Phase Of HfO2 Research Articles

Structure of Hf0.9La0.1O2 Ferroelectric Films Obtained by the Atomic Layer Deposition

Thin films of La-doped hafnium oxide synthesized by plasma-enhanced atomic layer deposition with subsequent rapid annealing have been studied. It has been found that the films under study have an orthorhombic noncentrosymmetric structure with the space group Pmn21. It has been shown that these films have ferroelectric properties. The ratio of atomic concentrations of elements in a film has been determined. It has been found that the film consists of the mixture of the HfO2 and La2O3 phases. It has been shown that argon ion etching results in the generation of oxygen vacancies with a concentration of about 1 at % in the surface region of the films. Vacancies are formed primarily through the knock-out of oxygen atoms to interstitial positions with the formation of a Frenkel pair.

Transformation of molecular compounds with Ba(Sr)/Al/Si and Ca(Sr, Ba)/Ti(Zr, Hf)/Si heteroelements as new efficient route to metal silicate materials.

Silicon-based moieties were anchored in heterometallic alkoxide platforms to obtain molecular clusters based on the M-O-M'-O-Si motif for the generation of mixed-metal silicate materials. Single-source molecular precursors with structures [M{(μ-ddbfo)2Al(OSiPh3)2}2], where dbbfoH = 2,3-dihydro-2,2-dimethylbenzofuran-7-ol and M = Ba (1), Sr (2), [Sr4Ti2(μ6-O)(μ3,η2-OCH2CH2OMe)8(η-OCH2CH2OMe)2(OSiPh3)4] (3), [Sr4M'2(μ4-OH)(μ3,η2-OCH2CH2OMe)4(μ,η2-OCH2CH2OMe)6(μ-X)(OSiPh3)4] (4, with M' = Zr and X = Cl; 5 with M' = Hf and X = OH), [Sr3Hf2(μ5-O)(μ3,η2-OCH2CH2OMe)4(OCH2CH2OMe)4(OSiPh3)4] (6), and [Ca2M'2(μ3-OH)2(μ,η2-OCH2CH2OMe)4(η2-HOCH2CH2OMe)2(η-OCH2CH2OMe)2(OSiPh3)4] for M' = Zr (7), Hf (8), were prepared by substitution of methyl groups, using Ph3SiOH or chloride atoms with KOSiPh3, in [M{(μ-ddbfo)2AlMe2}2] (M = Sr, Ba) or [M4M'2(μ6-O)(μ3,η2-OCH2CH2OMe)8(OCH2CH2OMe)2(HOCH2CH2OMe)xCl4] (M = Ca, Sr, Ba, M' = Ti, Zr, Hf and x = 0, 4). The precursors were characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray structural analysis. Thermal decomposition of compounds 1-8 at 1000 °C led to formation of ceramic materials that consisted of mixed-metal oxide nanocrystallites embedded in an amorphous SiO2 matrix. Compounds 1 and 2 decomposed to BaAl2Si2O8 and SrAl2Si2O8 aluminosilicates, 4 gave a mixture of SrSiO3, ZrO2, SrZrO3 and Sr7ZrSi6O21, 5 and 6 gave mixtures of various compounds, including SrHfO3, Hf0.96Si0.04O2, and SiO2, 7 lead to CaZrO3, ZrO2 and Ca3ZrSi2O9 and 8 gave a HfO2, Ca2HfSi4O12 and SiO2 phase. The thermolysis of compound 3 gave SiO2 particles of average size 40-90 nm, which contained spherical SrTiO3, SrSiO3 and Sr2TiSi2O8 nanocrystallites of diameter 5-16 nm.

