Nonvolatile Applications and Reliability Investigation of La-Doped ZrO₂ Antiferroelectric Capacitors

Abstract

The nonvolatile application of La-doped ZrO₂ (ZLO) antiferroelectric capacitors is demonstrated in this study, accompanied by systematic investigation of device reliability. A built-in electric field was successfully established through engineered work function modulation. The fabricated nonvolatile (NV) ZLO capacitor exhibits not only avoidance of wake-up and fatigue phenomena typically observed in ferroelectric systems but also demonstration of ultralow coercive voltage (2Vc = 1.2 V) and exceptional endurance exceeding 10¹² cycles. The inherent unique polarization reversal mechanism in NV ZLO device was identified as the origin of a unidirectional imprint effect. Accelerated testing at 85 °C for 10⁴ s yielded conclusive evidence of retention characteristic stability. This investigation provides a novel perspective for the engineering utilization of antiferroelectric materials and facilitates their potential incorporation into advanced integrated circuit architectures.

1. Introduction

In 2011, ferroelectric and antiferroelectric characteristics were first reported in Si-doped HfO₂ [1]. In 2012, these phenomena were also observed in Zr-doped HfO₂ (HZO) through the modification of the composition [2]. The discovery initiated the flourishing research on ferroelectric and antiferroelectric properties of HfO₂-based oxides compatible with current semiconductor process. In recent years, significant progress has been made in the study of HfO₂-based ferroelectric materials. In order to stabilize and fine-tune the ferroelectric properties of HfO₂, a broad spectrum of chemical elements, including Si [1], Al [3], Zr [4], Sc [5], La [6], Gd [7], Y [8], Ge [5], and N [9], have been investigated as potential dopant. In particular, HZO films have emerged as promising candidate for next-generation storage technologies due to the stability and scalability at the nanoscale.

Nevertheless, the subpar endurance and the existence of wake-up and fatigue effects during cycling have significantly impeded the development of HZO ferroelectric devices [10]. Antiferroelectric devices have recently been reported to exhibit high reliability; however, challenges such as imprint effect, retention limitation, and their inherent volatility still hinder their development, necessitating further research [10]. To mitigate these issues in ferroelectric and antiferroelectric, extensive investigations have been conducted on various aspects including electrode optimization [11], film thickness adjustment [12], annealing method [13], and multi-element doping [14]. The incorporation of La into HZO ferroelectric film was reported as an effective strategy for endurance enhancement [14,15,16,17,18]. However, La doping has not been recognized as an optimal solution, as the improvement in endurance performance is accompanied by hysteresis loop pinning and the dominant of antiferroelectric behavior [19,20,21,22,23]. This trade-off originates from La doping-induced elevation in oxygen vacancy formation energy, which drives preferential increase of the tetragonal phase associated with antiferroelectricity [24,25,26]. The antiferroelectric compatibility of La inspired the exploration of La-doped ZrO₂ (ZLO) antiferroelectric device. The emergence of ZLO has resolved the wake-up issue in HZO ferroelectric and antiferroelectric systems and exhibited enhanced reliability [27,28,29,30].

In this work, we mainly focused on the nonvolatile (NV) application and the reliability of ZLO antiferroelectric device. To realize nonvolatile application in antiferroelectric, a built-in electric field was induced through the work function tuning method [31,32,33,34]. Typically, TiN is employed as the electrode material due to its excellent thermal stability and compatibility with CMOS processes [35,36,37]. The working function of TiN electrodes can be modulated by varying the film thickness [38,39,40,41,42], which facilitates the introduction of the built-in electric field. The nonvolatile ZLO antiferroelectric capacitors were successfully fabricated through precise optimization of TiN thickness, accompanied by systematic reliability assessment. These capacitors not only overcome the challenges of wake-up and fatigue effects but also exhibit exceptional endurance and retention. This breakthrough broadens the avenue for the practical application of antiferroelectric technology.

