  Self-assembled InAs quantum dots (QDs), which have long hole-spin coherence
times and are amenable to optical control schemes, have long been explored as
building blocks for qubit architectures. One such design consists of vertically
stacking two QDs to create a quantum dot molecule (QDM) and using the
spin-mixing properties of "molecule-like" coupled hole states for all-optical
qubit manipulation. In this article, the first of two papers, we introduce the
incorporation of dilute GaBiAs alloys in the barrier region between the two
dots. GaBiAs is expected to increase the spin-mixing of the molecular states
needed for qubit operations by raising the barrier valence band edge and
spin-orbit splitting. Using an atomistic tight-binding model, we compute the
properties of GaBiAs and the modification of hole states that arise when the
alloy is used in the barrier of an InAs QDM. An atomistic treatment is
necessary to correctly capture non-traditional alloy effects such as the
band-anticrossing valence band. It also allows for the study of configurational
variances and clustering effects of the alloy. We find that in InAs QDMs with a
GaBiAs interdot barrier, electron states are not strongly affected by the
inclusion of Bi. However, hole states are much more sensitive to the presence
and configuration of Bi in the barriers. By independently studying the
alloy-induced strain and electronic scattering off Bi and As orbitals, we
conclude that an initial increase in QDM hole state energy at low Bi
concentration is caused by the alloy-induced strain. We further find that the
decrease in QDM hole energy at higher Bi concentrations can only be explained
when both alloy strain and orbital effects are considered. In our second
article, we use the understanding developed here to discuss how the alloyed
barriers contribute to enhancement in hole spin-mixing and the implications for
QDM qubit architectures.
