Optimisation des performances des piézo-générateurs à base de nanofils III-Nitrures par ajustement de leur dopage

par Tanbir Sodhi

Projet de thèse en Science des Matériaux

Sous la direction de Noelle Lebeau gogneau et de Frédéric Houzé.

Thèses en préparation à Paris Saclay , dans le cadre de Electrical, Optical, Bio: PHYSICS_AND_ENGINEERING , en partenariat avec Centre de Nanosciences et de Nanotechnologies (laboratoire) et de Université Paris-Sud (établissement de préparation de la thèse) depuis le 01-10-2018 .


  • Résumé

    Les dispositifs électroniques sont aujourd'hui omniprésents. Dans le contexte actuel de la diminution de leur consommation énergétique, et de ce fait de la réduction de notre empreinte écologique, la question de leur autonomie constitue un enjeu majeur. L'utilisation des énergies renouvelables devient également aujourd'hui essentielle. Cependant, le développement de nano-systèmes auto-alimentés performants implique de redimensionner et de repenser les sources d'énergie. Celles-ci doivent être intégrables, fournir durablement de l'énergie directement aux dispositifs tout en fonctionnant de manière indépendante, et surtout générer une puissance suffisante pour remplacer les batteries existantes. Les piezo-générateurs (PGs) à base de nanofils (NFs) piézoélectriques sont récemment apparu comme une solution très prometteuse. De par leurs propriétés spécifiques (grande flexibilité et résistance à la fracture, hauts coefficients piézoélectriques, grande sensibilité aux faibles forces appliquées), les NFs présentent des qualités indéniables pour convertir une énergie mécanique en énergie électrique. Les énergies mécaniques sont issues de sources renouvelables présentes dans l'environnement immédiat du PG et sont aussi variées que les mouvements du corps, les vibrations, les mouvements hydrauliques, le vent, la friction ... La génération d'énergie électrique à partir de la déformation de NFs de ZnO a été démontrée en 2006. Depuis, ce concept a été validé à partir de NFs de CdS, CdSe, PZT ou BaTiO3. La fabrication de PGs, essentiellement à base de NFs de ZnO, a été démontrée, ainsi que leur utilisation pour alimenter des capteurs chimiques et biologiques et des écrans LCD. Ces dernières années, les nanofils III-Nitrures sont explorés en raison de leur forte réponse piézoélectrique. Ainsi, notre équipe a démontré la forte capacité de conversion piézoélectrique des NFs de GaN (jusqu'à 440 mV par NF, résultat à l'état de l'art), qui est bien plus élevée que celle des autres matériaux 1D piézoélectriques (tels que les nanofils de ZnO). Nous avons établi la relation entre les propriétés des NFs et le mécanisme de conversion piézoélectrique, ainsi que l'impact des nanocontacts sur l'efficacité de récupération des énergies piézo-générées. Enfin, nous avons conçu un générateur piézoélectrique intégrant des NFs de GaN capable de générer une densité de puissance de 12,7 mW/cm3 (nouvel état de l'art pour les générateurs à base de NFs III-N). Cette densité de puissance offre déjà des applications possibles dans le monde réel avec, par exemple, l'alimentation d'émetteurs-récepteurs sans fil à distance. Malgré les importantes recherches menées sur les nanomatériaux et les récents développements de PGs à base de NFs, l'amélioration de la densité de puissance et de l'efficacité de conversion tout en minimisant la taille des PGs reste l'objectif essentiel poursuivi par la communauté scientifique internationale. L'augmentation des performances des générateurs requiert aujourd'hui de tenir compte de la résistance interne des NFs intégrés et donc de leur dopage. En effet, les porteurs libres sont connus pour écranter les charges piézoélectriques et ainsi détériorer la réponse des générateurs. Malgré l'importance de cet effet, la concentration des porteurs est souvent négligée dans la conception des piézo-générateurs. Dans ce contexte, l'objectif de cette thèse est de développer une nouvelle génération de piézo-générateurs en tenant compte des effets du dopage des nanofils. Le but est d'établir la relation existante entre la concentration des porteurs libres dans les nanofils et leurs propriétés de piézo-conversion. En établissant une telle relation, nous intégrerons des nanofils de GaN capables d'exalter la conversion d'énergie mécanique en énergie électrique et ainsi d'améliorer la conversion piézoélectrique des générateurs. Pour ce faire, nous nous baserons sur une approche multi-échelle allant de la caractérisation des propriétés piézoélectriques de NFs uniques jusqu'à la fabrication et la caractérisation de micro-dispositifs piézo-électriques à base de NFs.

