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Effects of Growth Conditions on Structure and Dielectric Properties of Bismuth-Magnesium Niobate Thin Films
Available online at www.sciencedirect.com
Physics Procedia 32 (2012) 340 – 344
18th International Vacuum Congress (IVC-18)
Effects of growth conditions on structure and dielectric properties of bismuth-magnesium niobate thin films Gao Libin, Jiang Shuwen, Li Yanrong State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China Chengdu 610054
Abstract The Bi1.5MgNb1.5O7 (BMN) thin films were prepared on platinum coated sapphire by rf magnetron sputtering deposition. Effects of substrate temperature, sputtering ambient and post rapid thermal annealing on structure, morphologies, and dielectric properties of films were investigated. Thin films have cubic pyrochlore structure when substrate temperature at 650ć or higher. Corresponding to tunable dielectric properties of bismuth based cubic pyrochlore thin films, voltage tunability of Bi1.5MgNb1.5O7 thin films increases with substrate temperature. Bi1.5MgNb1.5O7 thin films exhibit low dielectric loss of ~ 0.0018–0.004. A brief discussion is given on the enhanced tunability of Bi1.5MgNb1.5O7 thin films. The relatively high tunability and low dielectric loss suggest that Bi1.5MgNb1.5O7 thin films have potential application for tunable capacitor applications. Keywords: cubic pyrochlore; Bi1.5MgNb1.5O7 thin films; tunability; dielectric loss
1. Introduction Much attention has been paid to develop materials that have medium and higher permittivity with low dielectric loss. (BaxSr1-x)TiO3 (BST) thin films have been given a great importance many years to their high permittivity and large tunability [1~3]. However, BST thin films have the essential problem of high dielectric loss. Recently, bismuth-based pyrochlore have been paid attention due to their low loss as well as medium permittivity. The cubic pyrochlore Bi1.5ZnNb1.5O7 thin films have tunability of ~55% under a highly applied bias field of 2.4 MV/cm, loss tangent are about (tan¥~5h10-4)[4]DŽ Recently, another bismuth based pyrochlore Bi2O3-MgO-Nb2O5 system has begun to attract interests for low loss and high permittivity. The Bi2Mg2/3Nb4/3O7 composition was found to exhibit a high dielectric constant of ~ 210, while Bi2Zn2/3Nb4/3O7 is ~ 80[5, 6]. Another cubic pyrochlore Bi2O3-MgO-Nb2O5 system with Bi1.5MgNb1.5O7 composition exhibits tunability of 29.2% at bias voltage of 1.2 MV/cm which is higher than that of Bi1.5ZnNb1.5O7 thin films.[7, 8] In this paper, we prepared the cubic pyrochlore Bi1.5MgNb1.5O7 thin films on Pt-coated c-plane sapphire substrates by rf magnetron sputtering. The effects of substrate temperature, post rapid thermal annealing and deposition ambient on film structure and dielectric properties were investigated. We also investigated the dielectric properties of Bi1.5MgNb1.5O7 thin films under low temperatures. 2. Experimental Procedure The Bi1.5MgNb1.5O7 ceramic targets were prepared by solid state reaction process with Bi2O3, MgO and Nb2O5. The mixture materials in stoichiometry were planet-grinding for 7 h and sintered into discs at 1020ć for 2 h. BMN thin films were deposited on Ptcoated c-plane sapphire substrates by rf magnetron sputtering. The substrate temperatures varied from 400ć to 800ć. The sputtering ambient was a mixture of Ar and O2 with the O2 / (O2+Ar) mixing ratio of 15%, and rf power was 120 w. The thin film thickness, determined by cross-section scanning electron microscopy, was ~ 300 nm. Pt top electrodes with 0.2 mm in diameter were patterned by a lift-off process to form MIM-type capacitors. The crystalline structure of BMN films were characterized using XRD instrument. The dielectric measurements were performed on an Agilent 4284A precision impedance analyzer with a 500 mV test oscillation voltage. The permittivity and tunability were calculated from the film thickness and top electrode area.
