Rozprawy doktorskie na temat „Amorphous Silicon (a-Si:H)”

Les cellules à hétérojonctions de silicium (HET-Si) sont basées sur un substrat de silicium cristallin (c-Si) dopé n (p), une couche très fine de passivation (en général du silicium amorphe (a-Si:H) non dopé), et une couche d’une dizaine de nanomètres de silicium amorphe dopé p (n). Ces cellules atteignent aujourd’hui des rendements de l’ordre de 26% (record de 26,6% par l’entreprise Kaneka en 2017). Un des axes importants de recherche sur les cellules HET-Si porte sur l’étude de l’interface c-Si/a-Si:H qui est un élément clé dans le rendement des cellules. Ce rendement dépend en particulier de la présence d’états recombinants à l’interface c-Si/a-Si:H. Nous nous sommes donc tout particulièrement intéressés aux défauts d’interface en développant un calcul basé sur le modèle du réservoir de défauts (Defect-Pool Model ou DPM) dans le silicium amorphe et en corrélant nos résultats de modélisation avec des résultats expérimentaux de mesure de durée de vie. Afin de déterminer les caractéristiques de l’interface c-Si/a-Si:H, nous avons procédé comme suit : (1) Calcul de la densité d’états (DOS) volumique dans les couches de a-Si:H (dopé et non dopé), en nous appuyant sur le DPM. Dans ce modèle, la DOS varie en fonction notamment de la position du niveau de Fermi par rapport au bord de bande. La courbure des bandes de la jonction a-Si:H/c-Si implique ainsi une variation spatiale de la DOS dans le a-Si:H. (2) Calcul de la DOS surfacique à l'interface par projection des états volumiques présents à l’interface dans le a-Si:H. (3) Calcul des taux de recombinaison puis de la durée de vie effective sur des structures symétriques a-Si:H/c-Si/a-Si:H et comparaison avec des résultats expérimentaux. Nous avons ainsi pu étudier l’impact des paramètres matériaux du a-Si:H sur la durée de vie effective des porteurs minoritaires. L’évolution de la durée de vie avec les paramètres du a-Si:H est parfois contre-intuitive car deux phénomènes de passivation liés à la position du niveau de Fermi à l’interface s’opposent : passivation par la diminution de la densité d’états à l’interface et passivation par effet de champ. Le seul calcul de la DOS à l’interface ne suffit pas toujours à expliquer les variations de durées de vie, un calcul complet sous lumière est nécessaire. Nous avons montré que l’impact de certains paramètres du DPM peut-être grand sur la DOS mais faible sur la durée de vie effective à cause de cette compensation entre les phénomènes de passivation. Nous avons également étudié des structures correspondant aux faces avant : (p)a-Si:H/(i)a-Si:H/(n)c-Si(PIn) et arrière : (n)a-Si:H/(i)a-Si:H/(n)c-Si(NIn) des cellules HET-Si. Nos simulations permettent de montrer que les interfaces NIn sont moins critiques en terme de recombinaisons que les interfaces de type PIn. Nous montrons que la recombinaison aux interfaces PIn est dominée par la capture des électrons par les liaisons brisées de silicium chargées positivement. Nous montrons également que l’énergie d’Urbach est un paramètre qui joue de manière importante dans le calcul de la durée de vie effective et que l’utilisation de valeurs fixes de cette énergie d’Urbach dans la couche de passivation ne permet pas de reproduire les tendances expérimentales dans les structures avec des interfaces PIn. Nous proposons un modèle de variation de l’énergie d’Urbach avec l’épaisseur de la couche de passivation, qui permet de reproduire les tendances expérimentales pour les faibles épaisseurs de la couche de passivation mais qui demande à être complété pour de plus grandes épaisseurs
Silicon heterojunction (Si- HET) solar cells are based on an n-doped (p-doped) crystalline silicon (c-Si) substrate, a very thin (a few nanometers) passivation layer of undoped hydrogenated amorphous silicon (a-Si:H) and a layer of p-doped (n-doped) a-Si:H, approximately 10 nanometer- thick. These cells currently lead the performance of silicon solar cells with conversion efficiencies in the order of 26% (with a record of 26.6% being achieved by the Kaneka company in 2017). One of the major focal points of research in Si- HET cells is the study of the c-Si/a-Si:H interface, which is a key factor in the cells' efficiency. In particular, this efficiency is strongly dependent on the recombination states at the interface between c-Si and a-Si:H. We therefore focused on developing a model of recombination through interface defects, which were evaluated based on the Defect-Pool Model (DPM) in a-Si:H. We calculated the effective lifetime vs excess carrier density curves and their dependence on the undoped a-Si:H passivation layer thickness and compared them to experimental results.In order to determine the characteristics of the c-Si/a-Si:H interface, we proceeded as follows: (1) Calculation of the volumic density of states (DOS) in a-Si:H layers (doped and undoped) using the DPM. In this model, the DOS varies as a function of the position of the Fermi level in relation to the band edge. The band bending at the a-Si:H/c-Si interface thus implies a spatial variation of the DOS in a-Si:H. (2) Calculation of the surface DOS at the interface by projection from the volumic states present in a-Si:H at the interface. (3) Calculation of the recombination rates and of the effective lifetime curves for symmetrical a-Si:H/c-Si/a-Si:H structures and comparison with experimental results. Thus we were able to study the impact of material parameters of a-Si:H on the effective lifetime curves. The change in lifetime as a function of a-Si:H parameters is sometimes counter-intuitive because two passivation mechanisms, namely passivation by field-effect or by the reduction of the DOS at the a-Si:H/c-Si interface, have opposed behavior in relation to the position of the Fermi level at the interface. A simple calculation of the DOS at the interface is not, therefore, sufficient to explain variations in lifetime, and a complete calculation of effective lifetime under illumination is required and has been performed. We demonstrate that the impact of certain DPM parameters may have a significant effect on the DOS but only a minor effect on the effective lifetime due to the compensation by the field-effect passivation. Moreover we have studied both types of silicon heterojunctions, (p)a-Si:H/(i)a-Si:H/(n)c-Si(PIn), and (n)a-Si:H/(i)a-Si:H/(n)c-Si(NIn) that are used as front emitter and back surface field junctions, respectively, in double-side contacted silicon Si-HET solar cells. Our simulations allowed us to emphasize that NIn interfaces are less critical in terms of recombination than PIn interfaces. We demonstrate that recombination at PIn interfaces is dominated by the capture of electrons by positively charged silicon dangling bonds. We further show that the Urbach energy is the major a-Si:H parameter that determines the effective lifetime in Si-HET solar cells and that the use of fixed values for this Urbach energy in the passivation layer whatever the layer thickness does not permit the experimental trends of PIn interfaces to be reproduced. Instead, we propose a model featuring that the Urbach energy decreases with the thickness of the passivation layer, which does allow experimental trends to be reproduced for very thin passivation layers (< 10 nm), but which requires further elaboration for larger thicknesses, for instance with a combined bandgap variation

