How can the Surface Charge Effects be Advantageous for Enhancing the Piezo-Conversion Efficiency in GaN NWs?

The piezoelectric nanowires (NWs) are considered as promising nanomaterials to develop ultra-compact and high-efficient piezoelectric nanogenerators working under environmental mechanical deformations and vibrations (equivalent forces of few Newton in the 1-200 Hz frequency range) for supplying micro-sensors and medical implants. Especially sub-100 nm-wide NWs present the particularity to exhibit specific properties, non-existing or non-significant at micrometric scales, that can lead to a strong modulation/modification 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. In-depth understanding of the relationship between these new properties 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 quantify experimentally the influence of the SC on the piezoelectric response of the GaN NWs, we have developed a new advanced nano-characterization tool based on AFM probing the piezo-conversion properties of NWs axially compressed with a well-controlled applied force. This unique technique, in addition to mimic the main deformations undergone by the nanostructures when they are integrated into a device, allow to measure simultaneously and thus correlate the output voltage generated by each NW of the array and its NW deformation rate. In other words, we can quantify the electromechanical coupling coefficient of each tested GaN NWs, one of the figures of merit of the piezoelectricity.<br/>With this new system, we have measured the electromechanical coupling coefficient of GaN NWs characterized by an equivalent height and a diameter evolving between 20 and 110 nm. We established, for NW presenting the same compression degree, that the coupling coefficient is strongly affected by the surface charge effects. For large diameter (60 nm in the intrinsic doping conditions of our NWs), the SC have no effect on the piezo-conversion and thus this one is only controlled by the NW mechanical characteristics. By contrast, for lower diameters, the SC effects become favorable for piezo-conversion due to the strong reduction of screening effect of the piezo-charges into the fully depleted NWs. Thus, we observe an enhancement of the piezoelectric response of the NWs. Especially, we demonstrate that by finely architecting the NWs dimension, the SC effects can be advantageous for strongly improving the electromechanical conversion efficiency of GaN NWs up to 43,2%.