Control of complex quantum structures in droplet epitaxy

We report the controllable growth of GaAs quantum complexes in droplet molecular-beam epitaxy, and the optical properties of self-assembled Alalt;subagt;xalt;/subagt;Gaalt;subagt;1-xalt;/subagt;As quantum rings embedded in a superlattice. We found that Ga droplets on a GaAs substrate can retain their geometry up to a maximum temperature of 490alt;supagt;alt;/supagt;C during post-growth annealing, with an optimal temperature of 320alt;supagt;alt;/supagt;C for creating uniform and symmetric droplets. Through controlling only the crystallisation temperature under Asalt;subagt;4alt;/subagt; in the range of 450alt;supagt;alt;/supagt;C to 580alt;supagt;alt;/supagt;C, we can reliably control diffusion, adsorption and etching rates to produce various GaAs quantum complexes such as quantum dots, dot pairs and nanoholes. Alalt;subagt;xalt;/subagt;Gaalt;subagt;1-xalt;/subagt;As quantum rings are also realised within these temperatures via the adjustment of As beam equivalent pressure. We found that crystallisation using Asalt;subagt;2alt;/subagt; molecules in the place of Asalt;subagt;4alt;/subagt; creates smaller diameter quantum rings at higher density. The photoluminescence of Asalt;subagt;2alt;/subagt; grown Alalt;subagt;xalt;/subagt;Gaalt;subagt;1-xalt;/subagt;As quantum rings embedded in a superlattice shows a dominant emission from the quantum rings at elevated temperatures. This observation reveals the properties of the quantum ring carrier confinement and their potential application as efficient photon emitters.