- Email: [email protected]
Si substrate
February 2001
Materials Letters 47 Ž2001. 219–224 www.elsevier.comrlocatermatlet
Microstructural and electrical properties in textured lead calcium lanthanum titanate thin films deposited on a PtrTirSiO 2rSi substrate Zhitang Song a,) , Chenglu Lin a , Lianwei Wang a , Shixin Wang b, Lumin Wang b, Jianxia Gao a , Xiaorong Fu a a
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, 865 Changning RD, Shanghai 200050, People’s Republic of China b Department of Nuclear Engineering and Radiological Science, College of Engineering, The UniÕersity of Michigan, USA Received 21 June 1999; received in revised form 10 July 2000; accepted 12 July 2000
Abstract Textured Ca modified ŽPb,La.TiO 3 ŽPLCT. films were deposited on PtrTirSiO2rSi substrates using a metal-organic decomposition ŽMOD. process. The microstructure of the PLCT thin film was investigated by X-ray diffraction ŽXRD., atomic force microscopy ŽAFM., and transmission electron microscopy ŽTEM.. Electric properties were measured using the PtrPLCTrPt capacitor structure. The PLCT films exhibit good ferroelectric and dielectric properties. q 2001 Elsevier Science B.V. All rights reserved. Keywords: PtrTirSiO2 rSi substrate; Dielectric properties; MOD process
1. Introduction Ferroelectric thin films have been extensively studied for possible use in nonvolatile memories, pyroelectric detectors, surface acoustic wave substrates, optical waveguides, and spatial light modulators w1–4x. The dielectric properties of Ca modified ŽPb,La.TiO 3 ŽPLCT. films, including the dielectric constant, remnant polarization, coercive field, and
)
Corresponding author. Tel.: q86-21-625-11070; fax: q86-21625-13510. E-mail address: [email protected] ŽZ. Song..
pyroelectric, are sensitive to the relative crystallographic orientation of the films w5x. Textured films have better properties compared with those of randomly oriented films. Therefore, for most applications, a high degree of texture film would be sufficient. However, for PLCT Žor other oxides. deposited on silicon substrates coated with a polycrystalline platinum ŽPt., the development of orientation and the mechanisms of orientation development are far from clear. In the present study, we prepared textured PLCT films on PtrTirSiO2rSi substrate using the metal-organic decomposition ŽMOD. process. The microstructure and electrical properties of the PLCT films were investigated.
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 3 8 - X
220
Z. Song et al.r Materials Letters 47 (2001) 219–224
2. Experimental The platinized silicon ŽPtrTirSiO2rSi. substrates were synthesized on Ž100. Si wafers with a layer of thermally grown 600 nm SiO 2 , then coated with 40 nm Ti and 120 nm Pt using UHV electron beam evaporator ŽBalzers UMS 500p.. Before evaporation, the chamber was initially pumped down 10y7 Pa and during evaporation, the chamber pressure returned to 10 -6 Pa. Precursor solutions for PLCT films were synthesized according to the formula Pb 1y xy y La x Ca yTi 1yx r4 O 3 , in which 10 mol% La Ž x s 0.1. and 10 mol% Ca Ž y s 0.1. were adopted to synthesize PLCT precursor solution. 10 mol% of excess PbO was added to compensate for the Pb deficiency during heat treatment. The percentage of excess PbO was calculated relative to A-sites in the above formula, not to Ž1 y x y y . Pb in A-sites. The 0.5 M PLCT precursor solution was prepared, respectively, from lead acetate trihydrate, lanthanum nitrate, calcium nitrate, and titanum n-butoxide. 2Butoxyethanol was chosen as a solvent. The details of the preparation procedure of the PLCT precursor solutions are shown in Ref. w6x. All precursor solution steps were performed in a clean room with a dry atmosphere. The 0.5 M precursor solution was spin-coated on the Ž111. PtrTirSiO2rSi substrates by spinning the sol for 20 s at 3000 rpm using a photo-resist spinner. Deposited films were placed on a hot plate, with digital temperature readout and control, at 4008C for 10 min to evaporate and burn out the organics. This procedure was repeated eight times before further treatment. The films were crystallized at a final temperature of 6008C in flowing oxygen for 1 h. The total thickness of the PLCT films obtained after thermal treatment was about 0.7 mm. The crystal structure of the films was characterized by X-ray diffraction ŽXRD.. The microsmorphology of the film was analyzed using atomic force microscopy ŽAFM. and transmission electron microscopy ŽTEM.. The spreading resistance profile of the film was revealed using automatic spreading resistance ŽASR.. The P–E hysteresis loops of the films were evaluated by a modified Sawyer–Tower circuit system. The C–V curve and the values of
dielectric constanst Ž ´ . and tan d were measured by HP4192A LF impedance analyzer.
