  Bismide semiconductor materials and heterostructures are considered a
promising candidate for the design and implementation of photonic,
thermoelectric, photovoltaic, and spintronic devices. This work presents a
detailed theoretical study of the electronic and optical properties of
strongly-coupled GaBi$_x$As$_{1-x}$/GaAs multiple quantum well (MQW)
structures. Based on a systematic set of large-scale atomistic tight-binding
calculations, our results reveal that the impact of atomic-scale fluctuations
in alloy composition is stronger than the inter-well coupling effect, and plays
an important role in the electronic and optical properties of MQW structures.
Independent of QW geometry parameters, alloy disorder leads to a strong
confinement of charge carriers, a large broadening of the hole energies, and a
red shift in the ground-state transition wavelength. Polarisation-resolved
optical transition strengths exhibit a striking effect of disorder, where the
inhomogeneous broadening could exceed an order of magnitude for MQWs, in
comparison to a factor of about three for single quantum wells. The strong
influence of alloy disorder effects persists when small variations in the size
and composition of MQWs typically expected in a realistic experimental
environment are considered. The presented results highlight the limited scope
of continuum methods and emphasise on the need for large-scale atomistic
approaches to design devices with tailored functionalities based on the novel
properties of bismide materials.
