Unraveling the Influence of Surface Contaminants and Cleaning Protocols on Charge States of SiO2 Dielectrics and the Performance of Organic Field-Effect Transistors

Organic field-effect transistors (OFETs) fabricated on SiO2/Si wafers represent a crucial avenue for the development of organic-on-silicon technology and serve as a platform for materials science to characterize carrier mobility. The surface cleaning treatment of wafers is critical because OFETs are highly sensitive to surface states. In this study, the effects of intrinsic organic contaminants on wafers, as well as the functions of commonly used cleaning solvents and treatments on the surface states, electronic structure of SiO2, and the performance of OFETs were systematically investigated. The intrinsic organic contaminants on the wafer surface were identified as being in positively charged states, which led to high operational voltages of OFETs and degraded bias-stress stability. However, their impact on field-effect mobility was found to be minimal following any surface cleaning process. The conventional ultrasonic cleaning using acetone and isopropanol only partially removed contaminants, which provides limited improvement in reducing operational voltages and enhancing bias-stress stability. The subsequent ultrasonic cleaning with deionized water and UV-ozone treatment further removed contaminants. More importantly, these processes anchor hydroxyl species on the SiO2 surface, which impart negative charges for the neutralization of surface charge states, thereby comprehensively enhancing operational performance of devices. By leveraging the distinct properties of various solvents and treatments, an optimized surface cleaning protocol was proposed, in which the threshold voltage of OFETs was reduced to nearly 0 V, and the bias-stress stability was enhanced by approximately 300%.