Taehwan Moon

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.

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.

Study on the internal field and conduction mechanism of atomic layer deposited ferroelectric Hf0.5Zr0.5O2 thin films

The internal field (Eint) in ferroelectric films is an important factor which can affect the reliability of practical devices utilizing two memory states which results from the remanent polarizations of ferroelectric films. In the current work, the Eint in TiN/Hf0.5Zr0.5O2/TiN capacitors was controlled by changing the annealing atmosphere (N2, O2, and forming gas). The magnitude of negative Eint in O2-annealed samples was the largest, whereas that in the forming gas-annealed sample was the smallest. The magnitude of Eint can be understood based on the asymmetric distribution of oxygen vacancies near top and bottom TiN electrodes. Despite the large magnitude of Eint, the two remanent polarizations can be reliably retained due to the large coercive electric field of Hf0.5Zr0.5O2 films, and this is expected to be beneficial for application in semiconductor memory devices. During the repetitive electric field cycling for the wake-up process, the change in Eint in O2- and forming gas-annealed samples showed the opposite tendency: the magnitude of Eint in the O2-annealed Hf0.5Zr0.5O2 film decreased, whereas that in the forming gas-annealed film increased. This difference is believed to be due to the redistribution of oxygen vacancies with electric field high enough for the migration of oxygen vacancies. The conduction mechanism of electrons through Hf0.5Zr0.5O2 films was also examined, and the results fitted best with the Poole–Frenkel emission model with the shallow traps for all the samples with a reasonable optical dielectric constant value for Hf0.5Zr0.5O2.

Ferroelectricity in undoped-HfO2 thin films induced by deposition temperature control during atomic layer deposition

HfO2 thin films, extensively studied as high-k gate dielectric layers in metal-oxide-semiconductor field effect transistors, have attracted interest of late due to their newly discovered ferroelectricity in doped HfO2. The appearance of the ferroelectric orthorhombic phase of HfO2 was previously examined in variously doped and undoped systems, but the effects of process-variable changes on the physical and chemical characteristics of a thin film and the resulting ferroelectricity have not been studied systematically. Here, the evolution of ferroelectricity in HfO2 thin films through deposition temperature control during atomic layer deposition was systematically examined without the intentional doping of metallic elements other than Hf. The lower-temperature-deposited HfO2 showed an increased impurity concentration, which was mainly carbon, and the involvement of these impurities suppressed the lateral grain growth during the crystallization thermal treatment. The grain size reduction could stabilize the metastable orthorhombic phase, whose surface and grain boundary energies are lower than those of the room-temperature-stable monoclinic phase, by increasing the grain boundary areas. The 9 nm-thick HfO2 thin film deposited at 220 °C exhibited a remanent polarization value of 10.4 μC cm−2 and endured up to 108 switching cycles, which is a 102-fold improvement compared to the previously reported undoped 6 nm-thick HfO2. This can be ascribed to the decrease in the relative portion of defective interfacial layers by increasing the total film thickness. The strategy of using deposition temperature control is a feasible method for the fabrication of these new lead-free binary ferroelectric thin films.

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.