Development of solar cells based on AllnN/Si heterojunctions growth by RF-Sputtering

Hoy en día, los semiconductores III-Nitruros se están proponiendo para su aplicación en dispositivos electrónicos y optoelectrónicos debido a sus características únicas, como son su energía de gap, su alta estabilidad térmica y alta resistencia a la radiación. En ese sentido, las aleaciones de nitruro de aluminio e indio (AlInN) ofrecen un gran potencial para su uso como dispositivos fotovoltaicos dado que su energía de gap puede cubrir un amplio rango del espectro solar, yendo desde 0.7 eV (InN) hasta 6.2 eV (AlN), y además de las ya mencionadas resistencia a altas temperaturas y a la radiación. Además, a lo largo de las últimas décadas, los compuestos del grupo III-V se han desarrollado utilizando diferentes técnicas de crecimiento, como el crecimiento epitaxial por haces moleculares (MBE), la epitaxia en fase de vapor con precursores metalorgánicos (MOVPE) o la pulverización catódica. Esta tesis se centra en esta última técnica, que permite obtener dispositivos más baratos y con mayor área que otras técnicas de depósito y también permite el crecimiento en un amplio rango de temperaturas (en nuestro caso desde temperatura ambiente hasta valores por debajo de 650 ° C) y sustratos. A lo largo de esta tesis, se ha estudiado la influencia de varios parámetros de la heterounión basada en el AlInN y el silicio, como son la energía de gap del AlInN, su espesor y concentración de portadores, la recombinación en la superficie del silicio, densidad de defectos en la intercara y la calidad de oblea de Si, en sus propiedades fotoeléctricas mediante el uso del software Pc1d, con el objetivo de explorar su potencial como células solares a través del análisis y la optimización de dichos parámetros Además de ello, se ha estudiado el efecto de varios parámetros de depósito, como la temperatura de crecimiento o la potencia aplicada a los blancos de indio y aluminio, sobre las propiedades estructurales, morfológicas, eléctricas y ópticas de los compuestos AlxIn1-xN crecidos sobre sustratos de Si (111), Si (100) y zafiro; mostrando propiedades similares en ambos sustratos de silicio, independientemente de la orientación. Tras ello, se han estudiado las propiedades fotoeléctricas de los dispositivos basados en heterouniones AlxIn1-xN / Si en función de la temperatura de crecimiento y la potencia aplicada al blanco de Al. La curva I-V de los dispositivos revela la alta influencia de la temperatura de crecimiento en la eficiencia de conversión de los dispositivos.Nowadays III-Nitrides semiconductors are being proposed for electronic and optoelectronic devices due to their unique material characteristics, such as their wide direct bandgap, high thermal stability and high radiation hardness. Particularly, Aluminum Indium Nitride (AlInN) alloys offer great potential for photovoltaic devices thanks to their wide direct bandgap energy that covers the solar spectrum from 0.7 eV (InN) to 6.2 eV (AlN), and their superior resistance to high temperatures and high-energy particles. Besides, III-nitrides compounds has been grown using different deposition techniques, such as Molecular Beam Epitaxy (MBE), Metalorganic vapour phase epitaxy-(MOVPE) or Radio Frequency (RF) Sputtering. This thesis is focused on this latest technique, which allows obtaining cheaper and larger devices than other deposition techniques and also deposition in a wide range of temperatures (from room temperature to values below 650°C in our case) and substrates. Along this thesis, the influence of several parameters, such as AlInN bandgap energy, AlInN thickness and carrier concentration, silicon surface recombination, interface defects and Si wafer quality, on the photovoltaic properties of AlInN on silicon heterojunctions has been carried out using the Pc1d software, with the aim to explore their potential for solar cell devices through the analysis and optimization of the aforementioned parameters. Besides, the effect of several growth parameters, such as the deposition temperature or the power supply applied to the In and the Al targets, on the structural, morphological, electrical and optical properties of AlxIn1-xN compounds deposited on Si (111), Si (100) and sapphire substrates has been studied, showing similar properties of the AlxIn1-xN layers growth on both silicon substrates regardless the orientation. After that, the photoelectrical properties of solar cell devices based on AlxIn1-xN/Si heterojunctions has been studied as a function of the growth temperature and the Al power supply. Their J-V curve of the devices reveals the high influence of the growth temperature on the conversion efficiency of the devices

Advanced scanners and imaging systems for earth observations

Assessments of present and future sensors and sensor related technology are reported along with a description of user needs and applications. Five areas are outlined: (1) electromechanical scanners, (2) self-scanned solid state sensors, (3) electron beam imagers, (4) sensor related technology, and (5) user applications. Recommendations, charts, system designs, technical approaches, and bibliographies are included for each area

Design and analysis of integrated waveguide structures and their coupling to silicon-based light emitters

