  Transition metal impurities such as nickel, copper, and iron, in solid-state
materials like silicon have a significant impact on the electrical performance
of integrated circuits and solar cells. To study the impact of copper
impurities inside bulk silicon on the electrical properties of the material,
one needs to understand the configurational space of copper atoms incorporated
inside the silicon lattice. In this work, we performed ReaxFF reactive force
field based molecular dynamics simulations, studying different configurations
of individual and crystalline copper atoms inside bulk silicon by looking at
the diffusional behavior of copper in silicon. The ReaxFF Cu/Si parameter set
was developed by training against DFT data, including the energy barrier for an
individual Cu-atom inside a silicon lattice. We found that the diffusion of
copper atoms has a direct relationship with the temperature. Moreover, it is
also shown that individual copper atoms start to clusterize inside bulk silicon
at elevated temperatures. Our simulation results provide a comprehensive
picture of the effects of temperature and copper concentration on the
crystallization of individual copper inside silicon lattice. Finally, the
stress-strain relationship of Cu/Si compounds under uniaxial tensile loading
have been obtained. Our results indicate a decrease in the elastic modulus with
increasing level of Cu-impurity concentration. We observe spontaneous
microcracking of the Si during the stress-strain tests as a consequence of the
formation of a small Cu clusters adjacent to the Si surface.
