Anodic Oxidation in Aluminum Electrode by Using Hydrated

Apr 12, 2016 - (13, 14) For the tantalum solid electrolytic capacitor, the oxygen in the MnO2 is utilized to repair the breakdown in the Ta2O5 film an...
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Anodic Oxidation in Aluminum Electrode by Using Hydrated Amorphous Aluminum Oxide Film as Solid Electrolyte under High Electric Field Manwen Yao,*,† Jianwen Chen,† Zhen Su, Yong Peng, Pei Zou, and Xi Yao Functional Materials Research Laboratory, School of Materials Science and Engineering, Tongji University, 4800 Cao-an Hwy., Shanghai 201804, China S Supporting Information *

ABSTRACT: Dense and nonporous amorphous aluminum oxide (AmAO) film was deposited onto platinized silicon substrate by sol−gel and spin coating technology. The evaporated aluminum film was deposited onto the AmAO film as top electrode. The hydrated AmAO film was utilized as a solid electrolyte for anodic oxidation of the aluminum electrode (Al) film under high electric field. The hydrated AmAO film was a high efficiency electrolyte, where a 45 nm thick Al film was anodized completely on a 210 nm thick hydrated AmAO film. The current−voltage (I−V) characteristics and breakdown phenomena of a dry and hydrated 210 nm thick AmAO film with a 150 nm thick Al electrode pad were studied in this work. Breakdown voltage of the dry and hydrated 210 nm thick AmAO film were 85 ± 3 V (405 ± 14 MV m−1) and 160 ± 5 V (762 ± 24 MV m−1), respectively. The breakdown voltage of the hydrated AmAO film increased about twice, owing to the self-healing behavior (anodic oxidation reaction). As an intuitive phenomenon of the self-healing behavior, priority anodic oxidation phenomena was observed in a 210 nm thick hydrated AmAO film with a 65 nm thick Al electrode pad. The results suggested that self-healing behavior (anodic oxidation reaction) was occurring nearby the defect regions of the films during I−V test. It was an effective electrical self-healing method, which would be able to extend to many other simple and complex oxide dielectrics and various composite structures. KEYWORDS: amorphous aluminum oxide film, hydrated alumina, anodic oxidation, solid electrolyte, self-healing

1. INTRODUCTION

Two types of electrolytic capacitor on the market use a solid electrolyte: tantalum solid electrolytic capacitors, which use the pyrolytic manganese dioxide as the solid electrolyte material,12 and aluminum solid electrolytic capacitors, which use 7,7,8,8tetracyanoquinodimethane (TCNQ) or polypyrrole (PPY) as the solid electrolyte material.13,14 For the tantalum solid electrolytic capacitor, the oxygen in the MnO2 is utilized to repair the breakdown in the Ta2O5 film and the MnO2 is reduced to Mn2O3, thus increasing the resistance. But poor productivity and high cost of the tantalum solid electrolytic capacitor limit its application. For aluminum solid electrolytic capacitors, the contact point will be locally heated to a higher temperature if a leakage fault occurs at a local site. The polymer reaches its melting or vaporization point and then isolates the connection to the fault site.15 However, the high volume fraction of the electrolyte used in these solid state capacitors still limits the increase of the energy-storage density. Anodic aluminum oxide (AAO) thin films were a type of high−quality dielectric material which had long been used in

It has long being a dream to endow material with a self-healing or self-repairing function similar to that of the biological tissue of living creatures so that it can be recovered to its original state upon any damage. Inspired by nature, the self-healing idea has been successfully applied to develop smart materials, such as self-healing polymers,1−3 self-healing ceramics and concrete,4,5 and self-healing metal.6 In addition to mechanical self-healing behavior, electrical self-healing phenomena has long been known and is also attractive. The electrolytic capacitors as a basic electronic component widely used in electronic industry are one successful example of making use of the high insulation characteristics and selfrepairing behavior of the anodized aluminum oxide layer formed on the surface of aluminum foil.7,8 The unavoidable weak spots in the anodized aluminum oxide layer can be repaired under high electric field by further anodic oxidation of the aluminum foil in the presence of a liquid/paste electrolyte. However, the electrolytic capacitor using a wet electrolyte would suffer from the leakage risk specifically at high temperature.9 A solid-state electrolyte would be desirable for the self-repairing effect.10,11 © 2016 American Chemical Society

