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 as well as other optoelectronic devices. Sensors produced with the new process can detect ammonia at parts-per-billion levels and differentiate between nitrogen-containing gases.
The researchers begin the process by growing monolayers of boron nitride on two-inch sapphire wafers using an MOVPE process at approximately 1300 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 1100 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.
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 only 100 by 100 microns, sensors for multiple gases can be produced on a single integrated device.
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.
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 improvement in response time, changes beyond what could be expected by a simple thermal change in the devices. Properties of the substrate alone make the difference in the performance.
In future work, the researchers hope to boost the quality of the devices and demonstrate other sensing applications — to improve the quality of the materials so they can be extended to other applications that are very sensitive to the substrates, such as high-performance electronics.