Fast anodization fabrication of AAO and barrier perforation process on ITO glass
© Liu et al.; licensee Springer. 2014
Received: 22 November 2013
Accepted: 25 March 2014
Published: 3 April 2014
Thin films of porous anodic aluminum oxide (AAO) on tin-doped indium oxide (ITO) substrates were fabricated through evaporation of a 1,000- to 2,000-nm-thick Al, followed by anodization with different durations, electrolytes, and pore widening. A faster method to obtain AAO on ITO substrates has been developed, which with 2.5 vol.% phosphoric acid at a voltage of 195 V at 269 K. It was found that the height of AAO films increased initially and then decreased with the increase of the anodizing time. Especially, the barrier layers can be removed by extending the anodizing duration, which is very useful for obtaining perforation AAO and will broaden the application of AAO on ITO substrates.
Nanostructures with monodisperse arrangement nanopores have been used widely as template to fabricate various functional nanomaterials [1–4]. One of such nanostructures is well-known porous anodic aluminum oxide (AAO), which is considered as one of the most prominent template owing to its advantages of controllable diameter, high aspect ratio, and economical way in producing [1, 5–7]. To this day, a variety of synthetic methods have been developed to fabricate porous AAO, typically fabricated from anodizing bulk aluminum foils or plates at constant voltage or current density in various electrolytes such as sulfuric redacid, oxalic acid, phosphoric acid, etc [8–11]. However, it needs great care in the process of preparation of the aluminum substrate and the manipulation of the anodic film since the AAO is a brittle ceramic film grown on soft aluminum metal . Thus, direct fabricating AAO onto rigid substrates become a more convenient and important technique to prepare vertical nanostructures. The fabrication of AAO on Si substrates has been well established [12–17], while many photonic applications call for nanowire structures on transparent conductive substrates. The tin-doped indium oxide (ITO) glass is a good choice to satisfy this demand [18–20].
Recently, several articles have reported the fabrication and application of AAO in phosphoric acid [21–23]. Chu et al.  reported the successful fabrication of AAO is in phosphoric acid, from 2-µm thick aluminum films deposited by radio frequency (rf) sputtering, resulting in large-diameter AAO pores. An anodization duration of more than 40 min was observed in 10 vol.% phosphoric acid at a voltage of 130 V at 280 K. Small transverse holes appear regularly in the anodized films, which arose from the fact that the aluminum was deposited in two-step sputtering. The current density rapidly decreased to 0, indicating a loss of electrical conductivity. Moreover, the barrier layer still exists, preventing the physical and electrical contact between the pore and the substrate.
The barrier layer of AAO arouse many people’s attention since it makes the bottom of the AAO electrically isolated from the substrate. The method to get rid of the barrier layer has been proved to be the key to make electrical contact at the bottom. A current technology that removes the barrier layer is through immersion in dilute acid during which time the pores are also widened [12, 24–26]. Oh et al.  had an innovative method through selectively etching the penetrating metal oxide WO3, which was formed from the metal underlayer W, to open the base of the alumina pores. However, it calls for a more simple method to remove the barrier layer.
In this article, fast growth of the AAO film on ITO glass was successfully realized by employing high-field anodization technology of our group  and a distinct ‘Y’ branch morphology was observed. The evolution process of the AAO film on ITO glass has been explored by using current-time curves under high-field anodization. Furthermore, we find a friendly and simple method to remove the barrier layer.
Deposition of aluminum thin films
Thin films of aluminum on tin-doped indium oxide (ITO) glass were formed via radio frequency (rf) sputtering process. After, that AAO layer was fabricated via anodization of the rf-sputtered aluminum films. The transparent substrate of ITO glass has a sheet resistance <7Ω/□. Before magnetron sputtering, the ITO glass were degreased in acetone and alcohol, and then washed in deionized water. The substrates were first vacuumed to 4×10−5 Pa and then inlet argon gas to the pressure of 2.2×10−2 Torr, the highly pure aluminum (99.99%) was deposited with the power of 200 W at room temperature. The mainly sputtering process was sputtered in one step for 1 h, as a contrast, the rest was sputtered in two steps, each step for 30 min.
After deposition, the glass was cut to the dimensions of 1×1 cm2. Then, the samples were put into a Teflon holder with a certain contact surface exposed to the electrolyte solution. All anodization processes were carried out in an electrochemical cell equipped with a cooling system. At the same time, a DC digital controlled stirrer with a stirring rate of 400 rpm was employed to keep the temperature stable.
For the samples anodized at target voltages of 195 V, the electrolyte was the mixture of ethanol and water with a ratio of 1:4 in volume, in which the concentrations of phosphoric acid were 2.5 wt.% and the temperature was −4°C; the sample was anodized for an ultrashort time (30 to 150 s). To enlarge the holes, a phosphoric solution with the concentrations of 5 wt.% was employed at 45°C, with the time of 20 min and 30 min.
As for the rest of the samples, the target voltage was 40 V and the anodization process was performed in an electrolyte of water in which the concentrations of oxalic acid were 0.3 M. The temperature was 4°C, and the anodizing time range was 15 to 105 min.
The current-time transients of the anodization were record by a programmed power source (Agilent, N5752, Santa Clara, CA, USA) linked to a computer. Field emission scanning electron microscopy (FESEM) micrographs were obtained by FE-SEM Philips Sirion 200 (Amsterdam, The Netherlands) to analyse the structure of the AAO films.
Results and discussion
Fast anodization process in phosphoric acid
Anodization in oxalic acid
Figure 2 is the anodizing schematic of the former process. Figure 2a shows Al film sputtered on ITO glass. When immerged in electrolyte, the AAO layer is formed, as shown in Figure 2b. After anodizing for a long time, the barrier layer touches the bottom, reaching the ITO glass which can be seen in Figure 2c. As the anodizing time goes on, the barrier layer upturned and there is no aluminum left as Figure 2d shows. And the remaining barrier layer can be removed as this process goes on, leaving an AAO template without barrier layer, as shown in Figure 2e.
In this study, an efficient way to form AAO film on ITO glass is performed, reducing the anodizing time to about 30 s. The forming process of AAO on ITO has been explained based on the current-time curves. The thickness of the AAO film anodized in oxalic acid increased first and then decreased with the progress of the anodization process. Getting rid of barrier layer has been proved to be the key to make electrical contact at the bottom, which helps to assemble nanowire structures on ITO glass directly. Having enough anodizating time, the barrier layer could be eliminated. This method will be highly advantageous to form nanostructured photoelectric devices.
This work was supported by the National Major Basic Research Project of 2012CB934302, National 863 Program 2011AA050518, and the Natural Science Foundation of China (grant nos.11174197 and 61234005).
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