148 research outputs found
Predicting the Behaviour of Near-Critical and Supercritical Alcohols at Microwave Frequencies: Validation of Molecular Dynamic Simulations as a Tool that can Substitute for Measurements under Extreme Experimental Conditions
No full textEquilibrium and non-equilibrium molecular dynamic simulations, predicting the dielectric properties of near-critical and supercritical methanol and ethanol at microwave frequencies have been carried out. The autocorrelation functions of the dielectric relaxation, show dependency on the slow component at the near-critical region for both alcohols. At the supercritical region, two competing relaxation mechanisms are observed, related to the large breakdown of the hydrogen-bonding network and the degree of clustering between the molecules. This approach closely matches experimental data at microwave frequencies and identical temperature and pressure conditions, validating the predictions of how the molecular structure and dynamics manifest themselves into the complex permittivity and dielectric relaxation behaviour. Thus, introducing a modelling-based solution to deliver accurate dielectric property values for materials at supercritical conditions for “a priori” screening of solvents, whilst removing the need to overcome engineering and safety challenges associated with the development of experimental equipment to practically generate such data
Experimental and computational investigation of heat transfer in a microwave-assisted flow system
No full textMicrowave technology is gaining popularity as a tool for chemical process intensification and an alternative to conventional heating. However, in flow systems non-uniform temperature profiles are commonly encountered and hence methods to characterise and improve them are required. In this work, we studied the effects of various operational parameters-microwave power, inlet flow rate, tube orientation and pressure-on the electric field and temperature profiles of water flowing in a PTFE tube (2.4 mm internal diameter), placed in a commercial single-mode microwave applicator. A finite element model was developed to estimate the longitudinal temperature profiles and the absorbed microwave power, while in situ temperature monitoring was performed by a fibre optic probe placed at multiple locations inside the tube. The water temperature inside the tube increased by increasing the microwave power input and temperature profiles stabilised beyond 20 W, while the percentage absorbed microwave power showed the inverse trend. When changing the tube orientation or decreasing the inlet flow rate, microwave absorption decreased significantly. When the pressure was increased to 2.3 bara, water temperature increased by ~ 20 o C. Results from this study provide valuable insights on achievable temperature profiles and energy efficiency of microwave-assisted flow synthesis systems.
Microwave assisted sintering of Na-β’’-Al2O3 in single mode cavities: Insights in the use of 2450 MHz frequency and preliminary experiments at 5800 MHz
No full textMicrowave assisted sintering of Na-beta’’-Al2O3 in single mode cavities was accurately investigated. The use of single mode cavity allowed monitoring the parameters affecting the sintering process, like the forward power, together with the temperature evolution, making possible to perform energy efficiency and specific energy consumption evaluations. Experiments have been performed at the frequency of 2450 MHz, but preliminary results are also reported using the higher frequency of 5800 MHz, in order to investigate its effect on important parameters like the power density distribution as well as the penetration depth, which are responsible of the resulting heating rate and sintering outcome. Dielectric properties of the powders were measured as a function of temperature in order to partially predict and support the understanding of their experimental heating behaviour. Furthermore, dielectric properties provide the fundamental information needed for the multiphysics numerical simulation, performed with the aim to reach insights into the power density evolution in the specimen as sintering proceeds
Dielectric characterisation of solar salt for volumetric heating applications in Power-to-Heat-to-Power systems
