(Hf, Zr)O₂ ferroelectric materials & applications

Recently, it was reported that HfO₂ can show ferroelectric (FE) properties when doped with various dopants such as Zr, Si, Y, Al, Gd, La, and Sr. Compared to the conventional ferroelectrics based on perovskite or layered perovskite structure, such as Pb(Zr,Ti)O₃ and SrBi₂Ta₂O₉, the FE HfO₂-based films showed quite distinctive physical and electrical properties. The HfO₂-based films can be extremely thin (film thickness, t_f < 10 nm) with feasible ferroelectricity (remnant polarization, P_r ~ 10–40 μC/cm²), whereas the much thicker thicknesses (t_f > 100 nm) are required for the stable FE properties of conventional ferroelectrics thin films, especially with the metal-ferroelectric-metal (MFM) configuration. Due to their small thickness, the HfO₂-based films are considered promising for the three-dimensional capacitor structure which is highly required for the future ferroelectric random-access-memory (FeRAM) device according to International Technology Roadmap for Semiconductors. This results from the relatively large bandgap (E_g ~ 5.5 eV), high compatibility with Si, and matured atomic layer deposition (ALD) technique of HfO₂-based films. In fact, due to their high Si-compatibility, the non-FE HfO₂ thin film has been used as the buffer layer in ferroelectric field-effect-transistor (FeFET) to solve the interfacial problems of conventional ferroelectrics and Si substrate.

HZO Thin film (polar phase & electrical properties)

We examined the effect of t_f on the FE properties of the Hf_0.5Zr_0.5O₂ films. They tested the Hf_0.5Zr_0.5O₂ films with various t_f (5.5–25 nm) and annealing temperature (400–800 °C). When the t_f was ≈ 10 nm, the P_r of the Hf_0.5Zr_0.5O₂ films was the largest (≈ 15 μC cm⁻²), whereas the P_r decreased to ≈ 12 and ≈ 5 μC cm⁻² when the t_f increased to 17 and 25 nm, respectively. This could be understood from the fact that the portion of the m-phase increased as the t_f increased. Meanwhile, we also found that the insertion of thin (~1nm) dielectric Al₂O₃ inter-layer between 20-nm- thick Hf_0.5Zr_0.5O₂ layers can prevent the continuous growth of the whole Hf_0.5Zr_0.5O₂ films, and the FE properties was not degraded with increasing t_f.

The origin of the formation of an unexpected FE phase was systematically examined for Hf_0.5Zr_0.5O₂ films by our group. Since the growing nuclei are supposed to have t-phase structure prior to the zipping, and because of the surface energy effect, there must be several variations in the resulting strain states after the zipping, according to the original crystallographic directions of the nuclei. Using the in-plane stress condition of the film, we could calculate the strain along the a-, b-, and c-axis of the grains of various orientations using the in-plane stress conditions of the thin film. As a result, the 110- orientation was the most favorable for the formation of the o-phase from t-phase, whereas the 111- orientation was the worst, since almost equivalent strains were formed along the a-, b-, and c-axis (none of the a-, b-, and c-directions can be tensile while other direction is compressive).

Related Paper:

M. H. Park, et al., Advanced Materials, 11, 27, 1811-1831 (2015)

M. H. Park, et al., Appl. Phys. Lett. 104, 072901 (2014)

H. J. Kim, et al., Appl. Phys. Lett. 105, 192903 (2014)

M. H. Park, et al., Appl. Phys. Lett. 105, 072902 (2014)

M. H. Park, et al., Appl. Phys. Lett. 102, 242905 (2013)

Wake Up Effect of HZO Thin Film

The Hf_0.5Zr_0.5O₂ film shows mostly ferroelectric (FE)-like behavior in the pristine state, where the slight anti-ferroelectric (AFE)-like distortion could be ascribed to the anti-parallel distribution of some of the FE domains and the AFE phase. The field cycling of only 100 cycles almost completely removed such anti-parallel pinned domains. Further increase in the field cycling number more effectively poled the domains making the P_r increase significantly. Field cycling also transformed the non-FE phases, mostly at the electrode interface, to the FE-phase, which could be inferred from the increase in the interfacial capacitance. These results indicate that the physical state of the FE-phases in the HZO film is quite different from the conventional FE thin films, such as PZT, in a sense that it is the outcome of variously oriented domains, possibly including anti-parallel orientation. Field cycling plays a role in poling in conventional FE materials, but the high field-induced non-FE t-phase to FE o-phase transition could also contribute to the effective poling (depinning of the pinned FE domains).

