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Ferroelectrics
ISSN: 0015-0193 (Print) 1563-5112 (Online) Journal homepage: https://www.tandfonline.com/loi/gfer20
Composition characterization of Pb(Zr,Ti)O3 (PZT) thin films prepared by pulsed laser deposition:
Ambient gas and substrate position effect
X. Y. Chen , J. M. Liu & Z. G. Liu
To cite this article: X. Y. Chen , J. M. Liu & Z. G. Liu (1999) Composition characterization of Pb(Zr,Ti)O3 (PZT) thin films prepared by pulsed laser deposition: Ambient gas and substrate position effect, Ferroelectrics, 232:1, 35-40, DOI: 10.1080/00150199908015767
To link to this article: https://doi.org/10.1080/00150199908015767
Published online: 09 Mar 2011.
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Composition Characterization of Pb(Zr,Ti)03 (PZT) Thin Films Prepared by Pulsed Laser Deposition: Ambient Gas and Substrate Position
Effect
X. Y. CHENa*, J. M. LIUab and Z. G. LIUa
aL,aboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, P, R. China and blnstitute
of Materials Research and Engineering, National University of Singapore, Singapore 119260
(Received December 8, 1998; In final form February 28, 1999)
Spatial distribution of the cations in pulsed laser deposition of Pb(Zr,Ti)O, thin films under ambient oxygen and argon was characterized by using the inductively-coupIed plasma method. The results show that Zr and Ti have similar spatial distribution under both oxygen and argon ambient, while the spatial distribution of Pb under the two ambient gases is very different. The Pb/(Zr+Ti) ratio under the two ambient gases decreases with distance from the target. The results observed here are interpreted by identifying the mechanism of ablation and evaporation in interaction between laser beam and target.
Keywords: pulsed laser deposition; composition: spatial distribution
INTRODUCTION
Thin films of Pb(Zr, Ti)O3 (PZT) are very potential for use in microelectronic devices such as memories, microsensors, and microactuators. Pulsed laser deposition (PLD) provides a convenient and powerful tool to grow PZT films and related devices. Often PLD is considered to be able to highly keep the
* Corresponding author, email: xychen @nju,edu.Cn [915]/35
36/[916] X. Y. CHEN et al.
stoichiometry of target in as-deposited films"]. However, it is still not easy to obtain a stoichiometric PZT film. In the most former studies, only the Pb deficiency in films was emphasized, and was attributed to preferential vaporization of Pb on substrate surface during the film growth and post- annealing due to high temperature of the In fact, the composition of PZT thin films deposited by PLD shows a complex dependence on the preparing conditionsf4]. In this article, we present a study on the spatial distribution of each cation in pulsed laser deposition of PZT thin films under either oxygen or argon ambient. In order to understand the intrinsic mechanism of PLD to determine or affect the stoichiometry of thin films, it is necessary to rule out the evaporation of the film elements from the substrate surface. In this work the substrates remained at room temperature during deposition. The compositions of the films were measured by the inductively- coupled plasma (ICP) method. This method allows us to obtain absolute amount of every element composed in the films, and thus provides an important way to study the dependence of composition on the preparing conditions.
EXPERIMENTAL
The experimental arrangement used in this work is plotted in Fig.1. Briefly, the output from a KrF excimer laser (248nm wavelength, -3011s FWHM, 5Hz) was focused onto a stoichiometric Pb(Zr043Ti057)03 target with 45"
incident angle and an energy density of -2.5.J/(cmzpulse). The substrate temperature was kept at room temperature. The samples were deposited under either ambient oxygen or argon with pressure of 27Pa. The deposition time for all films was 60min. Silicon wafers with area of 1x0.5cm2 were used as substrates and positioned at six sites labeled as Sl-S6, as shown in Fig.1. The distance between the irradiated spot on target and site S4 was -4cm.
The film was dissolved into hydrochloric acid after deposition. The concentration of each cation in the solution was measured by ICP method with accuracy better than 5%. The concentration of each cation multiplied by the solution volume gives the absolute amount of the cation in the PZT film.
target mapk window
FIGURE 1 . Setup for studying the spatial distribution of elements in PLD of PZT films.
3 00 C 2 8 0 -
0 -
E 2 6 0 -
1 -
- 2 4 0 -
2
2 2 0 :0 -
F
* 180-
a .
a 1 6 0 - 1 4 0 -
0.90
5
t k
0.801::::
I ' 0 5 0
i# 211 3 1 411 51 6 X
Position
FIGURE 2. Amount of Pb, Zr and Ti in PZT films deposited at different positions under ambient oxygen.
RESULTS
Figure 2 shows the amount of Pb, Ti and Zr in micro-mole and ratio of Pb/(Ti+Zr) in PZT films deposited at different positions in ambient oxygen.
38/[918] X. Y. CHEN et a1
The spatial distribution of Zr and Ti is similar to each other and shows a peak near position S5, while the amount of Pb decreases monotonously with the substrate-target distance. The Pb/(Zr-{ 'li) decreases rapidly with the target- substrate distance, with the value from 1.35 at site S1 to 0.6 at site S 6 .
