“In the case of GaN we can reach several hundred mV per nanowire – for ZnO nanowires the maximum output is between 45 and 80mV per nanowire,” says Noelle Gogneau, a researcher at the Centre for Nanosciences and Nanotechnology - Marcoussis Campus. Despite this apparent advantage, so far GaN studies have focused on LED and laser applications and far less attention has been paid to their potential as piezoelectric generators compared with their ZnO counterparts.

Gogneau and colleagues at CNRS Université Paris-Saclay and Université Paris-Sud now report a multi-scale analysis of the piezoelectric response of GaN structures, from single nanowires to full arrays integrated into macroscopic devices. “Because we have understood the piezoelectric mechanism at play we can design a much better device,” says Gogneau. “Our device has an output density of 12.7mV/cm3 which is a new state of the art for nitride nanogenerators and opens the way for new ways of powering electronic devices, such as medical implants.”

What is more, the usual methods for growing GaN nanowires produce structures with an N-polarity, so that a positive potential is generated under compression. This is the opposite of ZnO nanostructures, where the polarity is that of zinc, the metal. The choice between the antiparallel polarities that the two materials offer can be very useful for optimising device design for specific applications depending on the compressive or tensile strain being harvested.

Comparative difficulties

As a relatively young field of research, reports of the piezoelectric response of nanowires require particularly rigorous analysis. “People do not use the same units so it is very difficult to compare the results,” says Gogneau. “Also, we are measuring the piezo-generated power through a load resistance and people don’t always use the same load resistance.” Gogneau and colleagues used an atomic force microscope to subject the nanowires to well controlled and monitored forces, so that they could measure the resulting potentials produced.

GaN nanowires offer an additional complication because they are much softer than ZnO. As a result they bend to the substrate or into other nanowires in the array under quite low loads, which then causes charge leakage and inhibits the piezoelectric response. To arm the structures against this the team embedded the array of nanowires in hydrogen silsesquioxane (HSQ) leaving just the top portion uncovered so that they would not bend. They also introduced a layer of AlN on the silicon (111) substrate so that a SiN layer did not form, as this is very insulating and diminishes the efficiency of the device.

Next they plan to look at ways of improving the piezoelectric conversion capacity of the nanowires and improve the efficiency of the conversion. This may involve optimising the device design or engineering the active materials themselves. Narrower nanowires for example are known to have a higher piezoelectric coefficient.

Full details are reported in a Nantoechnology article, which also features in Nanotechnology Select.