3. Results and discussion Fig. 1 shows an XRD of the PLCT thin films on a Ž111. PtrTirSiO2rSi substrate annealed at 6008C for 1 h. The XRD pattern of the PLCT thin films shows only the Ž100. peaks, indicating that the film is a textured PLCT film. The AFM image of the surface morphology for the PLCT film is presented in Fig. 2. Clusters consisted of grains are observed in wFig. 2Ža. and ŽaX .x, whose size and shape are similar to those of the Pt film annealed at 6008C w7x. We found that the clusters consisted of round-shaped grains with a diameter of about 60 nm in wFig. 2Žb. and ŽbX .x, which are very dense and smooth. Fig. 3 shows the microstructure of the PLCT layer characterized by bright-field cross-sectional TEM. The bright-field image shows that the PLCT film has a distinct lack of cra domain boundaries commonly seen in films processed at temperature greater than the Curie temperature. This indicates that the film is predominately of one orientation. All sharp and continuous interface of PLCTrPt films is observed. Thickness of the PLCT thin film is about 0.7 mm and that of PtrTi bottom electrode layer is about 0.1
Fig. 1. XRD patterns of PLCT thin film prepared with 0.2 M precursor solution, MOD process, and annealed at 6008C.
Z. Song et al.r Materials Letters 47 (2001) 219–224
221
Fig. 2. AFM photographs of the PLCT thin films.
mm. Selected area diffraction pattern of the PLCT thin film indicated that the film, which grown on the Ž111. Pt facet, do not show growth, but rather as Ž100. textured growth. This pattern is consistent with our XRD result. The electrical property of the PLCT thin film was measured with ASR Žsee Fig. 4.. The sample was at first polished at an angle of 34X to its original plane. ˚ Spreading The depth of each step is about 200 A. resistance was obtained by the two-probe mode. The spreading resistance of the PLCT layer is about 109 V, and that of the PtrTi film is between 100 and
200 V. At a depth of 0.7 mm Žcorresponding to PLCTrPt interface., the spreading resistance decreases abruptly from 109 to 100 V, indicating the sharp interface of the PLCTrPt films. The C–V and P–E properties of the PLCT film were examined and shown in Fig. 5. The film has a well-defined hysteresis loop in Fig. 5. The remnant polarization Ž Pr ., and coercive voltage Ž Ec . of the film are about 9.8 mCrcm2 and 74.3 kVrcm, respectively. The C–V curve has a normal butterfly shape. The peak positions at both positive and negative voltages are symmetric about a central line.
222
Z. Song et al.r Materials Letters 47 (2001) 219–224
Fig. 2 Ž continued ..
Fig. 6 shows the frequency dependence of dielectric constant Ž ´ . and tan d . The value of dielectric constant gradually decreases with an increase of frequency up to 106 Hz and then abruptly began to decrease, showing the dispersion around 106 Hz in accordance with the results of Kim et al. w8x, and Sayer et al. w9x. The value of dielectric constant ´ and tan d were 337 and 0.022 at 1 kHz. Tani et al. w10x suggested that whenever the Ti from the adhesion layer formed Pt 3Ti on the surface, a Ž111. perovskite PLZT texture was obtained. They also concluded that under the condition where the
top surface was free from Ti, a Ž100. PLZT texture was observed, since this texture represented minimum surface energy. These observations are consistent with our experimental result, in that a Ž100. textured PLCT thin film was observed on Ž111. Pt surface that did not contain Ti ions at EDX-detectable level. The absence of Ti was presumably because the PtrTi electrode, which was prepared by high vacuum electron beam evaporator, is very dense and continuous and is stable at 6008C. Thus, it is difficult to diffuse all Ti ions onto the Pt surface from the adhesion layer. Even if a part of the Ti ions diffused
Z. Song et al.r Materials Letters 47 (2001) 219–224
223
Fig. 3. HRTEM photographs of the PLCT thin films.
from the adhesion layer, they must have been oxidized to TiO 2 at the grain boundaries of the columnar Pt grains and thus did not reach the Pt surface. Therefore, the interface between PLCT film and Pt layer is very sharp in Fig. 2. Tani et al. w10x calculated the surface energies of Ž100., Ž110., and Ž111. planes for PLZT based on bond strengths and the crystallographic structure of each plane. Their results showed that the Ž100. plane has the lowest surface energy. Because of the structural similarity between PLZT and PLCT, it is rea-
Fig. 4. ASR measurement results of the PLCT films.
