Temperature-Dependent Ferroelectric Behaviors of AlScN-Based Ferroelectric Capacitors with a Thin HfO₂ Interlayer for Improved Endurance and Leakage Current

Abstract

The ferroelectric switching behavior of a metal–ferroelectric AlScN–HfO₂ interlayer–metal (MFIM) capacitor was investigated at variable temperatures and compared with an MFM capacitor. Although the MFIM capacitor demonstrated an inferior remnant polarization (2P_r value of 74 µC/cm²), it exhibited a reduced leakage current (×1/100) and higher breakdown field. The MFIM showed a stable change in 2P_r from room temperature to 200 °C and an enhanced endurance of ~10⁴ cycles at 200 °C; moreover, the leakage current was less degraded after the cycling tests. Thus, the ferroelectric AlScN with a thin HfO₂ interlayer can enhance the reliability of ferroelectric switching.

1. Introduction

Aluminum Scandium Nitride (AlScN) has attracted considerable interest in the field of ferroelectrics owing to its unique properties. Firstly, AlScN exhibits a large remnant polarization (P_r) and a high coercive electric field (E_c), attributed to its stable wurtzite structure [1,2,3]. These characteristics are crucial for the development of high-performance memory devices with reliable ferroelectric properties. Secondly, AlScN exhibits stable ferroelectric properties up to temperatures exceeding 600 °C, whereas PZT and doped HfO₂ are limited to temperatures below 500 °C [4,5]. This thermal stability makes AlScN an effective solution to the thermal constraints in nonvolatile memory technologies and highlights it as a promising candidate for memory devices in harsh environments. Thirdly, AlScN films can be deposited using various techniques, including molecular beam epitaxy [6,7], pulsed laser deposition [8], metal–organic chemical vapor deposition [9,10], and reactive sputter deposition [11,12]. However, sputter deposition is considered advantageous because of its ability to deposit large-area films efficiently and economically at typical growth temperatures below 400 °C, thus ensuring CMOS compatibility [13,14,15]. Based on these properties, extensive research has been conducted on the integration of AlScN-based memory devices with MOSFETs and HEMTs [16,17,18].

When AlScN is used alone as a gate stack, it encounters a challenge in memory implementation because of an unexpectedly large leakage current, which adversely affects the reliability and power consumption of devices [19,20]. Another challenge with AlScN is its relatively low endurance characteristics compared with those of fluorite ferroelectric materials [21,22]. A recent study proposed an AlScN gate stack with a thin Al₂O₃ interlayer for AlGaN/GaN HEMTs to mitigate leakage currents [18]. In addition, the inclusion of an interlayer in doped HfO₂-based capacitors has been shown to enhance field cycling endurance [23,24,25]. However, there are limited studies focusing on improving the ferroelectric properties of AlScN, and to our best knowledge, no research has been reported on ferroelectric devices fabricated by growing AlScN on an insulator. For transistor and diode device applications, in particular, a low off-current is of significant importance. However, the sputtered AlScN film is inherently leaky, and thus, a relatively high-k HfO₂ can be considered an interlayer to suppress leakage currents. Although there are reported studies on alumina or hafnia as a capping layer [26,27], it is timely to investigate the effects of an insulating interlayer for the AlScN gate stack, especially in terms of ferroelectric behaviors to be improved.

In this study, we fabricated and compared metal–ferroelectric–metal (MFM) and metal–ferroelectric–insulator–metal (MFIM) structures. First, the crystallinity of AlScN in both devices was investigated by comparing the intensities in the XRD patterns. Subsequently, electrical measurements were conducted to observe the changes in P_r and leakage currents depending on the HfO₂ interlayer. Furthermore, the variations in P_r and endurance were characterized at varying temperatures to assess the thermal stability provided by the HfO₂ interlayer. Finally, the degradation (if any) in the retention characteristics owing to the HfO₂ interlayer was examined.

2. Materials and Methods

The fabrication process began with the deposition of Ti/Pt (5/50 nm) on a p⁺⁺ Si substrate using E-beam evaporation for bottom contact. Next, the insulator, HfO₂ (4 nm), was deposited at 350 °C using atomic layer deposition (ALD), which is exclusively performed on MFIM capacitors. The deposition was carried out at a rate of 0.76 cycles/Å for 50 cycles. Subsequently, Al_0.72Sc_0.28N (65 nm) was deposited on an Al_0.57Sc_0.43 single alloy without post-annealing using RF magnetron sputtering at 290 W. The deposition was performed at a working pressure of 5 mTorr with Ar/N₂ (15/30 sccm) at 400 °C for 30 min. The composition ratio of deposited AlScN film is referenced in the reported results [12,28,29,30]. Finally, Ni (100 nm) was patterned as the top metal contact using the lift-off method via E-beam evaporation. The patterns for both devices were circular with a diameter of 40 µm. The entire process is summarized in the cross-sectional schematic and process flow shown in Figure 1a. The thickness of the AlScN layer was verified using a surface profiler (DEKTAKXT-A/BRUKER, Billerica, MA, USA). The peaks of the AlScN crystals in the MFM and MFIM structures were confirmed using XRD (SmartLab/Rigaku, Tokyo, Japan). All electrical and ferroelectric measurements were performed using the Keithley-4200A-SCS analyzer with a PMU-4225 module for pulse measurements.

