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The effect of anodizing voltage on the electrical properties of Al–Ti composite oxide film on aluminum
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Electroanalytical Chemistry Journal of Electroanalytical Chemistry 590 (2006) 26–31 www.elsevier.com/locate/jelechem
The effect of anodizing voltage on the electrical properties of Al–Ti composite oxide film on aluminum Jinju Chen *, Zhesheng Feng, Meilian Jiang, Bangchao Yang College of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, People’s Republic of China Received 1 September 2005; received in revised form 16 January 2006; accepted 21 February 2006 Available online 21 April 2006
Abstract The Al–Ti composite oxide film with high dielectric constant was prepared by hydrolysis precipitation and anodizing. The depth profiles of the Al–Ti composite oxide films formed at 50 V and 80 V have been detected by glow discharge optical emission spectrometry (GDOES). The electrical properties of the Al–Ti composite oxide film formed at different anodizing voltages have been studied by electrical measurements. The local electric breakdown occurs in the Al–Ti composite oxide films about the anodizing voltage of 70 V. The Al–Ti composite oxide film formed at 80 V presents a thicker outer layer containing titanium than that of the film formed at 50 V. It was established that the dependence of the retention voltage and reciprocal capacitance on the anodizing voltage consisted of two linear regions. It was found that more than one conduction mechanism was involved in the range of electric field used in the experiment. The current conduction is ohmic at low electric field and Schottky emission at high electric field. The film formed at 80 V reveals an additional electric field region when compared with the film formed at 50 V, and its conduction mechanism is space charge limited current. According to our results, the film formed above the anodizing voltage of 70 V exhibits inferior electrical properties than that of the film formed below 70 V. 2006 Elsevier B.V. All rights reserved. Keywords: Al–Ti composite oxide film; Structure; Electrical properties; Anodizing voltage
1. Introduction Recently, there has been an increasing demand for the high dielectric constant composite oxide films formed on aluminum to replace pure Al anodic oxide films for small electrolytic capacitors with high electric capacitance. Among these composite oxide films such as Al–(Ti, Nb, Ta, Zr) composite oxide films, the Al–Ti composite oxide films have attracted significant attention for that titanium oxide has relatively high permittivity. According to Wilhelmsen and Hurlen [1], anodic oxide films formed on titanium show a dielectric constant of about 60. The capacitance of the Al–Ti composite oxide film is the highest among the composite oxide films, and reveals 60% higher *
Corresponding author. Tel.: +86 028 83201460. E-mail address: [email protected] (J. Chen).
0022-0728/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.02.016
than that of barrier type anodic oxide films on aluminum [2,3]. Many chemical methods can be used to prepare the Al–Ti composite oxide films: anodizing after coating of TiO2 with MOCVD [2], anodizing after TiO2 deposition in pores of porous anodic oxide films (pore-filling method) [3], anodizing after coating of TiO2 by hydrolysis precipitation[4], anodizing in solution with Ti-bearing anions [5], and so on. Among these methods, the method of hydrolysis precipitation and anodizing has made the fabrication of the Al–Ti composite oxide films with low cost and high efficiency, and it is now experiencing a conversion from lab stage to industrial application. The effects of anodizing conditions on the formation, structure and electrical properties of the Al–Ti composite oxide films have been rarely experimented on aluminum, whereas it is obviously important to find out the effect for successfully applying the technology of fabricating
J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
composite oxide films to aluminum foils with high specific capacitance and perfect electrical properties. In the present study, the Al–Ti composite oxide films with high dielectric constant are prepared through hydrolysis precipitation and anodizing. The growth and electrical properties of the Al– Ti composite oxide films formed at different anodizing voltages are investigated. 2. Experimental 2.1. Formation of the Al–Ti composite oxide film Specimens of 2 cm2 with a handle were cut from highly pure etched aluminum foil (100 lm thick), and subsequently immersed in the solution containing Ti4+ compound with parameters of the hydrolysis precipitation process shown in Table 1. Consumption of H+ ions reacted with Al increased the pH value in micro pores of the etched aluminum foil during the immersion, which can accelerate the hydrolysis precipitation of Ti-bearing cation. Further details of the hydrolysis precipitation process have been described in Chen et al. [4]. Afterwards, the treated specimens were thoroughly rinsed in the double-distilled water, air-dried at 80 C, and eventually annealed at 550 C for 10 min to obtain the TiO2-coating layer. The specimens coated with TiO2 were anodized in 15% ammonium adipate solution at 85 C, and a 304 stainless steel plate was used as the counter electrode. A constant current density of 50 mA cm2 was run through the cell between the specimens and the counter electrode until the potential difference reached the selected anodizing voltage of 10–90 V. The voltage was then held constant for 10 min and the current was allowed to decay. Specimens without TiO2-coating were also anodized under the same conditions. The changes in the anode potential (vs. Ag/ AgCl sat. KCl) and current with anodizing time were followed separately and simultaneously on Agilent 34401 A digital multimeters connected to a PC system.