A major focus is on integrated Silicon-based optoelectronics for the creation of low-cost photonics for mass-market applications. Especially, the growing demand for sensitive and portable optical sensors in the environmental control and medicine follows in the development of integrated high resolution sensors [1]. In particular, since 2013 the quick onsite verification of pathogens, like legionella in drinking water pipes, is becoming increasingly important [2, 3]. The essential questions regarding the establishment of portable biochemical sensors are the incorporation of electronic and optical devices as well as the implementations of fundamental cross-innovations between biotechnology and microelectronics. This thesis describes the design, fabrication and analysis of high-refractive-index-contrast photonic structures. Besides silicon nitride (Si3N4) strip waveguides, lateral tapers, bended waveguides, two-dimensional photonic crystals (PhCs) the focus lies on monolithically integrated waveguide butt-coupled Silicon-based light emitting devices (Sibased LEDs) [4, 5] for use as bioanalytical sensor components. Firstly, the design and performance characteristics as single mode regime, confinement factor and propagation losses due to the geometry and operation wavelength (1550 nm, 541 nm) of single mode (SM), multi mode (MM) waveguides and bends are studied and simulated. As a result, SM operation is obtained for 1550 nm by limiting the waveguide cross-section to 0.5 μm x 1 μm resulting in modal confinement factors of 87 %. In contrast, for shorter wavelengths as 541 nm SM propagation is excluded if the core height is not further decreased. Moreover, the obtained theoretical propagation losses for the lowestorder TE/TM mode are in the range of 0.3 - 1.3 dB/cm for an interface roughness of 1 nm. The lower silicon dioxide (SiO2) waveguide cladding should be at least 1 μm to avoid substrate radiations. These results are in a good correlation to the known values for common dielectric structures. In the case of bended waveguides, an idealized device with a radius of 10 μm was developed which shows a reflection minimum (S11 = - 22 dB) at 1550 nm resulting in almost perfect transmission of the signal. Additionally, tapered waveguides were investigated for an optimized light coupling between high-aspect-ratio devices. Here, adiabatic down-tapered waveguides were designed for the elimination of higher-order modes and perfect signal transmission. Secondly, fabrication lines including Electron-beam (E-beam) lithography and reactive ion etching (RIE) with an Aluminum (Al) mask were developed and lead to well fabricated optical devices in the (sub)micrometer range. The usage of focused ion beam (FIB) milling is invented for smoother front faces which were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). As a result, the anisotropy of the RIE process was increased, but the obtained surface roughness parameters are still too high (10 – 20 nm) demonstrating a more advanced lithography technique is needed for higher quality structures. Moreover, this study presents an alternative fabrication pathway for novel designed waveguides with free-edge overlapping endfaces for improving fiber-chipcoupling. Thirdly, the main focus lies on the development of a monolithic integration circuit consisting of the Si-based LED coupled to an integrated waveguide. The light propagation between high-aspect-ratio devices is enabled through low-loss adiabatic tapers. This study shows, that the usage of CMOS-related fabrication technologies result in a monolithic manufacturing pathway for the successful implementation of fully integrated Si-based photonic circuits. Fourth, transmission loss measurements of the fabricated photonic structures as well as the waveguide butt-coupled Si-based LEDs were performed with a generated setup. As a result, free-edge overlapping MM waveguides show propagation loss coefficients of ~ 65 dB/cm in the range of the telecommunication wavelength. The high surface roughness parameters (~ 150 nm) and the modal dispersion in the core are one of the key driving factors. These facts clearly underline the improvement potential of the used fabrication processes. However, electroluminescence (EL) measurements of waveguide butt-coupled Si-based LEDs due to the implanted rare earth (RE) ion (Tb3+, Er3+) and the host material (SiO2/SiNx) were carried out. The detected transmission spectra of the coupled Tb:SiO2 systems show a weak EL signal at the main transition line of the Tb3+-ion (538 nm). A second emission line was detected in the red region of the spectrum either corresponding to a further optical transition of Tb3+ or a Non Bridging Oxygen Hole Center (NBOHC) in SiO2. Unfortunately, no light emission in the infrared range was established for the Er3+-doped photonic circuits caused by the low external quantum efficiencies (EQE) of the Er3+ implanted Si-based LEDs. Nevertheless, transmission measurements between 450 nm – 800 nm lead again to the result that an emission at 650 nm is either caused by an optical transition of the Er3+-ion or initialized by the NBOHC in the host. Overall, it is difficult to assess whether or not these EL signals are generated from the implanted ions, thus detailed statements about the coupling efficiency between the LED and the integrated waveguide are quite inadequate. Nevertheless, the principle of a fully monolithically integrated photonic circuit consisting of a Si-based LED and a waveguide has been successfully proven in this study

EUROSENSORS XVII : book of abstracts

Fundação Calouste Gulbenkien (FCG).Fundação para a Ciência e a Tecnologia (FCT)