Keum Do Kim

Ferroelectricity and Antiferroelectricity of Doped Thin HfO2‐Based Films

The recent progress in ferroelectricity and antiferroelectricity in HfO2-based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non-volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si-compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si-doped HfO2 thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferro-electricity or antiferroelectricity in thin HfO2 films. They have large remanent polarization of up to 45 μC cm(-2), and their coercive field (≈1-2 MV cm(-1)) is larger than conventional ferroelectric films by approximately one order of magnitude. Furthermore, they can be extremely thin (<10 nm) and have a large bandgap (>5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field-effect-transistors and three-dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin films is expected to be useful for various applications, including energy harvesting/storage, solid-state-cooling, and infrared sensors.

Study on the degradation mechanism of the ferroelectric properties of thin Hf0.5Zr0.5O2 films on TiN and Ir electrodes

Hf0.5Zr0.5O2 films could show excellent ferroelectricity with a large remanent polarization (Pr, > 16 μC/cm2) on TiN and Ir electrodes, but their Pr decreased with the increasing thickness and monoclinic phase portion. The critical thickness for the degradation of the ferroelectricity of Hf0.5Zr0.5O2 films was smaller on the Ir electrode than the TiN electrode. This was due to the formation of larger grains, favorable for the formation of the monoclinic phase, on the Ir electrode than on the TiN electrode. The oxygen supply from IrOx exaggerated the initial growth on the Ir electrode and formed the larger grains.

Giant Negative Electrocaloric Effects of Hf0.5Zr0.5O2 Thin Films

Hafnia (HfO2 )-zirconia (ZrO2 ) solid solution films show giant positive (ΔT = 13.4 K) and negative (ΔT = -10.8 K) electrocaloric effects that can be simply controlled by tuning the Hf/Zr ratio. It is expected that the combination of the electrocaloric effects with opposite signs in this lead-free, simple, binary oxide can significantly improve the efficiency of electrocaloric cooling.

Grain size engineering for ferroelectric Hf0.5Zr0.5O2 films by an insertion of Al2O3 interlayer

The degradation of ferroelectric (FE) properties of atomic layer deposited Hf0.5Zr0.5O2 films with increasing thickness was mitigated by inserting 1 nm-thick Al2O3 interlayer at middle position of the thickness of the FE film. The large Pr of 10 μC/cm2, which is 11 times larger than that of single layer Hf0.5Zr0.5O2 film with equivalent thickness, was achieved from the films as thick as 40 nm. The Al2O3 interlayer could interrupt the continual growth of Hf0.5Zr0.5O2 films, and the resulting decrease of grain size prevented the formation of non-ferroelectric monoclinic phase. The Al2O3 interlayer also largely decreased the leakage current of the Hf0.5Zr0.5O2 films.

Study on the size effect in Hf0.5Zr0.5O2 films thinner than 8 nm before and after wake-up field cycling

The effects of film thickness and wake-up field cycling on the ferroelectricity in Hf0.5Zr0.5O2 films thinner than 8 nm were carefully examined. The Hf0.5Zr0.5O2 films became more antiferroelectric-like with decreasing film thickness in pristine state, whereas all the Hf0.5Zr0.5O2 films showed ferroelectric characteristics after wake-up process. The decrease in the coercive field with decreasing film thickness could be understood based on the depolarization correction. From the temperature-dependent characterization, the tetragonal-to-orthorhombic phase transition during wake-up process is believed to be a thermally activated process, and the estimated activation energy was ∼3.42 ± 0.17 kJ/mol.

Frustration of Negative Capacitance in Al2O3/BaTiO3 Bilayer Structure.

Enhancement of capacitance by negative capacitance (NC) effect in a dielectric/ferroelectric (DE/FE) stacked film is gaining a greater interest. While the previous theory on NC effect was based on the Landau-Ginzburg-Devonshire theory, 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 5 nm-thick Al2O3/150 nm-thick BaTiO3 system shows the capacitance boost effect with no FE-like hysteresis behavior, which was inconsistent with the experimental results; the amorphous-Al2O3/epitaxial-BaTiO3 system showed a typical FE-like hysteresis loop in the polarization – voltage test. This was due to the involvement of the trapped charges at the DE/FE interface, originating from the very high field across the thin Al2O3 layer when the BaTiO3 layer played a role as the NC layer. Therefore, the NC effect in the Al2O3/BaTiO3 system was frustrated by the involvement of reversible interface charge; 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 remanent polarization.

