Germanium Negative Capacitance Field Effect Transistors: Impacts of Zr Composition in Hf_1−xZr_xO₂

Abstract

Germanium (Ge) negative capacitance field-effect transistors (NCFETs) with various Zr compositions in Hf_1−xZr_xO₂ (x = 0.33, 0.48, and 0.67) are fabricated and characterized. For each Zr composition, the NCFET exhibits the sudden drop in some points of subthreshold swing (SS), which is induced by the NC effect. Drive current I_DS increases with the increase of annealing temperature, which should be due to the reduced source/drain resistance and improved carrier mobility. The steep SS points are repeatable and stable through multiple DC sweeping measurement proving that they are induced by the NC effect. The values of gate voltage V_GS corresponding to steep SS are consistent and clockwise I_DS-V_GS are maintained through the multiple DC sweeps. At fixed annealing temperature, NC device with Hf_0.52Zr_0.48O₂ achieves the higher I_DS but larger hysteresis compared to the other compositions. NCFET with Hf_0.67Zr_0.33O₂ can obtain the excellent performance with hysteresis-free curves and high I_DS.

Background

The ferroelectric negative capacitance field-effect transistor (NCFET) with a ferroelectric film inserted into gate stack is a promising candidate for the low-power dissipation applications owing to its ability to overcome the fundamental limitation in subthreshold swing (SS) for the conventional metal-oxide-semiconductor field-effect transistor (MOSFET) [1]. The negative capacitance (NC) phenomena in NCFETs have been extensively studied in different channel materials, including silicon (Si) [2, 3], germanium (Ge) [4], germanium-tin (GeSn) [5], III–V [6], and 2D materials [7]. Also, the NC characteristics have been demonstrated in NCFETs with various ferroelectrics, such as BiFeO₃ [8], PbZrTiO₃ (PZT) [9], PVDF [10], and Hf_1−xZr_xO₂ [11]. Compared to other ferroelectrics, Hf_1−xZr_xO₂ has the advantage of being compatible with CMOS integration. Experimental studies have shown that the electrical performance of NCFETs can be optimized by varying the thickness and area of Hf_1−xZr_xO₂, which affects the matching between MOS capacitance (C_MOS) and ferroelectric capacitance (C_FE) [12, 13]. It is expected that the Zr composition in Hf_1−xZr_xO₂ also has a great impact on the performance of NCFETs, because it determines the ferroelectric properties of Hf_1−xZr_xO₂. However, there is still a lack of a detailed study on the impacts of Zr composition on the electrical characteristics of NCFETs.

In this paper, we comprehensively study the influences of the annealing temperature and the Zr composition on the performance of Ge NCFET.

Methods

Key process steps for fabricating Ge p-channel NCFETs with the different Zr compositions in Hf_1−xZr_xO₂ are shown in Fig. 1(a). After the pregate cleaning, n-Ge (001) substrates were loaded into the atom layer deposition (ALD) chamber. A thin Al₂O₃ (25 cycles) film was deposited, which was followed by the O₃ passivation. Then, the Hf_1-xZr_xO₂ films (x = 0.33, 0.48 and 0.67) were deposited in the same ALD chamber using [(CH₃)₂N]₄Hf (TDMAHf), [(CH₃)₂N]₄Zr (TDMAZr) and H₂O as the Hf, Zr, and O precursors, respectively. After that, the TaN metal gate was deposited using the reactive sputtering. After gate patterning and etching, boron ions (B⁺) were implanted into source/drain (S/D) regions at an energy of 20 keV and a dose of 1 × 10¹⁵ cm⁻². Non-self-aligned S/D metals were formed by lift-off process. Finally, rapid thermal annealing (RTA) was carried out at various temperatures for dopant activation, S/D metallization, and crystallization of Hf_1−xZr_xO₂ film. Ge control pMOSFETs with the Al₂O₃/HfO₂ stack was also fabricated.

