Nanoscale Res Lett (2006) 1:57–61 DOI 10.1007/s11671-006-9002-z N A N O E X P R E S S Self-organization of quantum-dot pairs by high-temperature droplet epitaxy Zhiming M. Wang Æ Kyland Holmes Æ Yuriy I. Mazur Æ Kimberly A. Ramsey Æ Gregory J. Salamo Published online: 25 July 2006 to the authors 2006 Abstract The spontaneously formation of epitaxial GaAs quantum-dot pairs was demonstrated on an AlGaAs surface using Ga droplets as a Ga nano- source. The dot pair formation was attributed to the anisotropy of surface diffusion during high-tempera- ture droplet epitaxy. Keywords Quantum dots Æ Droplet epitaxy Self-assembly of epitaxial semiconductor nanostruc- tures has been an intensive ﬁeld of research. In par- ticular, the Stranski-Krastanov (SK) growth mode based on the use of lattice-mismatched materials has played an important role in the formation of nano- structures, the investigation of quantum conﬁnement effects, and has made possible applications of nano- structures [1–3]. While the SK growth mode has been a very powerful and beautiful technique, there has been Z. M. Wang (&) Æ K. Holmes Æ Y. I. Mazur Æ K. A. Ramsey Æ G. J. Salamo Department of Physics, University of Arkansas, Fayetteville, AR 72701, USA e-mail: zmwang@uark.edu a signiﬁcant, but perhaps less well-known, parallel development using lattice matched materials as an alternative approach for the growth of nanostructures called ‘‘droplet epitaxy’’ [4–7]. Here liquid metal droplets are ﬁrst formed as an intermediate growth step before being converted into semiconductor nanostructures. While the two growth approaches are very different, both the SK and the droplet approach are similar in that they both suffer from the stochastic nature of self-assembly. As a result, the control of spatial ordering of semiconductor nanostructures has been extremely challenging while desirable for appli- cations, such as, the fabrication of quantum-dot (QD) molecules for quantum computing [8]. Consequently, there has been much recent effort to control the lateral arrangement of QDs using a range of techniques, such as, lithography [9–12], templating [13, 14], and modi- ﬁed versions of self-organization [15–19]. Recently, the approach of droplet epitaxy has shown promise to the achieve local ordering of quantum nanostructures [6, 7]. In this letter, we report on the use of droplet epitaxy and anisotropic surface diffusion to fabricate QD pairs. All of the samples used in this study were grown in a molecular beam epitaxy (MBE) system equipped with reﬂection high-energy electron diffraction (RHEED) and a highly accurate (±2 C) optical transmission thermometry system for substrate temperature deter- mination and control. Our growth approach was to ﬁrst grow a 500 nm GaAs buffer layer on a semi-insulating GaAs (100) substrate, followed by a 50 nm thick Al0.3Ga0.7As layer. This was followed by the deposition of Ga and the formation of Ga droplets at the substrate temperature of 550 C with the arsenic source fully closed. We used a valved arsenic source to provide precise and fast control for over three orders of