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Influence of thermal treatment on the properties of Ca-modified lead titanate thin films
ELSEVIER
Thin SolidFilms 279 (1996) 70-74
Influence of thermal treatment on the properties of Ca-modified lead titanate thin films F. Carmona, M.L. Calzada, E. R o m ~ , R. Sirera, J. Mendiola lnstituto de Oencia de Materiales, Serrano 144, 28006 Madrid, Spain
Received 19 December 1994;accepted 2 October 1995
Abstract Results are presented on the influence of thermal treatment on the final properties of Ca-modified lead titanate thin films, obtained by a solgel method and spin coating. X-ray diffractometry, electron microscopy, Auger spectroscopy, and electrical measurements have been used to characterize samples. Films obtained by rapidly heating ( > 500 °C rain- t ) the deposits show a lower than expected tetragonal distortion, but are free from pyrochlores and have better ferroelectric properties. Keywords: Dielectrics;Electricalproperties and measurements;Lead; Heat treatment
1. Introduction Ferroelectric thin films are attractive because of their potential application to non-volatile memories, due to their fast switching speeds, and to infrared detectors and surface acoustic wave devices [ 1,2]. Much work has been reported on the preparation of lead titanate thin films. However, the high coercive field of this material may limit its applicability. Ca-substituted lead titanate thin films should not show this drawback, because substitution leads to lower coercive fields without significantly decreasing remanence [ 3 ]. The preparation of thin films by sol-gel methods, depositing them by spin coating, has some advantages over other techniques: extensive and homogeneous films can be prepared with good stoichiometric and structural control [4]. Furthermore, there are now sol-gel methods with which it is possible to obtain, with a single deposition attd a single heat treatment, thicknesses of -- 0.5 p,m [5,6], thus overcoming the initial diffic'dty of the method, which required, for such thicknesses, successive depositions [7 ]. The films, as deposited, are amorphous and have to be thermally treated so as to obtain the desired crystalline structure. The nature of the treatment has a decisive influence on the crystallinity of lead titanate-based films [8]; and there seems to be unanimity as regards the advantages of rapid treatments, which minimize the formation of pyrochlore phases, that all too frequently go with the perovskite phase and spoil the final properties [9]. 0040-6090/96/$15.00 © 1996ElsevierScience S.A. All rights reserved SSD10040-6090 t "q ) 08 ! 40-2
It is the aim of this paper to contribute to what is known of the formation of Ca-substituted lead titanate films, obtained by a sol-gel method and deposited by spin coating, and, in particular, of the influence of thermal treatment on final properties. 2. Experimental method Precursor solutions for the deposition of Pbo.76Cao.24TiO3 thin films were prepared by a sol-gel method, described with some detail in Ref. [ 10]. Deposition was made by spin coating at 2000 rpm for 45 s on silicon wafers, of [ 100] orientation, previously sputtered at = 100 °C with the following conditions. I. An antidiffusion barrier [ I l ] of TiO2 of = 500 ,~ thickness. 2. APt layer of = 1 500 ~, to be used as a bottom electrode. Films thus deposited received, to begin with, a drying treatment at ~ 350 °C for I min. This produces evaporation of the solvent and partial pyrolysis of the organic compounds [ 10]. The final formation of the films was accomplished by a thermal treatment of one of these two types. I. Conventional. The sample is heated in the furnace at a moderate rate ( ~ l0 *(2 min- i ) to its maximum, maintained for ~ l h, and finally annealed or air-quenched down to room temperature. 2. Rapid heating treatment (RHT). The samp!e is rapidly introduced into the furnace, previously stabilized at the desired temperature, maintained for some minutes, and annealed or air-quenched down to room temperature.
71
F. Carmona et al. /Thin Solid Films 279 (1996) 70-74
VMAX .
.
.
.
.
.
.
.
.
.
.
.
o ......
.....
.
,,, _V_
....
.
.
.
.
.
.
.
.
.
.
.
.
.
.
IV ,5 ............
.....
)1 . . . .
