Transfer technique produces wearable gallium nitride gas sensors
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Transfer technique produces wearable gallium nitride gas sensors
A transfer technique based on thin
sacrificial layers of boron nitride could allow high-performance gallium
nitride gas sensors to be grown on sapphire substrates and then
transferred to metallic or flexible polymer support materials. The
technique could facilitate the production of low-cost wearable, mobile
and disposable sensing devices for a wide range of environmental
applications.
Transferring the gallium nitride sensors to metallic foils and
flexible polymers doubles their sensitivity to nitrogen dioxide gas, and
boosts response time by a factor of six. The simple production steps,
based on metal organic vapor phase epitaxy (MOVPE), could also lower the
cost of producing the sensors and other optoelectronic devices.
Sensors produced with the new process can detect ammonia at parts-per-billion levels and differentiate between nitrogen-containing gases. The gas sensor fabrication technique was reported November 9 in the journal Scientific Reports.
"Mechanically, we just peel the devices off the substrate, like peeling the layers of an onion," explained Abdallah Ougazzaden, director of Georgia Tech Lorraine in Metz, France and a professor in Georgia Tech's School of Electrical and Computer Engineering (ECE). "We can put the layer on another support that could be flexible, metallic or plastic. This technique really opens up a lot of opportunity for new functionality, new devices -- and commercializing them."
The researchers begin the process by growing monolayers of boron nitride on two-inch sapphire wafers using an MOVPE process at approximately 1,300 degrees Celsius. The boron nitride surface coating is only a few nanometers thick, and produces crystalline structures that have strong planar surface connections, but weak vertical connections.
Aluminum gallium nitride (AlGaN/GaN) devices are then grown atop the monolayers at a temperature of about 1,100 degrees Celsius, also using an MOVPE process. Because of the boron nitride crystalline properties, the devices are attached to the substrate only by weak Van der Waals forces, which can be overcome mechanically. The devices can be transferred to other substrates without inducing cracks or other defects. The sapphire wafers can be reused for additional device growth.
"This approach for engineering GaN-based sensors is a key step in the pathway towards economically viable, flexible sensors with improved performances that could be integrated into wearable applications," the authors wrote in their paper.
So far, the researchers have transferred the sensors to copper foil, aluminum foil and polymeric materials. In operation, the devices can differentiate between nitrogen oxide, nitrogen dioxide, and ammonia. Because the devices are approximately 100 by 100 microns, sensors for multiple gases can be produced on a single integrated device.
"Not only can we differentiate between these gases, but because the sensor is very small, we can detect them all at the same time with an array of sensors," said Ougazzaden, who expects that the devices could be modified to also detect ozone, carbon dioxide and other gases.
The gallium nitride sensors could have a wide range of applications from industry to vehicle engines -- and for wearable sensing devices. The devices are attractive because of their advantageous materials properties, which include high thermal and chemical stability.
"The devices are small and flexible, which will allow us to put them onto many different types of support," said Ougazzaden, who also directs the International Joint Research Lab at Georgia Tech CNRS.
To assess the effects of transferring the devices to a different substrate, the researchers measured device performance on the original sapphire wafer and compared that to performance on the new metallic and polymer substrates. They were surprised to see a doubling of the sensor sensitivity and a six-fold increase in response time, changes beyond what could be expected by a simple thermal change in the devices.
"Not only can we have flexibility in the substrate, but we can also improve the performance of the devices just by moving them to a different support with appropriate properties," he said. "Properties of the substrate alone makes the different in the performance."
In future work, the researchers hope to boost the quality of the devices and demonstrate other sensing applications. "One of the challenges ahead is to improve the quality of the materials so we can extend this to other applications that are very sensitive to the substrates, such as high-performance electronics."
The Georgia Tech researchers have previously used a similar technique to produce light-emitting diodes and ultraviolet detectors that were transferred to different substrates, and they believe the process could also be used to produce high-power electronics. For those applications, transferring the devices from sapphire to substrates with better thermal conductivity could provide a significant advantage in device operation.
Ougazzaden and his research team have been working on boron-based semiconductors since 2005. Their work has attracted visits from several industrial companies interested in exploring the technology, he said.
"I am very excited and lucky to work on such hot topic and top-notch technology at GT-Lorraine," said Taha Ayari, a Ph.D. student in the Georgia Tech School of ECE and the paper's first author.
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Sensors produced with the new process can detect ammonia at parts-per-billion levels and differentiate between nitrogen-containing gases. The gas sensor fabrication technique was reported November 9 in the journal Scientific Reports.
"Mechanically, we just peel the devices off the substrate, like peeling the layers of an onion," explained Abdallah Ougazzaden, director of Georgia Tech Lorraine in Metz, France and a professor in Georgia Tech's School of Electrical and Computer Engineering (ECE). "We can put the layer on another support that could be flexible, metallic or plastic. This technique really opens up a lot of opportunity for new functionality, new devices -- and commercializing them."
The researchers begin the process by growing monolayers of boron nitride on two-inch sapphire wafers using an MOVPE process at approximately 1,300 degrees Celsius. The boron nitride surface coating is only a few nanometers thick, and produces crystalline structures that have strong planar surface connections, but weak vertical connections.
Aluminum gallium nitride (AlGaN/GaN) devices are then grown atop the monolayers at a temperature of about 1,100 degrees Celsius, also using an MOVPE process. Because of the boron nitride crystalline properties, the devices are attached to the substrate only by weak Van der Waals forces, which can be overcome mechanically. The devices can be transferred to other substrates without inducing cracks or other defects. The sapphire wafers can be reused for additional device growth.
