Статьи на перевод PVDF_P(VDF-TrFE) / article1

I. INTRODUCTION

Researchers and developers have shown great interest in the development of nonvolatile memories based on ferroelectric materials. For nondestructive read-out ferroelectric random access memory a ferroelectric gate structure is required. However, it is still di cult to obtain a good quality ferroelectric thin film on a Si surface. Thus, a bu er layer with a high dielectric constant is often inserted between the ferroelectric film and the Si substrate [1,2]. There has been much research on a bu er insulator that has good electrical properties, such as low leakage current and high dielectric constant, for a metal- ferroelectric-insulator (MFIS) structure. Representative bu er insulators include HfO2, LaAlO3, SrTa2O6, ZrO2, and Dy2O3. They have high dielectric constants (10 50) with low leakage currents (below 10−6 A/cm2) and have good interface characteristics [3–7]. Good switching and memory characteristics have been obtained in MFIS structures by Kodama et al. [8]. The LaZrOx system is attractive as a bu er insulating layer. Because of both the lanthanum and zirconium atoms, the constituents of the LaZrOx thin film are considered to be thermally stable in contact with Si [9]. For this reason, we expected

E-mail: pbe@uos.ac.kr; Fax: +82-2-2249-6802

the LaZrOx thin film to be the most advantageous bu er insulator to investigate because a ferroelectric layer usually requires high-temperature annealing.

The ferroelectric polymer polyvinylidene fluoridetrifluoroethylene (P(VDF-TrFE)) film has an unique mechanism of ferroelectricity. There have been some e orts to apply a P(VDF-TrFE) thin film to a ferroelectric random access memory (FeRAM) [10]. In particular, a P(VDF-TrFE) thin film is a promising candidate for a FeRAM due to its low processing temperature. This temperature is much lower than that of inorganic ferroelectrics such as Pb(Zr,Ti)O3 (PZT), SrBi2Ta2O9 (SBT), and (Bi,La)4Ti3O12 (BLT) [11– 16]. In this study, we fabricated a LaZrOx/Si metal- insulator-semiconductor (MIS) structure and a P(VDFTrFE)/LaZrOx/Si MFIS structure by combining an organic ferroelectric gate layer with an inorganic bu er layer.

II. EXPERIMENTS

To fabricate the MFIS structure, we prepared a LaZrOx thin film and a P(VDF-TrFE) film by using a sol-gel method. The LaZrOx thin film was deposited on a Si substrate by using a spin-coating method at a rota-

Fig. 3. Memory window widths for three P(VDF-TrFE) film thicknesses as a function of the bias voltage. The film thickness was controlled by using its sol-gel solution concentration.

was formed from a solution with a higher concentration. Regardless of the weight fraction of the solution, all C-V curves showed hysteresis loops with a clockwise trace, as indicated by the arrows. The accumulation capacitance of the 1-wt% P(VDF-TrFE) film on LaZrOx was 230 nF/cm2 for a bias voltage sweep of ±5 V. As the concentration of the solution was increased, as can be seen from Fig. 2, the accumulation capacitance decreased to 90 and 42 nF/cm2 for the same bias voltage sweep. The memory window width of the Au/P(VDFTrFE)/LaZrOx/Si, measured for a voltage sweep of ±5 V, was about 0.7, 2.2, and 3 V for 1-, 3-, and 5-wt% P(VDF-TrFE) films on LaZrOx/Si structures, respectively. Contrary to the accumulation capacitance, the memory window width increased as the concentration of the solution increased, meaning that a large portion of the applied voltage could be distributed to the P(VDFTrFE) film in the MFIS structure. This happened as the film thickness increased, resulting in the decrease in a capacitance of the P(VDF-TrFE) thin film.

The large voltage distribution to the P(VDF-TrFE) film leads to an increase of the remnant polarization in the film. In a FeRAM, a reduction of the remnant polarization in the ferroelectric film results in a reduction of the programmable memory window. Therefore, a larger memory window width is better than a small one for this memory device. We also demonstrated that the memory window width and the threshold voltage shift gradually increased with increasing voltage. Figure 3 illustrates the variations in the memory window width with the bias voltage. The lines for 3- and 5-wt% P(VDF-TrFE) films in Fig. 3 show a rapid increase as compared with the one for 1 wt%-P(VDF-TrFE). From the above results, the P(VDF-TrFE) films, except for the 1-wt% film, are suitable to apply to a FeRAM from the point of view of the memory window width.

Fig. 4. J-V characteristic curves of Au/P(VDF-TrFE)/Si and Au/P(VDF-TrFE)/LaZrOx/Si structures. The LaZrOx bu er insulator dramatically improves the leakage property.

Fig. 5. AFM surface images for P(VDF-TrFE) films from

(a) 3 and (b) 5 wt%.