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

Les cellules à hétérojonctions de silicium fabriquées par croissance de couches minces de silicium amorphe hydrogéné (a-Si :H) à basse température sur des substrats de silicium cristallin (c-Si) peuvent atteindre des rendements de conversion photovoltaïque élevés (η=23 % démontré). Les efforts de recherche ayant principalement été orientés vers le cristallin de type p jusqu'à présent en France, ce travail s'attache à l'étude du type n pour d'une part déterminer les performances auxquelles s'attendre avec cette nouvelle filière et d'autre part les améliorer. Pour cela, nous avons mis en œuvre des techniques de caractérisation des matériaux composant la structure et de l'interface (a-Si :H/c-Si) couplées à des outils de simulations numériques afin mieux comprendre les phénomènes de transport électronique. Nous nous sommes également intéressés aux cellules à hétérojonctions avec substrats de silicium multicristallin de type n, le silicium multicristallin étant le matériau le plus répandu actuellement dans la fabrication des cellules photovoltaïques.

Ce travail est une contribution à l'étude de l'interaction entre des couches minces de silicium amorphe hydrogéné (a-Si:H) et un plasma d'hydrogène, dans un réacteur de dépôt par PECVD (Plasma Enhanced Chemical Vapor Deposition). Le suivi in situ de la cinétique de gravure par l'hydrogène atomique est réalisé par ellipsométrie UV-visble. Les différents paramètres de plasma (température, puissance radiofréquence, pression du gaz H2, type de dopage du matériau) pouvant impacter cette cinétique ont été sondés. L'analyse des spectres d'ellipsométrie spectroscopique, à l'aide d'un modèle optique approprié, a permis de mettre en évidence leurs effets sur le temps de formation de la couche modifiée par l'hydrogène, son épaisseur et son excès d'hydrogène, ont été analysés. Le même traitement au plasma d'hydrogène appliqué à des jonctions i/p et i/n, révèle un comportement particulier de la cinétique de gravure dans la zone de jonction. Ce comportement a été interprété dans le cadre d'un modèle simple de diffusion de l'hydrogène sous champ électrique
This work is a contribution to the study of the interaction between hydrogenated amorphous silicon (a-Si:H) thin films and hydrogen plasma in a PECVD (Plasma Enhanced Chemical Vapor Deposition) reactor. The kinetics of silicon etching by atomic hydrogen is monitored in situ by UV - visble ellipsometry .Several plasma parameters (temperature, RF power, H2 gas pressure, the doping of the material) that may impact the kinetics were probed. An analysis of the spectroscopic ellipsometry spectra, thanks to an appropriate optical model, allowed evidencing their effects on the time constant, the thickness and the hydrogen excess of the H-modified layer.The same hydrogen plasma treatment repeated on i/p and i/n H base junctions revealed a particular behavior of the etching kinetics in the junction zone. This effect is interpreted in the frame of a simple of hydrogen diffusion model under an electric field

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.

