Plasma-enhanced chemical vapor deposition of PbTiO3 thin films

November 2000

Materials Letters 46 Ž2000. 60–64 www.elsevier.comrlocatermatlet

Plasma-enhanced chemical vapor deposition of PbTiO 3 thin films Maosong Tong, Guorui Dai ) , Dingsan Gao Department of Electronic Engineering, Jilin UniÕersity, Changchun 130023, People’s Republic of China Received 15 April 2000; accepted 20 April 2000

Abstract Functional ceramic PbTiO 3 thin films have been prepared onto Si substrates by plasma-enhanced chemical vapor deposition ŽPECVD. technique at the substrate temperature of 1708C. Lead tetraethyl wPbŽC 2 H 5 .4 x, titanium tetrachloride ŽTiCl 4 ., and oxygen ŽO 2 . were used as precursors. The composition, structure and morphology of the thin films were investigated by means of X-ray fluorescence spectroscopy ŽX-FS., X-ray diffraction ŽXRD., X-ray photoelectron spectroscopy ŽXPS. and scanning electron microscopy ŽSEM. methods. q 2000 Elsevier Science B.V. All rights reserved. Keywords: PbTiO 3 ; Perovskites; PECVD technique; Thin films

1. Introduction Lead titanate ŽPbTiO 3 . is a well-known ferroelectric material showing remarkable pyroelectric and piezoelectric effects. There have been many proposals for application of its ceramic to electronic devices such as infrared sensor and piezoeelectroic transducer w1–4x, but few usable devices have been developed so far. Because large thickness of the ceramic requires high operating voltage, its small area is not adequate for large display devices and array sensors having many electrodes have difficulty in connecting with signal processing devices. Such problems can be overcome by thin film of this material w5x. Most of these thin films have been prepared by r.f. sputtering. But there are some disad-

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Corresponding author. Tel.: q86-431-8923189; fax: q86431-8923907. E-mail address: [email protected] ŽG. Dai..

vantages as follows, ŽI. low deposition rate, ŽII. generation of surface defects and damages of a substrate material by high deposition temperature, high-energy atoms, ions and molecules, ŽIII. stoichiometric change of the deposited film from source materials, and ŽIV. surface roughness of the film. Chemical vapor deposition ŽCVD. method can suppress these drawbacks compared with the sputtering method or the other deposition methods w6x. It should be noted that a conventional semiconductor plasmaenhanced chemical vapor deposition ŽPECVD. method of the deposited PbTiO 3 thin films has not been reported. Compared with the conventional CVD method, the PECVD technique of formation of the thin films have some other advantages, such as the low deposition temperature and simply control of deposition w7x. In the present work, we report for the first time the preparation of PbTiO 3 thin films by PECVD method at low temperature Ž1708C. using lead tetraethyl wPbŽC 2 H 5 .4 x and tin tetrachloride ŽSnCl 4 . reactants as the precursors.

00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 1 4 3 - 9

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2. Experimental procedure The thin film materials, PbTiO 3 , have been deposited onto the cleaned Ž111. surface of the single crystal silicon wafers by using PECVD technique. The schematic diagram of the PECVD reaction system is shown in Fig. 1. wPbŽC 2 H 5 .4 x and SnCl 4 reactants are used as the precursors, whose temperatures are y118C and y218C, respectively. The chamber was pumped down to approximately 0.52 Pa, then a desired amount of oxygen and precursor vapor were introduced into the chamber Žthe partial press ratio of PbŽC 2 H 5 .4 and TiCl 4 is 1.5:1. . The substrate temperature was maintained at 1708C during the thin film deposition. The radio frequency is 13.56 MHz with 150 W power. According to Mukherjee and Evan’s report w8x, oxygen may undergo the following reactions with electrons by the resonance capture process: O2 q e

™O

y 2

™OqO

Oy 2

y

Ž 1. at 4.53 eV net

Ž 2.

The PbTiO 3 thin films were deposited by the following plasma excite reaction: Pb Ž C 2 H 5 . 4 q TiCl 4 q 27 w O x

™ PbTiO q 8H O q 8CO q 4HCl. 3

2

2

Ž 3.

