Threshold shift mechanism in fluorine-doped indium-gallium-zinc-oxide thin film transistors via defect analysis

In this study, we systematically investigate the threshold voltage (V_th) shift mechanism in amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors subjected to varying fluorine (F) implantation doses. Fluorine was implanted into a-IGZO at a dose of 1 × 10²⁰ cm⁻³ with an implantation energy of 30 keV, resulting in a negative V_th shift compared to undoped samples. In contrast, higher doping concentrations (5 × 10²⁰ and 1 × 10²¹ cm⁻³) induced positive V_th shifts. To elucidate this mechanism, we conducted Current Transient Spectroscopy (CTS), X-ray Photoelectron Spectroscopy (XPS), and Reflection Electron Energy Loss Spectroscopy (REELS). The results indicate that moderate F doping shifts the Fermi level closer to the conduction band, causing a negative V_th shift. However, at higher doping levels, shallow defect states (D1) emerge, facilitating the recombination of conduction band electrons into these states. This process reduces the on-current (I_on) and leads to a positive V_th shift. Fluorine doping enhances device stability against negative bias temperature instability (NBTI), while positive bias temperature instability (PBTI) degrades increasingly with higher doping. While our experiments did not encompass the full range of doping concentrations required for simultaneous optimization of both, our results suggest that lower fluorine doses may offer a balanced approach. Through direct defect characterization, this study clarifies the critical role of such defects in the threshold voltage shift mechanism of oxide thin-film transistors, providing valuable guidance for reliability improvements.