Structural and Electrical Properties of Sol-Gel

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 6 No. 1, February 1999

69

Structural and Electrical Properties of Sol-Gel Synthesized PLZT Thin Films C. Vijayaraghavan, T. C. Goel and R. G. Mendiratta Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi, India

ABSTRACT Thin films of FLZT with PbTiO3 interlayers have been fabricated by using a sol-gel spin-on process. Three compositions of PLZT, namely 8/65/35,15/40/60 and 18/30/70, have been deposited on platinum and quartz substrates and heat treated at 650°C. All three compositions have crystallized in the perovskite phase. The 8/65/35 composition, which has been investigated in greater detail exhibits a low value of leakage current. It also exhibits polarization hysteresis, with Ec = 20 kV/cm and Pr = 25 fiClcm2.

1

INTRODUCTION

T

HIN films of lead lanthanum zirconate titanate (PLZT) Pbi-^La^ (Zri_ y Tij,)O3 have been investigated extensively over the last decade to exploit their high permittivities, large remanent polarization, electro-optic coefficients and optical transmittance for a wide variety of applications including decoupling capacitors, ferroelectric memories, optical modulators, displays, shutters and memories [1-3]. Much of the interest in PLZT has been for optical applications. However, reports on preparation of perovskite PLZT thin films for use in memory devices have been few [4,5]. Some of the advantages of PLZT thin films integrated on semiconductors would be high bit density, nonvolatile low voltage operation over a wide temperature range, short access and cycle times and retention of charge over long periods of time. PLZT thin films usually consist of a mixture of the perovskite and pyrochlore phases. Only the perovskite phase possesses desirable ferroelectric properties, because the crystal structure of the pyrochlore phase is centro-symmetric. The requirements for integration of these films into devices are low processing temperature, high relative permittivity, large remanent polarization, small coercive field and leakage current and low fatigue. A number of techniques have been used to obtain PLZT thin films in perovskite phase, including high vacuum processes such as sputtering [6] and electron-beam evaporation [7]. However, sol-gel processing of multicomponent thin films offers attractive features such as low processing temperature and ease of tailoring the structural and electrical properties by controlling the composition [8,9]. An attempt has been made to synthesize, by the sol-gel technique, PLZT thin films of the perovskite phase having good electrical properties and high optical transmittance. A seed layer of PbTiO3, was used. We have chosen for study, three PLZT compositions, 8/65/35,15/40/60 and 18/30/70.

2

EXPERIMENTAL PROCEDURE

Several precursor routes have been used to synthesize lead zirconate titanate (PZT) and PLZT based compositions, the most common of which

are those reported by Blum and Gurkovich [10], Schwartz et al. [11], and Vest and Zhu [12]. The present investigation uses lead acetate trihydrate, lanthanum acetate hydrate, zirconium acetylacetonate and titanium isopropoxide (Aldrich, USA) as precursors. 2-methoxyethanol has been used as solvent and acetic acid as catalyst. 10 mol% excess leed precursor was added to the solution to compensate for the loss of lead due to volatilization. Figure 1 illustrates the processing cycle for the PLZT sol and the deposition procedure for obtaining PLZT thin films. A 0.01 M precursor solution was prepared from lead acetate, lanthanum acetate, zirconium acetylacetonate and titanium isopropoxide. The choice of 2methoxyethanol as a solvent and zirconium acetylacetonate as a precursor reduces the viscosity of the solution and stabilizes the sol against gelation. Prior to deposition, the substrates were cleaned using standard semiconductor cleaning procedure. Organic contaminants were removed by boiling in trichloroethylene for 15 min. The substrates were then rinsed in acetone, ethanol and distilled water and dried with a jet of nitrogen. A thin interlayer of PbTiOs has been found to facilitate perovskite crystallization of PZT [13]. This buffer layer of PbTiO3 is important because it crystallizes into the perovskite phase irrespective of the substrate material [14], and thus substrate effects are minimized by depositing a thin interlayer. The thermal decomposition behavior of a PLZT 8/65/35 precursor solution was studied using thermogravimetric analysis (TGA) to determine the temperature at which the metal-organic compounds begin transforming into oxides. The solution was heated at a rate of 2 c C/min in air at a flow rate of 75 ml/min. At temperatures < 1 5 0 ^ , the weight loss of the sample was due to evaporation of the xylene solution. The decomposition of precursor compounds was complete at ^ 3 0 0 ^ , as is evident from Figure 2. Thin films of PLZT have therefore been heattreated at a temperature of 350°C to ensure complete removal of organics. The deposition process involves repeated application (10 to 15 times) of the sol to the substrate, spin coating at 3000 rpm for 30 s, drying at

M et al.; Structural and Electrical Properties of Sol-Gel Synthesized PLZT Thin Films

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100-'

Ftepest

100 200 300 400 500 600 700 Temperature (°C)

(

FURNACE ANNEAL at S00°c

900

Figure 2. Thermogravimetric analysis thermogram for the PLZT 8/65/ 35 precursor solution heated at a rate of 2°C/min in air. ments were carried out on a Hitachi UV-VIS-IR spectrophotometer.