Received: January 25, 2016 Accepted: April 12, 2016 Published: April 12, 2016 11100

DOI: 10.1021/acsami.6b00945 ACS Appl. Mater. Interfaces 2016, 8, 11100−11107

Research Article

ACS Applied Materials & Interfaces fabrication of electrolytic capacitors.16−19 There were two types of AAO films, barrier type and porous type, depending upon the preparation of the films using neutral electrolyte20 or corrosive electrolyte,21,22 as well as the processing conditions. The thickness of the barrier-type AAO films depend upon the applied voltage, which was usually quoted in terms of the anodizing ratio.19 The breakdown strength of the barrier-type AAO film was up to 700−1000 MV m−1.23−25 According to the self-healing technology in aluminum liquid electrolyte capacitor, the liquid electrolyte supposed the O2− anions, OH− anions, or both in time, which is required for the self-healing reaction (anodic oxidation). In this study, the sol− gel derived amorphous aluminum oxide (AmAO) films were confirmed to be quite effective to anodic oxidation of aluminum anode under the applied electric field. Structural and absorbed water of the hydrated AmAO film was supposed to be the supplier of the O2− anions, OH− anions, or both for the anodic oxidation, which acted as an effective solid state electrolyte.26,27 While the AmAO film could still behave as an excellent dielectric with very high breakdown strength close to its intrinsic value.24,25 A delicate balance as an effective solid state electrolyte and an excellent dielectric insulator had been achieved. To avoid any confusion, we refer to it as “dry anodic oxidation” in this work to distinguish it from the anodic oxidation behavior in liquid electrolyte.

Figure 1. FESEM micrographs of the AmAO film: (a) surface and (b) cross section.

Figure 2. Plot of I−V for the hydrated AmAO film with 45 nm thick Al electrode pad and the metallographic photograph of the Al electrode pad during I−V test.

2. EXPERIMENTAL SECTION The AmAO films were deposited onto platinized silicon wafer substrate (Si/SiO2/Ti/Pt) by a sol−gel and spin coating technology. The aluminum iso-propylate as the aluminum precursor and 2ethoxyethanol as the solvent were used for the sol-synthesis. The preparation process of the thin AmAO films was elaborated in ref 20. The thickness of the AmAO films in this work was about 210 nm, which was measured by an interferometer (Filmetrics F20, San Diego, CA). Aluminum films with the thickness of 45, 65, and 150 nm and the diameter of 1 mm were deposited onto the AmAO film as top electrode by using a vacuum evaporation instrument (ZHD-400, Technol Science, China). During the deposition of the Al electrode, the samples were held in a vacuum chamber at a pressure lower than 2 × 10−4 Pa at 353 K. The rotation of the substrates was 10 rpm. Before the electrical measurement, the AmAO films were hydrated at relative humidity of 50% for 60 min at room temperature in a humidity chamber (GP/TH-50, Guangpin, China). Electrical characteristics were measured by using a Keithley 2400 source meter (Keithley Instruments, Inc., Cleveland, OH). The morphology of the films was examined by using an optical microscope (BX51, Olympus) and an XL30 field-emission scanning electron microscope (FE-SEM, Philips). The chemical composition was identified using an Escalab 250Xi X-ray photoelectron spectrometer (XPS; Thermo Scientific).

