Transformation of Self-Assembled InAs/InP Quantum Dots into Quantum Rings without Capping Jaakko Sormunen,* Juha Riikonen, Marco Mattila, Jouni Tiilikainen, Markku Sopanen, and Harri Lipsanen Optoelectronics Laboratory, MicronoVa, Helsinki UniVersity of Technology, P.O. Box 3500, FIN-02015 TKK, Finland Received April 7, 2005; Revised Manuscript Received June 21, 2005 ABSTRACT Transformation of self-assembled InAs quantum dots (QDs) on InP(100) into quantum rings (QRs) is studied. In contrast to the typical approach to III-V semiconductor QR growth, the QDs are not capped to form rings. Atomic force micrographs reveal a drastic change from InAs QDs into rings after a growth interruption in tertiarybutylphosphine ambient. Strain energy relief in the InAs QD is discussed and a mechanism for dot-to-ring transformation by As/P exhange reactions is proposed. Ring-shaped semiconductor nanostructures, that is, quantum rings (QRs) or nanorings, have gathered growing attention in recent years. These structures have many interesting electronic properties 1 and have been shown experimentally to confine carriers into ringlike quantum states. 2 QRs also provide a means to study quantum effects involving magnetic flux. 3 Most of the experimental work in this field has been done on In(Ga)As/GaAs QRs. By capping self-organized InAs quantum dots (QDs) by a thin GaAs layer followed by a growth interruption, we find that a dot shape change into ring takes place. 4 Although the detailed mechanism of this transformation is still unknown, two models have been proposed. According to a thermodynamic model, 5 the mate- rial redistribution is due to a change in the InAs dot surface energy caused by the GaAs capping layer. A kinetic model, 4 however, explains ring formation by the different surface diffusion rates of Ga and In atoms. In addition to these models, a study has shown that strain relaxation of the dot has to be taken into account to explain the formation of SiGe/ Si QRs. 6 Other QR material systems also differ from the above growth considerations because CdTe/ZnTe 7 and GaSb/ GaAs 8 QRs have reportedly been achieved by direct deposi- tion. In a recent paper 9 in which InAs/InP QRs were fabricated by partially capping InAs QDs by InP, the experimental data was reported to disagree with the kinetic model. The difference in group III surface mobilities cannot explain the ring formation because, in contrast to InAs/GaAs QRs, both compounds here share the same group III atom. Instead, group V exchange reactions have been shown to be major contributors to the formation 10,11 and shape 12 of InAs nanostructures on InP. In this paper, quantum rings are fabricated by annealing as-grown InAs QDs in a phosphorus ambient. Unlike the typical method, no capping of dots is utilized to achieve the dramatic change in the QD morphology. We also study the effect of temperature on the InAs/InP dot-to-ring transforma- tion. The formation mechanism of QRs is discussed in terms of As/P exchange and the strain of the QD/QR system. The advantage of this QR fabrication process is that it separates the capping step from the ring formation. Thus, the capping of the rings can be studied separately. Moreover, it may allow the use of alternative capping materials, such as ternary or quarternary compounds in the design of InP-based QR structures. The samples were fabricated by metal-organic vapor phase epitaxy (MOVPE) under atmospheric pressure. Hydrogen was used as a carrier gas while utilizing trimethylindium (TMI), tertiarybutylphosphine (TBP), and tertiarybutylarsine (TBA) as precursors. The epitaxial layers were grown on semi-insulating InP(001) wafers. Prior to deposition, the substrates were annealed for 5 min at 650 °C after which a 100-nm InP buffer layer was grown at 640 °C. The temperature was subsequently decreased to 560 °C where 0.6-1.0 monolayers (MLs) of InAs were deposited at a growth rate of 0.8 ML/s. The self-organized growth of coherent InAs/InP QDs by MOVPE is described in more detail elsewhere. 11 Following the deposition of InAs QDs, a 10-s flush in TBA was performed after which the group V * Corresponding author. Tel: +358-9-451-5312; fax: + 358-9-451-3128; e-mail: jaakko.sormunen@hut.fi. NANO LETTERS 2005 Vol. 5, No. 8 1541-1543 10.1021/nl050646v CCC: $30.25 © 2005 American Chemical Society Published on Web 07/08/2005 © 2005 American Chemical Society. Reprinted with permission from Nano Letters 5, 1541-1543 (2005).