- Email: [email protected]
High-energy ion beam analysis of ferroelectric thin films
applied
surface science Applied Surface Science 117/l
18 (1997) 453-458
High-energy ion beam analysis of ferroelectric thin films Michio Watamori a3*, Shin-ichi Honda a, Osamu Kubo a, Isaku Kanno b, Takashi Hirao ‘, Kaoru Sasabe d, Kenjiro Oura a a Department of Electronic Engineering, Faculty of Engineering, Osaka University, 2-l Yamadaoka, Suita, Osaka 565, Japan b Human Environment Research Laboratory, Matsushita Electric Industrial Co., Ltd., 3-4 Hikaridai, Seikacho, Sorakugun, Kyoto 619-02, Japan ’ Matsushita Techno Research Co., Ltd., Yagumonakamachi, Moriguchi, Osaka 570, Japan ’ Ion Engineering, Center Corporation, Tsuda, Hirakata, Osaka 573.01, Japan
Abstract The composition and crystalline quality of ferroelectric thin films formed on Pt covered MgO(100) substrates have been investigated with combined use of ‘%(a, (Y)‘~O 3.045 MeV resonant backscattering and channelling techniques. Oxygen depth profiling has been estimated by the resonant backscattering, and crystalline quality was estimated by the chanfelling method. The ferroelectric thin films of PbTiO, @TO) and La-modified PTO (Pb,La, _ ,TiO,; PLT) ( y = 3) of 750 A thick were formed on Pt covered MgO substrates by multi-target ion beam sputtering method with introducing oxygen gas. Pt thin films beneath the film due to the under-electrode were also fabricated by the same method. For both PTO and PLT films, depth profiling of Pb and Ti elements showed relatively constant concentrations for whole film thicknesses, whereas the oxygen concentrations varied drastically. The extent of variation of oxygen concentration was larger for the PTO film than for the PLT film. This led to better crystalline quality of PLT, compared with PTO. The effect of oxygen depth profiling and crystalline quality on the electric properties is also discussed. Keywords: Backscattering
and channeling;
Oxygen beam; PbTiO,;
1. Introduction Recently, ferroctectric thin films have attracted considerable attention since their ferroelectric and dielectric properties are suitable for various applications such as memory, sensor and optical devices [ 1,2]. So far, a variety of dielectric thin films have been fabricated in order to obtain similar properties as bulk ceramics. For the fabrication of Pb-based
* Corresponding author. [email protected].
Fax:
0169.4332/97/$17.00 Copyright PIZ SO169-4332(97)00082-2
+81-6-8797780;
e-mail:
wata-
PbLaTiO,
dielectric thin films, remarkable progress has been made in recent years, and the epitaxial thin films of PbTiO, (PTO), PbZr, _ xTi,O, (PZT) and Pb,La,_,TiO, (PLT) could be grown on the single crystal substrates. On the other hand, growth of the dielectric superlattices is of great interest because of the exploration of new functional dielectric materials. Furthermore, it is expected that the characterization of the dielectric superlattices provides some microscopic understanding of the origin of the excellent ferroelectricity. To understand the origin of the electric properties fully, or to control it completely, characterization of
0 1997 Elsevier Science B.V. All rights reserved.
454
M. Watamori et al./Applied
Surface Science X17/118
the film atomically must be needed. It is often said that the oxygen is a key material to obtain deep understanding and to control the electric properties due to its reactivity, though it will be difficult to accurately obtain the oxygen information. In this work, we have investigated the oxygen depth profiling and crystalline quality for the PTO, PLT and both incorporated multi-layer ([PTO],/[PLT] >, films with use of newly developed 160(~,n)‘0 3.045 MeV resonant backscattering and channelling methods. The resonant backscattering is a rapid, quantitative and depth-sensitive. Moreover, oxygen detection sensitivity was enhanced, compared with normal Rutherford backscattering spectrometry (RBS) [3]. The method is suitable to analyze these kinds of oxide films [4-61. We discuss the effect of oxygen depth distribution on the film crystalline quality, and in addition, the effect of them on the electrical properties. The role of Pt films at the interface region for the film crystalline quality was also investigated.