"VMAX
Fig. I. Series of pulses applied to the samples after tracing the hysteresis loop. P*r and P , are the polarizations measured after the first and second positive pulses, respectively. The difference P ' r - P r measures the switching achieved with the first positive pulse.
The structure of the films was studied by X-ray diffraction (XRD), and the microtexture was examined by scanning electron microscopy (SEM). Compositional analysis, made by Auger spectroscopy, did not provide absolute values, but allowed us to determit~e how certain ratios varied through the films. Depth profile analysis by argon ion etching was performed in a commercial scanning Auger microscope. The electron gun was operated at a voltage of 5 keV, a current of 30 nA and a 10 Ixm diameter spot. Ion etching of the sample was performed with an Ar ÷ gun of 3 keV energy, 150 nA current, on a rastered area of 1 × 1 mm 2. Finally, films were characterized electrically with an RT66A Radiant ferroelectric testing device, designed to trace loops with an applied voltage of up to 20 V (which for -,, 0.5 ix thicknesses amounts to fields of up to 4 × 105 V cm- i ), and to measure the response of the films to pre-defined pulses (Fig. 1).
3. Results and discussion
Previous work [ 10] had shown differences between films obtained by conventional treatments and those obtained by RHT. 1. Tetragonal distortion (c/a) of films obtained by a conventional treatment was generally very similar to the expected value (,,, 1.038) for the nominal composition used; whereas for films obtained by RHT, values of c/a were clearly lower ( ,-, 1.025). 2. The presence of pyrochlores (intermediate products of a reaction with undesirable properties) was detected by XRD in films obtained by conventional treatments. 3. Films obtained by RHT showed better electrical properties. In order to further explore these differences a series of tests have been made. The first consisted of taking four samples from the same batch and treating them at 650 °C for 18 min: two of them rapidly introduced into the furnace, the other two heated at 10 °C min- i. From each pair one sample was airquenched and the other was annealed. Results, as regards tetragonal distortion, are summarised in Table 1. There is little difference between air-quenching or annealing, and the main difference is the heating rate. Another test was made so as to study in more detail the influence of the heating rate: four samples of the same batch were given conventional treatments with heating rates of 5,
Table ! Tetragonality (c/a) as a function of heat treatment Heating rate (°C rain- i )
l0
> 500
Annealed Air quenched
1.031 1.032
1.019 1.022
10, 20 and 40 °C min- ~, maintaining at 650 °C for 12 min, and air-quenching. Two additional samples from the same batch were treated by RHT. Results, as regards the quantity c/a, are given in Fig. 2, in which data have been included from other samples that, although not belonging to the same batch, should fit into the picture. The tendency to a decrease of c/a with heating rate seems clear. Different treatments produced different microtextures, as shown in Fig. 3. Average grain sizes, measured from the micrographs, for samples heated at 5, 40 and > 500 °C rain- i, were 60, 35 and 25 nm respectively. Diffractograms of samples that received RHT show line broadening in some peaks, due to the presence of a small grain size phase and strains. This, however, requires further and more detailed studies. The results of Auger spectroscopy have allowed us to evaluate composition inhomogeneities in the films. Values obtained by this technique are not absolute, for lack of an adequate standard. However, it is possible, for instance, to follow the evolution of the ratio Ca/Pb through the thickness of the sample. We thus have, in Fig. 4, values for two samples, one conventionally treated, the other subjected to RHT. A loss of lead is observed, in both cases, next to the surface and next to the substrate. A scale factor has been applied to the experimental values of Ca/Pb, so that the average composition agreed with the x
®
cla 1.03
TIME AT MAXIMUM TEMPERATURE 1.02
x
12 rain
+
18 min
0
48 rain
® 0
10
20
30 v('Clmin]
40
Fig. 2. Tetragonality (c/a) as a function of u (heating rate).