When the deposition is performed in ambient argon, Zr, Ti and also Pb have a peaked spatial distribution with the peak position at site near S4, as shown in Fig.3. The ratio of Pb/(Zr+Ti) shows similar trend as that observed under ambient oxygen, but the value is nearer to 1 than that in case of oxygen.
0
G 2 5 0 - Hrn
-
.-
4f 2 0 0 - /'
1 5 0 - I'
1 0 0 -
-
1
J 1
1n ZD 3L 4# 5u BU
Posi tian
FIGURE 3 . Distribution of Ph, Zr and Ti as well Pb/(Zr+Ti) in PZT films when the deposition was carried out in ambient argon.
DISCCUSION
In order to explain the results obscrved hcre, it is necessary to consider the mechanisni of PLD. Gcnerally, the whole processes of PLD can be divided into three stages: (1) interaction between laser beam and target and formation of plume on the target surface, ( 2 ) transportation of the plume from the target to the substrate, and (3) deposition and growlh of ejected species on the substrate. The mechanism to determine and affect the spatial distribution of the cjected species can onl! exist in first or second stage under present conditions. I n the second stage, the ejected species can be scattered by
molecules of the ambient gas. 1 lowever, spatial distribution of Pb, Zr, and Ti can little be affected due to scattering between these cations and much lighter
0 2 molecule. So the mechanism to decide the spatial distribution of the cations observed here should exist in the first stage". 61.The target-laser interaction leads to some very complex processes taking place on irradiated target surface, such as melting, evaporation, and formation of a plume One process is the so-called ablation. It leads to the plume plasma being distributed spatially in form of cos"0 (n>4), where 9 is the angle with respect to the normal direction of the target surface[']. The spatial distribution of the ablated species is characterized by a peak along the target normal. The spatial distribution of Zr and Ti under ambient oxygen and Pb, Zr and Ti under ambient argon observed herc is the result of the ablation. The composition is homogeneous everywhere in the plume. So the ablation is favorable to get a stoichiometric film.
On the other hand, the temperature of the target surface can be up to -104d7. *I. At such high temperature. the evaporation on the target surface will occur and those volatile elements, e.g. Pb in PZT, will first evaporate.
Evaporation leads to the isotropic spatial distribution of the species, so the amount of the evaporated species arriving on the substrate decreases with the substrate-target distance. Thus, evaporation is disadvantageous for the stoichiometric films. The spatial distribution of Pb in ambient oxygen mainly originates from the mechanism.
In fact, the two mechanisms always co-exist, but under common conditions used in PLD, the ablation should be dominant. So the films prepared by PLD are roughly stoichiometric when the films do not include the volatile elements, e.g. YBaCuzO;, while they are deficient in volatile elements when films include the volatile elements, such as Pb in PZT, Li in LiNb03 etc.. The difference in spatial distribution of cations under different ambient gas is attributed to the fact that the ambient gas has a significant effect on the laser-target interaction"'". By comparing Fig.2 and Fig.3, it can be seen that the growth rate of PZT film is larger in ambient argon than that in ambient oxygen at most substrate positions, M hich supports the conclusion.
40/[920] X. Y. CHEN et a1
CONCLUSION
The dependence of compositicin of 1’7’1’ t h i n lilms prepared by PLD on the substrate position in amhicnt o\ygen and argon has been studied. The results show that the Pb in tilms prcpared in both ambient oxygen and argon is surplus at position near the raiget. nhile is badly deficient at site away the target under rhc preparing conditions uscd here. The nonstoichiornetry in films is attributed to the c.\qx)r.ition during thc interaction between target and laser beam.
Acknowledgments
The uork is suppurtcd by h’Aonal Natural Scientific Foundation of China (normal and special projects)
References
[ I ] T. Venkatesan, in Pulsed Lclser Deposirion of Thirz Films, edited by D. B. Chrisey and G.K. Hubler. (John Wiley & Sons, Tnc., New York, 1994), p313
[2] J.S. Honvitz, K.S. Grabowski, D.B. Chrisey, and R.E. Leuchtner, Appl. Phys. Lett. 59, 1565 (1991)
[31 G.C. Tyrrel. T.H. York. L.G. Coccia, and 1. W. Boyd, Appl. Surf: Sci. 96-98,769, 1996 [4] D. Roy and S.B. Krupanidhi, J . Muter: Res. 7,2521, (1992)
[5] J. Gonzalo. C.N. Afonso. and J. Pemre. Appl. Phys. Lett. 67, 1325, 1995 161 M.C. Foote. B.B. Jones. and B.D. Hunt et al., Physica C201, 147, 1992
(71 R. Kelly and Miotello. in Pulsed Laser Deposition of Thin Films, edited by D.B. Chri- sey and G.K. Hubler. (John Wiley & Sons, Inc., New York, 1994). p55
181 R.K. Singh and J. Narayan. Phys. Rev. B42,233, 1990
[9] D.B. Geohegan. in Pulsed Laser Deposition of Thin Films. edited by D.B. Chrisey and G.K. Hubler, (John Wiley & Sons, Inc., New York, 1994), p115
[lo] X.Y. Chen. in Ph.D. thesis (1998). Chap. 1. (in Chinese)
