Gas Sensors Transfer Easily to Different Substrates Without Degrading Performance

A transfer technique based on thin 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 optoelectronic applications.

Abdallah Ougazzaden, director of Georgia Tech Lorraine in Metz, France, and Chris Bishop, a researcher at Institut Lafayette, example a sample being processed in a lab at Georgia Tech Lorraine. Courtesy of Rob Felt, Georgia Tech.

Researchers from Georgia Institute of Technology began by growing monolayers of boron nitride on two-inch sapphire wafers using a metal organic vapor phase epitaxy (MOVPE) process at approximately 1300 °C. The boron nitride surface coating, only a few nanometers thick, produced crystalline structures with strong planar surface connections, but weak vertical connections.

Aluminum gallium nitride (AlGaN/GaN) devices were then grown on top of the monolayers at a temperature of about 1100 °C, also using an MOVPE process. Because of the boron nitride crystalline properties, the devices were attached to the substrate only by weak Van der Waals forces, which can be overcome mechanically.

“Mechanically, we just peel the devices off the substrate, like peeling the layers of an onion. 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,” said professor Abdallah Ougazzaden.

So far, the team has transferred the sensors to copper foil, aluminum foil and polymeric materials.

Image shows wafer-scale processed AlGaN/GaN sensors being tested. Courtesy of Georgia Tech Lorraine.

To assess the effects of transferring the devices to a different substrate, the researchers measured device performance on the original sapphire wafer and compared it 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.

In operation, the devices have shown that they can detect ammonia at parts-per-billion levels and differentiate between nitrogen oxide, nitrogen dioxide and ammonia. Because the devices are approximately 100 × 100 μm, 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.

In future work, the researchers hope to boost the quality of the devices and demonstrate other sensing applications.

The devices can be transferred to other substrates without inducing cracks or other defects. The sapphire wafers can be reused for additional device growth.

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 MOVPE, could also lower the cost of producing the sensors and other optoelectronic devices.

The gallium nitride sensors could have a wide range of applications, from industry to vehicle engines to wearable sensing devices. The devices are attractive because of their advantageous materials properties, which include high thermal and chemical stability.

“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,” said Ougazzaden.

The researchers previously used a similar technique to produce LEDs and UV 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.

The research was published in Scientific Reports (doi:10.1038/s41598-017-15065-6).