When it comes to putting technology in space, size and mass are prime considerations. High-power gallium nitride-based high electron mobility transistors (HEMTs) are appealing in this regard because they have the potential to replace bulkier, less efficient transistors, and are also more tolerant of the harsh radiation environment of space. Compared to similar aluminum gallium arsenide/gallium arsenide HEMTs, the gallium nitride-based HEMTs are ten times more tolerant of radiation-induced displacement damage.

Until recently, scientists could only guess why this phenomena occurred: Was the gallium nitride material system itself so inherently disordered that adding more defects had scant effect? Or did the strong binding of gallium and nitrogen atoms to their lattice sites render the atoms more difficult to displace?

The answer, according to scientists at the Naval Research Laboratory, is none of the above.

Examining radiation response

In a recent open access article published in the ECS Journal of Solid State Science and Technologyentitled, “On the Radiation Tolerance of AlGaN/GaN HEMTs,” the team of researchers from NRL state that by studying the effect of proton irradiation on gallium nitride-based HEMTs with a wide range of initial threading dislocation defectiveness, they found that the pre-irradiation material quality had no effect on radiation response.

Additionally, the team discovered that the order-of-magnitude difference in radiation tolerance between gallium arsenide- and gallium nitride-based HEMTs is much too large to be explained by differences in binding energy. Instead, they noticed that radiation-induced disorder causes the carrier mobility to decrease and the scattering rate to increase as expected, but the carrier concentration remains significantly less affected than it should be.

Applications in space exploration

Because of their relative radiation hardness, gallium arsenide- and gallium nitride-based HEMTs are desirable for space application. Take, for example, the Juno Spacecraft.

On July 4, the Juno Spacecraft successfully entered orbit around Jupiter – a planet scientists still know very little about, which generates extreme levels of radiation. Without the proper technology, the radiation levels of Jupiter could destroy the sensitive electronics in the satellite upon approaching the planet. Better understanding of why gallium arsenide- and gallium nitride-based HEMTs are more tolerant of radiation could ultimately accelerate innovative and bolster projects where radiation levels prove to be barriers.

Novel advancements in HEMTs

The paper was designated ECS Editors’ Choice due to its significance and expected impact on the solid state science and technology community.

“Editors’ Choice articles are elite publications because they are deemed by reviewers and journal editors to demonstrate a transformative advance, discovery, interpretation, or direction in a field,” says Dennis Hess, Editor of the ECS Journal of Solid State Science and Technology. “They represent very high quality science and engineering and hold the promise of altering current technology practices.”

Unexpected answers

The explanation for this novel discovery turns out to be rather elegant.

In gallium nitride-based HEMTs, a piezoelectric field forms at the aluminum gallium nitride/gallium nitride interface due to lattice strain. The field gives rise to two-dimensional electron gas by which carriers travel across the transistor from source to drain. It also provides an electrically attractive environment that causes carriers that are scattered out of the two-dimensional electron gas by radiation-induced defects to be reinjected. In this way, the scattering rate can increase and the mobility can decrease without greatly affecting the two-dimensional electron gas carrier density.

In other words, it is the internal structure itself that renders aluminum gallium nitride/gallium nitride HEMTs rad-hard.

“Gallium nitride is such a complicated system – not like gallium arsenide at all,” says Bradley Weaver, co-author of the study. “We struggled for four years to figure out why it’s so rad-hard, expecting a complicated solution. But the answer turned out to be really simple. Science does that sometimes.”

Materials researchers at North Carolina State University have fine-tuned a technique that enables them to apply precisely controlled silica coatings to quantum dot nanorods in a day – up to 21 times faster than previous methods. In addition to saving time, the advance means the quantum dots are less likely to degrade, preserving their advantageous optical properties.

Morphological control of the silica shell on CdSe/CdS core/shell quantum dot nanorods is reported, giving single or double lobes of silica or a uniform silica shell. Credit: Joe Tracy

Quantum dots are nanoscale semiconductor materials whose small size cause them to have electron energy levels that differ from larger-scale versions of the same material. By controlling the size of the quantum dots, researchers can control the relevant energy levels – and those energy levels give quantum dots novel optical properties. These characteristics make quantum dots promising for applications such as opto-electronics and display technologies.

But quantum dots are surrounded by ligands, which are organic molecules that are sensitive to heat. If the ligands are damaged, the optical properties of the quantum dots suffer.

“We wanted to coat the rod-shaped quantum dots with silica to preserve their chemical and optical properties,” says Bryan Anderson, a former Ph.D. student at NC State who is lead author of a paper on the work. “However, coating quantum dot nanorods in a precise way poses challenges of its own.”

Previous work by other research teams has used water and ammonia in solution to facilitate coating quantum dot nanorods with silica. However, those techniques did not independently control the amounts of water and ammonia used in the process.

By independently controlling the amounts of water and ammonia used, the NC State researchers were able to match or exceed the precision of silica coatings achieved by previous methods. In addition, using their approach, the NC State team was able to complete the entire silica-coating process in a single day – rather than up to one to three weeks needed for other processes.

“The process time is important, because the longer the process takes, the more likely it is that the quantum dot nanorods being coated will degrade,” says Joe Tracy, an associate professor of materials science and engineering at NC State and senior author on the paper. “The time factor may also be important when we think about scaling this process up for manufacturing processes.”

That said, researchers still have a problem.

The process of applying the silica coating etches the cadmium sulfide surface of the quantum dot nanorods, which shortens the length of the nanorods by as much as four or five nanometers. That shortening is indicative of etching, which reduces the brightness of the light emitted by the quantum dot nanorods.

“We think ammonia may be the culprit,” Tracy says. “We have some ideas that we’re pursuing, focused on how to substitute another catalyst for ammonia in order to minimize the etching and better preserve the quantum dot nanorod’s optical properties.”

The paper, “Silica Overcoating of CdSe/CdS Core/Shell Quantum Dot Nanorods with Controlled Morphologies,” is published online in the journal Chemistry of Materials. The paper was co-authored by Wei-Chen Wu, a former Ph.D. student in Tracy’s lab. The work was done with support from the National Science Foundation under grant number DMR-1056653.

Tracy has previously published related research in Chemistry of Materials on coating gold nanorods with silica shells.