Dissertations / Theses on the topic 'Amorphous Silica Surface'

Developing a fundamental understanding of the interactions of chemical warfare agents (CWAs) with surfaces is essential for the rational design of new sorbents, sensors, and decontamination strategies. The interactions of chemical warfare agent simulants, molecules which retain many of the same chemical or physical properties of the agent without the toxic effects, with amorphous silica were conducted to investigate how small changes in chemical structure affect the overall chemistry. Experiments investigating the surface chemistry of two classes of CWAs, nerve and blister agents, were performed in ultrahigh vacuum to provide a well-characterized system in the absence of background gases. Transmission infrared spectroscopy and temperature-programmed desorption techniques were used to learn about the adsorption mechanism and to measure the activation energy for desorption for each of the simulant studied. In the organophosphate series, the simulants diisopropyl methylphosphonate (DIMP), dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), dimethyl chlorophosphate (DMCP), and methyl dichlorophosphate (MDCP) were all observed to interact with the silica surface through the formation of a hydrogen bond between the phosphoryl oxygen of the simulant and an isolated hydroxyl group on the surface. In the limit of zero coverage, and after defect effects were excluded, the activation energies for desorption were measured to be 57.9 ± 1, 54.5 ± 0.3, 52.4 ± 0.6, 48.4 ± 1, and 43.0 ± 0.8 kJ/mol for DIMP. DMMP, TMP, DMCP, and MDCP respectively. The adsorption strength was linearly correlated to the magnitude of the frequency shift of the ν(SiO-H) mode upon simulant adsorption. The interaction strength was also linearly correlated to the calculated negative charge on the phosphoryl oxygen, which is affected by the combined inductive effects of the simulants’ different substituents. From the structure-function relationship provided by the simulant studies, the CWA, Sarin is predicted to adsorb to isolated hydroxyl groups of the silica surface via the phosphoryl oxygen with a strength of 53 kJ/mol. The interactions of two common mustard simulants, 2-chloroethyl ethyl sulfide (2-CEES) and methyl salicylate (MeS), with amorphous silica were also studied. 2-CEES was observed to adsorb to form two different types of hydrogen bonds with isolated hydroxyl groups, one via the S moiety and another via the Cl moiety. The desorption energy depends strongly on the simulant coverage, suggesting that each 2-CEES adsorbate forms two hydrogen bonds. MeS interacts with the surface via a single hydrogen bond through either its hydroxyl or carbonyl functionality. While the simulant work has allowed us to make predictions agent-surface interactions, actual experiments with the live agents need to be conducted to fully understand this chemistry. To this end, a new surface science instrument specifically designed for agent-surface experiments has been developed, constructed, and tested. The instrument, located at Edgewood Chemical Biological Center, now makes it possible to make direct comparisons between simulants and agents that will aid in choosing which simulants best model live agent chemistry for a given system. These fundamental studies will also contribute to the development of new agent detection and decontamination strategies.
Ph. D.

L'hydrotraitement est un procédé catalytique important dans le raffinage du pétrole qui utilise des catalyseurs bimétalliques sulfurés NiWS ou NiMoS (ou CoMoS) supportés sur alumine. Leur mode conventionnel de préparation implique l’imprégnation d'une solution aqueuse de sels de Mo/W et de Ni/Co, puis l’activation par un agent sulfo-réducteur (H2S/H2). Pour répondre aux exigences environnementales et améliorer l'efficacité de l'hydrotraitement, des améliorations permanentes de la performance de ces systèmes catalytiques sont attendues. Ce travail se concentre sur la préparation de catalyseurs d'hydrotraitement hautement actifs par une approche de chimie de surface contrôlée (CSC) qui implique l'imprégnation successive de précurseurs moléculaires de MoV et NiII en solvant organique sur un support silice-alumine traité thermiquement. Dans la première partie de cette thèse, la genèse de la phase active du catalyseur CSC et conventionnel Mo et NiMo est étudiée par quick-XAS combinée à d’autres techniques (chimiométrie, XPS, RPE, STEM-HAADF, modélisation moléculaire). Nous proposons ainsi des structures moléculaires depuis les précurseurs oxydes de Mo et Ni supportés jusqu’aux nombreuses espèces intermédiaires (oxysulfure et sulfures) en fonction de la température. Cette analyse multi-technique permet d'abord de révéler les spécificités de la genèse des catalyseurs CSC et conventionnels qui peuvent expliquer leurs différentes activités catalytiques. Ensuite, elle révèle également de nouvelles connaissances sur les mécanismes d’insertion du Ni dans la phase NiMoS en fonction de la préparation. Dans la seconde partie, la possibilité de remplacer Co et Ni comme promoteurs est explorée. Ceci est entrepris en synthétisant des catalyseurs alternatifs de type XYMoS, où X et Y sont des métaux de transition 3d. Comme suggéré par des études de modélisation quantiques antérieures, certaines formulations XYMoS peuvent présenter un effet de synergie analogue à ceux des phases actives CoMoS et NiMoS. L’étude des formulations les plus prometteuses méritent d'être approfondies afin de mieux comprendre leur fonctionnement
Hydrotreating is an important catalytic process in petroleum refining which uses sulfided bimetallic catalysts NiWS or NiMoS (or CoMoS) supported on alumina. Their conventional preparation involves an incipient wetness impregnation of an aqueous solution of Mo/W and Ni/Co salts, and then activation by a sulfo-reductive agent (such as H2S/H2). To meet environmental regulations and improve the energy efficiency of hydrotreatment, permanent improvements on the performance of these catalytic systems are expected. This work is thus focused on the preparation of highly active hydrotreating catalysts through a controlled surface chemistry (CSC) approach; which involves the successive impregnation of Mo5+ and Ni2+ molecular precursors in an organic solvent on a thermally treated silica-alumina support. In the first part of this thesis, the active phase genesis of CSC and conventional Mo and NiMo catalysts is studied by in situ quick-XAS combined with various other techniques (chemometrics, XPS, EPR, STEM-HAADF, molecular modeling). We thus propose molecular structures from the oxide of supported Mo and Ni precursors up to the numerous intermediate sulfided species as a function of temperature. This multi-technique analysis enables first to reveal the specific features of the genesis of CSC and conventional catalysts which may explain their different catalytic activities. Then, it also reveals new insights into the mechanisms of Ni promoter incorporation into the NiMoS phase as a function of the preparation. In the second part, the feasibility of replacing Co and Ni as promoters is explored. Using the CSC method, we attempted to synthesize alternative catalysts of the form XYMoS ternary sulfides, where X and Y are 3d transition metals. As suggested by previous quantum simulations, certain XY formulations possibly reveal a synergy effect as observed in CoMoS and NiMoS active phases. The most promising formulations merit further investigations

