Crystallization and characterization of Pb2Nb2O7 thin films prepared at high pressure and low temperature

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Journal of Physics and Chemistry of Solids 69 (2008) 475–479

Crystallization and characterization of Pb

₂

Nb

₂

O

₇

thin ﬁlms

prepared at high pressure and low temperature

Chung-Han Wu, Mohammad Qureshi, Chung-Hsin Lu

Department of Chemical Engineering, Electronic and Electro-optical Ceramics Laboratory, National Taiwan University, Taipei, Taiwan, ROC

Abstract

A facile thin ﬁlm crystallization of pyrochlore Pb2Nb2O7at low temperatures has been demonstrated at high pressures over rapid

thermal annealing process (RTA). The crystallization of Pb2Nb2O7 has started at temperatures as low as 220 1C. Powder X-ray

diffraction patterns reveal the formation of pyrochlore phase without any intermediate phase formation. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) features show the temperature dependence of crystal growth in both high pressure and RTA methods. Using the high-pressure method, the crystallization temperatures of Pb2Nb2O7are reduced to 220 1C when compared to 600 1C

required for crystallization using RTA process. The uniform and dense structure that consisted of small grains with the size of 20–30 nm existed in the Pb2Nb2O7thin ﬁlms heated by RTA process, whereas the use of high pressure modiﬁed the crystallize size to approximately

40–45 nm with island structures observed throughout the Pb2Nb2O7ﬁlms.

Keywords: A. Thin ﬁlms; B. Chemical synthesis; D. Microstructure

1. Introduction

Electro-ceramic thin ﬁlms have attracted considerable interest for applications in non-volatile ferroelectric ran-dom access memories due to their large reversible spontaneous polarization [1,2] and high dielectric permit-tivity[3–7]. In particular, compounds with the pyrochlore structure possess the distinctive characters such as: the low synthetic temperature, the facility for processing, the good chemical stability, the paraelectric property, the relative high dielectric constant, and interesting optical properties

[8,9]. Therefore, the pyrochlore compounds become promising materials for the use in charge storage capacitors or in the application of memory thin ﬁlms. Pb2Nb2O7 ceramic is one of such paraelectric materials in room temperature having pyrochlore structure which has a dielectric constant of 190. Pb2Nb2O7 has octahedral NbO6 in the tetragonal structure with encircled Pb2O in annular conformation. This material has attracted great attention for its low-temperature formation while synthe-sizing ferroelectric Pb(Zr,Ti)O3–Pb(Ni1/3Nb2/3)O3ceramics

or other oxides of lead and niobium. Inspite of having technological importance of Pb2Nb2O7, there are very few literature reports available on the synthesis and character-ization of this pyrochlore [10,11]. In our early efforts, we have demonstrated the synthesis of nanosize Pb2Nb2O7 ceramic powder and other ceramic thin films by hydro-thermal process [12,13], where in the single phase, Pb2Nb2O7 powder was obtained at considerably low temperature such as 150 1C. The low crystallizing tempera-ture and high dielectric constant makes the Pb2Nb2O7 compound as the potential candidate for the application to thin film capacitors. In this paper we present a facile low temperature, high-pressure route for the crystallization of Pb2Nb2O7 thin films fabricated by metal–organic decom-position (MOD) method.

2. Experimental

In the present study, Pb2Nb2O7 thin ﬁlms were fabricated by MOD method. The starting materials lead 2-ethylhexanoate [Pb(C8H15O2)2], niobium ethoxide [Nb(OC2H5)4], with toluene as solvent mixed in stoichio-metric ratio in a glove box to prepare the precursors needed www.elsevier.com/locate/jpcs

Corresponding author. Tel.: +886 2 2365 1428; fax: +886 2 2362 3040. E-mail address:chlu@ntu.edu.tw (C.-H. Lu).

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for MOD method. The substrates were of the structure Pt/Ti/SiO2/Si. Pt layer acts as the bottom electrode while Ti layer is used to enhance the adhesion of Pt layer to the silicon substrate. The Pt bottom electrode exhibited (1 1 1) plane preferred orientation. Before coating, the substrates were cut into small chip of dimension of 1 cm 1 cm and then cleaned by RCA wet cleaning methods. The prepared precursor was spin-coated onto the cleaned substrates. Thin films were baked on the hot plate at 150 1C after coating in order to remove the excess of toluene solvent. Subsequently, the film with organic raw materials was pyrolyzed at 350 1C to form the amorphous phase. The spinning–baking–pyrolyzing cycle was repeated several times to obtain the proper film thickness. Thereafter, the amorphous films were annealed at the temperature range of 500–650 1C in flowing oxygen with different heating rates and soaking times.

