Complex quantum dots in III-As nanowires

Single quantum dots embedded in the core of freestanding semiconductor nanowires grown directly on Si offer a novel and promising scheme for the realization of on-demand sources of single photons or entangled photon pairs in quantum technology systems. Here, we have investigated the growth mechanism and optical properties of axial quantum dots embedded in GaAs nanowires grown in self-catalyzed vapor-liquid-solid mode while demonstrating the tuneabilty of their emission energy across a wide range of wavelengths with the potential for telecom band access.
We have incorporated GaAs complex quantum dots inside GaAs/In(x)Al(1-x)As and GaAs/Al(x)Ga(1-x)As core/shell nanowires grown via our nanowire growth technique called droplet-confined alternate pulsed-epitaxy (DCAPE)[1] (an adaptation of conventional molecular beam epitaxy (MBE)), which grants precise control over the axial growth rate and droplet composition. Using a combination of conventional MBE and DCAPE the growth of highly symmetrical quantum dots, as little as 10 nm in diameter and just a few nanometers in height, were made possible. Strong axial confinement was achieved in the form of a double axial heterostructure by incorporating two Al(x)Ga(1-x)As/Al(y)Ga(1-y)As short-period superlattices separated by a thin GaAs segment (figure 1(a)). The complexity of our quantum dots is derived from the unique possibility of using different ternary alloys for the axial and radial confinement. By introducing a latticed mismatched ternary alloy shell as the radial barrier (In(x)Al(1-x)As), we demonstrated controlled hydrostatic strain induced redshifts of the highly polarized quantum dot emission energy by adjusting the In content of the shell. For 39% In we measured a redshift in the quantum dot emission of 320 meV, from an unstrained quantum dot reference (Al(x)Ga(1-x)As shell), as shown by the photoluminescence measurements in figure 1 (b).
To optimize the emission quality, the compositional grading effect of the constituent superlattice materials across the interfaces must be addressed. High-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and nanowire growth models were utilized to gain an understanding of the compositional grading mechanism at the quantum dot/superlattice interface. We found that interfacial sharpness increases significantly by reducing the superlattice growth temperature and nanowire radius. Notwithstanding, limitations in what can be achieved ensue and possible strategies to overcome them will be presented.

[1] Balaghi et al., Nano Lett. 16, 4032 (2016)