Figure 4 shows the J-V characteristics of the same samples as those used in Fig. 2. The current densities for the P(VDF-TrFE) films obtained from the 1-, 3-, and 5-wt% solution, were lower than 10−6, 10−7, and 10−7 A/cm2, respectively. With the MFIS structure, we also made a MFS structure capacitor. As the weight

fraction of the P(VDF-TrFE) in the solution increased, the current density decreased. This observation might be related to the thickness. We observed the surface morphology of the P(VDF-TrFE) by using AFM. Figure 5 shows the surface AFM images of P(VDF-TrFE) films. We were not able to find the lamella structure at a 1-wt% solution, but we were able to find it at the 3- and 5-wt% solutions. The ferroelectric polymer basically consists of two-phase structures, amorphous and crystalline lamella. Thus the ferroelectricity of P(VDFTrFE) originates from the lamella structure in the film [17,18]. These AFM images agree with the C-V measurements. As mentioned above, the memory window width from the C-V curves for 3- and 5-wt% P(VDF-TrFE) is much larger than that of 1-wt% P(VDF-TrFE). We measured the root-mean-square roughnesses (Rrms) as 4.136 and 9.560 nm at 3- and 5-wt%, respectively. These values indicate that the P(VDF-TrFE) film has a good surface morphology, especially at 3-wt%.

IV. CONCLUSION

In this work, we applied a LaZrOx thin film as a bu er layer and a P(VDF-TrFE) film as a ferroelectric-gate layer. The C-V characteristics of the Au/LaZrOx/Si MIS structure revealed that the LaZrOx thin film had good electrical properties and was suitable for a bu er layer in a MFIS structure. We estimated the EOT value of the LaZrOx film as about 7.9 nm. For the Au/P(VDFTrFE)/LaZrOx/Si MFIS structures, the memory window width increased with increasing applied bias voltage and with increasing thickness of the P(VDF-TrFE) film. The 3- and 5-wt% P(VDF-TrFE) films, deposited on the LaZrOx/Si structure showed a memory window width of over 2 V at a bias sweep range of ±5 V. The value of the leakage current density was about 3.5 × 10−8 A/cm2 for a 5-wt% thick film. The good ferroelectricity of the 3- and 5-wt% P(VDF-TrFE) films was caused by the lamellae structure shown in the AFM images. From these results, we expect a combination of P(VDF-TrFE) and LaZrOx thin films to be useful and promising for a 1T-type FeRAM at a low voltage; however, more properties in term of parameters such as the retention time and the endurance should be verified.

ACKNOWLEDGMENTS

This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government Ministry of Education, Science, and Technology (MEST), (No. R01-2007-000-11985-0).

REFERENCES

[1]B. E. Park and H. Ishiwara, J. Korean Phys. Soc. 42, S1149 (2003).

[2]B. E. Park, K. Takahashi and H. Ishiwara, J. Korean Phys. Soc. 46, 346 (2005).

[3]B. E. Park and H. Ishiwara, Appl. Phys. Lett. 79, 806 (2001).

[4]E. Tokumistsu, G. Fujii and H. Ishiwara, Jpn. J. Appl. Phys. 39, 2125 (2000).

[5]H. S. Choi, E. H. Kim, I. H. Choi, Y. T. Kim, J. H. Choi and J. Y. Lee, J. Korean Phys. Soc. 39, 179 (2001).

[6]B. E. Park, K. Takahashi and H. Ishiwara, Appl. Phys. Lett. 85, 4448 (2004).

[7]C. Y. Chang, T. P. C. Juan and J. Y. M. Lee, Appl. Phys. Lett. 88, 072917 (2006).

[8]K. Kodama, M. Takahashi, D. Ricinschi, A. I. Lerescu, M. Noda and M. Okuyama, Jpn. J. Appl. Phys. 41, 2639 (2002).

[9]Y. Yamamoto, K. Kita, K. Kyuno and A. Toriumi, Appl. Phys. Lett. 89, 032903 (2006).

[10]D. W. Kim, J. H. Kim, J. N. Kim, H. J. Park, H. S. Jeon and B. E. Park, Integrated Ferroelectrics 98, 121 (2008).

[11]K. N. Narayanan, U. R. de Bettignies, S. Dabos-Seignon, and J.-M. Nunzi, Appl. Phys. Lett. 85, 1823 (2004).

[12]D. W. Kim, G. G. Lee and B. E. Park, J. Korean Phys. Soc. 51, 719 (2007).

[13]C. Y. Koo, J. H. Cheon, J. H. Yeom, J. Ha, S. H. Kim and S. K. Hong, J. Korean Phys. Soc. 49, S514 (2006).

[14]C. H. Chien, D. Y. Wang, M. J. Yang, P. Lehnen, C. C. Leu, S. H. Chuang, T. Y. Huang and C. Y. Chang, IEEE Elec. Dev. Lett. 24, 553 (2003).

[15]H. S. Jeon, K. H. Park, B. E. Park, C. J. Kim and Y. S. Choi, Ferroelectr. 353, 242 (2007).

[16]J. S. Park, G. G. Lee, B. E. Park and C. J. Kim, J. Korean Phys. Soc. 51, 727 (2007).

[17]A. J. Lovinger, Science 220, 1115 (1983).

[18]S. Fujisaki and H. Ishiwara, Appl. Phys. Lett. 90, 162902 (2007).