F. Nahif

Ab initio molecular dynamics of Al irradiation-induced processes during Al2O3 growth

Al bombardment induced structural changes in alpha-Al(2)O(3) (R-3c) and gamma-Al(2)O(3) (Fd-3m) were studied using ab initio molecular dynamics. Diffusion and irradiation damage occur for both polymorphs in the kinetic energy range from 3.5 to 40 eV. However, for gamma-Al(2)O(3)(001) subplantation of impinging Al causes significantly larger irradiation damage and hence larger mobility as compared to alpha-Al(2)O(3). Consequently, fast diffusion along gamma-Al(2)O(3)(001) gives rise to preferential alpha-Al(2)O(3)(0001) growth, which is consistent with published structure evolution experiments.

On the phase formation of sputtered hafnium oxide and oxynitride films

Hafnium oxynitride films are deposited from a Hf target employing direct current magnetron sputtering in an Ar-O(2)-N(2) atmosphere. It is shown that the presence of N(2) allows for the stabilizati ...

Ab initio study of the effect of Si on the phase stability and electronic structure of γ- and α-Al2O3

Using density functional theory, the effect of Si on the stability and electronic structure of γ- and α-Al2O3 has been investigated. The concentration range from 0 to 5 at.% is probed and the additive is positioned at different substitutional sites in the γ-phase. The calculations for (Al,Si)2O3 predict a trend towards spontaneous decomposition into α-/γ-Al2O3 and SiO2. Therefore, the formation of the metastable γ-(Al,Si)2O3 phase can only be expected during non-equilibrium processing where the decomposition is kinetically hindered. The Si-induced changes in stability of this metastable solid solution may be understood based on the electronic structure. As the Si concentration is increased, stiff silicon-oxygen bonds are formed giving rise to the observed stabilization of the γ-phase.

Effect of Si additions on thermal stability and the phase transition sequence of sputtered amorphous alumina thin films

Si-alloyed amorphous alumina coatings having a silicon concentration of 0 to 2.7 at. % were deposited by combinatorial reactive pulsed DC magnetron sputtering of Al and Al-Si (90-10 at. %) split segments in Ar/O2 atmosphere. The effect of Si alloying on thermal stability of the as-deposited amorphous alumina thin films and the phase formation sequence was evaluated by using differential scanning calorimetry and X-ray diffraction. The thermal stability window of the amorphous phase containing 2.7 at. % of Si was increased by more than 100 °C compared to that of the unalloyed phase. A similar retarding effect of Si alloying was also observed for the α-Al2O3 formation temperature, which increased by more than 120 °C. While for the latter retardation, the evidence for the presence of SiO2 at the grain boundaries was presented previously, this obviously cannot explain the stability enhancement reported here for the amorphous phase. Based on density functional theory molecular dynamics simulations and synchrotr...

Bimodal substrate biasing to control γ-Al2O3 deposition during reactive magnetron sputtering

Al2O3 thin films have been deposited at substrate temperatures between 500 °C and 600 °C by reactive magnetron sputtering using an additional arbitrary substrate bias to tailor the energy distribution of the incident ions. The films were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. The film structure being amorphous, nanocrystalline, or crystalline was correlated with characteristic ion energy distributions. The evolving crystalline structure is connected with different levels of displacements per atom (dpa) in the growing film as being derived from TRIM simulations. The boundary between the formation of crystalline films and amorphous or nanocrystalline films was at 0.8 dpa for a substrate temperature of 500 °C. This threshold shifts to 0.6 dpa for films grown at 550 °C.