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Leakage current in PZT films with sputtered RuOx electrodes
Solid-State Electronics 44 (2000) 1569±1571
Leakage current in PZT ®lms with sputtered RuOx electrodes C.W. Law a, K.Y. Tong a,*, J.H. Li b, K. Li b a b
Department of Electronic and Information Engineering, Hong Kong Polytechnic University, Hung Hom, Hong Kong Functional Materials Laboratory, Jiangsu Institute of Petrochemical Technology, Jiangsu, People's Republic of China Received 22 November 1999; received in revised form 12 May 2000; accepted 15 May 2000
Abstract We have studied the leakage current in sol±gel PZT capacitors with reactive sputtered RuOx electrodes under different oxygen partial pressures during sputtering. The amount of oxygen content in the electrodes has been found to have a signi®cant eect on the leakage current. Considerable decrease of leakage current at lower electric ®eld can be achieved by sputtering at a higher oxygen partial pressure of 20%. Correlation with degradation experimental results suggests that the presence of an oxygen de®cient layer near the electrode has crucial eect in controlling both the leakage current and degradation. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction PZT thin ®lms have been widely used in ferroelectric memories due to the low coercive force and high remanent polarization near the morphotropic phase boundary. Both platinum and conductive oxides such as ruthenium oxide have been applied to form the electrodes of PZT ®lms. RuOx electrodes have the advantages of excellent resistance to fatigue, but usually exhibit higher leakage current. There have been eorts trying to lower the leakage in ferroelectric capacitors with RuOx =Pt hybrid electrodes [1]. However, the mechanisms responsible for the leakage current at different electrodes are still not completely understood. Recently, we have investigated the eect of oxygen content in the RuOx electrodes on the ferroelectric and fatigue properties of PZT ®lms, by varying the partial pressure of oxygen during the reactive sputtering of the RuOx electrodes [2]. In this study, we report for the ®rst time the dependence of the leakage current in sol±gel PZT ®lms on the amount of oxygen content in the sputtered RuOx electrodes. Our results show that signi®cant reduction of leakage current can be achieved by increasing the oxygen proportion in the electrodes. A
*
Corresponding author. Fax: +85-2236-28439. E-mail address: [email protected] (K.Y. Tong).
current model for the ®lm is proposed to explain the observed characteristics of leakage current.
2. Experimental N-type silicon wafers with (1 0 0) orientation were chosen as the substrates. Silicon dioxide (SiO2 ) was grown by thermal oxidation to a thickness of 600 nm on the surface. RuOx thin ®lms were reactively sputtered by rf sputtering of ruthenium target in a mixture of Ar and O2 . The relative partial pressure of oxygen to the total gas pressure, p(O2 )=p(O2 Ar), was varied from 0% to 20%. Prior to sputtering, the system was pumped down to a base pressure of less than 5 10ÿ7 Torr. During sputtering, the input power was 150 W, the substrate temperature was 350°C and the total pressure was 6 m Torr. Post-annealing at 550°C in N2 for 30 min was performed to lower the resistivity of the RuOx bottom electrodes. Pb(Zrx Ti1ÿx )O3 ®lms were prepared from a precursor solution including a mixture of Pb(CH3 COO)2 , ZrO(CH3 COO)2 and Ti(OC4 H9 )4 . The solvent used was 2-methoxyethanol with 8% CHON(CH3 )2 . The hydrolysis process was performed using steam bubbling until the ratio of H2 O:Ti(OC4 H9 )4 was equal to 2:1. The composition was chosen to be near the morphotropic phase boundary x 0:52. The solution contained
0038-1101/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 0 ) 0 0 1 0 2 - 7
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C.W. Law et al. / Solid-State Electronics 44 (2000) 1569±1571
5 mol% excess Pb in order to account for the loss of Pb during the later thermal treatment. A ®ltered 0.5 mol/l precursor solution as described above was then applied to the RuOx coated substrate by spin-on coating at 3000 rpm for 30 s. The ®lm was then pyrolyzed at 400°C in oxygen ambience for 5 min in a rapid thermal processor. The above coating and pyrolysis process was repeated a few times in order to obtain a PZT thickness of about 400 nm. Finally, the PZT ®lm was annealed in oxygen at a temperature of 700°C for 10 min to crystallize it. The top RuOx electrode layer consists of an array of dots, each with 2 10ÿ2 cm2 area deposited by rf sputtering through a shadow mask. After applying a constant voltage across the capacitor for 1 s, the leakage current was measured using HP4140B pA meter.
