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Pulsed laser deposition of rhombohedral (Ba,Sr)TiO3 thin films on LiNbO3 substrates
Applied Surface Science 223 (2004) 275–278
Short communication
Pulsed laser deposition of rhombohedral (Ba,Sr)TiO3 thin films on LiNbO3 substrates D.M. Bubba,b,*, S.B. Qadrib, J.S. Horwitzb, D. Kniesb, D.M. Potrepkac a
Department of Physics, Seton Hall University, 400 South Orange Avenue, South Orange, NJ 07079, USA b Naval Research Laboratory, Washington, DC 20375, USA c Army Research Laboratory, Adelphi, MD 20783, USA Received 9 August 2003; received in revised form 16 September 2003; accepted 16 September 2003
Abstract (BaxSr1x)TiO3 (BST) thin films have been deposited on z-cut LiNbO3 (LNO) substrates using pulsed laser deposition (PLD). Both the structure and electrical properties of the film have been characterized. The structure of the film, determined by X-ray diffraction (XRD), is neither cubic nor tetragonal as exhibited in the bulk material. The film structure is rhombohedral, and exhibits a phase transition at a temperature that is similar to the corresponding Tc of the bulk ferroelectric material. This is the first report of a rhombohedral phase for BST. # 2003 Elsevier B.V. All rights reserved. PACS: 68.55.Nq; 77.80.-e; 81.15.Fg Keywords: Ferroelectric thin films; Pulsed laser deposition
1. Introduction The large electric field induced change in dielectric constant of (BaxSr1x)TiO3 (BST; 0 < x < 1), is currently being used to develop a new class low of loss, tunable microwave devices, such as tunable oscillators, delay lines and phase shifters. These devices will reduce the size and the operating power of the current semiconducting and ferrite based devices, and will soon have a significant impact on both radar and wireless communication systems. An issue in the fabrication of these devices is optimizing the processing of the material such that it exhibits a large change in the dielectric *
Corresponding author. Tel.: þ1-9737619058; fax: þ1-9737619772. E-mail address: [email protected] (D.M. Bubb).
constant with an applied bias while minimizing the dielectric loss at microwave frequencies. The dielectric properties of thin films are affected by many factors, such as Ba/Sr ratio, grain size, defect chemistry, oxygen vacancies, interfacial strain, and dopants. For BST films deposited on cubic substrates, such as MgO and LaAlO3, a strong relationship between film structure and dielectric properties has been reported. As-deposited (Ba,Sr)TiO3 films with minimal stress, grown in a relatively low pressure of O2, exhibited the highest microwave figure of merit (K ¼ %tuning Q0 V ) [1]. LiNbO3 (LNO) is an important optoelectronic material which exhibits a large piezoelectric effect. Since interfacial strain plays such a significant role in the microwave dielectric properties of BST of oriented BST films, it may be possible in the case of LNO to
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.09.024
276
D.M. Bubb et al. / Applied Surface Science 223 (2004) 275–278
use the substrate to correct for, or induce strain in the BST film and further enhance the dielectric properties of the film for the tunable microwave application. In addition, some truly novel devices could be fabricated through the use of thin film hetero-structures. We have used pulsed laser deposition (PLD) to investigate the properties of Ba0.6Sr0.4TiO3 films on z-cut LNO substrates which have rhombohedral symmetry. The in-plane symmetry of the z-cut substrate is hexagonal. Characterization of the film structure was accomplished through the use of X-ray diffraction (XRD). Film composition was determined from Rutherford backscattering (RBS). The dielectric properties were measured from gap capacitors deposited on top of the BST films. We observe, for the first time, a novel rhombohedral structure in the BST thin film which exhibits a phase transition at a temperature that is similar to the Tc of the corresponding bulk ferroelectric material.