Hafnium oxide nanoparticles fabricated by femtosecond laser ablation in water

We report the fabrication of colloidal hafnia nanoparticles (NPs) and nanoribbons (NRs) in deionized water achieved by femtosecond laser ablation. The average size of NPs and NRs varied in the range 13.5–18.0 nm and 10–20 nm, respectively, with varying input laser energy. At lower energies, the NPs were observed to be in pure monoclinic phase of HfO2. However, at higher input energies, interestingly, both monoclinic and hexagonal phases corresponding to HfO2 and Hf6O were observed. Hf6O is otherwise expected only at high pressures.

Atomic and electronic structure of ferroelectric La-doped HfO2 films

The atomic structure and optical properties of ferroelectric La-doped hafnium oxide (La:HfO2) thin films grown by the plasma-assisted atomic layer deposition were investigated. Using high resolution transmission electron microscopy, it was shown that the studied La:HfO2 film has a orthorhombic polar structure with the Pmn21 space group. It was found that the film exhibits ferroelectric properties. By means of x-ray photoelectron spectroscopy and specroellipsometry it was established that La:HfO2 consists of the HfO2 and La2O3 phases mixture. The specroellipsometry analysis with the Bruggeman effective medium approximation showed that the investigated La:HfO2 consists of 88% HfO2 and 12% La2O3. It is shown that etching La:HfO2 with argon ions leads to the oxygen vacancies generation in the near-surface region. These vacancies are generated mainly due to the knocking out of oxygen atoms to the interstitial positions, and the following annealing at 700 °C in vacuum for 1 h leads to the annihilation of that Frenkel defects.

The peculiarities of structural and optical properties of HfO2-based films co-doped with silicon and erbium

The effect of deposition conditions and further annealing treatment on microstructure and optical properties of (Si,Er)-codoped HfO2 thin films is investigated. The films are grown on silicon substrates by RF magnetron co-sputtering of Si and erbium oxide pellets on a HfO2 target. As-deposited and annealed samples are examined by means of spectroscopic ellipsometry, Fourier transform infrared spectroscopy and photoluminescence method. It is demonstrated that the variation of RF power density allows monitoring the dopant content. An annealing of the samples at 800–1100 °C for 10–60 min in nitrogen atmosphere results in the phase separation and the formation of HfO2, SiO2 and pure silicon phases. The films annealed at 900–950 °C demonstrate the red luminescence that is ascribed to the carrier recombination in Si nanocrystals. Annealing at higher temperatures causes an enhancement of rare-earth luminescence under non-resonant excitation providing an additional argument toward the formation of silicon nanoclusters. The mechanism of the excitation of Er ions is found to be similar to that of Si-rich-SiO2 films doped with rare-earths. The Si nanocrystals are considered as the main sensitizer at visible excitation while the energy transfer from host defects dominates at ultraviolet-deep blue illumination.

Nonlinear optical response of ZnO/HfO2 core/shell nanorod arrays under continuous wave laser irradiation

ZnO nanorods (NRs) were synthesized by the hydrothermal method and coated by HfO2 sell layers by using the atomic layer deposition to form ZnO/HfO2 core/shell NRs arrays. XRD showed the hexagonal wurtzite structure of ZnO NRs and tetragonal phase of HfO2. Scanning electron microscope and atomic force microscopy analysis illustrated vertically well-aligned ZnO/HfO2 core/shell NRs structure. Optical property was studied by PL which revealed that the near band-edge emission of the ZnO NRs improved after coating by HfO2 shells. The band gap energy was estimated about 3.51 eV by UV–Vis. For the first time, the measurements of nonlinear optical (NLO) properties of ZnO/HfO2 core/shell NRs structures have been performed by using a continuous-wave (cw) Nd:YAG laser at 532 nm by the Z-scan technique. A negative nonlinear refractive index and a positive absorption coefficient were calculated equal to 8.6 × 10−6 cm2/W and 1.75 cm/W for ZnO/HfO2 core/shell NRs, respectively. A third-order NLO susceptibility of ZnO/HfO2 core/shell NRs was measured of the order of 10−2 esu which is a large value. The results suggest that ZnO/HfO2 core/shell NRs are promising candidates for NLO applications.

