Growth and Properties of Oxide Coatings

Our recent research on oxide coatings mainly concerns fundamental studies related to thin film growth of alumina (Al₂O₃). Alumina 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. For example, crystalline alumina phases are commonly utilized in wear-resistant coatings due to their beneficial mechanical and thermal properties. Owing to the many appealing properties, 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. The research in this group is focused on gaining a fundamantal understanding of the phase formation and structure evolution of sputter deposited alumina films. This would not only be important in many existing applications, but also open the possiblity for new thin film applications of the material.

Some of our recent results are listed below. For further information, please see our publications or contact us.

• We published results demonstrating growth of a-alumina thin films by reactive sputtering at very low temperatures. It was shown that both nucleation and continuous bombardment need to be controlled in order to grow the a phase instead of the commonly forming g phase. An important source of bombardment in our experiments was energetic negative oxygen ions. The a phase of alumina is desired in many applications, due to its outstanding wear resistant properties.
  
  
• We reported results on energy-resolved mass spectrometry of the deposition flux (ions/atoms/molecules) during reactive Ar/O₂ magnetron sputtering of Al. The fraction of AlO molecules in the flux was found to be of the same order as the flux of atomic Al, a result which could be important when understanding alumina thin film growth. Also, as the Al target is oxidized a high flux of negative oxygen ions are formed which can bombard the growing films with high energies (a few hundred eV) and strongly influence growth (cf. previous point).
  
  
• The effects of H₂O vapor, which is often present in industrial growth systems, on alumina thin film growth were explored. Films deposited onto Cr₂O₃ nucleation layers at a substrate temperature of 500 °C expectedly exhibited a columnar structure consisting of crystalline a-alumina if deposited under ultra high vacuum conditions. However, as water vapor was introduced the microstructure changed to a structure consisting of small equiaxed grains. This structure change was accompanied by change in the phase composition of the films, where the g-alumina content was found to increase with increasing film thickness. The amount of hydrogen incorporated into the films was, however, found to be small. Hence, the results demonstrate that the effects of residual water on the film growth are considerable, also in cases where the amount of impurities incorporated into the film is small.
  
  
• To gain a better understanding of alumina growth on an atomistic level, ab initio calculations of adsorption processes on a-alumina surfaces were performed. The results show, e.g., that several metastable adsorption sites exist on the O-terminated (0001) surface. This provides a possible part of the explanation for the difficulties in growing crystalline a-alumina at lower temperatures. Moreover, the results provide important insights into how hydrogen adsorbing on growth surfaces might disturb the growth (cf. previous point). For example, it was shown that Al adsorption corresponding to the bulk stacking sequence is unstable, or considerably weakened, on a hydrogenated O-terminated surface (see figure below), indicating that hydrogen might hinder further a-alumina crystallite growth.
  
  
  Figure. A relaxed configuration after an attempt of Al adsorption in the bulk position (marked with X) on a hydrogenated O-terminated a-alumina (0001) surface. The adsorbed Al atom has moved away from the attempted adsorption site and only adsorbs weakly on the surface. O atoms are red, Al atoms green, H atoms grey, and the added Al atom is yellow.