Refined sputtering method could enable large-scale production of Scandium thin films for GaN transistors with higher carrier densities.
Researchers at the Tokyo University of Science have shown that high-temperature sputtering can produce scandium aluminium nitride (ScAlN) thin films suitable for gallium nitride (GaN) high electron mobility transistors (HEMTs).
Previous fabrication techniques such as Physical Vapour Deposition (PVD) offered limited crystalline quality, while molecular beam epitaxy achieved strong results but at slow throughput rates. ScAlN growth is difficult at production scale. Sputtering has been fast but lower quality, while molecular beam epitaxy produces high-quality films too slowly for commercial use.
GaN HEMTs rely on a Two-Dimensional Electron Gas (2DEG) at the GaN/AlGaN interface for high carrier mobility. ScAlN barriers can raise electron density in the 2DEG through high polarisation and ferroelectric properties, enabling adjustable control over electron flow. This may support new functionalities in GaN-based devices.
The Tokyo team used 10% scandium films on AlGaN-AlN-GaN heterostructures, applying atomic force microscopy, electron diffraction, and Hall-effect measurements. Film quality improved above 250°C deposition temperatures, with 750°C samples showing step-and-terrace surface features linked to high crystalline quality.
At 750°C, carrier density reached 1.1×10¹³ cm⁻², roughly three times higher than without ScAlN. Electron mobility was 900 cm²/V/s, lower than the 1,677 cm²/V/s of the reference due to roughness and defects introduced by the ScAlN barrier.
The findings indicate that sputtering, a common electronics manufacturing method, can be adapted for ScAlN at high temperatures. This could enable lower-cost, higher-density GaN HEMTs, though further work is needed to raise mobility performance to current benchmarks.