27
ite oxide films formed at different voltages. The measurements were performed in 15% aqueous ammonium adipate electrolyte at 50 C. A 304 stainless steel plate was used as the counter electrode. A constant current density of 0.5 mA cm2 was applied to the anodized specimen and the response voltage was recorded. As soon as the current was applied, the voltage increased immediately and then gradually reached a saturated value. The voltage after 180 s was recorded as the retention voltage of the Al–Ti composite oxide films. The specific capacitance of the Al–Ti composite oxide films was measured in 25 C – 15% aqueous ammonium adipate solution using a general LCR meter at 100 Hz. A pure aluminum sheet with a very large area was used as the counter electrode. The electronic current through the aluminum/Al–Ti composite oxide film/electrolyte system was measured during increasing the applied voltage which did not exceed 50% of the formation voltage. The value of the leakage current at any voltage was recorded after the voltage was applied for 1 min. The contact electrolyte was based on ammonium adipate and ethylene glycol. All experimental data were obtained at 50 C. 3. Results and discussion 3.1. Growth of the Al–Ti composite oxide film and its structure The specimens coated with TiO2 were anodized at different voltages of 10–90 V. The variations of voltage and current with time during anodizing to 50 V and 80 V are shown in Figs. 1 and 2 respectively. In Fig. 1, the rise of the curves corresponds to the stage where the anodization is conducted in constant current mode. An initial voltage surge of 2 V at a commencement of anodizing reflects the presence of an aluminum thermal oxide layer formed at
2.2. Structure analysis of the Al–Ti composite oxide film The depth profile of component elements in the Al–Ti composite oxide film was recorded with a Leco GDS 750A spectrometer, operated in the constant current–constant voltage mode with active pressure regulation to stabilize the discharge. 2.3. Electrical measurements The Agilent 34401A digital multimeter was used to determine the retention voltages (VR) of the Al–Ti composTable 1 Parameters of the hydrolysis precipitation process Ti4+, M
pH
Temperature, C
Time, min
0.002
1.5–2.0
65–70
8
Fig. 1. Voltage–time responses of the specimens during anodizing up to 50 V and 80 V at 50 mA cm2 in 15% ammonium adipate electrolyte at 85 C.
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J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
Fig. 2. Changes in the current with time during anodizing in 15% ammonium adipate electrolyte at 85 C.
550 C annealing. After the surge, the voltage increases at the same velocity for the two different films. The curve of 80 V in Fig. 1 reveals two parts in which the voltage shows sudden drop and subsequent raise at about 70 V, which are probably caused by the local dielectric breakdown in the Al–Ti composite oxide film during anodizing. The breakdown seems to take place by abrupt destruction of the oxide at a local weak area. The breakdown potential observed during anodization of the specimens has been proposed to be related to the crystallization of anodic titania which leads to compressive stress in the oxide or oxygen evolution [6–8]. It has been reported that in the case of anodization of titanium, the oxide exhibits a phase transition with increasing potential from an amorphous phase into a crystalline phase such as anatase or rutile [9,10]. The exact potential of the phase change depends on the anodization conditions such as electrolyte and anodizing temperature. Several results obtained in other electrolyte systems might be taken into account. For example, Arsov et al. [11] reported that during the anodization of titanium in sulfuric acid, fully crystalline anatase was formed at voltages higher than 50 V, and Choi et al. [12] found that the breakdown potential might be around 80 V in phosphoric acid. The increasing slopes of the two curves do not really present increscent formation rate of the oxides, but reveals the decrease in the effective surface area of etched aluminum foil with increasing thickness of the oxide film. In Fig. 2, the sudden drop of the anodic current from 0.20 A signifies the onset of constant voltage mode and the anodic current decaying. There are surges of current in constant current region due to the dielectric breakdown of the composite oxide film, which corresponds with the surges of voltage in Fig. 1. The dielectric breakdown does not occur in the curve of 50 V. Figs. 3 and 4 show depth profiles of the Al–Ti composite oxide films formed at 50 V and 80 V respectively, revealing distributions of aluminum, titanium and oxygen species. The total thickness of the Al–Ti composite oxide film
Fig. 3. GDOES depth profile of the Al–Ti composite oxide film formed at the anodizing voltage of 50 V.
Fig. 4. GDOES depth profile of the Al–Ti composite oxide film formed at the anodizing voltage of 80 V.
increases with the anodizing voltage increasing. For the two kinds of films the inner layer consists of pure aluminum oxide, and the outer layer is thought to be a Ti-containing layer. Under a high electric field, O2 ions transported inwards across the anodic oxide film to form pure aluminum oxide at the interface between the inner Ti-oxide layer and the metal substrate, and the outward transport of Al3+ ions formed the composite oxide in which titanium oxides were incorporated. From comparison of Figs. 3 and 4, The profile of Ti shows distribution with the maximum concentration at depth of 21 nm for the Al–Ti composite oxide film formed at 50 V, and at depth of 38 nm for the Al–Ti composite oxide film formed at 80 V. The Al–Ti composite oxide film formed at 80 V has a thicker outer layer containing titanium than that of the film formed at 50 V. This is supported by the fact that the Ti-bearing cations transport outward at a slower rate than that of Al3+ [3]. As is well
J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
known, the ionic conductivity is rather lower in the inner layer of pure alumina and higher in the outer layer contaminated by ion species due to its imperfections [13]. The thicker distribution of titanium species will induce more defects in films, which will affect the characteristics of the retention voltage and the leakage current. Furthermore, the anodizing voltage has greatly effect on the film composition. The ratio of aluminum to oxygen in the Al–Ti composite oxide film formed at 80 V has a bigger deviation from stoichiometric ratio than that in the film formed at 50 V, and accordingly nonstoichiometric defects in the film formed at 80 V get increased.
