ZrO_x Negative Capacitance Field-Effect Transistor with Sub-60 Subthreshold Swing Behavior

Abstract

Here we report the ZrO_x-based negative capacitance (NC) FETs with 45.06 mV/decade subthreshold swing (SS) under ± 1 V V_GS range, which can achieve new opportunities in future voltage-scalable NCFET applications. The ferroelectric-like behavior of the Ge/ZrO_x/TaN capacitors is proposed to be originated from the oxygen vacancy dipoles. The NC effect of the amorphous HfO₂ and ZrO_x 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 ZrO_x-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 ZrO_x. The suppressed NC effect of Al₂O₃/HfO₂ NCFET compared with ZrO_x 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.

Background

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 [3]. 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 HfZrO_x (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 ZrO_x 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 ZrO_x-based NCFET has not been carried out.

In this work, Ge NCFETs with 5 nm ZrO_x 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 ZrO_x (5 nm) NCFET, which can be attributed to the NC effect of ZrO_x ferroelectric layer. And we analyzed the polarization P as function of applied voltage V for the Ge/ZrO_x/TaN capacitors. The ferroelectric-like behavior of the Ge/ZrO_x/TaN capacitors is induced by the voltage-induced oxygen vacancy dipoles. Moreover, we attributed the improved I_DS and the sudden drop of I_G in the Al₂O₃/HfO₂ NCFETs and ZrO_x NCFETs to the NC effect. We also observed the NDR phenomenon in the Al₂O₃/HfO₂ NCFETs and ZrO_x NCFETs. In addition, we further analyzed the physical mechanism of interfacial dipoles-induced decreased NC effect in the Al₂O₃/HfO₂ NCFET. The ZrO_x 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”.

Methods

Key process steps for NCFETs with ZrO_x and Al₂O₃/HfO₂ fabrication are shown in Fig. 1a. Different gate dielectric insulators, including Al₂O₃/amorphous HfO₂ (5 nm) films and amorphous ZrO_x (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 ZrO_x 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 ZrO_x 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 ZrO_x (4.2 nm) film on Ge (001). The C–V curve of ZrO_x NCFETs and the X-ray photoelectron spectra (XPS) of TaN/ZrO_x (4.2 nm)/Ge capacitors were measured in Additional file 1: Fig. S1.

Fig. 1

a Key process steps for the fabrication of the Al₂O₃/5 nm HfO₂ NCFETs and 4.2 nm ZrO_x NCFETs. b Schematics and c HRTEM images of the fabricated ZrO_x NCFETs. d Schematics and e HRTEM images of the fabricated Al₂O₃/HfO₂ NCFETs

Results and Discussion

Figure 2a shows the measured curves of polarization P v.s. applied voltage V characteristics for the Ge/ZrO_x/TaN capacitors at 3.3 kHz. The gate length (L_G) of the capacitors are 8 μm. It is observed that the remnant polarization P_r of the Ge/ZrO_x/TaN capacitors can be enhanced with larger sweeping range of V. The ferroelectric-like behavior of the amorphous ZrO_x film in the Fig. 2a is proposed to be originated from the voltage-driven oxygen vacancy dipoles [35]. Figure 2b shows the measured P–V curves for the Ge/ZrO_x/TaN capacitors under different frequencies from 200 to 10 kHz. We can see that the ferroelectric-like behavior of the amorphous ZrO_x film remain stable for all frequencies. However, the P_r of the amorphous ZrO_x 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 ZrO_x film decreases. Thus, part of oxygen vacancy diploes switching is incomplete, providing decreased P_r.

