Researchers grow polar NbAlN films on GaN
Polar wurtzite nitride semiconductors, such as AlN and GaN are central to modern power and high-frequency devices because of their wide bandgaps, high breakdown electric fields, and strong spontaneous and piezoelectric polarisation.
At AlN/GaN heterointerfaces, differences in polarisation can create a two-dimensional electron gas (2DEG) with high carrier density and mobility without intentional doping. This 2DEG forms the operating basis of GaN HEMTs.
Across the broader family of wurtzite nitrides, polarisation underpins functions ranging from piezoelectric actuation to ferroelectric switching, creating an incentive to expand polar nitride materials compatible with GaN-based platforms. The discovery of ScAlN marked a major milestone .
Now in a new study, a team led by Atsushi Kobayashi, associate professor at the Department of Materials Science and Technology at Tokyo University of Science (TUS), Japan, together with researchers from the University of Tokyo and Mie University, has achieved the first successful epitaxial growth of single-crystalline polar wurtzite NbAlN thin films on GaN substrates.
The work 'Polar Wurtzite NbAlN: A Transition-Metal Nitride Alloy for Polarisation-Engineered GaN Heterostructures' is published in Advanced Materials (online publication date: September 4, 2026).
“This study demonstrates that NbAlN belongs to a previously unrecognised class of transition-metal-containing polar nitride semiconductors,” explains Kobayashi. “The NbAlN films preserve both the wurtzite crystal structure and the metal polarity of the underlying GaN.”
Using reactive sputter epitaxy, the researchers grew NbAlN films containing 11 percent to 37 percent Nb on GaN. Films containing up to 25 percent Nb maintained a smooth surface and a coherent wurtzite crystal structure aligned with GaN, whereas the sample with 37 percent Nb showed marked surface roughening and degraded crystal quality. X-ray diffraction further showed that the out-of-plane lattice parameter increased systematically with increasing Nb content.
To determine whether polarity survived Nb incorporation, the team examined the films using atomic-resolution scanning transmission electron microscopy. Observations of an approximately 25-nm-thick NbAlN film containing 23 percent Nb showed the same metal-polar stacking as the underlying GaN in the observed region. The researchers also found coherent Nb-rich nanoscale regions. Despite this compositional variation, the wurtzite lattice remained continuous without grain boundaries or a relaxed secondary phase.
To evaluate its functional impact, the researchers inserted a NbAlN barrier layer into an AlGaN/AlN/GaN reference heterostructure, forming an NbAlN/AlGaN/AlN/GaN stack. The optimised NbAlN heterostructure showed more than a threefold increase in sheet electron density while retaining room-temperature mobility. For a 13-nm-thick NbAlN barrier containing 10 percent Nb and grown at 775 °C, the sheet electron density increased from approximately 5.1 × 10¹² cm⁻² in the reference structure to 1.7 × 10¹³ cm⁻²—more than a threefold increase. The room-temperature electron mobility was 1,485 cm² V⁻¹ s⁻¹, compared with 1,690 cm² V⁻¹ s⁻¹ in the reference structure, and reached 6,800 cm² V⁻¹ s⁻¹ at 80 K. These results are consistent with polarisation-related carrier modulation introduced by the NbAlN barrier.
The results are relevant to researchers and manufacturers working on GaN materials and power/RF semiconductor devices. Future work will measure the polarisation constants and band alignment of NbAlN, examine interface and impurity effects in greater detail, and fabricate transistors to determine how the barrier translates into device performance.
“Because Nb tends to form metallic nitrides, incorporating it into the polar wurtzite structure of AlN is not straightforward. We confirmed at the atomic scale that Nb-rich nanoscale regions could be incorporated while preserving the continuous crystal lattice and the same polarity as GaN,” concludes Kobayashi. “This work establishes a new family of transition-metal-containing polar nitride semiconductors and expands the options for designing carrier density in GaN heterostructures.”