2. Experimental Experiment was performed in a high-vacuum chamber coupled to a Tandetron accelerator which produces He+ or He2+ ion beams collimated to 1 mm diameter over the range of 0.6-5.1 MeV. The chamber contains two SSDs for energy analysis of the scattered ions and a sample manipulator which has three-axis linear motion drive and three orthogonal and independent axes of rotation. Each rotation for the three axes can be controlled by a stepping motor with an accuracy of 0.05”. The scattering angles for incident beams to SSD are fixed at 100” and 170”. Other relevant experimental parameters are the resolution of SSD of 20 keV and an analysis beam current of l-5 nA. More detailed information has been described elsewhere [3,7]. The films were fabricated by the multi-ion-beam sputtering system. The substrates used were (lOO)Pt-covered (100) MgO single crystals which were heated at 400°C during the film deposition. Prior to the film deposition, Pt films was coated by the same sputtering method as an under-electrode. For the deposition of PTO and PLT films, Pb, La and Ti metal targets were simultaneously sputtered
(1997) 453-458
using individual Arf ion beams which were accelerated at 1100 V. Film composition was controlled by Ar+ ion beam currents. Oxygen gas was introduced near the substrate to oxidize the sputtered species. More detailed information of the film fabrication has been described elsewhere [8,9]. The film thickness was monitored by a quartz film thickness monitor. The structure of the films was examined by X-ray diffraction (XRD) measurements using CuK (Yradiation, and every film showed (001) structure (c-axis oriented film). The electric properties of the films were measured with gold top electrodes of 0.3 mm in diameter deposited on the films. The dielectric constants of the films were measured by an impedance analyzer, and the P-E hysteresis loops were observed using a SawyerTower circuit. The computer simulation code we used to estimate the compositional depth profiling from the resonant-RBS spectra is Gisa 3.99 developed by Saarlahti and Rauhara [lo]. In this program, we can adjust the oxygen scattering cross section in the ‘60(a,(r)‘60 3.045 MeV resonance using the standard sample, and we obtained suitable oxygen cross section set in our experimental geometry, using standard samples of TiO, and a 6039 A thick SiO, film.
3. Results and discussion To estimate depth profiling of oxygen concentration, the resonant backscattering method was adjusted and applied. Correction of the method and application to thin oxide films have been described in detail elsewhere [ 111. The result of depth profiling is obtained by comparison between a raw resonantRBS spectrum and a simulated spectrum produced by Gisa 3.99 code. It is more convenient for multi elemental films such as PTO and PLT to use the simulation code than to extract each elemental signal intensity from a raw spectrum and estimate the composition. But there should be paid more attention to estimate the film composition by comparison with the computer simulation, because extent of fitting strongly depends on one’s feeling. In these PTO and PLT cases, we can distinguish oxygen concentration of 3.0 and 3.1 when lead concentration is defined as 1.0. In addition to the error from RBS spectrum
455
M. Watamori et al. /Applied Surface Science 1 I7/ I18 (1997) 453-458
3060keV4He+ +
PbTiOr(750A)/Pt(l OO)/Mgo (8=170°
F5
simulation
-
42
experiment
-*-
s 0 .F
)
F
306Oke@He+ +
(8=170”
)
(Pb--80%,La--20%)TiOrC150~)/Pt(l00)/MgO
simulation
-
experiment
*. -
P 0
1000
,j 2000
3000
Energy[KeVj
Energy[KeV]
Fig. 1. Typical resonant backscattering spectra for 3.06OMeV helium ions incident on the PbTiO,/Pt/MgO (a) and the Pb,La, _,TiO,/Pt/MgO (b). Each elemental signal intensity starts from each arrow point. Dotted points indicate the experiment and the solid lines indicate simulation results.
