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Fabrication of polycrystalline aluminum oxide thin films via hydrolysis and hydrothermal reactions in solutions
Thin Solid Films 520 (2011) 25–29
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Fabrication of polycrystalline aluminum oxide thin films via hydrolysis and hydrothermal reactions in solutions XiaoFei Duan a, Irving Liaw a, Nguyen H. Tran b, Robert N. Lamb a,⁎ a b
School of Chemistry, The University of Melbourne, Victoria, 3010, Australia School of Natural Sciences, The University of Western Sydney, Parramatta, NSW, Australia
a r t i c l e
i n f o
Article history: Received 20 September 2010 Received in revised form 9 March 2011 Accepted 4 June 2011 Available online 14 June 2011 Keywords: Aluminum oxide Polycrystalline Thin films Sol–gel Hydrothermal
a b s t r a c t Polycrystalline Al2O3 thin films were fabricated through a method combining urea hydrolysis and hydrothermal reactions. The overall growth temperature of these films was achieved as low as 150 °C. Although cracks occurred across the gel film after hydrolysis, a subsequent nucleation under elevated pressure and temperature resulted in a closely packed morphology. Moreover, the hydrothermal treatment led to high oxide content and an increase in crystallinity within the films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Al2O3 thin films have been seen in a variety of applications from electronics, [1,2] biological implantation, [3,4] to mechanical coatings. [5,6] Conventional deposition methods require high thermal energy, i.e. high deposition temperature. A reduction in deposition temperatures opens up opportunities for coupling of electrical and mechanical properties of polycrystalline Al2O3 films with soft, thermally unstable substrates such as polymers. Even though atomic layer deposition can be used for a low temperature deposition (≤300 °C), [7,8] controls over multiple sources and instrumentation often result in experimental complications. Sol–gel deposition method, on the other hand, has been extensively employed for fabrication of Al2O3 thin films, [9] due to low deposition temperature and inexpensive equipment. The method involves the hydrolysis of precursors such as aluminum alkoxide to produce the hydrated Al(OH)3 films at temperatures below 100 °C. [10,11] However, high temperature annealing treatment was still required to form oxide films (N350 °C [12,13]), metastable crystalline phases (N800 °C [10]) and thermodynamically stable phase (N1000 °C [11,14,15]). Annealing films at high temperatures often causes microcracks across the film on the substrate due to differences in thermal expansion coefficients. An alternative transformation method is a low temperature hydrothermal treatment. At an elevated pressure, this method demonstrated effective formation of a polycrystalline ZrO2 thin film at a low temperature of ~200 °C [16]. ⁎ Corresponding author. Surface Science and Technology Group, School of Chemistry, The University of Melbourne, VIC, 3010, Australia. E-mail addresses: [email protected] (X. Duan), [email protected] (R.N. Lamb). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.06.003
Prior to the structural transformation, urea hydrolysis reaction is employed to produce Al(OH)3. This method can effectively deliver OH − and form Al hydroxide with Al 3+, which has been used for the synthesis of Al(OH)3 particles and powders. [17–19] But the formation of thin films has to date not been reported. In this work, we demonstrate the formation of polycrystalline Al2O3 thin films at a temperature as low as 150 °C using a hydrolysis reaction followed by a carefully controlled hydrothermal treatment. With this combined process, a transformation of thin films containing micro-cracks to closely packed, crack-free films were achieved.