2. Results and discussion PLD has been successful in stabilizing metastable phases in materials, as in the case of cubic and superconducting NbN [2]. Here, a primitive cubic phase was observed for the first time in PLD grown films on (1 0 0) MgO substrates. The phase was demonstrated to be metastable, and was presumably stabilized by the lattice match with the MgO substrate. Bulk BST is characterized as being either cubic (paraelectric) or tetragonal (ferroelectric). Similar structures are observed in thin films on both cubic (MgO and LaAlO3) or hexagonal (c-Al2O3) substrates. For BST on LNO, we have stabilized a rhombohedral structure and this has not been reported before. BaTiO3 has a low temperature rhombohedral phase [3] and a high temperature hexagonal phase [4]. The BST films were grown via PLD, utilizing a KrF excimer laser (248 nm) that was focused to reach a fluence of 2 J/cm2 and incident upon a rotating target of BST. Films were deposited in a partial pressure of 6.7 Pa O2 gas and the substrate temperature was 710 8C. At substrate temperatures >710 8C, we saw clear evidence of chemical interaction between the substrate and film. The deposited films were measured ˚ thick by profilometry on a stepto be 3400–3500 A edge created by a shadow mask. The deposition rate
Table 1 Structural parameters for films and substrates as determined by XRD 2y values (8)
(h k l)
LNO
BST
34.798 62.398 83.631 93.046 128.844
32.644 56.90 81.344 90.164 129.859
(1 1 0) (3 0 0) (0 0 12) (4 0 4) (1 0 16)
for a number of films deposited with 5000, 10,000 shots, or 20,000 shots was determined to be 0.34– ˚ per shot with consistency for both thick 0.35 A ˚ ) and thin films (<2000 A ˚ ). (>5000 A Films were characterized by for structure and composition using XRD and RBS. For the XRD measurements, a Huber 4-circle diffractometer using Cu Ka1 radiation in triple axis geometry was used. The monochromator and analyzer are very high quality Ge(1, 1, 1) crystals. In Table 1, a summary of the XRD results is shown. To the left of every substrate peak, we find a film peak. The combination of in-plane and out-ofplane measurements has led us conclude that the film is registering epitaxially with the substrate and is highly oriented in the space group R3c. There were no other peaks in the XRD spectrum. The calculated ˚, lattice parameters for this phase are a ¼ 5:486 A ˚ —relative to a hexagonal P63/mmc cell c ¼ 14:182 A for convenience. We report the lattice parameters in this fashion in order to facilitate comparison with the ˚, P63/mmc high temperature phase (a ¼ 5:735 A ˚ ) [5]. Also in Fig. 1, we display several c ¼ 14:05 A important diffraction peaks to expand on the results displayed in Table 1. RBS measurements were also performed upon a representative film. The Ba/Sr, Ba/Ti, and Sr/Ti ratios were found to be in good agreement, to within 2%, of the target composition. The film thickness may also be estimated using RBS and we determined a value of ˚ for a film deposited with 10,000 laser shots in 3400 A excellent agreement with our profilometry measurements. In order to evaluate the microwave dielectric response, interdigitated capacitors were deposited upon the film by photolithography. The 0.5 mm Au electrodes were deposited on top of a thin oxide layer
D.M. Bubb et al. / Applied Surface Science 223 (2004) 275–278
LNO (0 0 12)
277
LNO (0 0 3)
100000 1000 10000
BST (0 0 3) BST (0 0 12)
1000
100
100
Counts (a. u.)
10 10
1
1 75
78
81
84
87
90
30
32
34
36
38
θ (deg)
LNO (2 2 0)
100000
LNO (4 0 4)
10000
BST (2 2 0)
10000 1000
BST (4 0 4)
1000
100
100
10
10 86
88
90
92
94
96
98
60
62
64
70
72
74
76
θ (deg) Fig. 1. Selected X-ray diffraction peaks.
Ba0.6Sr0.4TiO3/LiNbO3 2.0
LiNbO3
0.14 0.12
Capacitance (pf)
1.5
0.10
D C[pF]
0.08 0.06
1.0
0.04 0.02 0.00 -0.02
0.5
-80 -60 -40 -20
0
20
40
60
80 100 120
Temperature (Celsius) 0.0
Difference -80
-60
-40
-20
0
20
40
60
80
100
120
Temperature (Celsius) Fig. 2. Capacitance of BST films as a function of temperature at a frequency of 1 MHz. The inset shows the difference in capacitance between the substrate and substrate–film combination which we assign to the film. There is a very broad transition centered at around 20 8C.