Related Paper:

H. J. Kim, et al., Nanoscale, 8, 1383-1389 (2016)

Two-Step Polarization Switching Mediated by a Nonpolar Phase in HZO Thin Film

The broken hysteresis loop in the P–E test and two-step polarization switching in pulse switching measure measurement from Hf_0.4Zr_0.6O₂ films were systematically examined based on the first-order-like phase transition theory. The broken hysteresis behavior Hf_0.4Zr_0.6O₂ film could be understood based on the classical theory on the first order phase transition, where the NP phase is involved during the transition from one polarization state to the opposite polarization state. The expected two-step (UP → NP (region I) and NP → DOWN (region II)) polarization switching could be also confirmed by dynamic pulse switching measurement.

Related Paper:

M. H. Park, and H. J. Kim, et al., Nanoscale, DOI:10.1039/c5nr08346j (2016)

Monolithic Device with Various Devives for Pyroelectric Energy Harvestor/Electro Caloric Device, High-Charge Capacitor/Electrostatic Energy Storage, and IR Sensing.

The schematic images of the HZO-based monolithic device for pyroelectric energy harvesting (PEH), electrocaloric effect (ECE), electrostatic energy storage (EES), a high-charge capacitor, and infrared (IR) sensing fabricated on Si substrates. The upper center panel shows the schematics of a PEH device based on microelectromechanical systems, which was designed by Oak Ridge National Laboratory. A cantilever is positioned between a hot surface and a cold surface, and the resonant motion of the cantilever can be produced due to the difference in the thermal expansion coefficient of the materials in the bimorph part. Furthermore, the appropriate combination of the cycling of the electrical field with the resonant motion can use the PEH or the ECE. The PEH and the ECE are inverse cycles, so they can be easily used in an equivalent structure merely by changing the sequence of the applying field and changing T. The upper right panel shows the structure of an EES or a high-charge capacitor, which are based on a simple capacitor structure with top and bottom electrodes. For HZO, TiN might be the best electrode material, and the three-dimensional capacitor structure was used to increase the ESD and the stored charge by increasing the area of the capacitor. With the appropriate external circuit, the EES capacitors with an HZO dielectric layer can store the energy harvested by a PEH device using another HZO film. The upper left panel shows the schematics of an array-based pyroelectric IR detector. It is believed that the Si compatibility of HZO films might make the IR detector more appropriate for mass production. These functional devices could be produced on Si wafers and could be stacked with the Si processors or memory chips that are the sources of the waste heat, or placed near the chips on the printed circuit board.

Related Paper:

M. H. Park, et al., Nano Energy, 12, 131-140 (2015)

Electrostatic Supercapacitors with Large Energy Storage Density

The energy storage characteristics of a new lead-free AFE Hf_xZr_1-xO₂ (0.1 < x < 0.4) materials system were examined. The thin HZO capacitors could have an ESD higher than ≈ 30 J cm⁻³ in the wide x-value range of 0.1–0.4, which is a crucial advantage for their mass production. The ESD of the 9.2-nm-thick Hf_0.3Zr_0.7O₂ capacitor was the largest among the HZO films with various compositions, and the maximum ESD was 46 J cm⁻³ at the 4.5 MV cm⁻¹ electric field. The ESD of the Hf_0.3Zr_0.7O₂ capacitor decreased with an increase in the film’s thickness, likely due to the increased formation of the m-phase, but was still as high as ≈ 21 J cm⁻³ when t_f was 29.0 nm. The ESD and energy efficiency of the Hf_0.3Zr_0.7O₂ capacitor did not decrease with the increase in the temperature up to 175 °C, which shows its robust thermal stability that makes it appropriate for practical application. Moreover, the large ESD decreased by only ≈ 4.5% even after 10⁹ times of field cycling at a switching electric field of 3.26 MV cm⁻¹ . All the dielectric films were deposited using the ALD method at a substrate temperature of 280 °C, which demonstrates the compatibility of the material and the process with nanostructured devices.