>500
F. Carmona et aL /Thin Solid Films 279 (1996) 70-74
72
Fig. 3. Microstmcture for different heating rates: ( a ) 5 °C rain- J; (b) 40 °C min - '; (c) > 500 °C min- *. o
"il
Pb/IPb.C,I
e
atomic ratio
01~
suRrtcE
,or\
.~3 , ,
,, .. ~
SUBSTRATE.74
.'/5
o .35
0
3O
® ~
'
I o,o
I o,1
I 0,2
I o,3
I I 0,4 o.s (t~ml Fig, 4, Ca/Pb atomic ratio in two samples of Pb,7~Ca,e4TiO3, x, distance to the surface of the film, ( ! ) Sample heated at 10 *C rain- '. (2) Sample heated at > 500"C rain- i, Curves are least-squares fits. Experimental values are only shown for curve 2, and correspond to Ca/Pb measurements, regularly spaced at ~ 50 nm from each other along the thickness,
nominal composition. Curves have been obtained by leastsquares fits. From the values of Ca/Pb, the Pb contents are easily calculated, such as those shown in the inset. These values should obviously be adequately corrected, because there are Pb losses; but in principle they constitute a good first approximation in order to quantify the level of inhomogeneity of composition in the films. In any case, total Pb losses are not expected to be higher than = 5% [ 12]. In Fig. 5 we have represented Pb contents next to the surface, around the centre of the film, and next to the substrate, for a number of films, as a function of heating rate. There is no significant variation with heating rate and a comparison is made via averages and standard deviations. Differences between the centre and the surfaces are meaningful. One cannot say, on the contrary, that there is any difference between Pb contents next to the surface and next to the substrate (levels of acceptance of the hypothesis that they are
equal are not surpassed in a Kolmogorov-Smirnov test with a=0.01). What presumably happens is that, when samples are heated at a low rate, the stability of pyrochlore phases, deficient in Pb, is enhanced [9], and this implies loss of PbO. This is the case of PbTi307, identified in some of our diffractograms. On the other hand, with rapid heating one passes rapidly over the temperature range at which pyrochlores are stable. This is confirmed by the fact that pyrochlores (and, namely, PhTi3OT) have been identified by XRD in films that received conventional treatments, but never in films that received RHT. The results of electrical measurements are, in turn, coherent with this. In Fig. 6 we have two hysteresis loops to represent two instances: a film that received conventional treatment (a), and a film obtained by RHT (b). Two features are worthwhile underlining. I. Both loops are displaced, and this suggests the existence of internal bias fields; but the displacement in a is much larger. 2. The polarizations involved in b are more important; but the difference is still larger if, instead of comparing the loops, we compare pulse responses: 8P, ffiP ' r - P is 2.5 I~C cm -2 in the first case and 10.9 p,C cm -2 for RHT.
0,80 12
.9
12
12
12
.,.J ~0.7S -
x
O
m--O'-- ~ O O
0.70
0
O'- .
.
.
x
.
.
'IP
.~ J.
x
*
10
I
I
20
I
I
30
I
I
40
S$ >500
v('Clmin) Fig. 5. Lead content of films as a function of heating rate. × , next to the surface; I"1, center of the film; O, next to the substrate. Dashed lines are averages, and vertical segments standard deviations.
F. Carmona et al. I Thin Solid Films 279 (1996) 70--74
saturation, and in sum higher polarizations. But variations in composition, sufficient to explain differences in c / a such as have been measured, would require PbO losses to be quite high, and this would imply meaningful quantities of free TiO2, that would show up by )~RD, and this is simply not the case. The reason for c / a values being lower than expected would rather have to be found in the strains arising when films are obtained by rapid heating [ 14].
clcm2 T
....
12 t
....
"J(
, ..~,oo/.'7 ~oo
a) ~
p.
,
kV/cm
Fig. 6. Hysteresis loops for two samples, treated with different heating rates: (a) 5 °C min-~; (b) >500°C min-'.
(6Pr),0.0s I~Clcmz 15-
10-
5x
x I
x ,I,
fo
x
20
30
60
# ,
>500
vl°Clminl Fig. 7. (BP,),9.gs, measuring the degree of switching achieved with Vmax= 19.95 V, as a function of heating rate.
