IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 5, SEPTEMBER/OCTOBER 2002 1025 Self-Assembled Nanoholes, Lateral Quantum-Dot Molecules, and Rolled-Up Nanotubes O. G. Schmidt, Ch. Deneke, S. Kiravittaya, R. Songmuang, H. Heidemeyer, Y. Nakamura, R. Zapf-Gottwick, C. Müller, and N. Y. Jin-Phillipp Invited Paper Abstract—We present a detailed investigation of novel strain-driven semiconductor nanostructures. Our examinations include self-assembled nanoholes, lateral quantum-dot (QD) molecules, and rolled-up nanotubes. We overgrow InAs QDs with GaAs and apply atomically precise in situ etching to fabricate homogeneous arrays of nanometer-sized holes with diameters of 40 to 60 nm and depths up to 6.2 nm. The structural properties of the nanoholes can be precisely tuned by changing the QD capping thickness and the in situ etching time. We show that strain fields surrounding the buried quantum dots drive the nanohole formation process. We overgrow the nanoholes with 0.2- to 2.5-ML InAs and observe the formation of compact lateral InAs QD molecules. The number of QDs involved in a lateral QD molecule can be tuned from two to six by changing the growth temperature. Our systematic photoluminescence study documents the QD molecule formation process step by step and helps to interpret our structural results. We also present the fabrication of laterally aligned lateral QD bimolecules by growing InGaAs on a GaAs (001) substrate patterned with a square array of nanometer sized holes. Charge carriers in such bimolecules might serve as quantum gates in a future semiconductor based quantum computer. Furthermore, we release strained semiconductor bilayers from their surface to fabricate individual rolled-up semiconductor micro- and nanotubes. We control the diameter of strain-driven In(Ga)As–GaAs tubes from the nanometer to micrometer range by simply changing the layer thicknesses and built-in strain. We propose to roll in metal strip lines to fabricate nanocoils and nanotransformers. To support our proposition, we fabricate homogeneous single and twin GaInP tubes. We present a straight GaInP microtube of more than 2 mm in length and a length-to-diameter ratio of about 2000, thus, elucidating the great potential of this technology. Index Terms—Hollow channel, inductor, MEMS, nanotubes, NEMS, quantum computing, quantum dot molecules, quantum dots, quantum gate, transformer, twin-tube. Manuscript received June 4, 2002; revised July 29, 2002. O. G. Schmidt, Ch. Deneke, R. Songmuang, H. Heidemeyer, R. Zapf-Gottwick, and C. Müller are with the Max-Planck-Institut für Festkörper- forschung, Stuttgart D-70569, Germany (e-mail: O.Schmidt@fkf.mpg.de). S. Kiravittaya is with the Chulalongkorn University, Bangkok 10330, Thailand. Y. Nakamura is with The Femtosecond Technology Research Association, Ibaraki 300-2635, Japan. N. Y. Jin-Phillipp is with the Max-Planck-Institut für Metallforschung, Stuttgart D-70569, Germany. Digital Object Identifier 10.1109/JSTQE.2002.804235 I. INTRODUCTION I N RECENT years, strained layer epitaxy has focused on the formation of self-assembled quantum-dot (QD) heterostruc- tures in a variety of different material systems [1]. Several devices that exploit the properties of a single or a well-defined number of nanostructures have been realized or proposed. This includes single-photon sources [2], QD qubits [3]–[5], and more conventional devices such as resonant tunneling diodes [6], [7] and field-effect transistors [8], [9]. An integration of single QD devices on a single chip seems possible, since the controlled long-range alignment of self-assembled nanostructures into two-dimensional (2-D) and three-dimensional (3-D) arrays has been accomplished on Si (001) and GaAs (001) surfaces [10]–[14]. Our recent efforts [10], [13], [14] to laterally and vertically align self-assembled Ge islands on flat Si (001) and In(Ga)As quantum dots on flat GaAs (001) are summarized in Fig. 1. The lateral alignment is based on the fact that a strained superlattice can translate the initial surface modulation of a prepatterned substrate into an ordered strain field modulation at the superlattice growth front, which eventually induces the formation of laterally aligned quantum dots. In Fig. 1(a), the superlattice consisted of strained undulated SiGe–Si quantum wells, whereas in Fig. 1(b)–(d), we used In(Ga)As–GaAs QD layers to create a 3-D QD crystal. There is great interest to not only fabricate single isolated quantum dots but also lateral QD molecules, since such molecules would theoretically allow the fabrication of, e.g., quantum gates in a quantum computer [3]–[5]. Only a few reports exist on clusters of laterally closely spaced QDs [10], [12], [14], [15], which in most cases occur during the growth on patterned substrates. In Fig. 1(b), for example, we find that several nucleation sites are decorated with two closely spaced InAs quantum dots. QD multimolecules, grown on patterned substrates, tend to incorporate straight QD chains [10], [12], [14]. Recently, the formation of SiGe hut clusters around the edges of square pits have been reported [15]. In this paper, we use InAs QD growth in combination with atomically precise in situ etching [16]–[18] to produce size ho- mogeneous arrays of self-assembled nanoholes and compact lateral QD molecules. The formation process is systematically studied with atomic force microscopy (AFM) and photolumi- nescence (PL), and it is shown that the number of QDs in the QD molecule can be tuned from two to six by simply changing 1077-260X/02$17.00 © 2002 IEEE