Layered lead zirconate titanate and lanthanum-doped ...

Layered lead zirconate titanate and lanthanum-doped lead zirconate titanate ceramic thin films Todd Myers, Parag Banerjee, Susmita Bose, and Amit Bandyopadhyaya) School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164-2920 (Received 5 September 2001; accepted 17 June 2002)

The physical layering of sol-gel-derived lead zirconate titanate (PZT) 52/48 and lanthanum-doped PZT (PLZT) 2/52/48 on platinized silicon substrates was investigated to determine if the ferroelectric properties and fatigue resistance could be influenced by different layering sequences. Monolithic thin films of PZT and PLZT were characterized to determine their ferroelectric properties. Sandwich structures of Pt/PZT/PLZT/PLZT/PZT/Au and Pt/PLZT/PZT/PZT/PLZT/Au and alternating structures of Pt/PZT/PLZT/PZT/PLZT/Au and Pt/PLZT/PZT/PLZT/PZT/Au were then fabricated and characterized. X-ray photoelectron spectroscopy depth profiles revealed that the layering sequence remained intact up to 700 °C for 45 min. It was found that the end layers in the multilayered films had a significant influence on the resulting hysteresis behavior and fatigue resistance. A direct correlation of ferroelectric properties and fatigue resistance can be made between the data obtained from the sandwiched structures and their end-layer monolithic thin film counterparts. Alternating structures also showed an improvement in the fatigue resistance while the polarization values remained between those for PZT and PLZT thin films.

I. INTRODUCTION

Lead zirconate titanate [Pb(ZrxTi1−x)O3; PZT] based thin films have been studied extensively due to their piezoelectric and ferroelectric properties. PZT films have been implemented for use in nonvolatile memories, microelectromechanical systems (MEMS), sensors, and actuators. One important issue that has evolved from the early research into ferroelectric nonvolatile memories is ferroelectric fatigue of the film. Polarization degradation in PZT-based thin film compositions as a result of an applied cyclic electric field has been shown to be quite extensive. One of the reasons for this degradation is charge transport via movement of oxygen vacancies (VÖ) due to an applied field.1–4 With this premise, it would seem that an obvious way to improve ferroelectric fatigue is to reduce the concentration and mobility of oxygen vacancies in the film. The hypothesis behind our work is to accomplish this through donor dopants that release extra electrons in the film to counteract VÖ. Common A-site donor dopants for PZT that substitute for Pb2+ include lanthanum (La3+) and yttrium (Y3+), while common B-site donor dopants that

a)

Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 9, Sep 2002

substitute for Ti4+ and Zr4+ include niobium (Nb5+) and tantalum (Ta5+). Along with doping come a number of changes to the ceramic that include altered P-E characteristics and changes to the crystal lattice.5 Significant research has been conducted to improve fatigue resistance using alternative electrodes. The general mechanism involved with conducting oxide electrodes is the belief that the VÖ can easily interchange with oxygen ions from the electrode, thereby reducing VÖ pileup at the interface and improving fatigue resistance. This approach can be very effective in improving fatigue resistance, but it may lead to additional issues of processing structures with thin-film PZT. Oxide electrodes, such as RuO2, IrO2, and (La,Sr)CoO3 (LSCO), that offer low resistance, chemical inertness, and good adhesion to the ferroelectric material have been the most widely studied. Work on RuO2 electrodes by Al-Shareef et al. has shown a dependence of B-site cation stoichiometry on the ferroelectric fatigue of RuO2/PZT/RuO2 capacitors6 and the benefits to a pre-deposition and postdeposition anneal of the bottom electrode.7 In two separate studies, Lee et al.8 and Bursill et al.9 compared Pt/PZT/Pt and RuO2/PZT/RuO2 thin film capacitors and suggest that the improved fatigue properties of the RuO2 electrodes are due to the reduction in pileup of oxygen vacancies at the electrode/PZT interface. Research on LSCO as an electrode for PZT-based thin film capacitors © 2002 Materials Research Society