2. Experimental A sequential three-step reaction was used to produce polycrystalline Al2O3 thin films. Firstly, Al(NO3)3 and urea (5.2 mmol : 5.8 mmol) were dissolved in demineralized water (20 ml). The clear solution was heated and maintained at 80 °C. A Si wafer (10 × 10 mm) was cleaned with demineralized water and immersed in the solution for 2 h. Secondly, after removing the Si wafer from the solution, the wafer was tilted against filter paper to remove excess solution. It was then heated and dried at 100 °C for N1 h in a furnace. A thermally dehydrated film (TDF) with observable diffraction bands was visible on the surface of the Si substrate. Finally, this sample was transferred into a Parr reactor, to which demineralized water (500 μl) was added. The reactions were run at 150 (4.7 atm), 200 (15.3 atm) and 250 °C (39.2 atm), respectively, for 24 h, and then gradually cooled at room temperature. The hydrothermally treated film (HTF) was then dried at 100 °C for 1 h. TDF and HTF were characterized using X-ray photoelectron spectroscopy (XPS),
26
X. Duan et al. / Thin Solid Films 520 (2011) 25–29
grazing incidence X-ray diffraction (GI XRD) and high-resolution scanning electron microscopy (HR SEM). XPS spectra were obtained using a VG ESCALAB 220i-XL spectrometer equipped with a monochromatic Al Kα X-Ray source, which emitted photon energy of 1486.6 eV at 10 kV and 12 mA. Spectra were obtained at a step size of either 1 eV (survey scans) or 0.1 eV (region scans). Film samples were secured onto Al holders and were measured in the analysis chamber at a typical operating pressure of ~1 × 10− 9 mbar. An electron flood gun was used to reduce the charging effect. Quantification and curve fitting of XPS spectra were performed using CasaXPS. O 1s peaks were curve-fitted using a Gaussian/Lorentzian (30) line shape with Shirley background type. The XPS spectra of the untreated surface of the films were calibrated with respect to the C 1s peak at a binding energy of 285 eV [20]. The untreated surface of the film was cleaned with Ar+ etching (at ~1 × 10− 7 mbar) and a depth of approximate 6 nm was removed. The XPS spectra of the etched surface of the films were calibrated by assuming Al 2p at binding energy of 74.6 eV. XRD measurements of the films were carried out using Bruker AXS D8. The Cu X-ray generator was operated at 40 kV and 35 mA and supplied a Kα emission with a wavelength of 1.5406 Å. Films were scanned for 2θ axis only at a step size of 0.02 2θ° in a continuous scan mode. The morphology and micro-structure of the films were determined using a FEI Quanta SEM. An Everhart–Thornley detector was used to collect secondary electrons. The film samples were sputter-coated with Au to enhance conductivity for acquiring images. HR SEM images were obtained at an operational voltage of 10 kV. 3. Results The compositions of the films were determined using XPS survey scans. Fig. 1(a) shows photoelectrons O 1s and Al 2p are observed at the binding energies of 532.4 eV and 74.6 eV, respectively, on the untreated surface of TDF. This result suggests the presence of Al hydroxide, which is typically formed by hydration. The bulk of the film has been revealed by etching off the outer surface. The binding energy of O 1s shifted to 531.5 eV by assuming the Al 2p peak remained at the same position. The
Fig. 1. XPS survey scans of TDF (a) and HTF (b) after Ar+ surface etching; insets reveal contamination-free bulk of the films. XRS region scans reveal the presence of O2− and OH− in both TDF (c) and HTF (d).
Fig. 2. XRD diffraction patterns of (a) the gel film TDF at 100 °C and films hydrothermally treated (b) at 150 °C, (c) at 200 °C and at 250 °C.
X. Duan et al. / Thin Solid Films 520 (2011) 25–29
difference in the binding energies of O 1s and Al 2p decreased to 456.9 eV, suggesting that the film consists of a mixture of O2− and OH−[20,21]. A hydrated layer was also found on the untreated surface of HTF as the binding energies of O 1s and Al 2p are 532.3 eV and 74.6 eV, respectively (Fig. 1b). After removing the outer surface, the O 1s peak shifted to 531.1 eV by assuming the Al 2p peak remained at 74.6 eV. The energy difference between O 1s and Al 2p peaks is 456.5 eV, suggesting the presence of major O 2− in the bulk of HTF. Region scan XPS spectrum of O 1s in TDF shows a broad and unsymmetrical peak (Fig. 1c). The fitted O curve contains two peaks at 531.1 eV and 532.4 eV, indicating the presence of O 2− and OH − in TDF, respectively. [21] In addition, the atomic percentage (at%) of the O 2− component is 67.9 at.