Impact of surface defects in electron beam evaporated ZnO thin films on FET biosensing characteristics towards reliable PSA detection

Abstract In this work, the impact of surface defects in zinc oxide (ZnO) thin films on the performance of field effect transistor (FET) biosensors has been explored. Here, ZnO thin films have been fabricated using electron beam evaporation with various deposition parameters to tune the surface roughness and defect state density. The films have been deployed for sensing prostate specific antigen (PSA) after anti-PSA antibody immobilization. Interestingly, it has been observed that though the transconductance of the sensors degrade monotonically with increasing defect density, the antibody binding density has a non-monotonic behaviour due to enhanced steric hindrance effect beyond a certain limit of surface roughness. There is a variation of sensitivity by 15 times with different physical properties of ZnO and the optimized sensor has a PSA detection limit of 0.06 fM. Further, the sensor with the best sensitivity retains its performance upto 2 months while a non-optimized sensor with initial PSA detection limit of 0.1 fM has longevity of 4 months. Thus, it can be envisaged that tuning the surface defect state in ZnO FET biosensor is an essential requirement not only to ensure a low detection limit and high sensitivity but also to assure a reliable long term performance.