3. Results

Figure 1b presents the XRD 2θ scans for the MFM and MFIM structures, indicating the (002) peak of the AlScN film to substantiate the origin of its ferroelectricity. In the MFM structure, a (002) peak was identified. Additionally, a reflection peak corresponding to the crystalline plane in the Si(100) orientation was observed in the MFM sample [31,32]. Similarly, the (002) peak was also observed in the MFIM structure; however, it exhibited a slight left shift and reduction in intensity compared with the MFM structure. The inset in Figure 1b shows a magnified view to observe the variations in peak intensity. Most AlScN thin films are grown on metal seed layers such as Pt or Mo, which ensures a decent c-axis orientation [33,34]. However, AlScN thin films grown on nonmetallic substrates exhibit a high density of misaligned grains, indicating a poor c-axis orientation, which consequently leads to relatively inferior ferroelectric properties [31]. This compromises the crystallinity of AlScN in the MFIM structure, as shown by the reduction in peak intensity.

To characterize the P_r values, we performed Positive-Up, Negative-Down (PUND) measurements using the pulse schemes shown in Figure 2a. Both the P and N pulses include switching and non-switching charges, whereas the U and D pulses capture only the non-switching charge. By subtracting the respective pulse types, the actual remnant polarization can be determined [17,35]. Therefore, instead of employing a P–E loop that includes non-switching charges, 1 kHz PUND pulses were applied using a bottom-drive, top-sense two-probing method. Polarization–Electric field (P–E) characteristics of the MFM and MFIM capacitors are extracted and shown in Figure 2b,c, respectively. The MFM capacitor with AlScN grown on Pt exhibited a 2P_r value of 250 µC/cm² at 39 V PUND measurement at 1 kHz. These P_r values are similar to those reported in previous studies on similar Al_0.72Sc_0.28N compositions [4,36,37]. In contrast, the MFIM capacitor with AlScN grown on a HfO₂ layer achieved a lower 2P_r of 74 µC/cm². This reduction in polarization can be attributed to the difference in the crystallinity of AlScN between the two devices, as discussed with the reduction in the (002) peak intensity shown in Figure 1b. Electrical analysis was conducted to analyze the cause of polarization degradation in the MFIM structure. Using C–V measurements, a dielectric constant (${\epsilon }_{IN}$) and thickness ($t_{IN}$) of HfO₂ were found to be 19.3 and 4 nm, respectively; the thickness ($t_{FE}$) and the dielectric constant (${\epsilon }_{FE}$) of AlScN were determined to be 65 nm and 18.5, respectively. Consequently, upon calculating the voltage applied to AlScN using Equation (1), it was determined that 36.8 V out of an applied voltage of 39 V was applied to the AlScN layer. This confirms that the reduction in polarization was not due to voltage division.

$${\displaystyle \frac{V_{FE}}{V_{IN}}}={\displaystyle \frac{t_{FE}{\epsilon }_{IN}}{t_{IN}{\epsilon }_{FE}}}$$

(1)

Figure 2d,e show the leakage current and breakdown characteristics, respectively, as a function of the applied voltage. The MFIM capacitor exhibited a reduced leakage current on the order of ~10² at 25 V, which was attributed to the thin HfO₂ interlayer. Moreover, the MFM capacitor exhibited a breakdown field of 4.7 MV/cm, whereas the MFIM capacitor reached a value of 6.9 MV/cm. Both the improved breakdown field and reduced leakage current of the device are attributed to the HfO₂ interlayer.

To assess temperature-dependent reliability, PUND measurements were conducted from room temperature (RT) up to 200 °C. As shown in Figure 3a,b, the MFM capacitor exhibited a 2P_r value of 82 µC/cm ² at 25 V for RT and a 2P_r value of 81 µC/cm² at 20 V for 200 °C. Similarly, the MFIM capacitor showed a 2P_r value of 8 µC/cm² at 30 V for RT, and it exhibited a 2P_r value of 6.8 µC/cm² at 20 V for 200 °C. For both devices, the coercive field (E_c) decreased with increasing temperature. This is consistent with the recent reports, suggesting that the energy barrier for switching in AlScN is reduced and, thus, there are lower coercive fields at elevated temperatures [38,39]. Figure 3c,d present 2P_r variation (∆2P_r1) as a function of voltage at 100 °C and 200 °C, respectively, in reference to RT. For the MFM capacitor, a significant increase in +2P_r was observed as the voltage of the pulse applied at high temperatures increased. This is evident from the steep slopes observed in the MFM graphs shown in Figure 3c,d. In contrast, the MFIM capacitor exhibited stable polarization variations. The thin HfO₂ layer effectively maintained stable polarization variations even at elevated temperatures. The results confirmed an enhancement in the high-temperature reliability of the MFIM capacitor through the HfO₂ interlayer.