Theoretical and Experimental Evidence for a Post-Cotunnite Phase Transition in Hafnia at High Pressures

Using first-principles density-functional theory (DFT) computations, we have predicted a new post-cotunnite (OII) phase of hafnia (HfO2) at high pressures. Our computations, using the generalized gradient approximation (GGA), predict a phase transition from OII to a Fe2P-type structure at ~ 120 GPa (~ 140 GPa) with a slight volume collapse at the transition pressure of ~ 0.2% (~ 0.1%) between the two phases using the second- (third-) order Birch-Murnaghan equation of state, respectively. The prediction of the new phase is consistent with recent experiments and computations performed on similar dioxides titania (TiO2) and zirconia (ZrO2) at extreme pressure-temperature (p-T) conditions. Importantly, our theoretical prediction for the OII → Fe2P transition in HfO2 is experimentally supported by the re-analysis of X-ray diffraction patterns of HfO2 at extreme pressure-temperature conditions. Additionally, the equation of state and hardness of the predicted phase have been computed and show that Fe2P-type phase while less compressible than the OII phase is nearly identical in hardness, indicating that none of the HfO2 phases qualify as superhard.

Pre‐Transitional Behavior in Tetragonal to Cubic Phase Transition in HfO2 Revealed by High Temperature Diffraction Experiments

Using the container‐less levitation technique, high‐temperature X‐ray diffraction experiments have been performed on HfO2 powders to confirm the temperature dependences of the lattice constants, thermal expansion character, and phase relations. A two‐dimensional imaging plate detector is used for short‐time observations; diffraction data in a wide area are projected in one dimension for Rietveld analysis, as large‐grain growth occurred at high temperatures. The lattice constants, thermal expansions, and c/a ratios for the tetragonal (space group P42/nmc) and cubic (space group Fm3m) HfO2 phases are measured at high temperature. The transition temperature of HfO2 from the tetragonal phase to the cubic phase is specified to be between 2500 and 2550 °C. The pre‐transitional behavior is clearly observed around 2200 °C. As no clear lattice volume change is observed at the transition point from the tetragonal phase to the cubic phase, a phase boundary between the tetragonal phase and the cubic phase has been proposed which does not exhibit a negative slope. It has been estimated that the stable region of the cubic phase is narrow in the higher‐pressure region.

(Invited) Surface Free Energy and Interfacial Strain in HfO2 and Hzo Ferroelectric Formation

The mechanism of stability of the phases of HfO2, ZrO2, and HZO (HfxZr1-xO2) were systematically investigated with density functional theory molecular dynamics (DFT-MD) to determine the mechanism for HZO having a much larger process window for formation the ferroelectric phase as compared to doped HfO2 or ZrO2. For the bulk states, the monoclinic phase (“m”) is about 80 mV per formula unit more stable than either the orthorhombic ferroelectric (“f”) phase or tetragonal (t-phase) for all three oxides. The surface free energies of the (001), (110), and (111) surfaces of all three oxides were calculated using an identical DFT technique. For all three oxides, the (111) face has the lowest surface free energies consistent with experimental data on columnar HZO grains showing [111] is the preferred growth direction. However, the surface free energy for all direction are nearly degenerate between HfO2, ZrO2, and HZO; therefore, even for nanocrystal formation the surface free energy does not favor f-phase formation. The effect of stress/strain was calculated by determining the free energy of formation as a function of the volume of the unit cell. When the oxides are grown in the low density amorphous phase but a post deposition anneal is perform for crystallization. The crystalline forms are more dense than the amorphous forms and the DFT calculation show that a higher surface area per unit cells will greatly favor f-phase formation. However, the effect is nearly identical for HfO2, ZrO2, and HZO; this is consistent with experiments showing the molar volumes of HfO2 and ZrO2 being within 2%. Instead, formation of nanocrystalites is hypothesized to be the source of the enhanced processing window for HZO. Experimental data is consistent with partial phase separation in HZO. Atom probe tomography imaging of the chemical composition of TiN/5 nm HZO/Si(001) ferroelectric films show an asymmetric distribution of the Hf and Zr within the HZO layer with the Zr being concentrated near the TiN/HZO interface; this is consistent with ZrO2 having a 100C lower crystallization temperature than HfO2 and therefore initiate the crystallization starting on the TiN(111) surface. It is hypothesized that the nanocrystals which template on TiN(111) can produce the interfacial stress/strain needed to stabilize f-phase formation; high resolution TEM shows regions of epitaxial alignment between HZO and TiN consistent with this mechanism. In addition atom probe tomography (APT) was performed on TiN/HZO/Si structures to determine the film composition of the interfaces for indication of possible phase separation of HZO since phase separation could promote nanocrystal formation.Funding by LAM Research is gratefully acknowledged.