showed the typical I−V characteristics of the AmAO films with aluminum film as top electrode. A few characteristic images of these two samples were recorded at different time/voltage nodes. In the low voltage regime, the leakage current increased exponentially up to a peak value of 4 μA at 20 V, then decreased slightly, and maintained around 3 μA with the voltage increasing from 20 to 38 V. Under optical microscope, the color of the aluminum electrode looked metaloid gray (Figure 2A). However, with the increasing of voltage, the edge of the aluminum electrode become blue at first and then all the grayish surface area of the aluminum electrode turned into blue gradually (Figure 2B). A sudden decrease of the leakage current from 4 μA to 5 nA took place at about 38 V. After that, with the increasing of voltage from 38 to 160 V, the leakage current increased in an unstable fluctuating manner from 5 nA to 1 μA. Meanwhile, the color of the aluminum electrode gradually turned into dark blue (Figure 2C). At last, a sudden escalation of the current from micro- to milli-amperes took place, which signified the occurrence of the final hard breakdown. A breakdown spot was found under the contact point of the probe (red circle in Figure 2D). The color change of the Al electrode pad was studied by a Xray Photoelectron Spectrum (XPS) analysis. Figure 3(a) was the XPS survey spectra of the AmAO dielectric film and the Al electrode film before and after the measurement. The peaks for all the films were corresponding to the binding energy of Al and O. Figure 3(b) shows the high-resolution XPS spectra of Al 2p peaks of the films. For the original metaloid grayish Al electrode film before the measurement, two peaks at 72.7 eV corresponding to the metal state of aluminum and 74.3 eV corresponding to the oxide state coexisted in the XPS spectra.28 The observed spectrum was quite similar to that of the metal

3. RESULTS AND DISCUSSION 3.1. Anodic Oxidation Phenomena. The patterns obtained by XRD investigations revealed that the AmAO thin films possessed an amorphous structure. Figure 1 showed the surface and cross-section micrographs of the AmAO film. The AmAO film exhibited dense, crack free and uniform structure. The tested film thickness was 210 nm, which was in good agreement with the value obtained with the interferometer (Filmetrics F20, San Diego, CA). High field phenomena of the AmAO films was carefully studied under a stepwise applied voltage at a constant ramping rate of 0.2 V/step and an interval of 0.5 s. The samples were monitored under an optical microscope and recorded in realtime during the whole examination course. The leakage current of the samples under test was taken simultaneously. Figure 2 11101

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Figure 3. (a) XPS survey spectra and (b) high resolution XPS spectra of Al 2p peaks for AmAO film and aluminum electrode film of the sample before and after the measurement.

calculated by the peaks at 72.7 eV (Al 2p Metal), 74.3 eV (Al 2p Oxide) and 531.1 eV (O 1s), respectively. No Al metal state was observed at less than the etch depth of 50 nm. It indicated that the Al electrode film had been anodized to the oxide state. But metal-state Al was found at the etch depth from 50 to 200 nm, which suggested that the cation Al3+ entered to while the anion O2−/OH− egressed the AmAO film under the high field. The Al3+ was formed at the metal/film interface by oxidation of aluminum atoms and the anion O2−/ OH− were derived from the hydrated AmAO film. As a result of the counter-migration of Al3+ and O2−/OH−, a new anodic oxide film layer was formed on the metal/film interface. Al3+ was partially consumed and partially transported to the inner layer of AmAO film. The cation partially counter-migrated with O2−/OH− in the inner layer, and partially gain electrons, and then was transformed back to the metallic state and left in the AmAO film. The anodic oxidation reaction of the aluminum electrode film was further analyzed by the XPS spectra of the anions involved. Figure 5(a) showed the experimental result of high resolution O 1s XPS peak of the AmAO dielectric film and Al electrode film. Because the O 1s peaks at 531.1 and 532.5 eV were attributed to the oxygen bonding energy of aluminum oxide and aluminum hydroxide, respectively,28,29 the experimental O 1s peak of the films was deconvoluted into two subpeaks contributed from oxide and hydroxide respectively (Figure 5(b,c,d). Suppose the fwhm (full width at half maximum) of the two peaks are the same, then the concentration ratio of the [oxide]/[hydroxide] of the films was estimated. From the experimental spectra of the films, the [oxide]/[hydroxide] ratio for the AmAO dielectric film was about 2.5, while the ratio for both the gray Al electrode film before the measurement and blue AAO film after the measurement was about 4. It suggested that both O2− and OH− anions were involved in the anodic oxidation reaction of the films. The concentration of the aluminum hydroxide in AmAO dielectric film was higher than that of the Al electrode film. In considering of the fact that the AmAO film was derived from a wet chemistry sol−gel route, the higher concentration of the hydroxide was not surprising. However, the aluminum hydroxide concentration of the electrode film had no significant change after the anodic oxidation reaction taking place during the measurement and still kept the same level as a naturally derived hydrated aluminum oxide surface layer. Most importantly, the high concentration of hydroxide in the

