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Effect of annealing conditions on the leakage current characteristics of ferroelectric PZT thin films grown by sol–gel process
Thin Solid Films 338 (1999) 149±154
Effect of annealing conditions on the leakage current characteristics of ferroelectric PZT thin ®lms grown by sol±gel process Seong Moon Cho 1,*, Duk Young Jeon Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-Gu, Taejon 305-701, South Korea Received 12 February 1998; accepted 7 August 1998
Abstract The microstructure and leakage current of lead zirconate titanate (PZT) thin ®lms were investigated to understand the mechanism of the leakage current and also to attempt to improve the leakage characteristics. The PZT thin ®lms were prepared using a sol±gel process followed by crystallization in three different gas ambients, i.e. N2, air and O2. The three kinds of crystallized samples show different microstructure and orientation. Electrical characterization of the PZT thin ®lms including the permittivity, P±E hysteresis, and the leakage current was carried out. It was found that the leakage current was affected not only by the microstructures of the ®lms but also by the interface between the Pt electrode and the PZT ®lm. Moreover, it was found that more than one conduction mechanism is involved in the range of electric ®eld used in the experiment. In the low electric ®eld region the current conduction is ohmic and its mechanism is found to be electron hopping among electron traps. In the high ®eld region the sample treated in N2 shows Schottky emission, the sample treated in O2 shows space charge limited conduction, and the sample treated in air shows both Schottky and space charge conduction together. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Sol±gel process; PZT; Annealing; Electrical properties and measurements
1. Introduction Ferroelectric lead zirconate titanate (PZT) is a well known functional material which has a great deal of potential as a non-volatile memory device [1,2]. It is expected that PZT will improve the limitations in storage density encountered in conventional Si memory technology. A variety of techniques has been proposed to fabricate ferroelectric PZT thin ®lms such as metalorganic chemical vapor deposition (MOCVD) [3], sputtering [4], sol±gel process [5] and laser ablation [6]. At present, however, even high quality PZT thin ®lms show some undesirable properties which affect the reliability of ferroelectric memory devices such as fatigue, retention, imprint, leakage current, time-dependent dielectric breakdown (TDDB) etc. [7,8]. The degradation in electrical properties is known to be due mostly to the existence of oxygen vacancies which move in the direction of the electric ®eld. Previous works done by other research-
* Correspoonding author; e-mail: [email protected]. 1 Present address: LG Corporate Institute of Technology, Devices and Materials Laboratory, MA group, 16 Woomyeon-Dong, Seocho-Gu, Seoul 137-724, South Korea.
ers reported that several conducting mechanisms are present in the ferroelectric thin ®lms and also they are closely related to the growth conditions, electrode materials, process conditions etc. [9±12]. There are few reports on the relationship between the conduction mechanism and various kinds of ambient gas used when crystallizing the ferroelectric ®lms. In the present study, we investigated the leakage current characteristics of PZT thin ®lms grown by the sol±gel process and crystallized subsequently using rapid thermal annealing (RTA) in various ambient gases, i.e. N2, air and O2. Microstructural differences were also examined. 2. Experimental details The PZT thin ®lms were prepared using mixed solutions of lead acetate, zirconium-tetra-n-butoxide and titaniumtetra-isopropoxide that were dissolved in methoxyethanol. The molar ratio of Zr/Ti was 52/48 and 10 mol% excess lead precursor was added to the solution to compensate for the de®ciency in lead composition. Oxidized silicon(100) single-crystal wafers were used as substrates on which the Pt/Ti layers were sputtered as a bottom electrode. The thick-
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Fig. 1. Surface image of PZT thin ®lms treated in (a) N2, (b) air and (c) O2 ambience.
Ê and 200 A Ê thick Ti was used as an ness of Pt was 1800 A adhesion promoter. The PZT thin ®lms were spin coated onto the substrates at 3000 rev min 21 for 15 s followed by drying on a hot plate at 3508C for 10 min. The coating and drying process was repeated six times to obtain appropriate thickness of the ®lms. As-dried ®lms were heat treated using rapid thermal annealing at 7508C for 60 s to crystallize the ®lms. Various gases were adopted as ambients during heat treatment, i.e. N2, air and O2. The resultant PZT ®lm thickÊ . Again, Pt was sputtered as a top ness was 3000±3500 A electrode in order to measure the electrical properties. The microstructure of the PZT thin ®lm was analyzed by scanning electron microscopy (SEM) to examine the surface and the cross-sectional morphologies, and by X-ray diffraction (XRD) to determine the structure of the ®lms. Electrical characterization of the ®lms was done by measuring the dielectric constant, P±E hysteresis, leakage current as a function of electric ®eld, temperature and time. The measurements were done using an HP4194A impedance
Fig. 2. XRD patterns of PZT thin ®lms heat treated in (a) N2, (b) air and (c) O2 ambience.
analyzer, RT66A standard ferroelectric test system and Keithley 236 source measure unit respectively.
