Innovative ternary alloy films pave the way for ultra-low-power memory devices

A recent study reports (Al,Ga,Sc)N thin films with record-high scandium levels, with exciting potential for ultra-low-power memory devices, as reported by researchers from Institute of Science Tokyo (Science Tokyo). Using reactive magnetron sputtering, they fine-tuned the composition of ternary alloys to overcome previous stability limits.

Beyond enabling efficient data storage, these films also show promise for noise filters for 6G communications and optical computing, thanks to attractive piezoelectric and optoelectric properties.

Electronic devices are becoming smaller yet more capable than ever, creating a demand for memory technologies that can store more data while consuming less power. Non-volatile ferroelectric memories have emerged as a promising solution to this issue. By maintaining an intrinsic electrical polarization, these devices can retain stored information without requiring constant electrical power—thus extending battery life and enabling more sophisticated mobile computing.

Gallium nitride (GaN) and aluminum nitride (AlN), materials already used in LEDs, possess unique crystal structures where positive and negative charge centers are naturally displaced. This displacement creates a switchable polarization that can be controlled by an applied external voltage, forming the foundation for non-volatile memory functionalities.

Scientists know that incorporating scandium (Sc) into these crystal structures could significantly reduce operating voltages and enable ultra-low power operation. However, increasing the concentration of Sc has proven extremely challenging due to fundamental limitations in stability for both GaN and AlN.

Against this backdrop, a research team led by Professor Hiroshi Funakubo from Institute of Science Tokyo (Science Tokyo), Japan, has achieved a significant breakthrough by successfully synthesizing (Al,Ga,Sc)N thin films with unprecedentedly high Sc concentrations. Their work, published online in APL Materials, demonstrates that alloying AlN with GaN in the right proportion can significantly expand the amount of Sc that can be incorporated into the final crystal structure.

First, the researchers employed reactive magnetron sputtering, a physical vapor deposition technique, to deposit thin films of (Al,Ga,Sc)N with carefully controlled compositions onto platinum- and titanium-coated silicon substrates. Through meticulous adjustment of the sputtering parameters and target power, they synthesized a diverse range of ternary alloys with different proportions of each element.

These films were then rigorously characterized using advanced techniques such as X-ray diffraction to determine their crystal structure, electron microscopy to examine their microstructure, and electrical measurements to evaluate their ferroelectric and dielectric properties. Their systematic approach enabled them to map out the so-called “phase diagram” of the AlN–GaN–ScN system, revealing a new region for the ferroelectrically active wurtzite crystal structure at higher Sc contents when a small fraction of gallium was present.

An important outcome of this research was the significant reduction in the material’s coercive field (E_c)—the electric field needed to switch polarization—that came with increasing Sc content in the ternary alloys. The team observed a remarkable decrease in E_c from 5.8 MV/cm to 1.8 MV/cm as the Sc ratio grew larger.

“This E_c value is much lower than most reported values in previous works for various dopants of AlN- and GaN-based wurtzite films, which is very promising for the development of memory devices,” remarks Funakubo. Further analysis of the results revealed that an entropy effect could be responsible for this effect.

Notably, the achieved voltage reduction could translate directly to lower power consumption in memory devices, addressing one of the most pressing challenges in modern electronics. Beyond memory applications, these novel ferroelectric films also exhibited superior piezoelectric and optoelectric properties.

“These properties open up potential applications in high-frequency noise filters and ultra-low-power optical computing systems, which are necessary for next-generation 6G smartphones and in optical computing devices that operate with ultra-low power,” says Funakubo.

Overall, the combination of reduced operating voltages, enhanced functional properties, and compatibility with existing semiconductor processing techniques makes (Al,Ga,Sc)N films promising materials for next-generation electronics.