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ZrOx Negative Capacitance Field-Effect Transistor with Sub-60 Subthreshold Swing Behavior
Nanoscale Research Letters volume 16, Article number: 21 (2021)
Here we report the ZrOx-based negative capacitance (NC) FETs with 45.06 mV/decade subthreshold swing (SS) under ± 1 V VGS range, which can achieve new opportunities in future voltage-scalable NCFET applications. The ferroelectric-like behavior of the Ge/ZrOx/TaN capacitors is proposed to be originated from the oxygen vacancy dipoles. The NC effect of the amorphous HfO2 and ZrOx films devices can be proved by the sudden drop of gate leakage, the negative differential resistance (NDR) phenomenon, the enhancement of IDS and sub-60 subthreshold swing. 5 nm ZrOx-based NCFETs achieve a clockwise hysteresis of 0.24 V, lower than 60 mV/decade SS and an 12% IDS enhancement compared to the control device without ZrOx. The suppressed NC effect of Al2O3/HfO2 NCFET compared with ZrOx NCFET is related to the partial switching of oxygen vacancy dipoles in the forward sweeping due to negative interfacial dipoles at the Al2O3/HfO2 interface.
As complementary metal oxide semiconductor (CMOS) devices scaling down constantly, the integrated circuit (IC) technique has entered into the era of “more than Moore era”. The driving force of IC industry and technology becomes the reduction of power consumption, instead of the miniaturization of transistors [1, 2]. However, the Boltzmann tyranny of MOSFETs, more than 60 mV/decade SS has restricted the energy/power efficiency . In recent years, many proposed novel devices have the ability to achieve sub-60 mV/decade threshold swing, including impact ionization MOSFETs, tunnel FETs and NCFETs [4,5,6,7]. Due to the simple structure, the steep SS and improved drive current, NCFETs with a ferroelectric (FE) film have been regarded as an attractive alternative among these emerging devices [8,9,10]. The reported experiments on NCFETs mainly include PbZrTiO3 (PZT), P(VDF-TrFE) and HfZrOx (HZO) [11,12,13,14,15,16,17]. However, the high process temperature and undesired gate leakage current along the grain boundaries of polycrystalline ferroelectric materials have restricted their development for the state-of-the-art technology nodes [18,19,20,21,22,23,24,25,26]. Recently, ferroelectricity in the amorphous Al2O3 and ZrOx films enabled by the voltage-modulated oxygen vacancy dipoles has been investigated [27,28,29]. Compared with the crystalline counterpart, the amorphous ferroelectric-like films have significant advantages in reduced process temperature and leakage current. Thus, there are mass researches on FeFETs with amorphous gate insulator for the non-volatile memory and analog synapse applications [27, 30,31,32,33,34]. However, the systematical investigation on one-transistor ZrOx-based NCFET has not been carried out.
In this work, Ge NCFETs with 5 nm ZrOx ferroelectric dielectric layer and 5 nm Al2O3/HfO2 ferroelectric dielectric layer have been proposed, respectively. We experimentally observed sub-60 mV/decade steep slope in ZrOx (5 nm) NCFET, which can be attributed to the NC effect of ZrOx ferroelectric layer. And we analyzed the polarization P as function of applied voltage V for the Ge/ZrOx/TaN capacitors. The ferroelectric-like behavior of the Ge/ZrOx/TaN capacitors is induced by the voltage-induced oxygen vacancy dipoles. Moreover, we attributed the improved IDS and the sudden drop of IG in the Al2O3/HfO2 NCFETs and ZrOx NCFETs to the NC effect. We also observed the NDR phenomenon in the Al2O3/HfO2 NCFETs and ZrOx NCFETs. In addition, we further analyzed the physical mechanism of interfacial dipoles-induced decreased NC effect in the Al2O3/HfO2 NCFET. The ZrOx NCFETs with sub-60 mV/decade steep slope, improved drain voltage and low operating voltage will be suit for the design of NCFETs with low power consumption in the “more than Moore era”.
Key process steps for NCFETs with ZrOx and Al2O3/HfO2 fabrication are shown in Fig. 1a. Different gate dielectric insulators, including Al2O3/amorphous HfO2 (5 nm) films and amorphous ZrOx (4.2 nm) films were grown on n-Ge (001) substrates by atomic layer deposition (ALD) at 300 °C. TMA, TDMAHf, TDMAZr and H2O vapor were used as the precursors of Al, Hf, Zr and O, respectively. The pulse time and purge time of the precursors of Hf and Zr are 1.6 s and 8 s, respectively. The pulse time and purge time of the precursors of Al are 0.2 s and 8 s, respectively. A TaN top gate electrode was then deposited on HfO2 or ZrOx surfaces by reactive sputtering. Source/drain (S/D) regions were defined by lithography patterning and dry etching. After that, boron (B+) and nickel (Ni) was deposited in source/drain (S/D) regions. Finally, rapid thermal annealing (RTA) at 350 °C for 30 s in a 108 Pa nitrogen ambient was carried out. Figure 1b, d show the schematics of the fabricated Al2O3/HfO2 NCFETs and ZrOx NCFETs. High-resolution transmission electron microscope (HRTEM) image in Fig. 1c depicts the amorphous HfO2 (5 nm) film on Ge (001) with Al2O3 interfacial layer. HRTEM image in Fig. 1e depicts the amorphous ZrOx (4.2 nm) film on Ge (001). The C–V curve of ZrOx NCFETs and the X-ray photoelectron spectra (XPS) of TaN/ZrOx (4.2 nm)/Ge capacitors were measured in Additional file 1: Fig. S1.