itself, we can say that the oxygen concentration can be estimated within an error of 10%. A more detailed discussion will be described elsewhere [12]. Fig. 1 shows typical resonant backscattering spectra for 3.060 MeV helium ions incident on a and PbTiO,/Pt/M gO(OO1) Pb,La,_,TiO,/Pt/MgO(OOl) (x ;=:0.8, y = 3). Film thicknesses of both samples were about 750 A. Each elemental signal intensity starts from each arrow point, shown in Fig. 1. The result of the com-
puter simulation is also plotted as a solid line. From resonant-RBS spectra with successive changes of incident energies, depth distributions of each element could be obtained, shown in Fig. 2. Fig. 2a is the depth distribution for the PTO, and Fig. 2b is for the PLT. The energy ranges of the helium ions for this depth profiling were from 3.045 MeV to 3.105 MeV for 20 keV steps. As can be seen in Fig. 2, depth profiling of Pb and Ti elements showed a relatively constant concentration for the whole film thick-
Depth Profiling of PbTiOY
Depth Profiling of PbxLa,_xTiOY
Oxygen
0
200
600
400
800
0
200
600
800
W
(a) Fig. 2. Depth distribution of each concentration varied extremely for the film thickness.
400
DEPTH(A)
DEPTH(A)
for tbe PbTiO,/Pt/MgO
(a) and the Pb,La,
_ xTiO,/Pt/MgO
(b). Oxygen concentration
456
M. Watamori et al./Applied
Surface Science 117/118
nesses, whereas the oxygen concentrations varied drastically. The extent of oxygen composition of about 7.5 for the PTO film feels extremely large. But for other oxide films having relatively resemble structures such as SrTiO, [4] and YBa,Cu,O, [l l] thin films these compositions were relatively stoichiometry, this oxygen concentration might be real. This might indicate the difficulty of fabrication of oxide films having correct stoichiometry. The extent of variation of oxygen concentration was larger for the PTO film than for the PLT film. This might be due to the effect of pinning of oxygen atoms in the structure, induced by La addition. It is noted that the RBS method also counts non structural oxygen atoms in addition to oxygen atoms in the lattice. As described later, a high minimum yield value of oxygen might be due to these non-structural oxygen atoms. In Fig. 2, strong oxygen deficiency was shown for both the PTO and PLT films. Though the reason is not yet clear, at the initial stage of film fabrication, not the epitaxial films but the amorphous metal films might be formed on the Pt covered substrate. Fig. 3 shows angular scans through the (001) direction of the films for the Pb and Ti elements, where the Pb and Ti signals are integrated for almost whole film thickness. Fig. 3a is for the PTO film, and Fig. 3b is for the PLT film. An angular scan of the under-electrode of Pt element was also plotted in each figure. The crystalline quality of the PTO and PLT films is reflected in the small value of the
c
Dip Curve of PbTiOy film
(1997) 453-458
minimum yield, xti,, defined as the ratio of the backscattering yields for perfect alignment along the major axis for random incidence [71. The xti, values of Pb and Ti show the crystalline quality for the whole film thickness. Using the resonant RBS-channelling method, the crystalline quality of the oxygen component can be estimated from the oxygen xti, value as a nearly same manner [13]. In this case, the integrated area for the oxygen0 element is a subsurface region of about 200-300 A, where the 3.045 MeV 160(a,a)160 resonance takes place for 3.060 MeV He+ ion incidence. It can be seen in Fig. 3, that xti, values of Pb, Ti, 0, and Pt elements for the PTO film were 0.38, 0.44, 0.78, and 0.24, respectively. On the other hand, xti, values of Pb, Ti, 0, and Pt elements for the PLT film were 0.27, 0.24, 0.70, and 0.22, respectively. For both films, the ,ymin values of Pb and Ti were nearly the same, though the xmin value of oxygen was extremely bigger than other xti, values. Obviously, this might be due to in-stoichiometric composition of oxygen element in the films. Moreover, it has been found that the crystalline quality of under coating films such as Pt thin films was better than that of ferroelectric films when the film was fabricated by our multi-ion-beam sputtering method. It can be said that the crystalline quality of the under-coating Pt film is very important, to fabricate good quality thin ferroelectric films. From the result of the xmin values of Pb element
I
I-
Dip Curve of PbXLa,+TiOy
film
(W Fig. 3. Angular scans of the PbTiO,/Pt/MgO film (a) and the Pb,La, _xTiO,/Pt/MgO curves for Pb, Ti and 0 of films and for Pt of under-coated films are shown.
film (b) through the (001) axis. The angular yield
M. Watamori et al. /Applied
Dip Curve
of PbTiOy, and
Surface Science 1 I7/
118 (1997) 453-458
451
was intermediate between value of ([PLT]/[PTO]) those of PTO and PLT. This means that the extent of the crystalline quality of the ([PLT]/[PTO]) film was intermediate between two films of PTO and PLT. To fabricate more excellent multi-layer ferroelectric films, it is important to improve the crystalline quality of each ferroelectric film.
