Size Dispersion Control of GaAs Quantum Dots Grown by Droplet Epitaxy

Droplet epitaxy (DE) is a growth technique proposed in 1991 by Dr. N. Koguchi [1] based on molecular beam epitaxy. In DE, first a beam of Ga atoms impinges on the sample surface in absence of arsenic, leading to the formation of nanometric size gallium droplets. Then, an As flux is supplied in order to crystallize the droplets into GaAs nanocrystals. The separation of the group III and group V atom irradiation on the substrate, makes possible to obtain a fine tuning of the quantum dot (QD) shape and density by simply changing the parameters that control the group III atom diffusion on the surface and allows for the fabrication of III-V QDs by self-assembly without wetting layer, lattice-matched with the barrier layer and strain-free. In this presentation we will show how it is possible to control the size dispersion of the Ga droplets, and then of the final GaAs QDs, by simply tuning during the Ga deposition step the substrate temperature and the Ga flux. This allow for the fabrication of nanostructures with narrow size dispersion (few percent) using optimal substrate temperatures and Ga fluxes. In our experiments performed in an MBE chamber on GaAs (001) surface, we changed the substrate temperature between 200 °C and 450 °C and the Ga flux between 0.01 and 1 ML/s, comparing the island size distribution (ISD) and the capture zone distribution (CZD). The CZD is estimated with Voronoi polygons, whose boundaries are defined as the locus of points equidistant from the two nearest island centers. This tessellation of the plane was carried out by using AFM images of each sample. The standard nucleation theory predicts that the minimum variance of ISD (σISD) is achieved when it matches the CZD (σCZD). In usual conditions, σISD > σCZD. For a substrate temperature of 200 °C during the Ga droplet deposition, the ISD variance for each sample is well above the σCZD. The fabricated QD thus show a rather broad emission due to the large size dispersion. If the substrate temperature is increased up to 300°C during Ga droplet deposition, it is possible to observe a range of Ga fluxes where σISD becomes comparable to σCZD . In these conditions an extremely narrow emission from the QDs can be achieved. As shown in figure 2, photoluminescence of two samples grown in different conditions (ISD variance much higher than CZD, black line, ISD variance comparable CZD, red line) shows a different full widht half maximum, reaching the value of 20 meV comparable with the best results present in scientific literature for Stranski Krastanow QDs. [1] N.Koguchi, S.Takahashi, T.Chikyow, Journal of Crystal Growth 111, 688 (1991) [2] Venables, Spiller, Hanbuenchen, Nucleation and growth of thin films, Rep. Prog. Phys. 47, 399 (1984)