Superlattice unit cells used in the calculation of the bond pair binding energy. Ga is green, As is yellow, Zn is blue, and Se is red. (a) An interface containing only Se–Ga bonds. (b) An interface containing only As–Zn bonds.
Superlattice unit cells used in the calculation of the bond pair binding energy for the case where the interface contains an equal mix of As–Zn and Se–Ga bonds. (a) Both the As atoms and Se atoms are aligned along the  direction giving a “” geometry to the anion arrangement adjacent to the interface. (b) The interfacial anions are aligned along the  and  directions giving a “” geometry.
Superlattice unit cells used in the bond pair binding energy. (a) The interface conceptually derived from ZnSe(100) heteroepitaxy on the “old” substrate. At the interface, the Se-to-As ratio is 1:3, and there is a charge deficit. (b) The interface conceptually derived from ZnSe(100) heteroepitaxy on a substrate. Here, the Se-to-As ratio 1:1. Note that the second layer Ga vacancies have been back-filled with Zn. Here, again, there is a charge deficit. (c) Same as (b) except that the second layer vacancy has been back-filled with Ga. This might be viewed as growth on a simple , rather than a structure, followed by displacement of one of the top-layer As dimers with Se. Note that this configuration produces a charge-neutral interface. (d) Same as (b), but with at 3:1 Se-to-As ratio. Experimentally, this corresponds to the displacement of one As atom from the structure with a Se atom. This interface is also charge neutral.
Superlattice unit cells used in the calculation of the bond-pair binding energy for the cases where the interface region contains a vacancy. (a) The vacancy site contains two As NBOs and two Se NBOs. The presence of this vacancy guarantees charge neutrality at the interface. (b) Same as (a) except that the vacancy is off-set into the GaAs bulk and contains four As NBOs, and no Se NBOs.
Superlattice unit cells used in the calculation of binding energy. (a) A Zn vacancy at the interface. (b) A Zn vacancy in the interior of the ZnSe bulk component.
Same cells as in Fig. 4, except that here each superlattice unit cell contains two vacancies. Here, even in the extended crystal, the vacancies are in a staggered configuration. (a) Two Se and two As NBOs in each vacancy. (b) Four Se NBOs in one vacancy, and four As NBOs in the second vacancy.
Numbers of atoms, vacancies, and bond pairs for interfaces studied.
Calculated DFT energies and energies estimated assuming bond pair additivity.
Energy per bond-pair and per nonbonding orbital pair.
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