X-ray Based in Situ Investigation of Silicon Growth Mechanism Dynamics—Application to Grain and Defect Formation

To control the final grain structure and the density of structural crystalline defects in silicon (Si) ingots is still a main issue for Si used in photovoltaic solar cells. It concerns both innovative and conventional fabrication processes. Due to the dynamic essence of the phenomena and to the coupling of mechanisms at different scales, the post-mortem study of the solidified ingots gives limited results. In the past years, we developed an original system named GaTSBI for Growth at high Temperature observed by Synchrotron Beam Imaging, to investigate in situ the mechanisms involved during solidification. X-ray radiography and X-ray Bragg diffraction imaging (topography) are combined and implemented together with the running of a high temperature (up to 2073 K) solidification furnace. The experiments are conducted at the European Synchrotron Radiation Facility (ESRF). Both imaging techniques provide in situ and real time information during growth on the morphology and kinetics of the solid/liquid (S/L) interface, as well as on the deformation of the crystal structure and on the dynamics of structural defects including dislocations. Essential features of twinning, grain nucleation, competition, strain building, and dislocations during Si solidification are characterized and allow a deeper understanding of the fundamental mechanisms of its growth

Development and properties of contacts in electronic devices

The present Ph.D thesis was part of a research program titled “Growth of photovoltaic technology based on thin films”. The aim of this program was the development of a multilayered chalcopyrite based (CIS / CIGS), thin film solar cell device. The purpose of the present Ph.D. thesis was the development and the characterization of the front ohmic contact (window layer - metal contact) of such a solar cell device (ZnO / ZnSe / chalcopyrite / Mo / Glass). Specifically, intrinsic ZnO and Al such as In doped ZnO thin films were deposited on soda lime glass substrates by the pulsed laser deposition technique (PLD). The deposition parameters such as laser wavelength and energy density, substrate temperature, oxygen pressure and dopant percentage addition were investigated, in order to produce thin films with optimum structural, optical and electrical properties. It has been found that the best deposition conditions were the following: · UV pulsed KrF excimer laser operated at 248nm with pulse frequency 10 Hz, pulse duration 10 ns, · Energy density 2.4 J/cm2 · Substrate temperature 300oC · Oxygen pressure 20 Pa · Compound ceramic target 3 wt% In:ZnO · Deposition time 