Source: https://nanoscalereslett.springeropen.com/articles/10.1186/s11671-019-2927-9
Timestamp: 2019-04-22 18:00:00+00:00

Document:
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.
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) . The negative capacitance (NC) phenomena in NCFETs have been extensively studied in different channel materials, including silicon (Si) [2, 3], germanium (Ge) , germanium-tin (GeSn) , III–V , and 2D materials . Also, the NC characteristics have been demonstrated in NCFETs with various ferroelectrics, such as BiFeO3 , PbZrTiO3 (PZT) , PVDF , and Hf1−xZrxO2 . Compared to other ferroelectrics, Hf1−xZrxO2 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 Hf1−xZrxO2, which affects the matching between MOS capacitance (CMOS) and ferroelectric capacitance (CFE) [12, 13]. It is expected that the Zr composition in Hf1−xZrxO2 also has a great impact on the performance of NCFETs, because it determines the ferroelectric properties of Hf1−xZrxO2. 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.
Key process steps for fabricating Ge p-channel NCFETs with the different Zr compositions in Hf1−xZrxO2 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 Hf1-xZrxO2 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 Hf1−xZrxO2 film. Ge control pMOSFETs with the Al2O3/HfO2 stack was also fabricated.
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 Hf0.52Zr0.48O2 ferroelectric. The thicknesses of Al2O3 and Hf0.52Zr0.48O2 layers are 2 nm and 7 nm, respectively.
To confirm the stoichiometries of Hf1−xZrxO2, 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 Hf0.67Zr0.33O2, Hf0.52Zr0.48O2, and Hf0.33Zr0.67O2 films. The material compositions were calculated based on the area ratio of the peaks and the corresponding sensitivity factors. The two peaks of Zr3d5/2 and Zr3d3/2 have a spin-orbital splitting of 2.4 eV, which is consisted with Refs. [14, 15]. With the increment of Zr composition in Hf1−xZrxO2, Zr3d, and Hf4f peaks shift to the lower energy direction.
The ferroelectric properties of the Hf1−xZrxO2 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/Hf1−xZrxO2(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 Hf1−xZrxO2 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].
Figure 4(a) shows the measured transfer characteristics of Ge NCFETs with Hf0.52Zr0.48O2 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 LG 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 VTH of the NC devices shift to the positive VGS 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 IDS-VGS loop, opposite to the results induced by ferroelectric switching . Point SS vs. IDS 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 IDS induced by the NC effect . 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 IDS-VDS curves of the NCFETs with Hf0.52Zr0.48O2 ferroelectric annealed at different temperatures are shown in Fig. 4(c). IDS-VDS curves of the NC transistor show the obvious NDR phenomenon, which is a typical characteristic of NC transistors [20–23]. Figure 4(d) is the plots of the IDS of the Ge NCFETs with the Hf0.52Zr0.48O2 ferroelectric layer annealed at 450, 500, and 550 °C, respectively, at VDS = − 0.05 V and − 0.5 V, and |VGS − VTH| = 1.0 V. Here, the VTH is defined as the VGS at IDS of 10−7 A/μm. IDS 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.
In addition to the Hf0.52Zr0.48O2 ferroelectric transistor, we also investigate the electrical characteristics of Ge NC transistors with the Hf0.33Zr0.67O2 ferroelectric. Figure 5(a) presents the IDS-VGS characteristics of the devices with Hf0.33Zr0.67O2 with the different annealing temperatures at VDS = − 0.05 V and − 0.5 V. Compared to the Hf0.52Zr0.48O2 NC transistors, even smaller hysteresis is obtained. Similar to the Hf0.52Zr0.48O2 NC transistors, as the annealing temperature increases from 450 to 550 °C, VTH of the device increase from − 0.63 V to 0.51 V in the forward sweeping at VDS = − 0.05 V. Point SS as a function of IDS characteristics for the Hf0.33Zr0.67O2 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 IDS of the Hf0.33Zr0.67O2 NCFETs at VDS = − 0.05 V and − 0.5 V, and |VGS–VTH| = 1.0 V. Whether for the forward or reverse sweeping, the IDS increases with the annealing temperature, which is consistent with the characteristic of the Hf0.52Zr0.48O2 device.
We also investigate the electrical performance of Ge NCFET with the smaller Zr composition. The transfer characteristics of the Hf0.67Zr0.33O2 NCFETs annealed at different annealing temperatures are presented in Fig. 6(a). No hysteresis phenomenon is observed. Compared to Hf0.33Zr0.67O2 and Hf0.52Zr0.48O2 devices, the VTH shift induced by varying annealing temperature is less pronounced in Hf0.67Zr0.33O2 NCFETs. Point SS vs. IDS curves in Fig. 6(b) show that the Hf0.67Zr0.33O2 NC transistor exhibits the sudden drop in some points of SS of NC transistor at VDS = − 0.05 V. Figure 6(c) presents the IDS of Hf0.67Zr0.33O2 Ge NCFETs annealed at 450 °C, 500 °C, and 550 °C, at VDS = − 0.05 V and − 0.5 V, and |VGS–VTH| = 1.0 V. Likewise, IDS enhances as the RTA temperature increases.
The stability of the NC effect induced by the ferroelectric layer of the Hf0.52Zr0.48O2 NCFET was verified by multiple DC sweeping measurements. The measured IDS-VGS curves over 100 cycles of DC sweeping are shown in Fig. 7(a). It can be seen that the values of VGS 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 Hf1−xZrxO2 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 VDS 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 IDS of NCFETs annealed at 450 °C is plotted in Fig. 8(b), at VDS = − 0.5 V and VGS − VTH = − 1.0 V. Open and solid represent the forward and reverse sweeping, respectively. The NC device with Hf0.52Zr0.48O2 achieves the highest IDS, but its hysteresis is serious. NCFET with Hf0.67Zr0.33O2 can obtain excellent performance with hysteresis-free curves and high IDS. As Zr composition increases, the ferroelectric capacitance Cfe (= 0.3849*Pr/(Ec*tfe) ) increases with the increasing of Pr, and meanwhile, the MOS capacitance (CMOS) rises as well due to the growing permittivity of the HZO film. The IDS and hysteresis are determined by |Cfe| and CMOS of the transistor. With Zr composition increasing from 0.33 to 0.48, the increase of |Cfe| is speculated to be slower than does the CMOS, leading to the widening of the hysteresis. Nevertheless, the larger CMOS produces a higher IDS. With the further increase of Zr composition, the increase of |Cfe| is faster than CMOS, which might provide |Cfe| ≥ CMOS, reducing the hysteresis of NCFET.
The impacts of the annealing temperature and Zr composition in Hf1−xZrxO2 on the electrical performance of the Ge NCFETs are experimentally studied. The stoichiometries and ferroelectric properties of Hf1−xZrxO2 were confirmed by XPS and P-V measurements, respectively. NCFETs demonstrate the steep point SS and improved IDS compared to the control device, due to the NC effect. The VTH and IDS of the Hf1−xZrxO2 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. Hf0.67Zr0.33O2 NCFET can more easily achieve the hysteresis-free characteristics than the devices with higher Zr composition.
The authors acknowledge support from the National Natural Science Foundation of China under Grant No. 61534004, 61604112, 61622405 and 61874081, and 61851406. This work was also supported by the 111 Project (B12026).
YP carried out the experiments and drafted the manuscript. YP and GQH designed the experiments. GQH and YL helped to revise the manuscript. JCZ and YH supported the study. All the authors read and approved the final manuscript.
State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi’an 710071, People’s Republic of China.

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