EB kV/cm X x x
150 X
X
100
50 X
10
20
X
30 vl'C/min|
73
1(
40
Fig. 8. Bias fields, Ea, as a function of heating rate.
In Fig. 7 we have represented 8Pr against v (heating rate) for all the samples referred to in this paper. It turns out that the degree of switching achieved is systematically higher for samples obtained by rapid heating. This may be ascribed to the absence of pyrochlores in films thus treated. And it is also supported by the results of Fig. 8, in which the displacement of the hysteresis loops, or bias field En, has been represented as a function of heating rate. Again the samples appear clearly divided into two groups. Bias fields, that may be originated by the presence of pyrochlores and other defects [ 13 ], would thus be, according to our data, higher for lower heating rates. The fact that samples obtained by RHT show, also systematically, lower than expected c / a values could perhaps induce one to think that, in these samples, lead losses would have favoured the formation of Pb~ _ xCaxTiO3 with x > 0.24. This would imply lower coercive fields, a closer approach to
4. Conclusions Properties of Ca-substituted lead titanate thin films, obtained by a sol-gel process and deposited by spin coating, depend markedly on the thermal treatment used for their crystallization. It can be established that the heating rate is determinant as regards some of the final properties of the films. Tetragonal distortion, measured by the ratio c l a , decreases from values next to those expected for the nominal composition (conventional treatments), to values 2% lower for rapid heating treatments (RHT). Electrical measurements by a conventional device show conclusively that the degree of switching achieved is systematically higher for films obtained by RHT. This may not be ascribed to a high loss of PbO (and the formation of Caenriched titanates) because, if this were the case, pyrochlores and/or TiO2 would have been observed by XRD. In samples obtained by conventional treatments pyrochlores have been detected, and this is coherent with the fact that the displacements of the hysteresis loops, or the corresponding bias fields, are bigger for smaller heating rates. Auger spectroscopy shows that films are compositionally inhomogeneous in depth, and that there is less lead both next to the surface and next to the substrate. No meaningful differences have been measured, however, between surface and substrate; nor between films obtained by conventional treatments and those obtained by RHT. Therefore, loss of PbO takes place in both cases, rapid or slow heating rates, but in the second case it implies the formation of pyrochlores, detrimental for the final properties.
Acknowledgements The authors acknowledge the support of the Spanish projects Mat91-422 (CICYT) and C 120 / 91 (CAM).
References [ ! ] S.K. Dey and R. Zuleeg, Integrated sol-gel PZT thin films on Pt, Si,
and GaAs for non-volatile memoryapplications, Ferroe!ectrics, 108 (1990) 37-46. [2] C.A. Paz de Araujo and G.W. Taylor, Integrated ferroelectrics, Ferroelectrics, 116 ( 1991) 215-228. [3] J. Mendiola, B. Jim6nez, C. Alemany,L. Pardo and L. dei OImo, Influence of calciumon the ferroelectdcity of modified lead titanate ceramics,Ferroelectrics, 94 (1989) 183-188. [4] Y. Xu and J.D. Mackenzie,Ferroelectricthin filmsprepared by solgel processing,Integrated Ferroelectrics, 1 (1992) 17-42.