This work deals with the experimental investigation of the surface of one of the most interesting and important new semiconductor materials, hydrogenated amorphous silicon (a-Si:H). Ultra-high vacuum surface spectroscopy methods, especially Auger lineshape analysis and X-ray photoelectron spectroscopy (XPS), have been used with a view to studying the effect on the local densities of states at the surface of various preparation methods and subsequent treatments. X-ray excited Si L2,3VV and Si L1L2,3V Auger lines as well as XPS valence band (XPS VB) spectra have been measured for a number of silicon materials and surfaces prepared in different ways. These materials included crystalline silicon (c-Si), amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H). Preparation techniques used included atmospheric pressure chemical vapour deposition (APCVD), glow discharge (GD) or plasma enhanced CVD and radio frequency (RF) sputtering. Surface treatments included disordering of c-Si by argon ion bombardment, hydrogenation by hydrogen ion bombardment, annealing and rehydrogenation from the bulk. Methods have been developed and thoroughly tested to enable X-ray excited silicon Auger spectra to be treated routinely using numerical debroadening and deconvolution to obtain an indication of the valence band transition densities of states (VBTDOS). These show good agreement with previously published results for electron initiated Auger spectra and theoretical results. In particular a method has been developed for treating the experimentally difficult Si L1L2,3V Auger line. Techniques have been developed to remove the sloping background and Coster-Kronig broadening to enable an indication of the transition densities of states. The L1L2,3V derived VBTDOS approximates closely to the theoretical DOS and the experimental results obtained from UPS and XPS. It is shown that because of this the Si L1L2,3V line is a more effective method of monitoring changes in the surface VBTDOS of a-Si:H due to various treatments than the more commonly used but harder to interpret Si L2,3VV line. A method based on the simplex algorithm has been applied to enable the Si L2,3VV and Si L1L2,3V spectra to be decomposed (decoupled) into their component (p-, sp- and s/L2H like) peaks. Changes in the relative contributions of these components have been compared with changes induced by disordering and hydrogenation. It is shown that both the Si L2,3VV and Si L1L2,3V lines give a semiquantitative method for monitoring hydrogen incorporation and changes in the localised states near the valence band edge. Results are presented for varying amounts of disorder (or amorphousness) produced by argon ion bombardment of the surface. A number of results are ·presented· for artificial and naturally hydrogenated surfaces as well as for different deposition techniques. AES and XPS are shown to be very sensitive to changes in disorder (amorphousness) and hydrogen bonding in a-Si:H. The L1L2,3V Auger spectrum is found to be particularly sensitive. Both the AES and XPS VB spectra for a disordered c-Si sample give new information on the affect of disorder on the DOS. The L1L2,3V Auger line is also shown to be sensitive to varying degrees of disorder. Si L2,3VV and L1L2,3V spectra are successfully used to study the effect of several rehydrogenation methods on a-Si. These methods are shown to lead to different amounts of hydrogen in the surface as well as differences in the type of hydrogen bonding. A-Si:H prepared using different techniques is shown to have differences in the amount of order and hydrogen present in the films produced. The deposition technique is also seen to effect the type of hydrogen bonding present in the surface. A novel transfer vessel has been constructed to enable samples prepared in one system to be analysed in another UHV system without exposure to air and the subsequent contamination of the surface. Results are presented for a pure, 'as deposited' surface of a-Si:H prepared by GD. The 'as deposited' surface is shown to be significantly different to one that has been argon ion cleaned and then rehydrogenated. Also using the transfer method changes in the Si L1L2,3V, Si L2,3VV and XPS VB spectra were studied for an a-Si:H surface after heating above the first desorption threshold for hydrogen. This enables the effect of different Si-H bonding configurations on the VBDOS to be studied.

En aquesta tesi s'estudia la passivació del silici cristal·lí per a la producció de cèl·lules solars d'alta eficiència (> 20%) a baix preu.
Actualment la indústria fotovoltaica empra capes de nitrur de silici crescut mitjançant la tècnica PECVD. Com a alternativa, es presenta el carbur de silici amorf (a-SiC), també crescut mitjançant PECVD. Resultats anteriors mostren que la passivacio del silici a partir de carbur de silici amorf son excel·lents quan el material és ric en silici i dopat amb fòsfor. L'alt contingut en silici provoca absorció de la llum a la capa, que no es tradueix en corrent elèctric, fent d'aquesta manera que el material sigui només útil quan s'aplica a la cara no il·luminada de la cèl·lula.
L'objectiu d'aquesta tesi és millorar les propietats de passivació del carbur de silici afegint els requisits indispensables en cèl·lules solars: uniformitat, transparència i propietats antireflectants, estabilitat a llarg termini i enfront altes temperatures. A part de les aplicacions tecnològiques també es pretèn entendre millor les propietats fonamentals de passivació.
Els principals resultats són:
- La passivació millora a mesura que s'incrementa el gruix de la capa de a-SiC, fins arribar a una saturació a partir de 50 nm. El mecanisme responsable es una millor saturació dels defectes de la interficie amb hidrogen. Al contrari del que es pensava a priori, la càrrega el·lèctrica emmagatzemada a la capa es manté constant amb el gruix.
- Experiments amb "corona charge" indiquen que l'origen de la càrrega el·lèctrica que produeix la passivació per efecte de camp es troba en la densitat d'estats a la interfície.
- No ha estat possible trobar una capa tranparent (rica en carboni) amb bona passivació. La millor aproximació per combinar passivació més transparència és emprar dues capes, una molt prima rica en silici per passivar i l'altra rica en carboni per aconseguir les propietats antireflectants adequades. S'ha optimitzat el gruix de la capa rica en silici per aconseguir un compromís entre la pèrdua de corrent degut a l'absorció de la llum a la capa i les propietats de passivació. Aquesta combinació de doble capa s'ha fet servir per passivar bases tipus p i emissors tipus n amb resultats excel·lents. Finalment, amb la doble capa es va poder fabricar la primera cèl·lula passivada amb carbur de silici amb una eficiencia > 20%.
- S'ha desenvolupat un material nou: l'al·leació de silici, carboni i nitrogen dopada amb fòsfor. Aquest material ha donat els millors resultats de passició fins ara obtingut dins el nostre grup en bases tipus p i tipus n i en emissors tipus n. La composició òptima és rica en silici i la combinació de capes dobles amb diferents composicions, com en el cas anterior, torna a donar bons resultats de passivació i transparència.
- S'han desenvolupat experiments d'estrès tèrmic a alta temperatura. Les propietats de passivació es veuen fortament afectades desprès de l'estrès si les capes són riques en silici. D'altra banda, les dobles capes mostren una estabilitat molt més alta a l'estrès tèrmic.
The thesis focuses on the study of surface passivation of crystalline silicon to produce high efficiency solar cells (with conversion efficiencies > 20%) at reduced prices. The state of the art in surface passivation is done by thin films of amorphous silicon nitride grown by Plasma Enhanced Chemical Vapour Deposition (PECVD) and it is a very well established material in the photovoltaic field.
In this thesis we offer an alternative that is based on amorphous silicon carbide (a-SiC), also grown by PECVD. The passivation properties of silicon carbide have been already studied in our group finding that excellent results can be obtained when the films are rich in silicon, especially for those doped with phosphorus to make a n-type material. Because this feature leads to undesirable absorption of solar light within the films that does not contribute to the photocurrent, silicon carbide would then be relegated to passivate only the rear side of the solar cell.
The aim of this work is to improve surface passivation properties developed previously and add compulsory requisites for the application of crystalline solar cells. These requisites are: uniformity, transparency and antireflective properties, stability under long term operation and stability under high temperature steps (allowing screen printing processes). Also it is the willing to provide a better understanding of the fundamental properties.
The main results achieved are enumerated hereafter:
- Surface passivation improves with the film thickness and then saturates for films thicker than 50 nm. The mechanism responsible for this improvement is not an increase of the electric charge in the film, as in principle could be thought, but a better saturation of defects by the presence of hydrogen. The amount of charge density seems to be independent of the film.
- Experiments of corona charge reveal some treats about the nature of the charge density to provide the field effect passivation. The origin of the charge seems to be a continuous density of states at the interface, rather a fixed charge allocated in the film.
- None of the attempts using carbon rich films, which are transparent and with antireflective properties, resulted in excellent surface passivation. Such attempts included variation of the deposition parameters, use of remote plasma PECVD with high incorporation of hydrogen, and introduction of nitrogen of in the phosphorus doped a-SiC films. Therefore, up to now it becomes apparent that it is a fundamental property of silicon carbide films the necessity to be rich in silicon to perform surface passivation.
- The way to combine surface passivation and antireflective properties was applying stacks of different a-SiC layers: one silicon rich and one carbon rich. The thickness of the silicon rich layer was optimized to reach a trade-off between level of passivation and lost of photocurrent due to the absorption in the film. The stacks were used to passivate p-type bases, with reasonably good results, and n+- type emitters, with very good results. The stacks provided the the first silicon solar a-SiC rear side passivated with efficiency above 20%.
- A new material was tested: a ternary alloy of silicon, carbon and nitrogen doped with phosphorus. This material was applied to n- and p-type bases and n+-type emitters, presenting the best results in surface passivation achieved by our group, and comparable to surface passivation record achieved by amorphous silicon carbide. Best composition was rich in silicon, and again stacks of silicon rich and carbon rich films was combined successfully.
- Stability against thermal processes was tested on different passivation schemes. After the treatment, the passivation is strongly reduced for single silicon rich films, which were offering good initial results. On the other hand, the stacks with a second carbon rich film maintain reasonably well the surface passivation properties.

