Transformation of concentric quantum double rings to single quantum rings with squarelike nanoholes on GaAs(0Â 0Â 1) by droplet epitaxy

The fabrication of self-assembled InxGa1−xAs nanostructures on GaAs(0 0 1) substrates grown by droplet epitaxy using molecular beam epitaxy is reported. The effect of In contents (0≤x≤0.2) for InxGa1−x droplets on their shape, dimension, density, and depth profile was investigated. The concentric quantum double rings (CQDRs) are transformed into quantum rings (QRs) with squarelike nanoholes when In content is increased. The transformation mechanism is explained by the strain relaxation arguments. As In content increases, crystallization can occur not only from the outer periphery, but also from the inner periphery of the rings. This is confirmed by decrease in outer dimension and change in the hole profile along the [1 −1 0] direction. In addition, low density QRs with shallow nanoholes were found on the surface for In content of 0.2. The surface morphology of InxGa1−xAs nanostructures was examined by atomic force microscopy (AFM).