Fig. 1

(a) Key process steps for the fabrication of Ge NCFETs with the different Zr compositions in Hf_1−xZr_xO₂ ferroelectrics. (b) Schematic of the fabricated NC transistor. (c) TEM image of the gate stack of NC device illustrating the 7 nm H_0.52Zr_0.48O₂ layer and 2 nm Al₂O₃ layer

Figure 1(b) shows the schematic of the fabricated NCFET. High-resolution transmission electron microscope (HRTEM) image in Fig. 1(c) shows the gate stack on Ge channel of device with Hf_0.52Zr_0.48O₂ ferroelectric. The thicknesses of Al₂O₃ and Hf_0.52Zr_0.48O₂ layers are 2 nm and 7 nm, respectively.

To confirm the stoichiometries of Hf_1−xZr_xO₂, the X-ray photoelectron spectroscopy (XPS) measurement was carried out. Figure 2(a) and (b) show the Hf4f and Zr3d photoelectron core level spectra, respectively, for the Hf_0.67Zr_0.33O₂, Hf_0.52Zr_0.48O₂, and Hf_0.33Zr_0.67O₂ films. The material compositions were calculated based on the area ratio of the peaks and the corresponding sensitivity factors. The two peaks of Zr3d_5/2 and Zr3d_3/2 have a spin-orbital splitting of 2.4 eV, which is consisted with Refs. [14, 15]. With the increment of Zr composition in Hf_1−xZr_xO₂, Zr3d, and Hf4f peaks shift to the lower energy direction.

Fig. 2

(a) Hf 4f and (b) Zr 3d core level spectra for the Hf_1−xZr_xO₂ samples with the different Zr compositions

The ferroelectric properties of the Hf_1−xZr_xO₂ films (x = 0.33, 0.48, and 0.66) were characterized by the polarization P vs. drive voltage V hysteresis loops measurement. P-V loops were recorded on the pristine devices. Figure 3 shows the curves of P vs. V for TaN/Hf_1−xZr_xO₂(10 nm)/TaN samples in a series of drive voltages. With the post-annealing temperature increases from 500 to 550 °C, the P-V curves of the Hf_1−xZr_xO₂ tend to be saturated in a sub-loop state. As the Zr composition increases, the remnant polarization of the film is obviously improved, and the thinning of the hysteresis loop at zero bias is observed, which can be phenomenologically best described as superimposed antiferroelectric-like characteristics [16, 17].

Fig. 3

Measured P-V curves of the Hf_1-xZrxO2 films with different Zr compositions annealed at 500 and 550 ^oC. (a) and (b) are the Hf_0.67Zr_0.33O₂ film annealed at 500 and 550 ^oC, respectively. (c) and (d) are the Hf_0.52Zr_0.48O₂ film annealed at 500 and 550 ^oC, respectively. (e) and (f) are the Hf_0.33Zr_0.67O₂ film annealed at 500 and 550 ^oC, respectively. With the post annealing temperature increases from 500 to 550 ^oC, the P-V curves of the Hf_1-xZr_xO₂ tend to be saturated in a sub-loop state. An evolution ferroelectric to an antiferroelectric-like behavior is observed with the Zr composition increased

Results and Discussion

Figure 4(a) shows the measured transfer characteristics of Ge NCFETs with Hf_0.52Zr_0.48O₂ ferroelectrics with different annealing temperatures and control device with Al₂O₃/HfO₂ stack dielectric. The control device was annealed at 500 °C. All the devices have a gate length L_G of 2 μm. The forward and reverse sweeping are indicated by the open and solid symbols, respectively. The NCFETs have a much higher drive current compared to the control device. It is seen that, with the annealing temperature increasing from 450 to 550 °C, the threshold voltage V_TH of the NC devices shift to the positive V_GS direction. The NCFETs exhibit a small hysteresis, which becomes negligible with the increasing of RTA temperature. The trapping effect also leads to the hysteresis, but that produces the counterclockwise I_DS-V_GS loop, opposite to the results induced by ferroelectric switching [18]. Point SS vs. I_DS curves in Fig. 4(b) show that the NC transistor exhibits the sudden drop in some points of SS, corresponding to the abrupt change of I_DS induced by the NC effect [19]. It is observed that NCFETs achieve the improved SS characteristics compared to the control device. We found that the sudden drop points of the devices are consistent at the different annealing temperatures. The measured I_DS-V_DS curves of the NCFETs with Hf_0.52Zr_0.48O₂ ferroelectric annealed at different temperatures are shown in Fig. 4(c). I_DS-V_DS curves of the NC transistor show the obvious NDR phenomenon, which is a typical characteristic of NC transistors [20,21,22,23]. Figure 4(d) is the plots of the I_DS of the Ge NCFETs with the Hf_0.52Zr_0.48O₂ ferroelectric layer annealed at 450, 500, and 550 °C, respectively, at V_DS = − 0.05 V and − 0.5 V, and |V_GS − V_TH| = 1.0 V. Here, the V_TH is defined as the V_GS at I_DS of 10⁻⁷ A/μm. I_DS increases with the increasing of RTA temperature, which is due to the reduced source/drain resistance and improved carrier mobility at the higher annealing temperature.