Origin of Pr3+ luminescence in hafnium silicate films: combined atom probe tomography and TEM investigations

Structural, chemical, and luminescence properties of Pr3+-doped HfSiOx layers fabricated by radio-frequency magnetron sputtering were examined as a function of annealing temperature. Phase separation between SiO2 and HfO2 as well as the location of Pr3+ dopants were investigated using atom probe tomography and transmission electron microscopy while optical properties of Pr3+ ions were studied using photoluminescence measurements. As a result, (i) we evidenced the location of the Pr3+ dopants in the HfO2 phase while the SiO2 phase was discovered to be free of these dopants, (ii) the HfO2 phase was identified to crystallize in the cubic phase until 1050 °C annealing, (iii) no Pr clusters were detected as function of annealing, and (iv) luminescence properties were discussed in regard to the location of Pr in the HfO2 cubic phase.

Short-Range Order in Amorphous and Crystalline Ferroelectric Hf0.5Zr0.5O2

The microstructures of amorphous and polycrystalline ferroelectric Hf0.5Zr0.5O2 films are studied by X-ray spectroscopy and ellipsometry. EXAFS spectra demonstrate that the amorphous film consists of an “incompletely mixed” solid solution of metallic oxides HfO2 and ZrO2. After rapid thermal annealing, the mixed Hf0.5Zr0.5O2 oxide films have a more ordered polycrystalline structure, and individual Hf and Zr monoxide islands are formed in the films. These islands are several nanometers in size and have a structure that is similar to the monoclinic structure of HfO2 and ZrO2. The presence of the HfO2 and ZrO2 phases in the Hf0.5Zr0.5O2 films is also detected by ellipsometry.

Al-, Y-, and La-doping effects favoring intrinsic and field induced ferroelectricity in HfO2: A first principles study

III-valent dopants have shown to be most effective in stabilizing the ferroelectric, crystalline phase in atomic layer deposited, polycrystalline HfO2 thin films. On the other hand, such dopants are commonly used for tetragonal and cubic phase stabilization in ceramic HfO2. This difference in the impact has not been elucidated so far. The prospect is a suitable doping to produce ferroelectric HfO2 ceramics with a technological impact. In this paper, we investigate the impact of Al, Y, and La doping, which have experimentally proven to stabilize the ferroelectric Pca21 phase in HfO2, in a comprehensive first-principles study. Density functional theory calculations reveal the structure, formation energy, and total energy of various defects in HfO2. Most relevant are substitutional electronically compensated defects without oxygen vacancy, substitutional mixed compensated defects paired with a vacancy, and ionically compensated defect complexes containing two substitutional dopants paired with a vacancy. The ferroelectric phase is strongly favored with La and Y in the substitutional defect. The mixed compensated defect favors the ferroelectric phase as well, but the strongly favored cubic phase limits the concentration range for ferroelectricity. We conclude that a reduction of oxygen vacancies should significantly enhance this range in Y doped HfO2 thin films. With Al, the substitutional defect hardly favors the ferroelectric phase before the tetragonal phase becomes strongly favored with the increasing concentration. This could explain the observed field induced ferroelectricity in Al-doped HfO2. Further Al defects are investigated, but do not favor the f-phase such that the current explanation remains incomplete for Al doping. According to the simulation, doping alone shows clear trends, but is insufficient to replace the monoclinic phase as the ground state. To explain this fact, some other mechanism is needed.