Fig. 2

Measured P versus V characteristics of the 4.2 nm ZrO_x capacitors with a different sweeping ranges of V and b different measurement frequences

Figure 3a shows the measured I_DS–V_GS curves of a ferroelectric Al₂O₃/HfO₂ NCFET at the V_DS of − 0.05 V and − 0.5 V. The L_G of the devices is 3 μm. The hysteresis loops of 0.14 V (V_DS = − 0.05 V, I_ds = 1 nA/μm) and 0.08 V (V_DS = − 0.5 V, I_ds = 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) V_GS. Therefore, the threshold voltage (V_TH) 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 |V_GS–V_TH| =|V_DS|= 0.8 V. The current enhancement is induced by the increased inversion charge intensity (Q_inv) 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 V_DS increases [40, 41]. Figure 3c compares the measured gate leakage I_G–V_GS curves for the 5 nm Al₂O₃/HfO₂ NCFET at the V_DS of − 0.05 V and − 0.5 V. The sudden drops of I_G only during the reverse sweeping indicate the decreased voltage in the amorphous HfO₂ film and the amplication of surface potential [42]. The absence of NC effect during the forward sweeping is caused by the partical switching of oxygen vacancy dipoles in the amorphous HfO₂ film [43]. 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).

Fig. 3

a Measured I_DS–V_GS curves of the 5 nm HfO₂ NCFET when V_DS = − 0.5 V and V_DS = − 0.05 V. b Measured I_DS–V_DS curves of the HfO₂ NCFET and the control MOSFET. c Measured I_G–V_GS curves of the 5 nm HfO₂ NCFET when V_DS = − 0.5 V and V_DS = − 0.05 V

Figure 4a shows the measured transfer curves of a ferroelectric ZrO_x NCFET at the V_DS of − 0.05 V and − 0.5 V. The L_G of the two devices are 4 μm. The clockwise hysteresis loops of 0.24 V (V_DS = − 0.05 V, I_ds = 1 nA/μm) and 0.14 V (V_DS = − 0.5 V, I_DS = 1 nA/μm) are demonstrated, respectively. As shown in Fig. 4b, the output characteristics of the ZrO_x NCFET and the control FET are compared. The saturation current of the ZrO_x NCFET exceeds 30 μA/μm, with a rise of 12% compared to that of the control FET at |V_GS–V_TH| =|V_DS|= 1 V. The improved current enhancement and more obvious NDR indicate the enhanced NC effect of the amorphous ZrO_x film (5 nm) contrast to that of 5 nm HfO₂ film. Figure 4c compares the measured gate leakage I_G–V_GS curves for the 5 nm ZrO_x NCFET at the V_DS of − 0.05 V and − 0.5 V. Compared to the sudden I_G drops of Al₂O₃/HfO₂ NCFET only during reverse sweeping in Fig. 3c, the sudden drops of I_G both in forward and reverse sweeping in Fig. 4c also prove the enhanced NC effect in the amorphous ZrO_x film.

Fig. 4

a Measured I_DS–V_GS curves of the 5 nm ZrO_x NCFET when V_DS = − 0.5 V and V_DS = − 0.05 V. b Measured I_DS–V_DS curves of the ZrO_x NCFET and the control MOSFET demonstraing the obvious NDR phenomenon. c Measured I_G–V_GS curves of the 5 nm ZrO_x NCFET when V_DS = − 0.5 V and V_DS = − 0.05 V

Figure 5a, b shows the point SS as function of I_DS for the Al₂O₃/HfO₂ and ZrO_x NCFET at the V_DS 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 V_GS at the V_DS of− 0.05 V and− 0.5 V. When V_DS 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 V_DS 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 ZrO_x 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 ZrO_x NCFET.

Fig. 5

Point SS as a function of I_DS for the a Al₂O₃/5 nm HfO₂ NCFETs and b 5 nm ZrO_x NCFETs

Conclusions

We report the demonstration of ferroelectric NC ZrO_x 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 ZrO_x films on the ferroelectric behavior is explained by the oxygen vacancy dipoles. The improved I_DS and NDR phenomenon are also obtained in Al₂O₃/HfO₂ NCFETs and ZrO_x 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 ZrO_x 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.