Metastable impurities in semiconductors: Si:Mg and Si:Be

Using 30-year-old ideas on Fermi-energy- (${\varepsilon }_F$) induced stability enhancements in impure semiconductors, it follows that if two impurity sites with different electrical levels exist, one can expect to find, inside the band gap, a critical Fermi energy ${\varepsilon }_F^b$ such that one site is stable for ${\varepsilon }_F>{\varepsilon }_F^b$ and the other for ${\varepsilon }_F<{\varepsilon }_F^b$. This bifurcation of the site-preference energies leads to general metastabilities, as demonstrated here for Si:Mg and Si:Be, using self-consistent local-density total-energy calculations.