs) exhibit a high transparency window in the IR range and large optical nonlinearities offering unique opportunities for elaboration of innovative mid-IR components [2]. Besides, a huge nonlinear behavior of conductivity is observed in some CGs under electrical field application. Such CGs appear to be promising materials for innovative OTS (Ovonic Threshold Switching) selector elements in 3D resistive memory arrays [3]. Indeed, among the different selector technologies developed in the last years [4], the OTS selector technology showed the capability to overcome key issues for crossbar application, as very recently demonstrated in the Intel/Micron OptaneTM memory technology [5]. The OTS mechanism discovered in the 60’s [6] consists in the switching between a high resistance (OFF state) and a low resistance state (ON state) when the voltage applied on the CG exceeds a critical value (threshold voltage Vth). When the current is reduced below the holding current density, Jh, the selector recovers its high resistance state. However, the underlying physical mechanism is still under debate with, up to now, two main classes of models, one involving a purely electronic effect [7] and the other invoking structural changes under field application [8]. In that context, we investigate the origin of the OTS effect by means of a structural analysis of some prototypical and state-of-the-art Ge/Sb/Se-based OTS glasses. The structure of selected thin films, differing significantly by the amplitude of the OTS effect and the performance of OTS devices, is studied by means of Fourier Transform Infrared (FTIR) and Raman spectroscopy as well as X-Ray Absorption Spectroscopy. As a result, we elucidate the role of Sb and N doping in changing the structure of Ge30Se70 glass, which leads to improved OTS selector performance. Finally, from this structural analysis and original ab initio molecular dynamics simulations, we will propose a new scenario explaining the origin of the OTS mechanism in such state-of-the-art CGs. References [1] P. Noé et al., PC Materials for NVM devices: From Technological Challenges to Materials Science Issues, Topical review in Sem. Sc. And Tech. (2017). http://iopscience.iop.org/article/10.1088/1361-6641/aa7c25 [2] D. Tsiulyanu et al., Sensor. Actuat. B-Chem. 223, 95-100 (2016). [3] A. Verdy et al., IEEE 9th Int. Mem. Work. 2017, IMW (2017). [4] [C. Kügeler et al., Solid State Electron. 53, pp. 1287-1292 (2009). [5] http://www.techinsights.com/about-techinsights/overview/blog/intel-3D-xpoint-memory-die-removed-from-intel-optane-pcm/ [6] S. R. Ovshinsky, Phys. Rev. Lett. 21 (20), 1968. [7] D. Ielmini, Y. Zhang. J. Appl. Phys. 102, 054517 (2007). [8] I. Karpov et al., Appl. Phys. Lett. 92, 173501 (2008)

Exciton-exciton interactions in CdWO4 irradiated by intense femtosecond vacuum ultraviolet pulses

Exciton-exciton interaction is experimentally revealed and quantitatively analyzed in a wide band-gap scintillator material CdWO4. Under high-intensity femtosecond vacuum ultraviolet excitation, the CdWO4 luminescence is quenched, while its decay becomes essentially nonexponential. We propose an analytical model, which successfully reproduces the decay kinetics recorded in a wide range of excitation densities. The dipole-dipole interaction between excitons leading to their nonradiative decay is shown to be the main cause of a nonproportional response common for many scintillators

Electronic Energy Transport in c-Si Irradiated with X-ray Beam Under Grazing Incidence Angles

The rapid development of a new generation of X-ray radiation sources providing ultrashort (from atto- to femtoseconds) pulses creates unique possibilities for generating high energy density states of matter. Instruments, like free-electron lasers (FELs) produce pulses of very high intensity and allow to extend the optical studies of radiation induced phase transitions of solids. The excitation of solid materials with x-ray femtosecond pulses offers a number of advantages over irradiation with femtosecond optical lasers. First of all the energy deposition process is not influenced by optical nonlinearities i.e. multiphoton absorption and free carrier absorption. Moreover the absorption depth can be varied over many orders of magnitude. E.g. for silicon it changes from a few nanometres up to hundreds of microns. Therefore, ultrashort X-ray pulses allow the preparation of well-defined excitation conditions in variable sample volumes and thus to study the energy transport processes. Single shot irradiations of the Si flat mirror were performed at SACLA FEL facilities in the range of 5.5 – 12 keV photon energies, at normal and grazing incidence angles. Observed radiation induced structural modification of materials is related to melting of silicon and its resolidification and a have threshold nature. The experimental damage thresholds are the highest in case of the irradiations below the critical angles. In these cases the energy density of the radiation absorbed at the sample’s surface can reach above a melting threshold (approx. 1eV/atom) without any structural modification. This may be explained by the transport of the energy out of the excitation volume (limited to the absorption skin depth) by hot electrons on the time scales shorter than the one typical for the electron-phonon coupling (~2 ps for Si). Modelling of the energy transport by ballistic electrons has been performed by means of the PENELOPE simulation code

Investigation of damage induced by intense femtosecond XUV pulses in silicon crystals by means of white beam synchrotron section topography

Silicon crystalline samples were exposed to intense single pulses of XUV radiation (λ=13.5 nm) what lead to melting and ablation of the surface material. The deformation field around craters along the whole thickness of silicon wafers was observed by means of the synchrotron transmission section topography using the beam perpendicular to the surface of the sample. The geometrical shape and depth extension around craters was evaluated based on numerous, dense series of section topographs spaced by 10 µm. In the topographs we observed the direct image connected with the boundary of the crater associated with some deformation of the Kato fringes. The evaluated depth extension, different for individual craters, was in the range of 30–100 µm. The depth values were confirmed also by evaluations based on the Bragg case section topographs.It was possible to reproduce the contrast of the craters in transmission section topographs by numerical simulation based on integration of the Takagi–Taupin equations. The damage of the crystal defects connected with craters was approximated as droplet-like inclusions