Epitaxy and characterization of InP/InGaAs tandem solar cells grown by MOVPE on InP and Si substrates

The integration of III-V multi-junction solar cells on Si substrates is currently one of the most promising possibilities to combine high photovoltaic performance with a reduction of the manufacturing costs. In this work, we propose a prospective study for the realization of an InP/InGaAs tandem solar cell lattice-matched to InP on a commercially available Si template by direct MOVPE growth. The InP top cell and the InGaAs bottom cell were firstly separately grown and optimized using InP substrates, which exhibited conversion efficiencies of 13.5% and 11.4%, respectively. The two devices were then combined in a tandem device by introducing an intermediate InP/AlInAs lattice-matched tunnel junction, showing an efficiency of 18.4%. As an intermediate step towards the realization of the tandem device on Si, the InP and InGaAs single junction solar cells were grown on top of a commercial InP/GaP/Si template. This transitional stage enabled to isolate and evaluate the effects of the growth of III-V on Si on the photovoltaic performance through the comparison with the aforementioned devices on InP. Each cell was electrically characterized by external quantum efficiency and dark and illuminated current-voltage under solar simulator. The material quality was also analyzed by means of X-ray diffraction, Atomic-Force Microscopy, Transmission Electron and Scanning Electron Microscopy. The III-V on Si devices showed efficiencies of 3.6% and 2.0% for the InP and InGaAs solar cells, respectively

Synthèse de films de diamant de haute qualité cristalline pour la réalisation de dosimètres pour la radiothérapie

Cette thèse vise à maitriser la synthèse MPCVD de films hétéroépitaxiés de diamant de haute qualité cristalline sur substrat d iridium pour la réalisation de dosimètres en radiothérapie. Cet objectif nous a conduits à élaborer la couche d iridium épitaxiée sur des substrats SrtiO3 (001). Un bâti sous vide équipé d un canon à électrons a donc été développé et calibré. Les couches obtenues ont été caractérisées par DRX et présentent une qualité structurale équivalente à l état de l art. Le procédé de nucléation (BEN) - MPCVD induit sur la surface de l iridium des domaines spécifiques à la nucléation du diamant sur iridium. Un travail important a été mené sur l optimisation du (BEN) - MPCVD de façon à obtenir un procédé fiable et reproductible pour obtenir des domaines homogènes sur une surface de 5x5mm2 d Ir/SrtiO3. Des études de caractérisation de surface (MEB, XPS, AES) des domaines nous ont permis de dresser leur carte d identité chimique et morphologique. Nous démontrons ainsi qu ils contiennent des nuclei de diamant. De plus, la propagation de ces domaines semble suivre des directions préférentielles [110] induites par l épitaxie de l iridium au cours du temps durant l étape de (BEN)-MPCVD. A partir de ces résultats, des films de diamant hétéroépitaxiés autosupportés de 100&#956-m ont été élaborés. La corrélation entre la qualité cristalline du diamant hétéroépitaxié et sa réponse en détection a été menée avec l équipe dosimétrie du LCD. Des inhomogénéités de la structure cristalline due à la présence de défauts structuraux ont été mises en évidence. Afin d étudier localement ces échantillons, une campagne de mesure par microfaisceau X a été réalisée sur la ligne Diffabs du Synchrotron Soleil. L assemblage des différentes connaissances acquises lors de cette thèse a permis de fabriquer et de caractériser un premier détecteur à base de diamant hétéroépitaxié au LCDThis thesis aims to master the MPCVD synthesis of heteroepitaxial diamond films of high crystalline quality on iridium substrate for radiotherapy dosimeters. This objective has led us to develop the epitaxial iridium layer grown on SrtiO3 substrates (001). A vacuum frame equipped with an electron gun has been developed and calibrated. The obtained layers characterized by XRD, possess a structural quality equivalent to the state of the art/in literature. Bias Enhanced Nucleation (BEN)- MPCVD induces nucleation of domains on the iridium surface, according a unique nucleation pathway. Significant work has been conducted on (BEN)-MPCVD optimization to obtain a reliable and reproducible method for generating homogeneous domains on a surface of 5x5mm2. Combined characterizations (SEM, XPS, AES) of domains surface enabled us to establish the identity card of their chemical and morphological properties. We demonstrate that they contain diamond nuclei. In addition, the temporal expansion of these domains seems to follow preferential directions of iridium lattice during the (BEN)-MPCVD stage. From these results, self-supported heteroepitaxial diamond films 100&#956-m thick have been grown. The correlation between their crystalline quality and their detection response was conducted with the LCD dosimeter team. The inhomogeneities in the crystal structure due to structural defects have been identified. To study more locally these samples, a measurement campaign was carried out by microbeam X on the DIFFABS line at Soleil Synchrotron. The combination of the different knowledges acquired during this thesis has allowed the fabrication and characterization of the first detector based on heteropitaxial diamond at the LCD laboratory.CACHAN-ENS (940162301) / SudocSudocFranceF