3. Results The O/Ru atomic ratio in the as-deposited RuOx ®lms was measured by Rutherford backscattering method and was found to be 2 and 2.2 at oxygen partial pressure of 10% and 20%, respectively (Fig. 1). Fig. 2 shows the current density plotted against the electric®eld intensity for the typical PZT capacitors with RuOx electrodes for dierent oxygen partial pressures (0% to 20%) during sputtering of the electrodes. Fig. 2 clearly shows the in¯uence of the oxygen content of the electrode on the leakage current. We observe in Fig. 2 that (i) at lower electric ®elds, below about 120 kV/cm, the current density is strongly aected by the oxygen partial pressure, with the current signi®cantly reduced at higher oxygen partial pressures; (ii) above about 120 kV/cm, the current density is quite independent of the oxygen partial pressure. We have found that the fatigue resistance in PZT ®lms can be greatly improved by increasing the oxygen content of the electrodes (Fig. 3). This is in accordance with the oxygen vacancy model for the fatigue property; oxygen vacancy defects migrate to the
Fig. 1. Variations of oxygen content and resistivity in the asdeposited RuOx ®lms with oxygen partial pressure.
Fig. 2. Current density versus electric ®eld for PZT capacitors with RuOx electrodes sputtered at dierent oxygen partial pressures.
Fig. 3. Fatigue curves of PZT capacitors with RuOx electrodes sputtered at dierent oxygen partial pressures.
region close to the electrodes due to asymmetrical electric-®eld cycling in the ®lm and results in loss of polarization [3±6]. RuOx electrodes improve the fatigue resistance by providing oxygen source to ®ll up the oxygen vacancy defects. Based on such oxygen vacancy model, we propose the following explanations on the leakage characteristics: (i) At lower electric ®elds, the current is determined by the contact properties at the electrode/PZT interface. Previous models simply assume that a Schottky contact exists between the electrode and the PZT (which is known to be p-type due to the loss of Pb atoms) [4,7,8]. The leakage current is then evaluated as in usual Schottky currents. However, it is reported that oxygen vacancies are n-type in nature, so that the accumulation of
C.W. Law et al. / Solid-State Electronics 44 (2000) 1569±1571
oxygen vacancies near the electrodes can result in the formation of a very thin n-type oxygen de®cient region [3]. This can greatly complicate the current conduction mechanism. Questions may be raised as to whether the current is controlled by the Schottky contact between the electrode and the n-type oxygen de®cient region or by the p±n junction between the oxygen de®cient region and the p-type bulk PZT. Our experiments have demonstrated that the presence of oxygen de®cient regions has crucial control on the leakage current, because different oxygen proportion in the RuOx electrodes are supposed to aect the amount of oxygen vacancies accumulating near the electrodes. By correlation with our results in fatigue degradation, we speculate that the leakage current is decreased when the amount of oxygen vacancies near the electrodes is reduced. This seems to agree with a very recent report that the work function of oxygen-de®cient RuOx ®lms deposited by spin coating is lowered due to the presence of oxygen vacancies [9]. (ii) At higher electric ®elds, the current is determined by the bulk conduction properties. As in previous reports [7,10], the explanation is due to the breakdown of the Schottky contact at the electrode interface by tunneling (though in our model, there is also the possibility of breakdown of the p±n junction between the bulk PZT and oxygen de®cient region). In our results, the fact that the leakage current at very high ®eld is quite independent of the oxygen content in the electrodes suggests that the current is not related to the interface properties at the electrodes. Other authors have also suggested that conduction is related to the bulk processes in this range of electric ®eld [10,11], probably due to space charge limited current. 4. Discussion For a PZT capacitor using RuOx electrode with excessive oxygen sputtered at 20% oxygen partial pressure,
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the leakage current is signi®cantly reduced and is only slightly inferior to the usual values obtained in samples using Pt electrodes. The RuOx electrode with 20% oxygen ¯ow rate also has better resistance to fatigue compared with 10% and 0% from measurements on the degradation of remanent polarization. The only slight disadvantage of using 20% oxygen ¯ow rate is that the remanent polarization is only 70% of that at 10% oxygen ¯ow rate [2]. These results lead to the conclusion that excessive oxygen content in the RuOx electrode can produce PZT capacitors with low leakage current and signi®cantly improved resistance to fatigue.
