Nov 10 2017
A transfer method based on thin sacrificial layers of boron nitride could enable high-performance gallium nitride gas sensors to be developed on sapphire substrates and after that transferred to flexible polymer or metallic support materials.
The method could facilitate the production of mobile, low-cost wearable and disposable sensing devices for a broad range of environmental applications.
Image shows wafer-scale processed AlGaN/GaN sensors being tested. (Credit: Georgia Tech Lorraine).
The transferring of the gallium nitride sensors to flexible polymers and metallic foils increases their sensitivity to nitrogen dioxide gas, and improves response time by six-fold. The simple metal organic vapor phase epitaxy (MOVPE) based production steps could minimize the production cost of sensors and other optoelectronic devices.
Sensors produced with the new method can detect ammonia at parts-per-billion levels as well as differentiate between nitrogen-containing gases. The gas sensor fabrication method was reported in the Scientific Reports journal on November 9th, 2017.
“Mechanically, we just peel the devices off the substrate, like peeling the layers of an onion,” described 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 use an MOVPE process to grow monolayers of boron nitride on 2 inch sapphire wafers at approximately 1,300 °C. The boron nitride surface coating is just a few nanometers thick, and generates crystalline structures that have solid planar surface connections, but poor vertical connections.
Then, the MOVPE process is used again to grow aluminum gallium nitride (AlGaN/GaN) devices on top of the monolayers at a temperature of about 1,100 °C. Due to the boron nitride crystalline properties, the devices are connected to the substrate only by weak Van der Waals forces, which may be mechanically overcome. Without causing cracks or other defects, the devices can be transferred to other substrates. The sapphire wafers can be used again 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.
Until now, the researchers have transferred the sensors to polymeric materials, aluminum foil and copper foil. During operation, the devices can differentiate between ammonia, nitrogen oxide and nitrogen dioxide. Since the devices are about 100 by 100 microns, sensors for different gases can be generated 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,” stated Ougazzaden, who expects that the devices could be customized to also detect carbon dioxide, ozone and other gases.
The gallium nitride sensors could have a broad range of applications from industry to vehicle engines as well as for wearable sensing devices. Due to their advantageous materials properties, which include high chemical and thermal stability, the devices are attractive.
“The devices are small and flexible, which will allow us to put them onto many different types of support,” stated Ougazzaden, who also directs the International Joint Research Lab at Georgia Tech CNRS.
The researchers measured the performance of the device on the original sapphire wafer in order to evaluate the effects of transferring the devices to a different substrate and compared that to performance on the new polymer and metallic substrates. Surprisingly, they saw a doubling of the sensor sensitivity and an increase in response time by six-fold, changes beyond what could be projected 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. Properties of the substrate alone makes the different in the performance.
Abdallah Ougazzaden, Professor, School of Electrical and Computer Engineering (ECE), Georgia Tech
The researchers hope to increase the quality of the devices and show other sensing applications in the future. “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.”
Earlier, the Georgia Tech researchers have used a similar technique to generate ultraviolet detectors and light-emitting diodes that were transferred to different substrates, and they consider that the process could also be employed to produce high-power electronics. For those applications, the transferring of devices from sapphire to substrates with excellent thermal conductivity could offer a considerable advantage in device operation.
Since 2005, Ougazzaden and his team of researchers have been working on boron-based semiconductors. Their work has attracted many industrial companies interested investigating the technology, he said.
I am very excited and lucky to work on such hot topic and top-notch technology at GT-Lorraine.
Taha Ayari, Ph.D. Student, School of ECE, The Georgia Tech
Besides Ougazzaden, the research team also includes Georgia Tech Ph.D. students Saiful Alam, Xin Li, Matthew Jordan, and Taha Ayari; Youssef ElGmili and Chris Bishop, researchers at Institut Lafayette; Gilles Patriarche, a researcher at the Centre de Nanosciences et de Nanotechnologies (C2N) at CNRS; Suresh Sundaram, a researcher at Georgia Tech Lorraine; Jean Paul Salvestrini, a professor at Georgia Tech Lorraine and adjunct professor in the Georgia Tech School of ECE; and Paul Voss, an associate professor in the Georgia Tech School of ECE.
ANR (Agence Nationale de Recherche), the National Agency of Research in France, supported the research through the “GANEX” Project.