No full textCarnot batteries, or Power-to-Heat-to-Power systems, rely on solar salt as a thermal energy storage medium and require efficient and controllable heating technologies. However, conventional resistive heating is constrained by the low thermal conductivity of solar salt, leading to temperature gradients, local overheating, and material degradation, which motivates the exploration of alternative volumetric heating approaches. In this context, this study evaluates the feasibility of microwave-based volumetric heating of solar salt by analysing its dielectric behaviour across both solid and molten states. Dielectric properties were measured using the cavity perturbation method at 912 MHz and 2.45 GHz with different sample volumes and electromagnetic field configurations. Under these conditions, the sharp increase in electrical conductivity in the molten state results in high effective dielectric losses that violate the small-perturbation assumption underlying this technique. Consequently, the microwave measurements were complemented by four-electrode electrochemical impedance spectroscopy from 100 Hz to 1 MHz up to 550 • C to confirm the dominance of ionic transport mechanisms. The results show activation energies of 0.810 eV in the solid state and 0.148 eV in the liquid state, while extrapolated conductivities of approximately 160-170 S m −1 correspond to microwave penetration depths of about 1.3 mm at 912 MHz and 0.8 mm at 2.45 GHz, providing an application-relevant measure of the interaction between molten solar salt and electromagnetic fields. These findings indicate that accurate dielectric characterisation of molten solar salt at microwave frequencies requires measurement systems specifically adapted to highly conductive liquids and suggest that effective microwave heating strategies may rely on solar salt-compatible ceramic materials combined with appropriately tailored electromagnetic field distributions
Phosphorous Diffusion in N2+ -Implanted Germanium during Flash Lamp Annealing: Influence of Nitrogen on Ge Substrate Damage and Capping Layer Engineering
No full textIn this work we present a systematic study on post-implantation phosphorous diffusion control in Ge by co-implanted nitrogen in combination with various surface capping layers (Al2O3, SiO2 and Si3N4). Phosphorous has been implanted at low energy (11 keV) and high dose (1015 cm−2) in p-Ge (100) already implanted or not with low energy (10 keV−5 × 1014 cm−2) N2+. Flash Lamp Annealing (FLA) at 800–850°C for 20 ms in inert ambient has been used as post-implantation annealing scheme. In the absence of nitrogen, significant substrate damage and capping layer deterioration prevents a reliable comparison among the three capping materials. The presence of nitrogen in the Ge substrate, effectively suppresses the damage observed after the FLA. In this case, P diffusion is additionally retarded in the presence of Al2O3 as compared to SiO2 and Si3N4. The experimental results constitute a direct evidence of the action of the three capping layers as sinks for Ge vacancies with different interface recombination velocities. On the contrary, the nitrogen diffusion data suggest that interface recombination velocities of Ge interstitials are almost independent of the capping layer choice
Emergence of ambient temperature ferroelectricity in meso-tetrakis(1- methylpyridinium-4-yl)porphyrin chloride thin films
No full textHere, we demonstrate that the meso-tetrakis(1-methylpyridinium-4-yl) porphyrin chloride, [H2TMPyP]4+Cl4, with a face-to-face orientation directed along a single direction displays ferroelectric properties at room temperature. This is attributed to its spontaneous polarization, due to an extensive hydrogen-bonded network. From C-V measurements, a remnant polarization of approximately 0.5 μC cm-2 was estimated for pristine porphyrin film, which increases linearly up to about 1.7 μC cm-2 after applying 2 V at the top electrode and further to 9.6 μC cm-2 after 5 V positive poling. This large - for practical utilization - level of remnant polarization of [H2TMPyP] 4+Cl4 makes it promising for future applications.</p
Semiconductor Nanocrystal Floating-gate Memory Devices
No full textABSTRACTCurrent research directions and recent advances in the area of semiconductor nanocrystal floating-gate memory devices are herein reviewed. Particular attention is placed on the advantages, limitations and perspectives of some of the principal new alternatives suggested for improving device performance and reliability. The attractive option of generating Si nanocrystal memories by ion-beam-synthesis (IBS) is discussed with emphasis on the ultra-low-energy (ULE) regime. Pertinent issues related to the fabrication of low-voltage memory cells and the integration of the ULE-IBS technique in manufactory environment are discussed. The effect on device performance of parasitic transistors that form at the channel corner of shallow trench isolated transistors is described in details. It is shown that such parasitic transistors lead to a substantial degradation of the electrical properties of the intended devices and dominates the memory behavior of deep submicronic cells.</jats:p
Nanoparticles for electronic devices
No full text358 σ.Στην παρούσα διατριβή διερευνήθηκε η σύνθεση ημιαγωγικών νανοκρυσταλιτών πυριτίου (νκ-Si) εντός πολύ λεπτών υμενίων διοξειδίου του πυριτίου, με σκοπό την κατασκευή ηλεκτρονικών διατάξεων μνήμης, στις οποίες οι νκ-Si χρησιμοποιούνται ως διακριτές θέσεις αποθήκευσης ηλεκτρικών φορτίων. Η σύνθεση των νκ-Si πραγματοποιήθηκε χρησιμοποιώντας ιοντική εμφύτευση πολύ χαμηλής ενέργειας (0.65-2keV).Έγιναν πειράματα εμφύτευσης σε εργαστηριακό και βιομηχανικό εμφυτευτή.