Acknowledgements This project is supported by a research grant from the Hong Kong Polytechnic University.
References [1] Al-Shareef HN, Auciello O, Kingon AI. J Appl Phys 1995;77:2146. [2] Law CW, Tong KY, Li JH, Poon MC. Thin Solid Films 1999;354:162. [3] Scott JF, Araujo CA, Meinick BM, McMillan LD, Zuleeg R. J Appl Phys 1991;70:382. [4] Lee JJ, Thio CL, Desu SB. J Appl Phys 1995;78:5073. [5] Yoo IK, Desu SB, Xing J. Mat Res Soc Sym Proc 1993; 310:165. [6] Mihara T, Watanabe H, Pax de Araujo CA. Jpn J Appl Phys 1994;33:5281. [7] Zheng L, Lin C, Ma T-P. J Phys D: Appl Phys 1996;29:457. [8] Mihara T, Watanabe H. Jpn J Appl Phys 1995;34:5664. [9] Hartmann AJ, Neilson M, Lamb RN, Watanabe K, Scott JF. Appl Phys A 2000;70:239. [10] Aoki K, Fukuda Y, Nishimura A. Jpn J Appl Phys 1993; 32:4147. [11] Melnick BN, Scott JF. Ferroelectrics 1992;135:163.
Leakage current in PZT ®lms with sputtered RuOx electrodes C.W. Law a, K.Y. Tong a,*, J.H. Li b, K. Li b a b
Department of Electronic and Information Engineering, Hong Kong Polytechnic University, Hung Hom, Hong Kong Functional Materials Laboratory, Jiangsu Institute of Petrochemical Technology, Jiangsu, People's Republic of China Received 22 November 1999; received in revised form 12 May 2000; accepted 15 May 2000
Abstract We have studied the leakage current in sol±gel PZT capacitors with reactive sputtered RuOx electrodes under different oxygen partial pressures during sputtering. The amount of oxygen content in the electrodes has been found to have a signi®cant eect on the leakage current. Considerable decrease of leakage current at lower electric ®eld can be achieved by sputtering at a higher oxygen partial pressure of 20%. Correlation with degradation experimental results suggests that the presence of an oxygen de®cient layer near the electrode has crucial eect in controlling both the leakage current and degradation. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction PZT thin ®lms have been widely used in ferroelectric memories due to the low coercive force and high remanent polarization near the morphotropic phase boundary. Both platinum and conductive oxides such as ruthenium oxide have been applied to form the electrodes of PZT ®lms. RuOx electrodes have the advantages of excellent resistance to fatigue, but usually exhibit higher leakage current. There have been eorts trying to lower the leakage in ferroelectric capacitors with RuOx =Pt hybrid electrodes [1]. However, the mechanisms responsible for the leakage current at different electrodes are still not completely understood. Recently, we have investigated the eect of oxygen content in the RuOx electrodes on the ferroelectric and fatigue properties of PZT ®lms, by varying the partial pressure of oxygen during the reactive sputtering of the RuOx electrodes [2]. In this study, we report for the ®rst time the dependence of the leakage current in sol±gel PZT ®lms on the amount of oxygen content in the sputtered RuOx electrodes. Our results show that signi®cant reduction of leakage current can be achieved by increasing the oxygen proportion in the electrodes. A
*
Corresponding author. Fax: +85-2236-28439. E-mail address: [email protected] (K.Y. Tong).
current model for the ®lm is proposed to explain the observed characteristics of leakage current.
