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Deposition of nano-composite Mg–F–Si–O films on optical grade silica glass was studied employing RF magnetron co-sputtering from magnesium fluoride (MgF2) and fused silica (SiO2) targets. The aim was to obtain a stable and reliable sputtering process for optical coatings exhibiting a refractive index lower than the one of quartz glass (1.46 at 550 nm) without adding gaseous fluorine to the deposition process. The two magnetrons were installed in a confocal way at 45° off-axis with respect to a static substrate, thus creating a lateral gradient in the thin-film composition. The deposited Mg–F–Si–O coatings were structurally analysed by electron dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The obtained films consist of MgF2 nanocrystals embedded in a SiO2-rich amorphous matrix. Spectroscopic ellipsometry and spectrophotometry measurements showed that they are highly transparent exhibiting a very-low extinction coefficient k and a refractive index n in the desired range between the one of MgF2 (1.38) and SiO2 (1.46). Films with n = 1.424 and 1.435 at 550 nm were accomplished with absorption below the detection threshold.

This dissertation thesis revolves around hydrogen economy and energy-storage electrochemical systems. More specifically, it investigates the possibility of using magnetron sputtering for deposition of efficient thin-film anode catalysts with low noble metal content for proton exchange membrane water electrolyzers (PEM-WEs) and unitized regenerative fuel cells (PEM-URFCs). The motivation for this research derives from the urgent need of minimizing the price of mentioned electrochemical devices should they enter mass production. Numerous experiments were carried out, correlating the actual in-cell performance with the varying position of thin-film catalyst within the membrane electrode assembly, with the composition of high-surface support sublayer and with the chemical structure of the catalyst itself. The wide arsenal of analytical methods ranging from electrochemical impedance spectroscopy through scanning electron microscopy to photoelectron spectroscopy allowed us to describe complex phenomena behind different obtained efficiencies. Consequent systematic optimizations led to the design of novel PEM-WE anode thin-film iridium catalyst with thickness of just 50 nm, supported on optimized TiC-based sublayer which performed similarly to standard counterparts despite using just a fraction of their noble metal...

Most thin-film techniques require a multiple vacuum process, and cannot produce high-coverage continuous thin films with the thickness of a few nanometers on rough surfaces. We present a new "paradigm shift" non-vacuum process to deposit high-quality, ultra-thin, single-crystal layers of coalesced sulfide nanoparticles (NPs) with controllable thickness down to a few nanometers, based on thermal decomposition. This provides high-coverage, homogeneous thickness, and large-area deposition over a rough surface, with little material loss or liquid chemical waste, and deposition rates of 10 nm/min. This technique can potentially replace conventional thin-film deposition methods, such as atomic layer deposition (ALD) and chemical bath deposition (CBD) as used by the Cu(In,Ga)Se₂ (CIGS) thin-film solar cell industry for decades. We demonstrate 32% improvement of CIGS thin-film solar cell efficiency in comparison to reference devices prepared by conventional CBD deposition method by depositing the ZnS NPs buffer layer using the new process. The new ZnS NPs layer allows reduction of an intrinsic ZnO layer, which can lead to severe shunt leakage in case of a CBD buffer layer. This leads to a 65% relative efficiency increase.

et al., The suitability of a particular material for use in magnetic devices is determined by the process of magnetization reversal/relaxation, which in turn depends on the magnetic anisotropy. Therefore, designing new ways to control magnetic anisotropy in technologically important materials is highly desirable. Here we show that magnetic anisotropy of epitaxial thin-films of half-metallic ferromagnet LaSrMnO (LSMO) is determined by the proximity to thermodynamic equilibrium conditions during growth. We performed a series of x-ray diffraction and ferromagnetic resonance (FMR) experiments in two different sets of samples: the first corresponds to LSMO thin-films deposited under tensile strain on (0 0 1) SrTiO by pulsed laser deposition (PLD; far from thermodynamic equilibrium); the second were deposited by a slow chemical solution deposition (CSD) method, under quasi-equilibrium conditions. Thin films prepared by PLD show fourfold in-plane magnetic anisotropy, with an overimposed uniaxial term. However, the uniaxial anisotropy is completely suppressed in the CSD films. This change is due to a different rotation pattern of MnO octahedra to accommodate epitaxial strain, which depends not only on the amplitude of tensile stress imposed by the STO substrate, but also on the growth conditions. Our results demonstrate that the nature and magnitude of the magnetic anisotropy in LSMO can be tuned by the thermodynamic parameters during thin-film deposition., This work was supported by the European Research Council (ERC StG-259082, 2DTHERMS), Xunta de Galicia (2012-Projet No. CP072) and by the Ministry of Science of Spain (Project No. MAT2013-44673-R). JMVF also acknowledges the same organization for an FPI grant. EW and JM thank UNCuyo Argentina for Grant No C011.

