Shape Transition in Self-Organized InAs/InP Nanostructures
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Shape Transition in Self-Organized InAs/InP Nanostructures H.R. Gutiérrez, M.A. Cotta, M.M.G. de Carvalho. Instituto de Física Gleb Wataghin, DFA/LPD, UNICAMP, CP 6165, 13081-970 Campinas-SP, Brazil. ABSTRACT In this letter we report the transition from self-assembled InAs quantum-wires to quantumdots grown on (100) InP substrates. This transition is obtained when the wires are annealed at the growth temperature. Our results suggest that the quantum-wires are a metastable shape originated from the anisotropic diffusion over the InP buffer layer during the formation of the first InAs monolayer. The wires evolve to a more stable shape (dot) during the annealing. The driving force for the transition is associated with variations in the elastic energy and hence in the chemical potential produced by height fluctuations along the wire. The regions along the wires with no height variations are more stable allowing the formation of complex, self-assembled nanostructures such as dots interconnected by wires.
INTRODUCTION InAs nanostructures in an InP matrix (mismatch ~ 3.2%) have received much attention in the last years. Their potential application in light-emitting devices in the 1.3-1.55 µm wavelength range makes it necessary to understand and control the formation of these structures. Both InAs wires and dots grown on InP substrates with random distribution have been reported1-3. Recent results suggest that the surface morphology2 and/or the chemical composition4 of the buffer layer (InP, InAlAs, or InGaAs) determine the final shape of the InAs nanostructures. Nevertheless, the conditions and mechanisms that originate each kind of nanostructure are not clear. Dots have been shown to form only on smooth surfaces, with no anisotropies in the buffer layer morphology. When this was not the case, InAs wires were obtained2. However, in other work InAs dots were obtained for both smooth (2D) as well as highly anisotropic in shape, rippled buffer layers1. In this letter we report the transition from self-assembled InAs quantum-wires (QWr) to quantum-dots (QD) grown on (001) InP substrates. We observe that this shape transition is obtained when the QWr’s are annealed at the growth temperature, for both smooth and rippled InP buffer layer surfaces. Our results suggest that the QWr is a metastable shape resulting from anisotropic diffusion over the InP buffer layer during the formation of the first InAs monolayer. The wires evolve to a more stable shape (dot) during annealing. EXPERIMENTAL DETAILS The samples were grown by Chemical Beam Epitaxy (CBE) using trimethylindium (TMI) diluted with H2 carrier gas as the group III source and thermally decomposed phosphine (PH3) and arsine (AsH3) as group-V sources. The substrate native oxides were removed by heating the substrate for 10 minutes at 535oC in the growth chamber under P2 overpressure. Two set of samples were prepared, the first consisting of 3 ML of InAs deposited on an InP buffer layer at N5.7.1
490oC and 0.25 Å/s. The second set was prepared under the same conditions but,
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