Three-dimensional AlZnO/Al2O3/AlZnO nanocapacitor arrays on Si substrate for energy storage
© Li et al.; licensee Springer. 2012
Received: 15 August 2012
Accepted: 24 September 2012
Published: 2 October 2012
High density three-dimensional AZO/Al2O3/AZO nanocapacitor arrays have been fabricated for energy storage applications. Using atomic layer deposition technique, the stack of AZO/Al2O3/AZO has been grown in the porous anodic alumina template which is directly formed on the Si substrate. The fabricated capacitor shows a high capacitance density of 15.3 fF/μm2 at 100 kHz, which is nearly 2.5 times over the planar capacitor under identical conditions in theory. Further, the charge-discharge characteristics of the capacitor are characterized, indicating that the resistance-capacitance time constants are equal to 300 ns for the charging and discharging processes, and have no dependence on the voltage supply. This reflects good power characteristics of the electrostatic capacitor.
KeywordsNanocapacitor arrays AAO Capacitance density RC time constant Energy storage
In recent years, the novel characteristics of nanostructures have attracted great attention from researchers; in particular, the nanostructure-based devices have been explored in many fields such as electronics, optoelectronics, and magnetism [1–3]. As one of the most important applications, the nanocapacitor arrays have been intensively studied for the next generation energy storage system due to increasing demands of high capacity, lightweight, and compact energy storage devices [4–7].
In terms of the electrostatic capacitor theory, the capacitor capacity is mainly determined by the electrode area. Therefore, to increase the effective area of electrodes, the three-dimensional nanocapacitor arrays are introduced to achieve a high capacitance density. As one of the most promising methods of fabricating nanocapacitor arrays, the porous nanostructure templates are used widely, including nanowire, nanopillar, anodic alumina (AAO), and so on [4–7]. For example, although InAs nanowire-based nanocapacitors (Au/Cr/HfO2/InAs) can achieve a larger electrode surface area, the poor mechanical strength of nanowires makes it unsuitable for energy storage . Moreover, a significant improvement in capacitance has been achieved for the template of silicon nanopillars, which was fabricated by Au metal-assisted etching in conjunction with interference lithography; however, the Au residue could cause oxide degradation and inferior device performance . On the other hand, porous AAO templates made from aluminum foils exhibit a high degree of regularity and uniformity in addition to a quite simple process. Therefore, the performance of the fabricated nanocapacitors has been improved significantly [6, 7]. However, it is hard to transfer the AAO template onto other substrates (e.g., Si substrate) due to its fragileness . Although Banerjee et al. reported that the aluminum foils were first bonded anodically to the glass substrate and then AAO template was formed by two steps of anodization , it faces a complex processing. Therefore, it is expected that the AAO template can be formed directly on the Si substrate without template transfer or complex bonding.
On the other hand, the electrode material of the capacitor also plays an important role in the performance of nanocapacitor arrays. As a transparent conductive oxide material in optoelectronics, Al-doped ZnO (AZO) has many attractive characteristics including excellent thermal stability, low resistivity, low manufacture cost, and so on . Therefore, the introduction of AZO as the electrode of nanocapacitor arrays could boost the energy storage device integrated with the optoelectronic device.
In this study, we demonstrate the successful fabrication of AZO/Al2O3/AZO nanocapacitor arrays in the porous AAO template, which is directly formed on Si substrate by two-step anodization. The resulting nanocapacitor arrays show a high capacitance density of 15.3 fF/μm2, which is nearly 2.5 times that of the planar capacitor. Furthermore, the charge-discharge characteristics of the nanocapacitor arrays are also discussed.
Results and discussion
Here, α represents the density of pores, which is close to 1 × 1010 cm−2 in the present experiment according to the scanning electron microscope image of the AAO template (not shown here); k is the dielectric constant of Al2O3 (k = 7.6), and tBE and tTE correspond to the thicknesses of the bottom AZO layer and the top AZO layer, respectively, i.e., 12 nm. The depth (L) and radius (rpore) of nanopores are approximately 150 and 40 nm, respectively. D represents the interpore distance, which is 10 nm. Therefore, the calculated Cplanar, Cbottom, and Cpore are equal to 1.3 × 10−2, 0.068 × 10−2, and 14.3 × 10−2 fF, respectively; thus, the total capacitance density (Ctotal) amounts to 15.7 fF/μm2, which is close to the measurement result. In addition, this also indicates that Ctotal is dominated by Cpore. Further, the capacitance density can be enhanced by increasing the height of nanopores. Moreover, the parameters of tBE, rpore, and tinsulator have positive or negative effects on the capacitance density. Therefore, to achieve a maximum capacitance density for practical applications, we have to consider the effects of various parameters, especially the influence of the insulator thickness on the leakage current, in order to make a balance between high capacitance and low leakage current.
On the other hand, although a high capacitance density has been achieved, the leakage current characteristic is not satisfactory (not shown here). This is due to the inner surface roughness and chemical contamination of the template, thus resulting in local high electric fields and degrading the leakage current characteristics. The aforementioned phenomena are also reported by other groups [7, 16]. Further, it is reported that the leakage current can be reduced remarkably by the introduction of barrier anodic alumina and/or ALD passivation layers in the AAO template. As an example, the leakage current density can decrease from 1×10−3 A/cm2 to 1×10−9 A/cm2 at 3 MV/cm .
In summary, high capacitance density nanocapacitor arrays have been fabricated via porous AAO template directly on silicon substrate and ALD processing. The nanocapacitor arrays based on the stack of AZO/Al2O3/AZO (12/10/12 nm) exhibit a high capacitance density of 15.3 fF/μm2, and the RC time constant is 300 ns, indicating good power characteristics of the electrostatic capacitor. As a result, in combination with flexible electronics and energy transformation components such as solar cells, nanocapacitor arrays could be a promising candidate as energy storage devices.
The authors thank the financial support of the National Natural Science Foundation of China (no. 61076076) and the Program for New Century Excellent Talents in University (NCET-08-0127).
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