Research Bits: Aug. 27

Ammonia-free GaN

Researchers from Nagoya University discovered a way to grow gallium nitride (GaN) semiconductors without using ammonia. The process is both more environmentally friendly and allows for high-quality growth of crystals at a lower cost.

Metal organic chemical vapor deposition (MOCVD) is the most common technique for GaN production, which uses ammonia (NH3) gas as the source of nitrogen added to the Ga. However, the efficiency is low, said Arun Dhasiyan of Nagoya University’s Center for Low-temperature Plasma Sciences, in a release. “As a result, a large consumption of ammonia is necessary. Ammonia is a very toxic and corrosive gas if inhaled that can cause severe damage to the eyes, skin, and respiratory system. Because it is so toxic, much of the ammonia has to be detoxicated and disposed of unused, requiring a lot of energy. This is a large part, up to half, of the total production cost.”

The team’s radical-enhanced metalorganic chemical vapor deposition (REMOCVD) technique uses hydrogen and nitrogen gas with a very high-frequency power source (100 MHz) to produce activated nitrogen.

“Our method, known as the REMOCVD method, solves three problems of GaN growth,” Dhasiyan explained in a release. “It enables the growth of GaN semiconductors at lower temperatures (about 800°C compared to over 1150°C); it uses nitrogen and hydrogen gas instead of ammonia gas, saving on raw materials and lowering the production cost; and also cuts out the extraction of nitrogen from ammonia step, making the process faster at lower temperatures.”

In addition to GaN, the researchers have used REMOCVD to grow aluminum nitride, indium nitride, and aluminum indium nitride layers. They have also installed a 300-mm diameter REMOCVD system to test large-scale production. [1]

AlYN/GaN heterostructures

Researchers from Fraunhofer IAF fabricated aluminum yttrium nitride (AlYN) using metal organic chemical vapor deposition (MOCVD). Previously, AlYN had only been deposited by magnetron sputtering.

“AlYN is a material that enables increased performance while minimizing energy consumption, paving the way for innovations in electronics that our digitally connected society and its ever-increasing technology demands urgently need,” said Stefano Leone, scientist at Fraunhofer IAF in the field of epitaxy, in a statement.

Of particular interest to the researchers was AlYN’s adaptability to gallium nitride (GaN) thanks to its wurtzite crystal structure. They fabricated AlYN/GaN heterostructures with a precisely adjustable yttrium concentration of up to 16%, which gives it promising electrical properties, according to Leone. “We were able to observe impressive values for sheet resistance, electron density and electron mobility. These results showed us the potential of AlYN for high-frequency and high-performance electronics.”

Material characterization results showed that AlYN can be used in high electron mobility transistors (HEMTs), with a significant increase in electron mobility at low temperatures (more than 3000 cm2/Vs at 7 K). AlYN’s ferroelectric properties also make it suitable for the development of non-volatile memory. [2]

Monitoring nuclear reactors

Researchers from Oak Ridge National Laboratory and the Ohio State University built gallium nitride (GaN) transistors that maintained operations near the core of a nuclear reactor. The high radiation resistance means the wide bandgap semiconductor could bring electronics closer to the sensors that monitor nuclear equipment.

Silicon-based electronics must be connected to the nuclear monitoring sensors through yards of cable. “When you have lengthy cables, you end up with a lot of noise, which can interfere with the accuracy of the sensor information. By placing electronics closer to a sensor, you increase its accuracy and precision,” said Kyle Reed, a member of the Sensors and Electronics group at ORNL, in a press release.

The team irradiated gallium nitride transistors for three days at temperatures up to 125 degrees Celsius close to the core of The Ohio State University Research Reactor. “We fully expected to kill the transistors on the third day, and they survived,” Reed said. They were also exposed to seven hours at 90% power, the safety threshold for the reactor.

The gallium nitride transistors were able to handle at least 100 times higher accumulated dose of radiation than a standard silicon device, according to Dianne Ezell, leader of ORNL’s Nuclear and Extreme Environment Measurements group. To support the normal maintenance window for nuclear equipment, the device would need to last for five years in the pool of a nuclear reactor – something the team says is possible since it has much lower radiation levels compared to the environment in which the device was tested.

Eventually, the team would like to develop gallium nitride circuits that could be used to transmit data from the sensors wirelessly.

References

[1] Dhasiyan, A.K., Amalraj, F.W., Jayaprasad, S. et al. Epitaxial growth of high-quality GaN with a high growth rate at low temperatures by radical-enhanced metalorganic chemical vapor deposition. Sci Rep 14, 10861 (2024). https://doi.org/10.1038/s41598-024-61501-9

[2] Isabel Streicher, Patrik Straňák, Lutz Kirste, Mario Prescher, Stefan Müller, Stefano Leone; Two-dimensional electron gases in AlYN/GaN heterostructures grown by metal–organic chemical vapor deposition. APL Mater. 1 May 2024; 12 (5): 051109. https://doi.org/10.1063/5.0203156