Sensors produced with the new process are said to detect ammonia at parts-per-billion levels and to differentiate between nitrogen containing gases.
In the process, thin sacrificial layers of boron nitride (BN) allow gallium nitride (GaN) gas sensors to be grown on sapphire substrates and then transferred to metallic or flexible polymer support materials.
According to the team, transferring the GaN sensors to metallic foils and flexible polymers doubles their sensitivity to nitrogen oxides (NOx) and boosts response time by a factor of six. The production steps, based on metal organic vapour phase epitaxy (MOVPE), could also lower the cost of the sensors and other optoelectronic devices.
“Mechanically, we just peel the devices off the substrate; like peeling the layers of an onion,” explained Professor Abdallah Ougazzaden. “We can put the layer on another support that could be flexible, metallic or plastic. This technique opens up a lot of opportunity for new functionality, new devices – and commercialising them."
In the process, monolayers of BN are grown on 2in sapphire wafers using MOVPE at approximately 1300°C. Aluminium gallium nitride devices are then grown on the BN monolayers at a temperature of about 1100°C, also using MOVPE. Because of the crystalline properties of BN, 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 then be reused to grow more sensors.
So far, the researchers have transferred the sensors to copper foil, aluminum foil and polymeric materials. In operation, the devices can differentiate between NO, NO2 and ammonia. Because the devices are approximately 100 x 100µm, sensors for multiple gases could be produced on one 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 Prof Ougazzaden, who expects the devices to also be able to detect other gases, including ozone and carbon dioxide.
“Not only can we have flexibility in the substrate,” he continued, “but we can also improve the performance of the devices just by moving them to a different support with appropriate properties. The properties of the substrate alone makes the difference in the performance.”
