Characterisation of droplet-epitaxial GaAs/AlGaAs quantum dot and quantum ring systems using grazing incidence X-ray diffraction

We study with grazing incidence X-ray diffraction GaAs quantum dot (QD) and quantum ring (QR) systems grown via droplet epitaxy. The measurements clearly reveal that QDs and QRs grown on AlGaAs(0 0 1) substrate crystals are single crystalline with the same crystal orientation as the substrate, and are free of elastic strain. The QD and QR systems are found to laterally arrange in chains of closely spaced objects with preferential orientation in 〈1 1 0〉 crystallographic directions. We find that for the QD system this ordering is anisotropic, showing intra-chain correlation only in [1 -1 0] direction. In the QR system the intra-chain arrangement is equal in both [1 1 0] and [1 -1 0] directions. The differences in the ordering behaviour of QD and QR chains are believed to originate from the different speeds of As incorporation in the Ga droplets during their crystallization at different As pressures.