Hafnium-Oxide 3-D Nanofilms via the Anodizing of Al/Hf Metal Layers

Hafnium-oxide films with self-organized nanostructured 3-D architectures and variable dimension (10 to 400 nm) are synthesized via the high-current anodizing of thin aluminum-on-hafnium layers in phosphoric, malonic, and oxalic acid electrolytes. In the approach, the self-organized growth of a porous anodic alumina (PAA) film is immediately followed by the fast PAA-assisted reanodizing of the hafnium underlayer. The PAA-dissolved films consist of arrays of upright-standing hafnium-oxide nanorods held on the substrate by the tiny needle-like “nanoroots” widespread over a continuous hafnium-oxide bottom layer. The roots are amorphous Hf2O3, while the rods are amorphous HfO2–Hf2O3–Al2O3 mixed oxides, the bottom layer being, however, highly textured nanocrystalline HfO2. The calculated transport numbers for O2– and Hf4+(3)+ ions are, respectively, ∼0.55 and ∼0.45, which is a unique situation for anodic hafnium oxide, which normally grows by O2– transport only. Annealing the films in air at 600 °C oxidizes the remaining Hf metal to polycrystalline HfO2, still leaving the roots and rods amorphous. The annealing in vacuum results in partial oxide reduction and crystallization of the roots and rods to stable orthorhombic and monoclinic HfO2 phases. A model of the anodic film growth and solid-state ionic transport is proposed and experimentally justified. Potential applications of the 3-D hafnium-oxide nanofilms are in advanced electronic, photonic, or magnetic micro- and nanodevices.

Structural and electrical properties of reactive magnetron sputtered yttrium-doped HfO2 films**Project supported by the National Natural Science Foundation of China (Grant Nos. 51272034 and 51672032) and the Fundamental Research Funds for the Central Universities, China (Grant No. DUT17ZD211).

Hafnium oxide thin films doped with different concentrations of yttrium are prepared on Si (100) substrates at room temperature using a reactive magnetron sputtering system. The effects of Y content on the bonding structure, crystallographic structure, and electrical properties of Y-doped HfO2 films are investigated. The x-ray photoelectron spectrum (XPS) indicates that the core level peak positions of Hf 4f and O 1s shift toward lower energy due to the structure change after Y doping. The depth profiling of XPS shows that the surface of the film is completely oxidized while the oxygen deficiency emerges after the stripping depths have increased. The x-ray diffraction and high resolution transmission electron microscopy (HRTEM) analyses reveal the evolution from monoclinic HfO2 phase towards stabilized cubic HfO2 phase and the preferred orientation of (111) appears with increasing Y content, while pure HfO2 shows the monoclinic phase only. The leakage current and permittivity are determined as a function of the Y content. The best combination of low leakage current of 10−7 A/cm2 at 1 V and a highest permittivity value of 29 is achieved when the doping ratio of Y increases to 9 mol%. A correlation among Y content, phase evolution and electrical properties of Y-doped HfO2 ultra-thin film is investigated.

The prediction of a new high-pressure phase of hafnia using first-principles computations

Using density functional theory (DFT) calculations, we predicted a new high- pressure phase of hafnia (HfO2). We found the hexagonal phase (Ni2In-type structure; space group: P63/mmc) to be the stable phase at ultrahigh pressures greater than ~386 GPa. Our findings are consistent with recent calculations performed on the similar dioxide ZrO2 [M. Durandurdu, J. Solid State Chem. 230, 233 (2015)] where this phase has been claimed to be the most stable at pressures greater than 380 GPa. The Birch-Murnaghan equation of state (BM- EOS) of the new phase shows that this phase is more compressible and less dense than Fe2P-type phase. Additionally, the hardness calculations using a scaling model confirmed that our newly predicted phase has a similar hardness compared to the other HfO2 phases, indicating that none of the HfO2 phases can be considered to be superhard.