%, which is 35.1 at.% greater than that of the OH −, indicating the major product is O 2− after the thermal treatment. High resolution XPS also detected both components in HTF (Fig. 1d). Although the hydroxide was not completely oxidized during the hydrothermal treatment, the concentration of O 2− component has increased to 73.4 at.%. In both TDF and HTF, the O 1s photoelectron of water were not observed, which is typically found at the binding energy of 533.5–533.7 eV [21]. The absence of water suggests that the drying process was effective and water did not incorporate into the bulk during the hydrothermal reaction. Both untreated surfaces of TDF and HTF show the presence of C and N impurities (Fig. 1a & b) that are removed by Ar + etching (Fig. 1a & b insets), indicating that these impurities were present initially on the untreated surfaces. The C content is 17.6 at.% on the untreated surface
27
of HTF. This carbonaceous contamination was from carbonates due to a surface reaction of CO2 with hydrates. In comparison, a high C concentration of 43.9 at.% was found on the untreated surface of TDF. This C attributed not only to the carbonaceous contamination from CO2, but also to the solution carbonate that remained as a residue after drying. The XRD measurement does not show any diffractions from the coated film despite strong diffraction from the Si substrate at 2θ° 33.2, 61.8, 66.0, 66.6 and 69.1 (Fig. 2a). After hydrothermal treatment at 150 °C, the film diffracts X-rays at 2θ° 14.6 and 43.7 (Fig. 2b). The diffraction intensity of the peak at 2θ° 14.6 enhanced substantially at higher temperature (200 °C) (Fig. 2c), which suggests a significant improvement in the crystallinity of the film. The crystalline structure was shown to improve further at 250 °C as diffraction peaks at 2θ° 28.1, 38.4 and 49.2 began to appear (Fig. 2d). The peaks at 2θ° 14.6, 28.1, 38.4 and 49.2 are denoted to crystalline γ-AlO(OH) (JCPDS211307). The presence of γ-AlO(OH) in HTF is in agreement to the findings of OH − from the XPS results above. The peak at 2θ° 43.7 showed an α-Al2O3 b1 1 3 N orientation. In relation, the α-Al2O3 could be transformed from γ-AlO(OH) at a temperature between 1000 and 1100 °C [15]. The SEM image of the dehydrated film shows micro-cracking across the film's surface (Fig. 3a). The formation of cracks was a result of thermal expansion during drying. This fragile ceramic layer partially adheres to the Si substrate (Fig. 3b). Conventional annealing treatment would not be a suitable method for polycrystalline
Fig. 3. The surface (a) and cross-section (b) of TDF, in comparison with the surface (c) and cross-section of HTF.
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X. Duan et al. / Thin Solid Films 520 (2011) 25–29
Scheme 1. The formation of Al2O3/AlO(OH) thin film through: (a) solution deposition of an Al(OH)3 network; (b) dehydroxylation and cracks formation during thermal condensation; (c) nucleation and crystallization results in a closely packed and polycrystalline thin film.
conversion. The heating rate of the film attached to Si would be different from that of the separated films. As a result, unattached films tended to be more fragile and unstable on the surface of the Si substrate. In contrast, cracks disappeared after the hydrothermal treatment, resulting in a uniform layer formed across the surface of the Si substrate (Fig. 3c). This layer, with an average thickness of 500 nm, is closely packed and adheres uniformly to the substrate (Fig. 3d).
4. Discussion The formation of Al2O3/AlO(OH) thin film is proposed in a threestep process (Scheme 1): the formation of an Al(OH)3 gel network (step 1); dehydroxylation and cracks formation during a thermal condensation (step 2); the formation of a closely packed and polycrystalline film via nucleation and crystallization. Al(OH)3 is produced via a hydrolysis reaction of urea. The hydrolysis results in the formation of hydroxide anions [22] (Eq. (1)) which subsequently facilitates the precipitation of Al(OH)3 by hydroxylation (Eq. (2)). −
þ
NH2 Cð¼ OÞNH2 þ 4H2 O→2OH þ 2NH4 þ H2 CO3
Al
3þ
−
þ 3OH →AlðOHÞ3 ðgelÞ
ð1Þ
ð2Þ