Alternative interpretations for decreasing voltage with increasing charge in ferroelectric capacitors

Recent claim on the direct observation of a negative capacitance (NC) effect from a single layer epitaxial Pb(Zr0.2,Ti0.8)O3 (PZT) thin film was carefully reexamined, and alternative interpretations that can explain the experimental results without invoking the NC effect are provided. Any actual ferroelectric capacitor has an interfacial layer, and experiment always measures the sum of voltages across the interface layer and the ferroelectric layer. The main observation of decreasing ferroelectric capacitor voltage (VF) for increasing ferroelectric capacitor charge (QF), claimed to be the direct evidence for the NC effect, could be alternatively interpreted by either the sudden increase in the positive capacitance of a ferroelectric capacitor or decrease in the voltage across the interfacial layer due to resistance degradation. The experimental time-transient VF and QF could be precisely simulated by these alternative models that fundamentally assumes the reverse domain nucleation and growth. Supplementary experiments using an epitaxial BaTiO3 film supported this claim. This, however, does not necessarily mean that the realization of the NC effect within the ferroelectric layer is impractical under appropriate conditions. Rather, the circuit suggested by Khan et al. may not be useful to observe the NC effect directly.

Time-Dependent Negative Capacitance Effects in Al2O3/BaTiO3 Bilayers

The negative capacitance (NC) effects in ferroelectric materials have emerged as the possible solution to low-power transistor devices and high-charge-density capacitors. Although the steep switching characteristic (subthreshold swing < sub-60 mV/dec) has been demonstrated in various devices combining the conventional transistors with ferroelectric gates, the actual applications of the NC effects are still some way off owing to the inherent hysteresis problem. This work reinterpreted the hysteretic properties of the NC effects within the time domain and demonstrated that capacitance (charge) boosting could be achieved without the hysteresis from the Al2O3/BaTiO3 bilayer capacitors through short-pulse charging. This work revealed that the hysteresis phenomenon in NC devices originated from the dielectric leakage of the dielectric layer. The suppression of charge injection via the dielectric leakage, which usually takes time, inhibits complete ferroelectric polarization switching during a short pulse time. It was demonstrated that a nonhysteretic NC effect can be achieved only within certain limited time and voltage ranges, but that these are sufficient for critical device applications.

Preparation and characterization of ferroelectric Hf0.5Zr0.5O2 thin films grown by reactive sputtering

HfO2-ZrO2 solid-solution films were prepared by radio frequency sputtering, and the subsequent annealing process was optimized to render enhanced ferroelectric behavior. The target power, working pressure and O2 partial pressure ratios were varied, along with the annealing gas, time and temperature. Then, the films structural and electrical properties were carefully scrutinized. Oxygen-deficient conditions were necessary during the sputter deposition to suppress grain growth, while annealing by O2 gas was critical to avoid defects and leakage problems. It is expected that the grain size difference under various deposition conditions combined with the degree of TiN top and bottom electrode oxidation by O2 gas will result in different ferroelectric behaviors. As a result, Hf0.5Zr0.5O2 prepared by radio frequency sputtering showed optimized ferroelectricity at 0% of O2 reactive gas, with a doubled remnant polarization value of ∼20 μC cm-2 at a thickness of 11 nm. Film growth conditions with a high growth rate (4-5 nm min-1) were favorable for achieving the ferroelectric phase film, which feasibly suppressed both the grain growth and accompanying monoclinic phase formation.

Interfacial charge-induced polarization switching in Al2O3/Pb(Zr,Ti)O3 bi-layer

Detailed polarization switching behavior of an Al2O3/Pb(Zr,Ti)O3 (AO/PZT) structure is examined by comparing the phenomenological thermodynamic model to the experimental polarization–voltage (P-V) results. Amorphous AO films with various thicknesses (2–10 nm) were deposited on the polycrystalline 150-nm-thick PZT film. The thermodynamic calculation showed that the transition from the ferroelectric-like state to the paraelectric-like state with increasing AO thickness occurs at ∼3 nm thickness. This paraelectric-like state should have exhibited a negative capacitance effect without permanent polarization switching if no other adverse effects are involved. However, experiments showed typical ferroelectric-like hysteresis loops where the coercive voltage increased with the increasing AO thickness, which could be explained by the carrier injection through the thin AO layer and trapping of the carriers at the AO/PZT interface. The fitting of the experimental P-V loops using the thermodynamic model considering the depolarization energy effect showed that trapped charge density was ∼±0.1 Cm−2 and critical electric field at the Pt electrode/AO interface, at which the carrier transport occurs, was ∼±10 MV/cm irrespective of the AO thickness. Energy band model at each electrostatic state along the P-V loop was provided to elucidate correlation between macroscopic polarization and internal charge state of the stacked films.