Excellent ferroelectricity of 50 nm-thick doped HfO2 thin films induced by annealing with a rapid-heating-temperature process

Zhiyong Quan,Xiaohong Xu,Xiao Zhang,Wei Zhang,Huihui Liu,Meimei Wang

doi:10.1063/5.0013511

Abstract

The ferroelectric HfO2 thin film has attracted a lot of research interest due to being Pb free and its excellent compatibility with the Si-based semiconductor process. However, methods to obtain thicker HfO2 thin films with strong ferroelectricity have yet to be explored. In this work, a 50 nm-thick La-doped HfO2 thin film was prepared using pulsed laser deposition, and significant room temperature ferroelectricity with a remnant polarization (Pr) of 27 µC/cm2 was achieved through annealing in N2 with a rapid-heating-temperature process. The ferroelectricity is mainly related to the increase in the content of the (002)-oriented orthogonal phase formed by the rapid-heating-temperature treatment. Furthermore, this special annealing process was verified in a 50 nm-thick Tm-doped HfO2 film, and the Pr of 48 µC/cm2 was observed. This value is the highest value reported so far in doped HfO2 films with a thickness of 50 nm or greater. These results provide a new approach to prepare thicker ferroelectric HfO2-based thin films.

Highlights

IntroductionCompared with the traditional perovskite ferroelectric films [such as Pb(Zr,Ti)O3], doped HfO2 films with room temperature ferroelectricity have aroused research interest due to their advantages of excellent compatibility with the Si-based semiconductor process and being Pb free. Ferroelectric HfO2 films have exhibited broad application prospects in many fields including neuromorphic, steep-slope transistor applications, piezoelectric energy harvesters, and nonvolatile semiconductor devices. The ferroelectricity of doped HfO2 films originates from the non-centrosymmetric orthogonal phase (o-phase) polar structure (Pca21). Formation of this structure is influenced by the film’s surface energy, asymmetric stress, thickness, annealing process, and the materials of the top and bottom electrodes. Generally, the thickness of HfO2 films with remarkable ferroelectricity is limited below 30 nm because the non-polarized monoclinic phases (m-phases) will grow at higher thickness, weakening the ferroelectricity of the films. The small film thickness limits its application in actuators and piezoelectric sensors, such as piezoelectric energy harvesters
One can see that the heating-temperature rate of rapid thermal annealing (RTA) significantly affects the ferroelectricity of the LHO films, and a rapidheating-temperature rate may be beneficial to the ferroelectricity, thereby increasing the Pr value
Combined with the polarization–electric field (P–E) curves shown in Figs. 1(c) and 1(d), one can see that the grain size of the enhanced ferroelectric LHO-ann(3 s) is slightly smaller than that of LHO-ann(6 s), which is broadly similar to the results reported in Ref. 22

Summary

Introduction

Compared with the traditional perovskite ferroelectric films [such as Pb(Zr,Ti)O3], doped HfO2 films with room temperature ferroelectricity have aroused research interest due to their advantages of excellent compatibility with the Si-based semiconductor process and being Pb free. Ferroelectric HfO2 films have exhibited broad application prospects in many fields including neuromorphic, steep-slope transistor applications, piezoelectric energy harvesters, and nonvolatile semiconductor devices. The ferroelectricity of doped HfO2 films originates from the non-centrosymmetric orthogonal phase (o-phase) polar structure (Pca21). Formation of this structure is influenced by the film’s surface energy, asymmetric stress, thickness, annealing process, and the materials of the top and bottom electrodes. Generally, the thickness of HfO2 films with remarkable ferroelectricity is limited below 30 nm because the non-polarized monoclinic phases (m-phases) will grow at higher thickness, weakening the ferroelectricity of the films. The small film thickness limits its application in actuators and piezoelectric sensors, such as piezoelectric energy harvesters. The ferroelectricity of doped HfO2 films originates from the non-centrosymmetric orthogonal phase (o-phase) polar structure (Pca21).. The ferroelectricity of doped HfO2 films originates from the non-centrosymmetric orthogonal phase (o-phase) polar structure (Pca21).10,11 Formation of this structure is influenced by the film’s surface energy, asymmetric stress, thickness, annealing process, and the materials of the top and bottom electrodes.. Park et al achieved high ferroelectric polarization in a 40 nm Zr-doped HfO2 film by inserting an ultrathin Al2O3 film.. Park et al achieved high ferroelectric polarization in a 40 nm Zr-doped HfO2 film by inserting an ultrathin Al2O3 film.15 This method of providing stress through the inserted layer was subsequently confirmed by Riedel et al. with the preparation of a 50 nm film with a remnant polarization (Pr) of 20 μC/cm.

Methods

Results

Conclusion