Caractérisation et fabrication des cellules solaires à base d’InGaN

Ce projet a pour ambition de concevoir et de réaliser une nouvelle filière de cellule photovoltaïque utilisant la conversion directe de l’énergie solaire en électricité à base de la filière InGaN permettant d'atteindre un rendement de 50% de conversion directe de l’énergie solaire en électricité. Cette nouvelle approche constitue un défi technologique majeur pour la recherche académique et les applications industrielles dans les années avenir. Les cellules solaires actuelles à base de Silicium approchent leur limite théorique de rendement de conversion d’énergie (environ 25%). Les cellules solaires multi-jonctions permettent de repousser ces limites en empilant plusieurs matériaux possédant différentes énergies de bande interdite, chacun absorbant une petite portion du spectre solaire de manière plus efficace. Alors que les LEDs violettes et bleu à base du matériau InGaN sont déjà commercialisées, il apparaît essentiel de relever le défi qui consiste à fabriquer et utiliser ce matériau InGaN avec de fort taux d’Indium (i.e. des énergies de bandes interdites plus faibles) afin de couvrir l’ensemble du spectre solaire et ainsi réaliser des cellules photovoltaïques à très haut rendement, bien au-delà de l’état de l’art international. Au vu des limitations des cellules au silicium, des travaux théoriques ont montrés que des cellules à jonctions multiples à base de couches absorbantes d’InGaN permettraient d’atteindre un rendement de 50%. L’amélioration du rendement des cellules solaires aura un impact majeur sur de nombreuses applications. L’objectif de ce travail concerne la conception et de réalisation d'une nouvelle génération de cellule solaire à base d’InGaN. Ce travail concerne dans une première phase : la caractérisation du matériau InGaN à fort taux d’Indium (> 20%) élaboré à l'EPFL en collaboration avec l'IEMN ayant pour but de démontrer une énergie de bande interdite en dessous de 2 eV. Dans une seconde phase, après la validation électrique et structurelle de ce nouveau matériau, il s’agit de concevoir et de réaliser une nouvelle génération de cellule solaire mono-jonction sur saphir et sur substrat GaN. Cette nouvelle cellule solaire pourra être intégrée au sein d’une microsource d’énergie pour réseau de capteur autonome.This PhD thesis reports on the structural and optical characterization of solar cell structures with various active region designs and different substrates as well as the subsequent fabrication and electrical characterization of InGaN solar cells. The epitaxial growth of solar cell designs with pGaN/i-InGaN/n-GaN structures were performed by metal-organic vapor phase epitaxy (MOCVD) by the company NovaGaN. The structural and optical characterization is assessed by X-Ray diffraction, scanning transmission electron microscopy, atomic force microscopy and photoluminescence spectroscopy. A structural comparison of solar cell designs including bulk 200 nm thick InGaN layer and InGaN/GaN multiple quantum wells (MQWs) with similar indium compositions (~30%) is presented. Furthermore, structural quality of designs with InGaN/GaN MQWs were analyzed with variation of the indium content, thickness of InGaN quantum wells and type of the substrate, i.e. (0001) sapphire or bulk GaN substrate. An optimized and reproducible processing has been developed for fabrication of InGaN based solar cells. The challenges in device processing such as mesa etching of GaN and contamination on the device sidewalls, which caused high reverse leakage currents were studied and solutions of using SiO2 mask and protection of sidewalls by SiO2 layers were proposed. An optimization study of thermal treatment of Ni/Au current spreading layer is also presented. The electrical activity in the active region and the spectral response of the solar cells are investigated by electron beam induced current (EBIC) analysis and external quantum efficiency measurements. EBIC analysis is used to clarify the origin of the S-shape behavior in illuminated current-voltage characteristics of the solar cell with 25×In0.15Ga0.85N/GaN MQWs, which has performed the best performance in this study with a conversion efficiency of 0.59% under 1sun illumination (AM1.5G)

GaN-on-silicon high-electron-mobility transistor technology with ultra-low leakage up to 3000 V using local substrate removal and AlN ultra-wide bandgap

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Substrate grounded GaN-on-Si HEMTs with record vertical breakdown above 2 kV

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High Electron Confinement under High Electric Field in RF GaN-on-Silicon HEMTs

We report on AlN/GaN high electron mobility transistors grown on silicon substrate with highly optimized electron confinement under a high electric field. The fabricated short devices (sub-10-nm barrier thickness with a gate length of 120 nm) using gate-to-drain distances below 2 µm deliver a unique breakdown field close to 100 V/µm while offering high frequency performance. The low leakage current well below 1 µA/mm is achieved without using any gate dielectrics which typically degrade both the frequency performance and the device reliability. This achievement is mainly attributed to the optimization of material design and processing quality and paves the way for millimeter-wave devices operating at drain biases above 40 V, which would be only limited by the thermal dissipation

Low RF losses up to 110 GHz in GaN-on-silicon HEMTs

International audienceWe report on low RF losses at the interface between the epitaxial structure and the silicon substrate (less than 0.8 dB/mm up to 110GHz) of AlN/GaN high electron mobility transistors (HEMTs) grown on silicon substrate. This stateof-the-art performance makes GaN-on-Silicon HEMTs competitive with GaN-on-SiC in terms of parasitic RF losses. Furthermore, a maximum dc output current close to 1 A/mm together with low leakage current of 1 μA/mm and low trapping effects are achieved while using a short gate length of 0.2 μm. The large signal measurements confirmed the high quality of the epitaxy and the device processing as well as the low parasitic RF losses. This is reflected by a high output power density of 4.5 W/mm achieved at 18 GHz

Degradation of InGaN-based MQW photodetectors under 405 nm laser excitation

International audienceWithin this paper we analyze the reliability of 25x multi quantum well InGaN-based devices, designed to be used as high power photodetectors or in multi-junction solar cells. Under stress with monochromatic excitation at 405 nm by means of a laser diode, we detect degradation at optical power levels significantly above the common AM1.5 spectrum; the main degradation modes are a reduction in short circuit current and in open circuit voltage, and an improvement in fill factor. The analysis of the wavelength-dependent EQE highlights, as a consequence of stress, the increase in concentration of a deep level compatible with the yellow luminescence in gallium nitride, suggesting that gallium vacancies may play a role in the degradation of the detectors

Degradation of InGaN-based solar cells under monochromatic photoexcitation

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Degradation of InGaN-based MQW solar cells under 405 nm laser excitation

International audienceWithin this paper we analyze the reliability of 25 × multi quantum well InGaN-based devices, designed to be used as high power photodetectors or in multi-junction solar cells. Under stress with monochromatic excitation at 405 nm by means of a laser diode, we detect degradation at optical power levels significantly above the common AM1.5 spectrum; the main degradation modes are a reduction in short circuit current and in open circuit voltage, and an improvement in fill factor. The analysis of the wavelength-dependent EQE highlights, as a consequence of stress, the increase in concentration of a deep level compatible with the yellow luminescence in gallium nitride, suggesting that gallium vacancies may play a role in the degradation of the detectors

Photon-driven degradation processes in GaN-based optoelectronic devices

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