Low-temperature fabrication of aluminium oxide thin films

© 2010 Dr. Xiaofei DuanChemical 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 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 formed at 10-6 Torr was different from δ 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