2. Experiments

The cross-sectional schematic diagram and the key fabrication procedure of nonvolatile Al/TiN/ZLO/TiN MIM antiferroelectric capacitors are shown in Figure 1. A 10 nm TiN bottom electrode (BE) was deposited on a heavily doped p-type Si substrate by atomic layer deposition (ALD). After that, 8 nm ZLO thin film with a Zr/La ratio up to 20:1 was grown at 290 °C by controlling the number of ALD cycles for ZrO₂ and La₂O₃. The precursors were TEMAZ and La(iPrCp)₃, respectively. Then, a 1 nm TiN tuning layer was deposited at 350 °C using TiCl₄ and NH₃ as reactants. After that, a thermal annealing process of 450 °C for 60 s in an N₂ atmosphere was performed. The contact electrode Al was deposited by thermal evaporation. In the end, the patterning process was carried out and the Al back contact was deposited.

The electrical characteristics were measured using a Keysight B1500A semiconductor analyzer and Liryder F3000 ferroelectric analyzer [43]. Grazing-incidence X-ray diffraction (GIXRD) technology was employed to confirm the change in phase composition within the antiferroelectric ZrO₂ films before and after La doping.

3. Results and Discussion

Initially, we conducted a comparison of the electrical characteristics between volatile ZLO and undoped ZrO₂ antiferroelectric devices, both featuring a 10 nm-thick top TiN electrode. The resultant P–V curves and the concurrent I–V curves are depicted in Figure 2a and Figure 2b, respectively. The hysteresis loops were measured using pulsed triangular waves with 10 µs rise time, 10 µs fall time, and 10 µs interval time between pulses. It can be clearly observed that the positive segments of the hysteresis loops are more pronounced and saturated than their negative counterparts, a trend evident in both ZLO and undoped ZrO₂ devices. This asymmetry likely stems from the asymmetric interface layers, which could be a consequence of the different process undergone by the top and bottom electrodes [44]. Compared to the undoped ZrO₂ antiferroelectric capacitor, the ZLO device exhibits superior polarization and a reduced coercive field, which is consistent with the characteristic of La of enhancing the tetragonal phase [19,25,45]. The relationship between leakage current and voltage was then examined for both types of capacitors. As illustrated in Figure 2c, ZLO devices exhibit a lower leakage current compared to undoped ZrO₂. The reduction in leakage current is attributed to the La-induced passivation of oxygen vacancies, as well as the modulation of the Fermi level through its role as acceptor defect center [19,24]. Then, the C–V curves were measured at a frequency of 100 kHz. As shown in Figure 2d, both capacitors exhibit the classic double-butterfly characteristic of antiferroelectricity. The C–V curve reveals a significant improvement in permittivity, with ZLO capacitor showing a 10% increase in relative permittivity compared to undoped ZrO₂ (from ε_r = 41.8 to ε_r = 46). This discrepancy can be attributed to the stabilization of the high-κ tetragonal phase achieved through La doping.

Grazing-incidence X-ray diffraction (GI-XRD) was employed to monitor this evolutionary process. As shown in Figure 3, a pronounced increase in the t-phase content was observed in ZLO, which correlates well with the enhanced antiferroelectric property. Following the methodology reported in prior studies [46], the main diffraction peaks for the o-phase (o 111) and t-phase (t 011) were identified at 30.5° and 30.8°, respectively. The refined calculations revealed that the t-phase content is 19% in pure ZrO₂ and 38% in ZLO. This finding reveals a clear structure–property relationship between oxygen vacancy concentration, crystalline phase evolution, and antiferroelectric electrical characteristics.

The fundamental electrical properties of conventional volatile ZLO devices have been briefly reviewed in the preceding sections. The following section will provide a detailed discussion of the device characteristics of nonvolatile ZLO antiferroelectric devices, realized through the employing of a designed 1 nm TiN tuning layer.

