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.

Abstract

The unexpected ferroelectric properties of nanoscale hafnia-zirconia are considered to be promising for a wealth of applications including ferroelectric memory, field effect transistors, and energy-related applications. However, the reason why the unexpected ferroelectric Pca2 phase can be stabilized has not been clearly understood although numerous extensive theoretical and experimental results have been reported recently. The ferroelectric orthorhombic phase is not a stable phase under processing conditions from the viewpoint of bulk free energy. Although the possibility of stabilization of the ferroelectric phase due to the surface energy effect has been theoretically suggested, such a theoretical model has not been systematically compared with actual experimental results. In this study, the experimental observations on polymorphism in nanoscale HfO-ZrO solid solution thin films of a wide range of film compositions and thicknesses are comprehensively related to the theoretical predictions based on a thermodynamic surface energy model. The theoretical model can semi-quantitatively explain the experimental results on the phase-evolution, but there were non-negligible discrepancies between the two results. To understand these discrepancies, various factors such as the film stress, the role of a TiN capping layer, and the kinetics of crystallization are systematically studied. This work also reports on the evolution of electrical properties of the film, i.e. dielectric, ferroelectric, anti-ferroelectric, and morphotropic phase changes, as a function of the film composition and thickness. The in-depth analyses of the phase change are expected to provide an important guideline for subsequent studies.

Quantitative phase analysis is first performed on doped Hafnia films to elucidate the structural origin of unexpected ferroelectricity.

Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped hafnia. La:HfO appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.

still smaller than that of electrochemical capacitors due mainly to the use of the low dielectric constant ( ε r ) Al 2 O 3 thin fi lm as the dielectric layer ( ε r ≈ 9). The concurrent high-power capacitors are made mainly of linear dielectric polymers with an ESD of ≈1-2 J cm −3 . [ 3 ] Although the ε r values of linear dielectric polymers are generally small (≈2-5), their electric breakdown fi elds are quite high, thereby allowing the application of high voltages, which result in a relatively high ESD. [ 3 ] Nonetheless, their ESD values are not high enough, so many other materials have been studied for the purpose. Most previous research on this topic focused on AFE Pb(Zr,Ti)O 3 (PZT)-based fi lms because their ESD values are as large as ≈1 and ≈10-15 J cm −3 for bulk and thin fi lms, respectively. [ 3 ] An ESD value as high as ≈50 J cm −3 has been reported for thin PZT-based fi lms when a large ≈3.5 MV cm −1 electric fi eld was applied. [ 7,8 ] However, such a high electric fi eld may induce a signifi cant reliability concern for PZT fi lms. [ 9 ] In addition, the commercial use of PZT is restricted in many countries due to its environmental impact. Another signifi cant problem with PZT thin fi lms is the decrease in their ESD value by ≈20-40% when their operating temperature increases to 150 °C. [ 10,11 ] Other candidate materials are FE poly(vinylidenefl uoride) (PVDF)-based materials, which can have a large breakdown fi eld and maximum polarization. The highest ESD value of FE PVDF-based fi lms has been reported to be as high as ≈20 J cm −3 when a ≈8 MV cm −1 electric fi eld was applied. [ 3 ] However, the problem with such fi lms is that their ESD is only ≈40% of their total stored energy because PVDF is an FE material meaning that ≈60% of the total stored energy is retained in the material. The presence of remanent polarization ( P r ) prohibits the full discharge of the stored charges in any FE material. [ 3 ] In the case of AFE PVDF-based fi lms, an ESD value of ≈14 J cm −3 has been reported with a higher effi ciency of ≈70%. [ 12 ] 3D nanostructures, such as nanoholes or nanotrenches, would be needed to eventually contain an even higher ESD for practical use in electric vehicles. [ 5 ] However, both PZT-and PVDF-based materials are inappropriate for such nanostructures because they are too thick (thickness ( t f ) ≈ 10 2 -10 4 nm) to be incorporated into nanoscale structures. Therefore, a dielectric material that has an ESD value as high as that of PZT and PVDF with a high breakdown fi eld at a fi lm thickness of less than ≈10 nm must be found. The fi lm must also be well grown using the facile atomic layer deposition (ALD) technique to ensure fl uent step coverage with atomic accuracy in thickness control.FE and AFE HfO 2 -based thin fi lms with various dopants, such as Si, Al, and Zr, were recently reported, where the conventional Useful energy sources and ways to effi ciently distribute energy have been extensively studied. With the development of new energy generation and handling technologies, h...

Abstract

HfZrO thin films are one of the most appealing HfO-based ferroelectric thin films, which have been researched extensively for their applications in ferroelectric memory devices. In this work, a 1 mol % La-doped HfZrO thin film was grown by plasma-assisted atomic layer deposition and annealed at temperatures of 450 and 500 °C to crystallize the film into the desired orthorhombic phase. Despite the use of a lower temperature than that used in previous reports, the film showed highly promising ferroelectric properties-a remnant polarization of ∼30 μC/cm and switching cycle endurance up to 4 × 10. The performance was much better than that of undoped HfZrO thin films, demonstrating the positive influence of La doping. Such improvements were mainly attributed to the decreased coercive field (by ∼30% compared to the undoped film), which allowed for the use of a lower applied field to drive the cycling tests while maintaining a high polarization value. La doping also decreased the leakage current by ∼3 orders of magnitude compared to the undoped film, which also contributed to the strongly improved endurance. Nonetheless, the La-doped film required a larger number of wake-up cycles (∼10 cycles) to reach a saturated remnant polarization value. This behavior might be explained by the increased generation of oxygen vacancies and slower migration of these vacancies from the interface to the bulk region. However, the maximum number of wake-up cycles was less than 0.01% of the total possible cycles, and therefore, initializing the film to the maximum performance state would not be a serious burden.