"This approach for engineering GaN-based sensors is a key step in the pathway towards economically viable, flexible sensors with improved performances that could be integrated into wearable applications," the authors wrote in their paper.
So far, the researchers have transferred the sensors to copper foil, aluminum foil and polymeric materials. In operation, the devices can differentiate between nitrogen oxide, nitrogen dioxide, and ammonia. Because the devices are approximately 100 by 100 microns, sensors for multiple gases can be produced on a single integrated device.
"Not only can we differentiate between these gases, but because the sensor is very small, we can detect them all at the same time with an array of sensors," said Ougazzaden, who expects that the devices could be modified to also detect ozone, carbon dioxide and other gases.
The gallium nitride sensors could have a wide range of applications from industry to vehicle engines -- and for wearable sensing devices. The devices are attractive because of their advantageous materials properties, which include high thermal and chemical stability.
"The devices are small and flexible, which will allow us to put them onto many different types of support," said Ougazzaden, who also directs the International Joint Research Lab at Georgia Tech CNRS.
To assess the effects of transferring the devices to a different substrate, the researchers measured device performance on the original sapphire wafer and compared that to performance on the new metallic and polymer substrates. They were surprised to see a doubling of the sensor sensitivity and a six-fold increase in response time, changes beyond what could be expected by a simple thermal change in the devices.
"Not only can we have flexibility in the substrate, but we can also improve the performance of the devices just by moving them to a different support with appropriate properties," he said. "Properties of the substrate alone makes the different in the performance."
In future work, the researchers hope to boost the quality of the devices and demonstrate other sensing applications. "One of the challenges ahead is to improve the quality of the materials so we can extend this to other applications that are very sensitive to the substrates, such as high-performance electronics."
The Georgia Tech researchers have previously used a similar technique to produce light-emitting diodes and ultraviolet detectors that were transferred to different substrates, and they believe the process could also be used to produce high-power electronics. For those applications, transferring the devices from sapphire to substrates with better thermal conductivity could provide a significant advantage in device operation.
Ougazzaden and his research team have been working on boron-based semiconductors since 2005. Their work has attracted visits from several industrial companies interested in exploring the technology, he said.
"I am very excited and lucky to work on such hot topic and top-notch technology at GT-Lorraine," said Taha Ayari, a Ph.D. student in the Georgia Tech School of ECE and the paper's first author.
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Molecular magnetism packs power with 'messenger electron'
- Summary:
- A UW-Madison lab has made a molecule that gains magnetic strength through an unusual way of controlling those spins, which could lead to a breakthrough in quantam computing.
- Electrons can be a persuasive bunch, or at least, a talkative bunch, according to new work from John Berry's lab at the University of Wisconsin-Madison.The spins of unpaired electrons are the root of permanent magnetism, and after 10 years of design and re-design, Berry's lab has made a molecule that gains magnetic strength through an unusual way of controlling those spins.
Berry says the new structure that graduate student Jill Chipman created could lead to a breakthrough in quantum computing, an approach with such great potential that it could undermine today's silicon-based supercomputers much as the telephone did the telegraph: A great leap forward that begins a slide into irrelevance.
The presence and activity, or "spin," of unpaired electrons sets the strength of a permanent magnet, so molecules with a high degree of spin are a desirable target for chemists. The unusually large spin in the new magnetic molecule, Berry explains, results from a "messenger electron" that shuttles between an unpaired electron at each end of the rod-shaped molecule and persuades all three of them to adopt the same spin.
That agreement of spin, "orthogonality" in the jargon, adds strength to a permanent magnet.
Berry, a UW-Madison professor of chemistry, notes that in other materials, a traveling electron tends to oppose the spins of magnetic centers, reducing the magnetic strength. In Chipman's new creation, however, the messenger electron is focused on harmony: like a traveling social worker, it causes the two remote unpaired electrons to take the same spin, adding strength and/or durability.
The new molecule, described in Chemistry -- A European Journal, contains carbon, nickel, chlorine, nitrogen, and molybdenum, but lacks the costly rare earth elements that have bedeviled efforts to commercialize super-strong new magnets. Its structure suggests that the molecule could be formed into a polymer -- a repeating chain of units like those found in plastics -- raising the possibility of cheaper, stronger magnets.
"We tried to remove electrons from this molecule 10 years ago so it had an unpaired electron at each end, but did not get far," Berry says. "We since learned that this made a chemical that is really temperature-sensitive, so Jill had to develop a low-temperature process that relies on dry ice to cool it to -78 degrees C."
The "traveling social worker" electron establishes "a design principle that could be used to create many new magnetic molecules that behave as little bar magnets," Berry says.
The discovery was also enabled by the arrival last summer an instrument called a SQUID magnetometer (Superconducting QUantum Interference Device) that can measure magnetism with great accuracy down to below 2 degrees above absolute zero.
Much of the focus of magnet innovation concerns greater strength, Berry says, "but there are all sorts of things people look for. We need both permanent magnets and those with ephemeral magnetization for different technical reasons. Magnets are widespread in ultra-cold refrigeration, motors, computer hard drives and electronic circuits."
By going the next step, and miniaturizing magnets to a single molecule, that could enable quantum computing, Berry says. Quantum computing could be especially beneficial to chemists, who confront staggering complexity in trying to model the chemical reactions that are their bread and butter.
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