The joint research group led by Prof. Atsushi Kobayashi in Tokyo University of Science, and the University of Tokyo and Mie University has achieved the first successful fabrication of niobium aluminum nitride (NbAlN), an Aluminum Nitride (AlN)-based semiconductor containing the transition metal niobium (Nb), as a single-crystal polar wurtzite thin film on Gallium Nitride (GaN).
Polar wurtzite nitride semiconductors, typified by AlN and GaN, combine a high breakdown field with strong polarization, and serve as fundamental materials for high-power and high-frequency electronic devices. It is known that adding elements such as scandium (Sc) to AlN can substantially modify its lattice constants and polarization-related properties. However, it has remained difficult to incorporate transition metals, which favor metallic bonding, while preserving both the wurtzite crystal structure and a uniformly oriented polarity.
In this work, NbAlN thin films were epitaxially grown on GaN by reactive sputtering epitaxy. Atomic-resolution annular bright-field scanning transmission electron microscopy (ABF-STEM) confirmed that an approximately 25-nm-thick NbAlN film containing 23% Nb maintains the same metal-polar stacking as GaN throughout the regions observed. The observations further revealed that the wurtzite lattice remains continuous and uninterrupted even where Nb-rich nanoscale regions are present.
Moreover, in an NbAlN/AlGaN/AlN/GaN heterostructure incorporating NbAlN as a barrier layer, the density of the two-dimensional electron gas (2DEG) at the AlN/GaN interface increased more than threefold, from approximately 5.1×10¹² cm⁻² in the structure without NbAlN to 1.7×10¹³ cm⁻². This is consistent with polarization-related carrier modulation induced by the metal-polar NbAlN barrier.
These results extend the range of material choices toward polar nitrides containing transition metals, and present a new option for engineering the 2DEG carrier density in GaN-based heterostructures. The findings are scheduled to be published in the international journal Advanced Materials.
Schematic illustration of the crystal structure of polar wurtzite NbAlN
Papers
Journal: Advanced Materials
Title: Polar Wurtzite NbAlN: A Transition-Metal Nitride Alloy for Polarization-Engineered GaN Heterostructures
Authors: Atsushi Kobayashi, Souta Kurogi, Tomoya Okuda, Riku Kikuchi, Yusuke Wakamoto, Kouei Kubota, Hikaru Sasaki, Kazuhisa Ikeda, Kanako Shojiki, Takahiro Kawamura, Toru Akiyama, Takehito Seki, Takuya Maeda
DOI: https://doi.org/10.1002/adma.74756