We present a novel control methodology to control the roughening processes of semilinear parabolic stochastic partial differential equations in one dimension, which we exemplify with the stochastic Kuramoto-Sivashinsky equation. The original equation is split into a linear stochastic and a nonlinear deterministic equation so that we can apply linear feedback control methods. Our control strategy is then based on two steps: first, stabilize the zero solution of the deterministic part and, second, control the roughness of the stochastic linear equation. We consider both periodic controls and point actuated ones, observing in all cases that the second moment of the solution evolves in time according to a power-law until it saturates at the desired controlled value.

This paper focuses on the understanding of the main mechanisms that participate in the growth process of an SiO 2 -like film in an atmospheric pressure Townsend discharge in N 2 with small ad-mixtures of HMDSO and N 2 O. The approach consists of analyzing the influence of operating parameters on the growth rate profile through a fluid dynamics model. The suggested chemical mechanism is constituted by one volume reaction and one surface reaction. This simple model enables us to assume which phenomena control the film growth process among the following mechanisms: HMDSO dissociation by N 2 (A 3 Σ + u ) in Si-containing radicals, radicals transport to the surface, and surface activation process by plasma.

Rate constants of surface diffusion processes in metals often depend on the local atomic environment. Accurate models that describe this local environment dependence for specific material systems are not easily available in the literature despite the need for such information in kinetic models. In this work, we find the prefactor and activation barrier associated with adatom hop processes for a large number of local environments and use this information to systematically train cluster expansion (CE) models for Ag, Al, Cu, Ni, Pd and Pt. We demonstrate that although the local environments used for training is a small subset of all possible local environments, the CE model can predict the rate constants for many local environments that have not been used while training the model with reasonable accuracy. It is found that up to fourth nearest neighbor positions can be relevant to the activation barrier. Furthermore, many-body effects play an important role. We also find that the CE model depends on the direction of the hop. (C) 2013 Elsevier B.V. All rights reserved.

