Jingwen Hou’s research while affiliated with Technical Institute of Physics and Chemistry, Chinese Academy of Sciences and other places

We conducted a comprehensive investigation on the influence of TiN thickness and stress on the ferroelectric properties of Hf0.5Zr0.5O2 thin films. TiN top electrode layers with varying thicknesses of 2, 5, 10, 30, 50, 75, and 100 nm were deposited and analyzed. It was observed that the in-plane tensile stress in TiN films increased with the thickness of the TiN top electrode. This is expected to elevate the tensile stress in the Hf0.5Zr0.5O2 film, consequently leading to an enhancement in ferroelectric polarization. However, the effect of stress on the ferroelectric behavior of Hf0.5Zr0.5O2 films exhibited distinct stages: improvement, saturation, and degradation. Our study presents novel findings revealing a saturation and degradation phenomenon of in-plane tensile stress on the ferroelectric properties of polycrystalline Hf0.5Zr0.5O2 films, thereby partially resolving the discrepancies between experimental observations and theoretical predictions. The observed phase transformation induced by tensile stress in Hf0.5Zr0.5O2 films played a crucial role in these effects. Furthermore, we found that the impact of the TiN top electrode thickness on other factors influencing ferroelectricity, such as grain size and oxygen vacancies, was negligible. These comprehensive results offer valuable insights into the influence of stress and TiN top electrode thickness on the ferroelectric behavior of Hf0.5Zr0.5O2 films.

Hf0.5Zr0.5O2 (HZO) is a promising candidate for low-power non-volatile memory due to its nanoscale ferroelectricity and compatibility with silicon-based technologies. Stress and oxygen vacancy (VO) are key factors that impact the ferroelectricity of HZO. However, their combined effects have not been extensively studied. In this study, we investigated the impact of the VO content on HZO thin films’ ferroelectricity under different electrode stresses by using TiN and tungsten (W) top electrodes and controlling ozone dose time during HZO deposition. The HZO thin films with W top electrodes exhibit elevated stress levels and a greater abundance of orthorhombic/tetragonal phases, and the HZO thin films with TiN top electrode shows an increase in the monoclinic phase with increasing ozone dose time. The residual polarization (Pr) of the capacitors with TiN and W top electrodes displayed different or even opposing trends with increasing ozone dose time, and the VO content decreases with increasing ozone dose time for both sets of capacitor samples. We propose a model to explain these observations, considering the combined influence of electrode stresses and VO on the free and formation energy of the crystalline phase. Increasing the VO content promotes the transformation of the tetragonal phase to the orthorhombic phase in HZO films with TiN top electrodes, and with W top electrodes, a higher VO content prevents the tetragonal phase from transforming into the orthorhombic/monoclinic phase. Additionally, an alternative explanation is proposed solely from the perspective of stress. These findings provide valuable insights into the regulation of ferroelectricity in HZO thin films.

The ferroelectric field effect transistor (FeFET) is a very promising candidate for low-power and non-volatile memory. However, the co-existing effect of ferroelectric polarization and interface charge trapping in the FeFETs is demonstrated and many efforts have been made to eliminate this charge-trapping effect, which is usually treated as a deleterious effect. In contrast, we have found that the charge-trapping effect can play a dominant role in ferroelectric gates. In this work, we have verified that the charge-trapping effect of the ferroelectric/insulator interface could induce a memory window as the main physical mechanism in the TiN/Hf0.5Zr0.5O2/SiO2/p-Si (MFIS) structure, in which the ferroelectric characteristics of HZO thin films was verified through a reverse-grown MFIS structure. We also demonstrated that 2.5 nm SiO2 is optimal for the charge tunneling effect and the device has the largest memory window. Moreover, in order to enlarge the memory window of MFIS capacitors, we utilized the stress-enhanced ferroelectric polarization characteristics of Hf0.5Zr0.5O2 to improve the charge-trapping effect. Such a finding demonstrates that the ferroelectric-aided charge-trapping devices are potential to be used in non-volatile memories.

Voltage controlled magnetic anisotropy (VCMA) has been considered as an effective method in traditional magnetic devices with lower power consumption. In this article, we have investigated the dual-axis control of magnetic anisotropy in Co2MnSi/GaAs/PZT hybrid heterostructures through piezo-voltage-induced strain using longitudinal magneto-optical Kerr effect (LMOKE) microscopy. The major modification of in-plane magnetic anisotropy of the Co2MnSi thin film is controlled obviously by the piezo-voltages of the lead zirconate titanate (PZT) piezotransducer, accompanied by the coercivity field and magnetocrystalline anisotropy significantly manipulated. Because in-plane cubic magnetic anisotropy and uniaxial magnetic anisotropy coexist in the Co2MnSi thin film, the initial double easy axes of cubic split to an easiest axis (square loop) and an easier axis (two-step loop). While the stress direction is parallel to the [1-10] easiest axis (sample I), the square loop of the [1-10] direction could transform to a two-step loop under the negative piezo-voltages (compressed state). At the same time, the initial two-step loop of the [110] axis simultaneously changes to a square loop (the easiest axis). Otherwise, we designed and fabricated the sample II in which the PZT stress is parallel to the [110] two-step axis. The phenomenon of VCMA was also obtained along the [110] and [1-10] directions. However, the manipulated results of sample II were in contrast to those of the sample I under the piezo-voltages. Thus, an effective dual-axis regulation of the in-plane magnetization rotation was demonstrated in this work. Such a finding proposes a more optimized method for the magnetic logic gates and memories based on voltage-controlled magnetic anisotropy in the future.

In 3D NAND, as the stack number increases, the process cost becomes higher and higher, and the stress problem becomes more and more serious. Therefore, the low cost and low stress plasma enhanced tetraethyl orthosilicate (PE TEOS), compared to high density plasma (HDP) oxide, shows its superiority as pre-metal dielectric (PMD) oxide layer in 3D NAND. This paper explores the challenges in the application of PE TEOS in 3D NAND PMD oxide layer. In our experiment both PE TEOS and HDP are employed as the PMD oxide for 3D NAND staircase protection. There is not any void found in the two oxide structures. However, oxide pitting is spotted in the subsequent diluted hydrofluoric acid wet etching in the PE TEOS split. Moreover, we observe that the top silicon nitride corrodes in hot phosphoric acid. We study the mechanism of PE TEOS oxide pitting and silicon nitride corroding, propose two solutions: (1) HDP oxide + PE TEOS, and (2) PE TEOS + dry etching. Experimental results demonstrate that our solutions can well address the issue of PE TEOS oxide pitting and effectively protect the staircase structure. This work extends the application of PE TEOS oxide of which the cost and the stress are both low in 3D NAND.