Coupled quantum nanostructures formed by droplet epitaxy T. Mano ⁎ , T. Noda, M. Yamagiwa, N. Koguchi Nanomaterials Laboratory, National Institute for Materials Science (NIMS) 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan Available online 27 January 2006 Abstract We demonstrate self-assembly of GaAs double quantum dots (DQDs) by droplet epitaxy in a lattice-matched system in addition to concentric quantum double rings (CQDRs). The growth mechanism of these complex nanostructures is understood by taking into account the two crystallization processes; the counter-migration (of Ga and As atoms)-induced crystallization and droplet-edge-enhanced crystallization. By tuning the balance between these two processes, completely different types of the coupled quantum nanostructures are created. © 2005 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.07.Ta; 81.07.Vb; 81.15.Hi; 68.55.Jk Keywords: Droplet epitaxy; GaAs; Molecular beam epitaxy; Quantum dot 1. Introduction Recently, coupled quantum dots (QDs) systems, often referred to as artificial molecules, have attracted great attention not only for quantum computing and spintronics applications but also for the studies of basic physics in molecular-bonded quantum nanostructures [1]. In contrast to the drastic develop- ments of theoretical works and measurement techniques, how- ever, many hurdles still remain in fabricating these complex nanostructures. Since fully lithography based methods generally impose the limitation of spatial resolution and crystal quality, ‘Stranski–Krastanow’ growth mode or its combination with various techniques have been widely used in lattice-mismatched systems, such as QD growth on artificially patterned substrates [2,3] or on self-organized templates [4,5], or vertical stacking of these strained QDs [6,7]. These techniques, however, can be only applied to the lattice-mismatched systems and, moreover, the strain fields in these nanostructures complicate their elec- tronic structures and their physical properties [8]. Therefore, it is also desired to develop self-assembling methods for unstrained coupled quantum nanostructures in lattice-matched systems. We have developed droplet epitaxy for the formation of high quality self-assembled nanostructures in lattice-matched sys- tems [9,10]. Droplet epitaxy can be widely applied to the various compound semiconductor systems because the mech- anism is based on the formation of liquid metal particles (droplets) and their crystallization. While droplet epitaxy has been used mainly for the formation of simple QDs, this method has a high potential for the growth of more complex nano- structures due to the unique properties of droplets during the crystallization [11–13]. In this paper, we realize GaAs double quantum dots (DQDs) by droplet epitaxy in lattice-matched system in addition to con- centric quantum double rings (CQDRs) [13]. By simply controlling the intensity of As 4 flux supplied to the hemispherical Ga droplets, we obtain controllability between the two types of GaAs coupled quantum nanostructures. The formation mecha- nism of these complex nanostructures is understood by taking into account the balance between two crystallization processes. 2. Experimental procedure The samples were grown by conventional solid-source molecular beam epitaxy (MBE) on semi-insulating GaAs (100) substrates. For the precise control of As 4 flux intensity, a valved-cell was used for the As source. After the growth of a 400 nm thick Al 0.3 Ga 0.7 As buffer layer at 580 °C, As-stabilized c(4 × 4) surface was formed by reducing the substrate temper- ature to 200 or 350 °C. For the Ga droplet formation, nominally 3.75 ML (monolayer) Ga (0.5 ML/s) was supplied to the c(4 × 4) surface at these temperatures without As 4 flux [10]. On the c(4 × 4) surface, there is 1.75 ML of excess As layer. The first 1.75 ML of Thin Solid Films 515 (2006) 531 – 534 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +81 29 859 2790; fax: +81 29 859 2701. E-mail address: MANO.Takaaki@nims.go.jp (T. Mano). 0040-6090/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.289