Fig. 5. Electric properties of the PLCT films. Ža. C – V curve, Žb. P – E curve.
224
Z. Song et al.r Materials Letters 47 (2001) 219–224
Acknowledgements This wake is supported by the National Advanced Materials Committee of China ŽNAMCC., No. 715002-0110 and National Nature Foundation of China, No. 69738020.
References
Fig. 6. Frequency dependence of dielectric constant Ž ´ . and tan d in the PLCT film.
sonable to postulate that in the PLCT film, Ž100. is the plane with the lowest surface energy. Our result of Ž100. textured PLCT film is in accordance with the calculation of Tani et al. w10x.
4. Conclusions We have successfully prepared Ž100. textured PLCT thin films on Pt-coated silicon substrates using MOD process. The Ž100. textured PLCT film observed on Ž111. Pt may be explained by a lowest surface energy argument. The PLCT films exhibit good ferroelectric and dielectric properties.
w1x J.F. Scott, A.P.D. Araujo, Science 246 Ž1989. 1400. w2x R. Takayama, Y. Tomita, Ferroelectrics 118 Ž1991. 325. w3x H. Ohtani, M. Okuyama, Y. Hamakawa, Jpn. J. Appl. Phys. 23 Ž1984. 133. w4x S.J. Martin, M.A. Bulter, C.E. Land, Electron. Lett. 24 Ž1988. 1486. w5x L.R. Zheng, P.X. Yang, W.P. Xu, C.L. Lin, W.B. Wu, M. Okuyama, Integr. Ferroelectr. 20 Ž1998. 73. w6x Z.T. Song, W. Ren, L.Y. Zhang, X. Yao, Acta Phys. Sin. 7 Ž1998. 273. w7x Z.T. Song, C.L. Lin, X. Zhu, and X. Fu, J. Material Science, in press. w8x T.S. Kim, D.J. Kim, J.K. Lee, H.J. Jung, J. Mater. Res. 13 Ž1998. 3436. w9x M. Sayer, A. Mansingh, A.K. Arora, A. Lo, Integr. Ferroelectr. 1 Ž1992. 129. w10x T. Tani, Z. Xu, D.A. Payne, Ferroelectric thin films III, Material Research Society Symposium Proceeding vol. 310, Material Research Society, Pittsburgh, PA, 1993, 263.
Materials Letters 47 Ž2001. 219–224 www.elsevier.comrlocatermatlet
Microstructural and electrical properties in textured lead calcium lanthanum titanate thin films deposited on a PtrTirSiO 2rSi substrate Zhitang Song a,) , Chenglu Lin a , Lianwei Wang a , Shixin Wang b, Lumin Wang b, Jianxia Gao a , Xiaorong Fu a a
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, 865 Changning RD, Shanghai 200050, People’s Republic of China b Department of Nuclear Engineering and Radiological Science, College of Engineering, The UniÕersity of Michigan, USA Received 21 June 1999; received in revised form 10 July 2000; accepted 12 July 2000
Abstract Textured Ca modified ŽPb,La.TiO 3 ŽPLCT. films were deposited on PtrTirSiO2rSi substrates using a metal-organic decomposition ŽMOD. process. The microstructure of the PLCT thin film was investigated by X-ray diffraction ŽXRD., atomic force microscopy ŽAFM., and transmission electron microscopy ŽTEM.. Electric properties were measured using the PtrPLCTrPt capacitor structure. The PLCT films exhibit good ferroelectric and dielectric properties. q 2001 Elsevier Science B.V. All rights reserved. Keywords: PtrTirSiO2 rSi substrate; Dielectric properties; MOD process
1. Introduction Ferroelectric thin films have been extensively studied for possible use in nonvolatile memories, pyroelectric detectors, surface acoustic wave substrates, optical waveguides, and spatial light modulators w1–4x. The dielectric properties of Ca modified ŽPb,La.TiO 3 ŽPLCT. films, including the dielectric constant, remnant polarization, coercive field, and
)
Corresponding author. Tel.: q86-21-625-11070; fax: q86-21625-13510. E-mail address: [email protected] ŽZ. Song..
pyroelectric, are sensitive to the relative crystallographic orientation of the films w5x. Textured films have better properties compared with those of randomly oriented films. Therefore, for most applications, a high degree of texture film would be sufficient. However, for PLCT Žor other oxides. deposited on silicon substrates coated with a polycrystalline platinum ŽPt., the development of orientation and the mechanisms of orientation development are far from clear. In the present study, we prepared textured PLCT films on PtrTirSiO2rSi substrate using the metal-organic decomposition ŽMOD. process. The microstructure and electrical properties of the PLCT films were investigated.
