**3. Results and Discussion**

Prior to the start of HZO thin film deposition, the growth conditions of single thin films of HfO2 and ZrO2 were confirmed. Each thin film showed self-limiting behavior when the source was injected for more than 2.5 s in the previous experiment. Accordingly, as shown in Table 1, the injection time of the source was set to 3 s to allow sufficient time. Figure 2 presents the results of analyzing the change in growth per cycle (GPC) and refractive index according to the number of cycles at various substrate temperatures. The results of GPC illustrated in Figure 2a,b show that the deposition thickness value is high in the initial cycles (that is, 10 cycles or less). This phenomenon is due to an overestimated measurement error that occurred during the planarization process because of the native roughness and curvature of the substrate at the initial stage; after 10 cycles, the thickness is almost constant regardless of temperature [24,25]. Furthermore, from the refractive index results shown in Figure 2c,d, a trend of change in the refractive index according to the number of cycles can be observed. Notably, the refractive index approaches 2.0 and 2.1, the bulk refractive index values of HfO2 and ZrO2, respectively, with an increase in the number of cycles at the different substrate temperatures. All the thin films deposited at 100 ◦C had a low refractive index, and the difference was particularly pronounced in the case of the ZrO2 thin film. It can be inferred from the Lorentz–Lorenz relation that the thin film density decreased at low deposition temperatures [26]. Based on the results in Figure 2, HfO2 and ZrO2 were deposited at rates of 0.123 nm and 0.112 nm per cycle, respectively, at the deposition temperature of 180 ◦C. The two materials were alternately deposited in each cycle, repeating the process 42 times, resulting in a Hf0.5Zr0.5O2 thin film with a thickness of approximately 10 nm.

Figure 3a is a cross-sectional image obtained by the high-resolution TEM (HR-TEM) of a PEALD HZO thin film that was deposited at 180 ◦C and underwent post-annealing at 600 ◦C. The thickness of the thin film was approximately 10 nm, and an o-phase crystalline structure was mainly observed. The disappearance of the o-phase structure near the interface is thought to be because of interface instability due to TiN diffusion [13,27]. Figure 3b shows the EDS-based elemental composition profiles of the same thin film, and it can be observed that some of the Ti and N atoms diffused into the HZO thin film. In addition, nitrogen and carbon contamination was observed inside the HZO thin film owing to the TEMA precursors. In particular, the carbon contamination was considerable; herein, carbon is considered to be a residual impurity because the precursor is not completely decomposed during deposition [27–30]. Figure 3c shows the change in the XRD patterns

of the PEALD HZO thin films according to the deposition temperature in the substrate temperature range of 100–280 ◦C. The peaks at 28.5 ◦ and 31.6 ◦ represent the m-phase, and the peaks at 30.5 ◦ and 35.4 ◦ represent the (111) and (200) planes of the o-phase [16]. At all deposition temperatures, the proportion of o-phase was greater than that of the mphase, and it can be observed that the phase transformation to the o-phase was successfully achieved during the post-annealing at 600 ◦C. The intensity of the XRD peak corresponding to the o-phase was the highest at 180 ◦C, and it decreased as the deposition temperature decreased or increased further. In particular, in the case of deposition at a high temperature of 280 ◦C, secondary phases such as the m-phase were included.

**Figure 2.** Changes in the (**a**,**b**) growth per cycle (GPC) and (**c**,**d**) refractive index of HfO2 and ZrO2 single thin films according to the number of cycles at different substrate temperatures.

**Figure 3.** (**a**) Cross−sectional high−resolution TEM (HR−TEM) image and (**b**) EDS composition cross-sectional profile and (**c**) XRD pattern change with respect to substrate temperature in the range of 100–280 ◦C for PEALD HZO thin films deposited at 180 ◦C.

Figure 4a presents the XRR data of the HZO thin film deposited at 180 ◦C, and the inset graph corresponds to raw data showing the reflectivity according to the X-ray incident angle. The density of the thin film is calculated based on the initial angle at which the reflectivity decreases. The thickness of the thin film is simulated through the oscillation period, and the thickness and the density of the deposited thin films constituting the sample can be calculated as shown in the outer graph. Figure 4b outlines the density change according to the substrate temperature of the PEALD HZO thin film obtained by this method. The density of the thin film was the highest (8.18 g/cm3) at the substrate temperature of 180 ◦C. This density exceeds the theoretical density of HZO [31]. In a multilayered structure, the density of each thin film is calculated by the reflectivity at each interface, but an error may occur if the thin film is too thin or the interface is not distinct. In this study, the deposited thin films were compared according to calculated density.