2. Experimental N-type silicon wafers with (1 0 0) orientation were chosen as the substrates. Silicon dioxide (SiO2 ) was grown by thermal oxidation to a thickness of 600 nm on the surface. RuOx thin ®lms were reactively sputtered by rf sputtering of ruthenium target in a mixture of Ar and O2 . The relative partial pressure of oxygen to the total gas pressure, p(O2 )=p(O2 Ar), was varied from 0% to 20%. Prior to sputtering, the system was pumped down to a base pressure of less than 5 10ÿ7 Torr. During sputtering, the input power was 150 W, the substrate temperature was 350°C and the total pressure was 6 m Torr. Post-annealing at 550°C in N2 for 30 min was performed to lower the resistivity of the RuOx bottom electrodes. Pb(Zrx Ti1ÿx )O3 ®lms were prepared from a precursor solution including a mixture of Pb(CH3 COO)2 , ZrO(CH3 COO)2 and Ti(OC4 H9 )4 . The solvent used was 2-methoxyethanol with 8% CHON(CH3 )2 . The hydrolysis process was performed using steam bubbling until the ratio of H2 O:Ti(OC4 H9 )4 was equal to 2:1. The composition was chosen to be near the morphotropic phase boundary x 0:52. The solution contained
0038-1101/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 0 ) 0 0 1 0 2 - 7
1570
C.W. Law et al. / Solid-State Electronics 44 (2000) 1569±1571
5 mol% excess Pb in order to account for the loss of Pb during the later thermal treatment. A ®ltered 0.5 mol/l precursor solution as described above was then applied to the RuOx coated substrate by spin-on coating at 3000 rpm for 30 s. The ®lm was then pyrolyzed at 400°C in oxygen ambience for 5 min in a rapid thermal processor. The above coating and pyrolysis process was repeated a few times in order to obtain a PZT thickness of about 400 nm. Finally, the PZT ®lm was annealed in oxygen at a temperature of 700°C for 10 min to crystallize it. The top RuOx electrode layer consists of an array of dots, each with 2 10ÿ2 cm2 area deposited by rf sputtering through a shadow mask. After applying a constant voltage across the capacitor for 1 s, the leakage current was measured using HP4140B pA meter.
3. Results The O/Ru atomic ratio in the as-deposited RuOx ®lms was measured by Rutherford backscattering method and was found to be 2 and 2.2 at oxygen partial pressure of 10% and 20%, respectively (Fig. 1). Fig. 2 shows the current density plotted against the electric®eld intensity for the typical PZT capacitors with RuOx electrodes for dierent oxygen partial pressures (0% to 20%) during sputtering of the electrodes. Fig. 2 clearly shows the in¯uence of the oxygen content of the electrode on the leakage current. We observe in Fig. 2 that (i) at lower electric ®elds, below about 120 kV/cm, the current density is strongly aected by the oxygen partial pressure, with the current signi®cantly reduced at higher oxygen partial pressures; (ii) above about 120 kV/cm, the current density is quite independent of the oxygen partial pressure. We have found that the fatigue resistance in PZT ®lms can be greatly improved by increasing the oxygen content of the electrodes (Fig. 3). This is in accordance with the oxygen vacancy model for the fatigue property; oxygen vacancy defects migrate to the
Fig. 1. Variations of oxygen content and resistivity in the asdeposited RuOx ®lms with oxygen partial pressure.
Fig. 2. Current density versus electric ®eld for PZT capacitors with RuOx electrodes sputtered at dierent oxygen partial pressures.
