Ferroelectric semiconductors, a new class of chips, can store information in electric fields and could enable computers that run on less power, sensors with quantum precision, and the conversion of signals between electrical, optical and acoustic forms.
However, chips are prone to falling apart under these conditions, and how ferroelectric semiconductors maintain two opposite electric polarisations in the same material has remained a mystery until now.
A team led by engineers at the University of Michigan has now discovered the reason why the materials, called wurtzite ferroelectric nitrides, don’t tear themselves apart.
Zetian Mi, the Pallab K. Bhattacharya Collegiate Professor of Engineering and co-corresponding author, explained: “The wurtzite ferroelectric nitrides were recently discovered and have a broad range of applications in memory electronics, RF (radio frequency) electronics, acousto-electronics, microelectromechanical systems and quantum photonics, to name just a few.
“But the underlying mechanism of ferroelectric switching and charge compensation has remained elusive. How is the material stabilised? It was largely unknown.”
Polarisation changes in ferroelectric semiconductors
Often, it’s not the whole material that switches polarisation. Instead, it’s divided into domains of the original polarisation and the reversed polarisation.
Where these domains meet, and especially where two positive ends come together, researchers didn’t understand why the repulsion didn’t create a physical break in ferroelectric semiconductors.
“In principle, the polarisation discontinuity is not stable,” said Danhao Wang, U-M postdoctoral researcher in electrical and computer engineering and co-corresponding author of the study.
“Those interfaces have a unique atomic arrangement that has never been observed before. What’s even more exciting is that we observed that this structure may be suitable for conductive channels in future transistors.”
The glue that holds the bonds together
With experimental studies, the team found that there is an atomic-scale break in the material; however, that break creates the glue that holds it together.
At the horizontal joint, where the two positive ends meet, the crystal structure is fractured, creating a bunch of dangling bonds.
These bonds contain negatively charged electrons that perfectly balance the excess positive charge at the edge of each domain within the ferroelectric semiconductors.
Emmanouil Kioupakis, U-M professor of materials science and engineering and corresponding author, commented: “It’s a simple and elegant result – an abrupt polarisation change would typically create harmful defects, but in this case, the resulting broken bonds provide precisely the charge needed to stabilise the material.”
“This makes it a universal stabilising mechanism in all ferroelectrics – a class of materials that’s rapidly gaining attention for its potential in next-generation microelectronic devices.”
Supporting high currents with gallium nitride
With electron microscopy, the team discovered that the atomic structure of the ferroelectric semiconductors was made up of scandium gallium nitride.
Where the domains met, the usual hexagonal crystal structure was buckled over several atomic layers, creating the broken bonds. The microscopy showed that the layers were closer together than normal, but density functional theory calculations were needed to reveal the dangling bond structure.
In addition to holding the material together, the electrons in the dangling bonds create an adjustable superhighway for electricity along the joint, with about 100 times more charge carriers than in a normal gallium nitride transistor.
That highway can be turned off and on, moved within the material, and made more or less conductive by reversing, moving, strengthening or weakening the electrical field that sets the polarisation.
The team immediately noticed its potential as a field-effect transistor that could support high currents and be good for high-power and high-frequency electronics. This is what they plan to build next.