Self-assembled quantum-dots (QDs) represent a distributed ensemble of zero dimensional structures, each presenting a near-singular density of states. The QD size, shape, areal density and optical properties depend on growth parameters, such as growth temperature, growth rate and amount of InAs deposed. QDs are very suitable for optoelectronic device applications as during the epitaxial QD growth the phase transition relieves the strain elastically without introducing defects. Embedded in the active medium of a semiconductor laser diode QD's unique properties lead to improved and often novel characteristics as compared to bulk or quantum-well (QW) devices. In particular In(Ga)As QD lasers on GaAs substrates are of largest interest as the spectral range of their emission wavelength reaches further into the infrared than that of QW lasers of the same material system. The emission wavelength of strained InGaAs QW lasers is limited to about 1150nm, whereas in In(Ga)As QD lasers an emission wavelength beyond 1.3µm is feasible. Such kind of QD lasers performed – as recently reported – ultralow threshold current densities combined with decreased temperature sensitivity, relatively high modulation bandwith, low chirp, which has been already predicted many years ago. Beside quantum dot's application in semiconductor lasers, it has been suggested to use QDs also for the optical active region in so-called semiconductor optical amplifier (SOA). So far, experimentally it was possible to demonstrate in QD SOAs a much faster gain recovery time due to more effective carrier re-filling from excited states than in quantum well structures. Also a smaller noise figure ratio compared to QWs was predicted in QD SOA, but has not been experimentally demonstrated yet. However, two major constraints related to the limit of the high-frequency modulation of QD lasers and the QD SOA's polarization sensitivity are required to be first resolved, before it is possible to implement both technologies replacing QW devices in optical broad telecommunications devices. The polarization properties of semiconductor quantum dot/-dash lasers and amplifiers have been addressed in this PhD thesis. Within this research work – studying systematically several quantum dot and -dash heterostructure of different material families – two major goals have been identified: (1) characterization of the optical polarization properties of quantum dot/-dash structures embedded in the active region of semiconductor optical amplifiers and (2) realization of polarization-insensitive quantum dot based SOAs. Moreover, this thesis focuses on the understanding of the underlying physics being responsible for the QD's polarization and the dependence of the optical gain on the QD's aspect ratio and compositional material contrast. In terms of the experimental work broad area laser devices have been fabricated by applying different clean room processing methods like photolithography, metal deposition, wet and plasma etching. Through a subsequent device characterization using optoelectronic measurement techniques the polarization characteristics of dots and dashes have been accessed. From broad area laser device characterization – measuring the polarization-resolved edge-emitted electroluminescence (EL) – the polarization properties of standard Stranski-Krastanov (SK), closely- and columnar stacked InAs/GaAs quantum dots as well of closely-stacked InAs/InP quantum dots and columnar-stacked InAs/InP quantum dashes – in total 32 devices – have been evaluated. The comparison of the transversal magnetic (TM) versus -electric (TE) integrated EL signal for above heterostructures provided very promising results in order to achieve polarization-independent QD SOA: Starting from standard SK QDs – emitting dominantly in TE mode – it was possible first to enhance the TM EL signal compared to TE by epitaxial shape-engineering of the dot/dash structure of the QD's aspect ratio and the compositional material contrast in columnar-stacked InAs/GaAs (InAs/InP) QDs (QDashes), and then inverting the polarization direction from dominant TE towards dominant TM. The latter polarization transition has been also confirmed by photovoltage spectroscopy (PVS), and as well from room-temperature TM-lasing demonstration – both carried out by our research partner from the Cardiff University, Wales (UK). Introducing the well-known segment contact method for gain measurements, which is based on the analysis of the amplified spontaneous emission (ASE) generated by cavities of different length, it was possible to access both the TM and TE gain of such columnar-stacked InAs/GaAs quantum dots. The work was motivated and funded by the European Union project "Zero Order Dimension based Industrial components Applied to teleCommunications" (ZODIAC) within the sixth framework program. Very fruitful collaborations in order to exchange scientific ideas/know-how and achieving joint results have been established in a pan-European and multicultural inspired environment between four industrial partners – Alcatel Thales III-V Lab (France), Bookham (GB), Innolume – GmbH (Germany), Nanoplus Nanosystems and Technologies-GmbH (Germany) – and three national research institutes – Laboratory for Photonics and Nanostructures (France), The Tyndall National Institute (Irland), Saint-Petersburg Physics and Technology Centre for Research and Education of the Russian Academy of Science (Russia) – and three Universities: École Polytechnique Fédérale de Lausanne (Switzerland), Politechnika Wroclawska (Poland), Würzburg University (Germany). Besides this European ZODIAC project it was possible to establish with the Cardiff University, Wales (UK) a very constructive collaboration.