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has shown that it also has desirable electrical properties, good adhesion, chemical inertness to PZT, and the ability to also act as an oxygen diffusion barrier.10 In this study, the physical layering of PZT with 2 mol% lanthanum-doped PZT (PLZT) on a platinum bottom electrode was investigated to determine if the ferroelectric properties of the film can be manipulated by controlled doping with respect to the thickness of the film. We observed and it has also been reported that increasing lanthanum doping in PZT 52/48 increases fatigue resistance but lowers polarization values.11 By creating the layered structures of PZT and PLZT, perhaps the best of each material’s properties can be utilized: high polarizations and good fatigue resistance, respectively. Physical layering of films with different compositions is very compatible with current techniques in making films because multilayering is already a standard practice in sol-gel-derived thin film fabrication. Films of different composition have been layered before using sol-gel techniques, primarily to influence the structure of the film.12,13 Swartz et al. reported the use of lead titanate as an initial layer to create highly crystalline PLZT films.13 II. EXPERIMENTAL A. Precursor solution

For Pb(Zr0.52Ti0.48)O3 thin films, a 0.5 M metallorganic precursor solution was produced using methods similar to those of Budd et al.14 Stoichiometric amounts of lead acetate trihydrate, titanium iso-propoxide, and zirconium n-propoxide were used with 2-methoxyethonal (2-MOE) as the solvent. Lead in excess of 10 mol% was added to account for any lead volatization during sintering. Anhydrous lead acetate was first produced by the distillation of lead acetate trihydrate and 2-MOE at 120 °C for 1 h. Zirconium n-propoxide and titanium isopropoxide were then sequentially added along with additional solvent, each followed by a reflux at 120 °C for 2 h in an argon environment. A final distillation was performed to remove excess solvent and any reaction byproducts and to bring the solution to 0.5 M. PZT thin films doped with 2 mol% lanthanum [(Pb0.98La0.02)(Zr0.52Ti0.48)O3; PLZT], were produced in a similar fashion with the addition of a stoichiometric amount of lanthanum 2methoxyethoxide and excess solvent followed by the same refluxing conditions.

B. Film fabrication

Film deposition was accomplished via spin coating the precursor solution on to platinized silicon substrates. A photoresist spin coater was used at 4000 rpm for 12 s. A two-temperature drying cycle was performed after each layer was applied using hotplates at 150 and 350 °C, each for 5 min. After each drying cycle, the deposition was repeated for up to four layers. Sintering was performed in a muffle furnace at 700 °C for 10 min. Gold pads were sputtered on as top electrodes using a metal mask. Six different combinations of 4-layered films were made. Monolithic films of PZT and PLZT were first produced and extensively tested to determine the properties of these films. Sandwich structures of Pt/PZT/PLZT/PLZT/ PZT/Au (PLLP) and Pt/PLZT/PZT/PZT/PLZT/Au (LPPL) and alternating structures of Pt/PZT/PLZT/PZT/PLZT/ Au (PLPL) and Pt/PLZT/PZT/PLZT/PZT/Au (LPLP) were then fabricated on platinized silicon substrates to determine the influence of the physical structuring of the films. The shorthand notation for layered films (PLLP, LPPL, LPLP, and PLPL) are written with P representing PZT 52/48 and L representing PLZT 2/52/48. They are listed in order of deposition; therefore the first letter listed is in contact with the bottom Pt electrode and the last letter is in contact with the top gold electrode. Figure 1 shows a schematic diagram of the layered structures. C. Characterization of films

Film thickness was measured using a profilometer, and x-ray diffraction (XRD) was performed on the films to verify the perovskite phase. Atomic force microscope (AFM) images were also taken to measure the grain size in the films. A compositional depth profile was performed using x-ray photoelectron spectroscopy (XPS) on monolithic PLZT and layered films to verify the layering effect after sintering. An argon gun was used to sputter through the film, while compositional profiles were recorded at timed intervals. Sintering times were extended to 15 and 45 min at 70 °C for the physically layered structures to ensure that the diffusion of the La3+ ions did not homogenize the layered films and the layering sequence remained even after longer sintering times. Ferroelectric properties were characterized using an RT66-A (Radiant Technologies, Albuquerque, NM). Hysteresis loops were recorded at 20 V. The apparent polarization

FIG. 1. Schematic diagram of the layered structures (a) PLLP, (b) LPPL, (c) PLPL, and (d) LPLP. 2380

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relaxation seen in all of the P-E plots on the negative polarization axis can be attributed to measurement artifact from the RT66A. All of the hysteresis measurements were taken with a period of 8.272 ms with a 1-s delay between the preset loop (nonmeasurement loop) and the measurement loop. Fatigue testing was done by applying an alternating pulse charge at 15 V and 35 kHz on a minimum of three different films of each layered structure to compare the resulting properties.