This study was undertaken to investigate the possibility of synthesis of nitride based semiconductors. To this end hydrogenated amorphous silicon nitride (a-SiNx:Hy) has been deposited as the starting material using PECVD (plasma enhanced chemical vapour deposition). Then the effects of implanting gallium into the a-SiNx:H target material have been studied with the aim of forming GaN compounds. Should this technique work, it opens the possibility of carrying out similar synthesis of Al, hi and other nitride based compounds. PECVD hydrogenated amorphous silicon nitride thin films are studied as a function of the ammonia/silane gas ratio. The power coupled to the plasma, pressure and the substrate temperature were held constant, while the gas flow ratio of NH3/SiH4 was varied. IV measurements on the metal- nitride-metal structures indicated that the conduction mechanism might be explained by Poole-Frenkel conduction. The composition of the a-SiNx:Hy films were analysed using Rutherford backscattering spectroscopy (RBS) and Elastic recoil detection analysis (ERDA). The Si content decreased in a logarithmic manner for 0 < NH3/SiH4 < 4 and saturated for higher NH3/SiH4, ratios at 27 at.%. The N content mirrored this trend and saturated to a maximum asymptotic value of 49 at.% for NH3/SiH4 > 4. Stress, refractive index and optical absorption studies were also conducted. A turning point for most of the properties was observed at a NH3/SiH4 ratio of 4. This corresponds to a N/Si ratio of about 1.4 with a hydrogen content of 22 at.% for the deposited films. Below this ratio, a-SiNx:H films have high growth rates, a refractive index between 1.9 and 2.7, a N/Si ratio between 0.5 and 1.5 and moderate values of compressive stress (~ 0.7 GPa). While, above this pivotal ratio, growth rates become significantly lower, the refractive index minimises to 1.8, N and Si concentrations in the films saturate and the compressive stress rapidly increases. Evidence has been found for Ga-N bonds by implanting gallium into amorphous silicon nitride films. The a-SiNx:H films grown at high gas ratios (R > 70) are highly stressed and the nitrogen content is saturated. They are, therefore, ideally suited to forming GaN bonds under the high energy conditions of Ga ion implantation, a phase that is not thermodynamically favourable. X-ray photoelectron spectroscopy (XPS), FTIR and RBS have been used to examine the bond structure, composition and the depth profile of the synthesised material. It has been found that the implanted Ga bonds with the N from the NH2, NH and SiN bonds and the released Si and H from these bonds combine to form additional SiH. From the RBS and XPS data, annealing at 200°C was shown to increase the thickness of the a-GaN and transform more of the target. The Ga profile moves deeper into the material and the stoichiometric phase of SiN that is thermodynamically stable, recovers. Annealing at a higher temperature (500°C) shows a significant reduction in the amount of H from the amorphous network (ERD), mainly from the NH bonds (FTIR), thus leaving the free N available to bond with the unbonded Ga in the material. Up to ~ 22 at.% Ga is present in the material and can be converted to GaN on annealing. Electron diffraction through the material shows no evidence of any crystallites in the synthesised material.

This thesis presents a work which has been devoted to the growth of silicon thin films on crystalline silicon for photovoltaic applications by means of RF PECVD. The primary goal of this work was to obtain an amorphous growth on any c-Si surface in order to provide an efficient passivation, as required in heterojunction solar cells. Indeed, we demonstrated that epitaxial or mixed phase growths, easy to obtain on (100) Si, would lead to poor surface passivation. We proved that growing a few nm thin a-Si1-xCx:H alloy film was an efficient, stable and reproducible way to hinder epitaxy while keeping an excellent surface passivation by the subsequent deposition of a-Si:H films. Process optimization mainly based on Spectroscopic Ellipsometry, Effective lifetime measurements (Sinton lifetime tester) and current-voltage characterization led us to demonstrate that it was possible to obtain a-Si:H/c-Si heterojunction solar cells with stable VOC of 710 mV and FF of 76 % on flat (n) c-Si wafers, with solar cells of 25 cm2 whose metallization was realized by screen-printing technology. This work has also demonstrated the viability of a completely dry process where the native oxide is removed by SiF4 plasma etching instead of the wet HF removal. Last but not least, the epitaxial growth of silicon thin films, undoped and n or p-type doped, on (100)-oriented surfaces has been studied by Spectroscopic Ellipsometry and Hall effect measurements. We have been able to fabricate homojunction solar cells with a p-type emitter as well as p-i-n structures with an undoped epitaxial absorber on a heavily-doped (p) c-Si wafers.

