Ferroelectric Polarization Enhancement in Hafnium-Based Oxides Through Capping Layer Engineering

In this work, we investigate that the capping layer (CL) engineering of aluminum oxide (AlOx) on the dopant-free hafnium oxide (HfOx) and the hafnium zirconium oxide (HfZrOx) ferroelectric metal-ferroelectric-metal (MFM) capacitors. The AlOx CL featuring large bandgap and excellent thermal stability offers a stable interface favorable for ferroelectric phase transition. Therefore, the ferroelectric polarization and high-temperature leakage current of HfZrOx MFM capacitor can be largely improved due to the combination of zirconium doping and AlOx capping effect. From the analysis of interface thermodynamic stability and leakage current mechanism, the AlOx CL effectively alleviates interface defect traps between electrode and ferroelectric HfZrOx, which lowers high-temperature leakage current, reduces ferroelectric domains pinning, enhances ferroelectric polarization, and stabilizes the long-term endurance cycling.


I. INTRODUCTION
To support the rapid development of neuromorphic systems, artificial intelligence chips and Internet of Thing (IoT) technologies, the ferroelectric random-access memory and ferroelectric field-effect transistor attract more attention and are the promising candidate for the applications of next-generation memory [1], [2], [3] and in-memory computing [4]. Recently, the HfO 2 -based ferroelectric materials have been widely investigated to replace the conventional ferroelectric perovskites, owing to the potential advantages including lowpower consumption, fast switching speed, high device scalability and complementary-metal-oxide-semiconductor friendly process [5], [6], [7], [8], [9], [10]. According to the previous works, using the dopants (Aluminum, Zirconium, etc.) [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] and metal gate stress engineering [20], [21] to stabilize the formation of the non-centrosymmetric orthorhombic phase transition are common-used approaches to achieve the ferroelectricity properties in HfO 2 film. It is worth to note that the ferroelectric HfZrO x films with ∼50% Zr doping ratio have successfully achieved the favorable ferroelectric phase transition, well-behaved ferroelectric polarization property and steep switched transistor with negative capacitance operation [15], [16], [17], [18], [19]. However, the requirement of slight element doping into HfO 2 possibly causes an undesired fluctuation on the device performance while the film thickness is continuously scaled down. The imprecise element doping in HfO 2 would affect the formation of ferroelectric orthorhombic phase during annealing and also increase the instability of ferroelectric domain switching. Our previous work has reported that the ferroelectric polarization of dopant-free HfO 2 can be significantly enhanced by mechanical stress of metal electrode [22], but also reveal the issues of interface traps and leakage current. To solve those above these problems, we adopt the AlO x capping layer to integrate with the ferroelectric HfZrO x film. The AlO x film owns large bandgap and excellent thermal stability, which can offer a stable interface desirable for ferroelectric phase transition during high-temperature annealing process. The ferroelectric crystallinity, ferroelectric polarization property, leakage current characteristics and switching stability will be discussed here.

