Concentric Multiple Rings by Droplet Epitaxy: Fabrication and Study of the Morphological Anisotropy
© The Author(s) 2010
Received: 9 July 2010
Accepted: 13 July 2010
Published: 1 August 2010
We present the Molecular Beam Epitaxy fabrication of complex GaAs/AlGaAs nanostructures by Droplet Epitaxy, characterized by the presence of concentric multiple rings. We propose an innovative experimental procedure that allows the fabrication of individual portions of the structure, controlling their diameter by only changing the substrate temperature. The obtained nanocrystals show a significant anisotropy between  and [1–10] crystallographic directions, which can be ascribed to different activation energies for the Ga atoms migration processes.
KeywordsGaAs/AlGaAs Molecular beam epitaxy Droplet epitaxy Quantum rings
In recent times, Droplet Epitaxy (DE) [1, 2] has extensively been used for the fabrication of III–V semiconductor quantum nanostructures that show a good rotational symmetry. Indeed with this MBE-based technique, many different quantum systems with cylindrical symmetry can be grown, ranging from quantum rings (QRs) [3, 4], to concentric multiple quantum rings (CMQRs) , to coupled rings/disks (CRDs) . The interest in fabricating such structures is both fundamental, for the investigation of quantum interference phenomena [7, 8] and practical, in the fields of optoelectronics  and quantum information technology . In GaAs DE, the fabrication of the compound semiconductor is basically achieved in two steps: first, Ga molecular beam is supplied on the substrate in absence of As, for the formation of nanometre-size droplets and then, an As flux is used to transform the droplets into GaAs. Depending on the conditions of the crystallization step, many different shapes of the GaAs nanocrystals can be obtained. In particular, caused by the surface migration of Ga atoms from the originally formed droplet during the As molecular beam supply, a lateral growth of GaAs will be observed, which gives rise to the formation of outer rings or disks. By exploiting this lateral growth, here we propose a fabrication method for the formation of GaAs concentric multiple rings, based on sequential short-time (few tenth of seconds) As molecular beam supplies at different substrate temperatures. Compared to the standard DE, the innovation of our growth protocol consists in supplying only a small amount of As atoms to achieve a partial crystallization of the available Ga, which is stored in the droplets; after changing the substrate temperature, the procedure is repeated.
Just after the growth, the sample was cooled at room temperature and the surface morphology was investigated by means of Atomic Force Microscopy (AFM).
Results and Discussion
After the procedure of Ga molecular beam supply, numerous nearly hemispherical Gallium droplets were formed, with an average diameter of around 160 nm and a density of around 1 × 108 cm−2. The size distribution of the Ga droplets is estimated to be around 10%. After the crystallization procedure, well-defined GaAs multiple ring structures appeared. It is worth noticing that inner ring diameter is nearly equal to that of the original Ga droplet and that the density of the GaAs structures is equal to that of the original droplets, confirming that all Ga droplets transformed into GaAs multiple rings at the end of the procedure.
As already described in Ref. , in concentric multiple rings structures, the inner ring has a different origin compared to the outer ones. Indeed, the inner ring is formed just after the Ga droplets formation, caused by a partial dissolution of As coming from the substrate or from the background inside the liquid gallium. This was confirmed by fabricating a sample where no As was intentionally supplied after the formation of Ga droplets and by subsequent selective chemical etching in order to remove only the metallic gallium, without damaging the GaAs structure. In this sample, a tiny GaAs ring was clearly found at the droplets edge. Therefore, we can conclude that the inner ring present at the centre of the final structure is the result of the accretion of the tiny ring, marking the original droplets perimeter. On the contrary, every outer ring was found to be formed as a result of each As supply, caused by the interplay between the adsorption of arsenic atoms on the substrate surface and the simultaneous migration of Ga atoms, which depart from the original droplets. Being the surface diffusion of Ga atoms a thermally activated process, depending exponentially on the substrate temperature, larger diameter for the outer rings will correspond to higher temperatures. Therefore, in our experimental procedure, the outer rings are the first to be formed, caused by the larger diffusion length of gallium atoms at 380°C.
We have shown the fabrication method and the morphological analysis of GaAs concentric multiple rings structures grown in by DE. A five ring structure was created by exploiting the lateral growth of GaAs around the originally formed Ga droplets. Controlling the substrate temperature during the arsenic supply for the crystallization, it is possible to govern the diffusion length of Ga atoms and therefore each ring diameter. The nanostructures showed an anisotropy along  and [1–10] directions, and two different values of the activation energy for the diffusion of gallium atoms along these directions were obtained.
An erratum to this article can be found athttp://dx.doi.org/10.1007/s11671-010-9816-6
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