Enhancement of remnant polarization in ferroelectric HfO2 thin films induced by mechanical uniaxial tensile strain after the crystallization process

The effect of strain on the ferroelectricity of HfO2 thin films after crystallization was investigated by applying uniaxial mechanical strains to Au/HfO2/TiN metal–ferroelectric–metal (MFM) capacitors. The remnant polarization (2P r) of MFM capacitors increased when tensile strain was applied during polarization switching. This phenomenon should not be attributed to phase transformation from the non-ferroelectric to the ferroelectric phase, taking account of the fast relaxation of 2P r after removal of the mechanical strain and the fact that the crystal structure of HfO2 thin films evaluated by grazing incidence X-ray diffraction measurement was not changed by the tensile strain.


S
][4][5] It has been reported that ferroelectricity also appears in HfO 2 films with other dopants such as Y, 6,7) Zr, 8) Ge, 9) La, 10) Sr, 11) Al 12) and N, 13) and even in undoped HfO 2 . 14)][20][21] Moreover, it has been pointed out that tensile strain is one of the important factors promoting the formation of the o-phase in HfO 2 -based thin films.][28] These studies focused on the effects of process-induced strain during the crystallization process.However, the effects of strain after the crystallization process at RT on the ferroelectricity of HfO 2 thin films are not yet clear.Here, we focused on the effects of mechanical strain after crystallizing HfO 2 thin films.
In this study, we directly and quantitatively investigated the effects of strain after the crystallization process on the ferroelectricity of undoped HfO 2 thin films by applying mechanical strain to Au/HfO 2 /TiN metal-ferroelectric-metal (MFM) capacitors using a four-point bending jig during polarization switching at RT. Au/HfO 2 /TiN MFM capacitors were fabricated as follows.As a substrate, a 500-μm thick 4in p-type Si(100) wafer was thinned down to a thickness of 200 μm so that a large mechanical strain could be applied.After thermal oxidation, a 60-nm thick TiN layer was deposited by DC sputtering in Ar and N 2 ambient at a substrate temperature of 400 °C as a bottom electrode.The sample was transferred to another chamber, and a 30-nm thick HfO 2 film was deposited on the TiN electrode by RF sputtering in an Ar ambient of 0.1 Pa at RT in the chamber with a base pressure of 1.0 × 10 −6 Pa followed by postdeposition annealing in a N 2 ambient at 650 °C for 100 s.The deposition rate of HfO 2 was 1.1 nm min −1 .An additional annealing process was subsequently carried out in an O 2 ambient at 400 °C for 30 s to reduce oxygen deficiency in the HfO 2 films.Finally, Au was deposited by resistive heating vapor deposition as the top electrode.A schematic diagram of the sample structure is shown in Fig. 1(a).A uniaxial mechanical strain was applied to the Au/HfO 2 /TiN MFM capacitors using a four-point bending jig during the electric field cycling test, as shown in Figs.1(b) and 1(c).The amount of uniaxial strain (ε) was estimated from the displacement at the force points of the jig (δ) with the following equation: where h, l and d are the thickness of the sample, the distance between the supporting points of the jig and the distance between the force points of the jig, respectively.The polarization-electric field (P-E) curves of the MFM capacitors were measured using a ferroelectric tester with an electric field of ±4 MV cm −1 at a frequency of 5 kHz, and the endurance properties were characterized by applying a fatigue wave of ±2.7 MV cm −1 at a frequency of 20 kHz.The crystal structure of HfO 2 thin films was analyzed by outof-plane grazing incidence X-ray diffraction (GIXRD).The incident angle of XRD measurement was set to 0.45°.Figure 2(a) shows the initial P-E properties of the Au/HfO 2 /TiN MFM capacitors with application of the various mechanical strains using four-point bending.The relationship between the strain and the remnant polarization (2P r = P r + −P r

