The elastic constants of a wide range of models of defected crystalline and amorphous Si were calculated, using the environment-dependent interatomic potential. The defected crystalline simulation cells contained randomly generated defect distributions. An extensive characterization of point defects was performed, including structure, energy and influence on elastic constants. Three important conclusions were drawn. Firstly, defects had independent effects upon the elastic constants of Si up to (at least) a defect concentration of 0.3%. Secondly, the linear effect of Frenkel pairs upon the <110> Young's modulus of Si was –1653GPa per defect fraction. Thirdly, 17 different point defect types caused a very similar decrease in the <110> Young's modulus: –0.28% when calculated in isolation using a 1728-atom cell. These principles were expected to be very useful for predicting the effect of radiation damage upon the elastic modulus of Si in the typical case in which point-defect concentrations could be estimated, but the exact distribution and species of defects was unknown. A study was also made of amorphous samples generated by quenching the liquid with environment-dependent interatomic potential, including an ideal structure of perfect fourfold coordination, samples with threefold and fivefold coordinated defects, one with a nanovoid, and one with an amorphous inclusion in a crystalline matrix. In the last case, a useful observation was that the change in the Young’s modulus was simply related to the volume fraction of amorphous material, as had also been observed by experiment.

Elastic Constants of Defected and Amorphous Silicon with the Environment-Dependent Interatomic Potential. C.L.Allred, X.Yuan, M.Z.Bazant, L.W.Hobbs: Physical Review B, 2004, 70[13], 134113 (13pp)