  • Titre traduit

    Optimization of the III-Nitride nanowire based piezo-generators by tuning the dopant concentration


  • Résumé

    Scientific context and objectives: The electronic devices are today ubiquitous. In the context of the decreasing of their consumption, and thus of the decreasing of our ecological footprint, the question concerning their autonomy is a key challenge. The use of renewable energies is also today essential. However, the development of efficient self-powered nano-systems requires developing new energy sources. They must be integrable, sustainably supply energy directly to the devices while operating independently, and generate sufficient power to replace existing batteries. These last years, the piezoelectric nanowires (NWs) have emerged as a promising solution. In fact, due to their specific properties (high flexibility and resistance to fracture, high piezoelectric coefficients and high sensitivity to small applied forces), NWs present the potential to achieve high mechanical-electric conversion. The mechanical energy comes from the renewable sources present in the direct vicinity of the generator such as body movements, sound vibrations, hydraulic movements, wind, friction… The first demonstration of piezo-conversion based on ZnO NWs has been published in 2006. Since this first proof of concept, several 1D-nanostructures have been studied, such as CdS, CdSe, PZT or BaTiO3. NW-based piezo-generators (essentially integrating ZnO NWs) have been successfully developed with notably the demonstration of the feasibility to use these energy sources to supply chemical and biological sensors and LCD screens. Within the last few years, the III-Nitrides nanowires have been explored due to their higher piezoelectric response. Hence, in our team, we have demonstrated the high piezo-conversion capability of GaN NWs (up to 440 mV per NW, state of the art), which largely exceeds the one of other 1D piezoelectric nanostructures (e.g. ZnO NWs). We have established the relation between the NW properties and the piezo-conversion mechanism and evidenced the impact of the Schottky nanocontact on the piezoelectric energy harvesting. Finally, we have designed GaN-NW based piezo-generator delivering a power density of 12.7 mW/cm3 (new state of the art for III-Nitride piezo-generators). This power density already offers possible real world applications, such as the powering of remote wireless transceivers. However, in spite of the recent researches on nanomaterials and the spectacular development of NW-based generators, the improvement of the energy conversion efficiency and of the performance of the final device still remains the key objectives pursued by the international community. The increase in the generator performances requires taking into account the internal resistance of the NWs and therefore their doping. Indeed, the free carriers are known to screen the piezoelectric charges and thus decrease the response of the generators. Despite the importance of this effect, the concentration of carriers is often neglected in the design of piezo-generators. The objective of the PhD project is to develop a new generation of piezo-generators by considering the influence of the NW doping. The aim is to establish the close relationship between the doping level of the NWs and their piezo-conversion properties. By understanding this relation, we will integrate optimized GaN NWs having the ability to exalt the mechanical-electrical energy conversion, and thus develop high-efficient piezoelectric generator devices. PhD objectives: To develop a new generation of piezo-generators by considering the influence of the NW doping, the PhD project is based on three main points: 1- The characterization of the electrical properties of the NWs, i.e. the doping level and the piezo-conversion properties. n- and p-doped GaN NWs with different level of doping will be considered. 2- The establishment of the close relationship between the concentration of free carriers of the NWs and their piezo-conversion properties. Due to the nanometric dimensions of the NWs (diameter less than 100 nm) and the presence of strong surface charges, the distribution and concentration of free carriers are modulated inside the NWs. In fact, the surface charges induce the formation of a depletion region at the NW surface, the thickness of which depends on the doping level. Thus, according to the diameter and the doping level of the NW, this latter is more or less depleted, then inducing a specific distribution and concentration of the free carriers. The relation between the NW doping and their piezo-conversion properties will be established by considering these effects of dimension and free carrier concentration. 