Electromechanical study of semiconductor piezoelectric nanowires. Application to mechanical sensors and energy harvesters

Orateur : Soutenance de Thèse de Ronan HINCHET

PhD Thesis : Smart systems are the combined result of different advances in microelectronics leading to an increase in computing power, lower energy consumption, the addition of new features, means of communication and especially its integration and application into our daily lives. The evolution of the field of smart systems is promising, and the expectations are high in many fields : Industry, transport, infrastructure and environment monitoring as well as housing, consumer electronics, health care services but also defense and space applications. Nowadays, the integration of more and more functions in smart system is leading to a looming energy crisis where the autonomy of such smart systems is beginning to be the main issue. Therefore there is a growing need of autonomous sensors and power sources. Developing energy harvesters and self-powered sensors is one way to address this energy issue. Among the technologies studied, piezoelectricity has the advantage to be compatible with the MEMS industry, it generates high voltages and it has a high direct coupling between the mechanic and electric physics. Among the piezoelectric materials, semiconductor piezoelectric nanowires (NWS) could be a promising option as they exhibit improved piezoelectric properties and higher flexibility. Among the different piezoelectric NWs, ZnO and GaN NWs are the most studied, their properties increase at the nanoscale by more than 2. They have the advantage of being IC compatible and reasonably synthesizable by top-down and bottom-up approaches. Especially we studied the hydrothermal growth of ZnO NWs. In order to use them we studied the behavior of ZnO NWs. We performed a basic analytical study and FEM simulations of a ZnO NW under bending. This study explains the piezoelectric potential distribution as a function of the force and exposes the scaling rules. Then we develop the mechanical AFM characterization of the young modulus of ZnO and GaN NWs. Following we perform piezoelectric AFM characterization of these NWs, verifying the behavior under bending stresses. Once their physics understood, we discuss the limits of our piezoelectric NWs models and a more realistic model is developed, closer to the experimental configurations. Using this model we evaluated the ZnO NW f or force and displacement sensors by measuring the potential generated, and from experiments, the use of GaN NW for force sensor by measuring the current through the NW. But energy harvesting is also necessary to address the looming energy crisis and we deeper investigate this solution. To fully understand the problematic we study the history of nanogenerator (NG) and their potential architectures. We analyze their advantages and disadvantages in order to define a reference NG structure. After a basic analytical study of this structure, understanding its operation and challenges, we perform multiple FEM simulations to defining optimization routs for a NG working in compression or in bending. The fabrication of prototypes and theirs preliminary characterization is presented.

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