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Fig. 5 shows the retention voltages of the Al–Ti composite oxide films versus the anodizing voltage curve. The observed dependences of the retention voltage on the anodizing voltage have two linear natures. For oxide formation at the anodizing voltage higher than 70 V the decrease in the slope takes place. The approximation by the leastsquares method has allowed the following equations for the dependence of the retention voltage of the Al–Ti composite oxide films on the anodizing voltage to be established as: VR = 1.116 V+4.128 (the range of voltage from 10 V to 70 V) and VR = 0.84 V+19.95 (the anodizing voltage higher than 70 V). The retention voltage was considered as a parameter to evaluate the reliability of the oxide film [14]. The decrease in the slope of the curve after the anodizing voltage of 70 V indicates that the dielectric property of the Al–Ti composite oxide film gets reduced. Crystallization of titanium oxide could lead to the creation of the defects such as voids and cracks in film. The presence of voids and pores results in the formation of imperfection region and is detrimental to the electrical properties of the oxide films, which can explain the decrease in the slope of the curve after the anodizing voltage of 70 V.
Fig. 6 shows the relationship between the reciprocal capacitance of the oxide films and the anodizing voltage for specimens with and without TiO2-coating. As can be seen from Fig. 6, the value of the reciprocal capacitance for the specimens with and without TiO2-coating is proportional to the anodizing voltage, and the capacitance of the Al–Ti composite oxide film is larger than that of pure aluminum oxide film at the same anodizing voltage. All the points obtained are located at the same line in the case of the pure aluminum oxide film and at two lines in the case of the Al–Ti composite oxide film. The slope of the curve increases for the Al–Ti composite oxide film formation at the anodizing voltage higher than 70 V. The variation in the slopes of the two lines for the Al–Ti composite oxide film may be explained as follows: above the anodizing voltage of 70 V, titania crystallization could lead to the creation of defects in film which result in the formation of a thick oxide film probably due to an increased ionic current through the breakdown sites [15,16]. The slope of the curve reveals an increasing tendency after the anodizing voltage of 70 V for that the film thickness probably increases more rapidly than the dielectric constant. One of the important electrical characteristics of electrolytic capacitors is their electronic conductivity, i.e. the leakage current. Electronic currents are expected to flow through the system (+) Al/composite oxide film/electrolyte when anodized aluminum is anodically polarized in the electrolyte of ammonium adipate and ethylene glycol. Typical current– voltage characteristics of the Al–Ti composite oxide films anodized at 50 V and 80 V are shown in Figs. 7 and 8 as a Schottky plot respectively. It was found that the leakage current of the Al–Ti composite oxide films increased with anodizing voltage, and there is a linear relationship between log J and E1/2 at comparatively high field (E > 5 · 105 V cm1). For the two different films investigated, a deviation from the linear dependence of log J on E1/2 is observed at comparatively low electric fields. In the log J versus log E plot of the
Fig. 5. The dependence of the retention voltage of the Al–Ti composite oxide film on the anodizing voltage.
Fig. 6. Changes in the reciprocal of the specific capacitance with the anodizing voltage for the pure aluminum oxide films and the Al–Ti composite oxide films.
3.2. Electrical properties depending on the anodizing voltage
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J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
Fig. 7. log J versus E1/2 curve for the Al–Ti composite oxide film formed at 50 V. The straight line behavior at high electric field (E > 5 · 105 V cm1) indicates Schottky emission mechanism. The insert shows log J versus log E curve at comparatively low electric field (E < 5 · 105 V cm1) which suggests ohmic conduction.
Fig. 9. log J versus E2 curve for the Al–Ti composite oxide film formed at 80 V. Space charge limited current mechanism is dominant at medium electric field (1 · 105 V cm1 < E < 5 · 105 V cm1).
There is a difference at medium electric field (1 · 105 V cm1 < E < 5 · 105 V cm1) in Fig. 8 compared with Fig. 7. A relationship between J and E at medium electric field in Fig. 8 could be fitted reasonably well with the equation [20]: er J ¼ 9hle0 3 V 2 ! J / E2 ð2Þ 8d
insets in Figs. 7 and 8, the current increases linearly with external electric field in the region of low electric field, which suggests ohmic conduction. An empirical equation [17] is developed for the dependence of the electronic current (J) through the system (+)Al/Al2O3/electrolyte on the electric field (E) at a constant temperature (T):
where l denotes carrier mobility in the absence of traps, d represents film thickness, h denotes ratio of free to trapped charges, er is relative dielectric constant of the insulator material, and thus at the medium electric field a straight line of the current density against the square of electric field can be obtained as shown in Fig. 9, which represents space charge limited current mechanism. It is known that space charge limited conduction appears in insulators in which the trap sites are filled with charges and these, consequently, result in a strong increase in the number of free charges at higher electric field [21]. In this study, the space charge limited conduction is in association with the defects such as voids existing in the film caused by the titanium oxide crystallization. Three models have been proposed for voids origin: (1) voids are produced by the volume contraction associated with the amorphous-to-crystalline transition [22]; (2) voids are formed by the trapped oxygen atoms near the crystalline particle during anodizing [23]; and (3) voids are formed by vacancy condensation [24]. It is supposed that voids existing in the film presumably play a key role in generation of the trap site.
J ¼ ae expðbe E1=2 Þ
4. Conclusion
Fig. 8. log J versus E1/2 curve for the Al–Ti composite oxide film formed at 80 V. The straight line behavior at high electric field (E > 5 · 105 V cm1) indicates Schottky emission mechanism. The insert shows log J versus log E curve at low electric field (E < 1 · 105 V cm1) which suggests ohmic conduction.