3- The best combinations in terms of NW doping, diameter and piezo-conversion capacity will be validated through the fabrication and characterization of piezo-generator devices. Methodology: To reach the PhD objectives, we will use a multi-scale approach ranging from the characterization of the piezoelectric properties of unique NWs to the fabrication and characterization of NW-based piezoelectric generators. The PhD project is organized under 4 axis: 1- The growth of GaN NWs: The candidate will synthesize the GaN NWs on Si(111) substrates by plasma-assisted molecular beam epitaxy (PA-MBE). Thanks to its low growth rate and ultra-high vacuum conditions, the PA-MBE is the technique of choice to produce nanostructures with a perfect control of their morphology, structuration and doping. With regard to the doping of the NWs, the candidate will study the growth conditions in the presence of Si (n-type doping) and Mg (p-type doping), the efficiency of the dopant incorporation and its impact on the quality of the nanostructures. 2- Quantification of the conversion of mechanical energy into electrical energy: The candidate will characterize the piezoelectric conversion properties of the NWs as a function of their characteristics (diameter and doping), by using an atomic force microscope (AFM) equipped with a Resiscope module specifically adapted to perform piezoelectric measurements on single NWs. The doping level will also be quantified by AFM electric measurements. 3- Understanding of the piezoelectric mechanisms in play in the NWs: The candidate will establish the relation between the piezo-conversion properties of the NWs and their dopant and diameter characteristics. The experimental measurements will be discussed and compared with the theoretical approach, using COMSOL simulation, realized by co-workers from CEA-Grenoble, France (Dr. J. Eymery). 4- Fabrication and testing of piezo-generator devices: After establishing the best combination between the NW characteristics and their piezo-conversion properties, the candidate will validate its choice by integrating the optimized NWs into piezo-generator devices. These latert will be tested under compression, traction, flexion and friction. Context: The project is built on a long-term collaboration existing between the C2N-CNRS and GeePs-CentraleSupelec. The project is based on a consortium, which gathers all necessary complementary skills in materials sciences, low-level electrical measurements and physic characterization at nanoscale, surface structuration engineering and device processing and testing. The partners of this consortium, with their world-class know-how in these different fields, have all the scientific and technological expertise necessary to supervise the candidate on this ambitious and motivating project. The Center for Nanosciences and Nanotechnologies (C2N-CNRS), is a joint unit between CNRS and University Paris-Saclay, which hosts one of the largest nanofabrication facilities in France. Its strong research activity in instrumentation establishes C2N as a key international state-of-the-art actor in the development of new advanced tools for nanotechnologies. The Material Dept. has developed a strong expertise in the growth of III-V and III-N nanostructures by molecular beam epitaxy. The group has also an important know-how on the characterization of nano-objects notably by using atomic force microscope for morphological and electrical characterization, X-Ray diffraction, and Transmission Electron Microscope. The Photonic Dept. has developed since 2006 a strong expertise in the field of NW devices, namely NW detectors and light emitting diodes. Finally, the MicroSystem Dept. has more than ten-years of experience on small-scale piezoelectric and pyroelectric materials, with current works conducting on piezoelectric and electrostatic MEMS for Energy Harvesting. The Laboratoire de Génie électrique et électronique de Paris (GeePs-) is a joint research unit (CNRS, CentraleSupelec, UPMC, U-PSud) whose research activity is in the field of physics and electrical engineering. The PHEMADIC team has a strong expertise on electrical characterization at a microscopic scale with the development of additional capabilities for atomic force microscopy, in order to get local resistance measurements (Resiscope module). The team has also developed an important experience both in characterization and analytical and numerical modelling of materials and interfaces. Within the project, GeePs-CentraleSupelec will provide nanoscale electrical characterizations to study the piezoelectric performance of nanowires. Based on this strong collaboration between the C2N-CNRS and GeePs-CentraleSupelec, we have established several important results in the field of NW-based piezo-generators (Cf. scientific part).