Ferroelectricity of nondoped thin HfO₂ films in TiN/HfO₂/TiN stacks

First, we focus on the ferroelectricity of nondoped HfO₂. Figure 1(a) shows the P–V characteristics of the TiN/HfO₂/TiN and Al/HfO₂/TiN capacitors with 20-nm-thick HfO₂ films which were thermally annealed at 600 °C for 30 s in N₂. As we previously reported,¹⁶⁾ the Al/HfO₂/TiN capacitor shows a linear relationship with no hysteresis, which indicates that the HfO₂ film in the Al/HfO₂/TiN stack is paraelectric, while the TiN/HfO₂/TiN capacitor clearly shows a ferroelectric hysteresis in P–V characteristics. The difference is discussed in the later part. The ferroelectricity of nondoped HfO₂ film in the TiN/HfO₂/TiN stack is qualitatively in good agreement with that recently reported by another research group.¹⁸⁾

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We also confirmed the ferroelectricity from C–V characteristics. The relationship of the area of hysteresis observed in P–V curve and in C–V characteristics is described as

$$\begin{equation} Q = \int C(V)\,dV = t_{\text{HfO2}}\int C(E)\,dE = P(E) + \varepsilon_{0}E, \end{equation} \tag{ 1 }$$

where Q and t_HfO2 are the accumulated charge and HfO₂ film thickness in the capacitor, respectively. Thus, the C–V curve is obtained from the differential of the P–V curve. However, since a small ac voltage at a high frequency (50 mV at 100 kHz in this work) is generally used for probing in the differential C–V measurement, it is noted that only the reversible polarization processes involving ionic and electronic displacements and domain wall motion¹⁹⁾ can be detected. The relationship between P–V and C–V measured by differential C–V is described in Fig. 1(b). The applied dc voltage dependence of capacitance in the TiN/HfO₂/TiN capacitor exhibits a characteristic curve with a large hysteresis showing ferroelectricity as shown in Fig. 1(c). The dielectric loss values [tan δ = (2πfCR_p)⁻¹, where f, C, and R_p are the measurement frequency, capacitance, and parallel resistance connected to the capacitor, respectively] at V_g = ±4.8 V in the parallel mode measurement were less than 0.07, which indicates that the leakage current was substantially low, and that the characteristic C–V with hysteresis in Fig. 1(c) was not an artifact. Therefore, it was confirmed that the hysteresis in the P–V curve is not caused by a property of the nonlinear lossy dielectric but by that of the ferroelectric film. Moreover, the k of the HfO₂ film in TiN/HfO₂/TiN stack was estimated to be 22–30. This value is higher than that of monoclinic phase of nondoped HfO₂²⁰⁾ and comparable to that of the ferroelectric doped HfO₂.^1,2,5,6)

To further examine the ferroelectricity of nondoped HfO₂ in TiN/HfO₂/TiN stacks, the temperature dependence of C–V characteristics was measured. Figure 2(a) shows the C–V characteristics measured at various temperatures ranging from 300 to 100 K. Two points that characterize the ferroelectricity of nondoped HfO₂ in TiN/HfO₂/TiN stacks are addressed. One is the voltage hysteresis shown by an arrow in Fig. 2(a) being almost independent of the measurement temperature. This suggests that the origin of hysteresis is not the diffusion of thermally activated charged species.²¹⁾ The other point is the temperature dependence of the electric susceptibility χ, where χ in the ferroelectric system is generally proportional to 1/(T − T_C), T_C being the Curie temperature in the ferroelectric phase transition. Since k = 1 + χ and χ ≫ 1 in ferroelectric films, χ should be proportional capacitance. Figure 2(b) shows the inverse of capacitance as a function of the measurement temperature, in which the characteristic capacitance is defined by the crossing value around the center of the C–V hysteresis in Fig. 2(a). The inverse of capacitance linearly decreases with the increase of measurement temperature. This fact also ensures that nondoped HfO₂ in the TiN/HfO₂/TiN stack is ferroelectric. Furthermore, the Curie temperature T_C of nondoped HfO₂ is estimated to be above 500 °C by extrapolating 1/C to zero. It has also been reported that the ferroelectricity of Si-doped HfO₂ was observed up to 200 °C,²²⁾ and that the Curie temperature of Y-doped HfO₂ was estimated to be around 450 °C.²³⁾ These results suggest that the ferroelectricity of HfO₂ may not be due to doping elements but to the intrinsic HfO₂ crystalline structure.

