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Electrochemical and Solid-State Letters, 12 4 K25-K28 2009

1099-0062/2009/12 4 /K25/4/$23.00 © The Electrochemical Society

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Atomic Layer Deposition of ZrO2 and HfO2 Nanotubes by Template Replication

Diefeng Gu,a,b,*,z Helmut Baumgart,a,b,* Gon Namkoong,a,b,* and Tarek M. Abdel-Fattahb,c,z

a b c

Department of Electrical Engineering, Old Dominion University, Norfolk, Virginia 23529, USA The Applied Research Center, Newport News, Virginia 23606, USA Department of Biology, Chemistry, and Environmental Science, Christopher Newport University, Newport News, Virginia 23606, USA

Highly ordered zirconia and hafnia nanotubes were fabricated by atomic layer deposition ALD within the anodic alumina oxide AAO template. Scanning electron microscopy and energy-dispersive spectroscopy were used to characterize the morphology and elemental compositions of the different ALD coatings. The diameters of the AAO pores are in the range of 200­300 nm with a thickness of 60 m. The results indicated that the freestanding nanotubes were uniformly grown through the entire template thickness. The ALD process conformally replicated the AAO template dimensions. The number of ALD cycles controlled the resultant nanotube wall thickness. © 2009 The Electrochemical Society. DOI: 10.1149/1.3070617 All rights reserved. Manuscript submitted November 9, 2008; revised manuscript received December 8, 2008. Published January 20, 2009.

Metal-oxide tubes, synthesized in the nano range, exhibit novel physical properties and play an important role in fundamental research. In addition, they play a role in practical applications because of their restricted size and high surface area of the one-dimensional structure.1-4 Hafnium oxide hafnia, HfO2 and zirconium oxide zirconia, ZrO2 are important materials widely used in ceramics, gas sensors, catalysts, optoelectronics, and as high-k dielectrics in microelectronics.5 Metal-oxide tubes of hafnia and zirconia, with high aspect ratio and a small size of nanotubes or nanowires, are expected to improve the sensitivity of chemical sensors and reinforce thermal stability and toughness of the materials analogous to carbon nanotubes.6 Anodic aluminum oxide AAO and other nanoporous materials are attractive as a template for nanofabrication.7-10 AAO is being formed by electrochemical oxidation of aluminum in acidic solutions to form regular porous channels, which are parallel to each other.11-13 The channel diameter is mainly defined by the anodization voltage. Diameter of the pore depends on the electrolyte nature, its temperature and concentration, and the current density and other parameters of the anodization process. It is possible to vary the diameters of the channels and the pore by variations of the electrolyte composition and anodization conditions. The pore diameter can also be enlarged by selective etching of cell walls.11-13 Atomic layer deposition ALD is the only method for the deposition of hafnia and zirconium within AAO, in a controlled fashion, to yield good composition control and film uniformity within AAO and excellent conformal step coverage on complex nonplanar surface topography. Conventional alkylamido precursors tetrakis dimethylamido hafnium IV and tetrakis dimethylamido zirconium IV were used in this study.14,15 Experimental AAO was prepared by a two-step anodization procedure as described previously. Aluminum sheets Alfa Aesar, 99.998% pure, 0.5 mm thick were degreased in acetone. The Al sheets were then electropolished in a solution of HClO4 and ethanol 1:4, v/v at 20 V for 5­10 min or until a mirrorlike surface was achieved. The first anodization step was carried out in a 0.3 M oxalic acid solution electrolyte under a constant dc voltage of 80 V at 17°C for 24 h. The porous alumina layer was then stripped away from the Al substrate by etching the sample in a solution containing 6 wt % phosphoric acid and 1.8 wt % chromic acid at 60°C for 12 h. The