1hr 30 min The deposited 3 wt% In:ZnO thin film presented high crystallinity, high transmittance (95%) and low resistivity (2x10-3 Ωcm). In order to complete the growth of such a front ohmic contact, different double layer metal contacts (Al/Au, Ag/Au, In/Au, Ti/Au) were deposited on the above In doped ZnO film. It has been observed that the front contact 3 wt% In:ZnO / Ti / Au present an ohmic behavior with the lowest resistance (800Ω).Η παρούσα Διδακτορική Διατριβή αποτέλεσε τμήμα του ερευνητικού έργου ΠΕΝΕΔ-2003 με τίτλο “Ανάπτυξη της Τεχνολογίας Φωτοβολταϊκών Βασισμένων σε Λεπτά Υμένια”. Το αντικείμενό της ήταν η ανάπτυξη της εμπρόσθιας ωμικής επαφής μιας ηλιακή κυψέλη χαλκοπυριτών (CIS/CIGS), βασισμένη στην τεχνολογία των φωτοβολταϊκών λεπτών υμενίων. Συγκεκριμένα, λεπτά υμένια ZnO και υμένια ZnO με προσμείξεις μετάλλου (Al, In), προς βελτίωση της αγωγιμότητας του υμενίου, αναπτύχθηκαν με τη μέθοδο της Εναπόθεσης με Δέσμη Παλμικού Laser (PLD) σε υπόστρωμα γυαλιού, καθώς αποτελούν μέρος της ετεροδομής ZnO/ZnSe/χαλκοπυρίτη/Mo/Glass. Για την ανάπτυξη των υμενίων ZnO χρησιμοποιήθηκε κεραμικός (ZnO) και μεταλλικός (Zn) στόχος ο οποίος αποδομήθηκε με παλμικό laser (Nd:YAG, KrF Excimer) σε ατμόσφαιρα O2. Διερευνήθηκε συστηματικά η επίδραση των ακόλουθων συνθηκών της διαδικασίας της εναπόθεσης: α) το μήκος κύματος και η πυκνότητα ενέργειας του laser (Nd:YAG (532nm), KrF Excimer (248nm)), β) η πίεση του φέροντος αερίου στο θάλαμο και γ) η θερμοκρασία του υποστρώματος στις δομικές, οπτικές και ηλεκτρικές ιδιότητες των υμενίων ZnO. Διαπιστώθηκε ότι η χρήση KrF Excimer laser μήκους κύματος ακτινοβολίας 248 nm, διάρκειας παλμού 10ns, συχνότητας 10Hz και πυκνότητας ενέργειας 2.4 J/cm2 για την αποδόμηση κεραμικού στόχου ZnO, με πίεση αντιδρώντος αερίου 20 Pa και θερμοκρασία υποστρώματος 300 οC, οδήγησε στην ανάπτυξη υμενίου ZnO καλής κρυσταλλικότητας, υψηλής διαπερατότητας και μικρής ειδικής ηλεκτρικής αντίστασης. Η διαδικασία της εναπόθεσης με Nd:YAG laser δεν απέδωσε ικανοποιητικά υμένια ZnO. Επίσης, διερευνήθηκε η επίδραση του ποσοστού της πρόσμειξης (Al ή In) στις δομικές, οπτικές και ηλεκτρικές ιδιότητες των υμενίων Al:ZnO και In:ZnO. Διαπιστώθηκε ότι υμένια ZnO με πρόσμειξη In 3 κ.β.% εξασφαλίζουν μικρή ειδική ηλεκτρική αντίσταση και διατηρούν την διαπερατότητα των υμενίων ZnO σε ιδιαίτερα υψηλά επίπεδα. Προς ολοκλήρωση της εμπρόσθιας ηλεκτρικής επαφής εναποτέθηκαν μεταλλικά ηλεκτρόδια δύο στρωμάτων (Al/Au, Ti/Au, In/Au, Ag/Au), στην επιφάνεια των υμενίων Al:ZnO και In:ZnO και διερευνήθηκε το είδος της ετεροεπαφής μετάλλου - ημιαγωγού (Ωμική επαφή ή Schottky) που αναπτύσσεται. Διαπιστώθηκε ότι οι ετεροεπαφές μετάλλου - ημιαγωγού που αναπτύχθηκαν, παρουσιάζουν ωμική συμπεριφορά