Ce travail contribue à l'étude des propriétés de surface des silices amorphes. La principale technique d'analyse mise en oeuvre pour caractériser les hydroxyles présents à la surface du matériau (silanol et eau physisorbée) est la spectrométrie proche infrarouge (PIR). Les types de silanols à la surface d'une silice, leurs interactions, leur condensation par traitement thermique sont décrits à partir des absorptions IR. Le coefficient molaire d'absorption intégrée pour les transitions vOH, (v+[delta])SiOH et 2vOH est obtenu en corrélant PIR et thermogravimétrie. Ce demier ne dépend pas de l'environnement des hydroxyles pour des transitions de combinaison ou harmonique d'ordre 2 alors qu'il augmente fortement par formation de liaison hydrogène pour la transition fondamentale vOH. Une méthode de mesure spectrométrique de la quantité totale de silanols d'une silice est proposée. A 700°C, la surface d'une silice présente majoritairement des silanols isolés (0,7 OH/nm2). Une attribution détaillée des absorptions IR du groupement SiOH de 100 à 11000 cm-1 est déduite des spectres PIR. Un site de surface est simulé à partir de calculs ab initio sur une série de molécules modèles comportant les groupements SiOH et Si(OH)2. Le PIR présente une région spectrale propre à l'absorption de H2O qui permet de suivre l'hydratation du solide. L'étude quantitative est réalisée en combinant spectres PIR et isothermes d'adsorption par gravimétrie. Les silanols sont les centres d'adsorption et nous les avons classés en fonction de leur affinité pour la molécule d'eau. Lors de l'hydratation, les molécules d'eau occupent en premier les sites préférentiels puis forment des agrégats autour des premiers sites. Le verre poreux Vycor contient du bore en impureté. Sa migration vers la surface par traitement thermique est mise en évidence. Elle s'accompagne de la formation de groupements boranol. Les propriétés du bore de surface (hydratation et dissolution) et les vibrations de BOH sont examinées.

Magister Scientiae - MSc (Physics)
An increasing energy demand and growing environmental concerns regarding the use of fossil fuels in South Africa has led to the challenge to explore cheap, alternative sources of energy. The generation of electricity from Photovoltaic (PV) devices such as solar cells is currently seen as a viable alternative source of clean energy. As such, crystalline, amorphous and nanocrystalline silicon thin films are expected to play increasingly important roles as economically viable materials for PV development. Despite the growing interest shown in these materials, challenges such as the partial understanding of standardized measurement protocols, and the relationship between the structure and optoelectronic properties still need to be overcome.

The focus of the presented research is to examine the fundamental physics and chemistry of low-energy electron-stimulated reactions on adsorbate covered single crystal surfaces. Specifically, condensed SiCl₄ on the Si(111) surface and condensed H₂O on the TiO₂ (110) surface have been studied. By varying adsorbate film thicknesses, the coupling strength of the target molecule to the substrate and surrounding media dictates the progression of the electron induced reactions. To investigate the electron interactions with SiCl₄ on the Si(111) surface, desorbing cations and neutrals were detected via time of flight mass spectrometry (ToF-MS) where neutral chlorine atoms were ionized using a resonance enhanced multi-photon ionization (REMPI) technique. Structure in the cation and neutral yields were assigned to molecular excitations. At an incident electron energy of 10 eV, a resonance structure in the neutral yields was attributed to a negative ion resonance and observed in thick and thin films of SiCl₄. With monoenergetic electrons, specific surface reactions can be controlled which have implications for film growth, surface patterning and masking, and etching. For the H₂O/TiO₂ (110) system, the water interactions with the TiO₂ surface are revealed through the strong electron induced reaction dependencies on the water coverage. Understanding the nonthermal reaction landscape of H₂O on the TiO₂ (110) surface is crucial for developing the system as a catalytic source of hydrogen. The electron-stimulated oxidation of the TiO₂ (110) surface and electron induced sputtering of H ₂O was investigated. Irradiation of water films ([coverage]< 3 ML) oxidized the TiO₂ (110) surface similarly as surface oxidation via O₂ deposition. Each H₂O molecule in the first monolayer seems to be a target for the incoming electron initiating the oxidation. However, water coverages greater than a monolayer limited the oxidation process. The electron-stimulated desorption and sputtering yields of water from the TiO₂ (110) surface were measured as a function of water coverage. Surprisingly, the amount of water sputtered from the surface is nonlinearly dependent on water coverage.

Parmi les technologies photovoltaïques à base de silicium, les cellules solaires à hétérojonction a-Si:H/c-Si (HJ) ont montré une attention croissante en ce qui concerne leur fort potentiel d’amélioration du rendement et de la réduction de coûts. Dans cette thèse, des investigations sur les cellules solaires à hétérojonction a-Si:H/c-Si de type (n) développées à l'Institut National de l'Énergie Solaire sont présentées. Les aspects technologiques et physiques du dispositif à HJ ont été revus, en mettant l'accent sur la compréhension du rôle joué par la face arrière. À travers le développement et la mise en œuvre des films de a-Si:H intrinsèques et dopés (n) de haute qualité des cellules solaires à HJ, les conditions requises en face arrière des dispositifs ont été établies. Une comparaison entre plusieurs types de champ surface arrière, avec et sans l’introduction d’une couche buffer, est présentée et les caractéristiques des cellules solaires résultants sont discutées. Une discussion autour du contact arrière de cellules solaires à HJ est aussi présentée. Une nouvelle approche d’oxyde transparent conducteur en face arrière basé sur les couches d’oxyde de zinc dopé au bore (ZnO:B) est étudié. Dans le but de développer des couches de ZnO:B de haute qualité bien adaptées à leur utilisation dans des dispositifs à HJ, différents paramètres de dépôt ainsi que des traitements après dépôt comme le post plasma d’hydrogène ou le recuit laser sont étudiés et leur influence sur des cellules solaires est évaluée. Au cours de ce travail il est montré que la face arrière des cellules solaires à HJ joue un rôle important sur l’accomplissement de hauts rendements. Cependant, l'augmentation de la performance globale du dispositif dû à l’optimisation de la face arrière de la cellule est toujours dépendante des phénomènes ayant lieu en face avant des dispositifs. L'utilisation des films optimisés pour la face arrière des HJs développées dans cette thèse, associée à des couches améliorées pour la face avant et une nouvelle approche de métallisation nous a permis d’atteindre un rendement de conversion record de plus de 22%, démontrant ainsi le grand potentiel de cette technologie à HJ de a-Si:H/c-Si
Amongst available silicon-based photovoltaic technologies, a-Si:H/c-Si heterojunctions (HJ) have raised growing attention because of their potential for further efficiency improvement and cost reduction. In this thesis, research on n-type a-Si:H/c-Si heterojunction solar cells developed at the Institute National de l’Énergie Solaire is presented. Technological and physical aspects of HJ devices are reviewed, with the focus on the comprehension of the back side role. Then, an extensive work to optimise amorphous layers used at the rear side of our devices as well as back contact films is addressed. Through the development and implementation of high-quality intrinsic and n-doped a-Si:H films on HJ solar cells, the needed requirements at the back side of devices are established. A comparison between different back surface fields (BSF) with and without the inclusion of a buffer layer is presented and resulting solar cell output characteristics are discussed. A discussion on the back contact of HJ solar cells is also presented. A new back TCO approach based on boron-doped zinc oxide (ZnO:B) layers is studied. With the aim of developing high-quality ZnO:B layers well-adapted to their use in HJ devices, different deposition parameters as well as post-deposition treatments such as post-hydrogen plasma or excimer laser annealing are studied, and their influence on solar cells is assessed. Throughout this work it is evidenced that the back side of HJ solar cells plays an important role on the achievement of high efficiencies. However, the enhancement of the overall device performance due to the back side optimisation is always dependent on phenomena taking place at the front side of devices. The use of the optimised back side layers developed in this thesis, together with improved front side layers and a novel metallisation approach have permitted a record conversion efficiency over 22%, thus demonstrating the great potential of this technology

