Fabrication of polycrystalline aluminum oxide thin films via hydrolysis and hydrothermal reactions in solutions
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 (> 350 °C [12], [13]), metastable crystalline phases (> 800 °C [10]) and thermodynamically stable phase (> 1000 °C [11], [14], [15]). Annealing films at high temperatures often causes micro-cracks 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].
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 Al3+, which has been used for the synthesis of Al(OH)3 particles and powders. [17], [18], [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.
Section snippets
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 >1
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
Discussion
The formation of Al2O3/AlO(OH) thin film is proposed in a three-step 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
Summary
We have demonstrated a method for preparing polycrystalline γ-AlO(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
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.
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