[ECS 214th ECS Meeting - Honolulu, HI (October 12 - October 17, 2008)] ECS Transactions - Synthesis of Zirconia and Hafnia Tubes by Atomic Layer Deposition (ALD) Template Method

Synthesis of Zirconia and Hafnia Nanotubes by Atomic Layer Deposition (ALD) Template Method

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

a Department of Biology Chemistry and Environmental Science, Christopher Newport University, Newport News, VA 23606

b Department of Electrical Engineering, Old Dominion University , Norfolk, VA 23529 a,b The Applied Research Center-CNU& ODU-Jefferson Lab, Newport News, VA 23606

*Corresponding Author, Email: [email protected]

Highly ordered zirconia and hafnia nanotubes are prepared by Atomic Layer Deposition (ALD) within the pores of an anodic alumina oxide (AAO) template. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are used to characterize the morphology and elemental compositions of the different and EDS mapping of the entire sample cross-section. The diameters of the AAO channels are in the range of 200 nm and thickness of 60 µm. The length and diameter of the hafnia and zirconia nanotubes are dependent upon the pore diameter, the thickness of the applied AAO template and the ALD deposition time. The results indicate that the tubes are very uniformly assembled and parallel to each other in the pores of the AAO template.

Introduction Metal oxides 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, opto-electronics and as high-k dielectrics in microelectronics (5). Metal oxides 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 very attractive as a template for nanofabrication (7-10). Anodic aluminum oxide 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, 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

ECS Transactions, 16 (4) 159-164 (2008)10.1149/1.2979990 © The Electrochemical Society

159 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.102.115.28Downloaded on 2014-11-12 to IP

http://ecsdl.org/site/terms_use

film uniformity within AAO, and excellent conformal step coverage on complex nonplanar surface topographie. Conventional alkylamido precursors (tetrakis (dimethylamido) hafnium (IV) and tetrakis (dimethylamido) zirconium (IV)) were used in this study (14,15).

Experimental

Anodic aluminum oxide (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 10-5 min or until a mirror like surface was achieved.

The first anodization step was carried out in a 0.3 M oxalic acid solution electrolyte under a constant direct current (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 direct current (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 of the nanopores. The ZrO2 and HfO2 deposition was done at 250 ºC 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. Due to the depth of the nanopores and the diffusivity of precursors, the entire nanopores may not be coated uniformly without any extended exposure time for the precursor during deposition. A 30 seconds exposure time was used for ZrO2 deposition to cover the nanopores.

Scanning electron microscopy (SEM) was used to characterize the AAO surface, 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 Figure 1. The cross-sectional SEM image (Figure 1(a)) shows that the pores are all parallel to each other and across the whole template. The higher magnification image, Figure 1 (b), 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 200-300 nm and the wall width between pores is around 50 nm from Figure 1 (c). 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 view top view of tube opening showed that the side connected to the cathode has 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.

ECS Transactions, 16 (4) 159-164 (2008)



(a) (b) (c) (d) 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.

Figure 2 shows the top view of the pores coated with 20 nm HfO2 (a) and 20 nm ZrO2 (b). The pore size is reduced after coating compared to the pore size shown in Figure 1 (b), indicating successful coating of hafnia and zirconia. However, the open pore size after hafnia and zirconia coating were different, as well as the surface morphology. The mechanism of the difference in the surface morphology is still not clear since the property of the precursors and deposition rates are similar. The hafnia layer on the AAO surface needs to be removed to see the structure inside of the pores. A two layer structure, 20 nm HfO2 followed by 20 nm ZrO2, deposition was also carried out.

(a) (b)

Figure 2. (a) SEM image top view of hafnia nanotubes within AAO (b) SEM image top view of zirconia nanotubes within AAO.

Figure 3 shows the surface morphology and tube size after two layer coating. The tube diameter was further reduced after zirconia deposition. However, the cross sectional EDS mapping in Figure 4 shows that Hf signal was detected into the depth of about 15




µm 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 3. SEM image top view of hafnia/zirconia double layer ALD deposition within AAO.

(a) (b)

Figure 4. (a) Cross-sectional SEM image of hafnia nanotubes (b) EDS Hf mapping corresponding to (a).

Figure 5 shows that the entire pores have been coated with zirconia by applying 30 s

exposure time for the Zr precursor. It can be seen from Figure 5 (b) 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 backside opening was blocked. The uniformity of coating can be definitely improved by




lifting the AAO template so that the precursor can access both sizes of the pore opening during ALD deposition.

In order to fabricate free standing hafnia and zirconia nanotubes, we had to dissolve the alumina walls between the pores by a 6M NaOH solution. However, we first had to expose the alumina walls, which were completely coated by the ALD process, to the chemical etch solution. The porous AAO surface was cleared off its HfO2 and ZrO2 films by ion milling with argon gas at 5 kV. Figure 6 shows the free standing HfO2 and ZrO2 nanotubes after ion milling and chemical dissolution of alumina walls. The SEM images also clearly show the empty trenches in place of the former alumina side walls.

(a) (b)

Figure 5. (a) Cross-sectional SEM image of zirconium nanotubes (b) EDS Zr mapping corresponding to (a).

(a) (b)

Figure 6. SEM images of free standing (a) hafnia nanotubes (b) zirconia nanotubes after ion milling followed by dissolving AAO templates in 6M NaOH.

Conclusions

Anodic aluminum oxide (AAO) membranes were characterized by SEM and EDS before and after coating the entire surface (including the interior pore walls) of the AAO membranes by atomic layer deposition (ALD). The resultant zirconia and hafnia




nanotubes are highly ordered. The ALD can be used for preparing zirconia and hafnia nanotubes may be important in many applications ranging from gas sensors to various engineering materials such as high-k materials for integration into future integrated circuits (IC) technologies and photonic crystals.

References 1. R. Kelsall, M. Geoghegan, Nanoscale Science and Technology, Wiley, Chichester,

(2006). 2. C. R. Martin, Acc. Chem. Mater. 28, 61 (1995). 3. J. Goldberger, R. He, Y. Zhang, S. Lee, H. Yan, H.-J. Choi, P. Yang, Nature, 422 599

(2003). 4. S. B. Lee, D. T. Mitchell, L. Trofin, T. K. Nevanen, H. Soderlund, C. R. Martin,

Science, 296, 2198 (2002). 5. G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys., 89, 10 (2001). 6. T. Abdel-Fattah; E. Siochie, and R. Crooks, Fullerenes, Nanotubes, and Carbon

Nanostructures, 14, 585 (2006). 7. R. Fan, Y. Wu, D. Li, M. Yue, A. Majumdar, P. Yang, J. Am. Chem. Soc., 125, 5254

(2003). 8. Y. Zhao, Y.-G. Guo, Y.-L. Zhang, K. Jiao, Phys. Chem. Chem. Phys. 6, 1766 (2004). 9. M. Lai, J.A.G. Martinez, M. Grätzel, 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, 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).




Documents

[ECS 214th ECS Meeting - Honolulu, HI (October 12 - October 17, 2008)] ECS Transactions - Synthesis of Zirconia and Hafnia Tubes by Atomic Layer Deposition (ALD) Template Method