We report the direct formation of self-assembled GaAs/AlGaAs quantum dots by low-temperature molecular beam epitaxy. To drive a three dimensional growth mode, the AlGaAs surface was exposed alternately to the Ga and As sources. The resulting GaAs nanocrystals having {111} facets were clearly identified by high-resolution transmission electron microscopy. The emission spectra also confirmed the formation of dots. The transition to a three-dimensional growth mode is attributed to the limited surface migration of Ga adatoms on the AlGaAs surface, which has excess As at low substrate temperature.
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