**1. Introduction**

Since the report of ferroelectric behavior in HfO2-based thin films, studies have been conducted on HfO2 thin films doped with different elements. Particularly, Hf*x*Zr1−*<sup>x</sup>*O2 (HZO) thin films, which exhibit ferroelectricity even at thicknesses of a few nanometers, have gained increasing attention [1]. Among the diverse types of available ferroelectric materials, metal oxides have attained considerable technological importance owing to their compatibility with current complementary metal–oxide–semiconductor (CMOS) technology as well as large-scale integration. Therefore, active research has been underway for the application of HZO thin films to a variety of devices such as ferroelectric transistors, synapse devices, and ferroelectric tunneling junctions [2–6].

For the practical application of HZO thin films to semiconductor memories, it is necessary to overcome the issues of remanent polarization (Pr) and fatigue endurance. Pb(Zr,Ti)O3-based materials having the crystal structure of perovskites, which are ferroelectric materials that have been studied extensively, exhibit a low wake-up effect, and show stable characteristics with exceptional fatigue endurance over 1010 cycles [7–9]. In this context, studies on improving the properties of HZO have been actively underway. In particular, research on the effects of crystal structure, oxygen defects inside thin films, grain size, and interface engineering using electrodes on changes in electrical properties has been mainly reported [10–14]. HZO thin films have a variety of crystalline phases such as tetragonal (t-, P42/nmc), monoclinic (m-, P21/c), and orthorhombic (o-, Pca21) phases,

**Citation:** Kim, H.-G.; Hong, D.-H.; Yoo, J.-H.; Lee, H.-C. Effect of Process Temperature on Density and Electrical Characteristics of Hf0.5Zr0.5O2 Thin Films Prepared by Plasma-Enhanced Atomic Layer Deposition. *Nanomaterials* **2022**, *12*, 548. https://doi.org/10.3390/ nano12030548

Academic Editors: Zhenxiang Cheng, Changhong Yang and Chunchang Wang

Received: 31 December 2021 Accepted: 2 February 2022 Published: 5 February 2022

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of which the o-phase exhibits ferroelectric properties. However, the m-phase has been generally reported to be the stable phase of HZO [10,15], and research has been conducted to achieve a high ratio and stability of the o-phase in HZO films [16–18].

For HZO deposition, thermal atomic layer deposition (THALD) is mainly used. Moreover, there has been insufficient investigation on the properties of HZO thin films deposited by plasma-enhanced atomic layer deposition (PEALD) [19]. PEALD is capable of the high-density deposition of thin films and has the advantage of enabling low-temperature deposition [20–22]. The regions wherein the o-, t-, and m-phases of HZO are formed vary depending on grain size and temperature. Therefore, density improvement in the deposition process is expected through the stabilization of the o-phase and increasing the grain size through low-temperature deposition using PEALD [10,23]. In addition, to the best of the authors' knowledge, there is no report on the relationship between the PEALD process temperature and changes in the electrical properties with respect to HZO density.

In this study, the initial process conditions for fabricating PEALD HZO thin films were set according to the deposition temperature, and the effect of the deposition temperature on the density and crystallinity of the thin films was analyzed. In addition, the optimal conditions for fabricating HZO thin films by PEALD were derived by examining the thin film density and crystallinity according to the post-annealing temperature. Furthermore, the effects of the variation of the HZO thin film density with the process temperature on the crystallinity of the o-phase exhibiting ferroelectricity, as well as on the electrical characteristics such as polarization hysteresis loops (P-E loops) and fatigue endurance, were investigated. Finally, process improvement methods for obtaining HZO thin films with high density and excellent electrical properties at a low deposition temperature of 100 ◦C were determined, and the results were comparatively analyzed.