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). doi:10.1016/j.phpro.2012.03.566
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3. Results and discussion Fig. 1 shows the XRD patterns of the samples deposited at different substrate temperature, annealing treatment and total gas pressure. The as-deposited BMN thin films do not exhibit crystalline cubic pyrochlore phase when the substrate temperature is below 650 ć[Fig.1(a)]. The result indicated that an amorphous phase is still dominant. The (222) peak is detected at substrate temperature of 650ć and becomes stronger with the increasing of substrate temperature. There is a peak of Nb2O5 detected in thin films when the substrate temperature above 700ć It may be attributed to the lack of bismuth due to the decomposition of Bi1.5MgNb1.5O7 at high temperature. Post annealing treatment can decrease crystalline temperature of (222) - oriented texture down to 550ć which is lower 100ć than those without annealing treatment as shown in Fig.1 (b). With the increase of total gas pressure, the peaks of cubic pyrochlore structure of thin films became weaker as shown in Fig1(c).
Al2O3
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(400) (331)
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Gao Libin et al. / Physics Procedia 32 (2012) 340 – 344
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Al2O3
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6Pa 5Pa 4Pa 3Pa 2Pa 1Pa 25
30
35
40
45
50
55
60
2T(degree) Figure 1. XRD patterern of BMN thin films grown on Pt-coated sapphire substrates as a function of (a) substrate temperature, (b) post annealing treatment at 700ć, 1Pa ,30min with O2 / (O2+Ar) mixing ratio of 15% and (c) total sputter gas pressure. All the films were deposited with O2 / (O2+Ar) mixing ratio of 15% and sputtering power was 120W. The films in (a) and (b) were deposited in total sputter gas pressure of 1Pa.
Temperature affected greatly the surface morphology of BMN thin films Fig. 2. shows AFM images of BMN films deposited at different substrate temperature. Surface morphologies of the films deposited at 400ć are smooth with the grain size ~84 nm .With the increase of the substrate temperature, the grain size increased. As shown in Fig. 2 the grain size of the BMN thin films grown to 140 nm at 750ć and an agglomerate morphology clustered by grains is detected with the average cluster size of ~595 nm at 800ć The surface roughness of BMN thin films also increased from 1.56 nm at 400ć to 21.8 nm 750ć with substrate temperature increased.
(a)D=84nm Ra=1.56nm
(d) D=115nm Ra=6.13nm
(b) D=100nm Ra=9.45nm
(c) D=105nm Ra=5.89nm
(e) D=140nm Ra=21.8nm
(f) Ra=9.37nm
Figure 2. AFM images of BMN thin films deposited at different substrate temperatures of (a) 400ćˈ˄b˅500ć, (c) 650ć, (d)700ć, (e)750ć, (f)800ć. All Those films deposited at total pressure is 1Pa and O2 / (O2+Ar) mixing ratio is 15%. The insects show AFM images that were measured with a scanning area of 2h2
Pm 2 . The bismuth-based pyrochlore structure can be described as A2B2O6O˃. In BMN system Bi3+ are always on the A sites and Nb5+ are always on the B sites, but Mg2+ can also take A sites and B sites. As a given electric field force, Mg2+ may disorder between A sites and B sites. A simple model of hopping dipoles under the influences of electric fields can describe large electric field dielectric tunability. In order to achieve large dielectric tunability, the cubic pyrochlore structure of BMN thin films must be achieved. Experimental results indicated that the substrate temperature of 750ć and total sputter gas pressure of 1Pa with O2 / (O2+Ar) mixing ratio of 15% are the
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appropriate deposition conditions. The typical field dependent dielectric properties of BMN thin films with cubic pyrochlore structure are shown in Fig.3. The thin films show an electric field tunability of ~ 50% at the maximum applied bias field of 1.5 MV/cm, and the maximum permittivity of ~ 134. The loss tangent is lower than 0.007 with the bias field below 1.0 MV/cm. The loss tangent increases with the increasing of the bias field higher than 1.0 MV/cm, which may be attributed to the increasing of leakage current.
