The grain size effect of Pb(Zr0.3Ti0.7)O3 thin films

The grain size effect of Pb(Zr0.3Ti0.7)O3 thin films

Thin Solid Films 406 (2002) 282–285 The grain size effect of Pb(Zr0.3Ti0.7)O3 thin films Feng Yana,b,*, Peng Baoa, Helen L.W. Chanb, Chung-Loong Choy...

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Thin Solid Films 406 (2002) 282–285

The grain size effect of Pb(Zr0.3Ti0.7)O3 thin films Feng Yana,b,*, Peng Baoa, Helen L.W. Chanb, Chung-Loong Choyb, Yening Wanga a

National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, PR China b Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, PR China Received 26 January 2001; received in revised form 20 September 2001; accepted 3 December 2001

Abstract The grain size strongly influences the fatigue properties of Pb(Zr0.3 Ti0.7 )O3 (PZT) thin films with platinum (Pt) electrodes. A film with smaller grain size has better fatigue properties. It was assumed that fatigue is mainly due to the pinning of domain walls by space charge or charged point defects near Pt electrodes. Therefore, the film with a lower fraction of the grains touching Pt electrodes has better fatigue properties. The permittivity of the thin film decreases with the decrease in grain size. This may be due to the decrease of mobility of domain walls with the decrease of grain size. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferroelectric properties; Dielectric properties; Interfaces; Domain wall

1. Introduction Recently, ferroelectric materials, especially in thin film form, have attracted the attention of many researchers w1–4x. They have been considered as candidates of the materials of non-volatile random access memory as they have two stable polarized states. Their large dielectric constants make them suitable for dielectric layers of microcapacitors in microelectronics. Therefore, the physical properties of ferroelectric thin films should be carefully studied. As reported in some papers w5–8x, the film thickness can influence the ferroelectric properties, such as the dielectric constant, coercive electric field, etc. However, the thickness effect includes two factors, the grain size effect and the influence of interface layer, which have not yet been studied separately. In this paper we will study the grain size effect on the ferroelectric properties and dielectric properties, especially on the fatigue property of Pb(Zr0.3Ti0.7)O3 (PZT) thin films with the same film thickness. 2. Experiment The Pb(Zr0.3Ti0.7)O3 film was prepared on a PtyTiy SiO2 ySi substrate by a metal organic decomposition *Corresponding author present address. Department of Engineering, Cambridge University, Cambridge CB2 1PZ, UK. E-mail address: [email protected] (F. Yan).

(MOD) method using lead acetate wPb(CH3COO)2 Ø3H2Ox, zirconium n-propoxide wZr(O–nC3H7 )4 x, and titanium isopropoxide wTi(O–iC3H7)4 x as starting materials, with an excess of 10 mol% concentration for lead (Pb). Acetic acid and n-propanol were selected as solvents. The clear solution was spin-coated onto the substrates. The coated film was dried at 350 8C for 5 min and this procedure was performed several times to achieve the desired film thickness (with two different thicknesses). Then the films were crystallized at different temperatures varied from 500 to 650 8C in oxygen ambient for 30 min to get different grain sizes. Pt top electrodes with a dot size of 0.03 mm2 were deposited by sputtering on the films. The ferroelectric properties of PZT thin films were measured with an RT6000HVS ferroelectric tester. The dielectric properties were measured with a HP4194A impedance analyzer. Through a scanning electron microscope (SEM), the grain size of the thin films annealed at different temperatures was measured, as well as the thickness of the films. As shown in Table 1, the grain size increased with the annealing temperature. The thickness of the thin films was 160 and 400 nm, respectively. 3. Results and discussion Figs. 1 and 2 show the hysteresis loops of the PZT thin films annealed at different temperatures. Fig. 3

0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 7 4 5 - X

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Table 1 The average grain size of PZT thin films annealed at different temperatures for 30 min Annealing temperature (8C) Average grain size (nm)

500 60

550 90

600 130

650 150

shows the coercive field of the PZT thin films. The coercive field Ec has little change with the annealing temperatures. Only the film with the thickness of 400 nm annealed at 650 8C has a slightly lower coercive field. As reported in some papers w6,8x, the ferroelectric thin films with a metallic top electrode showed higher Ec with the decrease of thickness. It was attributed to a low permittivity layer which existed at the PtyPZT interface. However, the grain size of their samples also varied with the thickness w6x. Thus, the grain size effect could not be excluded in their results. Our experiment results indicated that the grain size has little influence on the coercive field of the film, which confirmed the variation of the coercive field with film thickness was mainly due to the interface layer. Fig. 4 shows the relative dielectric permittivity ´r of the PZT thin films with different grain size in the frequency range of 100 Hz–100 kHz. ´r decreased with the increase of frequency and can be fitted with the relation ´rs´`qCØfny1 (n-1), which is the so-called Curie–Von Schweidler relaxation w9x. ´` is the relative dielectric permittivity at high frequency. As shown in Fig. 4, ´r of high frequency increases with the grain size, which is like the results of BaTiO3 ceramics w5x. As pointed out by Arlt w5x, the dielectric permittivity of the BaTiO3 ceramics showed a pronounced maximum at a grain size of 0.8–1 mm and strongly decreased with the decrease of grain size below 0.7 mm. Through theoretical calculation w5x, the density of 908 domain walls was inversely proportional to the square root of the grain size and thus the area of 908 domain walls contributed to the permittivity per volumes increased with the decrease of grain size. However, distancedependent repulsive forces existed between two neighboring domain walls as reported in our previous papers w10,11x on PbTiO3 thin films and La1yxNbxP5O14 single. The nearer the domain walls, the stronger the repulsive force which existed. Therefore, the mobility of domain walls decreases with the decrease of grain size. In our samples, it is obvious that the influence of repulsive force between domain walls on the permittivity is more important. However, we assume that the permittivity may decrease with the increase of grain size if the grain size is above a critical value for the distance between domain walls is large and thus the repulsive force is very weak. As reported in some papers w6,7,12x, the permittivity of PZT thin film decreased with the film thickness. We assume that the thickness effect on the dielectric properties of PZT thin films can be attributed