Fig. 4

(a) Measured I_DS-V_GS curves for NCFETs with Hf_0.52Zr_0.48O₂ ferroelectric and control device. (b) Point SS vs. I_DS curves showing that NCFETs have the steeper SS compared to control MOSFET. (c) I_DS-V_DS curves for the NCFETs demonstrating the typical NDR phenomena. (d) Comparison of the I_DS for the NCFETs annealed at various temperatures at a gate overdrive of 1 V

In addition to the Hf_0.52Zr_0.48O₂ ferroelectric transistor, we also investigate the electrical characteristics of Ge NC transistors with the Hf_0.33Zr_0.67O₂ ferroelectric. Figure 5(a) presents the I_DS-V_GS characteristics of the devices with Hf_0.33Zr_0.67O₂ with the different annealing temperatures at V_DS = − 0.05 V and − 0.5 V. Compared to the Hf_0.52Zr_0.48O₂ NC transistors, even smaller hysteresis is obtained. Similar to the Hf_0.52Zr_0.48O₂ NC transistors, as the annealing temperature increases from 450 to 550 °C, V_TH of the device increase from − 0.63 V to 0.51 V in the forward sweeping at V_DS = − 0.05 V. Point SS as a function of I_DS characteristics for the Hf_0.33Zr_0.67O₂ ferroelectric NCFETs are depicted in Fig. 5(b). In addition, devices with 450 °C and 500 °C annealing temperature obtains the more obvious sudden drop in SS in comparison with the 550 °C annealed transistor. The sudden drop points in different annealing temperatures occur at the same gate voltage. Figure 5(c) exhibits forward and reverse I_DS of the Hf_0.33Zr_0.67O₂ NCFETs at V_DS = − 0.05 V and − 0.5 V, and |V_GS–V_TH| = 1.0 V. Whether for the forward or reverse sweeping, the I_DS increases with the annealing temperature, which is consistent with the characteristic of the Hf_0.52Zr_0.48O₂ device.

Fig. 5

(a) Measured transfer characteristics of the Hf_0.33Zr_0.67O₂ NC Ge pFETs annealed from 450 to 550 °C. (b) Point SS as a function of I_DS for the Hf_0.33Zr_0.67O₂ devices. (c) I_DS for the ferroelectric NC transistors with different annealing temperatures at a gate overdrive of 1 V 

We also investigate the electrical performance of Ge NCFET with the smaller Zr composition. The transfer characteristics of the Hf_0.67Zr_0.33O₂ NCFETs annealed at different annealing temperatures are presented in Fig. 6(a). No hysteresis phenomenon is observed. Compared to Hf_0.33Zr_0.67O₂ and Hf_0.52Zr_0.48O₂ devices, the V_TH shift induced by varying annealing temperature is less pronounced in Hf_0.67Zr_0.33O₂ NCFETs. Point SS vs. I_DS curves in Fig. 6(b) show that the Hf_0.67Zr_0.33O₂ NC transistor exhibits the sudden drop in some points of SS of NC transistor at V_DS = − 0.05 V. Figure 6(c) presents the I_DS of Hf_0.67Zr_0.33O₂ Ge NCFETs annealed at 450 °C, 500 °C, and 550 °C, at V_DS = − 0.05 V and − 0.5 V, and |V_GS–V_TH| = 1.0 V. Likewise, I_DS enhances as the RTA temperature increases.