3. Results and discussion 3.1. Microstructures Ferroelectric PZT thin ®lms were crystallized by heat treatment in N2, air and O2, respectively. They had different surface morphologies distinguished from each other as shown in Fig. 1. All the three kinds of ®lm show densely packed grains. Among them, the ®lm heat treated in O2 ambient shows larger grains than the others, and voids appeared at the grain boundary junction. This implies that increasing the partial pressure of oxygen assists grain growth of the PZT thin ®lms. Fig. 2 shows XRD patterns of the PZT thin ®lms heat treated in various gas ambients. Each peak was obtained from u ± 2u scanning of ®lms with the Cu Ka line. All ®lms grown were perovskite phase with
S.M. Cho, D.Y. Jeon / Thin Solid Films 338 (1999) 149±154
151
metallic phase enhanced (111) oriented ®lms. In this work, it is suggested that oxygen ambient suppresses to some degree the growth of the (111) plane more than N2 does. 3.2. Ferroelectric properties Ferroelectric hysteresis loops of the PZT thin ®lms heat treated in various ambient gases are shown in Fig. 3. They exhibited similar shapes and values of remnant polarization except that the ®lm treated in O2 shows a slightly higher Pr. This might result from the larger grain size of the ®lm compared with that of the other two ambiences as shown in Fig. 1. Fig. 3. Hysteresis curves of PZT thin ®lms heat treated in various gases.
randomly oriented polycrystalline structure and no other intermediate phases such as pyrochlore were observed. They show some difference in XRD patterns in that the relative intensities of (100) and (111) diffraction peaks of the ®lms heat treated in the three different gases were changed sequentially. As the oxygen partial pressure was increased, the relative intensity of the (111) diffraction peak increased and that of the (100) diffraction peak decreased. This shows clearly that oxygen affects the orientation of the PZT thin ®lms. There are many factors that may affect the orientation of the thin ®lms during growth such as the substrates used [13], annealing temperature [14,15], heating rate [16±18], composition of the elements [16,17] etc. Thus the above conditions should be carefully chosen to control the orientation of the ®lms. It is reported that, during phase transition from the amorphous to perovskite phase, intermediate phases play a key role as nucleation sites to determine the orientation of PZT thin ®lm in which the intermediate phases are created during heat treatment as the temperature rises [18,19]. When the PZT ®lm was pyrolyzed at a lower temperature, i.e. below 5008C, a PbO layer formed initially followed by the formation of (100) preferred PZT. On the other hand, at higher temperature, i.e. above 6008C, PbO disappeared fast and a Pb±Pt inter-
Fig. 4. I±V characteristic of PZT thin ®lm heat treated in N2.
3.3. Leakage current characteristics Leakage current characteristics of the PZT thin ®lms heat treated in various ambient gases were examined as a function of electric ®eld and temperature. Their conduction mechanisms were also investigated. Fig. 4 shows the I±V characteristic of the PZT thin ®lm heat treated in N2. In the logJ versus logE plot, the current increases linearly with external electric ®eld in the region of low electric ®eld, which suggests ohmic conduction. With increasing external ®eld above 100 kV cm 21, the current increases non-linearly which implies that current conduction is governed by another mechanism. Fig. 5 shows the leakage current of the ®lm heat treated in N2 in the high ®eld region where there is a linear relationship between lnJ and E 1/2, implying thermionic emission. The slope of the graph, corresponding to bs =kT, was 1.04 £ 10 22 (cm V 21) 1/2 and it was found to be Schottky emission. Its current density dominated by Schottky emission is expressed as h p i 1 J A* T 2 exp 2q fB 2 bs E =kT where A* is the Richardson constant, f B is the barrier height and bs q= 4pe0 ei 1=2 . Fig. 6 illustrates the I±V characteristic of the PZT thin ®lms heat treated in air. The measured current increases linearly as the electric ®eld increases and
Fig. 5. I±V characteristic of PZT thin ®lm heat treated in N2 in the high electric ®eld region.