75
0.12
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60 55
0.06
50 0.04 45
Loss tangent
Capacitance (pF)
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0.02
40 35
0.00
30 -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Bias field (MV/cm) Figure 3. Effects of bias field on capacitance and loss tangent of BMN (Bi1.5MgNb1.5O7 ) thin film. The thin film was deposited at substrate temperature of 750ć, total sputter gas pressure of 1Pa with O2 / (O2+Ar) mixing ratio of 15%. ,
The values of the dielectric loss tangent (tan¥) and the dielectric constant ( H ) depend on the variation of external factors such as temperature and frequency. Fig.4. shows that the dielectric constant almost has no change with the frequency, but decreases linearly at higher temperature. The loss tangent detected has weak changes (0.002 – 0.01) when the temperature between 160 K and 240 K as shown in Fig.4. This may caused by the relaxation of BMN materials. The performance is in sharp contrast to the remarkable dielectric relaxation observed in the Bi2O3 – ZnO – Nb2O5 pyrochlore thin films.[9-11] No distinguished frequency dispersion of the loss tangent was observed when the temperature between 78 K and 400 K indicating a good temperature stability. The frequency independence and the temperature stability behavior over a wide temperature make BMN (Bi1.5MgNb1.5O7 ) thin films are potential candidate for tunable capacitor applications. 190
(a)
100kHz 300kHz 500kHz 700kHz
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H`
180 175 170 165 160 50
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tanG
(b) 0.02
0.01
0.00 50
100
150
200
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350
400
450
Temperature (K) Figure 4. The (a) dielectric constant and (b) dielectric loss varied at different frequencies.
4. Conclusion The BMN (Bi1.5MgNb1.5O7 ) thin film with cubic pyrochlore structure was investigated .The thin films were prepared on Pt-coated sapphire substrates by rf magnetron sputtering methods. The XRD measurements showed that the peaks of cubic pyrochlore structure
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became stronger with the increasing of substrate temperatures and the reducing of the total sputtering gas pressure. The maximum voltage tunability of ~50% at bias field 1.5 MV/cm was found on BMN (Bi1.5MgNb1.5O7 ) thin film with loss tangent lower than 0.007 at bias field below 1.0 MV/cm. The dielectric constant of BMN thin films changed with temperature slightly and loss tangent of BMN thin films were temperature stability. These properties make BMN thin films attractive candidates for tunable capacitor applications. 5. Acknowledgments The authors grateful acknowledge the support from the National Natural Science foundation of China Grant NO. 50972024. References 1. A. Vorobiev, P. Rundqvist, K. Khamchane, and S. Gevorgian, Appl. Phys. Lett. 83, 3144 (2003). 2. A. K. Tagantsev, V. O. Sherman, K. F. Astafiev, J. Venkatesh, and N. Setter, J. Electroceram. 11, 5 (2003). 3. Z. Yuan, Y. Lin, J. Weaver, X. Chen, C. L. Chen, J. C. Jiang, G. Subramanyam, and E. I. Meletis, Appl. Phys. Lett. 87, 2901 (2005). 4. J. Lu and S. Stemmer, Appl. Phys. Lett. 83, 2411 (2003). 5. W. Ren, S. Trolier-McKinstry, C. A. Randall, and T. R. Shrout, J. Appl. Phys. 89, 767 2001. 6. X. L. Wang, H. Wang, and X. Yao, J. Am. Ceram. Soc. 80, 2745 ,1997 7. S. W. Jiang,aĂY. R. Li, R. G. Li. Appl. Phys. Lett., 94, 162908ˈ˄ 2009˅ 8. Jiwei Lu and Susanne Stemmer. Appl. Phys. Lett., 83, 2411ˈ˄ 2003˅ 9. Alexander K. Tagantsev. Appl. Phys. Lett., 86, 032901ˈ˄ 2005˅ 10. R. L. Thayer, C. A. Randall, and S. Trolier-McKinstry. J. Appl. Phys. 94, 1941, 2003 Jiwei Lu, Dmitri O. Klenov, and Susanne Stemmer. Appl. Phys. Lett., 84, 957ˈ˄ 2004˅
