Low-cost route to next-gen GaN transistors?
GaN HEMTs, widely used in wireless comms, power switching devices, and power amplifiers, owe their excellent high-frequency performance to a heterojunction (typically based on GaN and AlGaN) to create a narrow region called the 2D electron gas (2DEG), where electrons have very high mobility.
In recent years, Scandium aluminum nitride (ScAlN) has attracted attention as a novel barrier material that could further enhance the performance of GaN HEMTs.
ScAlN exhibits large polarisation, which increases electron densities in the 2DEG. Additionally, its ferroelectric nature makes it suitable for use as a ferroelectric gate material in ferroelectric HEMTs. Such a gate enables dynamic control over the 2DEG, offering potential to diversify the functionality of GaN-based devices.
Conventional methods to grow ScAlN layers on GaN require complex techniques and high processing temperatures. In contrast, sputtering is a promising alternative, offering a minimal setup with the possibility of low-temperature processing. So far studies on sputtering-based growth of ScAlN on GaN remain limited, and the influence of growth temperature on its electrical and structural properties remains unclear.
In a new study, just published in APL Materials, a research team led by Atsushi Kobayashi from Tokyo University of Science (TUS), Japan, successfully grew ScAlN thin films on AlGaN/GaN heterostructures using sputtering and studied the effect of growth temperature on their properties.
"Compared to expensive and complex deposition techniques, sputtering, widely used in electronics manufacturing, can enable the mass production of ScAlN thin films at much lower costs, making high-performance devices more accessible," explains Kobayashi. The team also included Shunsuke Ota, a 2024 graduate of TUS.
The researchers epitaxially grew ScAlN films with 10 percent scandium content on AlGaN/AlN/GaN heterostructures through sputtering at varying temperatures. They then studied the structure of the films using atomic force microscopy (AFM) and high-energy electron diffraction.
Their analysis revealed that epitaxial growth was achieved even at a low temperature of 250 °C, and surface flatness of the films improved with increasing temperature. Notably, a clear step-and-terrace surface structure was observed for the sample grown at 750 °C, indicating high structural quality.
Hall-effect measurements revealed that the carrier density in the 2DEG for the sample grown at 750 °C reached 1.1 × 1013 cm−2, approximately three times that of AlGaN/AlN/GaN heterostructures without ScAlN. In contrast, films grown at lower temperatures exhibited lower carrier densities than those of the initial non-ScAlN heterostructures.
These results demonstrate that growth temperature is critical for 2DEG formation within the heterostructure. The improvement in carrier density at 750 °C was attributed to improved structural quality. However, the electron mobility in all ScAlN samples was reduced, compared to the initial heterostructure, likely due to the roughness and structural imperfections introduced by the ScAlN barrier near the AlGaN/AlN/GaN interfaces.
"The findings of this study highlight the key role of growth conditions in the sputter epitaxy of ScAlN films," says Kobayashi. "Importantly, our study demonstrates the viability of sputtering for growing high-quality ScAlN layers on GaN, offering a practical path towards the commercialisation of high-performance GaN HEMTs with ScAlN barriers. This will lead to the widespread use of these transistors, which are fundamental to the development of highly efficient, energy-saving devices that can operate under harsh conditions, including electric vehicles and space vehicles, leading to the development of a sustainable future."
The team hopes that this research can lead to large-scale adoption of ScAlN-based GaN devices, which will, in turn, enable the development of next-generation high-performance electronics for communications, energy systems, and beyond.
