Pure HfO2 Film Research Articles

Control of intrinsic ferroelectricity and structural phase in pure HfO2 films via crystalline orientation

The ferroelectricity of hafnia-based thin films has garnered considerable attention in both academic researches and industrial applications. However, the fundamental properties, such as high coercivity, the wake-up effect, and the mechanism of ferroelectricity have not been fully elucidated. Here we report the crystallization orientation control of structural phase and ferroelectricity in pure HfO2 thin films. Both (001)- and (111)-oriented HfO2 thin films exhibit a mixture of ferroelectric orthorhombic and non-ferroelectric monoclinic phases. With decreasing film thickness, the orthorhombic phase ratio increases for both orientations, with a consistently higher proportion for (111)-oriented film. Consequently, the ferroelectricity is significantly enhanced in thinner (111)-oriented film. Remarkably, both (001)- and (111)-oriented pure HfO2 thin films demonstrate an intrinsic ferroelectricity. Moreover, the coercive field of the (001)-oriented film appears to be lower than that of the (111)-oriented film. Additionally, oxygen ions migrate more easily in the (001)-oriented film, which exhibits distinct electronic structure and local atomic ordering compared to the (111)-oriented film. These results provide valuable insights into the ferroelectricity of HfO2 and suggest that crystalline orientation is an effective approach to explore the ferroelectric properties in hafnia-based films.

Independent Effects of Dopant, Oxygen Vacancy, and Specific Surface Area on Crystal Phase of HfO2 Thin Films towards General Parameters to Engineer the Ferroelectricity

Many factors have been confirmed to affect ferroelectric phase formation in HfO2-based thin films but there was still a lack of general view on describing them. This paper discusses the intrinsic parameters to stabilize the ferroelectric phase of HfO2 thin films to approach this general view by investigating the separate effects of dopant, oxygen vacancy (VO), and specific surface area on the crystal phase of the films. It is found that in addition to extensively studied dopants, the ferroelectric orthorhombic phase can also be formed in pure HfO2 films by only introducing sufficient VO independently, and it is also formable by only increasing the specific surface area. By analyzing the common physics behind these factors, it is found that orthorhombic phase formation is universally related to strain in all the above cases with a given temperature. To get a general view, a physical model is established to describe how the strain influences ferroelectric phase formation during the fabrication of HfO2-based films based on thermodynamic and kinetics analysis.

In Situ Characterization of Ferroelectric HfO2 During Rapid Thermal Annealing

Thermal annealing is critical in governing the phase distribution, and ultimately the electrical properties, of ferroelectric hafnium oxide films. In particular, rapid thermal annealing (RTA) has been shown to favor the formation of ferroelectric crystal phases, but the dynamic behavior of the film over the course of the anneal is not well understood. Herein, synchrotron X‐ray diffraction is used to characterize the phase distribution of HfO2 films deposited by atomic layer deposition (ALD) in situ during RTA, revealing complex phase transformations occurring on the scale of seconds. All samples investigated here transform into a nonmonoclinic phase, which is required for ferroelectric films. However, this phase often converts into the more stable monoclinic phase as annealing proceeds. The kinetic barrier to the transformation to the monoclinic phase during heating is higher than 1 eV f.u.−1 The initial crystallization into the ferroelectric phase and relatively high barrier to transformation to the nonferroelectric phase suggests that careful control of the thermal annealing profile can greatly increase the ferroelectric fraction of the film. By using a fast ramp rate and a short annealing time above the ferroelectric crystallization temperature, the remanent polarization of a pure HfO2 film can be increased more than twofold.

Whether Ge-Rich ZrO2 and Ge-Rich HfO2 Materials Have Similar Reaction on Annealing Treatment?