Hydroxide particles slowly precipitate and cross-link to form a gel network. The network separates from the solution and adsorbs onto the surface of the substrate. The gel formation is facilitated by evaporating water through a thermal dehydration. NH4± and CO32− decompose to gaseous NH3 and CO2 that escape from the solution. Their residues remain only on the outer surface of the film. During the drying process, the condensation removes water that incorporated in the network. This leads to an increase in the contraction of the network, and thus an increase in tensile stress at the drying site. Furthermore, due to a low permeability of the gel and various pore sizes, the evaporation of water is not uniform and produces pressure differences within the film. As the stress continues to rise, the pressure difference becomes greater. Upon reaching a critical point, film cracking occurs [23,24]. The thermal treatment also results in the change of film's composition. Al(OH)3 is converted to oxide via a thermal dehydroxylation (Eq. (3)). –Alð−OHÞ–Alð−OHÞ–→–Al–O–Al– þ H2 O
ð3Þ
When the film is treated under a hydrothermal condition, the background pressure increases to approximate 15.9 atm (at 200 °C). The dehydroxylation continues and results in an increase in oxide
content. Al(OH)3 dissolves into a homogeneous solution under the elevated condition. Subsequent nucleation leads to a closely packed film without incorporation of water. Therefore, cracks are not formed in the film after drying. 5. Summary We have demonstrated a method for preparing polycrystalline γAlO(OH)/α-Al2O3 films utilizing a novel urea hydrolysis and hydrothermal reaction. The film with an average thickness of 500 nm was made at an overall temperature as low as 150 °C. A proposed nucleation mechanism during the hydrothermal treatment led to a high oxide content and the formation of crystalline phase in the film. The morphology of the film after this treatment appeared crack-free and closely packed, in comparison with a thermal treatment. Further investigation would have to develop further control of the thicknesses and deposition on different substrates such as plastic which have lower melting points. Acknowledgements We would like to thank Dr. Bin Gong (University of New South Wales) and Mr. Frank Antolasic (Royal Melbourne Institute of Technology) for acquiring XPS and XRD results, respectively. We also acknowledge David Hay Memorial (University of Melbourne) Fund for supporting the writing-up of this work. References [1] R.G. Vitchev, J.J. Pireaux, T. Conard, H. Bender, J. Wolstenholme, C. Defranoux, Appl. Surf. Sci. 235 (2004) 21. [2] J. Robertson, Eur. Phys. J. Appl. Phys. 28 (2004) 265. [3] R. Sweitzer, C. Scholz, S. Montezuma, J.F. Rizzo, J. Bioact. Compatible Polym. 21 (2006) 5. [4] K. Mustafa, A. Oden, A. Wennerberg, K. Hultenby, K. Arvidson, Biomaterials 26 (2005) 373. [5] W.D. Sproul, Science 273 (1996) 889. [6] X. Nie, E.I. Meletis, J.C. Jiang, A. Leyland, A.L. Yerokhin, A. Matthews, Surf. Coat. Technol. 149 (2002) 245. [7] S.K. Kim, S.W. Lee, C.S. Hwang, Y.-S. Min, J.Y. Won, J. Jeong, J. Electrochem. Soc. 153 (2006) F69. [8] M. Ritala, K. Kukli, A. Rahtu, P.I. Raisanen, M. Leskela, T. Sajavaara, J. Keinonen, Science 288 (2000) 319. [9] O. Guillon, L. Weiler, J. Rodel, J. Am. Ceram. Soc. 90 (2007) 1394. [10] T. Hubert, S. Svoboda, B. Oertel, Surf. Coat. Technol. 201 (2006) 487. [11] Y. Kobayashi, T. Ishizaka, Y. Kurokawa, J. Mater. Sci. 40 (2005) 263. [12] Q. Fu, C.B. Cao, H.S. Zhu, Thin Solid Films 348 (1999) 99. [13] S. Riaz, S. Shamaila, B. Khan, S. Naseem, Surf. Rev. Lett. 15 (2008) 681. [14] E. Fredriksson, J.-O. Carlsson, J. Chem. Vap. Depos. 1 (1993) 333. [15] I. Levin, D. Brandon, J. Am. Ceram. Soc. 81 (1998) 1995. [16] S. Park, B.L. Clark, D.A. Keszler, J.P. Bender, J.F. Wager, T.A. Reynolds, G.S. Herman, Science 297 (2002) 65. [17] M.I.F. Macedo, C.C. Osawa, C.A. Bertran, J. Sol–gel Sci. Technol. 30 (2004) 135.
X. Duan et al. / Thin Solid Films 520 (2011) 25–29 [18] S. Ramanathan, S.K. Roy, R. Bhat, D.D. Upadhyaya, A.R. Biswas, Ceram. Int. 23 (1997) 45. [19] J.L. Shi, J.H. Gao, Z.X. Lin, Solid State Ionics 32–3 (1989) 537. [20] A.N. Buckley, A.J. Hartmann, R.N. Lamb, A.P.J. Stampfl, J.W. Freeland, I. Coulthard, Surf. Interface Anal. 35 (2003) 922.
[21] [22] [23] [24]
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J. van den Brand, W.G. Sloof, H. Terryn, J.H.W. de Wit, Surf. Interface Anal. 36 (2004) 81. W.H.R. Shaw, J.J. Bordeaux, J. Am. Chem. Soc. 77 (1955) 4729. H. Kozuka, M. Kajimura, J. Am. Ceram. Soc. 83 (2000) 1056. C.J. Brinker, G.W. Scherer, Sol–gel science: the physics and chemistry of sol–gel processing, Academic Press, Boston, 1990 (ch.8).