278
D.M. Bubb et al. / Applied Surface Science 223 (2004) 275–278
with a finger spacing of 6 mm. Capacitors were also deposited on a bare patch of substrate in order to verify that the response to applied fields was due to the film only. The response is measured using a HP 8510 network analyzer at 2 GHz. The tuning, De/e, is approximately 8%. The losses are high with Q (1/ tan d) on the order 10–15. These measurements indicate that the dielectric properties of the film and substrate are strongly coupled. There are several possible schemes in order to minimize this coupling and will be the subject of future work. The dielectric response was also measured using interdigitated capacitor structures which were deposited onto the BST film as a function of temperature at 1 MHz. In order to guard against effects arising out of the substrate, the capacitance was measured for the film/substrate combination and the substrate alone. These two were then subtracted in order to give the response of the film alone. This is displayed in the inset of Fig. 2. Clear evidence of a phase transition appears at about 20 8C (253 K). This is similar to, but slightly lower than values reported for bulk material [6]. The dielectric constant is then computed to be 165. This value is much smaller than is found in bulk material, however, it is consistent with what has been previously observed for highly strained films. 3. Conclusion (BaxSr1x)TiO3 (BST) thin films have been deposited on z-cut LiNbO3 substrates using pulsed laser
deposition (PLD). The combination of the ferroelectric thin film and a piezoelectric substrate offer additional degrees of freedom in tailoring the microwave dielectric properties of the materials for a tunable microwave device application. For the first time, a new phase of BST is observed with a rhombohedral structure. The new structure exhibits a phase transition as a function of temperature with a transition temperature that is similar to the Tc of the bulk ferroelectric material.
Acknowledgements We gratefully acknowledge W. Chang for measuring the dielectric properties at 2 GHz. DMB also gratefully acknowledges Dr. J.M. Joseph for useful discussions.
References [1] W. Chang, S.W. Kirchoefer, J.M. Pond, J.S. Horwitz, L. Sengupta, J. Appl. Phys. 92 (2002) 1528, and references therein. [2] R.E. Treece, M.S. Osofsky, E.F. Skelton, S.B. Qadri, J.S. Horwitz, D.B. Chrisey, Phys. Rev. B 51 (1995) 9356. [3] G.H. Kwei, A.C. Lawson, S.J.L. Billinge, S.W. Cheong, J. Phys. Chem. 97 (1997) 2368. [4] J. Iniguez, A. Garcia, J.M. Perez-Mato, Ferroelectrics 237 (2000) 25, and references therein. [5] JCPDS 08-0372. [6] H.D. Wu, F.S. Barnes, Int. Ferrolectr. 22 (1998) 291.
Short communication
Pulsed laser deposition of rhombohedral (Ba,Sr)TiO3 thin films on LiNbO3 substrates D.M. Bubba,b,*, S.B. Qadrib, J.S. Horwitzb, D. Kniesb, D.M. Potrepkac a
Department of Physics, Seton Hall University, 400 South Orange Avenue, South Orange, NJ 07079, USA b Naval Research Laboratory, Washington, DC 20375, USA c Army Research Laboratory, Adelphi, MD 20783, USA Received 9 August 2003; received in revised form 16 September 2003; accepted 16 September 2003
Abstract (BaxSr1x)TiO3 (BST) thin films have been deposited on z-cut LiNbO3 (LNO) substrates using pulsed laser deposition (PLD). Both the structure and electrical properties of the film have been characterized. The structure of the film, determined by X-ray diffraction (XRD), is neither cubic nor tetragonal as exhibited in the bulk material. The film structure is rhombohedral, and exhibits a phase transition at a temperature that is similar to the corresponding Tc of the bulk ferroelectric material. This is the first report of a rhombohedral phase for BST. # 2003 Elsevier B.V. All rights reserved. PACS: 68.55.Nq; 77.80.-e; 81.15.Fg Keywords: Ferroelectric thin films; Pulsed laser deposition
1. Introduction The large electric field induced change in dielectric constant of (BaxSr1x)TiO3 (BST; 0 < x < 1), is currently being used to develop a new class low of loss, tunable microwave devices, such as tunable oscillators, delay lines and phase shifters. These devices will reduce the size and the operating power of the current semiconducting and ferrite based devices, and will soon have a significant impact on both radar and wireless communication systems. An issue in the fabrication of these devices is optimizing the processing of the material such that it exhibits a large change in the dielectric *
Corresponding author. Tel.: þ1-9737619058; fax: þ1-9737619772. E-mail address: [email protected] (D.M. Bubb).
