Flexible germanium monotelluride phase change films with ultra-high bending stability for wearable piezoresistive sensors

Although non-stoichiometric chalcogenide phase change materials (e.g., Ge2Sb2Te5, Sb2Te3) have been used in wearable electronics, their non-stoichiometric vacancies will inevitably degrade their bending stability and electrical contrast. In this study, we demonstrated that stoichiometric germanium monotelluride films deposited on flexible polyimide substrates exhibited excellent phase transition characteristics and ultra-high bending stability under different bending states. The GeTe films showed improved bending stability under compressive strain than under tensile strain, and the GeTe films still exhibited high conductivity contrast (6 orders of magnitude) after 2000 bending cycles, with potential use in flexible memory devices. As a proof-of-concept, the bending stability of the stoichiometric GeTe films was shown to be significantly better than the non-stoichiometric Ge2Sb2Te5 films. Notably, we fabricated a flexible piezoresistive sensor based on the GeTe film, which exhibited good sensitivity (gauge factor is 38), low detection limit (0.1%), short response time (83.3 ms), good reliability and repeatability (more than 1200 cycles) and high heat resistance (200 ℃), and was used to track human pulse, as well as finger and face movements. The results showed that the piezoresistive sensor had good sensitivity and a stable time-resolved electrical response, and could be used to detect the movement of different human body parts and the small signals of different pressure points. These findings indicated that GeTe-based wearable piezoresistive sensors hold significant promise for medical health monitoring and biosensing applications.