−
) extracted from P-E curves at 0 MV cm −1 is shown in Fig. 2(b).In this figure, the positive and negative strains indicate tensile and compressive strain, respectively.It was found that 2P r increased by increasing the applied tensile strain.The same trend was observed in the 2P r estimated by the PUND (positive up negative down) method, which can exclude the component of the leakage current (data not shown).It should be noted that the maximum 2P r of 17 μC cm −2 was observed when a tensile strain of 0.08% was applied, which is 1.5 times larger than the one observed in the case without strain (0%).On the other hand, no significant difference in 2P r was observed under compressive strain compared with the case without strain.Therefore, we conclude that 2P r increases due to the application of tensile strain during polarization switching even after the crystallization of the film.
Next, the endurance properties of Au/HfO 2 /TiN MFM capacitors with the application of various mechanical strains during field cycling was investigated as shown in Fig. 3(a).
The ratio of waking-up (r woke ) was defined from the switching polarization (P sw ) in the pristine state and the maximum P sw during field cycling, and the relationship between the strain and r woke is shown in Fig. 3(b).No significant difference in r woke was observed regardless of the direction and amount of strain.On the other hand, the number of cycles required to obtain the maximum P sw was ∼3 × 10 3 when a tensile strain of 0.08% was applied, while the number of cycles at the maximum P sw was ∼1 × 10 4 in the case without strain.Therefore, we found that the mechanical tensile strain could have an effect on facilitating the waking-up of HfO 2 thin films.
After removing the tensile strain, the MFM capacitors were left at RT for various time up to 5600 min, then P-E 051003-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd measurements of the MFM capacitors were carried out.
Figure 4 shows the relationship between the time after removing the tensile strain and 2P r normalized by where 2P ro and 2P r∞ are the values of 2P r just after the tensile strain was removed and after infinite time, respectively.For the value of 2P r∞ , the observed 2P r after 5600 min was employed in this analysis.The 2P r gradually decreased with time and returned to the original value of the MFM capacitors before the application of the mechanical strain.We estimated that this relaxation phenomenon follows the equation where τ is the relaxation time.According to previous research, it took more than 20 h for phase transformation from o-and tetragonal (t) phases to the monoclinic (m) phase in HfO 2 thin films at a temperature of 200 °C. 29)Therefore, it would be natural to expect an even longer time for such phase transformation.On the other hand, the observed relaxation time of 2P r at RT after removal of the tensile strain was approximately 1-5 h, which was estimated from the calculated line in Fig. 4 assuming the relationship of Eq. ( 2).This faster relaxation should not be due to phase transformation from the ferroelectric to the non-ferroelectric phase.These results indicate that the increase in 2P r should be induced by the application of tensile strain with a smaller activation energy than that required for phase transformation.
To confirm the lack of phase transformation while such an anomalous increase of the 2P r occurs, GIXRD measurements were conducted for the HfO 2 /TiN samples without Au top electrodes under mechanical strain.Figure 5(a) shows the GIXRD patterns of HfO 2 films with tensile strain.The peaks originating from the m-, o-, t-and cubic (c) phases were observed.Note that the peaks of o(111), t(101) and c(111) are located so close each other that it is difficult to distinguish them.The relative ratio of the o-, t-or c-phases (r o/t/c ) was defined from the peak areas of m( 1   No significant increase in r o/t/c was observed regardless of the application of tensile strain, as shown in Fig. 5(b), suggesting that the increase in 2P r was not due to phase transformation from the non-ferroelectric to ferroelectric o-phase.These results are consistent with the above consideration on the observed faster relaxation phenomenon of 2P r after removing the tensile strain at RT in Fig. 4. We may have to take account of a possibility that the change in the film properties might not be observable just by the application of strain but only appear after polarization, if we assume the 2P r enhancement is triggered by the polarization cycling.][28] However, since the mechanically induced strain applied in this study (∼0.1%) was not so large as the indicated process-induced strain (∼1%) in our previous study, [26][27][28] the amount of strain in this study was not sufficient to cause the phase transformation in HfO 2 thin films during field cycling. Terefore, the anomalous increase in 2P r in this study should be attributed to the enhancement of polarization without increase in the volume fraction of the ferroelectric o-phase in the film.One of the possible explanations for these enhancements of polarization is that the pinning effect could be weakened by the tensile strain.It is known that the polarization of the ferroelectric domain in HfO 2 thin films is pinned by the trapped charges at the interface of HfO 2 thin films and the electrodes or by the oxygen defects in the HfO 2 thin films.30) It is possible that the pinning effect could be weakened by the polarization under tensile strain, resulting in the larger 2P r .This explanation is consistent with the result of the promoted waking-up effect under tensile strain, as shown in Fig. 3(a), because one of the origins of waking-up is the de-pinning of the ferroelectric domain during field cycling.30,31) Another possible explanation is that the orientation of the crystal grains of the o-phase in HfO 2 thin films is aligned by the application of tensile strain.For the ferroelectric o-phase in HfO 2 , the lattice constant along the a-axis is the largest and the direction of polarization is along the c-axis.When the in-plane tensile strain is applied to the crystal grains of the o-phase it is possible that the a-axis of the crystal grains of the o-phase tends to align in the in-plane direction, which could cause an increase in the alignment of c-axis orientation in the out-of-plane direction.As a result, o-phase grains can make a greater contribution to polarization along the out-ofplane direction of the HfO 2 films, which leads to the larger 2P r .Since these explanations can be applied to thinner HfO 2 films, we consider that the enhancement of 2P r by the application of tensile strain is a general effect for ferroelectric HfO 2 thin films.However, the magnitude of enhancement of 2P r might be affected by the thickness of the HfO 2 films, the crystallinity of the films and the crystal orientation.
In conclusion, the effect of uniaxial mechanical strain after the crystallization process on the ferroelectricity and crystallinity of HfO 2 thin films was investigated using the four-point bending method.It was found that the application of tensile strain resulted in a larger 2P r of HfO 2 thin films, while compressive strain did not induce significant change.This phenomenon should be attributed to the enhancement of polarization without phase transformation from the nonferroelectric to the ferroelectric o-phase, because fast relaxation was observed after the removal of tensile strain at RT and no significant phase transformation was observed in the GIXRD patterns with application of mechanical strain.051003-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Fig. 3 .
Fig. 3. (a) Endurance properties of Au/HfO 2 /TiN MFM capacitors measured under various mechanical strains.(b) Relationship between the strain and r woke .The positive and negative strains indicate tensile and compressive strain, respectively.The error bars indicate the standard errors of the data on the plots.

Fig. 4 . 3 ©
Fig. 4. Relationship between the time after removing the tensile strain and normalized 2P r .The dotted curve indicates the calculated line for the case of τ = 1 h.The inset shows the data taking the log of the normalized 2P r in the vertical axis for t = 0-100 min.

Fig. 5 .
Fig. 5. (a) GIXRD patterns for HfO 2 under various strains of 0%, 0.05% and 0.08%.(b) Relationship between the relative ratio of o-/t-/c-phases, calculated using peak areas of the o-/t-/c-phase (A o t c / / ) and the m-phase (A m 111 ( ̅ ) and A m 111 () ) and the applied strain during the GIXRD measurement.