HfO₂ based ferroelectric thin films have attracted wide interest since their first discovery in 2011^[1] due to their encouraging properties, including complementary metal−oxide−semiconductor (CMOS) compatibility, physical thickness scalability^[2], low power consumption and fast speed^[3], the HfO₂ based ferroelectric devices have demonstrated potential for broad applications including non-volatile memory^[4] and bio-inspired computing^[5]. Among various doped and undoped HfO₂ based ferroelectric thin films, the hafnia-zirconia solid solution (HZO) films show robust ferroelectricity within a wide composition range, and their relatively low annealing temperature (350−500 °C) makes them potentially compatible with the Back-End-of-Line (BEOL) process.

Nowadays, the rapid development of space and quantum computing applications calls for high-performance memory devices that can reliably operate at cryogenic temperatures (below about 120 K)^[6]. While the research on this subject is currently limited, previous works have proved that memory devices with HfO₂ based ferroelectric materials or amorphous dielectrics showing ferroelectric-like electrical behaviors can be one of the most promising candidates for cryo-electronics^[7−15]. Müller et al. investigated the temperature-dependent polarization-electric field (P−E) hysteresis in Hf_0.3Zr_0.7O₂ thin films and observed a transition from anti-ferroelectric-like to ferroelectric behavior with the temperature decreasing from 230 to 80 K^[7]. Henry et al. first demonstrated ferroelectric switching in Hf_0.58Zr_0.42O₂ thin films with NbN electrodes at 4 K^[8]. Moreover, Hur et al. thoroughly characterized the electrical performance of Hf_0.5Zr_0.5O₂ ferroelectric capacitors with TiN electrodes from 400 to 4 K^[9]. However, the initial findings were obtained using non-optimized HZO devices, which exhibited either anti- or weak-ferroelectric behavior in their pristine states, and required a wake-up operation using bipolar voltage cycling to achieve the commonly reported ferroelectric full-hysteresis loops. The wake-up process is not practical for real products and has been widely attributed to the redistribution of defects and/or the phase transformation from the non-ferroelectric phase to the ferroelectric one in previous literature^[16]. Efficient ways to mitigate the wake-up issue have been extensively explored and demonstrated, including defects and interfacial engineering, plasma treatment, and epitaxial growth^[17]. It has been observed that the wake-up process can be effectively suppressed under low temperatures^[9]. Consequently, to unlock the full potential of these materials, it is imperative to investigate the temperature dependence on the ferroelectric properties of wake-up free ferroelectric HZO capacitors (referred to as optimized HZO capacitors) down to cryogenic temperatures.

In this letter, we present systematic electrical characterizations on wake-up free TiN/HZO/TiN ferroelectric capacitors, showcasing their stable ferroelectric performance over a wide temperature range from 300 down to 30 K. Our work demonstrates the great potential of HZO ferroelectric capacitors for cryogenic electronics applications.

The TiN/Hf_0.5Zr_0.5O₂/TiN capacitor devices were fabricated on SiO₂/Si substrates. A blanket layer of 40 nm TiN as bottom electrodes (BEs) was deposited by sputtering. The 10-nm-thick HZO thin films were then prepared by thermal atomic layer deposition with a substrate temperature of 280 °C. [(CH₃)(C₂H₅)N]₄Hf (TEMAH), [(CH₃)(C₂H₅)N]₄Zr (TEMAZ) and H₂O were used as Hf precursor, Zr precursor and oxygen source, respectively. The Hf : Zr ratio of 1 : 1 is controlled by alternating HfO₂ and ZrO₂ deposition cycles. The 40 nm sputtered TiN top electrodes (TEs) with an area of 50 × 50 μm² were patterned by the standard photolithography and lift-off process. Finally, the fabricated capacitors were annealed at 500 °C for 30 s by rapid thermal annealing in nitrogen atmosphere.

The crystal structure of HZO thin films after TEs removal was analyzed by grazing incidence X-ray diffraction (GI-XRD) at an incident angle of 0.5°, employing Bruker D8 Advance equipment. The transmission electron microscope (TEM) image was acquired using the FEI 200 kV Titan Themis STEM equipment. The temperature-dependent electrical characterizations were performed in a Lake Shore cryogenic probe station. The capacitance−electric field (C−E) and leakage−electric field (J−E) characteristics were measured by a Keithley 4200 semiconductor parameter analyzer, while the polarization-electric field (P−E) hysteresis loops, positive-up-negative-down (PUND) behaviors and endurance were characterized by a Precision LC Ⅱ ferroelectric tester (radiant technologies). The direct-current J−E curves were collected during the voltage swept from 3 to 0 V and −3 to 0 V. As schematically shown in the inset of Fig. 1 (a), TEs were biased with BEs grounded during the electrical measurements.

Fig. 1(a) shows the P−E hysteresis curves of our optimized HZO capacitor at its pristine state (blue curve) and after 10³ cycles (red curve), confirming its wake-up free behavior. The crystal structure of our HZO thin film was analyzed by GI-XRD (Fig. 1(b)), where diffraction peaks that possibly correspond to (111) and (200) reflections of the orthorhombic (o-) phase can be identified. The cross-sectional TEM image of the HZO capacitor is shown in Fig. 1(c), which illustrates that the thickness of the HZO film is 10 nm, and the interfaces between HZO and electrodes are clean and uniform. The fast Fourier transform pattern of the selected region (yellow dash box in Fig. 1(c)) demonstrates the existence of a ferroelectric o-phase (space group: Pbc2₁) in our HZO thin film (Fig. 1 (d)).