PbxLa,_XTiOy
[(PLT) / @‘TO)] films Pb element
4. Conclusion
01 -2’
I
I
I
I
-1
0
1
2
I
ANGLE (DEG.) Fig. 4. Angular scan of the PbTiO,/Pt/MgO, the Pb,La, _,TiO, /Pt/MgO and the multi-layer ([PLTI,, /[PIOl,a), films through the (001) axis. The minimum yield values ( x,,,~,) arc 38%, 27% and 34%, respectively.
the PTO and PLT films, the crystalline quality of the PLT film was found to be better than that of the PTO film, shown in Fig. 3. This is in accordance with the result of XRD analysis, i.e., slight narrower peak width of (001) PLT peak was obtained compared with that of PTO. The dielectric constant of the PLT film was about 200, and the dielectric constant of the PTO film was about 100. La modulation caused the enhancement of the dielectric constant about the extent of twice. This is usually explained as the effect of buried charge [ 1,2]. In our experiment, this does not imply deterioration of the crystalline quality. Probably, La addition might have somewhat the effect of flattening of oxygen depth profiling. But, it is not yet clear. Finally, we have investigated the crystalline quality of the multi-layer ([PTO],/[PLT],),, thin film on the (100)Pt covered MgO( 100) substrate fabricated by the same sputtering method. Multi-layer ferroelectric films are very important because of some possibility to explore new functional dielectric materials. Fig. 4 shows an angular scan of Pb signal intensities through the (001) direction of the films for the PTO, PLT and ([PTO],/[PLT],), films. The numbers of layers and periodicity of the films were 10 layers of PLT and 10 layers of PTO and 7 times repeated (notation is ([PLT],,/[PTO],,),). The film thickness was about 600 A. As can be seen in Fig. 4, the xmin
for
The composition and crystalline quality of ferroelectric thin films formed on Pt covered MgO(100) substrates have been investigated with combined use of 160( (Y,(Y,)I60 3.045 MeV resonant backscattering and channelling techniques. For both PTO and PLT films, depth profiling of Pb and Ti elements showed relatively constant concentrations for whole film thicknesses, whereas the oxygen concentrations varied drastically. The extent of variation of oxygen concentration was larger for the PTO film than for the PLT films. This led to better crystalline quality of PLT, compared with PTO. Since the present method is simple and nondestructive one, it can be applied to the analysis of other ferro- and di-electric thin films.
Acknowledgements The authors would like to acknowledge the VTT, Technical Research Center of Finland for providing the RBS simulation code of Gisa 3.99. Part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. This work was carried out at the Ion Beam Surface Analysis Facility of Osaka University funded by the Ministry of Education, Science, Sports and Culture, Japan.
References [l] J.F. Scott, C.A. Paz de Araujo, Science 246 (1989) 1400. [2] R. Takayama, Y. Tomita, K. Iijima, I. Ueda, J. Appl. Phys. 61 (1987) 411. [3] M. Watamori, F. Shoji, K. Oura, Jpn. J. Appl. Phys. 33 (1994) 6039.
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M. Watawwriet al. /Applied Surface Science 1 I7/ I18 (1997) 453-458
[4] M. Watamori, K. Oura, T. Nakamura, J. Vat. Sci. Technol. A 13 (1995) 1293. [5] S. Honda, A. Tsujimoto, M. Watamori, K. Oura, Jpn. J. Appl. Phys. 33 (1994) L1257. [6] S. Honda, A. Tsujimoto, M. Watamori, K. Oura, Jpn. J. Appl. Phys. 34 (1995) L1386. [7] M. Watamori, F. Shoji, Y. Bando, T. Terashima, K. Oura, Jpn. J. Appl. Phys. 32 (1993) 42. [8] I. Kanno, S. Hayashi, M. Kjtagawa, R. Takayama, T. Hirao, Appl. Phys. Lett. 66 (1995) 145.
