Surface Charge Effects: An Advantage for Enhancing The Piezoelectric Conversion Efficiency in GaN NWs

For powering micro-devices with strong dimension constraints (smaller than 1 cm²or 1 cm³), it is crucial to develop ultra-compact piezoelectric nanogenerators characterized with a high conversion efficiency per surface/volume unit. To reach this objective, we must both improve the electromechanical conversion efficiency of the active material and optimize the generator design to maximize the piezoelectric system performances in response to the environmental mechanical and vibration inputs.<br/>1D-nanostructures have emerged as system of interest for developing ultra-compact piezoelectric generators. This interest lies in their quasi-lattice perfection and large surface-to-volume ratio, which confer to NWs larger degree of elastic deformation without mechanical deterioration and higher sensitivity to applied forces in comparison with their 2D-film counterparts and conventional bulk piezoelectric materials. In addition, sub-100 nm-wide NWs present the particularity to exhibit specific properties that can lead to a strong modulation of their characteristics. Among these “new properties”, we can cite the exaltation of the piezoelectric coefficients, the formation of nano-contact at the NW/electrode interface allowing an enhanced energy harvesting, or the modulation of the free carrier concentration due to the surface charge (SC) effects. Regarding this last property, simulations have recently established that these SC effects can be advantageous for improving the piezoelectric response of the NWs, since they can limit, in given conditions, the screening by the free carriers of the piezoelectric charges.<br/>Since the expression of the SC effects depends on the NW dimensions and on the NW environment (in other terms of the soft matrix embedding them into nanogenerator devices), the in-depth understanding of the relationship between the SC effects and the piezoelectric conversion capacities of the NWs is now a prerequisite for further improving the device performances and thus approaching a future technological transfer.<br/>To investigate the SC effects in sub-100 nm-wide GaN NWs, we use a systematic multi-scale analysis going from the growth of the GaN NWs by plasma-assisted Molecular beam epitaxy and their possible post-treatment for modulating their surface trap density, to the fabrication and testing of piezoelectric nanogenerators, passing through the characterization of single NWs using nano-characterization tools based on atomic force microscope equipped with electrical modules.<br/>We experimentally highlight that the electromechanical coupling coefficient of GaN NWs is strongly affected by the expression of the SC. By finely adjusting the NW dimensions, we demonstrate that the SC effects can be advantageous for strongly improving the electromechanical conversion efficiency of GaN NWs up to 43%. We also establish that the piezoelectric response of the GaN NWs is sensitive to their direct environment. We confirm experimentally that the SC are useful for improving the piezo-conversion, and evidence that by properly engineering the GaN NWs surfaces, the piezoelectric response of the GaN NW-based nanogenerators can be enhanced. Finally, by architecting the nanogenerator design with regard to the deformation mode (direct compression or vibration) and the targeted application, we demonstrate generated power density in the µW-mW/cm³ range in response to mechanical inputs miming the environmental ones (equivalent forces of few Newton in the 1-200 Hz frequency range).