AlN Nanowire-Based Vertically Integrated Piezoelectric Nanogenerators

In this study, a detailed analysis of the direct piezo-response of AlN nanowire-based vertically integrated nanogenerators (VINGs) is undertaken as a function of mechanical excitation frequency. We show that the piezo-charge, piezo-voltage, and impedance measured at the same position of the devices can be directly correlated through an equivalent circuit model in the whole frequency range of investigation. Our presented results are utilized to determine the performance figures of merit (FoM) of nanowire-based VINGs, namely, the piezoelectric voltage constant (g) for sensing and the product d · g for energy harvesting, where d is the piezoelectric charge constant. By comparison of these metrics with those of freestanding single-crystal GaN and quartz substrates as well as sputtered AlN thin films, we suggest that the nanowires can outperform their rigid counterparts in terms of mechanical sensing and energy generation. This work provides experimental guidelines for understanding the direct piezo-characteristics of VINGs and facilitates a quantitative comparison between nanostructured piezoelectric devices fabricated by using different materials or architectures.