Text update on polarization in gallium nitride to optimize wide bandgap semiconductors

An updated model reconciles the gap between recent experiments and theory about polarization in wurtzite semiconductors—paving the way for the development of smaller, faster, and more efficient electronic devices, according to a recent study by University of Michigan researchers .

The second most widely produced semiconductor after silicon, gallium nitride is now widely used in LED lighting and high-power, high-frequency electronics. The material is expected to transform the next generation of mobile phones and communication systems, and the polarization underpins its stellar electronic performance.

“Gallium nitride semiconductors are already ubiquitous in our daily lives, and the impact will continue to grow,” said Zetian Mi, a UM professor of electrical and computer engineering and senior author of the study published in Applied Physics Letters.

Here, the research team focused on the crystal structure of gallium nitride wurtzite – the phase most commonly used for electronic and optoelectronic devices. The hexagonal lattice formation of the crystal lacks inversion symmetry, which promotes spontaneous polarization and when mechanical strain is applied, known as piezoelectric polarization.

Both types of polarization, and more importantly, the resulting polarization gradient at the interface, can be exploited to optimize the electronic properties of semiconductor devices.

Until recently, polarization in gallium nitride and other wurtzite materials was understood only through theoretical modeling. Experiments then revealed that the spontaneous polarization is about 10 times larger and in the opposite direction compared to what previous theory suggested.

An incorrect reference frame was the root of the large discrepancy between theory and experiments. The previous theory used zinc alloy as a reference structure, but when it was replaced with a hexagonal reference structure, experiments and theory agreed very well.

“The previous theory chose an inadequate ruler to measure the polarization, which caused them to get incomplete results. Finding the right ruler, Professor Chris Van de Walle at the University of California Santa Barbara obtained drastically different theoretical results in 2016 , which have now been experimentally confirmed by us as well as others,” said Danhao Wang, a researcher in electrical and computer engineering at UM and co-corresponding author of the study.

The researchers arrived at the new standard by sifting through the literature and correlating the findings with experimental studies that directly measured ferroelectricity—the spontaneous polarization that can be restored when an external electric field is applied—in single-crystal III-nitride ferroelectric semiconductors.

Previously, separate communities of researchers studied ferroelectricity and III-nitride materials—boron, aluminum, gallium, or indium combined with nitrogen—and designed applications for those properties in insulation. Mi’s research group recently demonstrated, for the first time, ferroelectric switching in single crystal nitric semiconductors.

“By combining the physics and properties of III-nitride materials and ferroelectricity, we can develop the next generation of electronics and optoelectronics with higher power, capacity and speed to better support our world,” said Ding Wang, an assistant scientist research in the field of electricity and informatics. engineering at UM and co-corresponding author of the study.

These studies provide new direction and insights into electronic or optoelectronic devices based on gallium nitride.

“Beyond electronics and optoelectronics, this new understanding of polarization is an important resource to develop new nitride-based materials and devices for future clean energy catalysis as well as quantum research and technology,” said Mi.