We focused on a non-cooling room temperature microbolometer infrared imaging array device which includes a sensing layer of p-type a-Si:H component layers doped with boron. Boron incorporation and bonding configuration were investigated for a-Si:H films grown by plasma enhanced chemical deposition (PECVD) at varying substrate temperatures, hydrogen dilution of the silane precursor, and dopant to silane ratio using multiple internal reflection infrared spectroscopy (MIR-IR). This study was then confirmed from collaborators via Raman spectroscopy. MIR-IR analyses reveal an interesting counter-balance relationship between boron-doping and hydrogen-dilution growth parameters in PECVD-grown a-Si:H. Specifically, an increase in the hydrogen dilution ratio (H2/SiH4) or substrate temperature was found to increase organization of the silicon lattice in the amorphous films. It resulted in the decrease of the most stable SiH bonding configuration and thus decrease the organization of the film. The new chemical bonding information of a-Si:H thin film was correlated with the various boron doping mechanisms proposed by theoretical calculations. The study revealed the corrosion morphology progression on aluminum alloy (Al, 0.5% Cu) under acidic chloride solution. This is due to defects and a higher copper content at the grain boundary. Direct galvanic current measurement, linear sweep voltammetry (LSV), and Tafel plots are used to measure corrosion current and potential. Hydrogen gas evolution was also observed (for the first time) in Cu/Al bimetallic interface in areas of active corrosion. Mechanistic insight that leads to effective prevention of aluminum bond pad corrosion is explored and discussed. (Chapter 4) Aluminum bond pad corrosion activity and mechanistic insight at a Cu/Al bimetallic interface typically used in microelectronic packages for automotive applications were investigated by means of optical and scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electrochemistry. Screening of corrosion variables (temperature, moisture, chloride ion concentration, pH) have been investigated to find their effect on corrosion rate and to better understand the Al/Cu bimetallic corrosion mechanism. The study revealed the corrosion morphology progression on aluminum alloy (Al, 0.5% Cu) under acidic chloride solution. The corrosion starts as surface roughening which evolves into a dendrite structure and later continues to grow into a mud-crack type corrosion. SEM showed the early stage of corrosion with dendritic formation usually occurs at the grain boundary. This is due to defects and a higher copper content at the grain boundary. The impact of copper bimetallic contact on aluminum corrosion was explored by sputtering copper microdots on aluminum substrate. Copper micropattern screening revealed that the corrosion is activated on the Al/Cu interface area and driven by the large potential difference; it was also seen to proceed at much higher rates than those observed with bare aluminum. Direct galvanic current measurement, linear sweep voltammetry (LSV), and Tafel plots are used to measure corrosion current and potential. Hydrogen gas evolution was also observed (for the first time) in Cu/Al bimetallic interface in areas of active corrosion. Mechanistic insight that leads to effective prevention of aluminum bond pad corrosion is explored and discussed. Micropattern corrosion screening identified hydrogen evolution and bimetallic interface as the root cause of Al pad corrosion that leads to Cu ball lift-off, a fatal defect, in Cu wire bonded device. Complete corrosion inhibition can be achieved by strategically disabling the mutually coupled cathodic and anodic reaction cycles.

Les simulations numériques ont négligé jusqu’ici l’influence du support de silice amorphe sur la structure des nanoparticules métalliques déposées car l’interaction métal-silice amorphe est faible. Toutefois les études expérimentales montrent un effet de troncature sur la structure des nanoparticules. L’idée de ce travail a donc été d’étudier l’influence de ce support sur la structure et la morphologie des nanoparticules d’argent au moyen de la modélisation moléculaire (Monte Carlo et dynamique moléculaire). L’objectif de ce travail a été tout d'abord de déterminer le potentiel interatomique permettant de décrire l’interaction argent-silice. Ce potentiel a été obtenu sur la base des données expérimentales d'angles de mouillages en phase liquide et en phase solide. D’autre part, l'intensité d'interaction argent-silice a été déterminée par calculs DFT sur la cristobalite qui est un polymorphe de la silice cristalline présentant la même densité que la silice amorphe. Les énergies d'adhésions obtenues ont ainsi permis d'ajuster les paramètres du potentiel argent-silice de type Lennard-Jones. L’étude de la stabilité structurale des nanoparticules d'argent supportées à température nulle a été effectuée pour trois degrés d'approximation du support. (1) : un support parfaitement lisse décrit par un puits carré dont la profondeur est reliée à l’énergie d’adhésion, (2) : un support atomique de silice amorphe de surface plane et (3) : un support atomique de silice amorphe présentant une rugosité de surface. L’influence de la température sur la structure a été étudiée par fusion et recristallisation des nanoparticules d’argent sur les deux supports de silice amorphe. Afin d’étudier la stabilité structurale des nanoparticules en température, le calcul d’énergie libre des nanoparticules a été abordé
Numerical simulations have so far neglected the influence of amorphous silica substrate on the structure of metallic nanoparticles due to its relatively weak interaction with deposited nanoparticles. However, experimental studies have often shown a truncation effect on the structure of nanoparticles. The idea of this work was to study the influence of this substrate on the structure of silver nanoparticles using molecular modeling (Monte Carlo and molecular dynamics). The objective of this work was firstly to determine silver-silica interatomic potential. This was achieved using experimental data of wetting angles in solid and liquid phase. On the other hand, silver-silica interaction intensity was determined by DFT calculations on cristobalite which is a polymorph of crystalline silica having the same density as amorphous silica. The adhesions energies obtained were used to fit the Lennard-Jones parameters for the silver-silica interaction. The study of the structural stability of silver nanoparticles supported at zero temperature was performed for three levels of approximation of the support. (1): the smooth wall approximation where the support is described by a square-well whose depth is related to the adhesion energy of the nanoparticle, (2): an atomistic model of flat amorphous silica, (3): an atomistic model of rough amorphous silica. The influence of the temperature on the structure was investigated by melting and recrystallization of the silver nanoparticles deposited on the two silica supports. In order to study the temperature stability of the nanoparticles the free energy calculation of the nanoparticles was discussed

Des films de gaas polycristallin ont ete realises par depot en phase vapeur, a partir de molecules organometalliques. L'auteur a montre que les proprietes electriques de ce materiau semiconducteur sont controlees par les joints de grains. Une densite d'etats de surface uniforme entre les grains a ete mise en evidence. L'introduction entre les grains d'une phase amorphe de type si::(y)c::(1-y) modifie le mode de conduction. La resistivite des films augmente ou diminue suivant la valeur de la concentration de si::(y)c::(1-y). Un depot par plasma froid a partir des memes molecules que celles utilisees precedemment, conduit a un materiau amorphe polymerique. Celui-ci est compose d'atomes de gallium et d'arsenic. Ses proprietes electriques sont plutot celles d'un isolant que celles d'un semiconducteur. Enfin, il a ete montre qu'un film contenant des atomes de ga et d'as peut etre utilise pour la passivation de la surface de substrats de gaas monocristallin

