Morphology of Self-Assembled InAs Quantum Dots on GaAs(001).
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Morphology of Self-Assembled InAs Quantum Dots on GaAs(001). F.Arciprete, F.Patella, M.Fanfoni, S.Nufris, E.Placidi, D.Schiumarini, and A.Balzarotti Dipartimento di Fisica, Università di Roma "Tor Vergata", and Istituto Nazionale per la Fisica della Materia, Via della Ricerca Scientifica 1, 00133 Roma, Italy
ABSTRACT We have followed by Atomic Force Microscopy (AFM) the epitaxial growth of InAs on GaAs(001) starting from the initial formation of a strained two-dimensional wetting layer up to the self-assembled nucleation and growth of 3D nanoparticles. In this work we underline many aspects of the morphology of this system, which substantiate the role either of kinetics on thermodynamics in the process of growth as well as the role of surface instabilities in controlling lateral ordering of the nanoaggregates.
INTRODUCTION Self-organization of nanostructures is a promising way to produce Quantum Dots (QDs) with a narrow distribution of size in presence of lattice mismatch. Epitaxial Stranski-Krastanov (SK) growth on a substrate starts usually with the formation of a pseudomorphic strained layer, referred to as the wetting layer (WL), and proceeds with the spontaneous formation of dots above a critical thickness, θc. The strain energy accumulated in the thicker layers is partially relieved by misfit dislocations (MD) or by the formation of coherent, dislocation-free QDs. The morphology (size and shape) and density of these dots depend on the interplay of thermodynamic and kinetic effects, which, in turn, vary with the growth parameters, and the details of the heteroepitaxy (substrate temperature, growth rate, flux ratio, etc.). To simplify the matter, the structural properties of the nanostructures are often discussed in terms of an equilibrium phase diagram based on the minimization of the free-energy in a restricted space of parameters [1]. In this paper we focus on the InAs/GaAs(001) system having a lattice mismatch as high as 7%. It is clearly important to identify and understand the kinetic pathways to island formation, particularly at the atomic scale. The growth of InAs on GaAs(001) does not proceed following a pure SK mode since, for equal InAs depositions on substrates at increasing temperature, larger fractions of WL and substrate participate to the formation of the dots [2-3,11]. We have grown samples of InAs on GaAs(001) at different coverage below and above the critical thickness by molecular beam epitaxy (MBE) either in modified migration enhanced (MEG) and conventional continuous (CG) growth modes in order to vary the migration length of In atoms keeping the other growth parameters fixed. The AFM analysis evidenced how different growth procedures that effectively alter relevant kinetic factors - in the present case the migration length of the element III - strongly affect the final structure and composition of the film. This result substantiates the overwhelming role of kinetics on thermodynamics in the non-equilibrium MBE growth. Large scale morphology of the epitaxial GaAs(001) has also been studie
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