ð1Þ
where ae and be are constants. The validity of this equation has been proved also for Nb [18] and Bi [19], which shows its universal character. As is seen from Figs. 7 and 8, Eq. (1) proved to be valid for the high electric field, and the dominant conduction mechanism is therefore determined to be Schottky emission.
The formation of the Al–Ti composite oxide films on aluminum through hydrolysis precipitation and anodizing was attempted to increase the capacitance of anodic oxide films. The structure and electrical properties of the Al–Ti composite oxide film on aluminum were found to be
J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
dependent on film formation voltage. The following conclusions may be drawn. (1) The local electric breakdown of the Al–Ti composite oxide film occurs about anodizing voltage of 70 V in aqueous ammonium adipate solution at 85 C. (2) For the two kinds of composite films formed at 50 V and 80 V, the inner layer consists of pure aluminum oxide and the outer layer are thought to be a Ti-containing layer, but the film formed at 80 V indicates a thicker outer layer than that of the film formed at 50 V. The ratio of aluminum to oxygen in the film formed at 80 V has a bigger deviation from stoichiometric ratio than that in the film formed at 50 V. (3) The retention voltage and reciprocal capacitance of the Al–Ti composite oxide film are proportional to the anodizing voltage. The dependence of the retention voltage and reciprocal capacitance on the anodizing voltage demonstrates two different rectilinear regions. Above 70 V, the retention voltage increases with a lower rate and the reciprocal capacitance increases faster. These changes in the retention voltage and reciprocal capacitance with the anodizing voltage indicate inferior electrical properties of the Al–Ti composite oxide films formed above the anodizing voltage of 70 V. (4) Leakage current characteristics of the Al–Ti composite oxide films can be varied with the anodizing voltage. The film formed at 50 V shows ohmic conduction at low electric field and Schottky emission at high electric field, but the film formed at 80 V reveals an additional electric field region when compared with the film formed at 50 V, and its conduction mechanism is space charge limited current.
Acknowledgement Support for this research by the youth science and technology fund of the University of Electronic Science and
31
Technology of China under grant No. L0801301JX04018 is gratefully acknowledged. References [1] W. Wilhelmsen, T. Hurlen, Electrochim. Acta 32 (1987) 85. [2] H. Takahashi, H. Kamada, M. Sakairi, K. Takahiro, S. Nagata, S. Yamaguchi, in: Proc. Intern. Symp. of Dielectric Material Integration for Micro-electronics, 1, 1998. p. 253. [3] M. Shikanai, M. Sakairi, H. Takahashi, M. Seo, K. Takahiro, S. Nagata, S. Yamaguchi, J. Electrochem. Soc. 144 (1997) 2756. [4] J.J. Chen, D.E. Gu, Z.S. Feng, B.C. Yang, J. Funct. Mater. 36 (2005) 399. [5] M. Ue, F. Mizutani, T. Takahashi, S. Sekigawa, US Patent 5,733,661, 1998. [6] J.-L. Delplancke, R. Winand, Electrochim. Acta 33 (1988) 1539. [7] J. Yahalom, J. Zahavi, Electrochim. Acta 15 (1970) 1429. [8] J.F. MaAleer, L.M. Peter, J. Electrochem. Soc. 129 (1982) 1252. [9] T. Shibata, Y.C. Zhu, Corros. Sci. 37 (1995) 133. [10] C.E.B. Marino, E.M. de Oliveira, R.C. Rocha-Filho, S.R. Biaggio, Corros. Sci. 43 (2001) 1465. [11] L. Arsov, M. Froelicher, M. Froment, A.H.-L. Goff, J. Chim. Phys. 72 (1975) 275. [12] J. Choi, R.B. Wehrspohn, J. Lee, U. Go¨sele, Electrochim. Acta 49 (2004) 2645. [13] Y. Song, X. Zhu, X. Wang, J. Che, Y. Du, J. Appl. Electrochem. 31 (2001) 1273. [14] H. Konno, S. Kobayashi, H. Takahashi, M. Nagayama, Corros. Sci. 22 (1982) 913. [15] T. Ohtsuka, M. Masuda, N. Sato, J. Electrochem. Soc. 132 (1985) 787. [16] J.S.L. Leach, B.R. Pearson, Corros. Sci. 28 (1988) 43. [17] S. Ikonoposov, Electrochim. Acta 14 (1969) 761. [18] S. Ikonopisov, N. Elenkov, J. Electroanal. Chem. 88 (1978) 417. [19] N. Elenkov, S. Ikonopisov, V. Trifonova, A. Girginov, Surf. Technol. 9 (1979) 379. [20] F.C. Chiu, J.J. Wang, J.Y. Lee, J. Appl. Phys. 81 (1997) 6911. [21] G. Barbottin, A. Vapaille, Instabilities in Silicon Devices – Silicon Passivation and Related Instabilities, North-Holland, Amsterdam, 1986. [22] K. Kobayashi, K. Shimizu, in: R.S. Alwitt (Ed.), Aluminum Surface Treatment Technology, The Electrochemical Society, Pennington, NJ, 1986, p. 380. [23] C. Crevecoeurs, H.J. de Wit, J. Electrochem. Soc. 134 (1987) 808. [24] D.D. Macdonald, J. Electrochem. Soc. 140 (1993) L27.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 590 (2006) 26–31 www.elsevier.com/locate/jelechem
The effect of anodizing voltage on the electrical properties of Al–Ti composite oxide film on aluminum Jinju Chen *, Zhesheng Feng, Meilian Jiang, Bangchao Yang College of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, People’s Republic of China Received 1 September 2005; received in revised form 16 January 2006; accepted 21 February 2006 Available online 21 April 2006