II. EXPERIMENTS
In this work, we fabricated dopant-free HfO 2 MFM capacitors with AlO x CL. A 30-nm-thick TiN was grown as a bottom electrode on n + -type Si substrate, then 7-nm-thick dopant-free HfO 2 was deposited on the bottom TiN electrode by thermal atomic layer process (ALD) using tetraethylmethyl amino hafnium (TDMAH) and H 2 O precursor. After that, a 2-nm-thick AlO x film using trimethylaluminum (TMA) and H 2 O precursor was deposited on HfO 2 film as CL. After depositing AlO x CL, the annealing temperature of 600 • C was performed under nitrogen ambient for 30sec. Finally, the tantalum nitride (TaN) was deposited by sputtering system as a top electrode. The flow rates of Ar and N 2 are 100 sccm and 10 sccm, respectively. The area of MFM capacitor is 10000 µm 2 . On the other hand, in order to investigate the CL effect of doped-HfO 2 MFM capacitors, the 7.5-nm-thick ferroelectric HfZrO x MFM capacitors with Zr-doping ratio of 50% using tetrakis (dimethylamido) zirconium (TDMAZ) and H 2 O precursor were simultaneously fabricated. The selected thicknesses of AlO x CLs for ferroelectric HfZrO x were 1 nm, 1.5 nm, and 2 nm. The polarization hysteresis loops and electrical characteristics of MFM capacitor devices were measured by using a precision RT66C ferroelectric tester and a semiconductor characterization system (Keithley 4200-SCS), respectively. Fig. 1(a) shows the polarization hysteresis loop of HfO x MFM capacitors at an applied voltage of 2.5 V. It is clearly observed that the HfO x MFM capacitor presents a weak ferroelectric property. This is because non-ferroelectric phase within HfO x dominates the ferroelectric behavior of HfO x MFM capacitor. To improve the ferroelectric property, a thin AlO x film is introduced as a capping layer on HfO x layer. From the polarization switching property, the well-behaved hysteresis loop is significantly enhanced after using amorphous AlO x CL, which explain that the AlO x CL plays a key role in determining the ferroelectric crystallinity and interface-trap formation during the annealing of HfO x . The maximum doubled remanent polarization (2P r ) was shown in Fig. 1(b). The value of 2P r measured at ±2.5 V increases from 1.3 to 6.2 µC/cm 2 after using AlO x CL, which confirms the enhanced ferroelectric polarization under excluding the contribution of leakage current. To deeply understand the influence of AlO x CL effect on the ferroelectricity of dopant-free HfO x film, the film crystallinity of AlO x /HfO x film stack was analyzed by grazing-angle incidence x-ray diffraction (GI-XRD), as shown in Fig. 1(c). Compared to the control sample without CL, the HfO x MFM capacitor with CL shows the significant increase in the peak (111) at 30.3 • (orthorhombic phase) responsible for ferroelectric property. Moreover, the intensity of orthorhombic peak (020) at 35.6 • is also stronger than that of control sample. Thus, it can be found that the monoclinic phase of control sample can be partially suppressed under capping layer effect, as shown in Fig. 1(d). The reduction on the monoclinic phase ratio from 67% to 54% is important for facilitating the orthorhombic phase transformation during film annealing. As seen Fig. 1(e), the increase of orthorhombic phase ratio from 32% to 45% is achieved with the assistance of AlO x CL, which effectively provides enough ferroelectric domains in dopant-free HfO x .