74
F. Carmona et al. / Thin Solid Films 279 (1996) 70-74
[5] S.J. Milne and S. Pyke, Modified sol-gel process for the production of PbTiO3 films, 3, Am. Ceram. Soc., 74 (1991) 1407. [6] N.J. Phillips, M.L.CalzadaandS.J. Milne, Sol-gel derived lead titar, ate films, J. Non-Cryst. Solids, 147-148 (1992) 285-290. [7] K.D. Budd, S.K. Dey and D.A. Payne, Sol-gel processing of PbTiO~, PbZaO3, PZT and PLZTthin films, Brit. Cerarn. Proc., 36 (1985) 107121. [8] S.A. Mansour, D.A. Binford and R.W. Vest, The dependence of ferroelectric and fatigue behaviors of PZT films on annealing conditions, Integrated Ferroelectrics, 1 (1992) 43-56. [9] H. Hu, L. Shi, V. Kumar and S.B. Krupanidhi, Rapid thermal annealing processed ferroelectric Pb(Zr,Ti)O3 films, Ceramic Transactions, Ferroelectric films, Vol. 25, 1992, pp. 113-120. [ 10] M.L. Calzada, F. Carmona and R. Sirera, Synthesis of multicomponent solutions from a sol-gel method applied to the preparation of
ferroelectric thin films, Proc. IV International Workshop on Noncrystalline Solids, Madrid (Spain), 1994, World Scientific Publishing Co.. Singapore (1995) 289-297. [ I l ] H.N. AI-Shareef, K.D. Gifford, S.H. Rou, P.D. Hren, O. Auciello and A.l. Kingon, Electrodes for ferroelectric thin films, Integrated ferroelectrics, 3 (1993) 321-332. [ 12] M. Klee, A. de Veirman, P. van de Weijer, U. Mackens and H. van Hal, Analytical study of the growth of polycrystalline titanate thin films, Philips J. Res., 47 (1993) 263-285. [ 13] S.B. Desu and I.K. Yoo, Electrochemical models of failure in oxide perovskites, lntegrated ferroelectrics, 3 (1993) 365-376. [ 14] J. Mendiola, M.L. Calzada, R. Sirera and P. Ramos, Structural and ferroelectric dependence on thermal treatment of modified lead titanate thin films, to be published.
Thin SolidFilms 279 (1996) 70-74
Influence of thermal treatment on the properties of Ca-modified lead titanate thin films F. Carmona, M.L. Calzada, E. R o m ~ , R. Sirera, J. Mendiola lnstituto de Oencia de Materiales, Serrano 144, 28006 Madrid, Spain
Received 19 December 1994;accepted 2 October 1995
Abstract Results are presented on the influence of thermal treatment on the final properties of Ca-modified lead titanate thin films, obtained by a solgel method and spin coating. X-ray diffractometry, electron microscopy, Auger spectroscopy, and electrical measurements have been used to characterize samples. Films obtained by rapidly heating ( > 500 °C rain- t ) the deposits show a lower than expected tetragonal distortion, but are free from pyrochlores and have better ferroelectric properties. Keywords: Dielectrics;Electricalproperties and measurements;Lead; Heat treatment
1. Introduction Ferroelectric thin films are attractive because of their potential application to non-volatile memories, due to their fast switching speeds, and to infrared detectors and surface acoustic wave devices [ 1,2]. Much work has been reported on the preparation of lead titanate thin films. However, the high coercive field of this material may limit its applicability. Ca-substituted lead titanate thin films should not show this drawback, because substitution leads to lower coercive fields without significantly decreasing remanence [ 3 ]. The preparation of thin films by sol-gel methods, depositing them by spin coating, has some advantages over other techniques: extensive and homogeneous films can be prepared with good stoichiometric and structural control [4]. Furthermore, there are now sol-gel methods with which it is possible to obtain, with a single deposition attd a single heat treatment, thicknesses of -- 0.5 p,m [5,6], thus overcoming the initial diffic'dty of the method, which required, for such thicknesses, successive depositions [7 ]. The films, as deposited, are amorphous and have to be thermally treated so as to obtain the desired crystalline structure. The nature of the treatment has a decisive influence on the crystallinity of lead titanate-based films [8]; and there seems to be unanimity as regards the advantages of rapid treatments, which minimize the formation of pyrochlore phases, that all too frequently go with the perovskite phase and spoil the final properties [9]. 0040-6090/96/$15.00 © 1996ElsevierScience S.A. All rights reserved SSD10040-6090 t "q ) 08 ! 40-2
It is the aim of this paper to contribute to what is known of the formation of Ca-substituted lead titanate films, obtained by a sol-gel method and deposited by spin coating, and, in particular, of the influence of thermal treatment on final properties. 2. Experimental method Precursor solutions for the deposition of Pbo.76Cao.24TiO3 thin films were prepared by a sol-gel method, described with some detail in Ref. [ 10]. Deposition was made by spin coating at 2000 rpm for 45 s on silicon wafers, of [ 100] orientation, previously sputtered at = 100 °C with the following conditions. I. An antidiffusion barrier [ I l ] of TiO2 of = 500 ,~ thickness. 2. APt layer of = 1 500 ~, to be used as a bottom electrode. Films thus deposited received, to begin with, a drying treatment at ~ 350 °C for I min. This produces evaporation of the solvent and partial pyrolysis of the organic compounds [ 10]. The final formation of the films was accomplished by a thermal treatment of one of these two types. I. Conventional. The sample is heated in the furnace at a moderate rate ( ~ l0 *(2 min- i ) to its maximum, maintained for ~ l h, and finally annealed or air-quenched down to room temperature. 2. Rapid heating treatment (RHT). The samp!e is rapidly introduced into the furnace, previously stabilized at the desired temperature, maintained for some minutes, and annealed or air-quenched down to room temperature.