Orientador: Leandro Russovski Tessler
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Este trabalho fornece caracterização ótica e estrutural de filmes finos compostos por nitreto de silício amorfo hidrogenado dopado com térbio (a-SiNx:H) ¿ crescidos por deposição química a vapor assistida por plasma gerado através de ressonância ciclotrônica de elétrons (ECR PECVD) e por pulverização catódica reativa em radiofrequência (reactive RF-Sputtering) ¿ com o propósito de avançar a investigação em fabricação de novos materiais e dos mecanismos da emissão de luz de íons de Tb quando diluídos em materiais baseados em silício. A fotoluminescência (PL) atribuída aos filmes de a-SiNx:H foi investigada em termos das condições de deposição e correlacionadas com suas propriedades estruturais e de recozimento pós-deposição. Entre as propriedades caracterizadas estão: estequiometria, taxa de deposição, índice de refração, coeficiente de extinção, bandgap ótico E04, concentração de térbio e vizinhança química presente ao redor de íons Tb3+. Concentrações de Tb da ordem de 1.8 at.% ou 1.4×?10?^21 at/cm^3 foram obtidas em amostras crescidas por Sputtering enquanto que concentrações de 14.0 at.%, ou da ordem ?10?^22 at/cm^3, puderam ser obtidas em amostras crescidas por ECR PECVD. Em Sputtering, a incorporação de Tb varia linearmente com a área recoberta por pastilhas de Tb4O7 em pó, enquanto que em PECVD, a incorporação de Tb é inversamente proporcional e pode ser ajustada sensivelmente pelo fluxo de gás SiH4. Forte emissão de luz, atribuída às transições eletrônicas em Tb3+ (PL do Tb), foi obtida em filmes não-recozidos que possuíam bandgap estequiométrico (E04 = 4.7 ± 0.4 eV and x = 1.5 ± 0.2). Espectros de PL do Tb não mostraram mudanças significativas no formato e na posição dos picos de emissão devido a alterações na temperatura de recozimento, nas condições de deposição ou entre amostras crescidas por diferentes técnicas de deposição. Entretanto, esses parâmetros influenciaram fortemente a intensidade da PL do Tb. Estudos da estrutura fina de absorção de raios-X (XAFS) em filmes crescidos por sputtering mostraram a estabilidade da vizinhança química ao redor dos íons Tb3+ mesmo em altas temperaturas (1100ºC). Investigações por sonda atômica tomográfica (APT) não encontraram formação de nanoclusters envolvendo ou não Tb, mesmo após recozimentos em altas temperaturas. Isso sugere que a excitação de Tb3+ deve ocorrer através da própria matriz hospedeira amorfa e não por mudanças no campo cristalino e, portanto, na força de oscilador das transições eletrônicas do Tb3+. Caracterização da densidade de ligações Si-H por espectroscopia infravermelha a transformada de Fourier (FTIR) em filmes recozidos em diferentes temperaturas foi relacionada com a intensidade da PL do Tb. Ela mostra que um decréscimo na densidade das ligações Si-H, que está relacionada a um aumento na concentração de ligações pendentes de Si (Si-dbs), resulta em filmes com maior intensidade na PL do Tb. Portanto, isso sugere que a excitação de Tb3+ parece acontecer através de transições envolvendo Si-dbs e estados estendidos, o que é consistente com o modelo de excitação Auger por defeitos (DRAE)
Abstract: This work offers optical and structural characterization of terbium (Tb) doped hydrogenated amorphous silicon nitrides thin films (a-SiNx:H) grown by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) and reactive RF-Sputtering with the purpose of advancing the investigation in fabrication of novel materials and the mechanisms of light emission of Tb ions when embedded in Si-based materials. Photoluminescence (PL) of a-SiNx:H films were investigated and correlated with the deposition conditions, structural properties, and post-deposition thermal treatments (isochronal annealing under flow of N2). Among the characterized properties are: film stoichiometry, deposition rate, refractive index, extinction coefficient, optical bandgap, terbium concentration, and the chemical neighborhood around Tb ions. Tb concentrations of about 1.8 at.% or 1.4×?10?^21 at/cm^3 have been achieved in Sputtering system while concentrations of 14.0 at.%, or about ?10?^22 at/cm^3, could be achieved in ECR PECVD samples. In Sputtering, Tb incorporation varies linearly with the covered area of the Si target by Tb4O7 powder pellets, while in PECVD, Tb incorporation is inversely proportional to and can be sensitively adjusted through SiH4 gas flow. Bright PL attributed to Tb3+ electronic transitions (Tb PL) were obtained in as-deposited films with stoichiometric bandgaps (E04 = 4.7 ± 0.4 eV and x = 1.5 ± 0.2). The Tb PL spectra did not show any significant change in shape and in PL peak positions due to alterations in annealing temperature, deposition conditions or due to the used deposition method. However, these parameters strongly affected Tb PL intensity. Studies of X-ray absorption fine structure (XAFS) in Sputtering grown films show the stability of the chemical neighborhood around Tb3+ under annealing conditions even after thermal treatments at temperatures as high as 1100ºC. Atom probe tomography (APT) investigation also found no formation of nanoclusters of any type (involving Tb ions or not) after high temperature annealing treatments suggesting that Tb3+ excitation should come from the amorphous host matrix itself and not by changes in crystal field and thus in oscillator strength of Tb3+ electronic transitions. Fourier transform infrared spectroscopy (FTIR) characterization of Si-H bond density in films treated atin different annealing temperatures were crossed correlated with Tb PL intensity. It shows that a decrease in Si-H bond density, related to increase in Si dangling bonds (Si-dbs) concentration, results in greater Tb PL intensity. Thus, it suggests that excitation of Tb3+ happens through transitions involving silicon dangling bonds and extended states, consistent with the defect related Auger excitation model (DRAE)
Doutorado
Física
Doutor em Ciências
142174/2012-2
010308/2014-08
CNPQ
CAPES