Fig. 6

(a) Measured I_DS-V_GS of the Hf_0.67Zr_0.33O₂ NC Ge pFETs annealed at 450 °C, 500 °C, and 550 °C. (b) Point SS vs. I_DS characteristics of the devices. (c) I_DS for the ferroelectric NC transistors with different annealing temperatures at a gate overdrive of 1 V

The stability of the NC effect induced by the ferroelectric layer of the Hf_0.52Zr_0.48O₂ NCFET was verified by multiple DC sweeping measurements. The measured I_DS-V_GS curves over 100 cycles of DC sweeping are shown in Fig. 7(a). It can be seen that the values of V_GS corresponding to steep SS are consistent. In addition, the clockwise I-V loops are maintained through the multiple DC sweeps. The steep SS points are repeatable and stable through multiple DC sweeps, which further proves that they are induced by the NC effect. Figure 7(b) presents the best point SS and drive current across the number of sweeping cycles. Figure 7(c) shows the hysteresis characteristics as a function of the number of DC sweeping cycles. Stable I-V hysteresis window of ~ 82 mV are seen.

Fig. 7

(a) Measured I_DS-V_GS curves of a Hf_0.52Zr_0.48O₂ NC Ge pFET over 100 cycles of DC sweeping. (b) Best point SS and I_DS vs. cycle number. (c) Hysteresis characteristics as a function of the number of DC sweeping cycles

We summarize the hysteresis and drive current characteristics of Ge NCFETs with different Zr compositions in Hf_1−xZr_xO₂ in Fig. 8. As shown in Fig. 8(a), the hysteresis values are 70, 148, and 106 mV for devices with x = 0.33, 0.48, and 0.67, respectively, at a V_DS of – 0.5 V. As the composition increases from 0.33 to 0.48, the hysteresis of the NC device increases significantly. With the further increasing of Zr composition, the hysteresis decreases rapidly. The I_DS of NCFETs annealed at 450 °C is plotted in Fig. 8(b), at V_DS = − 0.5 V and V_GS − V_TH = − 1.0 V. Open and solid represent the forward and reverse sweeping, respectively. The NC device with Hf_0.52Zr_0.48O₂ achieves the highest I_DS, but its hysteresis is serious. NCFET with Hf_0.67Zr_0.33O₂ can obtain excellent performance with hysteresis-free curves and high I_DS. As Zr composition increases, the ferroelectric capacitance C_fe (= 0.3849*P_r/(E_c*t_fe) [24]) increases with the increasing of P_r, and meanwhile, the MOS capacitance (C_MOS) rises as well due to the growing permittivity of the HZO film. The I_DS and hysteresis are determined by |C_fe| and C_MOS of the transistor. With Zr composition increasing from 0.33 to 0.48, the increase of |C_fe| is speculated to be slower than does the C_MOS, leading to the widening of the hysteresis. Nevertheless, the larger C_MOS produces a higher I_DS. With the further increase of Zr composition, the increase of |C_fe| is faster than C_MOS, which might provide |C_fe| ≥ C_MOS, reducing the hysteresis of NCFET.

Fig. 8

Statistical plots of (a) hysteresis and (b) I_DS of Ge NCFET with Hf_1−xZr_xO₂ (x = 0.33, 0.48, and 0.67)

Conclusions

The impacts of the annealing temperature and Zr composition in Hf_1−xZr_xO₂ on the electrical performance of the Ge NCFETs are experimentally studied. The stoichiometries and ferroelectric properties of Hf_1−xZr_xO₂ were confirmed by XPS and P-V measurements, respectively. NCFETs demonstrate the steep point SS and improved I_DS compared to the control device, due to the NC effect. The V_TH and I_DS of the Hf_1−xZr_xO₂ NCFET are greatly affected by the annealing temperature. Multiple DC sweeping measurements show that the stability of the NC effect induced by the ferroelectric layer is achieved in NCFET. Hf_0.67Zr_0.33O₂ NCFET can more easily achieve the hysteresis-free characteristics than the devices with higher Zr composition.