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Alumina, Al2O3, is one of the technologically most important ceramic materials. Due to the existence of a variety of different polymorphs, it finds use in a wide range of applications. Consequently, alumina thin film growth has been studied intensely in the past. However, the mechanisms behind the formation of different phases and microstructures are still poorly understood, especially for physically vapor deposited films. An increased atomic scale understanding of alumina surface processes would thus be an important step towards a more complete understanding and control of the deposition process. In the present work, density functional theory based methods were used to study the adsorption of Al, O, AlO, and O2 on differently terminated α-alumina (0001) surfaces. The results show, e.g., that several metastable adsorption sites exist on the O-terminated surface, providing a possible explanation for the difficulties in growing α-alumina at lower temperatures, where the energy available for adatom diffusion to the bulk site might be insufficient. Moreover, the results show that Al adsorption in the bulk position is unstable or considerably weakened for completely hydrogenated surfaces, indicating that hydrogen, e.g. stemming from residual water in vacuum systems, might hinder crystalline α-alumina growth. Furthermore, energy barriers for different surface diffusion processes were investigated using the nudged elastic band method, showing, e.g., that the Al surface diffusion barrier is ~0.7 eV on the Al-terminated (0001) surface. This is lower than what might be expected for a phase where synthesis at low temperatures is difficult (which is the case for α-alumina), supporting previous experimental studies suggesting the nucleation step of growth to be of crucial importance [See, e.g., Andersson et al., J. Vac. Sci. Technol. A 22, 117 (2004)]. The computational results are discussed in the context of alumina growth and provide important insights into how different factors influence the growth and how deposition processes can be optimized in order to achieve the, in many cases desired, α-alumina polymorph. |