The surface charge effects: A route to the enhancement of the piezoelectric conversion efficiency in GaN nanowires

Nanowires (NWs) have emerged as a system of interest for developing ultra-compact piezoelectric nanogenerators. In addition to their quasi-crystalline perfection and large surface-to-volume ratio, which confer them a large degree of elastic deformation and high sensitivity to applied forces, sub-100 nm-wide NWs present the particularity to exhibit specific nanometer scale properties leading to a strong modulation of their characteristics. Among these “new properties”, we can cite the modulation of the free carrier concentration due to the surface charge (SC) effects. Regarding this last property, simulations have recently established that these SCs can improve the piezoelectric response of the NWs. The in-depth understanding of the relationship between the SCs and the NW piezoelectric conversion capacities is thus now a prerequisite. In this overview, we investigate the impact of the surface in sub-100 nm-wide GaN NWs, as a function of their diameter and direct environment - two characteristics known to strongly modulate the SC influence. By using a unique advanced nano-characterization tool derived from AFM equipped with a modified Resiscope module to quantify the piezo-conversion properties of NWs, we experimentally confirm that the SCs are useful for improving the piezo-response. By adjusting the NW dimensions and/or their direct environment to take advantage of the SCs, we demonstrate average outputs up to 528 mV generated per GaN NW and strongly improved electromechanical conversion efficiency, up to 43 %. We thus highlight the importance of the proper engineering of GaN NW surfaces, allowing to maximize the piezoelectric response of the GaN NW-based nanogenerators.