Usually, HfO2 and ZrO2 are considered as twin-oxides that have to demonstrate similar reaction on their doping with group IV elements as well as stable tetragonal “high-k” phase. However, in most cases the films produced with different approaches were compared. In this work, both Ge doped HfO2 and Ge-doped ZrO2 films were prepared using the same equipment that allows the same Ge content in both materials to be achieved via monitoring of deposition conditions. Thus, the effect of Ge content and thermal treatment on optical and structural properties of these films was studied to obtain direct experimental confirmation whether the formation of corresponding solid solutions and their phase separation stimulated by thermal treatment is similar for both oxides.The films were grown on 6-inch Si wafers by confocal magnetron sputtering using the deposition unit with installed Ge, HfO2 and ZrO2 targets simultaneously. After the deposition, the wafers with the films were cut on smaller samples and rapid thermal annealing in nitrogen atmosphere was performed at 300-900ºC during 30 s. The samples were investigated by means of spectroscopic ellipsometry, Raman scattering, FTIR and TEM methods.As-deposited pure ZrO2 and Ge-ZrO2 films as well as those annealed at TA≤600°C were found to be amorphous and homogeneous. Annealing at higher TA stimulates a phase separation and a formation of Ge nanoclusters (Ge-ncs). They crystallize at TC=640-700°C showing the tendency to the TC decrease with Ge content rise. An appearance of tetragonal ZrO2 phase in Ge-rich films was observed at TC=680-700°C. Such behavior of ZrO2 host shows the possibility to form Ge-ncs in amorphous matrix at TA=640-680°C that offers success application of such films.Contrary to pure ZrO2 films, as-deposited pure HfO2 films were found to be crystalline. Their doping with [Ge]≥5at% reduces the degree of film crystallinity towards amorphous structure that was found to be stable for TA£600°C. At higher TA the formation of Ge-ncs and tetragonal HfO2 phase was detected. However, this latter was found to be formed at TC=600-670 °C, while the Ge-ncs crystallize at TC=700-800°C. Note that the formation of larger Ge-ncs in Ge-ZrO2 films than that in Ge-HfO2 ones at the same annealing conditions was observed.Obtained results support the theoretical consideration about stability of tetragonal “high-k” phases in annealed films. However, they show also the difference in the reaction of Ge-HfO2 and Ge-ZrO2 materials on thermal treatment followed by phase separation. Its mechanism will be discussed in details.

Crystallization behaviors of ultrathin Al-doped HfO2 amorphous films grown by atomic layer deposition**Project supported by the National High Technology Research and Development Program of China (Grant No. 2015AA016501) and the National Natural Science Foundation of China (Grant Nos. 61574168 and 61504163).

In this work, ultrathin pure HfO2 and Al-doped HfO2 films (about 4-nm thick) are prepared by atomic layer deposition and the crystallinities of these films before and after annealing at temperatures ranging from 550 °C to 750 °C are analyzed by grazing incidence x-ray diffraction. The as-deposited pure HfO2 and Al-doped HfO2 films are both amorphous. After 550-°C annealing, a multiphase consisting of a few orthorhombic, monoclinic and tetragonal phases can be observed in the pure HfO2 film while the Al-doped HfO2 film remains amorphous. After annealing at 650 °C and above, a great number of HfO2 tetragonal phases, a high-temperature phase with higher dielectric constant, can be stabilized in the Al-doped HfO2 film. As a result, the dielectric constant is enhanced up to about 35. The physical mechanism of the phase transition behavior is discussed from the viewpoint of thermodynamics and kinetics.

Effect of Al addition in HfO2 on the optical properties of the dielectrics using spectroscopy ellipsometry

The imaginary part of the dielectric functions (ϵ2) and the band gap energy (Eg) of Al-HfO2 films with different Al incorporation levels prepared by atomic layer deposition (ALD) are extracted using variable angle spectroscopic ellipsometry (VASE). The analysis results show that pure HfO2 film exhibits crystallization features after annealing at 500 °C. When Al is incorporated, Al-HfO2 films can maintain amorphous at temperatures up to 700 °C after rapid thermal annealing, indicating a good thermal stability. In particular, as-deposition [1Al2O3 + 5HfO2] film shows similar optical properties to that of pure HfO2. Moreover, least disorder in the amorphous film structure and excellent thermal stability of Eg (5.5 ± 0.1 eV) during annealing are obtained for [1Al2O3 ± 5HfO2] films. However, increased Al incorporation levels induce more disorder in the amorphous film structure and thermal instability of Eg during the annealing procedure. Thus, [1Al2O3 ± 5HfO2] film displays the optimal optical properties and excellent thermal stability of Eg.

(Invited) Opportunity for Phase-Controlled Higher-k HfO2

Structural phase transformation of pure-HfO2 films to higher-k phase (cubic or tetragonal phase) is discussed in terms of both doping effect and dynamic post-deposition annealing (PDA). Y2O3 and SiO2 were doped into HfO2, while the rapid thermal treatment was performed, respectively. It was demonstrated that the k-value of cubic-HfO2 was ~30 in doped case, while ~50 in dynamic PDA case. A kinetic analysis of the recovery process from cubic to monoclinic phase in the dynamic PDA HfO2 was performed according to the Johnson-Mehl-Avrami law.