Στα πειράματα με βιομηχανικό εμφυτευτή μελέτες TEM απέδειξαν ότι (α) αύξηση της ενέργειας προκαλεί αύξηση του βάθους σχηματισμού του διδιάστατου στρώματος σχηματισμού των νκ-Si και (β) αύξηση της δόσης εμφύτευσης οδηγεί στο σχηματισμού νησίδων πυριτίου οι οποίες μπορούν να ανταλλάσσουν φορτία μεταξύ τους. Η καταλληλότερη ενέργεια εμφύτευσης είναι 1keV και η καταλληλότερη δόση 21016Si+cm-2. Με την βοήθεια ηλεκτρικών μεθόδων χαρακτηρισμού των πυκνωτών MOS προσδιορίσθηκε ο ρόλος της ενέργειας και της δόσης εμφύτευσης στα φαινόμενα μνήμης και προσδιορίσθηκε ο ρόλος των πλεοναζόντων ατόμων Si στην αγωγιμότητα των οξειδίων.
Μελέτες για την βελτιστοποίηση του παραθύρου μνήμης με ταυτόχρονη διατήρησή του για πάρα πολύ μεγάλο χρονικό διάστημα (10 χρόνια) έδειξαν ότι τα καλύτερα αποτελέσματα επιτυγχάνονται κατά την θερμική ανόπτηση στους 950ο C σε αέριο μίγμα Ν2/Ο2 με συγκέντρωση Ο2 1.5%-2% κατά όγκο για 30min για οξείδια πάχους 7nm εμφυτευμένα με 21016Si+cm-2 ενέργειας 1keV. Αύξηση της συγκέντρωσης οξυγόνου ή του χρόνου ανόπτησης είχαν σαν αποτέλεσμα την αυξημένη οξείδωση των νκ-Si και του υποστρώματος, την μείωση της συγκέντρωσής τους και της αγωγιμότητας φαινομένου σήραγγας προκαλώντας την εξαφάνιση των φαινομένων μνήμης.
Ακολουθώντας τις βέλτιστες συνθήκες επεξεργασίας που προσδιορίσθηκαν στα προκαταρκτικά πειράματα, έγινε η κατασκευή εργαστηριακών (τεχνολογία CMOS 1μm, 100mm δισκία Si) και βιομηχανικών (τεχνολογία CMOS 0.15μm, 200mm δισκία Si) πρωτοτύπων κυττάρων μνήμης MOSFET νκ-Si. Τα κύτταρα μνήμης MOSFET νκ-Si που κατασκευάσθηκαν πληρούν τις προϋποθέσεις για την κατασκευή ηλεκτρονικών μνημών με χαρακτηριστικά μη-προσωρινής διατήρησης της πληροφορίας (περισσότερο από 10 χρόνια στους 150οC) σε χαμηλές τάσεις λειτουργίας (+9V/-7V) και ανταγωνιστικούς χρόνους λειτουργίας (10ms) σχετικά με τις συμβατικές μνήμες αιωρούμενης πύλης. Πειράματα για τη μελέτη της απώλειας του αποθηκευμένου φορτίου σε υψηλές θερμοκρασίες απέδειξαν ότι η αποθήκευση των ηλεκτρονίων πραγματοποιείται σε βαθιές στάθμες εντός των νκ-Si, ενώ των οπών σε ρηχές στάθμες ή σε παγίδες του οξειδίου με πάρα πολύ μικρή ενέργεια ενεργοποίησης. Επιπλέον, αποδείχθηκε ότι με την επιλογή των κατάλληλων συνθηκών ανόπτησης και εμφύτευσης είναι δυνατό να κατασκευαστούν διατάξεις μνήμης οι οποίες προορίζονται για εφαρμογές γρήγορης (1μs) μη προσωρινής (11ημέρες στους 85οC) αποθήκευσης πληροφορίας με μικρές τάσεις λειτουργίας (+7V/-7V).
Τέλος, διερευνήθηκε η δράση του παρασιτικού τρανζίστορ το οποίο εμφανίζεται για διαφορετικούς λόγους τόσο στα εργαστηριακά όσο και στα βιομηχανικά πρωτότυπα. Αποδείχθηκε ότι σε υπομικρονικές διατάξεις μνήμης νκ-Si τα παρασιτικά φαινόμενα μνήμης είναι εξίσου σημαντικά με τα ενδογενή εξαιτίας της κυριαρχίας του παρασιτικού τρανζίστορ στις χαρακτηριστικές μεταφοράς.In this thesis, the synthesis of semiconductor nanocrystals (Si-ncs) embedded into thin silicon dioxide layers was studied, in order to fabricate electronic memory devices where the Si-ncs should act as discrete charge storage nodes. The synthesis of Si-ncs was achieved by very low energy ion beam synthesis (0.65 – 2 keV). Ion implantation experiments were performed in laboratory and industrial implanters. Results from devices prepared by the industrial implanter were reliable for both quantitative and qualitative results because the industrial implantation system (a) has a monoenergetic ion beam and (b) has a charge neutralization system.
