New Generation Lithographic (NGL) techniques have been recently investigated in order to overcome the limitations of the long-used UV lithography. Several techniques have been proposed during the last decades, but the continued improvement of UV lithography rendered them useful only for a limited number of applications. More recently, nanoimprint lithography (NIL), invented in the nineties, has been considered as the new NGL due to its extreme simplicity and high resolution. Thermal NIL consists in the deformation of a thermoplastic under pressure and temperature by a nanostructured mold, while UV-NIL consists in the polymerization by UV light of a monomer at room temperature and under a lower pressure than Thermal NIL. One of the main problems of this technique is mold-polymer separation after the process. This problem is especially important for UV-NIL, because the working treatments for Thermal NIL degrade with UV light. In order to assess this problem, thin diamond-like amorphous carbon films (DLC) have been proposed as an alternative to existing treatments for their low chemical reactivity and the possibility to incorporate other chemical elements to further reduce their surface energy. Amorphous carbon exists in different forms, depending on how it is grown. Its mechanical properties range from polymer or graphite-like to almost as resistant as diamond. Besides the excellent mechanical properties of DLC (high hardness, elasticity and wear resistance, and low dry friction), amorphous carbon has also been found useful in applications requiring inert and/or biocompatible surfaces. The project DPI2007-61349 of the Science and Innovation Department of Spain, named “Amorphous carbon molds for micro and nanoimprint of polymeric surfaces”, aims to study the effect of the incorporation of different elements in DLC films for the improvement of NIL molds. This thesis has focused on a series of objectives of this project: - Design and construction of a very high vacuum reactor for deposition processes and ionic etch - Incorporation of fluorine to amorphous carbon films and subseqüent characterization by different surface, mechanical and tribological techniques, as well as spectroscopy for the characterization of the plasma used for the process. - Set up and optimization of a deep ion etch technique with ion beam for the production of molds. - The use of different lithographic techniques oriented to the production in large scale of nanometric patterns. - The exploration of mold coating to increase its durability and antisticking properties in nanoimprint processes. The incorporation of fluorine in DLC films has demonstrated to be useful in the improvement of the properties of NIL molds, because it avoids the use of the current surface treatments, which in addition to being less durable, can react with polymers in presence of UV light. In this thesis, the influence of fluorine incorporation in the films has been studied. Fluorinated amorphous carbon films have been deposited by pulsed-DC plasma enhanced chemical vapor deposition, by progressively replacing methane by trifluoromethane. The experimental device used for deposition has been designed and built to allow a number of multiple processes in the same reactor. The results of the study demonstrate the feasibility of this technique, of easy industrial implementation, for the deposition of this type of coatings. The characterization of both the active species in the plasma and the groups incorporated into the deposited films has helped to understand the process of fluorine incorporation, as well as the change in the surface properties that it entails. / La dificultad de extender el uso de la litografía de luz ultravioleta (UV) a los cada vez más estrictos requisitos de resolución, llevaron desde hace ya un par de décadas, a plantearse la necesidad de buscar técnicas litográficas llamadas de “Nueva Generación” (NGL) que las superasen. Son diversas las técnicas se han ido proponiendo durante estos años, pero la mejora de la litografía UV las ha ido relegando fuera del ámbito industrial. Más recientemente, la litografía por nanoestampación (NIL), ha tomado fuerza como la nueva NGL por su extrema sencillez y por su demostrada elevada resolución. La NIL térmica (T-NIL) consiste en la deformación de un termoplástico bajo presión y temperatura por un molde con estructuras nanométricas, mientras que la NIL por UV (UV-NIL) consiste en la polimerización de un monómero a temperatura ambiente con menor presión ejercida por un molde transparente al UV. Uno de los principales problemas de esta técnica es la separación de molde y polímero, una vez finalizado el proceso. Como alternativa a los tratamientos existentes, se han propuesto los recubrimientos de carbono amorfo tipo diamante (DLC) por su baja reactividad química, elevada dureza y posibilidad de incorporación de otros elementos químicos a fin de reducir su energía superficial. El proyecto del Ministerio de Ciencia e Innovación DPI2007-61349, “Moldes de carbono amorfo para micro y nanograbado de superficies poliméricas”, en el cuál se ha enmarcado esta tesis, pretende estudiar los efectos de la incorporación de diferentes elementos en capas de DLC para la mejora de los moldes de NIL. La incorporación de flúor en capas de DLC ha demostrado recientemente ser útil en la mejora de las propiedades de los moldes de NIL, porque evita el uso de los actuales tratamientos superficiales (por ejemplo siloxanos), los cuales, además de ser menos duraderos, pueden reaccionar con los polímeros en presencia de luz UV. Así, en esta tesis se ha estudiado la influencia de la incorporación de flúor a capas de DLC en la composición y en las propiedades de superfície obtenidas.

