The surface diffusion of In atoms from an In droplet and their subsequent reaction with As4 molecules lead to the formation of InAs quantum dots elliptically distributed around the initially formed droplet. The size of this elliptic dot cluster depends sensitively on the growth conditions. For instance, as the substrate temperature Ts is raised from 220 to 350 °C, the shorter and longer diameters of the elliptic clusters, elongated in the [011] direction, increase exponentially with Ts, expanding from the range of 0.8−1.6 μm to 2−7 μm. When the pressure PAs of As4 is increased from 0.7 to 10 × 10−5 Torr at Ts = 300 °C, the cluster diameters shrink from 3−6 μm to 1−2 μm in inverse proportion to PAs0.3−0.4. These findings are interpreted in terms of anisotropic diffusion of In atoms from the droplet.

We examine in detail the spontaneous formation process of a cluster of InAs dots on GaAs (1 0 0) by droplet epitaxy. A cluster of dots was fabricated by irradiating an As flux for 30 s to In droplets. The dots were found to be formed mainly along a ring-like zone stretching in the [0 1 1] 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 (5 s). 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 °C. These peaks were attributed to the wetting layer and the dots, respectively.