  In this study, 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
using different materials or architectures.