aluminum with naturally hydroxidized surface layer because of the strong oxidation trend of aluminum in ambient air. However, for the blueish Al electrode film after the measurement, only one peak at 74.3 eV symbolized as aluminum oxide state could be observed, and no metal state can be found. The spectrum is similar to that of the amorphous aluminum oxide (AmAO) film, where only one Al 2p oxide state at 74.3 eV could be found. The above results strongly suggested that the thin aluminum electrode film had completely transformed into aluminum oxide layer under the applied electric field during the measurement. The transformation could be recognized as anodic oxidation reaction and the transformed layer could be called as anodized aluminum oxide (AAO) layer. XPS analysis of the binding energy of the Al cation confirmed the anodic oxidation reaction of the aluminum electrode film. In addition, the anodized Al electrode film was studied using X-ray photoelectron spectroscopy (XPS) depth profile showed in Figure 4. A fragment of the sample was depth profiled by

Figure 4. XPS depth profile of the blue Al electrode pads after applied stepwise positive voltage. The atomic concentration was calculated by the peaks at 72.7 eV (Al 2p Metal), 74.3 eV (Al 2p Oxide) and 531.1 eV (O 1s). The etch time (s) was converted to an etch depth (nm) using etch rate measured on a 30 nm Ta2O5/Ta standard.

rastering a beam of 3000 eV argon ions over a 3 mm by 4.5 mm area. Each etch cycle was 20 s, and after every etch level a survey spectrum was collected to detect all possible elements at every sample depth. The etch time (s) was converted to an etch depth (nm) using etch rate measured on a 30 nm Ta2O5/Ta standard. The atomic concentration of the samples was 11102

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Figure 5. (a) High-resolution XPS spectra of the O 1s peaks and (b) fitted curves of AmAO film, (c) Al electrode film before the measurement, and (d) AAO film after the measurement.

The above XPS and FTIR investigation of the film suggested that the 45 nm thin Al electrode film had completely transformed from metal state to oxide state under the applied electric field during the measurement. Then, the total thickness of the aluminum oxide of the sample should be increased, which was confirmed straightforward by the cross-section FESEM micrographs of the sample before and after the electric measurement, as shown in Figure 7. It was obvious that the

AmAO film might be an important driving factor for the anodic oxidation reaction of the aluminum electrode film. The hydration feature as well as its structural characteristics of the AmAO film was also investigated by Fourier transform infrared (FTIR) spectra. As shown in Figure 6, the peaks below

Figure 6. FTIR spectrum of the amorphous aluminum oxide (AmAO) film.

1000 cm−1 represent the general feature of alumina.30,31 Peaks at 440 and 543 cm−1 corresponded to the stretching vibration mode of Al−O bond in alumina, while the peak at near 910 cm−1 corresponds to the bending mode of Al−Al−OH bond.32 The peak at 1646 cm−1 and a broad band in the region of 2800−3500 cm−1 corresponded to the stretching vibrations mode of the hydrogen-bonded OH from the adsorbed H2O in alumina.33−35 Several peaks in the 1200−1700 cm−1 region corresponded to the chemisorbed CO2 molecules, while a peak centered at 2363 cm−1 in the region of 2350−2370 cm−1 corresponded to the physisorbed CO2 molecule.31 It was obvious that the AmAO film contained absorbed water and structural water in the form of hydroxyl. The absorbed water and structural water in the AmAO film provided enough amount of O2−/OH− anions required for the anodic oxidation reaction of the Al electrode and endowed the AmAO dielectric film with solid state electrolyte behavior.