III. RESULTS AND DISCUSSIONS

The final thickness of all films measured using a profilometer (Tencor/model Alpha Step 100) (SPN Technology, Goieta, CA) was 350 ± 10 nm. XRD investigations into the films revealed a (111) orientated Pt bottom electrode with polycrystalline perovskite PZT film. The strongest peak for the PZT was (110). No pyrochlore phase was present. From AFM images and the intercept method, grain size measurements indicated a 100 ± 15 nm grain size for all films. Figure 2 shows results of XPS depth profiles taken on three differently layered films. XPS is a surface-sensitive technique that uses x-rays to eject electrons from innershell orbitals. The emitted electrons are then collected in an energy analyzer where their kinetic energies are measured. Argon sputtering allows for compositional profiles to be taken at different depths. The amount of sputtering time varies between films because different-strength ion sputtering guns were used. The lanthanum concentration in the monolithic PLZT film [Fig. 2(a)] remained constant throughout the thickness of the film. This was expected because all four layers had the same concentration of lanthanum. In the layered structures, Figs. 2(b) and 2(c), there is an obvious lanthanum concentration profile showing the layering effect. The intended layering sequence remained intact up to 45 min at 700 °C, as can be seen in Fig. 2(b). In all three depth profiles, a large Pb concentration can be seen on the surface, indicating lead volatization and justifying the excess Pb added to the precursor solution. Figure 3 shows hysteresis loops obtained on monolithic PZT and PLZT thin films. The drop in polarization and coercive field of the film due to the lanthanum doping is clearly visible. Figure 4 is a fatigue plot showing normalized remnant polarization of the monolithic films after being electrically cycled by an alternating pulse charge up to 109 cycles at 15 V and 35 kHz. The doping of lanthanum shows an increase in polarization degradation resistance from 85% to 95% of the original saturation and remnant polarizations. Table I gives a statistical breakdown with the averages, standard deviations, and range of polarizations along with the fatigue results.

FIG. 2. XPS depth profile of lead and lanthanum on (a) 4-layer PLZT 2/52/48 film sintered at 700 °C for 10 min, (b) Pt/PLZT/PZT/ PZT/PLZT (LPPL) film sintered at 700 °C for 45 min, and (c) Pt/PZT/PLZT/PZT/PLZT (PLPL) film sintered at 700 °C for 15 min.

FIG. 3. Hysteresis loops of PZT 52/48 and PLZT 2/52/48. Measurements were recorded with a maximum voltage of 20 V.


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Lanthanum doping in bulk PZT leads to the generation of excess electrons, which tend to suppress oxygen vacancy formation (VÖ).15 2La

2Pb → 2La˙Pb + 2e ¨ + 2e → null VO

(1)

,

polarization versus cycles. Table II lists average polarization values, standard deviations, ranges, and fatigue behavior of the measurements recorded. The data from Tables I and II show a definite correlation between the sandwiched layered structures and their end layer monolithic counterparts. Polarization

(2)

.

This mechanism agrees with the fatigue data reported here assuming that VÖ diffusion is the main reason for polarization degradation under an alternating electric field. The presence of La3+ doping in the film improved ferroelectric fatigue resistance. La3+ doping in bulk PZT can also cause a decrease in domain-stabilizing effects due to the reduction in concentration of VÖ leading to improved domain mobility and lower coercive fields. Recently published data agrees with our findings in that polarization values decreased with La3+ doping in sol-gel derived thin films.11 Possible explanations provided by the authors include stresses in the film due to mismatches of thermal expansion coefficients between bottom electrode and film and a decrease in Curie temperature. Figure 5 shows hysteresis loops for the layered structures of LPPL and PLLP before and after electrical fatigue by an alternating pulse charge at 15 V and 35 kHz for 109 cycles. The LPPL structure shows no degradation in the polarization values and only a slight increase in coercive field, while the PLLP structure shows an 80% drop in polarization saturation and remnant polarization along with a slight increase in coercive field. Figure 6 illustrates this drop in remnant polarization by plotting