Abstract The Al–Ti composite oxide film with high dielectric constant was prepared by hydrolysis precipitation and anodizing. The depth profiles of the Al–Ti composite oxide films formed at 50 V and 80 V have been detected by glow discharge optical emission spectrometry (GDOES). The electrical properties of the Al–Ti composite oxide film formed at different anodizing voltages have been studied by electrical measurements. The local electric breakdown occurs in the Al–Ti composite oxide films about the anodizing voltage of 70 V. The Al–Ti composite oxide film formed at 80 V presents a thicker outer layer containing titanium than that of the film formed at 50 V. It was established that the dependence of the retention voltage and reciprocal capacitance on the anodizing voltage consisted of two linear regions. It was found that more than one conduction mechanism was involved in the range of electric field used in the experiment. The current conduction is ohmic at low electric field and Schottky emission at high electric field. The film formed at 80 V reveals an additional electric field region when compared with the film formed at 50 V, and its conduction mechanism is space charge limited current. According to our results, the film formed above the anodizing voltage of 70 V exhibits inferior electrical properties than that of the film formed below 70 V. 2006 Elsevier B.V. All rights reserved. Keywords: Al–Ti composite oxide film; Structure; Electrical properties; Anodizing voltage
1. Introduction Recently, there has been an increasing demand for the high dielectric constant composite oxide films formed on aluminum to replace pure Al anodic oxide films for small electrolytic capacitors with high electric capacitance. Among these composite oxide films such as Al–(Ti, Nb, Ta, Zr) composite oxide films, the Al–Ti composite oxide films have attracted significant attention for that titanium oxide has relatively high permittivity. According to Wilhelmsen and Hurlen [1], anodic oxide films formed on titanium show a dielectric constant of about 60. The capacitance of the Al–Ti composite oxide film is the highest among the composite oxide films, and reveals 60% higher *
Corresponding author. Tel.: +86 028 83201460. E-mail address: [email protected] (J. Chen).
0022-0728/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.02.016
than that of barrier type anodic oxide films on aluminum [2,3]. Many chemical methods can be used to prepare the Al–Ti composite oxide films: anodizing after coating of TiO2 with MOCVD [2], anodizing after TiO2 deposition in pores of porous anodic oxide films (pore-filling method) [3], anodizing after coating of TiO2 by hydrolysis precipitation[4], anodizing in solution with Ti-bearing anions [5], and so on. Among these methods, the method of hydrolysis precipitation and anodizing has made the fabrication of the Al–Ti composite oxide films with low cost and high efficiency, and it is now experiencing a conversion from lab stage to industrial application. The effects of anodizing conditions on the formation, structure and electrical properties of the Al–Ti composite oxide films have been rarely experimented on aluminum, whereas it is obviously important to find out the effect for successfully applying the technology of fabricating
J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
composite oxide films to aluminum foils with high specific capacitance and perfect electrical properties. In the present study, the Al–Ti composite oxide films with high dielectric constant are prepared through hydrolysis precipitation and anodizing. The growth and electrical properties of the Al– Ti composite oxide films formed at different anodizing voltages are investigated. 2. Experimental 2.1. Formation of the Al–Ti composite oxide film Specimens of 2 cm2 with a handle were cut from highly pure etched aluminum foil (100 lm thick), and subsequently immersed in the solution containing Ti4+ compound with parameters of the hydrolysis precipitation process shown in Table 1. Consumption of H+ ions reacted with Al increased the pH value in micro pores of the etched aluminum foil during the immersion, which can accelerate the hydrolysis precipitation of Ti-bearing cation. Further details of the hydrolysis precipitation process have been described in Chen et al. [4]. Afterwards, the treated specimens were thoroughly rinsed in the double-distilled water, air-dried at 80 C, and eventually annealed at 550 C for 10 min to obtain the TiO2-coating layer. The specimens coated with TiO2 were anodized in 15% ammonium adipate solution at 85 C, and a 304 stainless steel plate was used as the counter electrode. A constant current density of 50 mA cm2 was run through the cell between the specimens and the counter electrode until the potential difference reached the selected anodizing voltage of 10–90 V. The voltage was then held constant for 10 min and the current was allowed to decay. Specimens without TiO2-coating were also anodized under the same conditions. The changes in the anode potential (vs. Ag/ AgCl sat. KCl) and current with anodizing time were followed separately and simultaneously on Agilent 34401 A digital multimeters connected to a PC system.