Figure 4a,b show the endurance characteristics measured at three different temperatures to further evaluate high-temperature reliability. The pulse width and amplitude were set to 0.25 ms and 39 V, respectively. First, at room temperature, while the MFM capacitor achieved 10⁴ cycles, the MFIM capacitor demonstrated a significant improvement, reaching 10⁵ cycles, representing a ten-fold increase. Both devices exhibited wake-up characteristics, indicating that they were not measured at the voltage at which the polarization values saturated, as predicted [40]. Furthermore, when endurance measurements were conducted at high temperatures, MFM exhibited endurance characteristics of 2 × 10³ cycles at 100 °C and 10³ cycles at 200 °C, whereas MFIM achieved significantly improved endurance characteristics of 2 × 10⁴ cycles at 100 °C and 10⁴ cycles at 200 °C. Figure 4c shows the trend lines for the cycle counts at elevated temperatures, indicating that superior endurance characteristics were achieved for the MFIM capacitors. Figure 4d shows leakage properties before and after the 10³ cycle test. While MFM showed a significant increase in the leakage current after the cycling test, MFIM maintained a stable off-current and exhibited a negligibly increased current. This validates the efficacy of inserting the HfO₂ interlayer to enhance both the high-temperature and endurance operational reliability. The origin of this improvement is associated with nitrogen vacancies (V_N) in AlScN near the interface. V_N induces downward bending of the conduction band at the MFM interface, thereby increasing the probability of electron tunneling. As pulse cycling continues, this band bending effect persists, resulting in increased leakage and device failure due to Joule heating [21]. However, in the MFIM structure, the insertion of the HfO₂ interlayer can reduce electron tunneling caused by V_N at the interface, thus improving endurance. As shown in Figure 5, on the other hand, no discernible polarization decays up to 2000 s were observed for both capacitor structures regardless of the HfO₂ interlayer. This suggests that the HfO₂ layer does not degrade the retention characteristics. Finally, we compare our MFM and MFIM capacitors with reported ferroelectric-based two-terminal devices (

Table 1. Benchmarking table of a two-terminal ferroelectric capacitor with representative ferroelectric materials.

Ferroelectric	Structure	2Pr (μC/cm2)	Ferro/IL	Leakage Current
			Thickness (nm)	Density (A/cm2)
Al0.64Sc0.36N [26]	Al-AlScN-HfO2-Ti	n/a	10/4	2.4 × 10 to 8 × 10−8
Al0.72Sc0.28N [26]	Al-AlScN-Al2O3-Ti	50	20/4	7.9 to 8 × 10−7
Al0.7Sc0.3N [37]	AlN-Mo-AlN-AlScN-Pt	270	50/-	n/a
BiFeO3 [41]	Si-HfO2-BiFeO3-TiN	8	200/10	2.34 × 10−9
PZT [42]	Si-HfO2-PZT-Al	n/a	100/5	10−5 to 10−7
HZO [23]	TiN-HfO2-HZO-TiN	48	10/1	9 × 102 to 10−10
This work	Ti/Pt-AlScN-Ni	250	65/-	2 × 10−2 to 10−7
This work	Ti/Pt-HfO2-AlScN-Ni	74	65/4	3 × 10−4 to 4.4 × 10−9

). In addition to CMOS BEOL compatibility, our AlScN-based MFIM capacitor provides competitive leakage current properties.

4. Conclusions

In conclusion, we analyzed the ferroelectric switching behaviors of an AlScN-based MFIM capacitor in comparison with an MFM capacitor. The XRD results of the MFIM capacitor indicated the successful crystalline growth of AlScN on HfO₂. However, the MFIM exhibited an inferior 2P_r value of 74 µC/cm² compared with the 2P_r value of 250 µC/cm² of the MFM. The MFIM capacitor demonstrated improved insulation with a leakage current reduction of ~10² and an increased breakdown field from 4.7 MV/cm to 6.9 MV/cm. High-temperature reliability tests showed that the MFIM maintained stable polarization variations up to 200 °C, whereas the MFM exhibited a steep change in polarization. Endurance tests at RT revealed that the MFIM capacitor achieved 10⁵ cycles, a ten-fold increase over the 10⁴ cycles of the MFM capacitor; even at elevated temperatures, the MFIM capacitor continued to surpass the MFM capacitor. The retention measurements showed no significant polarization decay up to 2000 sec for either of the devices, suggesting that the HfO₂ layer did not affect retention. The results suggest that the thin HfO₂ interlayer enhances the insulation, breakdown field, high-temperature reliability, and endurance, making it a promising candidate for high-temperature AlScN-based memory devices.