CRYSTALLINE FILM of PLZT

Figure 1. Flow chart of processing cycle of PLZT films

150^ and heat treating at 3 5 0 ^ for 10 min. After achieving the desired thickness, the films were annealed at .650^ for 1 h. and transferred directly to room temperature environment. PbTiO3 precursor solution was used to deposit an 180 nm thick interlayer prior to the deposition of the PLZT films to facilitate crystallization. A homogeneous solution was synthesized from the chosen precursors with 2-methoxyethanol as solvent. However, it was found that some solid products precipitated from the solution after a few months. Several additives were investigated in an effort to avoid precipitation. Acetvlacetone was found to be the most suitable additive, since it is most easily removed during heat treatment, leading to smooth films without any cracks or voids. A 0.5 M equivalent of acetylacetone to total metal concentration was found to maintain the stoichiometry and stabilize the precursor solution. Crystallinity of the annealed films was examined after heat treatment by X-ray diffraction (Rigaku, CuKQ radiation, A = 0.541 nm, and microstructures were determined by scanning electron microscopy (Philips). Gold electrodes were sputter deposited through a shadow mask (210x 210 ^im2). Dielectric and ferroelectric properties were measured on an impedance analyzer (HP 4192A) and a modified SawyerTower circuit respectively. Film thickness was measured using a TaylorHobson Talystep Surface profilometer. Optical transmittance measure-

20

30

Figure 3. X-ray diffractograms of PbTiO3 and PLZT films on platinum heat-treated at 6 5 0 ^ for 1 h (a) PbTiO 3 , (b) PLZT 8/65/35, (c) PLZT 15/40/ 60 and (d) PLZT 18/30/70.

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RESULTS AND DISCUSSION

Figure 3 shows diffractograms of lead titanate (PT) film and composite PT/PLZT structures of the three compositions chosen for study. The prominent peaks of PT and PLZT corresponding to (100), (110), (111), (200), (002) and (211) have been identified in the Figure. The peak due to platinum substrate has also been identified. It follows from the diffractograms that all three compositions have crystallized in the perovskite tetragonal phase. The calculated lattice parameters are as follows: PbTiO3, a = b = 3.918, c = 4.122; PLZT 8/65/35, a = b =

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 6 No. 1, February 1999

3.918, c = 4.078; PLZT 15[40]60, a = b = 3.918, c = 4.026 and PLZT 18/30/70, a = b = 3.922,c = 4.008. These values are in agreement with those reported for that of perovskite PLZT [15]. Calculations have revealed that the average crystallite size is of the order of 20 nm in the films annealed at 650^C.

71

(/K/cm2)

400.0

.

.

/

AkV/cm) E

S 300.0-

i a. • 200.0-o

a

o

a

>

ioao0.04

Figure 5. Charge-field hysteresis loop in a PLZT 8/65/35 thin film, (vertical scale of polarization 15 /iC/cm 2 , horizontal scale 50 kV/cm per division).

70 95 T«mp»roture(°c)

for PZT ceramics (45 fiC/cm2), but Ec was larger as compared to those of bulk PZT (20 kV/cm) [5]. This is attributed to the small grain size in the films, the space-charge layers at the electrodes of the thin films and the clamping effect of the substrate.

