Electronics Engineering

Flexible ultraviolet sensors for use in wearables

To enable the development of wearable devices that possess advanced ultraviolet (UV) detection functions, scientists have created a new type of light sensor

September 14, 2021
The Scitech

In a study which was featured on the front cover of the peer-reviewed journal ACS Nano, the NTU researchers reported that their flexible UV light sensors were 25 times more responsive, and 330 times more sensitive, than existing sensors, exceeding the performance level required for optoelectronic applications – or light- based electronics. UV light sensors, also known as photodetectors, are used in a wide range of systems, from smartphones to biomedical imaging. Over the past decades, gallium nitride (GaN) has gained prominence as the ideal material to fabricate UV light sensors, largely due to its superior properties in emitting, regulating, transmitting, and sensing light. However, most GaN-based UV sensors today are built on rigid layers, limiting their use in flexible and wearable products. While researchers elsewhere have developed flexible GaN-based UV sensors, they have not attained the level of performance required for state-of-the-art use. Two of their biggest challenges are low responsivity and low sensitivity. The NTU team overcame these constraints by creating their flexible UV light sensors on a semiconductor wafer 8 inches in diameter, using free-standing single-crystalline layers of GaN and aluminium gallium nitride (AlGaN), arranged using membranes that consist of two different thin semiconductor layers (heterostructure membranes).

This type of semiconductor structure, which can be fabricated using existing industrial compatible methods, allows the material to be easily bent, making it ideal for use in flexible sensors. At the same time, the chemical composition of the material changes with depth, meaning that high performance is maintained even when it comes under strain. In lab tests, the NTU flexible UV light sensors created using the novel combined AlGaN and GaN operated at exceedingly high levels of responsivity and sensitivity. Subjected to multiple bending and high temperature tests, they also maintained good performance. Under a range of external strains (compressive, flat, and tensile), the sensors recorded a responsivity level of between 529 – 1340 Ampere/Watt (unit used to measure the ability of a device to transfer an optical signal to an electrical signal), which is about 100 times higher than existing UV sensors. This responsivity remained stable after 100 cycles of repetitive bending, demonstrating its potential to be integrated into wearables. Lead researcher, NTU Assistant Professor Kim Munho from the School of Electronic and Electrical Engineering, said the high performance of the team’s flexible UV light sensors proves that it would be feasible to manufacture large-scale lightweight and flexible electronics for use in future relevant light-based applications.

The NTU team’s achievement could lead to significant advances in UV optoelectronic devices and circuits added Asst Prof Kim, as product engineers could now look forward to developing UV-enabled wearable systems.

Source: NTU news release