Diffusion correlation effects of molybdenum and silicon in molybdenum disilicide

Abstract

Diffusion data for both principal directions of silicon and molybdenum as well as germanium are briefly summarised. Analysis is performed of the defect formation energies (available from previous ab initio calculations and experimental measurements) for diffusion mechanisms via home and foreign sublattices. The home sublattice mechanism is shown to be the preferred one for both silicon and molybdenum. Tracer correlation factors for silicon and molybdenum diffusion via sublattice vacancies in the respective sublattices of the tetragonal C11_b structure of molybdenum disilicide are calculated by a direct Monte Carlo simulation technique. Correlation factors for Si diffusion on its sublattice are compared with literature values that were calculated using a more complicated Monte Carlo method based on the matrix approach. It is shown that there is no need for this complicated approach and that the direct Monte Carlo simulation technique gives highly accurate correlation factors. Correlation factors and anisotropy ratios of vacancy-mediated diffusion in both sublattices are deduced and compared with experimental data. Tracer correlation in the tetragonal direction is shown to contribute 0.40 eV (i.e. over 55%) of the migration energy of the corresponding Si diffusivity. Two possible jump rates for Si diffusion are separately estimated. Mo diffusion correlation factors are calculated using the direct Monte Carlo technique. A comparison with experiment is made and the ratio of two possible jump rates is also estimated.

Keywords:

• molybdenum disilicide
• self-diffusion
• diffusion correlation factors
• diffusion anisotropy
• sublattice vacancy mechanism
• Monte Carlo simulations

Acknowledgements

One of the authors (HM) is grateful to Professors I.V. Belova and G.E. Murch for the invitation to join the ‘Centre for Mass and Thermal Transport in Engineering Materials’ at The University of Newcastle, Australia, as a visiting professor for several weeks. He also expresses thanks to all members of the Centre for their help and hospitality.