Electronic structures and optical properties of HfO2–TiO2 alloys studied by first-principles GGA + U approach**Project supported by the National Natural Science Foundation of China (Grant Nos. 11672087, 11502058, and 11402252).

The phase diagram of HfO2–TiO2 system shows that when Ti content is less than 33.0 mol%, HfO2–TiO2 system is monoclinic; when Ti content increases from 33.0 mol% to 52.0 mol%, it is orthorhombic; when Ti content reaches more than 52.0 mol%, it presents rutile phase. So, we choose the three phases of HfO2–TiO2 alloys with different Ti content values. The electronic structures and optical properties of monoclinic, orthorhombic and rutile phases of HfO2–TiO2 alloys are obtained by the first-principles generalized gradient approximation (GGA)+U approach, and the effects of Ti content and crystal structure on the electronic structures and optical properties of HfO2–TiO2 alloys are investigated. By introducing the Coulomb interactions of 5d orbitals on Hf atom (), those of 3d orbitals on Ti atom (), and those of 2p orbitals on O atom (Up) simultaneously, we can improve the calculation values of the band gaps, where , , and Up values are 8.0 eV, 7.0 eV, and 6.0 eV for both the monoclinic phase and orthorhombic phase, and 8.0 eV, 7.0 eV, and 3.5 eV for the rutile phase. The electronic structures and optical properties of the HfO2–TiO2 alloys calculated by (Up = 6.0 eV or 3.5 eV) are compared with available experimental results.

Phase variation and thermophysical properties of La2Hf2O7 with alumina addition

La2O3-Al2O3-HfO2 ceramics were prepared by solid reaction of La2Hf2O7 ceramic powders which were synthesized by solution combustion and commercial alumina powders at 1600°C in air for 10h. Phase variation and thermophysical properties of La2O3-Al2O3-HfO2 ceramics were investigated. With the increase of alumina content, LaAlO3, HfO2 and LaAl11O18 phase structures appeared. The thermal expansion coefficients gradually increased with the increase of alumina content from 1000°C to 1300°C. The thermal conductivities of La2O3-Al2O3-HfO2 ceramics had nonlinear variation with the increase of alumina content and reached the lowest value around 1.1W·m−1·K−1 when the ratio of La2Hf2O7 to Al2O3 was 1:0.25. Therefore, the La2O3-Al2O3-HfO2 ceramics with a proper content of alumina can be considered as a new thermal barrier materials. Meanwhile, as potential application as interface layer in TBC, La2O3-Al2O3-HfO2 ceramics are beneficial to improve the effectiveness of the thermal barrier coating.

Kinetic pathway of the ferroelectric phase formation in doped HfO2 films

The dopant-induced ferroelectric HfO2 formation has been systematically investigated by using cation (Sc, Y, Nb, Al, Si, Ge, and Zr) and anion (N) dopants. Both differences and similarities are discussed among various dopants by focusing on two major factors, the oxygen vacancy (Vo) and the dopant ionic size. First, the doping concentration dependence of the remanent polarization in 27 (±2) nm HfO2 films is quantitatively estimated. Then, by comparing the polarization result with the structural transformation in doped HfO2, the pathway of the dopant-induced HfO2 phase transition is discussed among monoclinic, ferroelectric orthorhombic, tetragonal, and cubic phases. Finally, it is addressed that a dopant species independent phase transition route may exist in HfO2 owing to the same kinetic transition process, in which the ferroelectric phase seems to be at an intermediate state between tetragonal and monoclinic phases.