**Figure 4.** (**a**) XRR data of PEALD HZO thin film deposited at 180 ◦C and (**b**) HZO thin film density according to substrate temperature in the range of 100–280 ◦C.

The density gradually decreased as the deposition temperature decreased or increased further. This trend is consistent with that of the o-phase peak intensity, as observed in the XRD patterns in Figure 3c. The decrease in density at low and high temperatures can be explained by the equation of Langmuir's adsorption isotherm [32]. *θ*, the fraction of the surface covered by the absorbate, can be expressed as a function of time as shown in Equation (1).

$$\frac{d\theta}{dt} = \gamma\_a P\_i(1 - \theta) - \gamma\_d \theta \tag{1}$$

Here, *γ<sup>a</sup>* denotes the adsorption coefficient, *γ<sup>d</sup>* is the desorption coefficient, and *Pi* is the pressure. Here, the adsorption coefficient and the desorption coefficient are exponentially proportional to the temperature with respect to the activation energy required for adsorption and desorption, respectively.

$$If \frac{d\theta}{dt} = 0 \text{ (equilibrium)}, \; \theta = \frac{P\_i}{P\_i + \gamma\_d / \gamma\_a} \tag{2}$$

Based on the assumption that the same pressure process applies in an equilibrium state where the adsorption rate becomes 0, if *Pi* is set to a constant value, the equilibrium value of *θ* is highly dependent on temperature, as shown in Equation (2). At low temperatures, the value of *γ<sup>a</sup>* becomes small; consequently, sufficient chemisorption does not occur, and the space where adsorption does not occur remains empty. Furthermore, as *θ* decreases, the density of the thin film decreases. At high temperatures, *γ<sup>d</sup>* increases, and it is thought that owing to the empty space formed by the atoms desorbed during the deposition process, both the values of *θ* and the density of the thin film decrease. Under the conditions of this experiment, the substrate temperature of 180 ◦C results in the minimum *<sup>γ</sup><sup>d</sup> <sup>γ</sup><sup>a</sup>* value and the

highest fraction of the surface covered by the absorbate; therefore, this condition is thought to be the optimal deposition condition that yields the highest density. In addition, the secondary phases, including the m-phase, appearing at a substrate temperature of 280 ◦C may cause density reduction [33,34].

Although the m-phase is the most stable phase of HZO thin films, the formation of the m-phase is suppressed while the ratio of the ferroelectric o-phase is increased because of thermal stress caused by the difference in the thermal expansion coefficient between the HZO thin film and the TiN electrode during the post-annealing process [35,36]. Figure 5 shows the changes in the XRD patterns and density of samples of the HZO thin films deposited at 180 ◦C, the optimal substrate temperature, which underwent post-annealing at 500 to 700 ◦C. At all annealing temperatures, HZO thin films almost purely consisting of o-phases without the m-phase and secondary phases were obtained. In the case of the sample obtained via 500 ◦C post-annealing, the X-ray peak intensity was slightly weak, but in the samples obtained via post-annealing at 600 ◦C or higher, the X-ray peak intensity was strong. With regard to the density, the sample annealed at 600 ◦C showed the highest value; thus, the optimum annealing temperature was determined to be 600 ◦C. It was confirmed that the HZO thin film was densified, with crystallization, through the post-annealing process. It can be inferred from the results shown in Figure 5b that the density of the thin film may decrease as the interdiffusion between the TiN electrode and the HZO thin film increases under high-temperature post-annealing conditions.

**Figure 5.** (**a**) XRD pattern and (**b**) density according to post-annealing temperature of PEALD HZO thin films deposited at 180 ◦C.