27
ite oxide films formed at different voltages. The measurements were performed in 15% aqueous ammonium adipate electrolyte at 50 C. A 304 stainless steel plate was used as the counter electrode. A constant current density of 0.5 mA cm2 was applied to the anodized specimen and the response voltage was recorded. As soon as the current was applied, the voltage increased immediately and then gradually reached a saturated value. The voltage after 180 s was recorded as the retention voltage of the Al–Ti composite oxide films. The specific capacitance of the Al–Ti composite oxide films was measured in 25 C – 15% aqueous ammonium adipate solution using a general LCR meter at 100 Hz. A pure aluminum sheet with a very large area was used as the counter electrode. The electronic current through the aluminum/Al–Ti composite oxide film/electrolyte system was measured during increasing the applied voltage which did not exceed 50% of the formation voltage. The value of the leakage current at any voltage was recorded after the voltage was applied for 1 min. The contact electrolyte was based on ammonium adipate and ethylene glycol. All experimental data were obtained at 50 C. 3. Results and discussion 3.1. Growth of the Al–Ti composite oxide film and its structure The specimens coated with TiO2 were anodized at different voltages of 10–90 V. The variations of voltage and current with time during anodizing to 50 V and 80 V are shown in Figs. 1 and 2 respectively. In Fig. 1, the rise of the curves corresponds to the stage where the anodization is conducted in constant current mode. An initial voltage surge of 2 V at a commencement of anodizing reflects the presence of an aluminum thermal oxide layer formed at
2.2. Structure analysis of the Al–Ti composite oxide film The depth profile of component elements in the Al–Ti composite oxide film was recorded with a Leco GDS 750A spectrometer, operated in the constant current–constant voltage mode with active pressure regulation to stabilize the discharge. 2.3. Electrical measurements The Agilent 34401A digital multimeter was used to determine the retention voltages (VR) of the Al–Ti composTable 1 Parameters of the hydrolysis precipitation process Ti4+, M
pH
Temperature, C
Time, min
0.002
1.5–2.0
65–70
8
Fig. 1. Voltage–time responses of the specimens during anodizing up to 50 V and 80 V at 50 mA cm2 in 15% ammonium adipate electrolyte at 85 C.
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J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
Fig. 2. Changes in the current with time during anodizing in 15% ammonium adipate electrolyte at 85 C.
550 C annealing. After the surge, the voltage increases at the same velocity for the two different films. The curve of 80 V in Fig. 1 reveals two parts in which the voltage shows sudden drop and subsequent raise at about 70 V, which are probably caused by the local dielectric breakdown in the Al–Ti composite oxide film during anodizing. The breakdown seems to take place by abrupt destruction of the oxide at a local weak area. The breakdown potential observed during anodization of the specimens has been proposed to be related to the crystallization of anodic titania which leads to compressive stress in the oxide or oxygen evolution [6–8]. It has been reported that in the case of anodization of titanium, the oxide exhibits a phase transition with increasing potential from an amorphous phase into a crystalline phase such as anatase or rutile [9,10]. The exact potential of the phase change depends on the anodization conditions such as electrolyte and anodizing temperature. Several results obtained in other electrolyte systems might be taken into account. For example, Arsov et al. [11] reported that during the anodization of titanium in sulfuric acid, fully crystalline anatase was formed at voltages higher than 50 V, and Choi et al. [12] found that the breakdown potential might be around 80 V in phosphoric acid. The increasing slopes of the two curves do not really present increscent formation rate of the oxides, but reveals the decrease in the effective surface area of etched aluminum foil with increasing thickness of the oxide film. In Fig. 2, the sudden drop of the anodic current from 0.20 A signifies the onset of constant voltage mode and the anodic current decaying. There are surges of current in constant current region due to the dielectric breakdown of the composite oxide film, which corresponds with the surges of voltage in Fig. 1. The dielectric breakdown does not occur in the curve of 50 V. Figs. 3 and 4 show depth profiles of the Al–Ti composite oxide films formed at 50 V and 80 V respectively, revealing distributions of aluminum, titanium and oxygen species. The total thickness of the Al–Ti composite oxide film
Fig. 3. GDOES depth profile of the Al–Ti composite oxide film formed at the anodizing voltage of 50 V.
Fig. 4. GDOES depth profile of the Al–Ti composite oxide film formed at the anodizing voltage of 80 V.
increases with the anodizing voltage increasing. For the two kinds of films the inner layer consists of pure aluminum oxide, and the outer layer is thought to be a Ti-containing layer. Under a high electric field, O2 ions transported inwards across the anodic oxide film to form pure aluminum oxide at the interface between the inner Ti-oxide layer and the metal substrate, and the outward transport of Al3+ ions formed the composite oxide in which titanium oxides were incorporated. From comparison of Figs. 3 and 4, The profile of Ti shows distribution with the maximum concentration at depth of 21 nm for the Al–Ti composite oxide film formed at 50 V, and at depth of 38 nm for the Al–Ti composite oxide film formed at 80 V. The Al–Ti composite oxide film formed at 80 V has a thicker outer layer containing titanium than that of the film formed at 50 V. This is supported by the fact that the Ti-bearing cations transport outward at a slower rate than that of Al3+ [3]. As is well
J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
known, the ionic conductivity is rather lower in the inner layer of pure alumina and higher in the outer layer contaminated by ion species due to its imperfections [13]. The thicker distribution of titanium species will induce more defects in films, which will affect the characteristics of the retention voltage and the leakage current. Furthermore, the anodizing voltage has greatly effect on the film composition. The ratio of aluminum to oxygen in the Al–Ti composite oxide film formed at 80 V has a bigger deviation from stoichiometric ratio than that in the film formed at 50 V, and accordingly nonstoichiometric defects in the film formed at 80 V get increased.