Figure 7. Cross-sectional FESEM micrographs of the sample (a) before and (b) after the electric measurement.

thickness of the formed AAO film was about 70 nm, which was transformed from 45 nm thick Al electrode film after the electric measurement. According to refs 36 and 37, the theoretical thickness ratio of the Al oxide to Al is 1.6, then the thickness of the newly generated AAO layer estimated from the theory should be 45 × 1.6 = 72 (nm). The measured value determined from the cross-section FESEM micrographs was well consistent with the theoretical estimation. According to the color generation of the anodic oxide film, Pashchanka et al.38 suggested that it was the interference color which was depended on the oxide film thickness. Therefore, the thickness change of the Al electrode film after applications of 11103

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absorbed water of the hydrated AmAO film were fundamental factors for the self-healing reaction, the current−voltage (I−V) characteristics of a dry and hydrated AmAO film with a 150 nm thick Al electrode pad were studied in this section. To obtain the I−V characteristics of a dry AmAO film, the hydrated AmAO film was heated at 300 °C for 60 min and then cooled down to room temperature to test the I−V characteristics. During these testing processes, the sample was maintained in a vacuum environment with the pressure lower than 5 × 10−1 Pa by a hot and cold stage (HCPP621VG, Instec, Naperville, IL). At the same time, I−V characteristics of the hydrated AmAO film with 150 nm thick Al electrode pad were tested at relative humidity of 50% at room temperature by a Humidity Chamber (GP/TH-50, Guangpin, China). As shown in Figure 9a, I−V characteristics of the dry AmAO film were significantly different from that of the hydrated AmAO film. For the hydrated one, the leakage current increased exponentially up to a peak value of 4 μA at 20 V (95 MV m−1), decreased slightly, and then was maintained around 3 μA until a final hard breakdown occurred at about 155 V (738 MV m−1). However, for the dry one, the leakage current increased exponentially up to a peak value of 2.5 μA at 80 V (380 MV m−1) and a final hard breakdown occurred at about 85 V (405 MV m−1). It is obvious that the leakage current of the AmAO film was strongly dependent upon the structural and/or absorbed water. By measuring breakdown voltage of 10 different sites on the same samples, breakdown voltage of the dry and hydrated AmAO films were 85 ± 3 V (405 ± 14 MV m−1) and 160 ± 5 V (762 ± 24 MV m−1) respectively. Compared with the dry AmAO film, breakdown strength of the hydrated AmAO film has increased about twice, owing to the self-healing behavior (anodic oxidation reaction). If the current efficiency of the anodic oxidation reaction is 100%, the thickness of the newly formed aluminum oxide film on Al/AmAO interface during I−V testing can be calculated by Faraday’s laws19 as follows:

electric field can be also confirmed by using an optical interferometer (Filmetrics F20, San Diego, CA). Figure 8 was

Figure 8. Reflectance spectroscopy of (a) the original bare AmAO film area and (b) the bluish Al electrode area after the electric measurement.

the reflectance spectra of the original bare AmAO film area and the bluish Al electrode area after the electric measurement. By Fourier fitting of the reflectance spectra, the thickness of the original AmAO film was 206.3 nm, and that of the bluish Al electrode area of the sample after the electric measurement was 279.1 nm. Therefore, the thickness of the newly transformed AAO layer from the oxidation of the 45 nm Al electrode was 73.8 nm. This result value determined from the interferometer was well consistent with the theoretical estimation (72 nm). Moreover, the observed color change of the Al electrode film from metaloid gray to dark blue after the measurement can also be traced back to the interference effect of the aluminum oxide film. The optical refractive index of the newly formed anodized aluminum oxide layer transformed from the Al electrode was the same as aluminum oxide. All the above results are strong evidence to confirm that the 45 nm thin aluminum electrode had completely transformed into anodized aluminum oxide layer during the measurement. 3.2. Self-Healing Phenomena. In the previous section, the sol−gel-derived hydrated AmAO films were confirmed to be quite effective to anodic oxidation of aluminum anode, where a 45 nm thick Al electrode film could be completely anodized during the measurement. Because structural and

dAAO =

QM ≈ 7.01 × 104 ·Q i(nm) nρFA

(1)

where i

Qi =

∫t

I(t )dt ≈

∑ Ij·Δt j=0

(2)