Related Paper:

M. H. Park, et al., Advanced Energy Materials, 16, 4, 1400610 (2014)

Ferroelectric/Dielectric Bilayer electronic devices - Theoretical work on ferroelectric/dielectric bilayer structure

Enhancement of capacitance by negative capacitance (NC) effect in a dielectric/ferroelectric (DE/FE) stacked film is gaining a greater interest as a feasible solution to miniaturize capacitors and field effect transistors for future semiconductor devices. Since its model was first suggested, several experimental proofs on the capacitance boost of DE/FE stacked films were reported. However, the previously reported theoretical background of this intriguing effect holds a self-contradictory problem. Therefore, this work suggests an alternative model to describe the negative capacitance effect in a DE/FE system quantitatively. This work adopted a modified formalism to incorporate the depolarization effect to describe the energy of the general DE/FE system. The model predicted that the SrTiO₃/BaTiO₃ system will show a capacitance boost effect. It was also predicted that the Al₂O₃/BaTiO₃ (AO/BTO) system shows the capacitance boost effect with no FE-like hysteresis behavior. However, the involvement of the trapped charges at the DE/FE interface, originating from the very high field across the thin Al₂O₃ layer when the BTO layer played a role as the NC layer, can frustrate, the NC effect in the AO/BTO system. Under this circumstance, the highly stored charge by the NC effect of the BaTiO₃ during the charging period could not be retrieved during the discharging process because integral part of the polarization charge was retained within the system as a remnant polarization.

Related Paper:

Y. J. Kim, et al., Scientific Reports, 6, 19039 (2016)

Tunnel Switch Device

Ferroelectric random access memory devices (FeRAM) have attracted a great deal of interest as a potential replacement for non-volatile stand-alone memory devices or as a memory element in a system-on-chip device where a high density (> 16 mega bit) is preferred for the device performance and cost effectiveness. However, scaling down the ferroelectric thin films has some practical problems. One of them is that Interfacial effects become increasingly severe as the ferroelectric layer thickness decreases, which has made ultra-thin ferroelectric film applications virtually impractical. On the contrary, an interfacial layer can be used positively. It is possible that this interfacial layer (tunnel switch layer) can be utilized constructively to improve the reliability of a FE capacitor if by-electrode charge injection is blocked by this layer. Of course, this layer should not interfere with polarization switching.

Related Paper:

A. Q. Jiang, et al., Advanced Materials, 21, 2870-2875 (2009)

Tri-state Memory

Ferroelectric memory has been one of the most attractive candidates for the next generation nonvolatile memory application. However, conventional researches on FeRAM(Ferroelectric Random Access Memory) is decreasing gradually due to its intrinsic problems, such as large size of unit cell(14~25 F²). Thus, there has been much efforts to reduce the cell size of the FeRAM. However, there were not so impressive advances in this field, and some innovative idea is needed to meet the requirement of the reduced sized FeRAM.

If reducing the cell size is impossible, writing more than two states could be a new solution for the problem of scaling. In this study, 3-states memory was fabricated and its mechanism was studied using PZT/Al₂O₃/ZnO trilayer. Even though, a unit cell of 3-states memory cannot be the multi-bit memory by itself, two unit cells of 3-states memory can substitute the three unit cells of 2-states memory. Thus, it is possible to memorize the 50% increased data with the help of peripheral circuit.

Related Paper:

M. H. Park, et al., Advanced Functional Materials, 22, 21, 4305-4313 (2011)