[9] I. Kanno, S. Hayashi, R. Takayama, T. Hirao, Appl. Phys. Lett. 68 (1996) 328. [lo] J. Saarilahti, E. Rauhala, Nucl. Instr. Meth. B 64 (1992) 734. [ll] M. Watamori, F. Shoji, K. Oura, Jpn. J. Appl. Phys. 33 (19941 6039. [12] M. Watamori et al., in preparation. [13] M. Watamori, K. Oura, T. Hirao, K. Sasabe, Nucl. Instr. Meth. B 118 (1996) 233.
surface science Applied Surface Science 117/l
18 (1997) 453-458
High-energy ion beam analysis of ferroelectric thin films Michio Watamori a3*, Shin-ichi Honda a, Osamu Kubo a, Isaku Kanno b, Takashi Hirao ‘, Kaoru Sasabe d, Kenjiro Oura a a Department of Electronic Engineering, Faculty of Engineering, Osaka University, 2-l Yamadaoka, Suita, Osaka 565, Japan b Human Environment Research Laboratory, Matsushita Electric Industrial Co., Ltd., 3-4 Hikaridai, Seikacho, Sorakugun, Kyoto 619-02, Japan ’ Matsushita Techno Research Co., Ltd., Yagumonakamachi, Moriguchi, Osaka 570, Japan ’ Ion Engineering, Center Corporation, Tsuda, Hirakata, Osaka 573.01, Japan
Abstract The composition and crystalline quality of ferroelectric thin films formed on Pt covered MgO(100) substrates have been investigated with combined use of ‘%(a, (Y)‘~O 3.045 MeV resonant backscattering and channelling techniques. Oxygen depth profiling has been estimated by the resonant backscattering, and crystalline quality was estimated by the chanfelling method. The ferroelectric thin films of PbTiO, @TO) and La-modified PTO (Pb,La, _ ,TiO,; PLT) ( y = 3) of 750 A thick were formed on Pt covered MgO substrates by multi-target ion beam sputtering method with introducing oxygen gas. Pt thin films beneath the film due to the under-electrode were also fabricated by the same method. For both PTO and PLT films, depth profiling of Pb and Ti elements showed relatively constant concentrations for whole film thicknesses, whereas the oxygen concentrations varied drastically. The extent of variation of oxygen concentration was larger for the PTO film than for the PLT film. This led to better crystalline quality of PLT, compared with PTO. The effect of oxygen depth profiling and crystalline quality on the electric properties is also discussed. Keywords: Backscattering
and channeling;
Oxygen beam; PbTiO,;
1. Introduction Recently, ferroctectric thin films have attracted considerable attention since their ferroelectric and dielectric properties are suitable for various applications such as memory, sensor and optical devices [ 1,2]. So far, a variety of dielectric thin films have been fabricated in order to obtain similar properties as bulk ceramics. For the fabrication of Pb-based
* Corresponding author. [email protected].
Fax:
0169.4332/97/$17.00 Copyright PIZ SO169-4332(97)00082-2
+81-6-8797780;
e-mail:
wata-
PbLaTiO,
dielectric thin films, remarkable progress has been made in recent years, and the epitaxial thin films of PbTiO, (PTO), PbZr, _ xTi,O, (PZT) and Pb,La,_,TiO, (PLT) could be grown on the single crystal substrates. On the other hand, growth of the dielectric superlattices is of great interest because of the exploration of new functional dielectric materials. Furthermore, it is expected that the characterization of the dielectric superlattices provides some microscopic understanding of the origin of the excellent ferroelectricity. To understand the origin of the electric properties fully, or to control it completely, characterization of
0 1997 Elsevier Science B.V. All rights reserved.