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Fig. 6. I±V characteristic of PZT thin ®lm heat treated in air.
the slope of the curves is 0.86. As the electric ®eld is increased, a non-linear relationship between J and E appeared as shown in Fig. 7a which could be ®tted reasonably well with a modi®ed Langmuir±Child law [20]: J E aE 1 bE2
2
Fig. 8. I±V characteristics of PZT thin ®lm heat treated in O2: (a) logJ vs. E plot, (b) J vs. E 1/2 plot.
Fig. 7. I±V characteristics of PZT thin ®lm heat treated in N2 in the high ®eld region: (a) J vs. E plot, (b) lnJ vs. E 1/2 plot.
Estimated values of a and b were 2:07 £ 10212 V 21 cm 21 and 1:62 £ 10216 V 21 V 21, respectively. However, when the electric ®eld is increased further, the I±V curve is not ®tted with Eq. (2). Rather it follows the Schottky emission again as shown in Fig. 7b. The slope of the curve is 1:10 £ 1022 (cm V 21) 1/2. On the other hand, the I±V curve of the PZT thin ®lm heat treated in O2 ambience in the high electric ®eld region is ®tted very well with Eq. (2), which implies space charge limited conduction at higher electric ®eld as shown in Fig. 8b. From the ®tting, the coef®cients a and b were found as 1:12 £ 10212 V 21 cm 21 and 1:07 £ 10216 V 21 V 21, respectively. It is very interesting to note that the space charge conduction is dominant in the ®lm heat treated in O2 and to some extent in air. It is suggested that the cause of space charge generation is the existence of oxygen vacancies [9]. It is known that space charge limited conduction appears in insulators in which the trap sites are ®lled with charges and these, consequently, result in a strong increase in the number of free charges at high electric ®eld [21]. In this study, it is supposed that voids came into existence in the ®lm heat treated in O2 and in air, which presumably play a key role in generation of the trap site. More detailed studies are required on this point. Also, timedependent leakage current characteristics of the PZT thin ®lm heat treated in N2 ambience were examined. As shown
S.M. Cho, D.Y. Jeon / Thin Solid Films 338 (1999) 149±154
153
Fig. 9. Time dependence of leakage current characteristics of PZT thin ®lm heat treated in N2.
Fig. 11. Time dependence of leakage current characteristics of PZT thin ®lm heat treated in O2.
in Fig. 9, below 440 K, logJ decreases linearly with logt which can be ®tted by a Curie±Von Schweidler type current J J0 t2 g (0 , g , 1), where g is 0.98 at room temperature and decreases as the temperature increases. Further, above 440 K, current relaxation almost disappears. The temperature dependence of the current after 200 s is shown as a logJT/E versus 1/T plot in Fig. 10. Two different slopes appear as the temperature varies. These reveal the activation energies of electrons which correspond to barrier heights. The calculated values of activation energies E1 and E2 are 1.59 eV and 0.37 eV respectively. The current density of electron hopping conduction is expressed as
tion under a low electric ®eld results from electron hopping. On the other hand, the time-dependent current behavior of the PZT thin ®lm heat treated in O2 ambience was somewhat different from that heat treated in N2. At room temperature, the current shows logarithmic decay with time as in the case of N2 heat treatment. However, at increased temperatures, the current initially decreases with time followed by an abrupt increase as shown in Fig. 11. This is very likely due to the conductivity increase caused by an increase in electron or hole density through space charge redistribution by the electric ®eld near the interface between the electrode and the PZT. It also supports the ®nding that the PZT thin ®lm heat treated in O2 ambient shows space charge limited conduction behavior in the region of high electric ®eld.
J
ÿ q2 * N Dexp f=kT E kT
3
where f * is an activation energy, N* the density of trap sites and D the diffusion coef®cient of electrons. Substitution of the applied electric ®eld (29 kV cm 21) and activation energy E2 for Eq. (3) gives J 7:22 £ 1028 A cm 22 which corresponds to the value of J in Fig. 4. This implies that conduc-
4. Conclusion Ferroelectric PZT thin ®lms were spin coated and heat treated in various ambient gases, i.e. N2, air and O2. The microstructures and electrical properties of the ®lms were different in the various ambient gases during crystallization. All ®lms heat treated at 7508C showed single perovskite phase and dense layers. However, PZT thin ®lm crystallized in air and O2 exhibited micropores on the surface. Leakage current characteristics of the PZT thin ®lms could be varied with the process conditions. In the high ®eld region the PZT thin ®lm heat treated in N2 shows Schottky emission while the ®lm heat treated in O2 revealed space charge conduction. On the other hand, air-ambient heated ®lm showed both space charge conduction and Schottky emission. In the low ®eld region, ohmic behavior of leakage current of the ®lms was proved to be hopping conduction of electrons.
