Impact of dimensionality, donor concentration, and surface charges on the piezoelectric response of ZnO nanowires grown by MOCVD

Defense of doctoral thesis by Lisa LEGARDINIER, for the  University  Grenoble Alpes, speciality  " NANO ELECTRONIC & NANO TECHNOLOGIES "

Abstract :
As an abundant and sustainable semiconductor with relatively high piezoelectric coefficients, ZnO nanowires (NWs) have emerged as a promising material for piezoelectric applications. ZnO NWs can be synthesized by a wide variety of chemical and physical deposition techniques, showing their distinctiveness in the family of semiconducting NWs. Among these methods, particular attention has been drawn to the pulsed-liquid injection metal-organic chemical vapor deposition (MOCVD) technique, standing out as a rapid growth method compatible with large surface area, while still growing ZnO NWs with great structural and optical quality. Although MOCVD has extensively been explored for growing ZnO NWs for optoelectronic devices, its potential for piezoelectric applications remains widely opened. Notably, the NW morphology achieved by MOCVD has demonstrated superior piezoelectric performances compared to thin films. However, using finite element method simulation, it has been further revealed that the dimensions of the NWs could be of critical importance to enhance their piezoelectric performance. Moreover, the control of the screening effect, by modifying both the concentration of free carriers responsible for it, and the amount of surface traps compensating for it, is critical in piezoelectric applications.

This work investigates the enhancement of piezoelectric properties of ZnO NWs grown by MOCVD through several strategies. The initial approach entails an investigation of the impact of dimensionality on the enhancement of piezoelectric properties. This involves the optimization of NW dimensions, including diameter and length, across a wide range of values by varying the growth time. The influence of various post-growth treatments, such as oxygen plasma, UV ozone and thermal annealing, is also examined to determine their impact on a multitude of parameters. Finally, the development of core-shell structures with Ga2O3 acting as a passivating layer is studied.

Jury members :

• Dr.N.Gogneau, C2N, CNRS, Palaiseau (France) : Rapporteur
• Dr.V.SalletGemac, CNRS, Versailles (France) : Rapporteur
• Prof.B.GautierINL, INSA Lyon, Villeurbanne (France) : Examiner
• Prof.J.Pernot, Institut Néel, UGA, Grenoble (France) : Examiner
• Dr.V.Consonni, LMGP, CNRS, Grenoble (France) : Supervisor
• Dr.G.ArdilaCROMA, UGA, Grenoble (France) :  Co-Supervisor