454
M. Watamori et al./Applied
Surface Science X17/118
the film atomically must be needed. It is often said that the oxygen is a key material to obtain deep understanding and to control the electric properties due to its reactivity, though it will be difficult to accurately obtain the oxygen information. In this work, we have investigated the oxygen depth profiling and crystalline quality for the PTO, PLT and both incorporated multi-layer ([PTO],/[PLT] >, films with use of newly developed 160(~,n)‘0 3.045 MeV resonant backscattering and channelling methods. The resonant backscattering is a rapid, quantitative and depth-sensitive. Moreover, oxygen detection sensitivity was enhanced, compared with normal Rutherford backscattering spectrometry (RBS) [3]. The method is suitable to analyze these kinds of oxide films [4-61. We discuss the effect of oxygen depth distribution on the film crystalline quality, and in addition, the effect of them on the electrical properties. The role of Pt films at the interface region for the film crystalline quality was also investigated.
2. Experimental Experiment was performed in a high-vacuum chamber coupled to a Tandetron accelerator which produces He+ or He2+ ion beams collimated to 1 mm diameter over the range of 0.6-5.1 MeV. The chamber contains two SSDs for energy analysis of the scattered ions and a sample manipulator which has three-axis linear motion drive and three orthogonal and independent axes of rotation. Each rotation for the three axes can be controlled by a stepping motor with an accuracy of 0.05”. The scattering angles for incident beams to SSD are fixed at 100” and 170”. Other relevant experimental parameters are the resolution of SSD of 20 keV and an analysis beam current of l-5 nA. More detailed information has been described elsewhere [3,7]. The films were fabricated by the multi-ion-beam sputtering system. The substrates used were (lOO)Pt-covered (100) MgO single crystals which were heated at 400°C during the film deposition. Prior to the film deposition, Pt films was coated by the same sputtering method as an under-electrode. For the deposition of PTO and PLT films, Pb, La and Ti metal targets were simultaneously sputtered
(1997) 453-458
using individual Arf ion beams which were accelerated at 1100 V. Film composition was controlled by Ar+ ion beam currents. Oxygen gas was introduced near the substrate to oxidize the sputtered species. More detailed information of the film fabrication has been described elsewhere [8,9]. The film thickness was monitored by a quartz film thickness monitor. The structure of the films was examined by X-ray diffraction (XRD) measurements using CuK (Yradiation, and every film showed (001) structure (c-axis oriented film). The electric properties of the films were measured with gold top electrodes of 0.3 mm in diameter deposited on the films. The dielectric constants of the films were measured by an impedance analyzer, and the P-E hysteresis loops were observed using a SawyerTower circuit. The computer simulation code we used to estimate the compositional depth profiling from the resonant-RBS spectra is Gisa 3.99 developed by Saarlahti and Rauhara [lo]. In this program, we can adjust the oxygen scattering cross section in the ‘60(a,(r)‘60 3.045 MeV resonance using the standard sample, and we obtained suitable oxygen cross section set in our experimental geometry, using standard samples of TiO, and a 6039 A thick SiO, film.
3. Results and discussion To estimate depth profiling of oxygen concentration, the resonant backscattering method was adjusted and applied. Correction of the method and application to thin oxide films have been described in detail elsewhere [ 111. The result of depth profiling is obtained by comparison between a raw resonantRBS spectrum and a simulated spectrum produced by Gisa 3.99 code. It is more convenient for multi elemental films such as PTO and PLT to use the simulation code than to extract each elemental signal intensity from a raw spectrum and estimate the composition. But there should be paid more attention to estimate the film composition by comparison with the computer simulation, because extent of fitting strongly depends on one’s feeling. In these PTO and PLT cases, we can distinguish oxygen concentration of 3.0 and 3.1 when lead concentration is defined as 1.0. In addition to the error from RBS spectrum
455
M. Watamori et al. /Applied Surface Science 1 I7/ I18 (1997) 453-458
3060keV4He+ +
PbTiOr(750A)/Pt(l OO)/Mgo (8=170°
F5
simulation
-
42
experiment
-*-
s 0 .F
)
F
306Oke@He+ +
(8=170”
)
(Pb--80%,La--20%)TiOrC150~)/Pt(l00)/MgO
simulation
-
experiment
*. -
P 0
1000
,j 2000
3000
Energy[KeVj
Energy[KeV]
Fig. 1. Typical resonant backscattering spectra for 3.06OMeV helium ions incident on the PbTiO,/Pt/MgO (a) and the Pb,La, _,TiO,/Pt/MgO (b). Each elemental signal intensity starts from each arrow point. Dotted points indicate the experiment and the solid lines indicate simulation results.