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Fabrication of polycrystalline aluminum oxide thin films via hydrolysis and hydrothermal reactions in solutions XiaoFei Duan a, Irving Liaw a, Nguyen H. Tran b, Robert N. Lamb a,⁎ a b
School of Chemistry, The University of Melbourne, Victoria, 3010, Australia School of Natural Sciences, The University of Western Sydney, Parramatta, NSW, Australia
a r t i c l e
i n f o
Article history: Received 20 September 2010 Received in revised form 9 March 2011 Accepted 4 June 2011 Available online 14 June 2011 Keywords: Aluminum oxide Polycrystalline Thin films Sol–gel Hydrothermal
a b s t r a c t Polycrystalline Al2O3 thin films were fabricated through a method combining urea hydrolysis and hydrothermal reactions. The overall growth temperature of these films was achieved as low as 150 °C. Although cracks occurred across the gel film after hydrolysis, a subsequent nucleation under elevated pressure and temperature resulted in a closely packed morphology. Moreover, the hydrothermal treatment led to high oxide content and an increase in crystallinity within the films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Al2O3 thin films have been seen in a variety of applications from electronics, [1,2] biological implantation, [3,4] to mechanical coatings. [5,6] Conventional deposition methods require high thermal energy, i.e. high deposition temperature. A reduction in deposition temperatures opens up opportunities for coupling of electrical and mechanical properties of polycrystalline Al2O3 films with soft, thermally unstable substrates such as polymers. Even though atomic layer deposition can be used for a low temperature deposition (≤300 °C), [7,8] controls over multiple sources and instrumentation often result in experimental complications. Sol–gel deposition method, on the other hand, has been extensively employed for fabrication of Al2O3 thin films, [9] due to low deposition temperature and inexpensive equipment. The method involves the hydrolysis of precursors such as aluminum alkoxide to produce the hydrated Al(OH)3 films at temperatures below 100 °C. [10,11] However, high temperature annealing treatment was still required to form oxide films (N350 °C [12,13]), metastable crystalline phases (N800 °C [10]) and thermodynamically stable phase (N1000 °C [11,14,15]). Annealing films at high temperatures often causes microcracks across the film on the substrate due to differences in thermal expansion coefficients. An alternative transformation method is a low temperature hydrothermal treatment. At an elevated pressure, this method demonstrated effective formation of a polycrystalline ZrO2 thin film at a low temperature of ~200 °C [16]. ⁎ Corresponding author. Surface Science and Technology Group, School of Chemistry, The University of Melbourne, VIC, 3010, Australia. E-mail addresses: [email protected] (X. Duan), [email protected] (R.N. Lamb). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.06.003
Prior to the structural transformation, urea hydrolysis reaction is employed to produce Al(OH)3. This method can effectively deliver OH − and form Al hydroxide with Al 3+, which has been used for the synthesis of Al(OH)3 particles and powders. [17–19] But the formation of thin films has to date not been reported. In this work, we demonstrate the formation of polycrystalline Al2O3 thin films at a temperature as low as 150 °C using a hydrolysis reaction followed by a carefully controlled hydrothermal treatment. With this combined process, a transformation of thin films containing micro-cracks to closely packed, crack-free films were achieved.