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 3 8 - X
220
Z. Song et al.r Materials Letters 47 (2001) 219–224
2. Experimental The platinized silicon ŽPtrTirSiO2rSi. substrates were synthesized on Ž100. Si wafers with a layer of thermally grown 600 nm SiO 2 , then coated with 40 nm Ti and 120 nm Pt using UHV electron beam evaporator ŽBalzers UMS 500p.. Before evaporation, the chamber was initially pumped down 10y7 Pa and during evaporation, the chamber pressure returned to 10 -6 Pa. Precursor solutions for PLCT films were synthesized according to the formula Pb 1y xy y La x Ca yTi 1yx r4 O 3 , in which 10 mol% La Ž x s 0.1. and 10 mol% Ca Ž y s 0.1. were adopted to synthesize PLCT precursor solution. 10 mol% of excess PbO was added to compensate for the Pb deficiency during heat treatment. The percentage of excess PbO was calculated relative to A-sites in the above formula, not to Ž1 y x y y . Pb in A-sites. The 0.5 M PLCT precursor solution was prepared, respectively, from lead acetate trihydrate, lanthanum nitrate, calcium nitrate, and titanum n-butoxide. 2Butoxyethanol was chosen as a solvent. The details of the preparation procedure of the PLCT precursor solutions are shown in Ref. w6x. All precursor solution steps were performed in a clean room with a dry atmosphere. The 0.5 M precursor solution was spin-coated on the Ž111. PtrTirSiO2rSi substrates by spinning the sol for 20 s at 3000 rpm using a photo-resist spinner. Deposited films were placed on a hot plate, with digital temperature readout and control, at 4008C for 10 min to evaporate and burn out the organics. This procedure was repeated eight times before further treatment. The films were crystallized at a final temperature of 6008C in flowing oxygen for 1 h. The total thickness of the PLCT films obtained after thermal treatment was about 0.7 mm. The crystal structure of the films was characterized by X-ray diffraction ŽXRD.. The microsmorphology of the film was analyzed using atomic force microscopy ŽAFM. and transmission electron microscopy ŽTEM.. The spreading resistance profile of the film was revealed using automatic spreading resistance ŽASR.. The P–E hysteresis loops of the films were evaluated by a modified Sawyer–Tower circuit system. The C–V curve and the values of
dielectric constanst Ž ´ . and tan d were measured by HP4192A LF impedance analyzer.
3. Results and discussion Fig. 1 shows an XRD of the PLCT thin films on a Ž111. PtrTirSiO2rSi substrate annealed at 6008C for 1 h. The XRD pattern of the PLCT thin films shows only the Ž100. peaks, indicating that the film is a textured PLCT film. The AFM image of the surface morphology for the PLCT film is presented in Fig. 2. Clusters consisted of grains are observed in wFig. 2Ža. and ŽaX .x, whose size and shape are similar to those of the Pt film annealed at 6008C w7x. We found that the clusters consisted of round-shaped grains with a diameter of about 60 nm in wFig. 2Žb. and ŽbX .x, which are very dense and smooth. Fig. 3 shows the microstructure of the PLCT layer characterized by bright-field cross-sectional TEM. The bright-field image shows that the PLCT film has a distinct lack of cra domain boundaries commonly seen in films processed at temperature greater than the Curie temperature. This indicates that the film is predominately of one orientation. All sharp and continuous interface of PLCTrPt films is observed. Thickness of the PLCT thin film is about 0.7 mm and that of PtrTi bottom electrode layer is about 0.1
Fig. 1. XRD patterns of PLCT thin film prepared with 0.2 M precursor solution, MOD process, and annealed at 6008C.
Z. Song et al.r Materials Letters 47 (2001) 219–224
221
Fig. 2. AFM photographs of the PLCT thin films.
mm. Selected area diffraction pattern of the PLCT thin film indicated that the film, which grown on the Ž111. Pt facet, do not show growth, but rather as Ž100. textured growth. This pattern is consistent with our XRD result. The electrical property of the PLCT thin film was measured with ASR Žsee Fig. 4.. The sample was at first polished at an angle of 34X to its original plane. ˚ Spreading The depth of each step is about 200 A. resistance was obtained by the two-probe mode. The spreading resistance of the PLCT layer is about 109 V, and that of the PtrTi film is between 100 and
200 V. At a depth of 0.7 mm Žcorresponding to PLCTrPt interface., the spreading resistance decreases abruptly from 109 to 100 V, indicating the sharp interface of the PLCTrPt films. The C–V and P–E properties of the PLCT film were examined and shown in Fig. 5. The film has a well-defined hysteresis loop in Fig. 5. The remnant polarization Ž Pr ., and coercive voltage Ž Ec . of the film are about 9.8 mCrcm2 and 74.3 kVrcm, respectively. The C–V curve has a normal butterfly shape. The peak positions at both positive and negative voltages are symmetric about a central line.