FIG. 5. Hysteresis loops of (a) LPPL (Pt/PLZT/PZT/PZT/PLZT/Au) and (b) PLLP (Pt/PZT/PLZT/PLZT/PZT/Au) initially and after being fatigued at 15 V and 35 kHz for 109 cycles.

FIG. 4. Fatigue plot of PZT 52/48 and PLZT 2/52/48 showing normalized polarization versus switching cycles. Cycling performed at 15 V and 35 kHz.

FIG. 6. Fatigue plot of PLLP and LPPL showing the normalized polarization versus switching cycles. Cycling performed at 15 V and 35 kHz.

TABLE I. Polarization values for PZT 52/48 and PLZT 2/52/48 along with the standard deviation and range of measurements based on a minimum of 50 measurements from five different films. Fatigue data is after 109 cycles at 15 V and 35 kHz. Psat ␮C/cm2

Prem ␮C/cm2

Fatigue (109 cycles)

Film

Average

␴

Range

Average

␴

Range

Psat

Prem

PZT 52/48 PLZT 2/52/48

53 27

3.3 4

46–58 22–34

30 10

3 2.9

25–34 7–17

80–85% 94–96%

82–87% 96–100%

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TABLE II. Polarization values for PLLP and LPPL long with the standard deviation and range of measurements based on a minimum of 50 measurements from five different films. Polarization degradation after 109 cycles at 15 V and 35 kHz is also listed. Psat ␮C/cm2

Prem ␮C/cm2


Film

Average

␴

Range

Average

␴

Range

Psat

Prem

PLLP LPPL

50 30

4.1 4.6

40–57 24–37

31 21

4.7 3.5

17–35 16–26

80–90% 90–98%

80–90% 90–99%

values for monolithic PZT and PLLP are very comparable, as is their fatigue behavior. The relationship between monolithic PLZT and LPPL is similar. Again, polarization values are nearly identical, statistically speaking, as are their fatigue behaviors. From these results, one can generalize that end layers in multilayer films have significant influence on ferroelectric properties and their fatigue behavior. Direct current resistance measurements mirrored this relation with monolithic PLZT and LPPL films having a higher resistivity than monolithic PZT and PLLP films. The excellent fatigue properties of the sandwich structure LPPL can be attributed to the lanthanum concentration in the film at the electrode contacts. Acting as oxygen vacancy sinks, the electrode/film interface was preserved for an extended time under cyclic electrical fatigue. Figure 7 illustrates this mechanism by placing an excess of electrons due to the lanthanum doping at the electrode film interfaces.8,9 This correlates very well with previous work done using oxide electrodes to place VÖ barriers/sinks at the electrode/film interface. If the VÖ sink is introduced on the thin film side of the interface, processing difficulties encountered with conductive oxides can be avoided. The structure PLLP also showed fatigue properties that appeared to be dictated by the end layers. Polarization degradation in the 80% to 85% range was typical of both monolithic PZT and PLLP, although some PLLP reached polarization values as high as 90% after fatigue. PLLP showed excellent polarization properties, values nearly identical to that of monolithic PZT. This leads to the generalization that the electrode/PZT interface has significant influence on the ferroelectric properties of thin films. The lower polarization values of PLZT and LPPL offer additional evidence. The film in the middle of the sandwich structures performs as its monolithic counterpart but does not have as much influence on the capacitors performance because it is not in contact with the electrodes. Figure 8 shows the resulting hysteresis loops for alternating structures of monolithic PLZT and PZT films. Plots are again presented showing the hysteresis loops before and after exposure to an alternating pulse charge to 109 cycles at 15 V and 35 kHz. PLPL (PZT deposited first) in Fig. 8(a) shows a very narrow loop with a small decrease in polarizations after fatiguing. LPLP (PLZT

FIG. 7. Schematic diagram of the oxygen vacancy sink introduced by the layering sequence Pt/PLZT/PZT/PZT/PLZT/Au.