TEM studies on samples fabricated by an industrial implantation system revealed that (a) the fabrication of a Si-ncs 2D array occurred deeper into the silicon dioxide matrix as the implantation energy increased and (b) elongated Si-ncs that were not mutually isolated and exchange charges were formed by increase of the implantation dose. In addition, it was found that the thinnest device grade silicon dioxide layer where Si-ncs could be achieved was 7nm. The best implantation conditions were 1keV energy and 21016Si+cm-2 implantation dose.
Experimental studies on the optimization of the memory window and its retention time (10 years standard) revealed that these could be achieved by thermal annealing at 950 οC for 30min in a mixture of Ν2/Ο2 with [Ο2]/[N2+O2] in the range 1.5%-2% per volume using 7nm thick silicon dioxide layers implanted with 1keV 21016Si+cm-2. Increase of the oxygen concentration or the annealing time resulted to the enhanced oxidation of the Si-ncs and the substrate, the reduction of the Si-ncs surface concentration and tunneling conduction current causing the elimination of the charge storage effects. Combined electrical characterization and structural TEM studies lead to a model describing of the role of oxygen during the thermal annealing process and thus the dose dependent action of oxygen was determined.
Following these optimized processing conditions, prototype memory cells were fabricated in a laboratory (1μm CMOS technology, 100mm Si wafer) and a manufacturing (1μm CMOS 0.15 technology, 200mm Si wafer) environment. The fabricated prototypes have (a) a highly uniform density of Si-ncs and (b) a highly uniform memory window, independently on the cell dimensions. The fabricated memory cells have nonvolatile properties (10-years retention time at 150οC) and their pulse operation was achieved utilizing low program/erase pulse voltages (+9V/-7V) with competitive pulse duration times (10ms) compared to the conventional floating gate memories. Further experiments on the charge retention characteristics revealed that the electrons (programming state) were stored at deep traps into the Si-ncs, while holes are stored either at shallow states into the Si-ncs or oxide traps. Additionally, it was found that proper selection of the implantation and annealing conditions could lead to the fabrication of memory devices with low voltage (+7V/-7V) and fast (1μs) operating characteristics, suitable for RAM applications with 11 days retention time at 85 οC.
Finally, the parasitic transistor action in laboratory and industrial memory cells was investigated. It was found that the parasitic transistor formed at the channel edges of the cells could be responsible for parasitic memory effects in Si-ncs memory devices. The effect was maximized for cells with deep submicronic gate lengths where the parasitic memory window was similar to the intrinsic one mainly because the cell’s transfer characteristics were governed by the action of parasitic transistor.Παναγιώτης Σ. Δημητράκη
Silicon Nanocrystals for Silicon Photonics
No full textIn the absence of suitable methods for integrating traditional semiconductor optoelectronic materials in CMOS microelectronic fabrication processes, nanostructured silicon has been actively explored as an alternative light emitter for silicon photonics. This thesis presents new experimental results in silicon nanocrystal photophysics and optoelectronics, including novel device designs for optical memory elements and light-emitting structures.
As quantum dots, silicon nanocrystals exhibit several interesting properties including size-tunable emission over visible and near-infrared wavelengths and improved oscillator strength for radiation. In contrast to bulk silicon, nanocrystals can emit light with quantum efficiencies approaching 100%. Through time-resolved photoluminescence measurements, we first quantitatively establish that the dense ensembles of nanocrystals that are attractive in device applications retain these advantages. We then describe the fabrication of fully CMOS compatible silicon nanocrystal optoelectronic test structures and show that such devices can function as room temperature optical memory elements.
We further demonstrate that electroluminescence can be achieved in our devices through a previously unreported process we call field effect electroluminescence, in which sequential charge carrier injection is used to create excitons in silicon nanocrystals. This mechanism is a promising approach for overcoming the difficulty inherent in electrically exciting silicon nanocrystals, which are necessarily surrounded by an electrical insulator. Finally, we present electrically excited infrared light sources that combine carrier injection through the field effect electroluminescence mechanism with near field energy transfer from silicon nanocrystals to infrared emitters.</p
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