second anodization step was carried out in a 0.3 M oxalic acid solution under a constant dc voltage of 80 V at 17°C for 24 h. The AAO substrates with highly ordered arrays of nanopores were then obtained by selectively etching away the unreacted Al in a saturated HgCl2 solution. The AAO substrates were then transferred to the ALD chamber for ZrO2 and HfO2 coating inside the nanopores. The ZrO2 and HfO2 deposition was done at 250°C using water vapor as the oxidant and tetrakis dimethylamido hafnium IV and tetrakis dimethylamido zirconium IV as the precursor, respectively. The deposition rate is about 1 Å/cycle at this temperature.15 Due to the depth of the nanopores and the diffusivity of precursors, the entire nanopore may not be coated uniformly without any extended exposure time for the precursor during deposition. A 30 s exposure time was used for ZrO2 deposition to cover the nanopores. Ion milling was used to remove the ALD film from the surface to expose the alumina pore walls to subsequent wet etch by NaOH. Scanning electron microscopy SEM was used to characterize the AAO surface characteristics, such as surface morphology, pore size and wall width, and cross-sectional structure. Energy-dispersive spectroscopy EDS was used to characterize the distribution of ZrO2 and HfO2 in the nanopores. Results and Discussion The SEM images of the pore structure of the AAO template without any coating of ZrO2 or HfO2 are shown in Fig. 1. The cross-sectional SEM image Fig. 1a shows that the pores are all parallel to each other and across the whole template. The highermagnification image, Fig. 1b, shows the formation of branches in some of the pores. These branches may not be developed if the anodization time is shorter, which results in a shorter pore length. The pore size is in the range of 200­300 nm, and the wall width between the pores is around 50 nm Fig. 1c . From the image of the tilted sample, it can be seen that some of the pores were connected through thinning of the wall. A closer top view of the tube opening showed that the side connected to the cathode has a smaller pore size, to a depth of a few micrometers. This thin layer can be removed by etching to achieve uniform pores across the entire substrate. Figure 2 shows the top view of the pores coated with 200 ALD cycles of HfO2 a and ZrO2 b , which produce 20 nm coatings according to our spectroscopic ellipsometry measurement.15 The pore size is reduced after coating compared to the pore size shown in Fig. 1c, indicating successful coating of hafnia and zirconia. However, the open pore size as well as the surface morphology were different after hafnia and zirconia coating. The mechanism of the difference in the surface morphology is still not clear, because the

* Electrochemical Society Active Member.

z

E-mail: [email protected]; [email protected]

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Electrochemical and Solid-State Letters, 12 4 K25-K28 2009

Figure 1. SEM front image of AAO template: a cross-sectional SEM image of AAO, b higher magnification of pores, c SEM top view of the pores, and d top view of the pores with sample tilted.

property of the precursors and deposition rates are similar. The HfO2 and ZrO2 coatings on the AAO surface need to be removed to see the consistency of the ALD coatings within the AAO. Figure 3a shows the original wall thickness after surface planarization by ion milling. For comparison, Fig. 3b and c illustrate the increase in wall thickness, which indicates that HfO2 and ZrO2 film 20 nm coatings have been added within the AAO pore structure. Figure 4 shows the surface morphology after 200 ALD cycles of HfO2 followed by 200 ALD cycles of ZrO2 within the AAO template. Figure 4 indicates that the pore diameter was further reduced after zirconia deposition compared to Fig. 2a, which received only 200 ALD cycles. However, the cross-sectional EDS mapping in Fig. 5 shows that a Hf signal was detected up to a depth of about 15 m from the surface, and no Zr signal was observed. The difficulty of zirconia coating inside of the pores is probably due to the diffusivity of the Zr precursor into the pores. This problem can be solved by increasing the exposure time of Hf and Zr precursor in the ALD chamber. Figure 6 shows that the entire pore length has been coated with 200 ALD cycles of ZrO2 by applying 30 s exposure time for the Zr precursor. It can be seen from Fig. 6a that there is still a gradient in the Zr signal along the nanotubes. This is because the AAO template was placed in the ALD chamber flat on one side so that access of the Zr precursor to the back-side opening was blocked. The uniformity

of coating within the AAO has been improved by lifting the AAO template so that the precursor can access the AAO from the top and bottom side, as shown in Fig. 6b. In order to fabricate freestanding HfO2 and ZrO2 nanotubes, we had to dissolve the alumina walls between the pores by a 6 M NaOH solution. The porous AAO surface was cleared off the HfO2 and ZrO2 films by ion milling. The SEM images of Fig. 7 show the freestanding HfO2 and ZrO2 nanotubes after ion milling and chemical dissolution of alumina template. Figure 8 shows the HfO2 tubes separated by sonication in isopropanol IPO and suspended on a copper grid. Figure 8b identifies a single HfO2 nanotube of 50 m length, which corresponds to the final AAO template thickness after ion milling. In summary, the length and diameter of the hafnia and zirconia nanotubes are dependent upon the thickness of the AAO template, the pore diameter, and the ALD deposition cycles. Combining our EDS analysis with the SEM analysis of Fig. 8b proves that for coatings of an extremely high aspect ratio of 300 nanopores, only ALD is capable to meet the challenges of nanotechnology by offering unique characteristics like outstanding thin-film conformality on complex patterned surface topographies and atomic-level control of film composition and film thickness. We fabricated various AAO templates ranging in pore diameter from 200 to 50 nm, which corresponds to an aspect ratio of 300 60 m