III. RESULTS AND DISCUSSION
In order to understand the difference of AlO x CL effect between dopant-free HfO x and doped-HfO x films, the HfZrO x MFM capacitors with different CL thicknesses were also carried out for a performance comparison. From thermodynamic analysis, the Gibbs free energies for ZrO 2 , HfO 2 , Al 2 O 3 and TaN are −1100 kJ/mol, −1010.8 kJ/mol, −1500 kJ/mol, and −604.96 kJ/mol, respectively [23], [24]. We can understand that the interface reaction at the TaN/HfO x and TaN/ZrO x interface is more significant than that of TaN/AlO x interface. Thus, the interface quality between TaN electrode and ferroelectric HfZrO x is a major concern for the stability of ferroelectric domain switching. The Fig. 2(a) shows the transmission electron microscope (TEM) images of HfZrO x films with 1-nm-and 2-nm-thick AlO x CLs. The AlO x CL is amorphous phase confirmed by fast Fourier transform (FFT) pattern of TEM image. Fig. 2(b) shows the hysteresis loops of HfZrO x MFM capacitors with different CL thicknesses. We can clearly observe that the ferroelectric polarization property is largely improved by AlO x CL. With increasing the thickness of CL, the value of 2P r increases from 11.9 µC/cm 2 to 36.9 µC/cm 2 , which exhibits at least three times increase in 2P r . In addition, the case of 2 nm-CL has larger instantaneous current than other conditions as shown in Fig. 2(c).
The GI-XRD spectrum of HfZrO x MFM capacitors with different thicknesses of CLs were shown in Fig. 3(a). It can be seen that the intensities of peaks at 30.3 • (111) and 35.6 • (020) corresponding to orthorhombic phase enhances with the increase of AlO x CL, especially for the case with a 2-nm-thick AlO x CL. Thus, we can confirm the strong ferroelectricity observed in 2-nm-thick CL case is mainly originated from the increase of ferroelectric crystalline phase. As shown in Fig. 3(b), the orthorhombic phase ratios significantly increase from 48% to 57% under the optimal thickness condition of 2-nm-thick AlO x CL. Compared to ferroelectric crystallization of dopant-free HfO x shown in Fig. 1(e), the HfO x film integrating with Zr doping and AlO x capping layer can optimize the orthorhombic phase ratio up to 57% and maximize the 2P r up to 36.9 µC/cm 2 at a low operating voltage of 3 V.  Fig. 4(a) shows the typical butterfly-shaped C-V curves measured from HfZrO x MFM capacitors with various thicknesses of CLs. The capacitance values measured at 100 kHz with bi-directional sweeps and showed significant increase as the thickness of AlO x CL increase. The dielectric permittivity derived from C-V curves is shown in Fig. 4(b). The extracted permittivity of HfZrO x film without CL is ∼ 21 and the permittivity of HfZrO x with 2-nm-thick AlO x CL is ∼ 29. It can be inferred that the AlO x CL lowers the stray electric field and enhances the polarization electric field, which increases the permittivity of ferroelectric film stack [25]. Thus, the amorphous AlO x CL has no adverse effect on the depolarization field due to the increase of orthorhombic phase stemming from capping effect to give rise to higher amount of switching domains, which fully agrees to the results of Fig. 2(a) and Fig. 3(b).
On the other hand, to understand the influence of interface traps on the leakage current characteristics of HfZrO x MFM capacitor before and after using AlO x CL, the temperature dependence of I-V measurement was also performed, as shown in Fig. 5(a). The leakage current at 2.5 V for HfZrO x MFM capacitor with AlO x CL is apparently lower than that of control sample without CL, especially at high temperature of 85 • C. The leakage current at 85 • C is effectively improved from 1.29 × 10 −6 A to 9.55 × 10 −8 A under the optimal thickness of 2-nm-thick AlO x CL. The high temperature leakage suppression can be ascribed to wide bandgap of AlO x with large conduction band offset with Si [26]. Fig. 5(b) presents the calculation result of the trapping level extracted from I-V curves of Fig. 5(a). It can be clearly observed that the trapping level gradually changes from 0.66 eV to 0.77 eV with increasing the CL thickness up to 2nm. The deeper trapping level of 0.77 eV is mainly ascribed to the effect of AlO x CL, which not only reduces the shallow traps near TaN/HfZrO x interface [27], [28], but also enhances the thermal stability of HfZrO x MFM capacitor. Therefore, the AlO x CL effectively improves interface quality to eliminate the interface shallow traps, possibly causing charge trapping/detrapping and domain wall pining during ferroelectric switching.
To further study the switching stability of HfZrO x MFM capacitors without and with AlO x CL, the endurance cycling test was performed under an applied voltage of 3V, as shown in Fig. 6. The wake-up process was performed under a pulse voltage of 3V at a frequency of 1 kHz. From the endurance characteristics, the HfZrO x MFM capacitor with CL is able to withstand 2.65 × 10 6 endurance cycles under repetitive field cycling. For the best condition of 2-nm-thick AlO x CL, the 2P r of 14.17 µC/cm 2 is still measured after 2.65 × 10 6 cycles, which presents significant improvement in 2P r value and endurance cycle as compared to control sample without CL. By contrast, the 2P r value measured at 10 6 cycles of HfZrO x MFM capacitor with CL is almost three time larger than that of control sample. The remarkable fatigue performance improvement is attributed to well-controlled interface quality between TaN electrode and ferroelectric HfZrO x due to the adoption of AlO x CL, which alleviates the generation of defect traps and provides a reliable domain switching during long-term endurance cycling.

IV. CONCLUSION
In this work, we demonstrated the ferroelectric polarization characteristics of dopant-free HfO x and HfZrO x MFM capacitors with AlO x CLs. For dopant-free HfO x film, the AlO x CL effectively reduces the formation of monocline phase within HfO x and promotes the ferroelectric phase transition during annealing. Furthermore, the ferroelectricity, leakage current and endurance cycling of HfZrO x MFM capacitors are also largely improved after using AlO x CLs. These improvements are due to the contribution of interface  engineering to obtain enhanced ferroelectric crystallinity and well-controlled interface with remarkably deeper trapping level than control sample.