Correlation between local structure and refractive index of e-beam evaporated (HfO2–SiO2) composite thin films

In the present work we have reported the results of investigations on local structures of e-beam evaporated (HfO2–SiO2) composite thin films by synchrotron based extended x-ray absorption fine structure measurements. It has been observed that for the composite film with 10% SiO2 content, both Hf–O and Hf–Hf bond lengths are less than their values in pure HfO2 film. However the bond lengths subsequently increase to higher values as the SiO2 content in the composite films is increased further. It has also been observed that at the same composition of 10% SiO2 content, the films have smallest grain sizes (as obtained from atomic force microscopy measurements) and highest refractive index (as obtained from spectroscopic ellipsometry (SE) measurements) which suggests that the e-beam evaporated HfO2–SiO2 composite films with 10% SiO2 content leads to the most compact amorphous thin film structure.

Experimental Demonstration of Higher-k Phase HfO2 Through Non-Equilibrium Thermal Treatment

Structural phase transformation of pure-HfO2 films in higher-k phase (cubic or tetragonal phase) was investigated. HfO2/SiO2/Si stacks were annealed at 600oC under various temperature programs. We found that the peak intensity ratio of the higher-k phase to monoclinic one in XRD was enhanced distinctly by increasing the ramping-up rate of temperature, even though the equilibrium phase at 600oC should be the monoclinic. Additionally, the ratio gradually decreased with extending the annealing time, but the higher-k phase was maintained by annealing after deposition of Si capping-layer on HfO2/SiO2/Si stacks.

Characterization upon potential properties of HfO2 stabilized by Y2O3 films as cubic phase

In this article, we have experimentally investigated the nanometer thick cubic HfO2 stabilized with 6 mol % Y2O3 (YSH) films deposited by pulsed laser deposition method in detail. Except the excellent dielectric properties, including a significant increase in dielectric constant as high as 27.2, a negative flatband voltage of −0.46 V, and a very small loop hysteresis, the YSH film has also shown an obvious response to magnetic field. The saturation magnetization of about 1.3 A m2 kg−1 and 5.8 A m2 kg−1 is presented from the YSH films at 300 K with parallel and perpendicular magnetic field, respectively, which is 20% and 9% larger than that of pure HfO2 film at corresponding magnetic field. It is an indicative approach to control the dielectric properties of hafnium-based oxide films with electric and/or magnetic operation.

Effect of annealing on characteristics of a HfOxNy−HfO2−HfOxNy sandwich stack compared with HfO2 film

HfOxNy–HfO2–HfOxNy sandwich-stack (SS) film was investigated in comparison with HfO2 film of the same thickness. Higher thermal stability and better surface morphology can be observed for the SS film. This structure also shows stronger immunity to interfacial oxidation compared with HfO2 film. Meanwhile, unlike the HfOxNy dielectric, the capacitance performance of SS film was not worse (but was even better) than a pure HfO2 film of the same thickness. The SS structure appears to be a promising high-k gate dielectric compared with both pure HfOxNy and HfO2 dielectrics for future ULSI devices. Additionally, PDA treatment plays an important role in improving the characteristics of SS film, which is confirmed by effective channel electron mobility and stress induced leakage current (SILC) investigations.

High-temperature phase stability of hafnium aluminate films for alternative gate dielectrics

Hafnium aluminate films with different compositions were deposited at room temperature by jet vapor deposition. The as-deposited films were amorphous. After annealing at 1100 °C, the microstructure of the films was analyzed by high-resolution transmission electron microscopy, electron diffraction and electron energy loss spectroscopy (EELS). The crystalline phase in pure HfO2 films was monoclinic. With an increase in the Al content in the films, the amount of metastable HfO2 with a tetragonal distorted fluorite structure increased. In addition, the grain sizes decreased, making the detection of crystallization by x-ray diffraction difficult. No crystalline Al2O3 phase could be unambiguously detected in electron diffraction patterns in films with up to 30 mol % Al2O3. However, an Al-rich intergranular phase was identified by EELS. Films with ∼64 mol % Al2O3 crystallized as tetragonal HfO2 and metastable cubic Al2O3 with the spinel structure. The complex microstructures of the films should be considered in the interpretation of their dielectric and electrical properties after high temperature anneals.

Change in the chemical state and thermal stability of HfO2 by the incorporation of Al2O3

Al 2 O 3 incorporated HfO2 films grown by atomic layer deposition were investigated using various measurement tools. The accumulation capacitance of the Al2O3 incorporated into HfO2 film increases as the postannealing temperature increases because of changes in interfacial and upper layer thickness and in interfacial stoichiometry. The core-level energy state of a 15 Å thick film shows a shift to higher binding energy, as the result of silicate formation and Al2O3 incorporation. The incorporation of Al2O3 into the HfO2 film has no effect on silicate formation at the interface between the film and Si, while the ionic bonding characteristics and hybridization effects are enhanced compared to a pure HfO2 film. Any dissociated Al2O3 on the film surface is completely removed by a vacuum annealing treatment over 850 °C, while HfO2 contributes to Hf silicide formation on the surface of the film.