Ionized physical vapor deposited Al₂O₃films: Does subplantation favor formation of α‐Al₂O₃?

Abstract

The broad energy distributions of the condensing particles typically encountered in ion assisted vapor deposition techniques are often a drawback when attempting to understand the effect of the energetic bombardment on the film properties. In the current study, a monoenergetic Al+ beam generated by a filtered cathodic arc discharge is employed for the deposition of alumina (Al2O3) films at well defined Al+ ion energies between 4 eV and 200 eV at a substrate temperature of 720 °C. Structural analysis shows that Al+ energies of 40 eV or larger favor the formation of the thermodynamically stable α‐Al2O3 phase at the expense of other metastable Al2O3 polymorphs. The well defined ion energies are used as input for Monte‐Carlo based simulations of the ion–surface interactions. The results of these simulations reveal that the increase of the Al+ ion energy leads to an increase in the fraction of ions subplanted into the growing film. These findings underline the previously not considered role of subsurface processes on the phase formation of ionized physical vapor deposited Al2O3 films. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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

“…In our previous work, we have used a monoenergetic Al + beam to synthesize ␣-Al 2 O 3 at energies of 40 eV. 16 It has been shown, using Monte Carlo simulations, that in this energy range a fraction of Al + ions is subplanted into the growing film highlighting, in addition to the above mentioned factors, the role of subsurface processes for the phase formation of Al 2 O 3 . 16 However, the effect of the ion irradiation in this energy range on the structure evolution of Al 2 O 3 has not yet been explored on the atomic and electronic level.…”

“…Diffusion and damage occur for both polymorphs in the kinetic energy range from 3.5 to 40 eV. This energy range has been chosen based on our previous experimental report, 16 where evidence for subplantation of impinging Al in ␥-Al 2 O 3 , which in turn causes significantly larger irradiation damage and hence larger mobility as compared to ␣-Al 2 O 3 , is presented. We suggest that Al bombardment induced fast diffusion along ␥-Al 2 O 3 ͑001͒ gives rise to preferential ␣-Al 2 O 3 ͑0001͒ growth, which is consistent with the previously reported structure evolution experiments.…”

“…We suggest that Al bombardment induced fast diffusion along ␥-Al 2 O 3 ͑001͒ gives rise to preferential ␣-Al 2 O 3 ͑0001͒ growth, which is consistent with the previously reported structure evolution experiments. 16 Ab initio MD was performed using the OPENMX code, 26 based on the density functional theory 27 and basis functions in the form of linear combination of localized pseudoatomic orbitals. 28 The electronic potentials were fully relativistic pseudopotentials with partial core corrections 29,30 and the generalized gradient approximation was applied.…”

“…The energy cutoff ͑150 Ry͒ and k-point grid ͑1 ϫ 1 ϫ 1͒ within the real space grid technique 33 were adjusted to reach the accuracy of 10 −6 H / atom. Canonical ensembles at 330 K ͑slightly above room temperature due to irradiation from plasma͒ were used to simulate Al bombardment of alumina slabs ͑vacuum thickness 10 16,34 Structural description of ␣-and ␥-Al 2 O 3 bulk/surfaces was adopted from literature. [35][36][37] The MD time step was 1.0 fs and the total MD simulation time was 400 fs, namely 100 fs for surface relaxations and 300 fs after bombardment events.…”

“…Obviously, subplantation occurs for ␥-Al 2 O 3 ͑001͒, but not for ␣-Al 2 O 3 ͑0001͒. To justify that 300 fs is large enough 16 In summary, we have studied irradiation-induced processes during alumina growth using ab initio MD at 330 K. We have correlated Al bombardment of ␣-Al 2 O 3 ͑0001͒ and ␥-Al 2 O 3 ͑001͒ with structure evolution thereof. Independent of kinetic energy of impinging Al and irradiated surface site, diffusion and local structural disorder occur.…”