Les atomes d'hydrogène sont introduits dans les couches durant leur élaboration par évaporation. L'influence des paramètres de préparation est mise en évidence sur les propriétés physiques de couches de SI pur. Les alliages SI::(1-X)SN::(X) et SI::(1-X)SN::(X) : H peuvent être préparés à l'état amorphe dans une large gamme de compositions. Des études de diffraction électronique, de spectrométrie moessbauer et des mesures de densité massique montrent que ces alliages possèdent une structure tétraédrique. Cette méthode a également permis d'élaborer des multicouches SI/SI : H, de l'étain semiconducteur et de l'hydrure de titane

L'etude fondamentale des parametres mis en jeu montre que la limitation de la quantite de cilane decompose et la formation importante d'hydrogene doivent etre associees a un mecanisme chimique en phase gazeuse et a la surface plutot qu'a la diminution de la transparence de la fenetre d'entree consecutive au depot du film si. Il est ainsi mis en evidence que la quantite relative de si depose est tres importante aux basses pressions et que la vitesse de depot peut etre accrue en operant a forte intensite lumineuse. La qualite des couches depend fortement des conditions de preparation, en particulier de la temperature du support. Les couches obtenues ont des proprietes essentielles pour la fabrication de dispositifs photovoltaiques et microelectroniques de dimensions reduites et a bon marche

Passivation of the crystalline silicon (c-Si) wafer surface and decreasing the number of interface defects are basic requirements for development of high efficiency a-Si:H/c-Si heterojunction solar cells. Surface passivation is generally achieved by development of detailed silicon wafer cleaning processes and the optimization of PECVD parameters for the deposition of intrinsic hydrogenated amorphous silicon layer. a-Si:H layers are grown in UHV-PECVD system. Solar cells were deposited on the p type Cz-silicon substrates in the structure of Al front contact/a-Si:H(n)/a-Si:H(i)/c-Si(p)/Al back contact. Solar cell parameters were determined under standard test conditions namely, using 1000 W/m2, AM 1.5G illumination at 25 oC. Growth of (i) a-Si:H, films on the clean wafer surface was investigated as a function of substrate temperature, RF power density, gas flow rate, hydrogen dilution ratio and deposition time and was characterized using SEM, HRTEM, AFM, SE, ATR-FTIR and I/V measurements. Structural properties of the films deposited on silicon wafer surface are directly effective on the solar cell efficiency. Morphological characterization of the grown films on the crystalline surface was found to be very complex depending on the deposition parameters and may even change during the deposition time. At 225 oC substrate temperature, at the beginning of the deposition, (i) a-Si:H films was found grown in epitaxial structure, followed by a simultaneous growth of crystalline and amorphous structure, and finally transforming to complete amorphous structure. Despite this complex structure, an efficiency of 9.2% for solar cells with total area of 72 cm2 was achieved. In this cell structure, TCO and back surface passivation do not exist. In the

