This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 uniform polarity. In a new study, a collaborative research team led by Dr. Atsushi Kobayashi, an associate professor in 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," Kobayashi explains. "The NbAlN films preserve both the wurtzite crystal structure and the metal polarity of the underlying GaN." The study was published in Advanced Materials. 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.
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