Visible photoluminescence from N-dot ensembles and the linewidth of ultrasmall ${\mathrm{Al}}_{\mathit{y}}$${\mathrm{In}}_{1\mathrm{-}\mathit{y}}$As/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum dots

Ensembles containing a few hundred quantum dots, ∼17 nm in diameter, were prepared by etching μm-size mesas in ${\mathrm{Al}}_{\mathit{y}}$${\mathrm{In}}_{1\mathrm{-}\mathit{y}}$As/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum-dot samples grown using the spontaneous island formation during molecular-beam epitaxy. The visible photoluminescence spectra display the reproducible statistical fluctuations of the ground states, which are emitting individual lines a fraction of a meV in width.