Fig. 3. Fatigue curves of PZT capacitors with RuOx electrodes sputtered at dierent oxygen partial pressures.
region close to the electrodes due to asymmetrical electric-®eld cycling in the ®lm and results in loss of polarization [3±6]. RuOx electrodes improve the fatigue resistance by providing oxygen source to ®ll up the oxygen vacancy defects. Based on such oxygen vacancy model, we propose the following explanations on the leakage characteristics: (i) At lower electric ®elds, the current is determined by the contact properties at the electrode/PZT interface. Previous models simply assume that a Schottky contact exists between the electrode and the PZT (which is known to be p-type due to the loss of Pb atoms) [4,7,8]. The leakage current is then evaluated as in usual Schottky currents. However, it is reported that oxygen vacancies are n-type in nature, so that the accumulation of
C.W. Law et al. / Solid-State Electronics 44 (2000) 1569±1571
oxygen vacancies near the electrodes can result in the formation of a very thin n-type oxygen de®cient region [3]. This can greatly complicate the current conduction mechanism. Questions may be raised as to whether the current is controlled by the Schottky contact between the electrode and the n-type oxygen de®cient region or by the p±n junction between the oxygen de®cient region and the p-type bulk PZT. Our experiments have demonstrated that the presence of oxygen de®cient regions has crucial control on the leakage current, because different oxygen proportion in the RuOx electrodes are supposed to aect the amount of oxygen vacancies accumulating near the electrodes. By correlation with our results in fatigue degradation, we speculate that the leakage current is decreased when the amount of oxygen vacancies near the electrodes is reduced. This seems to agree with a very recent report that the work function of oxygen-de®cient RuOx ®lms deposited by spin coating is lowered due to the presence of oxygen vacancies [9]. (ii) At higher electric ®elds, the current is determined by the bulk conduction properties. As in previous reports [7,10], the explanation is due to the breakdown of the Schottky contact at the electrode interface by tunneling (though in our model, there is also the possibility of breakdown of the p±n junction between the bulk PZT and oxygen de®cient region). In our results, the fact that the leakage current at very high ®eld is quite independent of the oxygen content in the electrodes suggests that the current is not related to the interface properties at the electrodes. Other authors have also suggested that conduction is related to the bulk processes in this range of electric ®eld [10,11], probably due to space charge limited current. 4. Discussion For a PZT capacitor using RuOx electrode with excessive oxygen sputtered at 20% oxygen partial pressure,
1571
the leakage current is signi®cantly reduced and is only slightly inferior to the usual values obtained in samples using Pt electrodes. The RuOx electrode with 20% oxygen ¯ow rate also has better resistance to fatigue compared with 10% and 0% from measurements on the degradation of remanent polarization. The only slight disadvantage of using 20% oxygen ¯ow rate is that the remanent polarization is only 70% of that at 10% oxygen ¯ow rate [2]. These results lead to the conclusion that excessive oxygen content in the RuOx electrode can produce PZT capacitors with low leakage current and signi®cantly improved resistance to fatigue.
Acknowledgements This project is supported by a research grant from the Hong Kong Polytechnic University.
References [1] Al-Shareef HN, Auciello O, Kingon AI. J Appl Phys 1995;77:2146. [2] Law CW, Tong KY, Li JH, Poon MC. Thin Solid Films 1999;354:162. [3] Scott JF, Araujo CA, Meinick BM, McMillan LD, Zuleeg R. J Appl Phys 1991;70:382. [4] Lee JJ, Thio CL, Desu SB. J Appl Phys 1995;78:5073. [5] Yoo IK, Desu SB, Xing J. Mat Res Soc Sym Proc 1993; 310:165. [6] Mihara T, Watanabe H, Pax de Araujo CA. Jpn J Appl Phys 1994;33:5281. [7] Zheng L, Lin C, Ma T-P. J Phys D: Appl Phys 1996;29:457. [8] Mihara T, Watanabe H. Jpn J Appl Phys 1995;34:5664. [9] Hartmann AJ, Neilson M, Lamb RN, Watanabe K, Scott JF. Appl Phys A 2000;70:239. [10] Aoki K, Fukuda Y, Nishimura A. Jpn J Appl Phys 1993; 32:4147. [11] Melnick BN, Scott JF. Ferroelectrics 1992;135:163.