Nanoscale Res Lett (2006) 1:57–61 59 Fig. 2 AFM image of surface nanostructures formed after 1 s annealing and the corresponding height proﬁles previously [7], droplet epitaxy can be described as GaAs growth supplied from a Ga nano-source (drop- let) under a uniform arsenic ﬂux. From the two line proﬁles in Fig. 2, the heights of the coin edges are different. In particular, the height is about 9 nm along the [01 – 1] direction and about 4 nm along the [011] direction. For a typical GaAs (100) surface, it has been well established that surface diffusion and incorporation are anisotropic [20]. With additional annealing, the mate- rial from the edges of the square holed nanostructure, tend to ﬁll the center hole but at different rates depending on direction. AFM images in Fig. 3a and b show a step-by-step evolution of the surface of the nanostructures after 5 and 15 s of annealing, respec- tively. For example, the ratio of the height of the edges along [01 – 1] compared to [011] are seen to increase from Fig. 3a to Fig. 3b forming a bridged pair after 15 s. Finally, after 45 s of annealing, the bridged nano- structure is encouraged to form a QD pair as shown in Fig. 4a. Additional very small dots indicated by arrows in Fig. 4a are often found to accompany the QD pair. These are the remnants of the edges along [011]. Fig- ure 4b is a 3D presentation of one single QD pair formed after 45 s annealing at 590 C. At this even higher temperature, the QDs in the dot pairs are bigger and more separated. The clear contour lines around each QD pair are steps on the GaAs hill. These QD pairs are very stable, as suggested by the observation at different growth temperatures. How- ever, the QD pairs tend to merge together with further annealing. For example, Fig. 5 shows an AFM image of the QD pair nanostructures after 600 s of annealing. Fig. 3 (a) and (b) are AFM images of surface nanostructures formed after 5 and 15 s annealing, respectively Compared to Fig. 4a, the paired QDs have connected together to form a rod sitting on a GaAs hill. The steps on the GaAs hills appear very clear due to the smoothing effects of a long-annealing period. This behavior is not surprising. The 3D nanostructures observed earlier in Figs. 1–4 form from the non-uni- form supply of Ga from nano droplets. After the Ga droplets are consumed, additional annealing tends to smear away the nanostructures by surface diffusion and incorporation forming the images in Fig. 5. In this sense, as a result of the anisotropic nature of the crystalline surface, we are able to engineer a variety of different shaped nanostructures ranging from Figs. 1 to 5. In contrast to the normal droplet epitaxy at low temperatures, high-temperature droplet epitaxy is