As extensively reported, the work function of TiN exhibits critical dependence on its thickness [38,39,40,41,42]. It decreases by ~1 eV when its thickness is reduced from 6 nm to 1 nm according to a previous report [38]. In this work, the work function difference between the bottom TiN (10 nm) and top TiN (1 nm) electrodes directly manifests as a ~1 V operational voltage shift, which aligns quantitatively with reported thickness-dependent trends. Figure 4a,b depict the I–V and P–V curves, respectively, of ZLO devices, demonstrating the effectiveness of the adopted approach for achieving nonvolatile applications.

The magnitude of E_built-in is governed by the work function difference between the top (WF_TE) and bottom (WF_BE) electrodes, as expressed by:

$$E_{built\text{-}in}={\displaystyle \frac{1}{t\times q}}({WF}_{TE}-{WF}_{BE})$$

(1)

where t is the AFE layer thickness and q is the elementary charge. The induced E_built-in shifts the AFE polarization–voltage (P–V) hysteresis loop, centering its right branch toward zero field. At low electric fields, a ferroelectric-like hysteresis loop (the right branch) is observed, whereas the full AFE hysteresis loop emerges under high electric fields. This critical adjustment stabilizes the constricted hysteresis loop, allowing deterministic switching between the polarized (binary “0”) and non-polarized (binary “1”) states without requiring sustained external bias.

Then, the endurance of the work function-modulated ZLO device was investigated. Given the achievement of a nonvolatile state, the endurance testing protocol aligns with that of ferroelectric devices, involving the application of bipolar stress at 100 kHz. After 10¹⁰ cycles, the P–V and I–V curves of the ZLO capacitor incorporating a 1 nm TiN tuning layer are presented in Figure 5a and Figure 5b, respectively. After 10¹⁰ cycles, the polarization intensity remains nearly unchanged (2Pr = 22.8 µC/cm²), with only a slight reduction in the positive coercive field. This decrease in the coercive field is advantageous for practical circuit applications.

To further optimize the ZLO device’s performance, a 6 nm ZLO film was accomplished, targeting a lower operating voltage. The P–V and I–V curves of the 6 nm nonvolatile ZLO capacitor were tested under varying applied voltages. The results reveal that when the measurement voltage is reduced from 2.1 V to 1.5 V, the polarization decreases by only 17%. However, a further reduction to 1.2 V leads to a significant polarization decay (35%), though the 2Pr value remains at a notable 15.6 µC/cm². In order to balance operate voltage and polarization strength, a bias of 1.5 V was chosen for the endurance process, as depicted in Figure 5c,d. After an extensive 10¹⁰ cycles, the NV ZLO device demonstrates exceptional robustness, with the endurance behavior exhibiting virtually no wake-up or fatigue effects.

Subsequently, we investigated the imprint effect of the NV ZLO devices. The imprint effect manifests as a persistent shift of the hysteresis loop. This phenomenon, predominantly driven by charge injection and interfacial charge trapping, represents a critical factor in the deterioration in retention performance in ferroelectric and antiferroelectric devices [47]. In Figure 6a, following positive polarization, the leftward shift of the hysteresis loop in the ZLO device, observed over different retention durations, is clearly illustrated. The presence of polarization charge leads to a large internal electric field in the interface layer, thereby causing the charge injection and electric field shift.

When a negative voltage is applied, the internal polarization charge undergoes a depolarization process and gradually vanishes, but cannot switch to another polarization state. This means that after the negative reversal, the device is hardly affected by the imprint effect during the retention process, as there is no polarization charge, as shown in Figure 6b. This is significantly different from ferroelectric devices, which tend to exhibit a rightward shift of the hysteresis loop after negative polarization. The difference arises from the distinct polarization reversal mechanisms of ferroelectric and antiferroelectric devices. The unique characteristic being unaffected by the imprint effect may provide inspiration for circuit design. Compared with ferroelectric devices, NV ZLO devices possess unparalleled potential in terms of retention characteristics.