71
F. Carmona et al. /Thin Solid Films 279 (1996) 70-74
VMAX .
.
.
.
.
.
.
.
.
.
.
.
o ......
.....
.
,,, _V_
....
.
.
.
.
.
.
.
.
.
.
.
.
.
.
IV ,5 ............
.....
)1 . . . .
"VMAX
Fig. I. Series of pulses applied to the samples after tracing the hysteresis loop. P*r and P , are the polarizations measured after the first and second positive pulses, respectively. The difference P ' r - P r measures the switching achieved with the first positive pulse.
The structure of the films was studied by X-ray diffraction (XRD), and the microtexture was examined by scanning electron microscopy (SEM). Compositional analysis, made by Auger spectroscopy, did not provide absolute values, but allowed us to determit~e how certain ratios varied through the films. Depth profile analysis by argon ion etching was performed in a commercial scanning Auger microscope. The electron gun was operated at a voltage of 5 keV, a current of 30 nA and a 10 Ixm diameter spot. Ion etching of the sample was performed with an Ar ÷ gun of 3 keV energy, 150 nA current, on a rastered area of 1 × 1 mm 2. Finally, films were characterized electrically with an RT66A Radiant ferroelectric testing device, designed to trace loops with an applied voltage of up to 20 V (which for -,, 0.5 ix thicknesses amounts to fields of up to 4 × 105 V cm- i ), and to measure the response of the films to pre-defined pulses (Fig. 1).
3. Results and discussion
Previous work [ 10] had shown differences between films obtained by conventional treatments and those obtained by RHT. 1. Tetragonal distortion (c/a) of films obtained by a conventional treatment was generally very similar to the expected value (,,, 1.038) for the nominal composition used; whereas for films obtained by RHT, values of c/a were clearly lower ( ,-, 1.025). 2. The presence of pyrochlores (intermediate products of a reaction with undesirable properties) was detected by XRD in films obtained by conventional treatments. 3. Films obtained by RHT showed better electrical properties. In order to further explore these differences a series of tests have been made. The first consisted of taking four samples from the same batch and treating them at 650 °C for 18 min: two of them rapidly introduced into the furnace, the other two heated at 10 °C min- i. From each pair one sample was airquenched and the other was annealed. Results, as regards tetragonal distortion, are summarised in Table 1. There is little difference between air-quenching or annealing, and the main difference is the heating rate. Another test was made so as to study in more detail the influence of the heating rate: four samples of the same batch were given conventional treatments with heating rates of 5,
Table ! Tetragonality (c/a) as a function of heat treatment Heating rate (°C rain- i )
l0
> 500
Annealed Air quenched
1.031 1.032
1.019 1.022
10, 20 and 40 °C min- ~, maintaining at 650 °C for 12 min, and air-quenching. Two additional samples from the same batch were treated by RHT. Results, as regards the quantity c/a, are given in Fig. 2, in which data have been included from other samples that, although not belonging to the same batch, should fit into the picture. The tendency to a decrease of c/a with heating rate seems clear. Different treatments produced different microtextures, as shown in Fig. 3. Average grain sizes, measured from the micrographs, for samples heated at 5, 40 and > 500 °C rain- i, were 60, 35 and 25 nm respectively. Diffractograms of samples that received RHT show line broadening in some peaks, due to the presence of a small grain size phase and strains. This, however, requires further and more detailed studies. The results