Physics Procedia 32 (2012) 340 – 344
18th International Vacuum Congress (IVC-18)
Effects of growth conditions on structure and dielectric properties of bismuth-magnesium niobate thin films Gao Libin, Jiang Shuwen, Li Yanrong State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China Chengdu 610054
Abstract The Bi1.5MgNb1.5O7 (BMN) thin films were prepared on platinum coated sapphire by rf magnetron sputtering deposition. Effects of substrate temperature, sputtering ambient and post rapid thermal annealing on structure, morphologies, and dielectric properties of films were investigated. Thin films have cubic pyrochlore structure when substrate temperature at 650ć or higher. Corresponding to tunable dielectric properties of bismuth based cubic pyrochlore thin films, voltage tunability of Bi1.5MgNb1.5O7 thin films increases with substrate temperature. Bi1.5MgNb1.5O7 thin films exhibit low dielectric loss of ~ 0.0018–0.004. A brief discussion is given on the enhanced tunability of Bi1.5MgNb1.5O7 thin films. The relatively high tunability and low dielectric loss suggest that Bi1.5MgNb1.5O7 thin films have potential application for tunable capacitor applications. Keywords: cubic pyrochlore; Bi1.5MgNb1.5O7 thin films; tunability; dielectric loss
1. Introduction Much attention has been paid to develop materials that have medium and higher permittivity with low dielectric loss. (BaxSr1-x)TiO3 (BST) thin films have been given a great importance many years to their high permittivity and large tunability [1~3]. However, BST thin films have the essential problem of high dielectric loss. Recently, bismuth-based pyrochlore have been paid attention due to their low loss as well as medium permittivity. The cubic pyrochlore Bi1.5ZnNb1.5O7 thin films have tunability of ~55% under a highly applied bias field of 2.4 MV/cm, loss tangent are about (tan¥~5h10-4)[4]DŽ Recently, another bismuth based pyrochlore Bi2O3-MgO-Nb2O5 system has begun to attract interests for low loss and high permittivity. The Bi2Mg2/3Nb4/3O7 composition was found to exhibit a high dielectric constant of ~ 210, while Bi2Zn2/3Nb4/3O7 is ~ 80[5, 6]. Another cubic pyrochlore Bi2O3-MgO-Nb2O5 system with Bi1.5MgNb1.5O7 composition exhibits tunability of 29.2% at bias voltage of 1.2 MV/cm which is higher than that of Bi1.5ZnNb1.5O7 thin films.[7, 8] In this paper, we prepared the cubic pyrochlore Bi1.5MgNb1.5O7 thin films on Pt-coated c-plane sapphire substrates by rf magnetron sputtering. The effects of substrate temperature, post rapid thermal annealing and deposition ambient on film structure and dielectric properties were investigated. We also investigated the dielectric properties of Bi1.5MgNb1.5O7 thin films under low temperatures. 2. Experimental Procedure The Bi1.5MgNb1.5O7 ceramic targets were prepared by solid state reaction process with Bi2O3, MgO and Nb2O5. The mixture materials in stoichiometry were planet-grinding for 7 h and sintered into discs at 1020ć for 2 h. BMN thin films were deposited on Ptcoated c-plane sapphire substrates by rf magnetron sputtering. The substrate temperatures varied from 400ć to 800ć. The sputtering ambient was a mixture of Ar and O2 with the O2 / (O2+Ar) mixing ratio of 15%, and rf power was 120 w. The thin film thickness, determined by cross-section scanning electron microscopy, was ~ 300 nm. Pt top electrodes with 0.2 mm in diameter were patterned by a lift-off process to form MIM-type capacitors. The crystalline structure of BMN films were characterized using XRD instrument. The dielectric measurements were performed on an Agilent 4284A precision impedance analyzer with a 500 mV test oscillation voltage. The permittivity and tunability were calculated from the film thickness and top electrode area.
