Researchers observe 2D-magnesium-intercalated GaN superlattices

A study led by Nagoya University has revealed that a thermal reaction of gallium nitride (GaN) with metallic magnesium (Mg) results in the formation of a distinctive superlattice structure. This is reportedly the first time that researchers have identified the insertion of 2D metal layers into a bulk semiconductor. By observing the materials through various characterisation techniques, the researchers uncovered insights into the process of semiconductor doping and elastic strain engineering. The research findings were published in the journal Nature.

GaN is an important wide bandgap semiconductor material that could replace traditional silicon semiconductors in applications that require higher power density and faster operating frequencies. These distinctive characteristics of GaN make is suitable for devices such as LEDs, laser diodes and power electronics, including critical components in electric vehicles and fast chargers.

In semiconductors, there are two essential and complementary types of electrical conductivity: p-type and n-type. The p-type semiconductor features primarily free carriers carrying positive charges, known as holes, whereas the n-type semiconductor conducts electricity through 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 alter its electrical and optical properties.

In the field of GaN semiconductors, Mg is thought to be the only known element to create p-type conductivity. The full mechanisms of Mg doping in GaN, especially the solubility limit and segregation behaviour of Mg, remain unclear. This uncertainty has limited further development for optoelectronics and electronics.

To improve the conductivity of p-type GaN, Jia Wang, the first author of the study, conducted an experiment in which the researchers deposited patterned metallic Mg thin films onto GaN wafers and heated them up at a high temperature — a process known as annealing. Using electron microscope imaging, the scientists observed the spontaneous formation of a superlattice featuring alternative layers of GaN and Mg. This is unusual since GaN and Mg are two types of materials with significant differences in their physical properties.

Wang said that although GaN is a wide-bandgap semiconductor with mixed ionic and covalent bonding and Mg is a metal featuring metallic bonding, these two dissimilar materials have the same crystal structure. “It is a strikingly natural coincidence that the lattice difference between hexagonal GaN and hexagonal Mg is negligibly small. We think that the perfect lattice match between GaN and Mg greatly reduces the energy needed to create the structure, playing a critical role in the spontaneous formation of such a superlattice,” Wang said.

The researchers determined that this unique intercalation behaviour, which they named interstitial intercalation, leads to compressive strain to the host material. Specifically, they found that the GaN being inserted with Mg layers sustains a high stress of more than 20 GPa. This is much more than the compressive stresses commonly found in silicon films (in the range of 0.1 to 2 GPa). Electronic thin films can undergo significant changes in electronic and magnetic properties because of this strain. The researchers found that the electrical conductivity in GaN via hole transport was enhanced along the strained direction.

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

Image caption: Thermal treating of metallic magnesium on gallium nitride semiconductor results in the formation of a distinctive superlattice structure. Magnesium, nitrogen, gallium atoms are shown in orange, blue and grey, respectively. Image credit: Jia Wang, Nagoya University.