In a striking natural coincidence, researchers discover that gallium nitride and magnesium form a superlattice when heated

A study led by Nagoya University in Japan found that a simple thermal reaction of gallium nitride (GaN) with metallic magnesium (Mg) results in the formation of a distinctive superlattice structure. This is the first time researchers have identified the insertion of 2D metal layers into a bulk semiconductor.

By carefully observing the materials using various advanced characterization techniques, the researchers discovered new insights into the process of semiconductor doping and elastic strain engineering. They published their findings in the journal Nature.

GaN is an important wide bandgap semiconductor material that is poised to replace traditional silicon semiconductors in applications requiring higher power densities and faster operating frequencies. These distinguishing characteristics of GaN make it valuable in devices such as LEDs, laser diodes and power electronics, including critical components in electric vehicles and fast chargers. The improved performance of GaN-based devices contributes to the realization of an energy-saving society and a carbon-neutral future.

In semiconductors, there are two essential and complementary types of electrical conductivity: p-type and n-type. The p-type semiconductor mainly has free carriers that carry positive charges, also called holes, while the n-type semiconductor conducts electricity via free electrons.

A semiconductor acquires p-type or n-type conductivity through a process called doping, which refers to the intentional introduction of specific impurities (known as dopants) into a pure semiconductor material to greatly alter its electrical and optical properties.

In the field of GaN semiconductors, Mg is so far the only known element that creates p-type conductivity. Despite 35 years since the first success of doping Mg into GaN, the full mechanisms of Mg doping into GaN, especially the solubility limit and segregation behavior of Mg, remain unclear. This uncertainty limits their optimization for optoelectronics and electronics.

To improve the conductivity of p-type GaN, Jia Wang, the first author of the study, and his colleagues conducted an experiment in which they deposited deposited metal Mg thin films on GaN wafers and heated them to a high temperature – a conventional process. known as glowing.

Using state-of-the-art electron microscope images, the scientists observed the spontaneous formation of a superlattice with alternating layers of GaN and Mg. This is especially unusual because GaN and Mg are two types of materials with significant differences in their physical properties.

“Although GaN is a wide bandgap semiconductor with mixed ionic and covalent bonds, and Mg is a metal with metallic bonds, these two dissimilar materials have the same crystal structure, and it is a striking natural coincidence that the lattice difference between hexagonal GaN and hexagonal Mg is negligibly small,” Wang said.

“We think that the perfect lattice match between GaN and Mg significantly reduces the energy required to create the structure, and plays a crucial role in the spontaneous formation of such a superlattice.”

The researchers determined that this unique intercalation behavior, which they called interstitial intercalation, leads to compressive loading on the host material. Specifically, they found that the GaN introduced with Mg layers undergoes a high stress of more than 20 GPa, equivalent to 200,000 times atmospheric pressure, making it the highest compressive stress ever measured in a thin-film material. This is much more than the compressive stresses commonly encountered in silicon films (in the range of 0.1 to 2 GPa).

Electronic thin films can undergo significant changes in electronic and magnetic properties due to this voltage. The researchers found that the electrical conductivity in GaN was significantly improved in the strained direction via hole transport.

“With such a simple and cheap approach, we could improve the transport of holes in GaN, which conducts more current,” Wang said. “This interesting finding in interactions between a semiconductor and a metal could provide new insights into semiconductor doping and improve the performance of GaN-based devices.”

University of Nagoya and GaN

The fact that this research took place at Nagoya University is appropriate, given the university’s reputation as the “cradle of GaN technology.” Hiroshi Amano, the corresponding author of the current study, and Isamu Akasaki of Nagoya University developed the first blue-light LEDs in the late 1980s, using Mg-doped GaN. Their contributions, for which they received the Nobel Prize in Physics in 2014, have played an important role in creating a more energy-efficient society.

“The discovery of Mg-intercalated GaN superlattice structures and the identification of the novel mechanism of 2D Mg doping provide a hard-earned opportunity to honor the pioneering achievements in III-nitride semiconductor research,” said Wang. Having developed the technology a decade after the Nobel Prize, Wang called this timely discovery a “true gift of nature” that could potentially open new paths and inspire more fundamental research in the field.

Authors of this study from Nagoya University included Jia Wang, Wentao Cai, Shun Lu, Emi Kano, Biplab Sarkar, Hirotaka Watanabe, Nobuyuki Ikarashi, Yoshio Honda and Hiroshi Amano. In addition to Nagoya University, other contributing authors of this study include researchers from Meijo University and an optical group led by Professor Makoto Nakajima at Osaka University.