Figure 6c,d depict the retention characteristics of the device under positive and negative polarization conditions, respectively. After 10⁴ seconds of retention testing, the remanent polarization exhibited remarkable stability in both switching states at 25 °C. Notably, at 85 °C, severe degradation in polarization intensity was observed, which is attributed to the temperature-induced enlarging of coercive field, as predicted by Landau theory [48,49]. With prolonged retention time, the device with a positive pre-switch displayed a gradual recovery in coercive field shift toward normal values due to the imprint effect, accompanied by a corresponding increase in 2Pr. In contrast, the negative polarization state maintained complete consistency with its initial configuration owing to its inherent immunity to imprint effect.

To evaluate the intrinsic endurance limit of the 6 nm-thick antiferroelectric NV-ZLO thin film, we specifically designed a small-scale test structure (2 × 2 μm²) featuring a ground–signal–ground (GSG) crossbar configuration [27]. This architecture, combined with ultrafast commercial testing equipment, enables sub-nanosecond ultrafast cycling while minimizing parasitic effects. The H_0.5Z_0.5O₂ ferroelectric device was also fabricated for comparison. After 10⁸ cycles, the ferroelectric polarization of the H_0.5Z_0.5O₂ devices began to exhibit signs of fatigue and ultimately failed after 10¹⁰ cycles, as illustrated in Figure 7. The modest reduction in remanent polarization observed in the NV ZLO antiferroelectric devices can be attributed to the fact that only half of the domains within the thin films were active. Notably, even after 10¹² cycles, the polarization of the NV ZLO devices maintained exceptional stability. The endurance performance underscores the immense potential of ZLO antiferroelectric devices.

To highlight the advantages of NV-ZLO systems, we present comparative performance metrics in

Table 1. Benchmark of Non-Volatile Antiferroelectric and Ferroelectric Devices.

Structure	Films	Wake-Up	2Ec (MV/cm)	2Pr (µC/cm2)	Endurance (Cycles)	Ref.
NV–AFE	Hf0.25Zr0.75O2	w/	1.5	~7	>1 × 1012	[50]
NV–AFE	ZrO2/Al2O3/ZrO2	w/o	1.6	18	1 × 109	[33]
NV–AFE	HfO2/ZrO2	w/	/	/	1 × 107	[51]
AFE	Hf0.2Zr0.8O2	w/	4	8–16	1 × 1012	[49]
NV-AFE	ZrO2	w/o	1.4	12–18	1 × 1010	[34]
FE	Hf0.5Zr0.5O2	w/	6	25	1 × 1010	[52]
FE	Hf0.5Zr0.5O2	w/	1.6	5–20	1 × 1010	[53]
FE	Hf0.5Zr0.5O2	w/	2	712	>1 × 1012	[54]
FE	HZO/CeO2/HZO	w/	3	30	1 × 1011	[55]
NV–AFE	ZrO2/La2O3/ZrO2	w/o	1.5	22	>1 × 1012	This work

, revealing superior characteristics of La-doped ZLO thin films in antiferroelectric applications. Notably, La doping effectively eliminates the wake-up effect, enabling immediate stable switching without preconditioning cycles. The optimized GSG devices demonstrate high endurance (>10¹² cycles) with minimal polarization loss, significantly outperforming existing nonvolatile antiferroelectric and ferroelectric devices. Furthermore, the coercive field shows a 50% reduction compared to conventional ferroelectrics, substantially lowering operational power requirements.

4. Conclusions

This paper provides an in-depth exploration on the nonvolatile applications and reliability of a ZLO antiferroelectric capacitor. The incorporation of La effectively passivates the formation of oxygen vacancies, thereby reducing leakage current and promoting the development of antiferroelectric phases. The nonvolatile application of ZLO antiferroelectric devices was achieved with designed work function engineering. In contrast to conventional ferroelectric devices, nonvolatile ZLO capacitors overcame wake-up and fatigue effects and revealed excellent endurance (>10¹² cycles) and retention performance. This study paves the way for new applications of antiferroelectric in integrated circuits, offering innovative perspectives for future technological advancements.