FIG. 8. Hysteresis loops of (a) PLPL (Pt/PZT/PLZT/PZT/PLZT/Au) and (b) LPLP (Pt/PLZT/PZT/PLZT/PZT/Au) initially and after being fatigued at 15 V and 35 kHz for 109 cycles.

deposited first) in Fig. 8(b) shows an improvement in ferroelectric properties after 109 cycles. Ferroelectric properties of these films are summarized in Table III, giving the average polarization values, standard deviations, ranges, and fatigue behavior of the measurements recorded. Hysteresis loops recorded from the alternating structures show polarization values that fall between the values of monolithic PZT and PLZT films. This is consistent


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TABLE III. Polarization values for PLPL and LPLP along with the standard deviation and range of measurements based on a minimum of 20 measurements from three different films. Fatigue data is after 109 cycles at 15 V and 35 kHz. Psat ␮C/cm2

Prem ␮C/cm2


Film

Average

␴

Range

Average

␴

Range

Psat

Prem

PLPL LPLP

33 39

1.2 3.8

31–35 32–43

15 23

1.3 3.8

13–18 17–28

90–93% +100%

85% +100%

FIG. 9. Fatigue plot of PLPL and LPLP showing the normalized polarization versus switching cycles. Cycling performed at 15 V and 35 kHz.

with the sandwich structures data indicating the influence of the end layers on ferroelectric properties. The coercive fields of the alternating films varied drastically, with PLPL characterized by narrow loops with low remnant polarization, while LPLP gave wide loops with much higher remnant polarizations. Fatigue resistance in the layered films also varied with PLPL films, giving improved properties over monolithic PZT, but not as dramatic as the improvement in the sandwich structure LPPL. The LPLP film actually showed improved ferroelectric properties on all films tested. The plots of polarization versus cycles for all LPLP films were very similar to the plot shown in Fig. 9, a sharp increase (greater than 10%) from the initial reading to the second reading, followed by random increases and decreases in values during the fatiguing. The results presented here support previous work on PZT thin film capacitors in that the electrode/film interface plays a significant role in the ferroelectric properties and fatigue resistance.16 Alternate electrodes offer the ability to introduce a VÖ sink/barrier at the electrode/film interface and the benefit of not having to alter the PZT films composition. However, these electrodes can add additional processing steps, affect crystal structure, and can limit applicable compositions.6,7 By introducing the VÖ sink/barrier on the thin film side of the interface to improve fatigue resistance, oxide electrodes can be avoided, but polarization values will also be affected. A comparison of the ferroelectric response between the Pt/ LPPL/Au films and RuO2/PZT/RuO2 structures indicates that the oxide electrode structures have slightly better fatigue resistance and polarization values. Often, PZT 2384

shows no ferroelectric degradation up to 1010 cycles and polarizations of 40 ␮C/cm2 at low applied electric fields on RuO2 electrodes.7 The Pt/LPPL/Au structure cited in this work had a lower polarization and a small amount of ferroelectric degradation at 109 cycles. The individual thickness of the layers in a 4-layer film shows only a small impact on the ferroelectric response of the capacitor. In some of our initial tests with 4-layer films and different molarity solutions, we found that the results were similar, showing end layer dependence. Due to the change in molarity, we were able to change the film thickness while keeping the grain size relatively constant. An exception—the end-layer dependence— would be expected if the end layer were too thin (spun at a very high rpm with very low molarity). At higher applied fields, the charge transport may be higher and the layering effect may diminish. We would also expect that, as the film increases in thickness (more than four layers), it would begin to behave as monolithic film and exhibit less of a dependence on the end layers. This would be similar to the effect seen in thick films, which behave like bulk PZT. The end-layer dependence on ferroelectric properties and ferroelectric fatigue resistance observed in this study can be applicable to other systems involving PZT-based thin films. The results of the PZT/PLZT layering suggest that ferroelectric properties of sol-gel-derived thin films can be controlled by layers deposited first and last during fabrication. The most-studied layering sequences in our laboratory were PLZT and PZT thin films which involve using an A-site donor dopant to introduce additional electrons to act as VÖ sink. These VÖ sinks were most effective in improving fatigue resistance when placed in contact with one or more electrodes (monolithic PLZT, LPPL, PLPL, and LPLP) but tended to lower polarization values. We have also done work with the B-site donor dopant niobium and fabricated layered structures of Nb5+ doped PZT (PNZT) and PZT. Similar end-layer dependence has been observed for both ferroelectric properties and fatigue resistance as reported here with the PZT/ PLZT system. IV. CONCLUSION