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). doi:10.1016/j.phpro.2012.03.566
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Gao Libin et al. / Physics Procedia 32 (2012) 340 – 344
(222)
3. Results and discussion Fig. 1 shows the XRD patterns of the samples deposited at different substrate temperature, annealing treatment and total gas pressure. The as-deposited BMN thin films do not exhibit crystalline cubic pyrochlore phase when the substrate temperature is below 650 ć[Fig.1(a)]. The result indicated that an amorphous phase is still dominant. The (222) peak is detected at substrate temperature of 650ć and becomes stronger with the increasing of substrate temperature. There is a peak of Nb2O5 detected in thin films when the substrate temperature above 700ć It may be attributed to the lack of bismuth due to the decomposition of Bi1.5MgNb1.5O7 at high temperature. Post annealing treatment can decrease crystalline temperature of (222) - oriented texture down to 550ć which is lower 100ć than those without annealing treatment as shown in Fig.1 (b). With the increase of total gas pressure, the peaks of cubic pyrochlore structure of thin films became weaker as shown in Fig1(c).
Al2O3
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Nb2O5
(400) (331)
Pt
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800 750 700 650 600 550 500 450 400
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Gao Libin et al. / Physics Procedia 32 (2012) 340 – 344
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50
55
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2T(degree) Figure 1. XRD patterern of BMN thin films grown on Pt-coated sapphire substrates as a function of (a) substrate temperature, (b) post annealing treatment at 700ć, 1Pa ,30min with O2 / (O2+Ar) mixing ratio of 15% and (c) total sputter gas pressure. All the films were deposited with O2 / (O2+Ar) mixing ratio of 15% and sputtering power was 120W. The films in (a) and (b) were deposited in total sputter gas pressure of 1Pa.
Temperature affected greatly the surface morphology of BMN thin films Fig. 2. shows AFM images of BMN films deposited at different substrate temperature. Surface morphologies of the films deposited at 400ć are smooth with the grain size ~84 nm .With the increase of the substrate temperature, the grain size increased. As shown in Fig. 2 the grain size of the BMN thin films grown to 140 nm at 750ć and an agglomerate morphology clustered by grains is detected with the average cluster size of ~595 nm at 800ć The surface roughness of BMN thin films also increased from 1.56 nm at 400ć to 21.8 nm 750ć with substrate temperature increased.
(a)D=84nm Ra=1.56nm
(d) D=115nm Ra=6.13nm
(b) D=100nm Ra=9.45nm
(c) D=105nm Ra=5.89nm
(e) D=140nm Ra=21.8nm
(f) Ra=9.37nm
Figure 2. AFM images of BMN thin films deposited at different substrate temperatures of (a) 400ćˈ˄b˅500ć, (c) 650ć, (d)700ć, (e)750ć, (f)800ć. All Those films deposited at total pressure is 1Pa and O2 / (O2+Ar) mixing ratio is 15%. The insects show AFM images that were measured with a scanning area of 2h2
Pm 2 . The bismuth-based pyrochlore structure can be described as A2B2O6O˃. In BMN system Bi3+ are always on the A sites and Nb5+ are always on the B sites, but Mg2+ can also take A sites and B sites. As a given electric field force, Mg2+ may disorder between A sites and B sites. A simple model of hopping dipoles under the influences of electric fields can describe large electric field dielectric tunability. In order to achieve large dielectric tunability, the cubic pyrochlore structure of BMN thin films must be achieved. Experimental results indicated that the substrate temperature of 750ć and total sputter gas pressure of 1Pa with O2 / (O2+Ar) mixing ratio of 15% are the
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Gao Libin et al. / Physics Procedia 32 (2012) 340 – 344
appropriate deposition conditions. The typical field dependent dielectric properties of BMN thin films with cubic pyrochlore structure are shown in Fig.3. The thin films show an electric field tunability of ~ 50% at the maximum applied bias field of 1.5 MV/cm, and the maximum permittivity of ~ 134. The loss tangent is lower than 0.007 with the bias field below 1.0 MV/cm. The loss tangent increases with the increasing of the bias field higher than 1.0 MV/cm, which may be attributed to the increasing of leakage current.