Figure 4. Temperature variation of er and loss tangent tan 8 of PLZT 8/65/35 thin film on platinum (x 8/65/35, D 15/40/60, O 18/30/70). The room temperature values of relative permittivity er and loss factor t a n S of the PT /PLZT structure heat treated at 6 5 0 ^ were measured to be approximately 250 and 0.016,154 and 0.026 and 360 and 0.029 for 8/65/35,15/40/60 and 18/30/70 compositions respectively. The temperature variation of the dielectric permittivity and loss is shown in Figure 4. The standard deviation in the average values of er measured for different areas of the same film, was < 2%, indicating uniformity of structure and composition. e r and t a n 5 measurements between room temperature and 120^0, after repeated thermal cycling and shelf-aging (six months) of specimens, were reproducible, indicating good adhesion to the substrate and stability of the films. The values of the relative permittivity are lower than those reported for PLZT films prepared by sputtering and other techniques [1] and for bulk PLZT [16]. The low value of relative permittivity for thin films as compared to that for bulk samples is attributed to the sub-micron grain size and stresses from the substrate. Because no change in the relative permittivity values is observed with temperature, it is most likely that the domain walls are pinned and therefore do not contribute to the permittivity. Similar results have been reported for fine grained barium titanate [17]. There is not much variation in the loss values to ~75 q C in all three compositions. Above this temperature, an increasing trend in the loss values is observed. This is attributed to the phase transition in these compositions which has been reported to be ~ 1 0 0 , 6 5 and 45^0 rrespectively for the three PLZT compositions [16]. Figure 5 shows a well-defined charge-field hysteresis loop measured on a 1 fjm thick 8/65/35 PLZT film coated on a platinum substrate, using a Sawyer-Tower circuit operating at 60 Hz at room temperature. The remanent polarization Pr — 25 /xC/cm 2 and the coercive field Ec = 30 kV/cm. These results contrast significantly with those reported for bulk PZT ceramics. Pr was smaller than the maximum value reported

3

* 5 Voltage (V)

Figure 6. Leakage current in a 1 fjm thick PLZT 8/65/35 thin film as a function of applied voltage. The leakage current characteristics, an important criterion for the application of ferroelectric thin films, were studied using dc I-V measurements in metal-insulator-metal (MIM) configuration. Figure 6 shows the I-V characteristics for a 1 /zm thick 8/65/35 PLZT film deposited on platinum substrate and annealed at 650^. A very low leakage current was observed, 3 x l O ~ 1 2 A/cm 2 at 2 V, which compares well with the values reported in the literature [5]. The excellent dielectric properties achieved in these films are attributed to the reduction in the concentration of oxygen vacancies in the films by proper annealing conditions and by the electrical compensation due to La doping. The morphology of the films was strongly dependent on the deposition conditions. Films coated with viscous solutions were translucent and rough and were easily scratched, whereas films coated from dilute solutions were transparent with interference colors. Thicker films coated using a repeated spinning-firing cycle were also transparent and without visible cracks. Optical transmittance of the films was measured in the 200 to 2000 nm range. The transmittance spectrum for PLZT 8/65/35, typical of all three compositions, is shown in Figure 7. The transmittance has a sharp absorption edge ( « 4 5 0 nm for 8/65/35 PLZT) and shows interference os-

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REFERENCES [I] S. Krishnakumar, V. H. Ozguz, C. Fan, C. Cozzolino, S. C. Esener and S. H. Lee, "Deposition and characterization of thin ferroelectric lead lanthanum zirconate titanate (PLZT) films on sapphire for spatial light modulator applications", IEEE Trans, on Ferro. and Freq. Control, Vol. 38, No. 6, pp. 585-590, Nov. 1991.

600

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1000 1200 1400 WAVELENGTH (nm)

1600

1800

2000

Figure 7. Optical transmittance spectrum of PLZT 8/65/35 thin film on quartz.

dilations caused by the structure of the film. The wavelength of this absorption edge is close to that of transparent bulk PLZT in the pe:rovskite phase [18]. Transparency in the visible range is one of the important characteristics of perovskite phase PLZT. The modified envelope method [18] was used to calculate the refractive index and extinction coefficient of the film. For 8/65/35 films deposited on quartz and heat-treated at 650°C, the refractive index and the extinction coefficient were observed to be 2.425 and 0.0008 respectively at 633 nm. The high value of refractive index is an indication of the high density of the film, and the low value of extinction coefficient illustrates the nature of the specular and highly transparent films.