Development and properties of contacts in electronic devices

247 σ.Η παρούσα Διδακτορική Διατριβή αποτέλεσε τμήμα του ερευνητικού έργου ΠΕΝΕΔ–2003 με τίτλο “Ανάπτυξη της Τεχνολογίας Φωτοβολταϊκών Βασισμένων σε Λεπτά Υμένια”. Το αντικείμενό της ήταν η ανάπτυξη της εμπρόσθιας ωμικής επαφής μιας ηλιακή κυψέλη χαλκοπυριτών (CIS/CIGS), βασισμένη στην τεχνολογία των φωτοβολταϊκών λεπτών υμενίων. Συγκεκριμένα, λεπτά υμένια ZnO και υμένια ZnO με προσμείξεις μετάλλου (Al, In), προς βελτίωση της αγωγιμότητας του υμενίου, αναπτύχθηκαν με τη μέθοδο της Εναπόθεσης με Δέσμη Παλμικού Laser (PLD) σε υπόστρωμα γυαλιού, καθώς αποτελούν μέρος της ετεροδομής ZnO/ZnSe/χαλκοπυρίτη/Mo/Glass. Για την ανάπτυξη των υμενίων ZnO χρησιμοποιήθηκε κεραμικός (ZnO) και μεταλλικός (Zn) στόχος ο οποίος αποδομήθηκε με παλμικό laser (Nd:YAG, KrF Excimer) σε ατμόσφαιρα O2. Διερευνήθηκε συστηματικά η επίδραση των ακόλουθων συνθηκών της διαδικασίας της εναπόθεσης: α) το μήκος κύματος και η πυκνότητα ενέργειας του laser (Nd:YAG (532nm), KrF Excimer (248nm)), β) η πίεση του φέροντος αερίου στο θάλαμο και γ) η θερμοκρασία του υποστρώματος στις δομικές, οπτικές και ηλεκτρικές ιδιότητες των υμενίων ZnO. Διαπιστώθηκε ότι η χρήση KrF Excimer laser μήκους κύματος ακτινοβολίας 248 nm, διάρκειας παλμού 10ns, συχνότητας 10Hz και πυκνότητας ενέργειας 2.4 J/cm2 για την αποδόμηση κεραμικού στόχου ZnO, με πίεση αντιδρώντος αερίου 20 Pa και θερμοκρασία υποστρώματος 300 οC, οδήγησε στην ανάπτυξη υμενίου ZnO καλής κρυσταλλικότητας, υψηλής διαπερατότητας και μικρής ειδικής ηλεκτρικής αντίστασης. Η διαδικασία της εναπόθεσης με Nd:YAG laser δεν απέδωσε ικανοποιητικά υμένια ZnO. Επίσης, διερευνήθηκε η επίδραση του ποσοστού της πρόσμειξης (Al ή In) στις δομικές, οπτικές και ηλεκτρικές ιδιότητες των υμενίων Al:ZnO και In:ZnO. Διαπιστώθηκε ότι υμένια ZnO με πρόσμειξη In 3 κ.β.% εξασφαλίζουν μικρή ειδική ηλεκτρική αντίσταση και διατηρούν την διαπερατότητα των υμενίων ZnO σε ιδιαίτερα υψηλά επίπεδα. Προς ολοκλήρωση της εμπρόσθιας ηλεκτρικής επαφής εναποτέθηκαν μεταλλικά ηλεκτρόδια δύο στρωμάτων (Al/Au, Ti/Au, In/Au, Ag/Au), στην επιφάνεια των υμενίων Al:ZnO και In:ZnO και διερευνήθηκε το είδος της ετεροεπαφής μετάλλου – ημιαγωγού (Ωμική επαφή ή Schottky) που αναπτύσσεται. Διαπιστώθηκε ότι οι ετεροεπαφές μετάλλου – ημιαγωγού που αναπτύχθηκαν, παρουσιάζουν ωμική συμπεριφορά.Thesis Title: “Development and properties of contacts in electronic devices”. The present Ph.D thesis was part of a research program titled “Growth of photovoltaic technology based on thin films”. The aim of this program was the development of a multilayered chalcopyrite based (CIS / CIGS), thin film solar cell device. The purpose of the present Ph.D. thesis was the development and the characterization of the front ohmic contact (window layer – metal contact) of such a solar cell device (ZnO / ZnSe / chalcopyrite / Mo / Glass). Specifically, intrinsic ZnO and Al such as In doped ZnO thin films were deposited on soda lime glass substrates by the pulsed laser deposition technique (PLD). The deposition parameters such as laser wavelength and energy density, substrate temperature, oxygen pressure and dopant percentage addition were investigated, in order to produce thin films with optimum structural, optical and electrical properties. It has been found that the best deposition conditions were the following: • UV pulsed KrF excimer laser operated at 248nm with pulse frequency 10 Hz, pulse duration 10 ns, • Energy density 2.4 J/cm2 • Substrate temperature 300oC • Oxygen pressure 20 Pa • Compound ceramic target 3 wt% In:ZnO • Deposition time 1hr 30 min The deposited 3 wt% In:ZnO thin film presented high crystallinity, high transmittance (95%) and low resistivity (2x10-3 Ωcm). In order to complete the growth of such a front ohmic contact, different double layer metal contacts (Al/Au, Ag/Au, In/Au, Ti/Au) were deposited on the above In doped ZnO film. It has been observed that the front contact 3 wt% In:ZnO / Ti / Au present an ohmic behavior with the lowest resistance (800Ω).Μαρία Γ. Τσούτσουβ