Radiation therapy is continuously increasing in complexity due to technological innovation in delivery techniques, necessitating thorough dosimetric verification. Comparing accurately predicted portal dose images to measured images obtained during patient treatment can determine if a particular treatment was delivered correctly. The goal of this thesis was to create a method to predict portal dose images that was versatile and accurate enough to use in a clinical setting. All measured images in this work were obtained with an amorphous silicon electronic portal imaging device (a-Si EPID), but the technique is applicable to any planar imager. A detailed, physics-motivated fluence model was developed to characterize fluence exiting the linear accelerator head. The model was further refined using results from Monte Carlo simulations and schematics of the linear accelerator. The fluence incident on the EPID was converted to a portal dose image through a superposition of Monte Carlo-generated, monoenergetic dose kernels specific to the a-Si EPID. Predictions of clinical IMRT fields with no patient present agreed with measured portal dose images within 3% and 3 mm. The dose kernels were applied ignoring the geometrically divergent nature of incident fluence on the EPID. A computational investigation into this parallel dose kernel assumption determined its validity under clinically relevant situations. Introducing a patient or phantom into the beam required the portal image prediction algorithm to account for patient scatter and attenuation. Primary fluence was calculated by attenuating raylines cast through the patient CT dataset, while scatter fluence was determined through the superposition of pre-calculated scatter fluence kernels. Total dose in the EPID was calculated by convolving the total predicted incident fluence with the EPID-specific dose kernels. The algorithm was tested on water slabs with square fields, agreeing with measurement within 3% and 3 mm. The method was then applied to five prostate and six head-and-neck IMRT treatment courses (~1900 clinical images). Deviations between the predicted and measured images were quantified. The portal dose image prediction model developed in this thesis work has been shown to be accurate, and it was demonstrated to be able to verify patients’ delivered radiation treatments.

Dünne Schichten aus hydrogenisiertem amorphem Silizium a-Si:H spielen für die Photovoltaik eine wichtige Rolle. Einerseits kommt für die Dünnschicht-Photovoltaik unterschiedlich dotiertes a-Si:H in den Schichten einer p-i-n-Solarzelle zur Anwendung, andererseits stellen Heterokontakt-Solarzellen aus amorphem und kristallinem Silizium (a-Si:H/c-Si) wegen ihres hohen Wirkungsgrades derzeit ein sehr aktuelles Forschungsthema dar. Die Abscheidung der a-Si:H-Schichten im Rahmen dieser Arbeit erfolgt mit der Methode des Magnetronsputterns (Kathodenzerstäubung). Dieses für die in-line-Beschichtung etablierte Verfahren wird speziell für die Photovoltaik noch nicht in industriellem Maßstab eingesetzt (lediglich für transparente leitfähige Oxide TCO). Insbesondere existiert nur eine geringe Zahl von Veröffentlichungen zu Heterokontakten, welche mittels Magnetronsputtern hergestellt wurden. Ein Schwerpunkt der vorliegenden Arbeit ist daher die Herstellung sowie Charakterisierung solcher Heterokontakte unter dem Aspekt variierter Abscheide- und Prozessparameter (Substrattemperatur, Wasserstoffflussrate, Ionenbeschuss). Das für das Sputtern erforderliche Plasma wird mit einer im Mittelfrequenzbereich gepulsten Gleichspannung angeregt. Ein dadurch mehr oder weniger ausgeprägter Ionenbeschuss der wachsenden Schichten in Abhängigkeit der Pulsparameter wird hier analysiert. Die Charakterisierung der Heterokontakte erfolgt hauptsächlich anhand deren Strom-Spannung-Kennlinien, welche auch bei variierter Temperatur gemessen werden. Erzielte Gleichrichtungsverhältnisse um 10000:1 sowie Diodenidealitätsfaktoren η ≈ 1,3 kennzeichnen (p)a-Si:H/(n)c-Si-Heterokontakte mit den besten halbleiterphysikalischen Eigenschaften. Bei zu schwacher Schichthydrogenisierung wurde ein Ladungstransportmechanismus nachgewiesen, welcher in der Literatur als multi-tunneling capture-emission MTCE bekannt ist. Eine erhöhte Hydrogenisierung unterdrückt diesen Mechanismus nahezu vollständig. Durch Abscheidung unterschiedlich stark bordotierter a-Si:H-Schichten wird außerdem die Dotiereffizienz beurteilt. Hohe Werte sind bei amorphen Halbleitern im Allgemeinen schwer zu erreichen. Die mit stärkerer Dotierung erhöhte Gleichrichterwirkung lieferte hier ein Indiz für eine nachweisbare Dotiereffizienz.