In recent years, the application of hydrogenated amorphous silicon (a-Si:H) to crystalline silicon (c-Si) solar cells for the purpose of surface passivation has begun to move rapidly forward following early innovations by Sanyo. The bulk of the research conducted throughout this thesis has been performed prior to this new drive in the development of a-Si:H/c-Si devices. Understanding the underlying principles and the essential physics concerning the interaction of these two materials has been often overlooked, making further improvements difficult, and limiting new technological developments therein. In this light, the strategies towards merging a-Si:H with c-Si to achieve high{u00AD} efficiency, low-cost photovoltaics are studied in this thesis, with a focus on the interface and lowering interface states. Plasma-Enhanced chemical vapour deposition (PECVD) of a-Si:H has commonly been an effective method for achieving uniform coverage of the c-Si surface. However, many deposition parameters have been reported as optimal, stemming from the limited range of experimental conditions examined. In this study, a more complete range of deposition conditions are tested, with the nature of the a-Si:H across a broad array of parameters being investigated. Ideally, a-Si:H layers which are most likely to result in high quality surface passivation should be deposited using temperatures of 225{u00B0}C, applied rf-power at 4W (SlmW/cm{u00B2} ), and partial pressure of 650mT. Notably, the ranges for deposition that can be ideally utilised, are with temperatures between 200{u00B0}C and 250{u00B0}C, rf-power up to 8W (100mW/cm{u00B2}), and partial pressures between 400mT and 750mT. Although the ideal values are somewhat system specific, these broader ranges are common to many PECVD systems. Previously overlooked in many studies on a-Si:H and indeed most hydrogenated materials is the influence of hydrides on the surface passivation. A widespread belief is that layers hydrogen{u00AD} rich in their bulk are best for passivation, due to a plentiful source of hydrogen. Analysis of hydride density by IR-spectroscopy has revealed several interesting results which identify some misconceptions concerning surface passivation and the influence of hydrides. In particular, this thesis clarifies the function of the composition and distribution of hydrides throughout the layer and their influence on the quality of the surface passivation; the existence of bulk and interface regions within the a-Si:H layer; and the influence of deposition conditions on the composition and density of hydrides. Ideally, a hydride-rich interface region is shown to yield the most reliable results. The diffusion of hydrogen from within the a-Si:H bulk towards the interface with c-Si at an energy of l.SeV has been a widely accepted mechanism governing surface passivation. However, experimental evidence to support this preferential diffusion through a high-defect material such as a-Si:H has been somewhat absent. In this work, an Arrhenius relationship between temperature and surface passivation is revealed, providing evidence that disputes the a-Si:H bulk-diffusion hypothesis in favour of a surface-diffusion mechanism at the a-Si:H/c-Si interface. The thermally activated surface passivation is shown to have energy of 0.7 {u00B1} O.leV, below that required for bulk diffusion or spontaneous release of hydrogen. From this experimental study, new insight into a surface-related transport model governing the passivation of the c-Si surface by hydrogen already present at or near the interface is presented in this thesis. Determined in this physical model, is the relationship between the likelihood of hydrogen diffusion across a c-Si surface and temperature. Following from early experimentation using post-deposition thermal annealing to improve surface passivation by a-Si:H, a new plasma-enhanced chemical vapour deposition (PECVD) technique was developed as part of this work. Multi-Layer-PECVD involves the deposition of sub{u00AD} layers of a-Si:H with thermal cycling, to build up a total layer thickness. This technique of sub-layer deposition is shown to improve the control of hydride density, composition, surface coverage and reduce the inherent thin-film stresses for very thin a-Si:H layers. Comparison of layers deposited by ML-PECVD in-place of standard PECVD showed improved reliability and stability thanks to this new approach to deposition of a-Si:H. With a greater understanding for the properties of a-Si:H in passivating c-Si and improvements in deposition technique, stacked a-Si:H structures which combine n-type or p-type a{u00AD} Si:H with a thin intrinsic a-Si:H layer in a HiT-like design are investigated from the perspective of passivating c-Si. Results here show that high-quality surface passivation can be maintained, with recombination velocities and saturation current densities at the c-Si surface as low as 3cms{u207B}{u00B9} and averaged below 30fA/cm{u00B2} respectively, which are equivalent to those achieved with SiOx and SiN layers. In a world first application, the a-Si:H(i) and stacked a-Si:H layer structure have been applied in this thesis to the mc-Si surface; whereby, excellent surface passivation results are achieved using both n- and p-type mc-Si. Recombination velocities below lOOcms{u207B}{u00B9} using only a-Si:H(i) were reduced further to approximately 40cms{u207B}{u00B9} with stacked a-Si:H(i/n) or a-Si:H(i/n) layers, without a diffused emitter. In addition, low current saturation densities of 4.5 x 10{u207B}{u00B9}{u2074}Acm{u207B}{u00B2} and implied open{u00AD}circuit voltages of 670mV were achieved. In the case of 100{u03BC}m mc-Si, further reductions are shown to be possible, opening the doorway for simple, high-efficiency mc-Si based photovoltaic designs at low-cost. The work in this thesis has yielded an improved understanding relating to a-Si:H/c-Si devices. Fundamental misconceptions concerning the hydrogen passivation mechanism, hydride content and configuration have been identified and a more accurate understanding has been proposed. Although many of the principles in the Sanyo HIT design have recently been reproduced by other groups, the implications of this research remain applicable. Importantly, the research regarding the optimisation of a-Si:H, development of ML-PECVD and many of the preliminary findings of this research are focused on high-efficiency, low-cost next generation photovoltaic designs yet to be developed.

The probability of recombination of photogenerated electron hole pairs in crystalline silicon is governed by the density of surface defect states and the density of charge carriers. Depositions of intrinsic hydrogenated amorphous silicon (a-Si:H) in dc saddle field (DCSF) PECVD system and hydrogenated amorphous silicon nitride (SiNx) in rf PECVD system forms a dual layer stack on c-Si, which results in an excellent passivation of the surface and an anti-reflection coating. Response Surface Methodology is used in this work to optimize the deposition conditions of SiNx. Optimization of the response surface function yielded deposition conditions that materialized in a surface recombination velocity of less than 4cm/s. The BACH (Back Amorphous Crystalline silicon Heterojunction) cell concept makes use of this dual layer a-Si:H/SiNx stack to form a high efficiency photovoltaic device. The high quality passivating structure can result in the BACH solar cell device with more than 20% conversion efficiency.

碩士
國立臺灣科技大學
化學工程系
100
In this paper, we studied the passivation quality of intrinsic amorphous silicon sub-oxides (a-SiOx:H) thin layers deposited on n-type FZ Si wafers. Conventional RF plasma enhanced chemical vapor deposition (RF-PECVD) system were applied to deposit a-SiOx:H using SiH4, CO2, and H2 as the reactant gases. We investigated to effects of passivation quality of c-Si wafers sandwiched by 30-nm thick a-SiOx:H layers deposited at various CO2 partial pressures. We found that by increasing the CO2 concentration from 0.1 to 0.5% the corresponding effective carrier lifetime of Si wafers increase from 1100 to 2360s, and the implied open circuit voltage increase from 608 to 740 mV. Optical analysis of the a-SiOx:H layers showed that the addition of CO2 concentration from 0.1 to 0.5% increase the optical band gap of a-SiOx:H layers from 1.72 to 1.85eV. The application of the a-SiOx:H layer with 1.85eV band gap to the fabrication of Si heterojunction solar cells showed a high cell Voc of 707mV.