In the designed process of altering crystallization, a high-pressure apparatus with an elaborate temperature controller was employed so as to change the process pressure with the temperature. A sealed stainless-steel (T316-SS) bomb was used as the autoclave container. The bottom of the bomb was ﬁlled with distilled water to produce a high vapor-pressure environment at elevated temperatures. The crystallization temperature of this developed process was adopted in the range of 220–300 1C with the heating rate of 10 1C/min. The composing phases of the synthesized ﬁlms were characterized by X-ray diffraction (XRD) at room temperature using a MAC Science MXP3 XRD system with Cu Ka radiation at 40 kV and 30 mA. The surface morphologies were analyzed by scanning electron micro-scopy (SEM) (Hitachi model S-800 microscope, 20 kV) and atomic force microscopy (AFM) using tapping mode with amplitude modulation (Nanoscope IIIa, Digital Instru-ments Company, Santababara).

3. Results and discussion

3.1. Crystallization of Pb2Nb2O7thin films annealed by rapid thermal annealing (RTA) process

The solution of prepared precursors was spin-coated onto the Pt/Ti/SiO2/Si substrates and then baked at 150 1C to remove the solvent. In order to burn out the organic species, the as-coated thin films were subsequently pyrolyzed at 350 1C for 30 min. By XRD analysis, there was no crystal structure found on the as-deposited thin films and the post-annealing was required for crystallizing Pb2Nb2O7 thin films. Fig. 1 shows the XRD patterns of Pb2Nb2O7thin films annealed at various temperatures for 10 min using RTA equipment. From

Fig. 1, it is evident that crystallized structure started to form at the temperature higher than 500 1C while the crystallization obtained was proved to be the phase of pyrochlore Pb2Nb2O7 by the comparison of JCPDS ﬁle no. 40-828. As the annealing temperature increased to 600 1C, the crystallization of Pb2Nb2O7thin ﬁlms was enhanced and only the diffraction peaks of Pb2Nb2O7 were detected. However, no apparent

improvement for the crystallization of Pb2Nb2O7thin film was observed as the annealing temperature rose to 650 1C. From the above results, the Pb2Nb2O7thin films were completely crystallized at the annealing temperature of 600 1C. There was no intermediate phase generated at the temperature below 500 1C, indicating the formation of crystallized Pb2Nb2O7thin films was nucleated directly from the amorphous precursor phase. The main diffraction peaks of these films were (2 2 2), (4 0 0), and (4 4 0). By comparing the intensity of diffraction peaks with those in JCPDS file, there was no special ori-entation existing in the crystallized Pb2Nb2O7thin films.

The SEM features of Pb2Nb2O7 thin films annealed at various temperatures are shown in Fig. 2(a). The micro-structure of amorphous thin films annealed at 450 1C was dense and flat and gradual increase in the grain growth was observed with increase in temperature up to 600 1C.

Fig. 2(a) shows the SEM features when the annealing temperature rose to 600 1C, showing a uniform and dense film feature consisting of well-developed small grains, and no apparent difference of microstructure was made by the subsequent increase of annealing temperature to 650 1C on the thin film with the morphology of small sphere (around 20–30 nm). Inset inFig. 2(a)shows the representative SEM features of a 550 1C annealed sample which shows the initial stage of Pb2Nb2O7film crystallization.

The surface morphology and ﬁlm roughness of

Pb2Nb2O7thin ﬁlms heated by RTA process was observed by AFM.Fig. 2(b)shows the representative AFM image of

20 25 30 35 40 45 50 2θ Intensity (222) (400) Pt Pt (440) (a) (b) (c) (d) (e) (f) Pb2Nb2O7

Fig. 1. Powder X-ray diffraction patterns of Pb2Nb2O7thin ﬁlms annealed by rapid thermal annealing at varied temperatures: (a) as-pyrolyzed, (b) 450 1C, (c) 500 1C, (d) 550 1C, (e) 600 1C, (f) 650 1C.

