We examine in detail the spontaneous formation process of a cluster of InAs dots on GaAs (100) by droplet epitaxy. A cluster of dots was fabricated by irradiating an As flux for 30s to In droplets. The dots were found to be formed mainly along a ring-like zone stretching in the [011] direction, showing that In atoms migrated from the droplet and reacted with As atoms. This indicates the presence of not only the Surface diffusion of In atoms but also the inward diffusion of As atoms, This is supported by the fact that such a ring was not visible when the As irradiation time was shortened (5s). For photoluminescence measurements the dots were capped with a 50-nm-thick GaAs layer. Two peaks at 1.46 and 1.3 eV were observed in the sample annealed at 750 degrees C. These peaks were attributed to the wetting layer and the dots, respectively. (C) 2008 Elsevier B.V. All rights reserved.