2. Experimental A sequential three-step reaction was used to produce polycrystalline Al2O3 thin films. Firstly, Al(NO3)3 and urea (5.2 mmol : 5.8 mmol) were dissolved in demineralized water (20 ml). The clear solution was heated and maintained at 80 °C. A Si wafer (10 × 10 mm) was cleaned with demineralized water and immersed in the solution for 2 h. Secondly, after removing the Si wafer from the solution, the wafer was tilted against filter paper to remove excess solution. It was then heated and dried at 100 °C for N1 h in a furnace. A thermally dehydrated film (TDF) with observable diffraction bands was visible on the surface of the Si substrate. Finally, this sample was transferred into a Parr reactor, to which demineralized water (500 μl) was added. The reactions were run at 150 (4.7 atm), 200 (15.3 atm) and 250 °C (39.2 atm), respectively, for 24 h, and then gradually cooled at room temperature. The hydrothermally treated film (HTF) was then dried at 100 °C for 1 h. TDF and HTF were characterized using X-ray photoelectron spectroscopy (XPS),
26
X. Duan et al. / Thin Solid Films 520 (2011) 25–29
grazing incidence X-ray diffraction (GI XRD) and high-resolution scanning electron microscopy (HR SEM). XPS spectra were obtained using a VG ESCALAB 220i-XL spectrometer equipped with a monochromatic Al Kα X-Ray source, which emitted photon energy of 1486.6 eV at 10 kV and 12 mA. Spectra were obtained at a step size of either 1 eV (survey scans) or 0.1 eV (region scans). Film samples were secured onto Al holders and were measured in the analysis chamber at a typical operating pressure of ~1 × 10− 9 mbar. An electron flood gun was used to reduce the charging effect. Quantification and curve fitting of XPS spectra were performed using CasaXPS. O 1s peaks were curve-fitted using a Gaussian/Lorentzian (30) line shape with Shirley background type. The XPS spectra of the untreated surface of the films were calibrated with respect to the C 1s peak at a binding energy of 285 eV [20]. The untreated surface of the film was cleaned with Ar+ etching (at ~1 × 10− 7 mbar) and a depth of approximate 6 nm was removed. The XPS spectra of the etched surface of the films were calibrated by assuming Al 2p at binding energy of 74.6 eV. XRD measurements of the films were carried out using Bruker AXS D8. The Cu X-ray generator was operated at 40 kV and 35 mA and supplied a Kα emission with a wavelength of 1.5406 Å. Films were scanned for 2θ axis only at a step size of 0.02 2θ° in a continuous scan mode. The morphology and micro-structure of the films were determined using a FEI Quanta SEM. An Everhart–Thornley detector was used to collect secondary electrons. The film samples were sputter-coated with Au to enhance conductivity for acquiring images. HR SEM images were obtained at an operational voltage of 10 kV. 3. Results The compositions of the films were determined using XPS survey scans. Fig. 1(a) shows photoelectrons O 1s and Al 2p are observed at the binding energies of 532.4 eV and 74.6 eV, respectively, on the untreated surface of TDF. This result suggests the presence of Al hydroxide, which is typically formed by hydration. The bulk of the film has been revealed by etching off the outer surface. The binding energy of O 1s shifted to 531.5 eV by assuming the Al 2p peak remained at the same position. The
Fig. 1. XPS survey scans of TDF (a) and HTF (b) after Ar+ surface etching; insets reveal contamination-free bulk of the films. XRS region scans reveal the presence of O2− and OH− in both TDF (c) and HTF (d).
Fig. 2. XRD diffraction patterns of (a) the gel film TDF at 100 °C and films hydrothermally treated (b) at 150 °C, (c) at 200 °C and at 250 °C.
X. Duan et al. / Thin Solid Films 520 (2011) 25–29
difference in the binding energies of O 1s and Al 2p decreased to 456.9 eV, suggesting that the film consists of a mixture of O2− and OH−[20,21]. A hydrated layer was also found on the untreated surface of HTF as the binding energies of O 1s and Al 2p are 532.3 eV and 74.6 eV, respectively (Fig. 1b). After removing the outer surface, the O 1s peak shifted to 531.1 eV by assuming the Al 2p peak remained at 74.6 eV. The energy difference between O 1s and Al 2p peaks is 456.5 eV, suggesting the presence of major O 2− in the bulk of HTF. Region scan XPS spectrum of O 1s in TDF shows a broad and unsymmetrical peak (Fig. 1c). The fitted O curve contains two peaks at 531.1 eV and 532.4 eV, indicating the presence of O 2− and OH − in TDF, respectively. [21] In addition, the atomic percentage (at%) of the O 2− component is 67.9 at.