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Fig. 5 shows the retention voltages of the Al–Ti composite oxide films versus the anodizing voltage curve. The observed dependences of the retention voltage on the anodizing voltage have two linear natures. For oxide formation at the anodizing voltage higher than 70 V the decrease in the slope takes place. The approximation by the leastsquares method has allowed the following equations for the dependence of the retention voltage of the Al–Ti composite oxide films on the anodizing voltage to be established as: VR = 1.116 V+4.128 (the range of voltage from 10 V to 70 V) and VR = 0.84 V+19.95 (the anodizing voltage higher than 70 V). The retention voltage was considered as a parameter to evaluate the reliability of the oxide film [14]. The decrease in the slope of the curve after the anodizing voltage of 70 V indicates that the dielectric property of the Al–Ti composite oxide film gets reduced. Crystallization of titanium oxide could lead to the creation of the defects such as voids and cracks in film. The presence of voids and pores results in the formation of imperfection region and is detrimental to the electrical properties of the oxide films, which can explain the decrease in the slope of the curve after the anodizing voltage of 70 V.
Fig. 6 shows the relationship between the reciprocal capacitance of the oxide films and the anodizing voltage for specimens with and without TiO2-coating. As can be seen from Fig. 6, the value of the reciprocal capacitance for the specimens with and without TiO2-coating is proportional to the anodizing voltage, and the capacitance of the Al–Ti composite oxide film is larger than that of pure aluminum oxide film at the same anodizing voltage. All the points obtained are located at the same line in the case of the pure aluminum oxide film and at two lines in the case of the Al–Ti composite oxide film. The slope of the curve increases for the Al–Ti composite oxide film formation at the anodizing voltage higher than 70 V. The variation in the slopes of the two lines for the Al–Ti composite oxide film may be explained as follows: above the anodizing voltage of 70 V, titania crystallization could lead to the creation of defects in film which result in the formation of a thick oxide film probably due to an increased ionic current through the breakdown sites [15,16]. The slope of the curve reveals an increasing tendency after the anodizing voltage of 70 V for that the film thickness probably increases more rapidly than the dielectric constant. One of the important electrical characteristics of electrolytic capacitors is their electronic conductivity, i.e. the leakage current. Electronic currents are expected to flow through the system (+) Al/composite oxide film/electrolyte when anodized aluminum is anodically polarized in the electrolyte of ammonium adipate and ethylene glycol. Typical current– voltage characteristics of the Al–Ti composite oxide films anodized at 50 V and 80 V are shown in Figs. 7 and 8 as a Schottky plot respectively. It was found that the leakage current of the Al–Ti composite oxide films increased with anodizing voltage, and there is a linear relationship between log J and E1/2 at comparatively high field (E > 5 · 105 V cm1). For the two different films investigated, a deviation from the linear dependence of log J on E1/2 is observed at comparatively low electric fields. In the log J versus log E plot of the
Fig. 5. The dependence of the retention voltage of the Al–Ti composite oxide film on the anodizing voltage.
Fig. 6. Changes in the reciprocal of the specific capacitance with the anodizing voltage for the pure aluminum oxide films and the Al–Ti composite oxide films.
3.2. Electrical properties depending on the anodizing voltage
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J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
Fig. 7. log J versus E1/2 curve for the Al–Ti composite oxide film formed at 50 V. The straight line behavior at high electric field (E > 5 · 105 V cm1) indicates Schottky emission mechanism. The insert shows log J versus log E curve at comparatively low electric field (E < 5 · 105 V cm1) which suggests ohmic conduction.
Fig. 9. log J versus E2 curve for the Al–Ti composite oxide film formed at 80 V. Space charge limited current mechanism is dominant at medium electric field (1 · 105 V cm1 < E < 5 · 105 V cm1).
There is a difference at medium electric field (1 · 105 V cm1 < E < 5 · 105 V cm1) in Fig. 8 compared with Fig. 7. A relationship between J and E at medium electric field in Fig. 8 could be fitted reasonably well with the equation [20]: er J ¼ 9hle0 3 V 2 ! J / E2 ð2Þ 8d
insets in Figs. 7 and 8, the current increases linearly with external electric field in the region of low electric field, which suggests ohmic conduction. An empirical equation [17] is developed for the dependence of the electronic current (J) through the system (+)Al/Al2O3/electrolyte on the electric field (E) at a constant temperature (T):
where l denotes carrier mobility in the absence of traps, d represents film thickness, h denotes ratio of free to trapped charges, er is relative dielectric constant of the insulator material, and thus at the medium electric field a straight line of the current density against the square of electric field can be obtained as shown in Fig. 9, which represents space charge limited current mechanism. It is known that space charge limited conduction appears in insulators in which the trap sites are filled with charges and these, consequently, result in a strong increase in the number of free charges at higher electric field [21]. In this study, the space charge limited conduction is in association with the defects such as voids existing in the film caused by the titanium oxide crystallization. Three models have been proposed for voids origin: (1) voids are produced by the volume contraction associated with the amorphous-to-crystalline transition [22]; (2) voids are formed by the trapped oxygen atoms near the crystalline particle during anodizing [23]; and (3) voids are formed by vacancy condensation [24]. It is supposed that voids existing in the film presumably play a key role in generation of the trap site.
J ¼ ae expðbe E1=2 Þ
4. Conclusion
Fig. 8. log J versus E1/2 curve for the Al–Ti composite oxide film formed at 80 V. The straight line behavior at high electric field (E > 5 · 105 V cm1) indicates Schottky emission mechanism. The insert shows log J versus log E curve at low electric field (E < 1 · 105 V cm1) which suggests ohmic conduction.
