Polar wurtzite nitride semiconductors, such as aluminum nitride (AlN) and gallium nitride (GaN), are central to modern high-power and high-frequency electronic devices because of their wide band gaps, high breakdown electric fields, and strong spontaneous and piezoelectric polarization. At AlN/GaN heterointerfaces, differences in polarization 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 high-electron-mobility transistors (HEMTs). Across the broader family of wurtzite nitrides, polarization underpins functions ranging from conventional piezoelectric actuation to ferroelectric switching.
An important objective in this field is to expand the family of polar nitride materials compatible with GaN-based platforms. The discovery of scandium aluminum nitride (ScAlN) marked a major milestone and stimulated broader exploration of polar wurtzite alloys. Niobium (Nb) is an intriguing but challenging alloying element: it is a transition metal with partially filled d orbitals and usually forms metallic rock-salt NbN. Nb is widely available and compatible with sputter deposition. It was unclear whether Nb could be incorporated into AlN while preserving a long-range wurtzite crystal structure, epitaxial quality, and a uniform polarity.
In a new study, a collaborative research team led by Associate Professor Atsushi Kobayashi of 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, achieved the first successful epitaxial growth of single-crystalline polar wurtzite NbAlN thin films on GaN substrates. "This study demonstrates that NbAlN belongs to a previously unrecognized class of transition-metal-containing polar nitride semiconductors," explains Dr. Kobayashi. "The NbAlN films preserve both the wurtzite crystal structure and the metal polarity of the underlying GaN." The study is scheduled for publication in Advanced Materials (online publication date: September 4, 2026).
Using reactive sputter epitaxy, the researchers grew NbAlN films containing 11% to 37% Nb on GaN. Films containing up to 25% Nb maintained a smooth surface and a coherent wurtzite crystal structure aligned with GaN, whereas the sample containing 37% 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% 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 optimized 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% 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 polarization-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 polarization 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 Dr. 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."
