“Logarithmic spirals are also known as growth spirals since they appear in naturally-grown objects,” explains team leader Danilo Bürger of the Chemnitz University of Technology. “Although researchers have observed the growth of spirals or spiral cracks in materials science before now, they have never seen the growth of logarithmic spirals, a specific class of spirals with self-similar properties.

“We have now detected two different types of nearly-perfect logarithmic crystallization spirals in a Ge:Mn layer sputtered on a (001)-oriented Ge wafer. When annealed with a flashlamp light pulse, small-amplitude deformations of the Ge:Mn layer form perpendicular to the surface of the Ge substrate. These deformations can be classified as ripples, tangents and spirals on micron length scales in the crystallized Ge:Mn layer.”

Explosive crystallization mechanism

The two types of spirals observed by Bürger and colleagues are: double spirals in the centre of the sample; and perfectly symmetric spirals starting at the sample edge. Germanium droplets at the surface act as “ignition points” for the formation of the first type of spiral.

According to the researchers, these deformations point to a so-called explosive crystallization mechanism, which is an autocatalytic process that occurs when a metastable amorphous film is heated up and crystallizes laterally. Explosive crystallization only stops if more heat is dissipated than actually generated from the crystallization itself.

Crystallization phenomena during flashlamp annealing

“Understanding such crystallization phenomena during flashlamp annealing is very important, since this is one of the annealing technologies employed in the semiconductor industry,” explains Bürger. “This technology seems to be able to process materials with a variety of adjustable parameters that can contribute to optimizing different material systems. Understanding the dominant forces at play during the self-organization of the spirals we have seen will be a step forward in this context, but we will need help from theoreticians here to advance further.

“Understanding the topography of the sample will also be important since we observed large smooth areas around the spirals,” he tells nanotechweb.org. “The smooth topography of the explosive crystallized layer – especially for thinner samples – is another unique feature of our samples and points perhaps to the fact that lots of different, hitherto unconsidered, structures might undergo explosive crystallization during the annealing process.”

The team, which also includes researchers from Leibniz IFW Dresden and the Institute of Ion Beam Physics and Materials Research at the Helmholtz-Zentrum Dresden-Rossendorf, details its work in the Journal of Applied Physics 121 184901.