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Next, we discuss the effect of film thickness on ferroelectricity. Since it is known that the structural phase of HfO₂ is sensitive to the film thickness,²⁴⁾ it is expected that the ferroelectricity of nondoped HfO₂ will be also strongly affected by the film thickness. Figure 3(a) shows XRD patterns of 40-, 20-, and 13-nm-thick nondoped HfO₂ in TiN/HfO₂/TiN stacks. Note that the XRD of HfO₂ films was measured through the TiN electrode on HfO₂. The peak intensity is normalized by the height of the peaks at around 30.5°, which is assigned to highly symmetric phases (i.e., cubic, tetragonal, and orthorhombic phases²⁵⁾). It is clearly shown that the crystallization into the monoclinic phase is significantly suppressed by decreasing the HfO₂ film thickness. We analyzed the ferroelectricity of HfO₂ in TiN/HfO₂/TiN stacks from C–V characteristics at 100 kHz to reduce possible leakage current effects. Figure 3(b) shows C–V characteristics of TiN/HfO₂/TiN capacitors with various HfO₂ thicknesses ranging from 13 to 40 nm. The hysteresis in C–V characteristics was clearly made visible by decreasing the film thickness. Furthermore, we discuss the ferroelectricity from the hysteresis in C–V curve. The analysis of ferroelectricity was performed based on the relationship between P–V and (C · t_HfO2)–V characteristics in ferroelectrics as shown in Fig. 3(c). Since point A in Fig. 3(c) is in the polarization inversion process, the ferroelectric domain wall motion, which is reversible in a small ac voltage, is detectable in addition to the electron motion and ion displacement. On the other hand, since point B in Fig. 3(c) is not in the polarization inversion process, only the electron motion and ion displacement are detected. Considering the fact that C · t_HfO2 corresponds to k, the Δ(C · t_HfO2) shown in Fig. 3(c) represents a change in k in the polarization inversion. Therefore, it is possible to evaluate the ferroelectricity of film by estimating Δ(C · t_HfO2). Figure 3(d) shows the dependence of film thickness on Δ(C · t_HfO2). It is clearly observed that Δ(C · t_HfO2) is significantly increased by decreasing the HfO₂ thickness down to 40 nm. In addition, we note that the static coercive field (E_C) can be estimated from the peak distance in the hysteresis of C–V characteristics.²⁶⁾ Figure 3(e) shows that E_C is almost constant for the HfO₂ thickness between 13 and 30 nm, which suggests that the ferroelectric domain structure in the thickness direction is not significantly affected by the reduction in film thickness.

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From these results, we would like to discuss the effect of film thickness on the crystallization into the ferroelectric phase. Considering that the interface often acts as an inhomogeneous nucleation center and thin HfO₂ films in the TiN/HfO₂/TiN stack exhibit ferroelectricity, it is inferred that the TiN interface might work as the nucleation center of the ferroelectric phase of HfO₂. When the grain size of HfO₂ becomes larger with an increase in film thickness, the paraelectric monoclinic phase becomes more stable than the ferroelectric phase owing to more volume energy gain than surface energy. As shown in Fig. 3(d), the ferroelectricity estimated from C–V characteristics drastically disappears above 40 nm. This view is consistent with the observed crystallization into monoclinic phase in the thick HfO₂ film as shown in Fig. 3(a). On the other hand, Δ(C · t_HfO2) seems to decrease below 20 nm as shown in Fig. 3(d). Considering the fact that the k of the thin HfO₂ film below 20 nm is also estimated to be 25–32 from the C–V characteristics in Fig. 3(b), it might be caused by the partial formation of highly symmetric paraelectric phase with higher k due to its lower surface energy. In addition, since it is known that O₂ PDA easily causes the monoclinic phase transition,²⁷⁾ it is inferred that an effect of oxygen blocking into HfO₂ might be involved in this monoclinic phase destabilization. However, other effects such as mechanical or doping effects from the interface might be intuitively probable causes of the phase destabilization. The physical mechanism of the TiN interface effect on the HfO₂ ferroelectricity should obviously be further investigated to understand and control the HfO₂ ferroelectricity.

Finally, we discuss the stability of the ferroelectric phase of nondoped HfO₂. According to the results shown in Fig. 1, a phase transition of HfO₂ film to the paraelectric phase might be induced in the thermal treatment the HfO₂/TiN stack, even if the HfO₂ film is initially crystallized into the ferroelectric phase. We directly examined the effects of annealing on the ferroelectric HfO₂ films with and without the top TiN electrode. We initially fabricated ferroelectric 25-nm-thick HfO₂ films in the TiN/HfO₂/TiN stack by crystallization at 500 °C for 30 s, and removed top TiN electrode with diluted H₂O₂+NH₄ solution for the samples without the TiN electrode. We confirmed by XRD measurement that no structural transition of HfO₂ was detected in the TiN removal process. We then annealed ferroelectric HfO₂ films in HfO₂/TiN and TiN/HfO₂/TiN stacks in N₂ at 500 °C, and then analyzed the crystalline structure of the HfO₂ films. Figures 4(a) and 4(b) show XRD patterns of the ferroelectric HfO₂ films and the monoclinic phase ratio of HfO₂ films estimated from three XRD peak areas at around 30°, respectively. The crystalline structure of the ferroelectric HfO₂ film in the TiN/HfO₂/TiN stack does not change at all through additional annealing, while that in the HfO₂/TiN stack exhibits the obvious structural phase transition into the monoclinic phase with an increase of the annealing time. Furthermore, we investigated the effect of additional annealing on the ferroelectricity of HfO₂ film. For C–V measurement, we deposited TiN electrodes on the additionally annealed HfO₂ film again. The Δ(C · t_HfO2) in C–V characteristics is drastically reduced in the additional annealing (not shown). These results directly prove that the ferroelectric nondoped HfO₂ film is stabilized by capping the HfO₂ surface with a TiN film. This fact is the same as the phase stabilization due to the capping in martensitic phase transition reported previously.¹⁵⁾ Thus, the ferroelectric phase in nondoped HfO₂ seems to be metastable, but it is successfully stabilized thanks to the TiN interfaces.

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