Kinetics of Magnesiothermic Reduction of Natural Quartz

In this work, the kinetics of natural quartz reduction by Mg to produce either Si or Mg2Si was studied through quantitative phase analysis. Reduction reaction experiments were performed at various temperatures, reaction times and Mg to SiO2 mole ratios of 2 and 4. Rietveld refinement of X-ray diffraction patterns was used to obtain phase distributions in the reacted samples. SEM and EPMA examinations were performed to evaluate the microstructural change during reduction. The results indicated that the reduction reaction rate was slower at a mole ratio of 2 than 4 at the same temperature, as illustrated by the total amount of Si formed (the percent of Si that is reduced to either Si or Mg2Si to total amount of Si) being 59% and 75%, respectively, after 240 min reaction time for mole ratios of 2 and 4. At the mole ratio of 4, the reaction rate was strongly dependent on the reaction temperature, where SiO2 was completely reduced after 20 min at 1273 K. At the lower temperatures of 1173 and 1073 K, total Si formed was 75% and 39%, respectively, after 240 min reaction time. The results of the current work show that Mg2Si can be produced through the magnesiothermic reduction of natural quartz with high yield. The obtained Mg2Si can be processed further to produce silane gas as a precursor to high purity Si. The combination of these two processes offers the potential for a more direct and low carbon method to produce Si with high purity

Kinetics of Magnesiothermic Reduction of Natural Quartz

In this work, the kinetics of natural quartz reduction by Mg to produce either Si or Mg2Si was studied through quantitative phase analysis. Reduction reaction experiments were performed at various temperatures, reaction times and Mg to SiO2 mole ratios of 2 and 4. Rietveld refinement of X-ray diffraction patterns was used to obtain phase distributions in the reacted samples. SEM and EPMA examinations were performed to evaluate the microstructural change during reduction. The results indicated that the reduction reaction rate was slower at a mole ratio of 2 than 4 at the same temperature, as illustrated by the total amount of Si formed (the percent of Si that is reduced to either Si or Mg2Si to total amount of Si) being 59% and 75%, respectively, after 240 min reaction time for mole ratios of 2 and 4. At the mole ratio of 4, the reaction rate was strongly dependent on the reaction temperature, where SiO2 was completely reduced after 20 min at 1273 K. At the lower temperatures of 1173 and 1073 K, total Si formed was 75% and 39%, respectively, after 240 min reaction time. The results of the current work show that Mg2Si can be produced through the magnesiothermic reduction of natural quartz with high yield. The obtained Mg2Si can be processed further to produce silane gas as a precursor to high purity Si. The combination of these two processes offers the potential for a more direct and low carbon method to produce Si with high purity

Chemical and microstructural characterisation of HfO2-Y2O3 ceramics with high amount of Y2O3 (33, 40 and 50 mol. %) manufactured using spark plasma sintering

International audienceHafnia based ceramics are potential promising candidates to be used as thermal barrier coatings (TBC) for applications in the field of propulsion. In this study, Spark Plasma Sintering (SPS) of fully stabilised hafnia with yttrium oxide (yttria) was investigated to provide a better understanding of the effect of manufacturing parameters, on the crystallography, chemistry and microstructure of the material. Several hafnia powders, containing different amounts of yttria (33 mol. %, 40 mol. % or 50 mol. %), were sintered by SPS at different temperature levels ranging from 1600 • C to 1850 • C. On these materials, X-ray diffraction patterns associated with scanning electron micrographs have highlighted the influence of both the sintering temperature and the amount of yttria on the final composition, the lattice parameter and the microstructure of hafnia-based materials. In the end, it is established that, for all quantities of yttrium employed, the main phase is Y 2 Hf 2 O 7 with very high densification levels

In situ and real-time investigation of the solidification of silicon by X-ray imaging

Aiming to the production of low cost and high efficiency solar cells based on silicon material, all processes either innovative or conventional face challenges linked to formation and development of the grain structure, distortion of the crystal and crystalline defects during the solidification step. Our contribution consists in studying these key and fundamental solidification mechanisms by conducting in situ and time-resolved investigations. Two imaging techniques based on X-ray synchrotron radiation are combined during solidification: X-ray radiography and Bragg diffraction (topography). X-ray radiography brings information on the morphology and kinetics of the solid/liquid (S/L) interface. X-ray Bragg diffraction (topography) gives complementary information about misorientations, structural defect formation and the global and local level of distortion of the crystal. The dynamics of twinning, grain competition, sub-grain formation and of the related crystal distortions have been studied in silicon with this method as well as the effect of the impurities on grain nucleation and defect formation