In order to optimize operating conditions, the effect of various parameters such as the radio fre-

Fig. 2. X-ray fluorescence spectra of the as-deposited thin film.

quency and power, the ration of PbŽC 2 H 5 .4 and TiCl 4 , the temperature of the substrates on the films preparation were examined. In general, the deposition rate of thin films is about 20 nmrmin under the above mentioned typical deposition conditions. For the analysis, the thin films were deposited onto the cleaned Si wafers and annealed at different temperature. The structure and crystal state of the thin films were determined by a Rigaku DrMAX-RA X-ray diffractometer, and the X-ray source was CuK a radiation. The composition and oxidizing state of the thin films were investigated using X-ray fluorescence spectroscopy ŽX-FS. and X-ray photoelectron spectroscopy ŽXPS., which were carried out on a VRA-20 X-FS spectrometer and ESCALAB MARK-II XPS spectrometer, respectively. Particle size and morphology of the thin films were deter-

Fig. 1. Experimental set-up for PECVD.

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3. Results and discussion

Fig. 3. XRD patterns of the thin films annealed at different temperatures for 5 h, Ža. as-deposited thin films Ž1708C., Žb. 4008C and Žc. 6008C.

mined using a Hitachi ŽX-650. scanning electron microscopy ŽSEM..

Fig. 2 shows the X-ray fluorescence spectra of the as-deposited thin film. Two peaks of 33.93 and 86.04 cmy1 can be observed in the figure, which are assigned to Pb 2q and Ti 4q, respectively. It indicates that the Pb 2q and Ti 4q are presenting in the as-deposited thin films. Fig. 3 shows the X-ray diffraction ŽXRD. patterns for the as-deposited thin film and the films annealed at 4008C and 6008C for 5 h. The figure shows that the XRD pattern of the as-deposited thin films Žabout 1708C. displays only very weak peaks, indicating an amorphous state or the presence of very small crystallites ŽFig. 3Ža... For the specimens annealed at different temperature, the relative intensities of the peaks in the diffraction patterns increase with increasing annealing temperature ŽFig. 3Žb. and Žc... Comparing these data with Powder Diffraction File

Fig. 4. X-ray photoelectron spectra of the thin films annealed at 6008C for 5 h, Ža. peak of O1s, Žb. peaks of Pb4f 7r 2 and Pb4f 5r2 and Žc. peaks of Ti2p 3r 2 and Ti2p1r2 .

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Fig.5. Scanning electron photomicrographs of PbTiO 3 thin films annealed at different temperature for 5 h, Ža. 4008C and Žb. 6008C.

Card Index, the characteristic peaks of the diffraction pattern of the thin films annealed at 6008C for 5 h can be assigned to PbTiO 3 Ž100., Ž101., Ž110., Ž111., Ž200. and Ž211. crystal planes. The deposited thin films are found to be composed of PbTiO 3 with a perovskite-type crystalline structure. No other crystalline phases can be observed in the diffraction patterns ŽFig. 3Žc... Fig. 4 shows the X-ray photoelectron spectra of the thin films annealed at 6008C for 5 h. As shown in Fig. 4Ža., the peak of O1s is found to lie at 530.3 eV, and it is assigned to lattice oxygen in PbTiO 3 . As shown in Fig. 4Žb., the photoelectron peaks of Pb4f 7r2 and Pb4f 5r2 are found at 138.7 and 143.8 eV, respectively, and they are attributed to lead lattice in a PbTiO 3 crystal. As shown in Fig. 4Žc., photoelectron peaks of Ti2p 3r2 and Ti2p1r2 are found to lie at 458.7 and 464.6 eV, respectively, and they are assigned to the lattice titanium in PbTiO 3 crystal. Fig. 5 shows the scanning electron micrographs of the PbTiO 3 thin films annealed at 4008C ŽFig. 5Ža.. and 6008C ŽFig. 5Žb.. for 5 h. It was found that the surfaces of the obtained thin films are very smooth, and the particle size is quite different for films annealed at different temperature. The mean particle size of PbTiO 3 annealed at 4008C and 6008C for 5 h are about 30 and 600 nm, respectively. These results

indicate that the particle size increases with increasing in annealing temperature. 4. Conclusion In summary, lead titanate thin films have been prepared by using PECVD technique from lead tetraethyl wPbŽC 2 H 5 .4 x, tin tetrachloride ŽSnCl 4 . reactants and oxygen ŽO 2 .. The X-FS result indicates that the Pb 2q and Ti 4q were in the as-deposited thin films. The XRD and XPS results indicate that the PbTiO 3 crystalline phase was formed above 4008C. The SEM result shows that the mean particle size of PbTiO 3 annealed at 4008C for 5 h is about 30 nm and increases with increasing in annealing temperature. When a satisfactory regime has been established, further experiments will be carried out to investigate the effect of the partial pressure ratio of the reactants, the temperature of the reactants and the substrates, and the ratio frequency on the reaction mechanisms, film growth and electrical properties. Acknowledgements This work was supported by the Natural Science Foundation of China ŽNo. 69776038..

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