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Pb2Nb2O7thin film annealed at 600 1C. The Pb2Nb2O7thin films annealed at the rising temperatures above 600 1C had no variation in the surface morphology. In contrast with the results of high-pressure process, the surface morphol-ogy of Pb2Nb2O7thin films crystallized by RTA process depended less on the heating temperature. The similar subject on the relation between the crystallite size and the roughness of the thin films crystallizing by RTA process is also discussed. The crystallite size was calculated by the Debye–Scherrer formula. As observed by AFM, the roughness of Pb2Nb2O7 thin films increased from 0.40 to 1.56 nm as the annealing temperature rose from 500 to 650 1C. The crystallite size of Pb2Nb2O7thin films annealed at 500 1C was about 23.6 nm, and the small crystallite was attributed to the low heating temperature. However, the variation in crystallite size with the increase in annealing

temperature was opposite from the variation in roughness while the heating temperature rose above 550 1C. The phenomenon observed that the crystallite size decreased as the annealing temperature increased was due to the greater heating rate of RTA process. The heating rate of 30 1C/s induced the shortening of heating time from room temperature to the annealing temperature. By the transient heating progress, the assumption that no crystallite formed while process temperature increasing was reasonable.

The Pb2Nb2O7 crystallite nucleated after reaching the annealing temperature and the rise of annealing tempera-ture led to the increase of Pb2Nb2O7nuclei. Thus, the grain growth of Pb2Nb2O7 was inhibited by the increase of nuclei, and the crystallite sizes of Pb2Nb2O7thin ﬁlms were decreased with the increase of annealing temperature. The increase of roughness was not due to the increase of cry-stallite size but was ascribed to the increase of Pb2Nb2O7 grains that were crystallized by RTA process with the steep rise of the heating rates. The shortcomings of using RTA process is the small crystallite size and high-crystallization temperature which we tried to overcome by using high-pressure process described in the later section.

3.2. Crystallization of Pb2Nb2O7thin films under high-pressure condition

The high-pressure process was adopted to decrease the crystallization temperature and to increase the crystallite size of Pb2Nb2O7 thin films. The as-deposited thin films were placed in a high-pressure apparatus after being pyrolyzed at 350 1C for the removal of organic compo-nents.Fig. 3displays the XRD patterns of Pb2Nb2O7thin films crystallized at various temperatures within the range of 220–300 1C by high-pressure process. As the thin films heated at 220 1C with the process pressure of 300 psi, crystallization started on the Pb2Nb2O7 thin films. The obvious crystallization of Pb2Nb2O7 thin films was obtained as the heating temperature and process pressure rises to 260 1C and 624 psi. According to the diffraction pattern of Pb2Nb2O7 thin films heated at 280 1C and 624 psi, the crystallization of Pb2Nb2O7 thin films was enhanced with the increase of heating temperature and process pressure. The Pb2Nb2O7thin films crystallized by high-pressure process remained single phase even when the heating temperature and process pressure rose to 300 1C and 1150 psi. Similar to the RTA process, it could be seen that no intermediate phase was observed as thin films heated at the temperature range of 220–260 1C that Pb2Nb2O7started to crystallize, the phenomenon indicated that the formation of crystallized Pb2Nb2O7 thin films is nucleated directly from the amorphous phase. The main diffraction peaks of these thin films were (2 2 2), (4 0 0), and (4 4 0) without the occurrence of special orientation. The crystallinity of Pb2Nb2O7 thin films by high-pressure process coincided well with that of the RTA samples, though the crystallization temperature was reduced from 600 to 280 1C. 500 550 600 650 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Crystallite Size (nm) Mean Roughness (nm) Annealing Temperature (°C) 18 20 22 24 26 28 30 32 34

Fig. 2. (a) Scanning electron microscope images of Pb2Nb2O7on Pt/Ti/ SiO2/Si substrate annealed by rapid thermal annealing at 600 1C. Inset shows the representative SEM feature of Pb2Nb2O7by RTA process at 550 1C. (b) Temperature dependence of crystallite size and roughness as Pb2Nb2O7thin ﬁlms annealed by RTA process. Inset shows the atomic force microscope image of Pb2Nb2O7 thin ﬁlms crystallized by RTA process at 600 1C.