ð1Þ
where ae and be are constants. The validity of this equation has been proved also for Nb [18] and Bi [19], which shows its universal character. As is seen from Figs. 7 and 8, Eq. (1) proved to be valid for the high electric field, and the dominant conduction mechanism is therefore determined to be Schottky emission.
The formation of the Al–Ti composite oxide films on aluminum through hydrolysis precipitation and anodizing was attempted to increase the capacitance of anodic oxide films. The structure and electrical properties of the Al–Ti composite oxide film on aluminum were found to be
J. Chen et al. / Journal of Electroanalytical Chemistry 590 (2006) 26–31
dependent on film formation voltage. The following conclusions may be drawn. (1) The local electric breakdown of the Al–Ti composite oxide film occurs about anodizing voltage of 70 V in aqueous ammonium adipate solution at 85 C. (2) For the two kinds of composite films formed at 50 V and 80 V, the inner layer consists of pure aluminum oxide and the outer layer are thought to be a Ti-containing layer, but the film formed at 80 V indicates a thicker outer layer than that of the film formed at 50 V. The ratio of aluminum to oxygen in the film formed at 80 V has a bigger deviation from stoichiometric ratio than that in the film formed at 50 V. (3) The retention voltage and reciprocal capacitance of the Al–Ti composite oxide film are proportional to the anodizing voltage. The dependence of the retention voltage and reciprocal capacitance on the anodizing voltage demonstrates two different rectilinear regions. Above 70 V, the retention voltage increases with a lower rate and the reciprocal capacitance increases faster. These changes in the retention voltage and reciprocal capacitance with the anodizing voltage indicate inferior electrical properties of the Al–Ti composite oxide films formed above the anodizing voltage of 70 V. (4) Leakage current characteristics of the Al–Ti composite oxide films can be varied with the anodizing voltage. The film formed at 50 V shows ohmic conduction at low electric field and Schottky emission at high electric field, but the film formed at 80 V reveals an additional electric field region when compared with the film formed at 50 V, and its conduction mechanism is space charge limited current.
Acknowledgement Support for this research by the youth science and technology fund of the University of Electronic Science and
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Technology of China under grant No. L0801301JX04018 is gratefully acknowledged. References [1] W. Wilhelmsen, T. Hurlen, Electrochim. Acta 32 (1987) 85. [2] H. Takahashi, H. Kamada, M. Sakairi, K. Takahiro, S. Nagata, S. Yamaguchi, in: Proc. Intern. Symp. of Dielectric Material Integration for Micro-electronics, 1, 1998. p. 253. [3] M. Shikanai, M. Sakairi, H. Takahashi, M. Seo, K. Takahiro, S. Nagata, S. Yamaguchi, J. Electrochem. Soc. 144 (1997) 2756. [4] J.J. Chen, D.E. Gu, Z.S. Feng, B.C. Yang, J. Funct. Mater. 36 (2005) 399. [5] M. Ue, F. Mizutani, T. Takahashi, S. Sekigawa, US Patent 5,733,661, 1998. [6] J.-L. Delplancke, R. Winand, Electrochim. Acta 33 (1988) 1539. [7] J. Yahalom, J. Zahavi, Electrochim. Acta 15 (1970) 1429. [8] J.F. MaAleer, L.M. Peter, J. Electrochem. Soc. 129 (1982) 1252. [9] T. Shibata, Y.C. Zhu, Corros. Sci. 37 (1995) 133. [10] C.E.B. Marino, E.M. de Oliveira, R.C. Rocha-Filho, S.R. Biaggio, Corros. Sci. 43 (2001) 1465. [11] L. Arsov, M. Froelicher, M. Froment, A.H.-L. Goff, J. Chim. Phys. 72 (1975) 275. [12] J. Choi, R.B. Wehrspohn, J. Lee, U. Go¨sele, Electrochim. Acta 49 (2004) 2645. [13] Y. Song, X. Zhu, X. Wang, J. Che, Y. Du, J. Appl. Electrochem. 31 (2001) 1273. [14] H. Konno, S. Kobayashi, H. Takahashi, M. Nagayama, Corros. Sci. 22 (1982) 913. [15] T. Ohtsuka, M. Masuda, N. Sato, J. Electrochem. Soc. 132 (1985) 787. [16] J.S.L. Leach, B.R. Pearson, Corros. Sci. 28 (1988) 43. [17] S. Ikonoposov, Electrochim. Acta 14 (1969) 761. [18] S. Ikonopisov, N. Elenkov, J. Electroanal. Chem. 88 (1978) 417. [19] N. Elenkov, S. Ikonopisov, V. Trifonova, A. Girginov, Surf. Technol. 9 (1979) 379. [20] F.C. Chiu, J.J. Wang, J.Y. Lee, J. Appl. Phys. 81 (1997) 6911. [21] G. Barbottin, A. Vapaille, Instabilities in Silicon Devices – Silicon Passivation and Related Instabilities, North-Holland, Amsterdam, 1986. [22] K. Kobayashi, K. Shimizu, in: R.S. Alwitt (Ed.), Aluminum Surface Treatment Technology, The Electrochemical Society, Pennington, NJ, 1986, p. 380. [23] C. Crevecoeurs, H.J. de Wit, J. Electrochem. Soc. 134 (1987) 808. [24] D.D. Macdonald, J. Electrochem. Soc. 140 (1993) L27.