75
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0.06
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Capacitance (pF)
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0.02
40 35
0.00
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-0.5
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1.0
1.5
Bias field (MV/cm) Figure 3. Effects of bias field on capacitance and loss tangent of BMN (Bi1.5MgNb1.5O7 ) thin film. The thin film was deposited at substrate temperature of 750ć, total sputter gas pressure of 1Pa with O2 / (O2+Ar) mixing ratio of 15%. ,
The values of the dielectric loss tangent (tan¥) and the dielectric constant ( H ) depend on the variation of external factors such as temperature and frequency. Fig.4. shows that the dielectric constant almost has no change with the frequency, but decreases linearly at higher temperature. The loss tangent detected has weak changes (0.002 – 0.01) when the temperature between 160 K and 240 K as shown in Fig.4. This may caused by the relaxation of BMN materials. The performance is in sharp contrast to the remarkable dielectric relaxation observed in the Bi2O3 – ZnO – Nb2O5 pyrochlore thin films.[9-11] No distinguished frequency dispersion of the loss tangent was observed when the temperature between 78 K and 400 K indicating a good temperature stability. The frequency independence and the temperature stability behavior over a wide temperature make BMN (Bi1.5MgNb1.5O7 ) thin films are potential candidate for tunable capacitor applications. 190
(a)
100kHz 300kHz 500kHz 700kHz
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H`
180 175 170 165 160 50
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tanG
(b) 0.02
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Temperature (K) Figure 4. The (a) dielectric constant and (b) dielectric loss varied at different frequencies.
4. Conclusion The BMN (Bi1.5MgNb1.5O7 ) thin film with cubic pyrochlore structure was investigated .The thin films were prepared on Pt-coated sapphire substrates by rf magnetron sputtering methods. The XRD measurements showed that the peaks of cubic pyrochlore structure
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became stronger with the increasing of substrate temperatures and the reducing of the total sputtering gas pressure. The maximum voltage tunability of ~50% at bias field 1.5 MV/cm was found on BMN (Bi1.5MgNb1.5O7 ) thin film with loss tangent lower than 0.007 at bias field below 1.0 MV/cm. The dielectric constant of BMN thin films changed with temperature slightly and loss tangent of BMN thin films were temperature stability. These properties make BMN thin films attractive candidates for tunable capacitor applications. 5. Acknowledgments The authors grateful acknowledge the support from the National Natural Science foundation of China Grant NO. 50972024. References 1. A. Vorobiev, P. Rundqvist, K. Khamchane, and S. Gevorgian, Appl. Phys. Lett. 83, 3144 (2003). 2. A. K. Tagantsev, V. O. Sherman, K. F. Astafiev, J. Venkatesh, and N. Setter, J. Electroceram. 11, 5 (2003). 3. Z. Yuan, Y. Lin, J. Weaver, X. Chen, C. L. Chen, J. C. Jiang, G. Subramanyam, and E. I. Meletis, Appl. Phys. Lett. 87, 2901 (2005). 4. J. Lu and S. Stemmer, Appl. Phys. Lett. 83, 2411 (2003). 5. W. Ren, S. Trolier-McKinstry, C. A. Randall, and T. R. Shrout, J. Appl. Phys. 89, 767 2001. 6. X. L. Wang, H. Wang, and X. Yao, J. Am. Ceram. Soc. 80, 2745 ,1997 7. S. W. Jiang,aĂY. R. Li, R. G. Li. Appl. Phys. Lett., 94, 162908ˈ˄ 2009˅ 8. Jiwei Lu and Susanne Stemmer. Appl. Phys. Lett., 83, 2411ˈ˄ 2003˅ 9. Alexander K. Tagantsev. Appl. Phys. Lett., 86, 032901ˈ˄ 2005˅ 10. R. L. Thayer, C. A. Randall, and S. Trolier-McKinstry. J. Appl. Phys. 94, 1941, 2003 Jiwei Lu, Dmitri O. Klenov, and Susanne Stemmer. Appl. Phys. Lett., 84, 957ˈ˄ 2004˅