The polarization characteristics of PEALD HZO thin films deposited at various substrate temperatures were evaluated. Figure 6a shows the PE hysteresis curves measured after 10<sup>5</sup> cycles for each sample in which wake-up had occurred and the value of coercive field (2Ec) was stabilized. Figure 6b shows the dynamic polarization switching current with respect to the electric field. The value of remanent polarization 2Pr measured based on each P-E hysteresis curve increased significantly from 12 μC/cm2 to 38.2 μC/cm2 as the deposition temperature increased from 100 ◦C to 180 ◦C. Further, as the strength of the coercive field (2Ec) increased from 1 MV/cm to 1.97 MV/cm, the total area of the hysteresis curve increased significantly. Thereafter, as the deposition temperature increased to 280 ◦C, both the 2Pr and 2Ec values decreased, and this trend was consistent with the X-ray intensity of the o-phase and the density of the thin film. The maximum remanent polarization value of 38.2 μC/cm2 of the sample obtained at the deposition temperature of 180 ◦C is higher than the values reported in previous papers [14,16,19]. In Figure 6b, the maximum polarization current is observed near the coercive field. In the sample obtained at a deposition temperature of 180 ◦C, a dynamic switching current density of up to 8.8 × <sup>10</sup>−<sup>3</sup> A/cm2 was measured in an electric field of 1 MV/cm, and an almost symmetrical current pattern

was also observed in negative electric fields. The trend of the polarization characteristics according to the deposition temperature is thought to be related to an increase in defects inside the HZO thin films deposited at low and high temperatures, as discussed above. These defects can limit the growth of the grain size and cause pinning of the switching of the ferroelectric domain under the external electric field [37–40].

**Figure 6.** (**a**) P−E hysteresis curve and (**b**) polarization switching current curve with respect to electric field of HZO thin films deposited at various substrate temperatures.

Figure 7 shows the results of analyzing the electrical properties of the HZO thin films deposited at 180 ◦C with post-annealing at various temperatures. The best 2Pr value and the largest dynamic switching current density were obtained at an annealing temperature of 600 ◦C. The HZO thin film annealed at 500 ◦C showed a 2Pr value of 21.6 μC/cm2 and a dynamic switching current density of up to 3.85 × <sup>10</sup>−<sup>3</sup> A/cm2, showing inferior characteristics compared to those of the samples annealed at 600 ◦C or higher. In addition, the P-E hysteresis curve and polarization switching current curve show asymmetry according to the sign, which indicates that a built-in potential is formed inside the thin film. From this, it can be inferred that at a low post-annealing temperature of 500◦C, the distribution of defects inside the film was not symmetrical or that the phase change to the o-phase was not fully completed inside the thin film [41,42].

**Figure 7.** (**a**) P−E hysteresis curve and (**b**) polarization switching current curve with respect to electric field of HZO thin films deposited at substrate temperature of 180 ◦C with different postannealing temperatures.

Figure 8 compares the results of fatigue endurance evaluation of the HZO thin films according to the above-mentioned (a) deposition temperature and (b) post-annealing temperature. In Figure 8a, the HZO thin films deposited at 180 ◦C and 230 ◦C showed the highest level of endurance of 1.6 × <sup>10</sup><sup>7</sup> cycles. The cases of deposition at the lowest temperature of 100 ◦C and the highest temperature of 280 ◦C showed relatively low endurances of 2.5 × 105 cycles and 1.6 × <sup>10</sup><sup>6</sup> cycles, respectively. In addition, in the case of these two samples, a wake-up phenomenon, in which the 2Pr value increased with the number of cycles, was clearly exhibited. Fatigue endurance is caused by the accumulation of impurities and oxygen vacancies inside the thin film at the electrode interface or crystal defects, and it is reported that the wake-up effect occurs in the process of redistribution of oxygen vacancies according to the application of an electric field [43,44]. It is inferred that samples with low density in the previous experiment contain many defects, resulting in low fatigue endurance or a marked wake-up effect. Figure 8b shows the results according to the annealing temperature, and it can be observed that the endurance of the thin film annealed at 600 ◦C is the highest. In addition, as the annealing temperature increases, the wake-up effect is improved; notably, the sample annealed at 700 ◦C shows the best improvement of the wake-up effect. However, the sample annealed at 700 ◦C showed the lowest fatigue endurance (1.6 × 106 cycles), which is attributed to diffusion at the electrode interface [45,46].

**Figure 8.** (**a**) Comparison of fatigue endurance of HZO films fabricated at different deposition temperatures after 600 ◦C annealing and (**b**) comparison of fatigue endurance of HZO films, deposited at a substrate temperature of 180 ◦C, with respect to annealing temperature.

The preparation methods and electrical properties of HZO films are summarized in Table 3 to compare our work with previous studies. The HZO thin film prepared at optimized PEALD conditions in this study showed relatively good remanent polarization and fatigue endurance performances despite being under the lowest deposition temperature.



