Self-organization of InAs quantum-dot clusters directed by droplet homoepitaxy.

Recently, droplet heteroepitaxy of GaAs/AlGaAs has attracted much attention due to its demonstrated capability of producing nanostructures with interesting geometrical shapes such as quantum single and double ringlike structures. In the droplet-heteroepitaxy approach, Ga is deposited on an AlGaAs surface to create liquid Ga droplets, and these droplets are exposed to an As flux that transforms the droplets into GaAs nanocrystals. In principle, this same approach can be modified into a similar process of GaAs homoACHTUNGTRENNUNGepitaxy, which also allows one to structure GaAs surfaces at the nanometer scale. In this droplet-homoepitaxy approach, the nanostructures formed by the droplets can then be used to encourage the growth of subsequent heteronanostructures such as self-assembled InAs quantum dots (QDs). In this self-assembly process, the lattice mismatch between InAs and GaAs encourages the spontaneous formation of QDs once InAs deposition reaches a critical thickness. These self-assembled InAs QDs have been extensively investigated for the past decade because of their applications in advanced devices such as lasers, infrared detectors, and quantum computers. However, the stochastic nature of self-assembly only permits a random distribution of InAs QDs on GaAs surfaces, which presents a major obstacle for applications such as quantum computing. Recently, much effort has been put forth to localize QDs on designed regions of substrates. By combining the self-assembly approach with certain lithography techniques, InAs QDs can be restricted to growth in specific substrate regions due to GaAs surface height modulations induced by the lithography. In this paper, however, we propose and demonstrate an approach using only self-organizing techACHTUNGTRENNUNGniques to localize the QDs, a hybrid process of GaAs droplet epitaxy and strain-driven self-assembly of InAs QDs. By using droplet epitaxy, GaAs ACHTUNGTRENNUNG(100) surfaces are structured with nanoscale mounds elongated along the [011] direction. The GaAs mounds then induce localized formations of InAs QDs, and these quantum-dot clusters (QDCs) are observed around the GaAs mounds. The density of QDCs and the number of QDs in the clusters can be adjusted by controlling the growth conditions of the Ga droplets. Photoluminescence (PL) measurements reveal atypically good optical properties of these QDCs. The samples were grown by molecular-beam epitaxy (MBE) on epitaxy-ready GaAsACHTUNGTRENNUNG(100) substrates. All growth began with a 300-nm GaAs buffer layer, which was followed by 10 monolayers (ML) of Ga deposition to form the liquid droplets. Subsequently, the Ga droplets were exposed to an As molecular flux and annealed for 2 min at the substrate temperature of 530 8C to fully crystallize the droplets into GaAs surface nanostructures. At the same substrate temperature of 530 8C, InAs was then deposited until the QDs formed after reaching a critical coverage of 1.7 ML. The resulting surface morphologies were imaged by atomic force microscopy (AFM) under ambient conditions. For PL meaACHTUNGTRENNUNGsurements, the resulting InAs QDs were capped with 100 nm of GaAs. Each PL measurement was performed at a temperature of 10 K by using 532-nm laser excitation. While Ga deposition with a concurrent As molecular flux encourages two-dimensional homoepitaxial growth on GaAs surfaces, Ga deposition without the concurrent As flux results in the formation of liquid droplets, as schematically represented in Figure 1a. After these droplets subsequently react with the As, they are transformed into GaAs nanostructures, normally taking an anisotropic shape due to the dropletsA orientation-dependent surface diffusion on the GaAs crystal surfaces. Figure 1b shows a schematic repre-