The ferroelectric properties of thin-film PZT 52/48 and PLZT 2/52/48 were characterized as monolithic films as well as in physically layered structures of the



two. Doping PZT with 2 mol% lanthanum caused the polarizations of the resulting monolithic 4-layer films to drop almost 50%, while the fatigue resistance was increased a minimum of 10%. An XPS depth profile verified that physical layering of PZT and PZTdoped thin films was possible, and the structures remained compositionally layered up to 700 °C for 45 min. Physically layered sandwich structures showed an end layer dependence on both polarization values and ferroelectric fatigue. These findings support other work by introducing an oxygen vacancy sink at the electrode/ film interface to improve fatigue resistance. Alternating layered structures of PZT and PLZT gave polarization values that mediated between their monolithic counterparts. These films also showed good fatigue resistance. ACKNOWLEDGMENTS

The authors would like to acknowledge financial support from the Washington Technology Center, Seattle, WA for this research. The authors would also like to acknowledge helpful discussions with Dr. John Fraser of Philips, Bothell, WA.

2. W.L. Warren, D. Dimos, and R.M. Waser, MRS Bull. 21, 40 (1996) 3. D. Dimos, W.L. Warren, and H.N. Al-Shareef, in Thin Film Ferroelectric Materials and Devices, edited by R. Ramesh (Kluwer Academic Press, London, United Kingdom, 1997), p. 199. 4. C.K. Kwok and S.B. Desu, in Ferroelectric Thin Films II, edited by A.I. Kingon, E.R. Myers, and B. Tuttle (Mater. Res. Soc. Symp. Proc. 243, Pittsburgh, PA, 1992), p. 393. 5. G.H. Haertling, J. Am. Ceram. Soc. 82, 797 (1999). 6. H.N. Al-Shareef, B.A. Tuttle, W.L. Warren, T.J. Headley, D. Dimos, J.A. Voigt, and R.D. Nasby, J. Appl. Phys. 79, 1013 (1996). 7. H.N. Al-Shareef, K.R. Bellur, O. Auciello, and A.I. Kingon, Thin Solid Films 256, 73 (1995). 8. J.J. Lee, C.L. Thio, and S.B. Desu, J. Appl. Phys. 78, 5073 (1995). 9. L.A. Bursill, I.M. Reaney, D.P. Vijay, and S.B. Desu, J. Appl. Phys. 75, 1521 (1994). 10. S. Aggarwal, B. Yang, and R. Ramesh, in Thin Film Ferroelectric Materials and Devices, edited by R. Ramesh (Kluwer Academic Press, London, United Kingdom, 1997), p. 221. 11. M. Es-Souni, M. Abed, A. Piorra, S. Malinowski, and V. Zaporojtchenko, Thin Solid Films 389, 99 (2001). 12. S.G. Lee, K.T. Kim, and Y.H. Lee, Thin Solid Films 372, 45 (2000). 13. S.L. Swartz, S.J. Bright, P.J. Melling, and T.R. Shrout, Ferroelectrics 108, 71 (1990). 14. K.D. Budd, S.K. Dey, and D.A. Payne, Br. Ceram. Proc. 36, 107 (1985). 15. A.J. Moulson and J.M. Herbert, Electroceramics (Chapman & Hall, London, United Kingdom, 1990), pp. 280–282. 16. H.N. Al-Shareef, A.I. Kingon, X. Chen, K.R. Bellur, and O. Auciello, J. Mater. Res. 9, 2968 (1994).

REFERENCES 1. W.L. Warren, B.A. Tuttle, and D. Dimos, Appl. Phys. Lett. 67, 1426 (1995).


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