%, which is 35.1 at.% greater than that of the OH −, indicating the major product is O 2− after the thermal treatment. High resolution XPS also detected both components in HTF (Fig. 1d). Although the hydroxide was not completely oxidized during the hydrothermal treatment, the concentration of O 2− component has increased to 73.4 at.%. In both TDF and HTF, the O 1s photoelectron of water were not observed, which is typically found at the binding energy of 533.5–533.7 eV [21]. The absence of water suggests that the drying process was effective and water did not incorporate into the bulk during the hydrothermal reaction. Both untreated surfaces of TDF and HTF show the presence of C and N impurities (Fig. 1a & b) that are removed by Ar + etching (Fig. 1a & b insets), indicating that these impurities were present initially on the untreated surfaces. The C content is 17.6 at.% on the untreated surface
27
of HTF. This carbonaceous contamination was from carbonates due to a surface reaction of CO2 with hydrates. In comparison, a high C concentration of 43.9 at.% was found on the untreated surface of TDF. This C attributed not only to the carbonaceous contamination from CO2, but also to the solution carbonate that remained as a residue after drying. The XRD measurement does not show any diffractions from the coated film despite strong diffraction from the Si substrate at 2θ° 33.2, 61.8, 66.0, 66.6 and 69.1 (Fig. 2a). After hydrothermal treatment at 150 °C, the film diffracts X-rays at 2θ° 14.6 and 43.7 (Fig. 2b). The diffraction intensity of the peak at 2θ° 14.6 enhanced substantially at higher temperature (200 °C) (Fig. 2c), which suggests a significant improvement in the crystallinity of the film. The crystalline structure was shown to improve further at 250 °C as diffraction peaks at 2θ° 28.1, 38.4 and 49.2 began to appear (Fig. 2d). The peaks at 2θ° 14.6, 28.1, 38.4 and 49.2 are denoted to crystalline γ-AlO(OH) (JCPDS211307). The presence of γ-AlO(OH) in HTF is in agreement to the findings of OH − from the XPS results above. The peak at 2θ° 43.7 showed an α-Al2O3 b1 1 3 N orientation. In relation, the α-Al2O3 could be transformed from γ-AlO(OH) at a temperature between 1000 and 1100 °C [15]. The SEM image of the dehydrated film shows micro-cracking across the film's surface (Fig. 3a). The formation of cracks was a result of thermal expansion during drying. This fragile ceramic layer partially adheres to the Si substrate (Fig. 3b). Conventional annealing treatment would not be a suitable method for polycrystalline
Fig. 3. The surface (a) and cross-section (b) of TDF, in comparison with the surface (c) and cross-section of HTF.
28
X. Duan et al. / Thin Solid Films 520 (2011) 25–29
Scheme 1. The formation of Al2O3/AlO(OH) thin film through: (a) solution deposition of an Al(OH)3 network; (b) dehydroxylation and cracks formation during thermal condensation; (c) nucleation and crystallization results in a closely packed and polycrystalline thin film.
conversion. The heating rate of the film attached to Si would be different from that of the separated films. As a result, unattached films tended to be more fragile and unstable on the surface of the Si substrate. In contrast, cracks disappeared after the hydrothermal treatment, resulting in a uniform layer formed across the surface of the Si substrate (Fig. 3c). This layer, with an average thickness of 500 nm, is closely packed and adheres uniformly to the substrate (Fig. 3d).
4. Discussion The formation of Al2O3/AlO(OH) thin film is proposed in a threestep process (Scheme 1): the formation of an Al(OH)3 gel network (step 1); dehydroxylation and cracks formation during a thermal condensation (step 2); the formation of a closely packed and polycrystalline film via nucleation and crystallization. Al(OH)3 is produced via a hydrolysis reaction of urea. The hydrolysis results in the formation of hydroxide anions [22] (Eq. (1)) which subsequently facilitates the precipitation of Al(OH)3 by hydroxylation (Eq. (2)). −
þ
NH2 Cð¼ OÞNH2 þ 4H2 O→2OH þ 2NH4 þ H2 CO3
Al
3þ
−
þ 3OH →AlðOHÞ3 ðgelÞ
ð1Þ
ð2Þ