constant with an applied bias while minimizing the dielectric loss at microwave frequencies. The dielectric properties of thin films are affected by many factors, such as Ba/Sr ratio, grain size, defect chemistry, oxygen vacancies, interfacial strain, and dopants. For BST films deposited on cubic substrates, such as MgO and LaAlO3, a strong relationship between film structure and dielectric properties has been reported. As-deposited (Ba,Sr)TiO3 films with minimal stress, grown in a relatively low pressure of O2, exhibited the highest microwave figure of merit (K ¼ %tuning Q0 V ) [1]. LiNbO3 (LNO) is an important optoelectronic material which exhibits a large piezoelectric effect. Since interfacial strain plays such a significant role in the microwave dielectric properties of BST of oriented BST films, it may be possible in the case of LNO to
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.09.024
276
D.M. Bubb et al. / Applied Surface Science 223 (2004) 275–278
use the substrate to correct for, or induce strain in the BST film and further enhance the dielectric properties of the film for the tunable microwave application. In addition, some truly novel devices could be fabricated through the use of thin film hetero-structures. We have used pulsed laser deposition (PLD) to investigate the properties of Ba0.6Sr0.4TiO3 films on z-cut LNO substrates which have rhombohedral symmetry. The in-plane symmetry of the z-cut substrate is hexagonal. Characterization of the film structure was accomplished through the use of X-ray diffraction (XRD). Film composition was determined from Rutherford backscattering (RBS). The dielectric properties were measured from gap capacitors deposited on top of the BST films. We observe, for the first time, a novel rhombohedral structure in the BST thin film which exhibits a phase transition at a temperature that is similar to the Tc of the corresponding bulk ferroelectric material.
2. Results and discussion PLD has been successful in stabilizing metastable phases in materials, as in the case of cubic and superconducting NbN [2]. Here, a primitive cubic phase was observed for the first time in PLD grown films on (1 0 0) MgO substrates. The phase was demonstrated to be metastable, and was presumably stabilized by the lattice match with the MgO substrate. Bulk BST is characterized as being either cubic (paraelectric) or tetragonal (ferroelectric). Similar structures are observed in thin films on both cubic (MgO and LaAlO3) or hexagonal (c-Al2O3) substrates. For BST on LNO, we have stabilized a rhombohedral structure and this has not been reported before. BaTiO3 has a low temperature rhombohedral phase [3] and a high temperature hexagonal phase [4]. The BST films were grown via PLD, utilizing a KrF excimer laser (248 nm) that was focused to reach a fluence of 2 J/cm2 and incident upon a rotating target of BST. Films were deposited in a partial pressure of 6.7 Pa O2 gas and the substrate temperature was 710 8C. At substrate temperatures >710 8C, we saw clear evidence of chemical interaction between the substrate and film. The deposited films were measured ˚ thick by profilometry on a stepto be 3400–3500 A edge created by a shadow mask. The deposition rate
Table 1 Structural parameters for films and substrates as determined by XRD 2y values (8)
(h k l)
LNO
BST
34.798 62.398 83.631 93.046 128.844
32.644 56.90 81.344 90.164 129.859
(1 1 0) (3 0 0) (0 0 12) (4 0 4) (1 0 16)
for a number of films deposited with 5000, 10,000 shots, or 20,000 shots was determined to be 0.34– ˚ per shot with consistency for both thick 0.35 A ˚ ) and thin films (<2000 A ˚ ). (>5000 A Films were characterized by for structure and composition using XRD and RBS. For the XRD measurements, a Huber 4-circle diffractometer using Cu Ka1 radiation in triple axis geometry was used. The monochromator and analyzer are very high quality Ge(1, 1, 1) crystals. In Table 1, a summary of the XRD results is shown. To the left of every substrate peak, we find a film peak. The combination of in-plane and out-ofplane measurements has led us conclude that the film is registering epitaxially with the substrate and is highly oriented in the space group R3c. There were no other peaks in the XRD spectrum. The calculated ˚, lattice parameters for this phase are a ¼ 5:486 A ˚ —relative to a hexagonal P63/mmc cell c ¼ 14:182 A for convenience. We report the lattice parameters in this fashion in order to facilitate comparison with the ˚, P63/mmc high temperature phase (a ¼ 5:735 A ˚ ) [5]. Also in Fig. 1, we display several c ¼ 14:05 A important diffraction peaks to expand on the results displayed in Table 1. RBS measurements were also performed upon a representative film. The Ba/Sr, Ba/Ti, and Sr/Ti ratios were found to be in good agreement, to within 2%, of the target composition. The film thickness may also be estimated using RBS and we determined a value of ˚ for a film deposited with 10,000 laser shots in 3400 A excellent agreement with our profilometry measurements. In order to evaluate the microwave dielectric response, interdigitated capacitors were deposited upon the film by photolithography. The 0.5 mm Au electrodes were deposited on top of a thin oxide layer
D.M. Bubb et al. / Applied Surface Science 223 (2004) 275–278
LNO (0 0 12)
277
LNO (0 0 3)
100000 1000 10000
BST (0 0 3) BST (0 0 12)
1000
100
100
Counts (a. u.)