L'etude porte sur les proprietes optiques de puits nanometriques multiples a-si:h/a-sio#2 elabores par pecvd (he+sih#4+o#2). Les caracteristiques des materiaux de reference sont determinees par diverses techniques de spectroscopie optique. Nous montrons la necessite de simuler les spectres en tenant compte des reflexions multiples afin d'eviter des erreurs importantes sur la valeur du coefficient d'absorption. Des mesures in situ de la teneur en oxygene dans le plasma nous ont permis de determiner une procedure de depot des multicouches reduisant la largeur des interfaces. La modulation de la composition chimique dans l'epaisseur est observee par des mesures de met, de rayons x et de spectrometrie infrarouge. La simulation des spectres de transmission infrarouge de la multicouche consideree comme un milieu effectif montre que l'interface a-sio#2/a-si:h est la plus large. La variation de l'epaisseur des motifs pour des series de multicouches de largeur de puits (d#a#-#s#i#:#h) ou de barriere constante permet d'elaborer des profils-modele de la composition d'oxygene en epaisseur x(d) en tenant compte de la reduction de la couche de silice et de l'oxydation de celle du silicium amorphe par le plasma lors du depot des couches ulterieures. Nous presentons enfin les mesures d'absorption et de photoluminescence des multicouches. La reduction de la largeur du puits conduit a une augmentation apparente du gap (e#g) mais une analyse plus fine s'appuyant sur le modele physico-chimique montre que la valeur apparente de e#g ne correspond pas a la separation minimale entre les bandes de valence et de conduction. Une emission de lumiere visible a l'il a temperature ambiante est observee pour des largeurs de puits nominales inferieures a 20a. Contrairement aux predictions du modele de confinement quantique, la position du pic de photoluminescence est independante de d#a#-#s#i#:#h. Nous concluons que la presence de sous-oxydes au fond du puits est a l'origine de l'emission observee.

碩士
國立臺灣大學
電子工程學研究所
97
With the advances in the fabrication technology, the liquid-crystal display (LCD) with amorphous-silicon (a-Si) active-matrix thin-film transistors (TFTs) is the most popular one due to its unparallel advantages in fabrication cost and productivity. In order to provide the control and driving signals for the display, a CMOS chipset is generally required. Recently, the concept of a system on glass (SOG) has been proposed. As the peripheral circuits are integrated with the pixel array on the same glass substrate, a compact system with low cost and high reliability can be realized. However, limited by the characteristics of the TFT devices, it is still a challenging task for designers to implement the integrated circuits in an a-Si TFT technology, especially for analog and mixed-signal building blocks. In this thesis, to alleviate the device limitations imposed on a-Si technologies, novel circuit topologies are developed. In Chapter 3, a gain-enhancement technique is introduced to the operational amplifier (OPAMP) while the small-signal gain can be boosted, facilitating circuit implementation in a-Si technologies. In Chapter 4, a differential-difference amplifier (DDA) and a low-pass filter (LPF) based on the proposed OPAMP are developed. In Chapter 5, a touch panel system modified by the conventional charge amplifier is presented for demonstration.

碩士
國立臺灣大學
電子工程學研究所
97
Conventionally, the amorphous-silicon (a-Si) thin-film-transistor (TFT) technology is primarily employed to realize the display panel excluding the driving circuits of LCDs. Recently, the concept of the system on glass (SOG) is introduced, and some analog circuits are integrated on the glass substrate. In this thesis, to demonstration the potential of the a-Si process for analog integrated circuits (ICs), novel design strategies are proposed to alleviate the intrinsic limitations imposed on this semiconductor. In Chapter 3, a novel operational amplifier (OPAMP) with a gain- enhancement technique is presented while the low-frequency gain can be effectively boosted, facilitating the realization of advanced analog circuits with a-Si TFTs. In Chapter 4, based on the OPAMP provided in Chapter 3, a prototype of the resistor-string digital-to-analog converter (DAC) is implemented in the a-Si technology while this DAC can be further applied in the source driver of the TFT-LCD. In Chapter 5, in cooperation of the fully differential amplifier in Chapter 3, a fully integrated touch panel on the glass substrate is developed, making the system inexpensive in cost and compact in layout area.

碩士
國立成功大學
電機工程研究所
84
In this thesis , the amorphous silicon-germanium / crystal silicon heterojunction high-speed IR photodetector was studied in detail. In preparing the samples , the amorphous silicon- germanium alloys were grown on the crystal silicon subtrate by plasma enhanced chemical vapor deposition (PECVD). Both advantages of the low resistivity and high mobility characteristics of crystal silicon and the low temperature preparation processing,high optical absorption ,large area device feasibility and low cost of amorphous silicon are employed to prepare the heterojunction structure photodetector for faster response speed and lower cost. Compared with the traditoinal amorphous silicon germanium structure,the device with the structure of Al/n-a-Si:H/i-a- Si0.6 Ge0.4:H/p-c-Si has the following advantages: 1.The absorption wavelength peak moves to a higher value(865 nm ) than that 710 nm of the traditional amorphous silicon germanium structure. 2.The device has a faster response speed ( with a rise time of 195μs ) than that ( with a rise time of 465μs ) of the traditional amorphous silicon germanium structure . 3.The dark current has been decreased to a lower value (3.3μA under a reverse bias of 5V ) than that ( 50μA under the same bias)of the amorphous silicon germanium structure.

