Engineered Nanosystems

Centre of Excellence in Computational Complex Systems Research

 

Grain Boundaries

Researchers: Sebastian von Alfthan ,Kimmo Kaski and Adrian P Sutton

Grain boundaries are formed when two crystalline slabs are put into contact with each other, so that the crystals are rotated with respect to each other. These are formed in polycrystalline silicon, which is used widely for solars cells and thin-film transistors. In pure twist grain boundaries the axis around which the crystals are rotated, is perpendicular to the interface. In Figure 1 one can see an example of how a twist grain boundary is formed. After the two crystals have been put into contact the atoms at the interface will self-organize into their minimum energy configuration. One cannot experimentally see the atomistic structure of twist grain boundaries, only the grain boundary energy can be measured. Therefore the atomistic structure of the interface is still considered controversial.

Figure 1

Figure 1. A twist grain boundary has been formed after two crystalline slabs of silicon, which have been rotated with respect to each other, have been put into contact.

In this work we are using computational methods to study the atomistic structure of twist grain boundaries of different rotation angles. We are interested in knowing if the interface of all grain boundaries is comprised of a thin amorphous layer, or if there are certain angles for which the interface is crystalline. We are also interested in the grain boundary energy, since it can be compared to experimental results.

Using molecular dynamics methods we have found crystalline structures for several twist grain boundaries in silicon . These results are supported by experimental evidence since these grain boundaries have been measured to have a lower grain boundary energy. We have also validated the model by calculting the energy for some of the structures using ab initio methods. These results contradict some previous computational studies which suggested that all grain boundaries are amorphous. These structures have never before to this authors knowledge been presented previously, and as such are of great interest.

References

  • Sebastian von Alfthan, Peter Haynes, Kimmo Kaski, Adrian P. Sutton (2006). Are the structures of twist grain boundaries in silicon ordered at 0 K? Phys. Rev. Lett. 96, 055505.