10 10
1
1 75
78
81
84
87
90
30
32
34
36
38
θ (deg)
LNO (2 2 0)
100000
LNO (4 0 4)
10000
BST (2 2 0)
10000 1000
BST (4 0 4)
1000
100
100
10
10 86
88
90
92
94
96
98
60
62
64
70
72
74
76
θ (deg) Fig. 1. Selected X-ray diffraction peaks.
Ba0.6Sr0.4TiO3/LiNbO3 2.0
LiNbO3
0.14 0.12
Capacitance (pf)
1.5
0.10
D C[pF]
0.08 0.06
1.0
0.04 0.02 0.00 -0.02
0.5
-80 -60 -40 -20
0
20
40
60
80 100 120
Temperature (Celsius) 0.0
Difference -80
-60
-40
-20
0
20
40
60
80
100
120
Temperature (Celsius) Fig. 2. Capacitance of BST films as a function of temperature at a frequency of 1 MHz. The inset shows the difference in capacitance between the substrate and substrate–film combination which we assign to the film. There is a very broad transition centered at around 20 8C.
278
D.M. Bubb et al. / Applied Surface Science 223 (2004) 275–278
with a finger spacing of 6 mm. Capacitors were also deposited on a bare patch of substrate in order to verify that the response to applied fields was due to the film only. The response is measured using a HP 8510 network analyzer at 2 GHz. The tuning, De/e, is approximately 8%. The losses are high with Q (1/ tan d) on the order 10–15. These measurements indicate that the dielectric properties of the film and substrate are strongly coupled. There are several possible schemes in order to minimize this coupling and will be the subject of future work. The dielectric response was also measured using interdigitated capacitor structures which were deposited onto the BST film as a function of temperature at 1 MHz. In order to guard against effects arising out of the substrate, the capacitance was measured for the film/substrate combination and the substrate alone. These two were then subtracted in order to give the response of the film alone. This is displayed in the inset of Fig. 2. Clear evidence of a phase transition appears at about 20 8C (253 K). This is similar to, but slightly lower than values reported for bulk material [6]. The dielectric constant is then computed to be 165. This value is much smaller than is found in bulk material, however, it is consistent with what has been previously observed for highly strained films. 3. Conclusion (BaxSr1x)TiO3 (BST) thin films have been deposited on z-cut LiNbO3 substrates using pulsed laser
deposition (PLD). The combination of the ferroelectric thin film and a piezoelectric substrate offer additional degrees of freedom in tailoring the microwave dielectric properties of the materials for a tunable microwave device application. For the first time, a new phase of BST is observed with a rhombohedral structure. The new structure exhibits a phase transition as a function of temperature with a transition temperature that is similar to the Tc of the bulk ferroelectric material.
Acknowledgements We gratefully acknowledge W. Chang for measuring the dielectric properties at 2 GHz. DMB also gratefully acknowledges Dr. J.M. Joseph for useful discussions.
References [1] W. Chang, S.W. Kirchoefer, J.M. Pond, J.S. Horwitz, L. Sengupta, J. Appl. Phys. 92 (2002) 1528, and references therein. [2] R.E. Treece, M.S. Osofsky, E.F. Skelton, S.B. Qadri, J.S. Horwitz, D.B. Chrisey, Phys. Rev. B 51 (1995) 9356. [3] G.H. Kwei, A.C. Lawson, S.J.L. Billinge, S.W. Cheong, J. Phys. Chem. 97 (1997) 2368. [4] J. Iniguez, A. Garcia, J.M. Perez-Mato, Ferroelectrics 237 (2000) 25, and references therein. [5] JCPDS 08-0372. [6] H.D. Wu, F.S. Barnes, Int. Ferrolectr. 22 (1998) 291.