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Fig. 4(a)shows the SEM micrographs of Pb2Nb2O7thin films crystallized by high-pressure process. The microstruc-ture of amorphous thin films was flat without the observa-tion of grains. As the Pb2Nb2O7thin films heated at 220 1C with the process pressure of 300 psi, few special clusters of grains with an irregular shape were generated, the similar phenomenon was observed as crystallizing the Ta2O5 thin films by high-pressure process. As the heating temperature and process pressure increased to 260 1C and 624 psi, the number of clusters was obviously increased. With the in-crease in temperature to 280 1C at the process pressure of 864 psi, the surface of Pb2Nb2O7thin film was covered with the clusters of grains while the size of these clusters were around 100–200 nm. Special phenomenon that the cluster combined to form the larger one was observed as the heating temperature increased to 300 1C with the process pressure of 1150 psi. From the results obtained by XRD and SEM analysis, the clusters of grains are supported to be the crystals of Pb2Nb2O7, since the well-crystallized Pb2Nb2O7 thin films possess the microstructure of grains in clusters.

Fig. 4(b)illustrates the relation between the crystallite size and the roughness of Pb2Nb2O7thin films as adopting high-pressure process for thin film crystallization. The roughness of the thin films was observed by AFM analysis while the crystallite size of Pb2Nb2O7was calculated from the width at the half maximum of the (2 2 2) peak by the Debye–Scherrer formula. As shown in Fig. 4(b), the crystallite size of Pb2Nb2O7increased with the increase of heating temperature. As the crystallization temperature rose from 220 to 300 1C, the crystallite size increased from 46.0 to 48.9 nm though the difference of these crystallites size was merely around 6% in the temperature range of 80 1C. Moreover, the roughness of

thin films was also increased with the increase in the temperature, the mean roughness increased from 2.72 to 4.96 nm. In comparison with the SEM results, the increase of roughness was ascribed to the increased number of grain clusters that formed on the thin films with the rising of heating temperature. Furthermore, there were similar ten-dencies for both crystallite size and roughness to increase as the crystallization temperature increased. The increment of roughness could also be partially attributed to the enlarge-ment of crystallite size while adopting the high-pressure process for Pb2Nb2O7 thin film crystallization. Inset in

Fig. 4(b) shows the representative AFM image of surface morphology of Pb2Nb2O7 thin ﬁlms crystallized by high-pressure process at 300 1C. The scan area is 5 mm 5 mm. The Pb2Nb2O7 crystallites formed on the surface of ﬁlms

20 30 40 50 60 300°C 280°C 260°C 220°C Pt Pt (440) (400) (222) Intensity 2θ

Fig. 3. Powder X-ray diffraction analysis for the Pb2Nb2O7 thin ﬁlms crystallized by high-pressure process at varied heating temperatures.

(a) 220 240 260 280 300 3 4 5 6 Crystallite Size (nm) Mean Roughness (nm) Heating Temperature (°C) 45 46 47 48 49

Fig. 4. (a) Scanning electron microscope images of Pb2Nb2O7thin films crystallized by high-pressure process at 260 1C. Inset shows the micro-scopic features of the film processed at 300 1C. (b) Temperature dependence of crystallite size and roughness as Pb2Nb2O7 thin films crystallized by high-pressure process. Inset shows the atomic microscopic image of the sample processed at 300 1C.

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possessed the similar island structure that was also found as crystallizing the Ta2O5 thin ﬁlms [14]. As the heating temperature and pressure increased to 260 1C and 624 psi, denser morphology was obtained with the increase of island number and crystallite size. The size of island structure was enhanced obviously with the increase of temperature and pressure. The rougher surface of Pb2Nb2O7 thin ﬁlms was generated when the heating temperature reached 300 1C with the process pressure of 1150 psi. The rougher morphology observed was attributed to the combination of cluster as contrast to the SEM micrograph.

4. Conclusions

We have demonstrated a facile low temperature, high-pressure crystallization process of pyrochlore Pb2Nb2O7 comparing the well-known RTA with that of the high-pressure method. Powder X-ray patterns of Pb2Nb2O7 synthesized by both the methods show initial crystallization temperatures of 500 and 200 1C by RTA and high-pressure methods, respectively. SEM and AFM features reveal that the high-pressure method is superior in forming special large crystals at low temperatures compared to the small dense crystallites using RTA method. With the increase in temperatures, the rate of increase in crystallinity is high in case of high-pressure method, whereas RTA method is not highly temperature sensitive after certain temperature range. By adopting the high-pressure process, the starting tempera-ture for the crystallization was reduced from 600 to 220 1C. References

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