A surprisingly natural coincidence: Researchers discover that heating gallium nitride and magnesium forms a superlattice

A study led by Nagoya University in Japan has 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 that researchers have identified the insertion of 2D metal layers in a massive semiconductor.

By carefully observing materials through various cutting-edge characterization techniques, researchers have 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 density and faster operating frequencies. These distinctive 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-efficient 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 primarily has free carriers carrying positive charges, called holes, while the N-type semiconductor conducts electricity through free electrons.

A semiconductor acquires P- or N-type conductivity through a process called doping, which refers to the intentional introduction of specific impurities (called dopants) into a pure semiconductor material to significantly change its electrical and optical properties .

In the field of GaN semiconductors, Mg is so far the only element known to create p-type conductivity. Despite 35 years since the first successful doping of Mg into GaN, the complete mechanisms of Mg doping into GaN, particularly 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 patterned thin films of metallic Mg deposited on GaN wafers and heated them to high temperature, a conventional process. known as annealing.

Using cutting-edge electron microscope imaging, scientists observed the spontaneous formation of a superlattice comprising alternating layers of GaN and Mg. This is particularly unusual since 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 a metallic bond, these two different materials have the same crystal structure, and it is a surprisingly natural coincidence that the lattice difference between hexagonal GaN and hexagonal magnesium is negligible,” said Wang.

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

The researchers determined that this unique intercalation behavior, which they called interstitial intercalation, results in compressive stress on the host material. Specifically, they found that GaN embedded in layers of Mg experiences a high stress of more than 20 GPa, equivalent to 200,000 times atmospheric pressure, making it the highest compressive stress ever recorded in a material. thin layer. This is much higher than the compressive stresses commonly encountered in silicon films (of the order of 0.1 to 2 GPa).

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

“Using such a simple and inexpensive approach, we were able to improve hole transport in GaN, which conducts more current,” Wang said. “This exciting discovery about interactions between a semiconductor and a metal could provide new insights into semiconductor doping and improve the performance of GaN-based devices.”

Nagoya University and GaN

The fact that this study took place at Nagoya University is entirely appropriate, given its reputation as the “birthplace 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 magnesium-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 GaN superlattice structures intercalated with Mg and the identification of the novel 2D-Mg doping mechanism provide a hard-won opportunity to honor pioneering achievements in the field of nitride semiconductor research III,” Wang said. Having advanced the technology 10 years after winning the Nobel Prize, Wang called the timely discovery a “true gift of nature” that could potentially open new avenues and inspire more fundamental research in the field.

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