Gallium nitride (GaN) has attracted attention for its excellent electrical conductivity, outperforming silicon by multiple measures when used as a semiconductor in devices. However, concerns about its electrical and structural reliability have prevented GaN-based devices from reaching their full potential and commercialization. 

Although combining GaN with AlGaN into a multi-layered semiconductor (AlGaN/GaN) can further improve electrical mobility, this dual material also suffers from inherent and use-related stress due to mismatches in the lattice spacing and differences in how the two materials expand when heated, in addition to other defects which gradually degrade this material’s electrical properties under a high electric field. These defects produce stresses in AlGaN/GaN’s crystal that can eventually cause catastrophic failure. 

Crystallographic defects in semiconductor materials and devices are typically mitigated with thermal annealing, a process in which a material is heated above its recrystallization temperature and then cooled. However, thermal annealing can also degrade multi-layer, multi-material semiconductors through a variety of different mechanisms. Seeking a different way to heal crystallographic defects, researchers from Penn State University and Argonne National Laboratory tested a low-temperature annealing technique utilizing electron wind force (EWF) to “push” defects out of the material. 

Evaluation using the 34-ID-E beamline at the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, showed that EWF is a viable, low-temperature way to reduce defects in degraded AlGaN/GaN devices and partially recover electrical performance.

Using commercially available AlGaN/GaN high electron mobility transistors (HEMTs), the researchers intentionally degraded these devices in the OFF-state condition by applying a gate voltage of -5V and a drain voltage of 5V for 48 hours. They then generated an EWF by applying a high density pulsed current with a low duty cycle for one minute. The researchers tested the electrical properties of the HEMTs while they were pristine, when they were degraded, and after they were annealed with EMF. 

To determine the effects on the AlGaN/GaN crystal structure, they also assessed the devices during these three states with the 34-ID-E beamline using differential aperture X-ray microscopy (DAXM). This technique combines a focused X-ray beam and a differential aperture to isolate diffraction from sub-mm³ volumes to determine local crystallographic orientation, lattice plane spacing, and structural phases. Here, DAXM was used to probe residual stresses in the HEMTs’ GaN layer. Although EWF has been reported to reduce defects in irradiated GaN HEMTs, thin film transistors, two-dimensional field effect transistors, and thin films, it had not been evaluated on electrically degraded GaN HEMTs.

Compared to the pristine devices, analysis of electrical properties showed that electrical degradation increased ON-state resistance by ~182% and reduced drain saturation current to ~86% at a gate voltage of 0 V. Annealing with EWF partially recovered ON-state resistance to ~122% and drain saturation current to ~93%. Peak transconductance, degraded to ~76.58% of the pristine material at the drain voltage of 3 V, returned to ~92% after EWF annealing. 

These changes suggested that EWF annealing induced partial healing of degradation-related crystallographic defects. Using DAXM, the researchers confirmed this hypothesis. Their results showed that EWF annealing reduced lattice plane spacing of (001) planes and stress within the GaN layer, particularly under the gate region, suggesting some mitigation of defects.

Tests showed that the maximum temperature reached during EWF annealing was 30.1 °C, confirming that this approach is truly low-temperature. Additionally, because the induced degradation of the GaN HEMTs was irreversible under light illumination and even after one week of resting at room temperature, the study authors suggest that the recovery of electrical function in these devices was not due to spontaneous room-temperature annealing due to de-trapping of temporarily trapped electrons. Rather, the EWF itself was responsible for removing lattice defects.

They note that this strategy could be used to heal crystallographic defects in a variety of other multi-layer, multi-material microelectronic devices that cannot be annealed under high heat and might even be applied while a device is in use.  – Christy Brownlee

See: N.S. Al-Mamun¹, D. Sheyfer², W. Liu², A. Haque¹, D. E. Wolfe³, D.C. Pagan³, “Low-temperature recovery of OFF-state stress induced degradation of AlGaN/GaN high electron mobility transistors,” Appl. Phys. Lett. 124, 013507 (January 2024)

Author affiliations: ¹Department of Mechanical Engineering, Penn State University; ²Argonne National Laboratory; ³Department of Materials Science and Engineering, Penn State University.

This work was funded by the Defense Threat Reduction Agency (DTRA) as a part of the Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA) under Contract No. HDTRA1-20-2-0002. A.H. also acknowledges support from the U.S. National Science Foundation (ECCS No. 2015795). The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

The U.S. Department of Energy's APS at Argonne National Laboratory is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries and solve more vital biological protein structures than users of any other x-ray light source research facility. APS x-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC, for the U.S. DOE Office of Science.

The U.S. Department of Energy's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit the Office of Science website.