Figure 2. SEM top-view images of coated AAO template by 200 ALD cycles of a as-grown HfO2 and b as-grown ZrO2.

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Electrochemical and Solid-State Letters, 12 4 K25-K28 2009

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Figure 3. SEM images of planarized ion milling: a original AAO template surface, b AAO template coated with 200 ALD cycles of HfO2, and c AAO template coated with 200 ALD cycles of ZrO2.

pore length vs 200 nm pore diameter and 1200, respectively. The inner pore walls can be coated by HfO2 and ZrO2 uniformly by letting the chemical precursor penetrate the porous template from both sides. Conclusions Nanotubes of ZrO2 and HfO2 grown by ALD in AAO membranes were characterized by SEM and EDS. Our EDS analysis proves that ALD is capable to penetrate the 60 m deep porous membranes with aspect ratios of 300 by employing extended exposure times. This ALD technique can be used for fabricating ZrO2 and HfO2 nanotubes by template replication with either porous AAO

Figure 6. Color online a EDS Zr mapping of cross-sectional AAO template coated with 200 ALD cycles of ZrO2 from top side only. b EDS Zr mapping of cross-sectional AAO coated with 200 ALD cycles of ZrO2 from both top and bottom side.

or polycarbonate nanoporous membranes. The ALD process conformally replicates any given template structure. In addition, the number of ALD cycles controls the resultant nanotube wall thickness with a sub-nanometer accuracy. The resultant ZrO2 and HfO2 nanotubes are highly ordered and absolutely conformal to the template pores. These ALD nanotubes are suitable for applications ranging from gas sensors to nanocapacitors and photonic crystals.16

Figure 4. SEM top-view image of 200 ALD cycle of HfO2 followed by 200 ALD cycles of ZrO2 within AAO template.

Figure 7. SEM images of freestanding nanotubes after ion milling followed by dissolving the AAO template walls for a HfO2 and b ZrO2.

Figure 5. Color online a Cross-sectional SEM image of AAO template coated with 200 ALD cycles of HfO2 with no exposure time, and b EDS Hf mapping corresponding to a .

Figure 8. a SEM image of separated HfO2 nanotube after removing the AAO template followed by sonication in IPO, and b a single HfO2 nanotube of 50 m length.

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Electrochemical and Solid-State Letters, 12 4 K25-K28 2009

8. Y. Zhao, Y.-G. Guo, Y.-L. Zhang, and K. Jiao, Phys. Chem. Chem. Phys., 6, 1766 2004 . 9. M. Lai, J. A. G. Martinez, M. Grätzel, and D. J. Riley, J. Mater. Chem., 16, 2843 2006 . 10. M. Steinhart, J. H. Wendorff, A. Greiner, R. B. Wehrspohn, K. Nielsch, J. Schilling, J. Choi, and U. Gösele, Science, 14, 1997 2002 . 11. H. Masuda and K. Fukuda, Science, 268, 1466 1995 . 12. V. P. Menon and C. R. Martin, Anal. Chem., 67, 1920 1995 . 13. M. A. Cameron, I. P. Gartland, J. A. Smith, S. F. Diaz, and S. M. George, Langmuir, 16, 7435 2000 . 14. D. M. Hausmann, E. Kim, J. Becker, and R. G. Gordon, Chem. Mater., 14, 4350 2002 . 15. D. Gu, K. Tapily, P. Shrestha, M. Y. Zhu, G. Celler, and H. Baumgart, J. Electrochem. Soc., 155, G129 2008 . 16. B. Wang, G. T. Fei, M. Wang, M. G. Kong, and L. D. Zhang, Nanotechnology, 18, 365601 2007 .

Old Dominion University assisted in meeting the publication costs of this article.

References

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