A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons and valence band holes in all three spatial directions, thus creating fully discrete energy levels. The confinement in the InAs/GaAs material system is generated by the bandgap difference for the two materials (∼ 0.4 eV for InAs, and ∼ 1.4 eV for GaAs) which provides a minimum of potential energy for both electrons and holes inside the InAs nanoparticle. The appearance of new nanoscale effects modifies the electro-optical properties of such a system which makes possible the improvement of existing device performance or even fascinating novel device applications. For the creation of coherent structures with very high purity and high density, as it is required for application in light emitting devices, self-assembling is a very important pathway. The quantum dot creation mostly relies on the relaxation of the strain accumulated during the growth of lattice mismatched materials, which happens in a non-equlibrium condition. This results in a statistical distribution of all the parameters characterizing the InAs nanocrystals as size, composition and strain, whose direct manifestation is the inhomogeneously broadened light emission deriving from quantum dot ensembles. At the dawning of the quantum dot development the broadening was considered as a limiting factor for application in semiconductor lasers. Today, it is considered favorably for all the applications where a broad gain spectrum is required as optical amplifiers, monolithic and external-cavity tunable lasers, and superluminescent diodes. The objective of this thesis is the application of InAs/GaAs self-assembled quantum dots to the active region of high power and broad-band superluminescent diodes emitting in the 1.3 μm wavelength region. Superluminescent diodes are edge-emitting semiconductor light sources based on the amplification of spontaneous emission along a waveguide where optical gain is achieved through current injection. They combine the high power and brightness of laser diodes with the low coherence of LEDs. The latter is a fundamental feature for the application in optical coherence tomography medical imaging, where the image resolution is related to the spectral bandwidth of the light source used. The achievement of bandwidths larger than 100 nm are demonstrated in this work through the optimization of molecular beam epitaxy growth conditions. This may be done optimizing the statistical size-dispersion of the dot ensemble, or through the use of different dot layers emitting at slightly different wavelengths. The large bandwidth emission is obtained also due to the peculiar energy structure of this material, for which a multi-state emission appears under particular injection conditions. As a term of comparison, best commercial single-chip superluminescent diodes for the imaging application at 1.3 μm show bandwidths around 60 nm. Moreover, the discrete nature of the energy levels in InAs quantum dots together with the implementation of a GaAs-based technology can be beneficial for the device temperature stability (the competing technology, based on InP, suffers from strong thermal degradation). The analysis and understanding of the device temperature characteristics will be detailed in the last chapter of the manuscript. Even though standard devices show an important temperature dependence, a better stability may be achieved through the use of p-doped active regions. In the last few years quantum dot devices have shown outstanding properties if compared to the competing quantum well technology. Low threshold currents, high temperature stability and low chirp in lasers, large bandwidths and low gain saturation in amplifiers have been demonstrated. However, in spite of the large interest among the scientific community, many of the microscopic processes at the origin of the electro-optical characteristics of this material are not completely understood yet. The physics of the quantum dot active material, will be modeled in this work through the use of systems of rate equations with different complexity depending on the system they are applied to. We will provide a mean-field rate equation model allowing to get insights about carrier dynamics in QD lasers. Also, we will develop two different traveling-wave rate equation models, one for the modeling of the L-I characteristics and the other for the modeling of the spectral characteristics of quantum dot superluminescent diodes. Considering the carrier and photon density distributions across the device cavity is essential in superluminescent diodes, where the high single pass gain results in large non-homogeneities of photon and carrier distributions. The work is motivated by an industrial cooperation with EXALOS AG, a leading company in the development, manufacturing, and sales of broadband superluminescent diodes.

Although the plasma-enhanced chemical vapor deposition (PECVD) of amorphous silicon films (a-Si:H, aSi:H,F) and related materials (a-Si:Ge, a-Si:C, a-Si:N) is already used in the industry for solar cells and optoelectronic devices, considerable research efforts have been carried out, in the last few years, on some aspects which are still considered unsolved. In particular, the identity of the growth precursors, the plasmasurface interaction, and the microscopic parameters affecting the film quality are problems still debated in the scientific community (refs.l,2).

The use of thin-film deposition in the fabrication of antireflection-coated diffractive optical elements is discussed. The antireflection coatings for these diffractive elements are optimized on the basis of an angular spectrum approach and the method of characteristic matrices. A minimum reflectivity as low as 1 x 10 -4 is realized using in situ controlled multilayers of TiO 2 and SiO 2 . The blazed profile of the diftractive optical elements is approximated by a stepped profile with up to 32 phase levels. The highest measured diffraction efficiency for 32-level Fresnel zone lenses was 97%.