Germanium Negative Capacitance Field Effect Transistors: Impacts of Zr Composition in Hf1−xZrxO2

Germanium (Ge) negative capacitance field-effect transistors (NCFETs) with various Zr compositions in Hf1−xZrxO2 (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 IDS 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 VGS corresponding to steep SS are consistent and clockwise IDS-VGS are maintained through the multiple DC sweeps. At fixed annealing temperature, NC device with Hf0.52Zr0.48O2 achieves the higher IDS but larger hysteresis compared to the other compositions. NCFET with Hf0.67Zr0.33O2 can obtain the excellent performance with hysteresis-free curves and high IDS.


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 BiFeO 3 [8], PbZrTiO 3 (PZT) [9], PVDF [10], and Hf 1−x Zr x O 2 [11]. Compared to other ferroelectrics, Hf 1 −x Zr x O 2 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−x Zr x O 2 , 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−x Zr x O 2 also has a great impact on the performance of NCFETs, because it determines the ferroelectric properties of Hf 1 −x Zr x O 2 . 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. deposited in the same ALD chamber using [(CH 3 ) 2 N] 4 Hf (TDMAHf ), [(CH 3 ) 2 N] 4 Zr (TDMAZr) and H 2 O 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 × 10 15 cm −2 . Non-selfaligned 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−x Zr x O 2 film. Ge control pMOSFETs with the Al 2 O 3 /HfO 2 stack was also fabricated. Figure 1(b) shows the schematic of the fabricated NCFET.
High-resolution transmission electron microscope (HRTEM) image in Fig. 1 To confirm the stoichiometries of Hf 1−x Zr x O 2 , the X-ray photoelectron spectroscopy (XPS) measurement was carried out. Figure 2 [14,15]. With the increment of Zr composition in Hf 1−x Zr x O 2 , Zr3d, and Hf4f peaks shift to the lower energy direction.
The ferroelectric properties of the Hf 1−x Zr x O 2 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−x Zr x O 2 (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−x Zr x O 2 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]. 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 0.52 Zr 0.48 O 2 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]. 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.

Results and Discussion
In addition to the Hf 0.52 Zr 0.48 O 2 ferroelectric transistor, we also investigate the electrical characteristics of Ge NC transistors with the Hf 0.33 Zr 0.67 O 2 ferroelectric. Figure 5 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. We summarize the hysteresis and drive current characteristics of Ge NCFETs with different Zr compositions in Hf 1−x Zr x O 2 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 0.52 Zr 0.48 O 2 achieves the highest I DS , but its hysteresis is serious. NCFET with Hf 0.67 Zr 0.33 O 2 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.

Conclusions
The impacts of the annealing temperature and Zr composition in Hf 1−x Zr x O 2 on the electrical performance of the Ge NCFETs are experimentally studied. The stoichiometries and ferroelectric properties of Hf 1−x Zr x O 2 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−x Zr x O 2 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 0.67 Zr 0.33 O 2 NCFET can more easily achieve the hysteresis-free characteristics than the devices with higher Zr composition.