Figure 9. (a) Plot of I−V for dry and hydrated AmAO film with 150 nm thick Al electrode pad; (b) the thickness of the formed AAO film on the Al/ AmAO interface calculated by Faraday’s law. 11104

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ACS Applied Materials & Interfaces where M is the molecular weight of the aluminum oxide film; ρ is the density of the newly formed aluminum oxide film, and it is assumed to be 3.2 g/cm3;36 n is the needed charge to form an oxide molecule; F is the Faraday constant; and A is the surface area of the specimen covered by the electrolyte, which is 1 calculated as A = 2 πr 2 = 0.785 cm 2 in this work. As shown in Figure 9b, the thickness of the formed aluminum oxide film increased linearly with the increasement of time or voltage, and the slope is 0.456 ± 0.001 nm/V. It has been suggested that leakage current of the hydrated AmAO film maintaining around 3 μA from 20 to 155 V is attributed to the diffusion control of the cation and anion during test. Therefore, the formed AAO layer, superimposed on the original AmAO film, increased the total thickness of the aluminum oxide film and helped to repair defects at the interface and the original AmAO film. Consequently, the breakdown strength of the aluminum oxide film was significantly enhanced. As shown in Figure 10, the I−V Plots of the samples with 65 nm thick and 150 nm thick Al electrode pads were carefully

behavior) then a blue band appeared around the edge of Al electrode pad. This phenomenon was also found in 150 nm thick Al electrode pad after I−V test (region D in Figure 11b). These blue spots dispersed on the Al electrode pad suggested that priority anodic oxidation behavior (self-healing reaction) occurred on the AmAO/Al interface nearby these defect regions. A breakdown phenomenon occurred at about 89 V(424 MV m−1) at the contact point of the probe (region C in Figure 11a). For the sample with 150 nm thick Al electrode pad, the Al electrode film was thick enough to maintain its good electrical conductivity during testing, regardless of priority anodic oxidation behavior nearby defect regions. Therefore, the self-healing reaction (anodic oxidation) model of the amorphous aluminum oxide film can be illustrated in Figure 11c. Platinum is the bottom electrode serving as the cathode of the AmAO sample. Aluminum is on the top surface of the AmAO film serving as the anode of the sample. Under the driving force of the applied electric field during the measurement, the O2−/OH− anions in the hydrated AmAO film migrate toward the aluminum anode and oxidized the aluminum at the Al/AmAO interface region, transformed the Al0 atoms into Al3+ cations forming aluminum oxide and/or hydroxide. The overall reaction can be described as anodic oxidation

O2 −and/or OH−

Al ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Al3 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Al 2O3and/or AlO(OH) (3)

With the progress of the anodic oxidation reaction, an anodized aluminum oxide AAO layer started to grow in the interface region at the expense of the aluminum anode. It has been widely recognized from the experience of aluminum electrolytic capacitor that the AAO film is a high quality dielectric material with breakdown strength very close to its intrinsic value about 700−1000 MV m−1.24,25 Hence, the formation of AAO layer in this case significantly enhanced the breakdown strength of the film. Furthermore, even some of the original micro/macro defects at the surface of the AmAO film can be mended by the AAO layer. As to the defects inside the AmAO film, they can also be remended. Because the newly oxidized and activated Al3+ cations in the interface region would also migrate toward the cathode, penetrating into the AmAO film under the driving force of the applied electric field. The counter migration tracks of the Al3+ cations and O2−/OH− anions are governed by the local field. Usually, the local fields around the defects are distorted and deviated from that of the normal state. The defected region is thermodynamically activated. The migration of the Al3+ cations and O2−/OH− anions has a self-adaptive trend to reduce and minimize the local energy pushing the structure restoring back to its normal state. From materials science point of view, it can be regarded as a kind of selfrepairing process similar to that of the self-healing behavior of the biological tissues of living creatures discussed at the beginning of this paper. From the physico-chemistry point of view, the self-repairing or self-remending process of this case is based on an anodic oxidation reaction preferentially takes place at the defected region to restore the normal aluminum oxide or hydroxide structure.

