Min Hyuk Park

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.

Evolution of phases and ferroelectric properties of thin Hf0.5Zr0.5O2 films according to the thickness and annealing temperature

The effects of annealing temperature (Tanneal) and film thickness (tf) on the crystal structure and ferroelectric properties of Hf0.5Zr0.5O2 films were examined. The Hf0.5Zr0.5O2 films consist of tetragonal, orthorhombic, and monoclinic phases. The orthorhombic phase content, which is responsible for the ferroelectricity in this material, is almost independent of Tanneal, but decreases with increasing tf. In contrast, increasing Tanneal and tf monotonically increases (decreases) the amount of monoclinic (tetragonal) phase, which coincides with the variations in the dielectric constant. The remanant polarization was determined by the content of orthorhombic phase as well as the spatial distribution of other phases.

Effect of forming gas annealing on the ferroelectric properties of Hf0.5Zr0.5O2 thin films with and without Pt electrodes

The effects of forming gas annealing (FGA) on the ferroelectric properties of Hf0.5Zr0.5O2 (HZO) films were examined. Although the H-incorporation during FGA degrades the ferroelectric properties of Hf0.5Zr0.5O2 films, the degree of degradation was much lower compared with other ferroelectrics, such as Pb(Zr,Ti)O3. Pt worked as a catalyst for H-incorporation, and maximum 2Pr loss of ∼40% occurred. However, the insertion of a ∼20-nm-thick TiN layer between Pt and Hf0.5Zr0.5O2 decreased the degradation to ∼12%. Hf0.5Zr0.5O2 is more resistant to degradation by FGA compared with the conventional ferroelectrics, which is a highly promising result for next-generation ferroelectric memory.

The effects of crystallographic orientation and strain of thin Hf0.5Zr0.5O2 film on its ferroelectricity

To elucidate the origin of the formation of the ferroelectric phase in Hf0.5Zr0.5O2 films, the effects of film strain and crystallographic orientation on the properties were examined. Using a (111)-textured Pt bottom electrode, Hf0.5Zr0.5O2 films with a (111)-preferred texture inappropriate for transforming their phase from non-ferroelectric tetragonal to ferroelectric orthorhombic phase were deposited. In contrast, randomly oriented Hf0.5Zr0.5O2 films, grown on the TiN electrode, showed feasible ferroelectric properties due to their transformation to the ferroelectric orthorhombic phase. The origin of such transformation is the large in-plane tensile strain for the elongation of the c-axis of the tetragonal phase.

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.

A comprehensive study on the structural evolution of HfO2 thin films doped with various dopants

The origin of the unexpected ferroelectricity in doped HfO2 thin films is now considered to be the formation of a non-centrosymmetric Pca21 orthorhombic phase. Due to the polycrystalline nature of the films as well as their extremely small thickness (∼10 nm) and mixed orientation and phase composition, structural analysis of doped HfO2 thin films remains a challenging task. As a further complication, the structural similarities of the orthorhombic and tetragonal phase are difficult to distinguish by typical structural analysis techniques such as X-ray diffraction. To resolve this issue, the changes in the grazing incidence X-ray diffraction (GIXRD) patterns of HfO2 films doped with Si, Al, and Gd are systematically examined. For all dopants, the shift of o111/t101 diffraction peak is observed with increasing atomic layer deposition (ALD) cycle ratio, and this shift is thought to originate from the orthorhombic to P42/nmc tetragonal phase transition with decreasing aspect ratio (2a/(b + c) for orthorhombic and c/a for the tetragonal phase). For quantitative phase analysis, Rietveld refinement is applied to the GIXRD patterns. A progressive phase transition from P21/c monoclinic to orthorhombic to tetragonal is confirmed for all dopants, and a strong relationship between orthorhombic phase fraction and remanent polarization value is uniquely demonstrated. The concentration range for the ferroelectric properties was the narrowest for the Si-doped HfO2 films. The dopant size is believed to strongly affect the concentration range for the ferroelectric phase stabilization, since small dopants can strongly decrease the free energy of the tetragonal phase due to their shorter metal–oxygen bonds.

Improved ferroelectric property of very thin Mn-doped BiFeO3 films by an inlaid Al2O3 tunnel switch

A thin Al2O3 layer was atomic layer deposited on a 50 - or 70 nm-thick, polycrystalline Mn-doped BiFeO3 (BFMO) thin film to fabricate a dielectric/ferroelectric bi-layer capacitor. The thin Al2O3 layer worked as a tunnel switch to improve the ferroelectric performance of the BFMO films. The BFMO thin film and BiFeO3 film contains a high density of charged defects, such as oxygen vacancies, Bi vacancies, and Fe2+ ions reduced from Fe3+, which induce a large leakage current and cause instability of ferroelectric polarization in one direction. An asymmetric hysteresis loop and severe depolarization was observed in the BFMO capacitor due to the local field induced by the defect complexes that are mainly formed near the Pt/BFMO interface from interactions between charged defects. Depositing a thin Al2O3 tunnel switch layer reduced these detrimental phenomena, which could be attributed to a decrease in the amount of oxygen vacancies as well as the suppression of polarization back-switching after the polarizatio...