Strained Si/Ge superlattices: Structural stability, growth, and electronic properties

We present detailed results of self-consistent-field pseudopotential calculations for the stability, structural phase transitions, growth, and the electronic properties of strained Si/Ge semiconductor superlattices and alloys. The metastable structure of (Si${)}_n$/(Ge${)}_n$ (n≤6) superlattices pseudomorphically restricted to the Si(001) surface are determined through total energy minimization and inter- atomic force calculations, and their formation energies are calculated. A simple model for the formation energy of superlattices is developed, whereby the energy of the activation barrier to form a misfit dislocation is estimated. A neostructural phase transition in the strained Si-Ge alloy is studied, and the order of instability for various possible structures is given. It is found that during growth the atoms of the topmost grown layer are dimerized, resulting in a possible short-range order in the alloy. The energy gap of all (Si${)}_n$/(Ge${)}_n$ superlattices is found to be indirect. A more significant finding, however, is that the energy separation between the direct and indirect gap continues to decrease with increasing n, and is only 0.07 eV for n=6. Conduction-band states of an extended nature located below the confined states point to a new feature of the band offset and quantum-size effect. Localized states lying deep in the valence- and conduction-band continua are another novel result of this study.