Density limits of high temperature and multiple local droplet etching on AlAs

The density of nanoholes created by self-assembled Al droplet etching of AlAs surfaces during molecular beam epitaxy is studied. We find a clear decrease of the hole density with increasing etching temperature T up to a threshold temperature T=620 C. At T>620 C, the hole density saturates at a minimum of 2×10⁸cm-². We attribute this saturation to a change of the AlAs surface reconstruction. On the other hand, at reduced T, hole densities up to 2×10⁹cm-² have been achieved. However, this hole density increase is accompanied by a reduction of the hole depth. To generate high density holes with larger depth suited for quantum dot fabrication, we have studied the effect of repeated etching steps.