Incorporation of random alloy GaBixAs1-x barriers in InAs quantum dot molecules: Energy levels and confined hole states
Arthur Lin, Matthew Doty, Garnett W. Bryant
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 InAs QDs to create a quantum dot molecule (QDM) and using the spin-mixing properties of "molecule-like" coupled hole states for ultrafast 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 impact the spin-mixing of the molecular states needed for qubit operations by increasing the valence band edge and spin-orbit splitting in the barrier region. 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. However, hole states are much more sensitive to the presence and configuration of Bi in the barriers. At 10\% Bi, a significant reduction in the energy of confined hole states emerges due to energetic overlap with GaBiAs barrier valence band states. 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.