Room-temperature deposition of ferroelectric HfO2-based films by the sputtering method

Abstract

Ferroelectricity has been demonstrated in epitaxial 7%Y-doped HfO2 (0.07YO1.5–0.93HfO2, YHO7) films grown by the RF magnetron sputtering method at room temperature without any subsequent annealing. The x-ray diffraction patterns of such films suggested that the decrease in RF power and in the partial oxygen pressure changes the crystal structures of the films from the monoclinic phase to the tetragonal/orthorhombic phase. Clear polarization-electric-field (P–E) hysteresis loops were observed for these epitaxial films with the tetragonal/orthorhombic phase. The obtained remanent polarization (Pr) and coercive field (Ec) values were 14.5 and 12.8 μC/cm2 and 2300 and 2200 kV/cm for the epitaxial films on (111) indium tin oxide (ITO)//(111) yttria-stabilized zirconia (YSZ) and (100)ITO//(100)YSZ substrates, respectively. Moreover, ferroelectricity was also observed in room-temperature-deposited polycrystalline YHO7 films prepared on Pt/TiOx/SiO2/(100)Si, crystallized ITO/soda glass, and amorphous ITO/polyethylene terephthalate substrates, namely, crystalline ferroelectric HfO2-based films were prepared at room temperature on various substrates, including organic flexible substrates, by using the RF magnetron sputtering method. The present results open a path to novel applications of ferroelectric HfO2-based films such as ferroelectric flexible memory.

Device Feasibility of Ferroelectric Field‐Effect Transistors Using Al‐Doped HfO2 Gate Insulator Deposited with H2O Oxygen Precursor during Atomic Layer Deposition Process

“…The oxygen vacancies generated within the films have been regarded as one of the main origins stabilizing the ferroelectric o phase of the doped HfO 2 thin films during the phase transitions from m to o phases. [ 14 ] Moreover, mechanical confinement induced by top‐capping electrode provides the in‐plane tensile strain induced between the TEs and the ferroelectric thin films, [2j,10,12] effectively suppressing the m phase volume fraction, and thereby facilitating o phase formation. [ 14 ] Both effects were more markedly reflected with increasing the PMA temperature in the properties of the ferroelectric Al:HfO 2 thin films as well as the device operations of MFM capacitors by readily stabilizing the ferroelectric o phase.…”

“…It is also important to investigate the ferroelectric polarization switching kinetics of the Al:HfO 2 thin films prepared with H 2 O oxygen precursor during the ALD process in this work, as the polarization switching characteristics including the switching time are sensitively dependent on various control parameters, such as film thickness, types of dopants, and ALD process conditions [2j,19] . The ferroelectric switching dynamics of the Al:HfO 2 thin films were examined by double‐pulse switching method and Kolmogorov–Avrami–Ishibashi's (KAI) model, [2j,2k,20] as shown in Figure , in which leakage current components can be effectively removed to accurately obtain ferroelectric reversal currents of the MFM capacitors for determining the polarization switching kinetics. Actually, for the polycrystalline ferroelectric thin films, the nucleation limited switching (NLS) model was verified to more properly explain the ferroelectric switching kinetics rather than the conventional KAI model.…”

“…[ 1 ] As the device performance of the FeFETs can be primarily dominated by ferroelectric natures of the HfO 2 ferroelectric thin films, it is important to investigate the process parameters for controlling and improving the ferroelectric properties of the doped HfO 2 thin films, such as types of dopants, deposition methods, and crystallization annealing conditions. [ 2 ] The use of cation dopants with a smaller atomic radius than Hf, such as Al and Si, has been suggested to reduce the difference in volume‐free energies between the tetragonal (t)/cubic (c) phase and orthorhombic (o) phase, facilitating the phase transition from monoclinic (m) phase to t/c phase and contributing to stabilization of o phase. [ 3 ] Therefore, the suitable choice of dopant has been commonly considered as one of the most effective methods to ensure the superior ferroelectricity of the HfO 2 thin films.…”

“…[ 3 ] Therefore, the suitable choice of dopant has been commonly considered as one of the most effective methods to ensure the superior ferroelectricity of the HfO 2 thin films. In particular, we conducted various works characterizing the ferroelectric capacitors [2j,2k] and FeFETs using Al‐doped HfO 2 (Al:HfO 2 ) ferroelectric thin films for the nonvolatile memory [1f] and neuromorphic synapse device [1a] applications, in which the Al:HfO 2 thin films were prepared by atomic layer deposition (ALD) process with ozone (O 3 ) oxygen precursor.…”

“…Therefore, to minimize the technical drawbacks of H 2 O oxygen precursor and properly use the doped HfO 2 thin films as ferroelectric gate insulators of the FeFETs, various control parameters such as deposition temperature, doping concentration, and annealing temperature have been investigated in previous reports. [ 8 ] Particularity, the crystallization annealing process is closely related to the formation of the ferroelectric phase within the film, which plays an important role in determining the characteristics of the ferroelectric gate insulator [2c,2d] . With the introduction of appropriate annealing conditions, values of ferroelectric remnant polarization (2 P r ) were obtained to be larger than 40 μC cm −2 for the doped HfO 2 thin films deposited by ALD process with H 2 O oxygen precursor [8e] .…”