Oxidation mechanisms under water vapour conditions of ZrB2-SiC and HfB2-SiC based materials up to 2400 °C

This study aims at observing and understanding the oxidation mechanisms of ZrB2-20 vol%SiC (ZS), HfB2-20 vol%SiC (HS) and HfB2-20 vol%SiC- 3 vol%Y2O3 (HSY) materials up to 2400°C under water vapour conditions. After SPS sintering, fully densified samples were oxidized at several temperatures with 30 vol% H2O/70 vol% Ar during 20 s. Weight variations as well as post-test microstructural and XRD analyses allowed understanding the influence of the composition on the oxidation behavior and the evolution of each oxide sublayer. Below 1550°C, oxidation is limited, and thin oxide layers are observed. At 1900 and 2200°C, ZS and HS show mechanical damage (cracks, spallation), while HSY keeps its structural integrity and interlayer adherence. The addition of Y2O3 reduces the damages due to thermal stresses in the material due to the stabilization of the cubic phase of HfO2, and the formation of a Y2Si2O7 interphase that mitigates thermal expansion mismatch between the SiC-depleted layer and the HfO2 layer.

(Invited) High-k, Higher-k and Ferroelectric HfO2

We have studied high-k gate stacks since 2000. We have presented our results on high-k and also learned a lot at ECS symposium. We gave a talk concerning doped HfO2 in order to achieve higher-k [1], and then also discussed a number of high-k properties including the interface dipole formation at high-k/SiO2 [2], advanced ALD process [3], the interface dipole formation mechanism [4, 5], high-k/Ge gate stacks [6], non-equilibrium PDA-induced HfO2 phase transformation [7], and the interlayer scavenging kinetics in HfO2 gate stacks [8]. In this talk, we would like to look back on our way, and discuss the origin of versatile properties of HfO2. The dielectric constant is important in high-k technology, and ferroelectric HfO2 recently reported is also exciting for functional device applications [9]. Why does HfO2 show such variety of properties? We would like to understand HfO2 in more detail. HfO2 has already been used in advanced CMOS. It is technically important whether a new higher-k material should be searched for or HfO2 should be further used. We have been interested in the structural phase control of HfO2 in order to enhance the dielectric constant. The 1st-principles calculation predicted that high temperature phase HfO2 such as cubic or tetragonal one had higher-k than amorphous or monoclinic one [10]. The monoclinic phase, however, is thermodynamically stable at Si processing temperatures, so usually cubic or tetragonal phase HfO2 is not available. Nevertheless, the significant enhancement of the dielectric constant in HfO2 was experimentally demonstrated through the stabilization of the cubic or tetragonal phase. The trick is the structural phase transformation by doping another metal-oxide to HfO2 [1]. The typical example is Y2O3-doped HfO2. In the ceramics community, it had been actually well recognized since long before that Y2O3 doping could induce the oxygen vacancy in ZrO2 to maintain the charge neutrality, which stabilized the cubic or tetragonal phase [11]. Furthermore, another way to realize the symmetric phase HfO2 was through a non-equilibrium thermal process of amorphous HfO2 [7]. In particular, the fast ramping-up in the RTA process enabled us to achieve the symmetric phase HfO2. It means that the nucleation rather than growth process is more important for implementing high symmetric phase HfO2. In addition, the size effect has also been well investigated thermodynamically for characterizing the structural phase [12]. Very unfortunately for us, we did not notice that HfO2 ferroelectricity came out by doping other oxides. An interesting point in the ferroelectric HfO2 is that the ferroelectric orthorhombic phase is not in the conventional phase diagram. Very recently we found that ferroelectric HfO2 was also made by anion doping into HfO2 [13]. The key is how to realize non-centrosymmetric phase HfO2 structure. Ferroelectric orthorhombic phase might be between monoclinic and tetragonal phases as a metastable state. This makes it more difficult to recognize such unexpected fabulous functionality easily. Materials are so challenging but so exciting.