222
Z. Song et al.r Materials Letters 47 (2001) 219–224
Fig. 2 Ž continued ..
Fig. 6 shows the frequency dependence of dielectric constant Ž ´ . and tan d . The value of dielectric constant gradually decreases with an increase of frequency up to 106 Hz and then abruptly began to decrease, showing the dispersion around 106 Hz in accordance with the results of Kim et al. w8x, and Sayer et al. w9x. The value of dielectric constant ´ and tan d were 337 and 0.022 at 1 kHz. Tani et al. w10x suggested that whenever the Ti from the adhesion layer formed Pt 3Ti on the surface, a Ž111. perovskite PLZT texture was obtained. They also concluded that under the condition where the
top surface was free from Ti, a Ž100. PLZT texture was observed, since this texture represented minimum surface energy. These observations are consistent with our experimental result, in that a Ž100. textured PLCT thin film was observed on Ž111. Pt surface that did not contain Ti ions at EDX-detectable level. The absence of Ti was presumably because the PtrTi electrode, which was prepared by high vacuum electron beam evaporator, is very dense and continuous and is stable at 6008C. Thus, it is difficult to diffuse all Ti ions onto the Pt surface from the adhesion layer. Even if a part of the Ti ions diffused
Z. Song et al.r Materials Letters 47 (2001) 219–224
223
Fig. 3. HRTEM photographs of the PLCT thin films.
from the adhesion layer, they must have been oxidized to TiO 2 at the grain boundaries of the columnar Pt grains and thus did not reach the Pt surface. Therefore, the interface between PLCT film and Pt layer is very sharp in Fig. 2. Tani et al. w10x calculated the surface energies of Ž100., Ž110., and Ž111. planes for PLZT based on bond strengths and the crystallographic structure of each plane. Their results showed that the Ž100. plane has the lowest surface energy. Because of the structural similarity between PLZT and PLCT, it is rea-
Fig. 4. ASR measurement results of the PLCT films.
Fig. 5. Electric properties of the PLCT films. Ža. C – V curve, Žb. P – E curve.
224
Z. Song et al.r Materials Letters 47 (2001) 219–224
Acknowledgements This wake is supported by the National Advanced Materials Committee of China ŽNAMCC., No. 715002-0110 and National Nature Foundation of China, No. 69738020.
References
Fig. 6. Frequency dependence of dielectric constant Ž ´ . and tan d in the PLCT film.
sonable to postulate that in the PLCT film, Ž100. is the plane with the lowest surface energy. Our result of Ž100. textured PLCT film is in accordance with the calculation of Tani et al. w10x.
4. Conclusions We have successfully prepared Ž100. textured PLCT thin films on Pt-coated silicon substrates using MOD process. The Ž100. textured PLCT film observed on Ž111. Pt may be explained by a lowest surface energy argument. The PLCT films exhibit good ferroelectric and dielectric properties.
w1x J.F. Scott, A.P.D. Araujo, Science 246 Ž1989. 1400. w2x R. Takayama, Y. Tomita, Ferroelectrics 118 Ž1991. 325. w3x H. Ohtani, M. Okuyama, Y. Hamakawa, Jpn. J. Appl. Phys. 23 Ž1984. 133. w4x S.J. Martin, M.A. Bulter, C.E. Land, Electron. Lett. 24 Ž1988. 1486. w5x L.R. Zheng, P.X. Yang, W.P. Xu, C.L. Lin, W.B. Wu, M. Okuyama, Integr. Ferroelectr. 20 Ž1998. 73. w6x Z.T. Song, W. Ren, L.Y. Zhang, X. Yao, Acta Phys. Sin. 7 Ž1998. 273. w7x Z.T. Song, C.L. Lin, X. Zhu, and X. Fu, J. Material Science, in press. w8x T.S. Kim, D.J. Kim, J.K. Lee, H.J. Jung, J. Mater. Res. 13 Ž1998. 3436. w9x M. Sayer, A. Mansingh, A.K. Arora, A. Lo, Integr. Ferroelectr. 1 Ž1992. 129. w10x T. Tani, Z. Xu, D.A. Payne, Ferroelectric thin films III, Material Research Society Symposium Proceeding vol. 310, Material Research Society, Pittsburgh, PA, 1993, 263.