Fig. 2 (a) shows the measured curves of polarization P v.s. applied voltage V characteristics for the Ge/ZrO2/TaN capacitors at 3.3 kHz. The gate length (LG) of the capacitors are 8 μm. It is observed that the Pr of the Ge/ZrO2/TaN capacitors can be enhanced with larger sweeping range of V. The ferroelectric-like behavior of the amorphous ZrO2 film in the Fig. 2 (a) is proposed to be originated from the oxygen vacancy dipoles . Fig. 2 (b) shows the measured P-V curves for the Ge/ZrO2/TaN capacitors under different frequencies from 200 Hz to 10 k Hz. We can see that the ferroelectric-like behaviorof the amorphous ZrO2 film remain stable for all frequencies. However, the Pr of the amorphous ZrO2 film is reduced with the increased frequencies. This phenomenon can be explained by the incomplete dipoles switching under high measurement frequencies [37,38].
Fig. 3 (a) shows the measured IDS-VGS curves of a ferroelectric Al2O3/HfO2 NCFET at the VDS of - 0.05 V and - 0.5 V. The LG of the devices are 3 μm. The hysteresis loops of 0.14 V (Vds = - 0.05 V, Ids = 1 nA/μm) and 0.08 V (Vds = - 0.5 V, Ids = 1 nA/μm) are demonstrated, respectively. The clockwise hysteresis loops are attributed to the migration of oxygen vacancies and accompanied negative charges. The oxygen vacancy dipoles accumulate (deplete) in the Ge/Al2O3 interface under positive (negative) VGS. Therefore, the threshold voltage (VTH) increases (decreases) under forward (reverse) sweeping of gate voltages. As shown in Fig. 3 (b), the output characteristics of the Al2O3/HfO2 NCFET and the control FET are compared. The saturation current of the Al2O3/HfO2 NCFET exceeds 26 μA/μm, with a rise of 23 % compared to that of the control FET at |VGS-VTH| = |VDS| = 0.8 V. The current enhancement is induced by the increased inversion charge intensity (Qinv) in the reverse polarization electric field and the amplication of surface potential [39,40]. In addition to current enhancement, the obtained obvious NDR proves the NC effect of the amorphous HfO2 film. The NDR effect is caused by the incomplete switching of oxygen vacancy dipoles due to the coupling of drain-to-channel as VDS increases [41,42]. Fig. 3 (c) compares the measured gate leakage IG-VGS curves for the 5 nm Al2O3/HfO2 NCFET at the VDS of - 0.05 V and –0.5 V. The sudden drops of IG onlyduring the reverse sweeping indicate the decreased voltage in the amorphous HfO2 film and the amplication of surface potential . The absence of NC effect during the forward sweeping is caused by the partical switching of oxygen vacancy dipoles in the amorphous HfO2 film . The different ability to contain oxygen atoms between Al2O3 and HfO2 layer leads to oxygen redistribution and negative interfacial dipoles at the Al2O3/HfO2 interface [45–47]. Due to the presense of negative interfacial dipoles, it is difficult for the amorphous HfO2 film to realize complete polarization swiching (NC effect) in the forward sweeping.
Fig. 4 (a) shows the measured transfer curves of a ferroelectric ZrO2 NCFET at the VDS of - 0.05 V and - 0.5 V. The LG of the two devices are 4 μm. The clockwise hysteresis loops of 0.24 V (Vds = - 0.05 V, Ids = 1 nA/μm) and 0.14 V (Vds = - 0.5 V, Ids = 1 nA/μm) are demonstrated, respectively. As shown in Fig. 4 (b), the output characteristics of the ZrO2 NCFET and the control FET are compared. The saturation current of the ZrO2 NCFET exceeds 30 μA/μm, with a rise of 12 % compared to that of the control FET at |VGS-VTH| = |VDS| = 1 V. The improved current enhancement and more obvious NDR indicate the enhanced NC effect of the amorphous ZrO2 film (5 nm) contrast to that of 5 nm HfO2 film. Fig. 3 (c) compares the measured gate leakage IG-VGS curves for the 5 nm ZrO2 NCFET at the VDS of - 0.05 V and - 0.5 V. Compared to the sudden IG drops of Al2O3/HfO2 NCFET only during reverse sweeping in Fig. 3 (c), the sudden drops of IG both in forward and reverse sweeping also prove the enhanced NC effect in the amorphous ZrO2 film.
Fig. 5 (a) and (b) shows the point SS as function of IDS for the Al2O3/HfO2 and ZrO2 NCFET at the VDS of - 0.05 V and - 0.5 V. Sub–60 mV/decade subthreshold swing (SS) can be achieved during forward or reverse sweeping of VGS at the VDS of - 0.05 V and - 0.5 V. When VDS is - 0.05 V, a point forward SS of 45.1 mV/dec and a point reverse SS of 55.2 mV/dec were achieved. When VDS is - 0.5 V, a point forward SS of 51.16 mV/dec and a point reverse SS of 46.52 mV/dec were achieved. Due to the different ability of scavenging effect for the Al2O3/HfO2 and ZrO2 layer, the partical dipoles switching is caused in the Al2O3/HfO2 NCFET. Therefore, the more obvious NC effect with sub–60 mV/decade SS is achieved in 5 nm ZrO2 NCFET.