of Auger spectroscopy have allowed us to evaluate composition inhomogeneities in the films. Values obtained by this technique are not absolute, for lack of an adequate standard. However, it is possible, for instance, to follow the evolution of the ratio Ca/Pb through the thickness of the sample. We thus have, in Fig. 4, values for two samples, one conventionally treated, the other subjected to RHT. A loss of lead is observed, in both cases, next to the surface and next to the substrate. A scale factor has been applied to the experimental values of Ca/Pb, so that the average composition agreed with the x
®
cla 1.03
TIME AT MAXIMUM TEMPERATURE 1.02
x
12 rain
+
18 min
0
48 rain
® 0
10
20
30 v('Clmin]
40
Fig. 2. Tetragonality (c/a) as a function of u (heating rate).
>500
F. Carmona et aL /Thin Solid Films 279 (1996) 70-74
72
Fig. 3. Microstmcture for different heating rates: ( a ) 5 °C rain- J; (b) 40 °C min - '; (c) > 500 °C min- *. o
"il
Pb/IPb.C,I
e
atomic ratio
01~
suRrtcE
,or\
.~3 , ,
,, .. ~
SUBSTRATE.74
.'/5
o .35
0
3O
® ~
'
I o,o
I o,1
I 0,2
I o,3
I I 0,4 o.s (t~ml Fig, 4, Ca/Pb atomic ratio in two samples of Pb,7~Ca,e4TiO3, x, distance to the surface of the film, ( ! ) Sample heated at 10 *C rain- '. (2) Sample heated at > 500"C rain- i, Curves are least-squares fits. Experimental values are only shown for curve 2, and correspond to Ca/Pb measurements, regularly spaced at ~ 50 nm from each other along the thickness,
nominal composition. Curves have been obtained by leastsquares fits. From the values of Ca/Pb, the Pb contents are easily calculated, such as those shown in the inset. These values should obviously be adequately corrected, because there are Pb losses; but in principle they constitute a good first approximation in order to quantify the level of inhomogeneity of composition in the films. In any case, total Pb losses are not expected to be higher than = 5% [ 12]. In Fig. 5 we have represented Pb contents next to the surface, around the centre of the film, and next to the substrate, for a number of films, as a function of heating rate. There is no significant variation with heating rate and a comparison is made via averages and standard deviations. Differences between the centre and the surfaces are meaningful. One cannot say, on the contrary, that there is any difference between Pb contents next to the surface and next to the substrate (levels of acceptance of the hypothesis that they are
equal are not surpassed in a Kolmogorov-Smirnov test with a=0.01). What presumably happens is that, when samples are heated at a low rate, the stability of pyrochlore phases, deficient in Pb, is enhanced [9], and this implies loss of PbO. This is the case of PbTi307, identified in some of our diffractograms. On the other hand, with rapid heating one passes rapidly over the temperature range at which pyrochlores are stable. This is confirmed by the fact that pyrochlores (and, namely, PhTi3OT) have been identified by XRD in films that received conventional treatments, but never in films that received RHT. The results of electrical measurements are, in turn, coherent with this. In Fig. 6 we have two hysteresis loops to represent two instances: a film that received conventional treatment (a), and a film obtained by RHT (b). Two features are worthwhile underlining. I. Both loops are displaced, and this suggests the existence of internal bias fields; but the displacement in a is much larger. 2. The polarizations involved in b are more important; but the difference is still larger if, instead of comparing the loops, we compare pulse responses: 8P, ffiP ' r - P is 2.5 I~C cm -2 in the first case and 10.9 p,C cm -2 for RHT.
