Source/drain metal work-function-dependent memory characteristics governed by ferroelectric switching-assisted current modulation in a-IGZO FeFETs

초록

This study investigates how source/drain (S/D) contact properties affect the transfer characteristics, hysteresis behavior, and current modulation of amorphous InGaZnO (a-IGZO) ferroelectric field effect transistors (FeFETs). TiN/HZO/a-IGZO FeFETs with Mo and Pd S/D electrodes were fabricated and comparatively analyzed. The intended device structure was confirmed by cross-sectional transmission electron microscopy (TEM), and fast Fourier transform (FFT) analysis of the HZO layer revealed reflections consistent with the orthorhombic phase (o-phase). Positive-up-negative-down (PUND) measurements of Metal/Ferroelectric/Semiconductor/Metal (MFSM) capacitors further showed that the Mo top structure exhibited a larger polarization response than the Pd top counterpart. Despite this, the Pd contact FeFET displayed a pronounced increase in drain current (IDS) near gate bias (VGS) approximate to 4 V, together with a larger memory window (MW) with enlarged IDS sensing margin. The gate current (IGS) also exhibited a more pronounced switching-induced contrast in the Pd device than in the Mo device. These results indicate that the higher work function (WF) of Pd initially suppresses electron injection at the Pd/a-IGZO contact, resulting in a lower current than that of the Mo contact device before ferroelectric switching. When ferroelectric switching is activated at high positive VGS, polarization-induced electrostatic modulation locally reduces the effective barrier width at the Pd/a-IGZO junction, producing a large switching-induced increase in IDS from the initially injection-limited state. Therefore, the larger apparent MW and IDS sensing margin in the Pd contact FeFET originate not from a larger intrinsic polarization response or inherently more favorable electron injection, but from stronger differential current modulation before and after ferroelectric switching. In contrast, the Mo contact device allows more efficient electron injection before switching, but its smaller switching-induced current increment makes the extracted hysteresis more susceptible to trap-related distortion at high VGS. The results highlight contact engineering as a key design parameter for controlling both current drive and apparent switching behavior in oxide-semiconductor FeFETs.