Results and Discussion
Figure 2a shows the measured curves of polarization P v.s. applied voltage V characteristics for the Ge/ZrOx/TaN capacitors at 3.3 kHz. The gate length (LG) of the capacitors are 8 μm. It is observed that the remnant polarization Pr of the Ge/ZrOx/TaN capacitors can be enhanced with larger sweeping range of V. The ferroelectric-like behavior of the amorphous ZrOx film in the Fig. 2a is proposed to be originated from the voltage-driven oxygen vacancy dipoles . Figure 2b shows the measured P–V curves for the Ge/ZrOx/TaN capacitors under different frequencies from 200 to 10 kHz. We can see that the ferroelectric-like behavior of the amorphous ZrOx film remain stable for all frequencies. However, the Pr of the amorphous ZrOx film is reduced with the increased frequencies. This phenomenon can be explained by the incomplete dipoles switching under high measurement frequencies [36, 37]. As measurement frequencies increasing, the time for the direction change of electric field in the amorphous ZrOx film decreases. Thus, part of oxygen vacancy diploes switching is incomplete, providing decreased Pr.
Figure 3a 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 is 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. 3b, 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 amplification of surface potential [38, 39]. 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 [40, 41]. Figure 3c 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 only during 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 [44, 45]. Due to the presence of negative interfacial dipoles, it is difficult for the amorphous HfO2 film to realize complete polarization switching (NC effect) in the forward sweeping (Additional file 1).
Figure 4a shows the measured transfer curves of a ferroelectric ZrOx 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. 4b, the output characteristics of the ZrOx NCFET and the control FET are compared. The saturation current of the ZrOx 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 ZrOx film (5 nm) contrast to that of 5 nm HfO2 film. Figure 4c compares the measured gate leakage IG–VGS curves for the 5 nm ZrOx 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. 3c, the sudden drops of IG both in forward and reverse sweeping in Fig. 4c also prove the enhanced NC effect in the amorphous ZrOx film.
Figure 5a, b shows the point SS as function of IDS for the Al2O3/HfO2 and ZrOx NCFET at the VDS of − 0.05 V and− 0.5 V. As shown in Fig. 5b, 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 ZrOx 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 ZrOx NCFET.
We report the demonstration of ferroelectric NC ZrOx pFETs with the sub-60 mV/decade SS, low operating voltage of 1 V and a hysteresis of less than 60 mV. The impact of the amorphous ZrOx films on the ferroelectric behavior is explained by the oxygen vacancy dipoles. The improved IDS and NDR phenomenon are also obtained in Al2O3/HfO2 NCFETs and ZrOx NCFETs compared to the control device. The suppressed NC effect of the Al2O3/HfO2 NCFET can be attributed to partical dipole swiching due to interfical dipoles at the Al2O3/HfO2 interface. The ZrOx NCFETs with sub-60 mV/decade steep slope, improved drain voltage and low operating voltage pave a new way for future low power consumption NCFETs design.
Availability of Data and Materials
The datasets supporting the conclusions of this article are included in the article.
- ZrOx :
Tetrakis (dimethylamido) zirconium
- P r :
- E c :
Coercive electric field
Metal–oxide–semiconductor field-effect transistors
Atomic layer deposition
- B+ :
- Al2O3 :
High-resolution transmission electron microscope
Repaid thermal annealing
- I DS :
- V GS :
- V TH :
Negative capacitance field-effect transistor
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The authors acknowledge support from the National Key Research and Development Project (Grant No. 2018YFB2202800, 2018YFB2200500) and the National Natural Science Foundation of China (Grant No. 62025402, 62090033, 91964202, 92064003, 61874081, 61851406, 62004149 and 62004145).
The authors declare that they have no competing interests.
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. From the C–V curve of ZrOx NCFETs in Fig. S1 (a), we can see that the threshold voltage of the ZrOx NCFETs is around 0.5 V. From the XPS of TaN/ZrOx (4.2 nm)/Ge capacitors in Fig. S1 (b), we can see that a TaOx interfacial layer formed in the TaN/ZrOx interface and oxygen vacancies (ZrOx) in ZrOx because of the scavenging effect.
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Zhang, S., Liu, H., Zhou, J. et al. ZrOx Negative Capacitance Field-Effect Transistor with Sub-60 Subthreshold Swing Behavior. Nanoscale Res Lett 16, 21 (2021). https://doi.org/10.1186/s11671-020-03468-w
- Amorphous ZrOx
- Subthreshold swing
- Negative capacitance