Fig. 1. The hysteresis loops of PZT thin films with the thickness of 160 nm annealed at different temperatures.

to both the existence of the low permittivity layer at the PtyPZT interface and the grain size effect. Fig. 5 shows the fatigue properties of PZT thin films. The fatigue rate strongly depends on the grain size and film thickness. The smaller the grain size is, the faster the fatigue rate can be measured. The fatigue rate increases with the decrease of film thickness as well. Since PZT thin films with Pt electrodes have apparent fatigue while the films with oxide electrodes have no fatigue w3x, we assume the fatigue is due to the reduction of switchable polarization of the grains near Pt electrodes and the grains not touching the Pt electrodes almost have no fatigue after switching cycles. Therefore, the film with the lower fraction of grains touching the Pt electrode has the better fatigue property. The smaller the grain size and the thicker the film, the lower the fraction of grains touched Pt electrodes. Therefore, the fatigue rate depends on the grain size and film thickness can be understood. Another most important factor that influences the fatigue rate is the density of charged point defects and

Fig. 2. The hysteresis loops of PZT thin films with the thickness of 400 nm annealed at different temperatures.

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F. Yan et al. / Thin Solid Films 406 (2002) 282–285

Fig. 3. The coercive field Ec of PZT thin films with different grain sizes.

space charges that can pin the ferroelectric domain walls and induce fatigue. It has been reported in some papers w13,14x that the increase of space charge may induce the increase of coercive field and the decrease of remnant polarization. As shown in Figs. 1 and 2, every sample has a typical hysteresis loop. The coercive field slightly decreased with the increase of annealing temperature and the remnant polarization increased with the annealing temperature. Therefore, there is no evidence of the increase of charged point defects and space charge with the increase of annealing temperature in the PZT thin films. In fact, the crystal quality normally increased with the increase of annealing temperature. Thus, the increase of the fatigue rate with the increase of annealing temperature is not due to the increase of density of space charge and charged pointed defects. Therefore, the effect of grain size on the fatigue endurance is very important for our samples. We assume that the internal electrical field in the Schottky barrier plays an important role on the fatigue of the PZT grains near the Pt electrode. The internal electric field is very strong especially near the PZTyPt interface and points to Pt electrodes if it is n-type doped. It can be calculated that the strength of the internal electrical field near the PZTyPt interface is approximately 3=105 Vycm if the Schottky barrier width is 100 nm and the barrier height is 1.5 V, which almost

Fig. 4. The relative dielectric permittivity ´r of PZT thin films with different grain sizes vs. frequency. The thickness of the PZT films is 400 nm.

Fig. 5. The fatigue property of PZT thin films with different grain size. P* is the switchable polarization of PZT thin films. P0* is the original switchable polarization of the PZT film. The measurement frequency is 5=104 Hz. (a) Films with the thickness of 160 nm. The applied voltage is 5 V. (b) Films with the thickness of 400 nm. The applied voltage is 12 V.

equals to the applied electric field. Therefore, the positive space charge and charged point defects near the PZTyPt interface will move to the electrodes and accumulate near the Pt electrode if the applied electric field has the same direction as the internal electric field. If the applied electric field is opposite to the internal electric field, the positive charges cannot be moved back for the electric field becomes very weak. Therefore, after some switching cycles, more and more positive charges accumulate near bottom and top Pt electrodes. The internal electric field becomes stronger and the height of the barrier decreases. Therefore, most of ferroelectric domains near the Pt electrode are pinned by the charges and the film shows fatigue. Through the standard Schottky equation w15x, the leakage current will increase with the decrease of Schottky barrier height. Some papers w7x have reported that the leakage current increased after fatigue happened, which also proved the Schottky barrier height decreased after fatigue. The internal electric field in the Schottky barrier can influence the capacitance as well. According to the Landau–Ginzburg–Devonshire theory w16x, the permittivity of a thin PZT film will decrease with the increase of applied bias voltage. Therefore, the presence of the internal electric field can decrease the permittivity near the interface and thus decrease the capacitance of the film. We assume that the interface layer with relative low dielectric permittivity was due to the existence of the internal electric field near the Pt electrode.

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Acknowledgments The authors thank the Hong Kong Polytechnic University for financial support of Dr Feng Yan. The work was also supported by the National Nature Science Foundation and the Climbing Project of China. This work was supported by Ke-li Fellowship financed by Sanzhu Co. Ltd. in Shandong. References w1x J.F. Scott, C.A. Araujo, Science 246 (1989) 1400. w2x O. Auciello, J.F. Scott, R. Ramesh, Phys. Today 51 (1998) 22. w3x W.L. Warren, D. Dimos, B.A. Tuttle, R.D. Nasby, G.E. Pike, Appl. Phys. Lett. 65 (1994) 1018. w4x E.L. Colla, D.V. Taylor, A.K. Tagantsev, N. Setter, Appl. Phys. Lett. 72 (1998) 2478. w5x G. Arlt, D. Hennings, G. de With, J. Appl. Phys. 58 (1985) 1619.

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