Boron Diffusion in Silicon in the Presence of Grain Boundaries and Voids

We have investigated the behavior of one B impurity in the presence of dislocations, nanovoids and grain boundaries (GBs) in Si. Our study has been motivated by recent experiments on highly B-doped polycrystalline Si where a very significant thermoelectric power factor enhancement was observed. We study the influence of nanovoids on B diffusivity and activation in Si. We compare with the grain-boundary of pure tilt Si[001](120) poly-Si, where the presence of dislocations is dominant, similarly as in the case of nanovoids in c-Si. We performed DFT calculations to investigate minimum energy configurations, energy barriers and activation or deactivation of one B impurity in the proximity of the above structures. We found that, B preferably occupies substitutional lattice sites at the edge of the cavities. Thus the presence of small voids reduces its diffusivity and mobility, but the electrical activation of B is not significantly influenced. In the case of Si[001](120) GB, B is found to form Boron interstitials clusters in the close vicinity of a GB. The formation energy of those clusters is lower when the cluster is closer to center of the GB. Our results are in agreement with previously reported experimental data. It is provided insight to the effect of structural inhomogeneity on the dopant properties in Si structures that are currently of interest for thermoelectric applications.