Thin film cadmium telluride (CdTe) technology is the most important competitor for silicon (Si) based solar cells. Pulsed direct current (DC) magnetron sputtering is a new technique has been developed for thin film CdTe deposition. This technique is industrially scalable and provides uniform coating. It is also possible to deposit thin films at low substrate temperatures. A series of experiments are presented for the optimisation of the cadmium chloride (CdCl2) activation process. Thin film CdTe solar cells require CdCl2 activation process to improve conversion efficiencies. The role of this activation process is to increase the grain size by recrystallisation and to remove stacking faults. Compaan and Bohn [1] used the radio-frequency (RF) sputtering technique for CdTe solar cell deposition and they observed small blisters on CdTe layer surface. They reported that blistering occurred after the CdCl2 treatment during the annealing process. Moreover, void formation was observed in the CdTe layer after the CdCl2 activation process. Voids at the cadmium sulphide (CdS)/CdTe junction caused delamination hence quality of the junction is poor. This issue has been known for more than two decades but the mechanisms of the blister formation have not been understood. One reason may be the stress formation during CdTe solar cells deposition or during the CdCl2 treatment. Therefore, the stress analysis was performed to remove the defects observed after the CdCl2 treatment. This was followed by the rapid thermal annealing to isolate the CdCl2 effect by simply annealing. Small bubbles observed in the CdTe layer which is the first step of the blister formation. Using high resolution transmission electron microscopy (HR-TEM), it has been discovered that argon (Ar) working gas trapped during the deposition process diffuses in the lattice which merge and form the bubbles during the annealing process and grow agglomeration mainly at interfaces and grain boundaries (GBs). Blister and void formation were observed in the CdTe devices after the CdCl2 treatment. Therefore, krypton (Kr), neon (Ne) gases were used as the magnetron working gas during the deposition of CdTe layer. The results presented in this thesis indicated that blister and void formation were still existing with the use of Kr an Ne. Xe, which has a higher atomic mass than Kr, Ne, Ar, Cd and Te, was used as the magnetron working gas and it resulted in surface blister and void free devices.

This thesis describes the development of a fast rate method for the deposition of high quality CdS and CdTe thin films. The technique uses Pulsed DC Magnetron Sputtering (PDCMS). Surprisingly, the technique produces highly stable process conditions. CREST is the first laboratory worldwide to show that pulsed DC power may be used to deposit CdS and CdTe thin films. This is a very promising process technology with potential for eventual industrial deployment. The major advantage is that the process produces high deposition rates suitable for use in solar module manufacturing. These rates are over an order of magnitude faster than those obtained by RF sputtering. In common with other applications it has also been found that the energetics of the pulsed DC process produce excellent thin film properties and the power supply configuration avoids the need for complex matching circuits. Conventional deposition methodologies for CdS, Chemical Bath Deposition (CBD) and CdTe thin films, Electrodeposition (ED), have been chosen as baselines to compare film properties with Pulsed DC Magnetron Sputtering (PDCMS). One of the issues encountered with the deposition of CdS thin films (window layers) was the presence of pinholes. A Plasma cleaning process of FTO-coated glass prior to the deposition of the CdS/CdTe solar cell has been developed. It strongly modifies and activates the TCO surface, and improves the density and compactness of the deposited CdS thin film. This, in turn, improves the optical and morphological properties of the deposited CdS thin films, resulting in a higher refractive index. The pinhole removal and the increased density allows the use of a much thinner CdS layer, and this reduces absorption of blue spectrum photons and thereby increases the photocurrent and the efficiency of the thin film CdTe cell. Replacing the conventional magnetic stirrer with an ultrasonic probe in the chemical bath (sonoCBD) was found to result in CdS films with higher optical density, higher refractive index, pinhole and void-free, more compact and uniform along the surface and through the thickness of the deposited material. PDCMS at 150 kHz, 500 W, 2.5 μs, 2 s, results in a highly stable process with no plasma arcing. It allows