Rochelle Salt-Based Ferroelectric and Piezoelectric Composite Produced with Simple Additive Manufacturing Techniques

International audienceMore than one century ago, piezoelectricity and ferroelectricity were discovered using Rochelle salt crystals. Today, modern societies are invited to switch to a resilient and circular economic model. In this context, this work proposes a method to manufacture piezoelectric devices made from agro-resources such as tartaric acid and polylactide, thereby significantly reducing the energy budget without requiring any sophisticated equipment. These piezoelectric devices are manufactured by liquid-phase epitaxy-grown Rochelle salt (RS) crystals in a 3D-printed poly(Lactic acid) (PLA) matrix, which is an artificial squared mesh which mimics anatomy of natural wood. This composite material can easily be produced in any fablab with renewable materials and at low processing temperatures, which reduces the total energy consumed. Manufactured biodegradable samples are fully recyclable and have good piezoelectric properties without any poling step. The measured piezoelectric coefficients of manufactured samples are higher than many piezoelectric polymers such as PVDF-TrFE

Improving the understanding of the KPFM technique through analyses of InP multilayers with associated modelling

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Two-dimensional carrier profiling on InP:Zn MOVPE epitaxially grown layers by Kelvin Probe Force Microscopy with associated modelling

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Cross-sectional Kelvin probe force microscopy on III–V epitaxial multilayer stacks: challenges and perspectives

International audienceMultilayer III–V-based solar cells are complex devices consisting of many layers and interfaces. The study and the comprehension of the mechanisms that take place at the interfaces is crucial for efficiency improvement. In this work, we apply frequency-modulated Kelvin probe force microscopy under ambient conditions to investigate the capability of this technique for the analysis of an InP/GaInAs(P) multilayer stack. KPFM reveals a strong dependence on the local doping concentration, allowing for the detection of the surface potential of layers with a resolution as low as 20 nm. The analysis of the surface potential allowed for the identification of space charge regions and, thus, the presence of several junctions along the stack. Furthermore, a contrast enhancement in the surface potential image was observed when KPFM was performed under illumination, which is analysed in terms of the reduction of surface band bending induced by surface defects by photogenerated carrier distributions. The analysis of the KPFM data was assisted by means of theoretical modelling simulating the energy bands profile and KPFM measurements

Revealing of InP multi-layer stacks from KPFM measurements in the dark and under illumination

Solar cells are complex devices, being constituted of many layers and interfaces. The study and the comprehension of the mechanisms that take place at the interfaces is crucial for efficiency improvement. This paper applies Kelvin probe force microscopy (KPFM) to study materials and interfaces with nanometer scale imaging of the surface potential in the dark and under illumination. KPFM measurements are highly sensitive to surface states and to the experimental measurement environment influencing the atomic probe operating conditions. Therefore, in order to develop a quantitative understanding of KPFM measurements, we have prepared a dedicated structured sample with alternating layers of InP:S and InP:Fe whose doping densities were determined by secondary-ion mass spectroscopy. We have performed KPFM measurements and shown that we can spatially resolve 20 nm thick InP layers, notably when performed under illumination which is well-known to reduce the surface band-bending

Integrated lasers on silicon for optical communications

We review our work on integrated lasers for optical communications. An InP-based multilayer stack containing Al-based quantum wells with optical gain in the telecom window is bonded onto a silicon-on-insulator wafer with patterned photonic circuits and cavities. Ring-based widely tunable lasers and narrow linewidth DFB lasers are demonstrated

As-Grown InGaAsN Subcells for Multijunction Solar Cells by Molecular Beam Epitaxy

International audienceIn this article, we investigate the molecular beam epitaxy growth of unannealed 1.12 eV InGaAsN solar cells. The impact of the growth temperature, the As/III ratio and the bismuth used as a surfactant is reported. An in-situ curvature measurement setup enables to monitor and ensure a constant N incorporation during the InGaAsN growth. Ex-situ characterization results suggest that a high As/III ratio ensures good optoelectronic properties and that the growth temperature has a strong influence on the residual doping of the dilute nitride layer. Under AM0> 870 nm and without anti-reflection coatings, our best InGaAsN solar cells exhibit Jsc and Voc values of 7.94 mA/cm 2 and 0.375 V respectively. Considering no internal reflection and no grid shading, generation up to 12 mA/cm² in a multijunction solar cell can be expected, which is the highest value ever reported for as-grown InGaAsN cells to our knowledge