Acknowledgements Fig. 10. Temperature dependence of leakage current densities of PZT thin ®lm heat treated in N2.
This work was done with technical help provided by LG
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Corporate Institute of Technology and the authors would like to acknowledge this. References [1] J.E. Evans, R. Womack, IEEE J. Solid-State Circuits 23 (6) (1988) 1171. [2] J.F. Scott, C.P.D. Araujo, Science 240 (1989) 140. [3] C.H. Peng, S.B. Desu, Appl. Phys. Lett. 61 (1) (1992) 16. [4] S. Krupanidhi, N. Maffei, M. Sayer, K. El-Assal, J. Appl. Phys. 54 (11) (1983) 6601. [5] K.D. Budd, S.K. Dey, D.A. Payne, Proc. Br. Ceram. Soc. 36 (1985) 107. [6] K. Ramkuman, J. Lee, A. Safari, S. Danforth, Mater. Res. Soc. Symp. Proc. 200 (1991) 121. [7] L.H. Parker, A.F. Tasch, IEEE Circuit Device Mag., (January 1990) 17. [8] P.K. Larsen, R. Cuppens, G.M.C.M. Spierings, Ferroelectrics 128 (1992) 265. [9] J.F. Scott, C.A. Araujo, B.M. Melnick, L.D. McMillan, R. Zuleeg, J. Appl. Phys. 70 (1) (1991) 382.
[10] H. Hu, S.B. Krupanidhi, J. Mater. Res. 9 (6) (1994) 1484. [11] C. Sudhama, A.C. Campbell, P.D. Maniar, R.E. Jones, R. Moazzami, C.J. Mogab, J.C. Lee, J. Appl. Phys. 75 (2) (1994) 1014. [12] Y. Fukuda, K. Aoki, K. Numata, A. Nishimura, Jpn. J. Appl. Phys. 33 (9B) (1994) 5255. [13] B.A. Tuttle, J.A. Voigt, D.C. Goodnow, D.L. Lamppa, T.J. Headley, M.O. Eatough, G. Zender, R.D. Nasby, S.M. Rodgers, J. Am. Ceram. Soc. 76 (6) (1993) 1537. [14] S.A. Mansour, G.L. Liedl, R.W. Vest, J. Am. Ceram. Soc. 78 (6) (1995) 1617. [15] N. Tohge, S. Takahashi, T. Minami, J. Am. Ceram. Soc. 74 (1) (1991) 67. [16] S.Y. Chen, I.W. Chen, J. Am. Ceram. Soc. 77 (9) (1994) 2337. [17] I.M. Reaney, K. Brooks, R. Klissurska, C. Pawlaczyk, N. Setter, J. Am. Ceram. Soc. 77 (5) (1994) 1209. [18] H. Hu, C.J. Peng, S.B. Krupanidhi, Thin Solid Films 223 (1993) 327. [19] S.Y. Chen, I.W. Chen, J. Am. Ceram. Soc. 77 (9) (1994) 2332. [20] R.H. Tredgold, Space Charge Conduction in Solids, Elsevier, Amsterdam, 1966. [21] G. Barbottin, A. Vapaille, Instabilities in Silicon Devices ± Silicon Passivation and Related Instabilities, North-Holland, Amsterdam, 1986.
