Low-temperature fabrication of aluminium oxide thin films
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2010
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Melbourne Authors
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Department of Chemistry
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PhD thesis
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Duan, X. (2010). Low-temperature fabrication of aluminium oxide thin films . PhD thesis, Department of Chemistry, The University of Melbourne.
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© 2010 Dr. Xiaofei Duan
Abstract
Chemical and structural characteristics of aluminium oxide (Al2O3) exhibit key properties of the material. Metastable γ, δ and θ-Al2O3 are important in catalytic industry, gas-sensing and optical devices. These transitional phases can be converted to the thermally stable α phase, which has a hexagonal close-packing structure and exhibits excellent thermal stability, chemical and wearing resistance, and rigidity.
Processing conditions for crystalline films can vary depending upon the degree of crystallinity required. Typical formation of θ-Al2O3 from boehmite (γ-AlOOH) by heating requires a significantly high temperature exceeding 800°C.
This thesis reports the fabrication of Al2O3 thin films on Si <1 0 0> wafers at a temperature of 500 °C (amorphous) and 550°C (crystalline). This was achieved through a unique approach utilising a novel single-source chemical vapour deposition (SSCVD) technique. The precursor was designed to have structurally octahedral Al-O cores enshrouded by large carbamate ligands, which increase the molecule’s volatility but are easily removable upon decomposition. It also showed the growth orientation α <1 1 3> formed at 10-6 Torr was different from δ <4 0 0> formed at 10-2 Torr, suggesting the thermally stable phase was produced at a lower background pressure.
For comparison, a solvothermal approach using the same precursor in a high pressure autoclave produced Al2O3 thin films at temperatures as low as 65°C. The low-temperature decomposition of aluminium diisopropyl-carbamate was elucidated to be via the β-elimination of the carbonate ligand. A range of similar precursors such as aluminium diethylcarbamate, aluminium dibutylcarbamate, and aluminium di-sec-butylcarbamate were subsequently synthesised using Schlenk chemistry and analysed.
In an inorganic sol-gel approach, Al(NO3)3 was used as an Al3+ source in an aqueous (or glycerol) solution. OH- ions generated by the urea hydrolysis enhanced the production of Al(OH)3, formed an amorphous film on the Si substrate. By combining a high-pressure hydrothermal technique, the hydroxide film was then demonstrated to convert to a polycrystalline Al2O3/AlOOH film at an overall temperature of 200°C. In addition, the hydrothermally treated films appeared as a densely packed and uniform surface, in contrast to crack formations by a conventional annealing treatment.
All deposited films were investigated using a range of laboratory and synchrotron based analytical techniques. These included X-ray photoelectron spectroscopy and diffraction and near edge X-ray absorption fine structure spectroscopy.
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Keywords
aluminium oxide, low temperature, thin films