by Tsang Kin Hung.
Title also in Chinese.
Thesis (M.Phil.)--Chinese University of Hong Kong, 1992.
Includes bibliographical references.
Table of Contents
Acknowledgements
Abstract
Part I --- p.1
Chapter Chapter 1 --- Introduction --- p.2
Chapter Chapter 2 --- Viscosity of molten P40Ni40P20 --- p.6
Chapter Chapter 3 --- Viscosity of molten Pd77Cu6.5Si16.5 and the principle of corresponding state --- p.26
Chapter Chapter 4 --- Viscosity of molten Pd82Si18 and the scaling of viscosities of glass forming systems --- p.40
Part II --- p.53
Chapter Chapter 5 --- Installation of Dual Electron Gun Evaporator --- p.54
Chapter Chapter 6 --- Calorimetric studies of the Heat Capacity and Relaxation of Amorphous Si prepared by electron beam evaporation --- p.67

The development of functional flexible electronics is essential to enable applications such as conformal medical imagers, wearable health monitoring systems, and flexible light-weight displays. Intensive research on thin-film transistors (TFTs) is being conducted with the goal of producing high-performance devices for improved backplane electronics. However, there are many challenges regarding the performance of devices fabricated at low temperatures that are compatible with flexible plastic substrates. Prior work has reported on the change in TFT characteristics due to mechanical strain, with especially extensive data on the effect of strain on field-effect mobility. This thesis investigates the effect of gate-bias stress and elastic strain on the long-term stability of flexible low-temperature hydrogenated amorphous silicon (a-Si:H) TFTs, as the topic has yet to be explored systematically. An emphasis was placed on bias-stress measurements over time in order to obtain information on the physical mechanisms of instability. Drain current was measured over various intervals of time to track the degradation of devices due to metastability, and results were then compared across devices of various sizes under tensile, compressive, and zero strain. Transfer characteristics of the TFTs were also measured under the different conditions, to allow for extraction of parameters that would provide insight into the instability mechanisms. In addition to parameter extraction, the degradation and recovery of TFT output current was quantitatively compared for various bias-stress times across the different levels of strain. Finally, the instability mechanisms are modelled with a Markov system to further examine the effect of strain on long-term TFT operation. From the analysis of results, it was found that shallow charge trapping in the dielectric is the main mechanism of instability for short bias stress times, and did not seem to be greatly affected by strain. For longer bias stress times of over 10000 seconds, defect creation in the a-Si:H becomes a more significant contributor to instability. Both tension and compression increased defect creation compared to TFTs with zero applied strain. Compression appeared to cause the greatest increase in the rate of defect formation, likely by weakening Si-Si bonds in the a-Si:H. Tension appeared to cause a less significant increase, possibly due to a strengthening of some proportion of the Si-Si bonds caused by the slight elongation of bond length or because the applied tension relieves intrinsic compressive stress in a-Si:H film. A longer conduction path and greater dielectric area appears to increase the bias-stress and strain-related effects. Therefore reducing device size should increase the reliability of flexible TFTs.

A Voltage Controlled Oscillator (VCO) based pixel and array architecture is reported using amorphous silicon (a-Si) technology for large area digital imaging applications. The objectives of this research are to (a) demonstrate photon quantum noise limited pixel operation of less than 30 input referred noise electrons, (b) theoretically explore the use of the proposed VCO pixel architecture for photon quantum noise limited large area imaging applications, more specifically protein crystallography using a-Si, (c) to implement and demonstrate experimentally a quantum noise limited (VCO) pixel, a small prototype of quantum noise limited (VCO) pixelated array and a quantum noise limited (VCO) pixel integrated with direct detection selenium for energies compatible with a protein crystallography application. Electronic noise (phase noise) and metastability performance of VCO pixels in low cost, widely available a-Si technology will be theoretically calculated and measured for the first time in this research. The application of a VCO pixel architecture in thin film technologies to large area imaging modalities will be examined and a small prototype a-Si array integrated with an overlying selenium X-ray converter will be demonstrated for the first time. A-Si and poly-Si transistor technologies are traditionally considered inferior in performance to crystalline silicon, the dominant semiconductor technology today. This work v aims to extend the reach of low cost, thin film transistor a-Si technology to high performance analog applications (i.e. very low input referred noise) previously considered only the domain of crystalline silicon type semiconductor. The proposed VCO pixel architecture can enable large area arrays with quantum noise limited pixels using low cost thin film transistor technologies.

