First-principles studies of intrinsic point defects in magnesium silicide.

We have studied intrinsic point defects in magnesium silicide, Mg(2)Si, by density-functional theory. Evaluating the formation energies of point defects, we show that n-type electric conductivity of Mg(2)Si originates from formations of positively charged Mg ions at interstitial sites, regardless of the chemical composition in crystal growth. Moreover, we have calculated the Born effective charge tensors and the valence charge density distribution. They show Mg(2)Si is an ionic crystal composed of Mg(2+) and Si(4-) which have very different ionic radii, 0.6 Å and 2.1 Å, respectively. We have concluded that the unfavorable antisite defect, Mg(Si), is due to the dissimilar ionic radii.