close control of film thickness using time only. The CdS films exhibited a high level of texture in the <001> direction. The grain size was typically ~50 nm. Pinholes and voids could be avoided by reducing the working gas pressure using gas flows ii below 20 sccm. The deposition rate was measured to be 1.33 nm/s on a rotating substrate holder. The equivalent deposition rate for a static substrate is 8.66 nm/s, which is high and much faster than can be achieved using a chemical bath deposition or RF magnetron sputtering. The transmission of CdS can be improved by engineering the band gap of the CdS layer. It has been shown that by adding oxygen to the working gas pressure in an RF sputtering deposition process it is possible to deposit an oxygenated CdS (CdS:O) layer with an improved band gap. In this thesis, oxygenated CdS films for CdTe TF-PV applications have been successfully deposited by using pulsed DC magnetron sputtering. The process is highly stable using a pulse frequency of 150 kHz and a 2.5 μs pulse reverse time. No plasma arcing was detected. A range of CdS:O films were deposited by using O2 flows from 1 sccm to 10 sccm during the deposition process. The deposition rates achieved using pulsed DC magnetron sputtering with only 500 W of power to the magnetron target were in the range ~1.49 nm/s ~2.44 nm/s, depending on the oxygen flow rate used. The properties of CdS thin films deposited by pulsed DC magnetron sputtering and chemical bath deposition have been studied and compared. The pulsed DC magnetron sputtering process produced CdS thin films with the preferred hexagonal <001> oriented crystalline structure with a columnar grain growth, while sonoCBD deposited films were polycrystalline with a cubic structure and small grainy crystallites throughout the thickness of the films. Examination of the PDCMS deposited CdS films confirmed the increased grain size, increased density, and higher crystallinity compared to the sonoCBD CdS films. The deposition rate for CdS obtained using pulsed DC magnetron sputtering was 2.86 nm/s using only 500 W power on a six inch circular target compared to the much slower (0.027 nm/s) for the sonoChemical bath deposited layers. CdTe thin films were grown on CdS films prepared by sonoCBD and Pulsed DC magnetron sputtering. The results showed that the deposition technique used for the CdS layer affected the growth and properties of the CdTe film and also determined the deposition rate of CdTe, being 3 times faster on the sputtered CdS. PDCMS CdTe layers were deposited at ambient temperature, 500 W, 2.9 μs, 10 s, 150 kHz, with a thickness of approximately 2 μm on CdS/TEC10 coated glass. The layers appear iii uniform and smooth with a grain size less than 100 nm, highly compact with the morphology dominated by columnar grain growth. Stress analysis was performed on the CdTe layers deposited at room temperature using different gas flows. Magnetron sputtered thin films deposited under low gas pressure are often subject to compressive stress due to the high mobility of the atoms during the deposition process. A possible way to reduce the stress in the film is the post-deposition annealing treatment. As the lattice parameter increased; the stress in the film is relieved. Also, a changing the deposition substrate temperature had an effect on the microstructure of CdTe thin films. Increasing the deposition temperature increased the grain size, up to ~600 nm. CdTe thin films with low stress have been deposited on CdS/TEC10 coated glass by setting the deposition substrate temperature at ~200°C and using high argon flows ~ 70 sccm Ar. Finally, broadband multilayer ARCs using alternate high and low refractive index dielectric thin films have been developed to improve the light transmission into solar cell devices by reducing the reflection of the glass in the extended wavelength range utilised by thin-film CdTe devices. A four-layer multilayer stack has been designed and tested, which operates across the wavelength range used by thin-film CdTe PV devices (400 850 nm). Optical modelling predicts that the MAR coating reduces the WAR (400-850 nm) from the glass surface from 4.22% down to 1.22%. The application of the MAR coating on a thin-film CdTe solar cell increased the efficiency from 10.55% to 10.93% or by 0.38% in absolute terms. This is a useful 3.6% relative increase in efficiency. The increased light transmission leads to improvement of the short-circuit current density produced by the cell by 0.65 mA/cm2. The MAR sputtering process developed in this work is capable of scaling to an industrial level.

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.

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.