[2] Yukio Watanabe, Mitsuru Tanamura and Yasuaki Matsumoto, "Memory retention and switching speed of ferroelectric field effect in (Pb,La)(Ti,Zr)O:s/La2CuO4t Sr Heterostructure", Japan ]. Appl. Phys., Vol. 35, pp. 1561-1568,1996. [3] Vinay K Seth and Walter A Schulze, "Fabrication and characterization of ferroelectric PLZT 7/65/35 ceramic thin films and fibers", Ferroelectrics, Vol. 112, pp. 283-307, 1990. [4] Joon Sung Lee, Chang Jung Kim, Dae Sung Yoon, Chaun Gi Choi, Jae Myung Kim and Kwandsoo No, "Effects of seeding layer on perovskite transformation, microstructure and transmittance of sol-gel processed lanthanum modified lead zirconate titanate films", Japan J. Appl. Phys., Vol. 33, pp. 260-265,1994. [5] W. Zhu, Z. Q. Liu, W. Lu, M. S. Tse, H. S. Tan and X. Yao, "A systematic study on structural and dielectric properties of lead zirconate titanate/(Pb,La)(Zr(1_a,)Ti(a.))O3 thin films deposited by metallo-organic deposition technology", J. Appl. Phys., Vol. 79, no. 8, pp. 4283-4290,15 April 1996. [6] M. Okuyama, I. Usuki, Y. Hamakawa and T. Nakagawa, "Epitaxial growth of ferroelectric PLZT thin film and their optical properties", Appl. Phys., Vol. 21, pp. 339-343, 1980. [7] B. Panda, S. K. Ray, A. Dhar, A. Sarkar, D. Bhattacharya and K. L. Chopra, "Electron beam deposited lead-lanthanum-zirconate-titanate thin films for silicon based device applications", J. Appl. Phys., Vol. 79, (2), pp. 1008-1012,15 January 1996. [8] M. Sayer, G. Yi and M. Sedlar, "Comparative sol-gel processing of PZT thin films", Integ. Ferroelectrics, Vol. 7, pp. 247-258,1995. [9] C. D. E. Lakeman and D. A. Payne, "Sol-gel processing of electrical and magnetic ceramics", Materials Chemistry and Physics, Vol. 38, pp. 305-324,1994. [10] J. B. Blum and S. R. Gurkovich, "Sol-Gel derived PbTiO3", J. Mater. Sci., 20, pp. 44794483,1985. [II] Schwartz, R. A. Assink and T. J. Headley, "Spectroscopic and microstructural characterization of solution chemistry effects in PZT thin film processing", in Materials

Research Society Symposia Proceedings, Vol. 243, pp. 345-354. Ferroelectric Thin Films II

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CONCLUSIONS

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ILMS of PLZT of pure perovskite phase with pore-free nanometer sized grains and remanent polarization of the order of 25 /xC/cm2 have been fabricated successfully by a sol-gel technique at a low processing temperature. Three different compositions crystallized in the tetragonal phase and show good dielectric properties. In particular, the 8/65/ 35 composition, which has been studied in greater detail, shows promise for ferroelectric memory applications on account of its high remanent polarization, small coercive field and low leakage current. The optical transparency of these films makes them attractive for use in electro-optic devices as well. Detailed ferroelectric and optical studies on the other two compositions are in progress and will be reported later

ACKNOWLEDGMENT One of the authors (CV) would like to thank the Council of Scientific and Industrial Research for the grant of a Senior Research Fellowship. We also acknowledge the help of Mr. Mahaveer Jain of the Thin Film Laboratory for the X-ray measurements.

Symposium, Ed. A. I. Kingon, E. R. Myers and B. A. Turtle, Materials Research Society, Pittsburgh, PA, 1992. [12] R. W. Vest and W. Zhu, "Films of 60/40 PZT by the MOD process for memory applications", Ferroelectrics, Vol. 119, pp. 61-75,1991. [13] Jiin-Jyn Shyu and Po-Chien Lee, "Sol-gel derived Pb(Zr, Ti)O3 thin films: Effects of PbTiO3 interlayer", Japan J. Appl. Phys., Vol. 35, pp. 3954-3959,1996. [14] S. L. Swartz, S. J. Bright, P. J. Melling and T. R. Shrout, "Sol-gel processing of composite PT/PLZT thin films", Ferroelectrics, Vol. 108, pp. 71-76,1990. [15] G. H. Haertling, "Piezoelectric and electro-optic ceramics", pp. 169 in Ceramic Materials for Electronics, 2nd ed. Edited by R. C. Buchannan, Marcel Dekker, New York, 1991.

[16] M. E. Lines and A. M. Glass, Principles and applications of ferroelectric and related materials, Clarendon Press, Oxford, 1977, p. 541 [17] G. Arlt, D. Hennings and G. de With, "Dielecric properties of fine grained BaTiO3 ceramics", J. Appl. Phys., Vol. 58, pp. 1619-1625,1984. [18] Chien H. Peng and Seshu B. Desu, "Modified envelope method for obtaining optical properties of weakly absorbing thin films and its application to thin films of lead zirconium titanate solid solutions", J. Am. Ceram. Soc, Vol. 77, no. 4, pp. 929-938, April 1994.

This manuscript is based on a paper given at the 9th International Symposium on Electrets, Shanghai, China, 25-27 September 1996. Manuscript was received on 7December 1997, in revised form 31 March 1998.