Author: Simon Olsson, Fredrik Eriksson, Jens Jensen, Magnus Garbrecht, Jens Birch, Lars Hultman

Institute: Department of Thin Film Physics, IFM, Linkoping University, SE-581 83 Linkoping, Sweden

In recent years, the study of Al(Si)CuFe-based thin films obtained by diffusion into silicon substrate has attracted considerable attention due to potential applications in microelectronics and functional materials. The diffusion method provides the possibility of forming heterogeneous structures with unique physical and chemical properties.

Aluminum, silicon, copper, and ferrum, which are part of such films, interact with the substrate, creating complex interphase boundaries. This leads to the formation of multi-level clusters, which significantly improves the mechanical and electrical characteristics of the final material. The complex structure of these films is determined by the synthesis conditions, including temperature, diffusion time, and initial composition.

Electron microscopy and X-ray diffraction analysis allow us to investigate the morphology and crystal lattice of the films, as well as to verify the presence of the desired phase states. In addition, such studies open up a new avenue for the creation of multifunctional materials that can be used in various fields, from energy to photonics, emphasizing the importance of understanding their structure and composition for further technological development.

Multilayer Al/Cu/Fe thin films with a composition close to quasicrystalline phases were obtained using the magnetron sputtering method. Annealing at 600 °C formed a homogeneous film of the cubic α-approximant phase due to the diffusion of silicon from the substrate, which prevents the formation of a quasicrystalline structure. After 4 hours of annealing, the film contained 8 at.% Si, which corresponds to the predicted value for the α-approximant. During the annealing process (64 hours), it was noted that the Si content in the films gradually increased to about 12 at.%, while the α-approximant phase was preserved. The lateral lattice parameters continuously decreased with increasing substitution of Al for Si. It was also noted that the film structure was polycrystalline, with individual crystals experiencing varying degrees of deformation and having no preferred orientation with respect to the substrate or to each other.

Approximants are classes of phases associated with quasicrystals. They have a local atomic structure similar to quasicrystals and, as a result, have many similar physical properties. The thermodynamic and chemical stability of approximants is often higher than that of quasicrystals, which allows them to replace quasicrystals in compositional and temperature ranges where the latter cannot form or persist. In a number of application areas, the use of thin films of approximants can significantly improve surface properties. However, the number of studies devoted to thin films remains limited.

In recent works it has been demonstrated that thin films of the icosahedral quasicrystalline phase ψ-Al62.5Cu25Fe12.5 can be obtained by annealing Al/Cu/Fe multilayers on an inert Al2O3 substrate. However, the use of Si as a substrate promotes the formation of a cubic α-approximant (with a nominal composition of Al55Si7Cu25.5Fe12.5) through the diffusion of Si from the substrate in the solid state. From studies carried out on bulk samples it is known that the addition of Si above 3 at.% gradually reduces the proportion of the quasicrystalline phase, and 9 at.% Si leads to the formation of a single-phase approximant. The range of Si content in the stable bulk approximant phase is from 3 to 15 at.%. In this paper, the microstructure and distribution of elements in thin films of the α-approximant phase before and after annealing are investigated.