To examine the leakage and its temperature dependence of our HZO capacitors, the direct-current J−E characteristics at various temperatures are shown in Fig. 2 (a). The low leakage current density of 4.8 A/cm² (leakage current of 1.2 nA) at 300 K and 2 MV/cm indicates a good insulating quality of the HZO thin films. The leakage decreases with the decrease in temperature. Figs. 2 (b) and 2(c) show the fitting results of the leakage current at temperatures from 300 K to 130 K by the Poole−Frenkel (P−F) emission and at 30 K by the Fowler−Nordheim (F−N) tunneling, respectively. The P−F emission, where the current density is directly dependent on the temperature, dominates the leakage in the HZO capacitor when the temperature decreases from 300 K^[18], which explains the observed decrease in leakage with temperature, while the F−N tunneling becomes dominant when the temperature is lower than 77 K. Fig. 2(d) shows the P−E hysteresis loops measured under 3.0 MV/cm from room temperature to 30 K. To accurately determine switching polarization while mitigating non-switching interference, the PUND scheme is employed (inset in Fig. 2(e)). The 2P_r of our HZO ferroelectric capacitors remains almost unchanged 36 µC/cm² at 30 K with an increase in the coercive electric field (E_c) as the temperature decreases, which will be discussed further in the following paragraph. Fig. 2(e) shows the statistical results of ±P_r values at varied temperatures from 10 randomly picked devices, demonstrating the great uniformity of the HZO capacitors, which is consistent with the studies by Jiang et al. across a 12-inch wafer^[19]. As expected, permittivity- electric field (ε_r−E) measurements (Fig. 2(f)) of the HZO capacitors show typical butterfly shaped hysteresis across the whole temperature range. The ε_r−E curves were measured under 2 MV/cm to avoid the device breakdown during measurements. The capacitance peaks in the ε_r−E curves that correlate with +E_c and −E_c, shifted to the right-hand and left-hand, respectively, which aligns with the trend observed in P−E measurements. In agreement with previous observations^[13], the permittivity of our capacitor (@ 2 MV/cm) monotonically decreases from 31.1 to 28.1 when the temperature decreases from 300 to 30 K.

To draw more solid conclusions on the temperature dependence of 2P_r and the averaged E_c (defined as (|+E_c| + |−E_c|)/2), we performed PUND measurements on our HZO ferroelectric capacitors under different electric fields. Figs. 3(a)−3(c) show the PUND hysteresis loops measured from 300 to 30 K under 2.5, 2.0, and 1.5 MV/cm, respectively. These show that the temperature dependence of ferroelectric properties is strongly influenced by the applied electric field^[11]. As shown in Fig. 3(d), the averaged E_c monotonically increases as the temperature decreases, suggesting a higher barrier for ferroelectric switching at lower temperatures^[20]. In particular, under 3.0 MV/cm, the averaged E_c increases from 1.1 MV/cm (@ 300 K) to 1.4 MV/cm (@ 30 K) as the temperature decreases. It should be noted that here we have defined the electric field when the polarization becomes zero as ±E_c for all measurements with different temperatures and maximum E-fields. For cases that the polarization cannot be fully switched, e.g. at 1.5 MV/cm and 30 K, ±E_P=0 can be a more applicable alternative for the definition above. In Fig. 3(e), the HZO capacitor shows constant 2P_r of ~36 µC/cm² under 3.0 MV/cm and ~30 µC/cm² under 2.5 MV/cm down to 30 K. Meanwhile, with an applied electric field amplitude (1.5 MV/cm) comparable to coercive electric field, the decrease in temperatures leads to a clear reduction in 2P_r (from 12.5 µC/cm² at 300 K to 2.9 µC/cm² at 30 K). Fig. 3(f) summarizes the temperature-dependent 2P_r of HZO ferroelectric capacitors from previous literatures and our work. Our devices exhibited negligible changes and achieved the highest 2P_r at cryogenic temperatures, which can be attributed to the high quality of the ferroelectric HZO thin film and the well-optimized oxide/electrode interfaces, highlighting the benefit of their nature of wake-up free.

Finally, the endurance of our TiN/HZO/TiN capacitors at 300 and 30 K is characterized and compared, as shown in Fig. 4. For the endurance measurements, repeated pulses (electric field of 2.5 MV/cm and pulse width of 1 µs) are applied. At room temperature, the 2P_r of the HZO capacitor dropped significantly from 36.4 µC/cm² at its pristine state to 22.4 µC/cm² after 3 × 10⁶ cycles. While at 30 K, there was only a slight reduction in the 2P_r value from 36.9 to 31.5 µC/cm² after 3 × 10⁶ cycles, similar to the observation by Hsiang et al^[21]. The fatigue in HfO₂ based ferroelectrics has been widely attributed to the generation and distribution of charged defects, e.g. oxygen vacancies^[22]. These two processes can be suppressed under low temperatures, which gives rise to mitigated fatigue effect and enhances the suitability of HZO ferroelectric capacitors for cytogenetic applications.

In summary, our study comprehensively investigates the ferroelectric characterization of wake-up free TiN/HZO/TiN capacitors, including J−E, P−E, ε_r−E, and endurance measurements, spanning a wide temperature range from 300 to 30 K. Our findings highlight the promising ferroelectric and endurance properties of HZO films at cryogenic temperatures. Notably, the 2P_r values of HZO thin films remain mostly stable down to 30 K. Moreover, the HZO capacitors show weaker fatigue characteristics at 30 K compared to room temperature, indicating their potential for cryogenic applications. Precise computational analysis and microscopic characterizations under different temperatures are further required to establish the relationship between electrical properties and structural evolution to better understand and advance the application of these materials in various technological fields.