itself, we can say that the oxygen concentration can be estimated within an error of 10%. A more detailed discussion will be described elsewhere [12]. Fig. 1 shows typical resonant backscattering spectra for 3.060 MeV helium ions incident on a and PbTiO,/Pt/M gO(OO1) Pb,La,_,TiO,/Pt/MgO(OOl) (x ;=:0.8, y = 3). Film thicknesses of both samples were about 750 A. Each elemental signal intensity starts from each arrow point, shown in Fig. 1. The result of the com-
puter simulation is also plotted as a solid line. From resonant-RBS spectra with successive changes of incident energies, depth distributions of each element could be obtained, shown in Fig. 2. Fig. 2a is the depth distribution for the PTO, and Fig. 2b is for the PLT. The energy ranges of the helium ions for this depth profiling were from 3.045 MeV to 3.105 MeV for 20 keV steps. As can be seen in Fig. 2, depth profiling of Pb and Ti elements showed a relatively constant concentration for the whole film thick-
Depth Profiling of PbTiOY
Depth Profiling of PbxLa,_xTiOY
Oxygen
0
200
600
400
800
0
200
600
800
W
(a) Fig. 2. Depth distribution of each concentration varied extremely for the film thickness.
400
DEPTH(A)
DEPTH(A)
for tbe PbTiO,/Pt/MgO
(a) and the Pb,La,
_ xTiO,/Pt/MgO
(b). Oxygen concentration
456
M. Watamori et al./Applied
Surface Science 117/118
nesses, whereas the oxygen concentrations varied drastically. The extent of oxygen composition of about 7.5 for the PTO film feels extremely large. But for other oxide films having relatively resemble structures such as SrTiO, [4] and YBa,Cu,O, [l l] thin films these compositions were relatively stoichiometry, this oxygen concentration might be real. This might indicate the difficulty of fabrication of oxide films having correct stoichiometry. The extent of variation of oxygen concentration was larger for the PTO film than for the PLT film. This might be due to the effect of pinning of oxygen atoms in the structure, induced by La addition. It is noted that the RBS method also counts non structural oxygen atoms in addition to oxygen atoms in the lattice. As described later, a high minimum yield value of oxygen might be due to these non-structural oxygen atoms. In Fig. 2, strong oxygen deficiency was shown for both the PTO and PLT films. Though the reason is not yet clear, at the initial stage of film fabrication, not the epitaxial films but the amorphous metal films might be formed on the Pt covered substrate. Fig. 3 shows angular scans through the (001) direction of the films for the Pb and Ti elements, where the Pb and Ti signals are integrated for almost whole film thickness. Fig. 3a is for the PTO film, and Fig. 3b is for the PLT film. An angular scan of the under-electrode of Pt element was also plotted in each figure. The crystalline quality of the PTO and PLT films is reflected in the small value of the
c
Dip Curve of PbTiOy film
(1997) 453-458
minimum yield, xti,, defined as the ratio of the backscattering yields for perfect alignment along the major axis for random incidence [71. The xti, values of Pb and Ti show the crystalline quality for the whole film thickness. Using the resonant RBS-channelling method, the crystalline quality of the oxygen component can be estimated from the oxygen xti, value as a nearly same manner [13]. In this case, the integrated area for the oxygen0 element is a subsurface region of about 200-300 A, where the 3.045 MeV 160(a,a)160 resonance takes place for 3.060 MeV He+ ion incidence. It can be seen in Fig. 3, that xti, values of Pb, Ti, 0, and Pt elements for the PTO film were 0.38, 0.44, 0.78, and 0.24, respectively. On the other hand, xti, values of Pb, Ti, 0, and Pt elements for the PLT film were 0.27, 0.24, 0.70, and 0.22, respectively. For both films, the ,ymin values of Pb and Ti were nearly the same, though the xmin value of oxygen was extremely bigger than other xti, values. Obviously, this might be due to in-stoichiometric composition of oxygen element in the films. Moreover, it has been found that the crystalline quality of under coating films such as Pt thin films was better than that of ferroelectric films when the film was fabricated by our multi-ion-beam sputtering method. It can be said that the crystalline quality of the under-coating Pt film is very important, to fabricate good quality thin ferroelectric films. From the result of the xmin values of Pb element
I
I-
Dip Curve of PbXLa,+TiOy
film
(W Fig. 3. Angular scans of the PbTiO,/Pt/MgO film (a) and the Pb,La, _xTiO,/Pt/MgO curves for Pb, Ti and 0 of films and for Pt of under-coated films are shown.