In the case of the thin film deposited at the substrate temperature of 280 ◦C in the previous experiment, the presence of secondary phases such as the m-phase was confirmed. Therefore, it can be inferred that for increasing the ratio of the o-phase in HZO thin

films, low-temperature deposition is advantageous. However, the density was greatly reduced in the thin films deposited at low temperatures. Therefore, this study investigated process improvement methods that can increase the density of thin films deposited at low temperatures. Four types of process improvement experiments (A to D) were performed for deposition at a substrate temperature of 100 ◦C, followed by RTA at 600 ◦C, and each process is outlined as follows. Process A is the process of improving *θ*, the fraction of the surface covered by the absorbate, by using the discrete feeding method (DFM). In the DFM, a purge step was included in the source injection step to refine the process, thereby removing the impurities and byproducts in the precursor injection step. This increased the initial chemisorption efficiency and fraction of the surface covered by the absorbate. In this experiment, the purge step was executed twice in the middle of the process. In process B, the plasma discharge time and oxygen injection time were increased by 4 s each; thus, the discharge time in this experiment was 6 s, and the oxygen injection time was 8 s. In process C, the total flow rate in the chamber was increased from 600 sccm to 900 sccm while maintaining the pressure. Finally, in process D, all of the aforementioned process improvement methods (A to C) were applied in combination.

Figure 9 shows the changes in the XRD patterns and thin film density of the HZO thin films obtained with the various process improvement methods. In the results corresponding to processes A to D, both the o-phase peak intensity and the thin film density are significantly higher than those of the HZO thin film deposited at 100 ◦C without applying the process improvement. The crystallinity and density of the thin film improved upon applying process A. Specifically, *θ*, the fraction of the surface covered by the absorbate, increased because the application of the DFM eliminated unnecessary physical adsorption, thus stably providing the chemical adsorption sites to the precursor [25]. By applying process B, the density was significantly improved to 9.7 g/cm3, which is thought to be due to the reduction of oxygen vacancies based on the increase in the reaction time corresponding to the precursor-reactant reaction [20,47]. The results of process C confirmed that both the crystallinity and density were improved, and it is believed that the reduced boundary layer and increased diffusion rate due to the increase in the flow rate of the precursor enhanced the fraction of the surface covered by the absorbate of the precursor [48,49]. For process D, the o-phase peak intensity and thin film density were lower than those of processes A and C, which is inferred to be due to the interaction between process parameters. The results confirmed that by applying these process improvement methods, the *θ* for low-temperature deposition can be improved, and with an increased RF power supply time to promote the reaction with oxygen radicals, the oxygen vacancies can be reduced, resulting in properties similar to those of thin films deposited at high temperatures.

The electrical properties of the low-temperature-deposited HZO thin films obtained with different process improvement methods were measured. Figure 10a shows the P-E hysteresis curves, and the HZO thin film obtained with process C showed the highest 2Pr value, 18.6 μC/cm2. Although this value was higher than the 2Pr value of 12 μC/cm<sup>2</sup> of the thin film deposited at 100 ◦C without applying any process improvement, it was significantly lower than that of the thin film deposited at 180 ◦C. This is thought to be because although the physical properties of the thin films obtained by applying the process improvement methods were similar to those of the thin films deposited at 180 ◦C, the formation of a ferroelectric domain inside the thin films was hindered, and further investigation is needed to identify the cause. As shown by the fatigue endurance measurement results in Figure 10b, dielectric breakdown occurred at 107 cycles or more, indicating that the service life was greatly improved through the process improvement. In particular, when process B was applied, the highest endurance of 2.5 × <sup>10</sup><sup>7</sup> cycles was measured, which was superior to that of the thin film deposited at 180 ◦C. This increase in endurance is thought to be because the oxygen vacancies inside the thin film, which cause fatigue, were greatly reduced by the effects of the applied process improvement.

**Figure 9.** (**a**) XRD patterns and (**b**) thin film density according to the process improvement method of low-temperature-deposited HZO thin films; A: use of discrete feeding method, B: increase in RF plasma time, C: increase in gas flow, D: combined application of A, B, and C.

**Figure 10.** (**a**) P−E hysteresis curves and (**b**) fatigue endurance characteristics according to the process improvement method of low-temperature-deposited HZO thin films; A: use of discrete feeding method, B: increase in RF plasma time, C: increase in gas flow, D: combined application of A, B, and C.