Effect of annealing conditions on the leakage current characteristics of ferroelectric PZT thin ®lms grown by sol±gel process Seong Moon Cho 1,*, Duk Young Jeon Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-Gu, Taejon 305-701, South Korea Received 12 February 1998; accepted 7 August 1998
Abstract The microstructure and leakage current of lead zirconate titanate (PZT) thin ®lms were investigated to understand the mechanism of the leakage current and also to attempt to improve the leakage characteristics. The PZT thin ®lms were prepared using a sol±gel process followed by crystallization in three different gas ambients, i.e. N2, air and O2. The three kinds of crystallized samples show different microstructure and orientation. Electrical characterization of the PZT thin ®lms including the permittivity, P±E hysteresis, and the leakage current was carried out. It was found that the leakage current was affected not only by the microstructures of the ®lms but also by the interface between the Pt electrode and the PZT ®lm. Moreover, it was found that more than one conduction mechanism is involved in the range of electric ®eld used in the experiment. In the low electric ®eld region the current conduction is ohmic and its mechanism is found to be electron hopping among electron traps. In the high ®eld region the sample treated in N2 shows Schottky emission, the sample treated in O2 shows space charge limited conduction, and the sample treated in air shows both Schottky and space charge conduction together. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Sol±gel process; PZT; Annealing; Electrical properties and measurements
1. Introduction Ferroelectric lead zirconate titanate (PZT) is a well known functional material which has a great deal of potential as a non-volatile memory device [1,2]. It is expected that PZT will improve the limitations in storage density encountered in conventional Si memory technology. A variety of techniques has been proposed to fabricate ferroelectric PZT thin ®lms such as metalorganic chemical vapor deposition (MOCVD) [3], sputtering [4], sol±gel process [5] and laser ablation [6]. At present, however, even high quality PZT thin ®lms show some undesirable properties which affect the reliability of ferroelectric memory devices such as fatigue, retention, imprint, leakage current, time-dependent dielectric breakdown (TDDB) etc. [7,8]. The degradation in electrical properties is known to be due mostly to the existence of oxygen vacancies which move in the direction of the electric ®eld. Previous works done by other research-
* Correspoonding author; e-mail: [email protected]. 1 Present address: LG Corporate Institute of Technology, Devices and Materials Laboratory, MA group, 16 Woomyeon-Dong, Seocho-Gu, Seoul 137-724, South Korea.
ers reported that several conducting mechanisms are present in the ferroelectric thin ®lms and also they are closely related to the growth conditions, electrode materials, process conditions etc. [9±12]. There are few reports on the relationship between the conduction mechanism and various kinds of ambient gas used when crystallizing the ferroelectric ®lms. In the present study, we investigated the leakage current characteristics of PZT thin ®lms grown by the sol±gel process and crystallized subsequently using rapid thermal annealing (RTA) in various ambient gases, i.e. N2, air and O2. Microstructural differences were also examined. 2. Experimental details The PZT thin ®lms were prepared using mixed solutions of lead acetate, zirconium-tetra-n-butoxide and titaniumtetra-isopropoxide that were dissolved in methoxyethanol. The molar ratio of Zr/Ti was 52/48 and 10 mol% excess lead precursor was added to the solution to compensate for the de®ciency in lead composition. Oxidized silicon(100) single-crystal wafers were used as substrates on which the Pt/Ti layers were sputtered as a bottom electrode. The thick-
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Fig. 1. Surface image of PZT thin ®lms treated in (a) N2, (b) air and (c) O2 ambience.
Ê and 200 A Ê thick Ti was used as an ness of Pt was 1800 A adhesion promoter. The PZT thin ®lms were spin coated onto the substrates at 3000 rev min 21 for 15 s followed by drying on a hot plate at 3508C for 10 min. The coating and drying process was repeated six times to obtain appropriate thickness of the ®lms. As-dried ®lms were heat treated using rapid thermal annealing at 7508C for 60 s to crystallize the ®lms. Various gases were adopted as ambients during heat treatment, i.e. N2, air and O2. The resultant PZT ®lm thickÊ . Again, Pt was sputtered as a top ness was 3000±3500 A electrode in order to measure the electrical properties. The microstructure of the PZT thin ®lm was analyzed by scanning electron microscopy (SEM) to examine the surface and the cross-sectional morphologies, and by X-ray diffraction (XRD) to determine the structure of the ®lms. Electrical characterization of the ®lms was done by measuring the dielectric constant, P±E hysteresis, leakage current as a function of electric ®eld, temperature and time. The measurements were done using an HP4194A impedance
Fig. 2. XRD patterns of PZT thin ®lms heat treated in (a) N2, (b) air and (c) O2 ambience.
analyzer, RT66A standard ferroelectric test system and Keithley 236 source measure unit respectively.