This dissertation describes the development of a process for the precise positioning of semiconductor nanoparticles grown by hot wire chemical vapor deposition and thermal chemical vapor deposition on amorphous dielectrics, and it presents two studies that demonstrate the process. The studies entailed growth and characterization using surface science techniques and scanning electron microscopy. The two systems, Ge nanoparticles on HfO₂ and Si nanoparticles on Si₃N₄, are of interest because their electronic properties show potential in flash memory devices. The positioning technique resulted in nanoparticles deposited within 20 nm diameter feature arrays having a 6x10¹⁰ cm⁻² feature density. Self-assembling diblock copolymer poly(styrene-b-methyl methacrylate) thin films served as the patterning soft mask. The diblock copolymer features were transferred using a CHF₃/O₂ reactive ion etch chemistry into a thin film SiO₂ hard mask to expose the desired HfO₂ or Si₃N₄ deposition surface underneath. Selective deposition upon exposed pore bottoms was performed at conditions where adatom accumulation occurred on the HfO₂ or Si₃N₄ surfaces and not upon the SiO₂ mask template. The selective deposition temperatures for the Ge/HfO₂ and Si/Si₃N₄ systems were 700 to 800 K and 900 to 1025 K, respectively. Germanium nucleation on HfO₂ is limited from hot wire chemical vapor deposition by depositing nanoparticles within 67% of the available features. Unity filling of features with Ge nanoparticles was achieved using room temperature adatom seeding before deposition. Nanoparticle shape and size are regulated through the Ge interactions with the SiO₂ feature sidewalls with the adatom removal rate from the features being a function of temperature. The SiO₂ mask limited Ge nanoparticle growth laterally to within ~5 nm of the hard mask at 800 K. Silicon deposition on patterned Si₃N₄ has multiple nanoparticles, up to four, within individual 20 nm features resulting from the highly reactive Si₃N₄ deposition surface. Silicon nucleation and continued nanoparticle growth is a linear function of deposition flux and an inverse function of sample temperature. Diblock copolymer organization can be directed into continuous crystalline domains having ordered minority phases in a process known as graphoepitaxy. In graphoepitaxy forced alignment within microscopic features occurs provided certain dimensional constraints are satisfied. Graphoepitaxy was attempted to precisely locate 20 nm diameter features for selective Ge or Si deposition and initial studies are presented. In addition to precise nanoparticle positioning studies, kinetic studies were performed using the Ge/HfO₂ material system. Germanium hot wire chemical vapor deposition on unpatterned HfO₂ surfaces was interpreted within the mathematical framework of mean-field nucleation theory. A critical cluster size of zero and critical cluster activation energy of 0.4 to 0.6 eV were estimated. Restricting HfO₂ deposition area to a 200 nm to 100 [mu]m feature-width range using SiO₂ decreases nanoparticle density compared to unpatterned surfaces. The studies reveal the activation energies for surface diffusion, nucleation, and Ge etching of SiO₂ are similar in magnitude. Comparable activation energies for Ge desorption, surface diffusion and cluster formation obscure the change with temperature an individual process rate has on nanoparticle growth characteristics as the feature size changes.
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碩士
國立臺灣科技大學
化學工程系
102
In this paper, we studied several important issues concerning fabrication of crystalline germanium (Ge) hetero-junction using amorphous Si as the passivation layers. First of all, surface cleaning procedure of Ge wafers was established through a comparison with the conventional RCA cleaning procedure for Si wafers. An efficient way for surface cleaning of Ge included a series of organic solvents, HCl, and HF treatments with suitable concentrations. Then, a surface oxide layer was fabricated with intention through an immediate dipping in H2O2 solution after HF treatment. Finally a very clean Ge(100) was obtained, which was verified by RHEED, by removing the oxide layer using thermal annealing in a high vacuum chamber at temperatures ranging 450 ℃. After surface cleaning process, we use PECVD to grow 16 nm hydrogenated amorphous silicon (a-Si:H) for germanium surface passivation. The best minority carrier lifetime of the Ge wafer after a-Si:H double-side coated was 291.3 μs, which was further reduced to 112.7 μs after completion of n+ a-Si:H/i a-Si:H/c-Ge/i a-Si:H/p+ a-Si:H.

abstract: Silicon solar cells with heterojunction carrier collectors based on a-Si/c-Si heterojunction (SHJ) have a potential to overcome the limitations of the conventional diffused junction solar cells and become the next industry standard manufacturing technology of solar cells. A brand feature of SHJ technology is ultrapassivated surfaces with already demonstrated 750 mV open circuit voltages (Voc) and 24.7% efficiency on large area solar cell. Despite very good results achieved in research and development, large volume manufacturing of high efficiency SHJ cells remains a fundamental challenge. The main objectives of this work were to develop a SHJ solar cell fabrication flow using industry compatible tools and processes in a pilot production environment, study the interactions between the used fabrication steps, identify the minimum set of optimization parameters and characterization techniques needed to achieve 20% baseline efficiency, and analyze the losses of power in fabricated SHJ cells by numerical and analytical modeling. This manuscript presents a detailed description of a SHJ solar cell fabrication flow developed at ASU Solar Power Laboratory (SPL) which allows large area solar cells with >750 mV Voc. SHJ cells on 135 um thick 153 cm2 area wafers with 19.5% efficiency were fabricated. Passivation quality of (i)a-Si:H film, bulk conductivity of doped a-Si films, bulk conductivity of ITO, transmission of ITO and the thickness of all films were identified as the minimum set of optimization parameters necessary to set up a baseline high efficiency SHJ fabrication flow. The preparation of randomly textured wafers to minimize the concentration of surface impurities and to avoid epitaxial growth of a-Si films was found to be a key challenge in achieving a repeatable and uniform passivation. This work resolved this issue by using a multi-step cleaning process based on sequential oxidation in nitric/acetic acids, Piranha and RCA-b solutions. The developed process allowed state of the art surface passivation with perfect repeatability and negligible reflectance losses. Two additional studies demonstrated 750 mV local Voc on 50 micron thick SHJ solar cell and < 1 cm/s effective surface recombination velocity on n-type wafers passivated by a-Si/SiO2/SiNx stack.
Dissertation/Thesis
Ph.D. Electrical Engineering 2013

This work aimed to improve aqueous drug solubility by amorphization upon loading in silica porous matrixes and stabilize it in the amorphous form. Naproxen was chosen as the target material, a practically insoluble pharmaceutical drug, with anti-pyretic and anti-inflammatory properties. To evaluate the influence of guest-host interactions in the drug delivery, two silica matrixes were synthesized differing in their surface composition: unmodified MCM-41 mainly with surface silanol groups and MCM-41_Func caped with methyl groups. The surface area modification with methyl groups was confirmed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), thermogravimetric analysis (TGA) and nuclear magnetic resonance (NMR). Textural analysis showed narrow pore diameter distributions centered at 3.0 and 2.9 nm, respectively. To evaluate the guest’s physical state, different techniques were used as: differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. These analyses showed that naproxen was successful incorporated in the both silica. The naproxen’s amorphization was confirmed by the DSC detection of the glass transition, located in between ~0ºC and 22ºC. However, crystallization and melting are always observed, nonetheless in low extent (~6 % of crystallization degree). The mobility of the amorphous pharmaceutical drug incorporated inside these silica pores, was probed by DRS, allowing estimating a dielectric glass transition temperature in good agreement with the calorimetric one and revealing a higher mobility for the hydrated unmodified composite. It was shown that this mobility enhancement controls the drug delivery, monitored by ultraviolet spectroscopy, which revealed to be faster in the unmodified matrix. The studied composites show promising behavior as controlled drug delivery systems.