Figure 10. I−V Plots of the samples with 65 nm thick and 150 nm thick Al electrode pad.

studied with a stepwise applied voltage at a constant ramping rate of 0.2 V/step and an interval of 0.5 Sec. In the low voltage, the leakage current of both two samples increased exponentially up to a peak value of 4 μA at 20 V (95 MV m−1) and then decreased slightly and finally maintained around 3 μA with the voltage ranging from 20 to 65 V (95−181 MV m−1). For the sample with 65 nm thick Al electrode pad, a sudden decrease of the leakage current from 3 μA to 50 nA took place at about 69 V (329 MV m−1) and a breakdown occurred at about 89 V (424 MV m−1). However, the leakage current of the sample with 150 nm thick Al electrode pad maintained around 3 μA until a final hard breakdown occurred at about 155 V (738 MV m−1). Figure 11a showed the metallographic photograph of 65 nm thick Al electrode pad after application of positive voltage. It was obvious that some blue spots (region A) and a blue band were around the aluminum electrode edge (region B). According to the discussion in the previous section, the change in color of Al electrode films was attributed to the anodic oxidation of Al electrode film. Therefore, the priority anodic oxidation phenomena occurring at some local regions suggested that self-healing reaction (anodic oxidation) was occurring during the I−V test. Because the electric field concentrates at the edge owing to the edge effect, priority anodic oxidation phenomena occurred at the electrode edge (self-healing

4. CONCLUSIONS The hydrated sol−gel derived amorphous aluminum oxide film is confirmed to be very effective to anodic oxidation of aluminum anode. The hydrated amorphous aluminum oxide film was the supplier of the O2− anions, OH− anions, or both 11105

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Figure 11. (a) Metallographic photograph of the sample with 65 nm thick Al electrode pad after I−V testing. (b) Metallographic photograph of Priority anodic oxidation phenomena on the edge of 150 nm thick Al electrode pad after I−V test. (c) The self-healing reaction (anodic oxidation) model of the AmAO film with aluminum electrode under high electric DC field.

Author Contributions

required for the anodic oxidation reaction acting as an effective solid state electrolyte. The hydrated AmAO film is a high efficiency electrolyte, where an aluminum electrode film in 45 nm thickness can be anodized on the AmAO film in 210 nm thickness. Breakdown voltage of the dry and hydrated 210 nm thick AmAO film with 150 nm thick Al electrode pad are 85 ± 3 V (405 ± 14 MV m−1) and 160 ± 5 V (762 ± 24 MV m−1), respectively. The breakdown strength of the hydrated 210 nm thick AmAO film increases about twice owing to the selfhealing behavior (anodic oxidation reaction). An intuitive phenomenon of the self-healing behavior (priority anodic oxidation phenomena) are observed in a 210 nm thick hydrated AmAO film with a 65 nm thick Al electrode pad. The selfrepairing or self-remending process based on an anodic oxidation reaction preferentially takes place at the defected region to restore the normal aluminum oxide or hydroxide structure, it would be able to extend to many other simple and complex oxide dielectrics and various composite structures.

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†

These authors contributed equally to this work and should be considered cofirst authors Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

This work is supported by the Ministry of Science and Technology of China through 973-project (Grant No. 2015CB654601), International Science & Technology Cooperation Program of China (Grant No. 2013DFR50470) and National Science Foundation of China (Grant No. 51272177).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00945.

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Details on the XRD pattern of the sol−gel derived AmAO thin film with thickness of 210 nm. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 11106

DOI: 10.1021/acsami.6b00945 ACS Appl. Mater. Interfaces 2016, 8, 11100−11107

Research Article

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DOI: 10.1021/acsami.6b00945 ACS Appl. Mater. Interfaces 2016, 8, 11100−11107