3. Results and discussion 3.1. Microstructures Ferroelectric PZT thin ®lms were crystallized by heat treatment in N2, air and O2, respectively. They had different surface morphologies distinguished from each other as shown in Fig. 1. All the three kinds of ®lm show densely packed grains. Among them, the ®lm heat treated in O2 ambient shows larger grains than the others, and voids appeared at the grain boundary junction. This implies that increasing the partial pressure of oxygen assists grain growth of the PZT thin ®lms. Fig. 2 shows XRD patterns of the PZT thin ®lms heat treated in various gas ambients. Each peak was obtained from u ± 2u scanning of ®lms with the Cu Ka line. All ®lms grown were perovskite phase with
S.M. Cho, D.Y. Jeon / Thin Solid Films 338 (1999) 149±154
151
metallic phase enhanced (111) oriented ®lms. In this work, it is suggested that oxygen ambient suppresses to some degree the growth of the (111) plane more than N2 does. 3.2. Ferroelectric properties Ferroelectric hysteresis loops of the PZT thin ®lms heat treated in various ambient gases are shown in Fig. 3. They exhibited similar shapes and values of remnant polarization except that the ®lm treated in O2 shows a slightly higher Pr. This might result from the larger grain size of the ®lm compared with that of the other two ambiences as shown in Fig. 1. Fig. 3. Hysteresis curves of PZT thin ®lms heat treated in various gases.
randomly oriented polycrystalline structure and no other intermediate phases such as pyrochlore were observed. They show some difference in XRD patterns in that the relative intensities of (100) and (111) diffraction peaks of the ®lms heat treated in the three different gases were changed sequentially. As the oxygen partial pressure was increased, the relative intensity of the (111) diffraction peak increased and that of the (100) diffraction peak decreased. This shows clearly that oxygen affects the orientation of the PZT thin ®lms. There are many factors that may affect the orientation of the thin ®lms during growth such as the substrates used [13], annealing temperature [14,15], heating rate [16±18], composition of the elements [16,17] etc. Thus the above conditions should be carefully chosen to control the orientation of the ®lms. It is reported that, during phase transition from the amorphous to perovskite phase, intermediate phases play a key role as nucleation sites to determine the orientation of PZT thin ®lm in which the intermediate phases are created during heat treatment as the temperature rises [18,19]. When the PZT ®lm was pyrolyzed at a lower temperature, i.e. below 5008C, a PbO layer formed initially followed by the formation of (100) preferred PZT. On the other hand, at higher temperature, i.e. above 6008C, PbO disappeared fast and a Pb±Pt inter-
Fig. 4. I±V characteristic of PZT thin ®lm heat treated in N2.
3.3. Leakage current characteristics Leakage current characteristics of the PZT thin ®lms heat treated in various ambient gases were examined as a function of electric ®eld and temperature. Their conduction mechanisms were also investigated. Fig. 4 shows the I±V characteristic of the PZT thin ®lm heat treated in N2. In the logJ versus logE plot, the current increases linearly with external electric ®eld in the region of low electric ®eld, which suggests ohmic conduction. With increasing external ®eld above 100 kV cm 21, the current increases non-linearly which implies that current conduction is governed by another mechanism. Fig. 5 shows the leakage current of the ®lm heat treated in N2 in the high ®eld region where there is a linear relationship between lnJ and E 1/2, implying thermionic emission. The slope of the graph, corresponding to bs =kT, was 1.04 £ 10 22 (cm V 21) 1/2 and it was found to be Schottky emission. Its current density dominated by Schottky emission is expressed as h p i 1 J A* T 2 exp 2q fB 2 bs E =kT where A* is the Richardson constant, f B is the barrier height and bs q= 4pe0 ei 1=2 . Fig. 6 illustrates the I±V characteristic of the PZT thin ®lms heat treated in air. The measured current increases linearly as the electric ®eld increases and
Fig. 5. I±V characteristic of PZT thin ®lm heat treated in N2 in the high electric ®eld region.
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Fig. 6. I±V characteristic of PZT thin ®lm heat treated in air.
the slope of the curves is 0.86. As the electric ®eld is increased, a non-linear relationship between J and E appeared as shown in Fig. 7a which could be ®tted reasonably well with a modi®ed Langmuir±Child law [20]: J E aE 1 bE2
2
Fig. 8. I±V characteristics of PZT thin ®lm heat treated in O2: (a) logJ vs. E plot, (b) J vs. E 1/2 plot.