Hydroxide particles slowly precipitate and cross-link to form a gel network. The network separates from the solution and adsorbs onto the surface of the substrate. The gel formation is facilitated by evaporating water through a thermal dehydration. NH4± and CO32− decompose to gaseous NH3 and CO2 that escape from the solution. Their residues remain only on the outer surface of the film. During the drying process, the condensation removes water that incorporated in the network. This leads to an increase in the contraction of the network, and thus an increase in tensile stress at the drying site. Furthermore, due to a low permeability of the gel and various pore sizes, the evaporation of water is not uniform and produces pressure differences within the film. As the stress continues to rise, the pressure difference becomes greater. Upon reaching a critical point, film cracking occurs [23,24]. The thermal treatment also results in the change of film's composition. Al(OH)3 is converted to oxide via a thermal dehydroxylation (Eq. (3)). –Alð−OHÞ–Alð−OHÞ–→–Al–O–Al– þ H2 O
ð3Þ
When the film is treated under a hydrothermal condition, the background pressure increases to approximate 15.9 atm (at 200 °C). The dehydroxylation continues and results in an increase in oxide
content. Al(OH)3 dissolves into a homogeneous solution under the elevated condition. Subsequent nucleation leads to a closely packed film without incorporation of water. Therefore, cracks are not formed in the film after drying. 5. Summary We have demonstrated a method for preparing polycrystalline γAlO(OH)/α-Al2O3 films utilizing a novel urea hydrolysis and hydrothermal reaction. The film with an average thickness of 500 nm was made at an overall temperature as low as 150 °C. A proposed nucleation mechanism during the hydrothermal treatment led to a high oxide content and the formation of crystalline phase in the film. The morphology of the film after this treatment appeared crack-free and closely packed, in comparison with a thermal treatment. Further investigation would have to develop further control of the thicknesses and deposition on different substrates such as plastic which have lower melting points. Acknowledgements We would like to thank Dr. Bin Gong (University of New South Wales) and Mr. Frank Antolasic (Royal Melbourne Institute of Technology) for acquiring XPS and XRD results, respectively. We also acknowledge David Hay Memorial (University of Melbourne) Fund for supporting the writing-up of this work. References [1] R.G. Vitchev, J.J. Pireaux, T. Conard, H. Bender, J. Wolstenholme, C. Defranoux, Appl. Surf. Sci. 235 (2004) 21. [2] J. Robertson, Eur. Phys. J. Appl. Phys. 28 (2004) 265. [3] R. Sweitzer, C. Scholz, S. Montezuma, J.F. Rizzo, J. Bioact. Compatible Polym. 21 (2006) 5. [4] K. Mustafa, A. Oden, A. Wennerberg, K. Hultenby, K. Arvidson, Biomaterials 26 (2005) 373. [5] W.D. Sproul, Science 273 (1996) 889. [6] X. Nie, E.I. Meletis, J.C. Jiang, A. Leyland, A.L. Yerokhin, A. Matthews, Surf. Coat. Technol. 149 (2002) 245. [7] S.K. Kim, S.W. Lee, C.S. Hwang, Y.-S. Min, J.Y. Won, J. Jeong, J. Electrochem. Soc. 153 (2006) F69. [8] M. Ritala, K. Kukli, A. Rahtu, P.I. Raisanen, M. Leskela, T. Sajavaara, J. Keinonen, Science 288 (2000) 319. [9] O. Guillon, L. Weiler, J. Rodel, J. Am. Ceram. Soc. 90 (2007) 1394. [10] T. Hubert, S. Svoboda, B. Oertel, Surf. Coat. Technol. 201 (2006) 487. [11] Y. Kobayashi, T. Ishizaka, Y. Kurokawa, J. Mater. Sci. 40 (2005) 263. [12] Q. Fu, C.B. Cao, H.S. Zhu, Thin Solid Films 348 (1999) 99. [13] S. Riaz, S. Shamaila, B. Khan, S. Naseem, Surf. Rev. Lett. 15 (2008) 681. [14] E. Fredriksson, J.-O. Carlsson, J. Chem. Vap. Depos. 1 (1993) 333. [15] I. Levin, D. Brandon, J. Am. Ceram. Soc. 81 (1998) 1995. [16] S. Park, B.L. Clark, D.A. Keszler, J.P. Bender, J.F. Wager, T.A. Reynolds, G.S. Herman, Science 297 (2002) 65. [17] M.I.F. Macedo, C.C. Osawa, C.A. Bertran, J. Sol–gel Sci. Technol. 30 (2004) 135.
X. Duan et al. / Thin Solid Films 520 (2011) 25–29 [18] S. Ramanathan, S.K. Roy, R. Bhat, D.D. Upadhyaya, A.R. Biswas, Ceram. Int. 23 (1997) 45. [19] J.L. Shi, J.H. Gao, Z.X. Lin, Solid State Ionics 32–3 (1989) 537. [20] A.N. Buckley, A.J. Hartmann, R.N. Lamb, A.P.J. Stampfl, J.W. Freeland, I. Coulthard, Surf. Interface Anal. 35 (2003) 922.
[21] [22] [23] [24]
29
J. van den Brand, W.G. Sloof, H. Terryn, J.H.W. de Wit, Surf. Interface Anal. 36 (2004) 81. W.H.R. Shaw, J.J. Bordeaux, J. Am. Chem. Soc. 77 (1955) 4729. H. Kozuka, M. Kajimura, J. Am. Ceram. Soc. 83 (2000) 1056. C.J. Brinker, G.W. Scherer, Sol–gel science: the physics and chemistry of sol–gel processing, Academic Press, Boston, 1990 (ch.8).