film (b) through the (001) axis. The angular yield
M. Watamori et al. /Applied
Dip Curve
of PbTiOy, and
Surface Science 1 I7/
118 (1997) 453-458
451
was intermediate between value of ([PLT]/[PTO]) those of PTO and PLT. This means that the extent of the crystalline quality of the ([PLT]/[PTO]) film was intermediate between two films of PTO and PLT. To fabricate more excellent multi-layer ferroelectric films, it is important to improve the crystalline quality of each ferroelectric film.
PbxLa,_XTiOy
[(PLT) / @‘TO)] films Pb element
4. Conclusion
01 -2’
I
I
I
I
-1
0
1
2
I
ANGLE (DEG.) Fig. 4. Angular scan of the PbTiO,/Pt/MgO, the Pb,La, _,TiO, /Pt/MgO and the multi-layer ([PLTI,, /[PIOl,a), films through the (001) axis. The minimum yield values ( x,,,~,) arc 38%, 27% and 34%, respectively.
the PTO and PLT films, the crystalline quality of the PLT film was found to be better than that of the PTO film, shown in Fig. 3. This is in accordance with the result of XRD analysis, i.e., slight narrower peak width of (001) PLT peak was obtained compared with that of PTO. The dielectric constant of the PLT film was about 200, and the dielectric constant of the PTO film was about 100. La modulation caused the enhancement of the dielectric constant about the extent of twice. This is usually explained as the effect of buried charge [ 1,2]. In our experiment, this does not imply deterioration of the crystalline quality. Probably, La addition might have somewhat the effect of flattening of oxygen depth profiling. But, it is not yet clear. Finally, we have investigated the crystalline quality of the multi-layer ([PTO],/[PLT],),, thin film on the (100)Pt covered MgO( 100) substrate fabricated by the same sputtering method. Multi-layer ferroelectric films are very important because of some possibility to explore new functional dielectric materials. Fig. 4 shows an angular scan of Pb signal intensities through the (001) direction of the films for the PTO, PLT and ([PTO],/[PLT],), films. The numbers of layers and periodicity of the films were 10 layers of PLT and 10 layers of PTO and 7 times repeated (notation is ([PLT],,/[PTO],,),). The film thickness was about 600 A. As can be seen in Fig. 4, the xmin
for
The composition and crystalline quality of ferroelectric thin films formed on Pt covered MgO(100) substrates have been investigated with combined use of 160( (Y,(Y,)I60 3.045 MeV resonant backscattering and channelling techniques. For both PTO and PLT films, depth profiling of Pb and Ti elements showed relatively constant concentrations for whole film thicknesses, whereas the oxygen concentrations varied drastically. The extent of variation of oxygen concentration was larger for the PTO film than for the PLT films. This led to better crystalline quality of PLT, compared with PTO. Since the present method is simple and nondestructive one, it can be applied to the analysis of other ferro- and di-electric thin films.
Acknowledgements The authors would like to acknowledge the VTT, Technical Research Center of Finland for providing the RBS simulation code of Gisa 3.99. Part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. This work was carried out at the Ion Beam Surface Analysis Facility of Osaka University funded by the Ministry of Education, Science, Sports and Culture, Japan.
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