Fig. 7. I±V characteristics of PZT thin ®lm heat treated in N2 in the high ®eld region: (a) J vs. E plot, (b) lnJ vs. E 1/2 plot.
Estimated values of a and b were 2:07 £ 10212 V 21 cm 21 and 1:62 £ 10216 V 21 V 21, respectively. However, when the electric ®eld is increased further, the I±V curve is not ®tted with Eq. (2). Rather it follows the Schottky emission again as shown in Fig. 7b. The slope of the curve is 1:10 £ 1022 (cm V 21) 1/2. On the other hand, the I±V curve of the PZT thin ®lm heat treated in O2 ambience in the high electric ®eld region is ®tted very well with Eq. (2), which implies space charge limited conduction at higher electric ®eld as shown in Fig. 8b. From the ®tting, the coef®cients a and b were found as 1:12 £ 10212 V 21 cm 21 and 1:07 £ 10216 V 21 V 21, respectively. It is very interesting to note that the space charge conduction is dominant in the ®lm heat treated in O2 and to some extent in air. It is suggested that the cause of space charge generation is the existence of oxygen vacancies [9]. It is known that space charge limited conduction appears in insulators in which the trap sites are ®lled with charges and these, consequently, result in a strong increase in the number of free charges at high electric ®eld [21]. In this study, it is supposed that voids came into existence in the ®lm heat treated in O2 and in air, which presumably play a key role in generation of the trap site. More detailed studies are required on this point. Also, timedependent leakage current characteristics of the PZT thin ®lm heat treated in N2 ambience were examined. As shown
S.M. Cho, D.Y. Jeon / Thin Solid Films 338 (1999) 149±154
153
Fig. 9. Time dependence of leakage current characteristics of PZT thin ®lm heat treated in N2.
Fig. 11. Time dependence of leakage current characteristics of PZT thin ®lm heat treated in O2.
in Fig. 9, below 440 K, logJ decreases linearly with logt which can be ®tted by a Curie±Von Schweidler type current J J0 t2 g (0 , g , 1), where g is 0.98 at room temperature and decreases as the temperature increases. Further, above 440 K, current relaxation almost disappears. The temperature dependence of the current after 200 s is shown as a logJT/E versus 1/T plot in Fig. 10. Two different slopes appear as the temperature varies. These reveal the activation energies of electrons which correspond to barrier heights. The calculated values of activation energies E1 and E2 are 1.59 eV and 0.37 eV respectively. The current density of electron hopping conduction is expressed as
tion under a low electric ®eld results from electron hopping. On the other hand, the time-dependent current behavior of the PZT thin ®lm heat treated in O2 ambience was somewhat different from that heat treated in N2. At room temperature, the current shows logarithmic decay with time as in the case of N2 heat treatment. However, at increased temperatures, the current initially decreases with time followed by an abrupt increase as shown in Fig. 11. This is very likely due to the conductivity increase caused by an increase in electron or hole density through space charge redistribution by the electric ®eld near the interface between the electrode and the PZT. It also supports the ®nding that the PZT thin ®lm heat treated in O2 ambient shows space charge limited conduction behavior in the region of high electric ®eld.
J
ÿ q2 * N Dexp f=kT E kT
3
where f * is an activation energy, N* the density of trap sites and D the diffusion coef®cient of electrons. Substitution of the applied electric ®eld (29 kV cm 21) and activation energy E2 for Eq. (3) gives J 7:22 £ 1028 A cm 22 which corresponds to the value of J in Fig. 4. This implies that conduc-
4. Conclusion Ferroelectric PZT thin ®lms were spin coated and heat treated in various ambient gases, i.e. N2, air and O2. The microstructures and electrical properties of the ®lms were different in the various ambient gases during crystallization. All ®lms heat treated at 7508C showed single perovskite phase and dense layers. However, PZT thin ®lm crystallized in air and O2 exhibited micropores on the surface. Leakage current characteristics of the PZT thin ®lms could be varied with the process conditions. In the high ®eld region the PZT thin ®lm heat treated in N2 shows Schottky emission while the ®lm heat treated in O2 revealed space charge conduction. On the other hand, air-ambient heated ®lm showed both space charge conduction and Schottky emission. In the low ®eld region, ohmic behavior of leakage current of the ®lms was proved to be hopping conduction of electrons.
Acknowledgements Fig. 10. Temperature dependence of leakage current densities of PZT thin ®lm heat treated in N2.
This work was done with technical help provided by LG
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