Magister Scientiae - MSc
Technological advancement has created a market for large area electronics such as solar cells and thin film transistors (TFT’s). Such devices now play an important role in modern society. Various types of conducting, semiconducting and insulating thin films of the order of hundreds, or even tens of nanometres are combined in strata to form stacks to create interactions and phenomena that can be exploited and employed in these devices for the benefit of mankind. One such is for the generation of energy via photovoltaic devices that use thin film technology; these are known as second and third generation solar cells. Silicon and its alloys such as silicon germanium (SiGex), silicon oxide (SiOx), silicon carbide (SiCx) and silicon nitride (SiNx) play an important role in these devices due to the fact that each material in its different structures, whether amorphous, micro or nano-crystalline or completely crystalline, has its own range of unique optical, mechanical and electrical properties. These structures and their material properties can thus exert a huge influence over the overall device performance. viii Chemical vapour deposition (CVD) techniques are most widely used in industry to obtain thin films of silicon and silicon alloys. Source gases are decomposed by the external provision of energy thereby allowing for the growth of a thin solid film on a substrate. In this study a variant of CVD, namely Hot Wire Chemical Vapour Deposition (HWCVD) will be used to deposit thin films of silicon nitride by the decomposition of silane (SiH4), hydrogen (H2) and ammonia (NH3) on a hot tantalum filament (~1600 C). Hydrogenated amorphous silicon nitride (a-SiNx:H) has great potential for application in optoelectronic devices. In commercial solar cell production its potential for use as anti-reflection coatings are due to its intermediate refractive index combined with low light absorption. An additional benefit is the passivation of interface and crystal defects due to the bonded hydrogen. This can lead to better photon conversion efficiency. Its optical properties including optical band gap, Urbach tail, and wavelength-dependent optical constants such as absorption coefficient and refractive index are crucial for the design and application in the relevant optoelectronic device. The final firing step in the production of micro-crystalline silicon solar cells, allows hydrogen to effuse into the solar cell from the a-SiNx:H, and drives bulk passivation of the grain boundaries. We therefore propose the exploration of annealing effects on the thin film structure. The study undertakes a comparison of optical and bonding structure of the as deposited thin film compared to that of the annealed thin film which would have undergone changes due to high temperature annealing under vacuum. However, it is difficult to simultaneously obtain all of these important ix optical parameters for a-SiNx:H thin films. Ultraviolet visible (UV-vis) spectroscopy will be the method of choice to investigate the optical properties. Infrared (IR) spectroscopy is a source of useful information on the microstructure of the material. In particular, the local atomic bonding configurations involving Si, N, and H atoms in a-SiNx:H films can be obtained by Fourier Transform Infrared Spectroscopy (FTIR). Therefore, this study will attempt to establish a relationship between film microstructure of a-SiNx:H thin films and their macroscopic optical properties.

碩士
國立臺灣大學
光電工程學研究所
99
By using the commercial software COMSOL Multiphysics which is based on the finite element method (FEM), the absorption effects of the thin-film amorphous silicon solar cell with one-dimensional or two-dimensional metal grating structures are numerically investigated. The solar cell structure consists of three parts: an ITO layer as the top contact, an amorphous silicon layer and a metal Ag grating layer as the back contact. The light source adopted is with the AM1.5G solar spectrum, and different incident angles and grating heights are changed to investigate the influences on the absorption of the solar cell. The thin-film solar cell with metal grating back contact can form the graded-refractive index layer on the surface. Based on the characteristic of the amorphous silicon, the poorly absorbed red light can couple into the surface plasmon mode in the back metal grating contact. The absorption of the solar cell can be enhanced due to the generation of surface wave resonance and scattering. The device combines advantages of both reduced reflection and enhanced absorption over a broad spectral range. The solar cells with the grating structures compared with the reference case of a flat metal surface back contact. The absorption enhancement of the solar cell can reach 56% at best with the appropriate grating design.

Nous présenterons le procédé de fabrication, la caractérisation, ainsi qu’un modèle numérique permettant l’optimisation d’un nouveau dispositif permettant d’effectuer des mesures de nanocalorimétrie sur un échantillon de silicium monocristallin. Ce dernier possède entre autre des propriétés thermiques nous permettant d’effectuer des mesures à des températures supérieures à 900 C, avec une résolution meilleure que 16 C. Ceci nous a permis d’étudier la dynamique des défauts induits par implantation ionique dans le silicium monocristallin. Deux comportements différents sont observés dans la germination de la phase amorphe induite par implantation à 10 et 80 keV. Ces résultats ont été confrontés à des simulations Monte-Carlo basées sur le modèle des paires lacunesinterstitiels. La comparaison entre les simulations et les mesures expérimentales ont montré que ce modèle est incomplet car il ne reproduit qualitativement que certaines caractéristiques observées expérimentalement. Des mesures réalisées à partir de -110 C dans le silicium monocristallin et amorphisé implanté avec des ions légers, ont mis en évidence des différences claires entre la relaxation dans le silicium amorphe et le recuit des défauts dans le silicium monocristallin. Deux processus à des énergies d’activation de 0.48 et 0.6 eV ont été observés pour les implantations réalisées dans le silicium monocristallin tandis qu’un relâchement de chaleur uniforme ne révélant qu’un spectre continu d’énergie d’activation a été observé dans le silicium amorphe.
We present the fabrication process, characterization and numerical model allowing the optimization of a new device that allows us to perform nanocalorimetry measurements on a silicon single crystals. The thermal properties of this device allows us to perform measurements at temperature higher than 900 C with a resolution better than 16 C. The device is used to study the ion implantation induced defect dynamic in monocrystalline silicon. Two different behaviours regarding the nucleation of the amorphous phase are observed at 10 and 80 keV. These results are confronted to Monte Carlo simulations based on the interstitial vacancy pair model. The comparison between simulations and measurements show that the model is incomplete as it reproduces only qualitatively some features of the experimental observations. Measurements performed from -110 C in monocrystalline and amorphized silicon implanted with light ions revealed clear differences between structural relaxation in amorphous silicon and defect annealing in monocrystalline silicon. Two processes with activation energies of 0.48 and 0.6 eV are observed after implantation performed in monocrystalline silicon while a uniform heat release associated with a continuous spectrum in terms of activation energy is observed in amorphous silicon.