Σκοπός αυτής της διπλωματικής εργασίας είναι να εμβαθύνουμε στη λειτουργία των φωτοβολταϊκών πλαισίων λεπτού φίλμ (και συγκεκριμένα των πλαισίων άμορφου πυριτίου - a-Si - και CIS) και μέσα από τα αριθμητικά δεδομένα, να αποφανθούμε πώς η λειτουργία σε πραγματικές συνθήκες μπορεί να επηρεάσει την παραγόμενη ισχύ τους. Στα πλαίσια αυτά, πραγματοποιήθηκαν πειραματικές μετρήσεις, στο χώρο της ταράτσας του κτιρίου του τμήματος των Ηλεκτρολόγων Μηχανικών και Τεχνολογίας Υπολογιστών, με φωτοβολταϊκά πλαίσια άμορφου πυριτίου και CIS ισχύος αιχμής 32 και 75 W αντίστοιχα. Οι μετρήσεις πραγματοποιούνταν μια φορά την εβδομάδα κατά τη διάρκεια ενός ημερολογιακού έτους (Μάιος 2009-Απρίλιος 2010) υπό διάφορες συνθήκες ακτινοβολίας και θερμοκρασίας και για αρκετές γωνίες κλίσης, με σκοπό να αποκτήσουμε μια ολοκληρωμένη εικόνα της ενεργειακής τους συμπεριφοράς. Οι μετρήσεις πραγματοποιήθηκαν με τη βοήθεια του μηχανήματος PVPM 2540C το οποίο αποτυπώνει τη χαρακτηριστική ρεύματος-τάσης του προς μέτρηση πλαισίου για μια χρονική στιγμή (σε χρόνο δυο δευτερολέπτων περίπου), ενώ επιπλέον σημειώναμε την ακτινοβολία, τη θερμοκρασία του περιβάλλοντος, καθώς και την κλίση τοποθέτησής τους. Επίσης μελετούσαμε πώς επηρεάζει τη χαρακτηριστική καμπύλη I-V, και κατά συνέπεια την απόδοση, τυχόν σκίαση από παρακείμενο αντικείμενο. Ο προσανατολισμός των πλαισίων ήταν πάντα προς το Νότο, ώστε να έχουμε περισσότερες ώρες ηλιοφάνειας, διότι η Ελλάδα είναι χώρα του βόρειου ημισφαιρίου. Κατά την επεξεργασία των μετρήσεων καταλήξαμε στην βέλτιστη κλίση τοποθέτησης των πλαισίων ανά εποχή καθώς και σε μία βέλτιστη κλίση τοποθέτησης για όλο τo χρόνο για την περιοχή της Πάτρας. Επιπλέον, υπολογίσαμε με τη μέγιστη δυνατή ακρίβεια την ετήσια ενεργειακή απόδοση του κάθε πλαισίου για όλο το έτος και συγκρίναμε τα παραγόμενα αποτελέσματα. Τέλος, με τη βοήθεια του προγράμματος PV*Sol, κάναμε μια μοντελοποίηση του χρησιμοποιούμενου συστήματος για να συγκρίνουμε μ’ αυτή τα πειραματικά μας αποτελέσματα.
The aim of this diploma thesis is to understand deeply the operation of thin-film (specifically amorphous silicon and CIS ) modules and through the numerical data of measurements and calculations, to make a conclusion considering how the operation in real conditions can influence their produced power. Measurements of current and voltage have been realized on the roof of the building of the department of Electrical and Computer Engineering using an amorphous silicon and a CIS photovoltaic module of 32 and 75 W peak power respectively. The measurements took place once a week during one a year (May 2009-April 2010) and our goal was to obtain measurements under various conditions of radiation and temperature and for some tilt angles so that we acquire enough knowledge on their energy behaviour. The measurements were taken by the “pve PVPM 2540C“ device, which plots the characteristic curve of current and voltage of a module (in space of two seconds) and we also noted down the radiation, the ambient temperature, as well as the tilt angle of the modules. Moreover, we have tested how a possible natural shading from an adjacent object influences the characteristic I-V curve, and as a result the efficiency of the module. The orientation of the module was always South, in order to gain more hours of sunlight, as Greece is a country of the northern hemisphere. While processing the measurements, we found the optimal tilt angle of the modules per season as well as per year for Patras area. Moreover, we tried to calculate with the maximum possible accuracy, the annual energy yield by the two different types of modules and compare the results. Finally, by using the computer modelling system “PV*sol”, we tried to simulate our photovoltaic system, in order to compare the measured results to the experimental.