First-principles study of intervalley mixing: Ultrathin GaAs/GaP superlattices

First-principles pseudopotential calculations are reported for the structural properties, band offsets, and single-particle electronic states of (GaP${)}_{\mathit{n}}$(GaAs${)}_{\mathit{n}}$ (001) and (111) superlattices with n≤3. With the aim of developing a systematic theory to relate superlattice levels in these ultrathin systems to those of their constituents, the superlattice is treated as evolving from a virtual-crystal parent zinc-blende compound. A semiquantitative description of the resulting superlattice states evolves from this theory, which is based on (i) the small perturbative nature of the superlattice ordering potential with respect to this parent compound, (ii) a generic n dependence found for the intervalley coupling strengths, (iii) generic selection rules governing intervalley mixing, and (iv) observations of the dependence of coupling strengths on the anion versus cation character of the involved states. Application of these principles to (110) and (201) GaAs/GaP, and to the previously studied GaAs/AlAs systems, demonstrates their usefulness. Calculated energy gaps (and their direct versus indirect and type-I versus type-II character) are presented for the superlattices studied.