New NbAlN Semiconductor Boosts Electron Density in GaN Electronics

Researchers from the Tokyo University of Science, the University of Tokyo, and Mie University have successfully grown single-crystalline polar wurtzite niobium aluminum nitride (NbAlN) thin films. This development introduces a new material capable of enhancing the electronic properties of gallium nitride (GaN) platforms, which are critical for next-generation power electronics.

Using reactive sputter epitaxy, the team incorporated niobium into the wurtzite crystal structure of aluminum nitride (AlN) while growing the films on GaN substrates. The researchers found that NbAlN films containing up to 25% niobium maintained a coherent wurtzite crystal structure and a smooth surface. However, quality degraded and surface roughening occurred when niobium concentrations reached 37%. The resulting films preserve both the wurtzite structure and the metal polarity of the underlying GaN substrate.

The Role of Polar Nitrides

Polar wurtzite nitrides, such as AlN and GaN, are foundational to high-frequency and high-power electronics due to their wide band gaps and strong polarization. This polarization is used to create a two-dimensional electron gas (2DEG), the conductive layer that enables high-electron-mobility transistors (HEMTs) to operate efficiently. While scandium aluminum nitride (ScAlN) has previously been used to expand this material family, incorporating other transition metals has proven difficult. Niobium, in particular, typically forms metallic rock-salt structures rather than the semiconductor structures required for these applications.

Impact on Electron Density

The ability to integrate niobium into a polar semiconductor allows for precise modulation of carrier density. In a practical test, the researchers utilized a 13-nm-thick NbAlN barrier containing 10% niobium. This configuration increased the sheet electron density from 5.1 × 10¹² cm⁻² to 1.7 × 10¹³ cm⁻², representing a more than threefold increase over standard configurations. This suggests that NbAlN can be used to significantly amplify the conductivity of the 2DEG in GaN-based heterostructures.

Future Implications

This discovery expands the available library of polar nitride materials compatible with existing GaN technology. By providing a new method to control electronic properties, the material could lead to the development of more efficient radio-frequency (RF) and power semiconductor devices. NbAlN belongs to a previously unrecognized class of transition-metal-containing polar nitride semiconductors, opening the door for further exploration of similar materials to optimize semiconductor performance.