0,80 12
.9
12
12
12
.,.J ~0.7S -
x
O
m--O'-- ~ O O
0.70
0
O'- .
.
.
x
.
.
'IP
.~ J.
x
*
10
I
I
20
I
I
30
I
I
40
S$ >500
v('Clmin) Fig. 5. Lead content of films as a function of heating rate. × , next to the surface; I"1, center of the film; O, next to the substrate. Dashed lines are averages, and vertical segments standard deviations.
F. Carmona et al. I Thin Solid Films 279 (1996) 70--74
saturation, and in sum higher polarizations. But variations in composition, sufficient to explain differences in c / a such as have been measured, would require PbO losses to be quite high, and this would imply meaningful quantities of free TiO2, that would show up by )~RD, and this is simply not the case. The reason for c / a values being lower than expected would rather have to be found in the strains arising when films are obtained by rapid heating [ 14].
clcm2 T
....
12 t
....
"J(
, ..~,oo/.'7 ~oo
a) ~
p.
,
kV/cm
Fig. 6. Hysteresis loops for two samples, treated with different heating rates: (a) 5 °C min-~; (b) >500°C min-'.
(6Pr),0.0s I~Clcmz 15-
10-
5x
x I
x ,I,
fo
x
20
30
60
# ,
>500
vl°Clminl Fig. 7. (BP,),9.gs, measuring the degree of switching achieved with Vmax= 19.95 V, as a function of heating rate.
EB kV/cm X x x
150 X
X
100
50 X
10
20
X
30 vl'C/min|
73
1(
40
Fig. 8. Bias fields, Ea, as a function of heating rate.
In Fig. 7 we have represented 8Pr against v (heating rate) for all the samples referred to in this paper. It turns out that the degree of switching achieved is systematically higher for samples obtained by rapid heating. This may be ascribed to the absence of pyrochlores in films thus treated. And it is also supported by the results of Fig. 8, in which the displacement of the hysteresis loops, or bias field En, has been represented as a function of heating rate. Again the samples appear clearly divided into two groups. Bias fields, that may be originated by the presence of pyrochlores and other defects [ 13 ], would thus be, according to our data, higher for lower heating rates. The fact that samples obtained by RHT show, also systematically, lower than expected c / a values could perhaps induce one to think that, in these samples, lead losses would have favoured the formation of Pb~ _ xCaxTiO3 with x > 0.24. This would imply lower coercive fields, a closer approach to
4. Conclusions Properties of Ca-substituted lead titanate thin films, obtained by a sol-gel process and deposited by spin coating, depend markedly on the thermal treatment used for their crystallization. It can be established that the heating rate is determinant as regards some of the final properties of the films. Tetragonal distortion, measured by the ratio c l a , decreases from values next to those expected for the nominal composition (conventional treatments), to values 2% lower for rapid heating treatments (RHT). Electrical measurements by a conventional device show conclusively that the degree of switching achieved is systematically higher for films obtained by RHT. This may not be ascribed to a high loss of PbO (and the formation of Caenriched titanates) because, if this were the case, pyrochlores and/or TiO2 would have been observed by XRD. In samples obtained by conventional treatments pyrochlores have been detected, and this is coherent with the fact that the displacements of the hysteresis loops, or the corresponding bias fields, are bigger for smaller heating rates. Auger spectroscopy shows that films are compositionally inhomogeneous in depth, and that there is less lead both next to the surface and next to the substrate. No meaningful differences have been measured, however, between surface and substrate; nor between films obtained by conventional treatments and those obtained by RHT. Therefore, loss of PbO takes place in both cases, rapid or slow heating rates, but in the second case it implies the formation of pyrochlores, detrimental for the final properties.
Acknowledgements The authors acknowledge the support of the Spanish projects Mat91-422 (CICYT) and C 120 / 91 (CAM).
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