New Tools to Control Morphology of Self-Organized Quantum Dot Nanostructures
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ABSTRACT We consider several approaches to control morphology of self-organized quantum dot (QD) nanostructures. (i) We study effects of temperature and of temperature ramping on formation of QD arrays. The theory of equilibrium distribution of island volumes is developed predicting an entropy-driven decrease of island volume at higher temperatures. Experiments on InAs/GaAs(001) obtained both at submonolayer deposition and in Stranski-Krastanow (SK) growth mode reveal the decrease of island volume with temperature increase that agrees with the thermodynamic picture of island formation. (ii) We show a reversibility of temperature-driven changes in island volume, shape, and density for SK InAs/GaAs(001) islands and a new possibility to control QDs. (iii) We consider an advanced way of formation of complex QD structures. For multisheet arrays of strained islands a transition between correlation and anticorrelation driven by the spacer thickness is predicted theoretically and confirmed experimentally. (iv) The overgrowth of InAs/GaAs islands by an InGa(Al)As alloy leads to alloy phase separation in the capping layer and an effective increase of both the lateral size and the height of the QDs. These complex growth approaches enable us to tune efficiently electronic spectra of the QD systems. INTRODUCTION The recent breakthrough in quantum dot (QD) technology allowing a new generation of semiconductor devices such as lasers, photodetectors, infrared emitters, single electron transistors, etc., relies on effects of self-organization at semiconductor surfaces [1, 2]. A high research activity in this area is related. first, to the fact that very basic effects in QD formation are still to be understood. Second, complexity of QD structures is steadily increasing according to necessary device design ensuring the control of wavelength, polarization, oscillator strength, gain, etc. Here we consider several approaches to control morphology of self-organized QD structures. A remarkable effect observed in many systems is a high uniformity of QDs in shape and size and the existence of a limiting size of the QDs. This apparently contradicts to the classical picture of Ostwald ripening where large islands grow in volume at the expense of the vanishing of small islands [3]. In this connection, one of key basic issues of the QD formation is to explain the effect and to understand the relative role of thermodynamics and kinetics in the process of self-organized formation of strained islands which is at present a subject of intense debates [2]. In the present paper, we develop a theory of an equilibrium distribution of island volumes for arrays of flat islands of a fixed height. This is a simple model system which allows us to emphasize the role of entropy in island formation and to propose an experimental tool for distinguishing thermodynamically-controlled arrays of islands from kinetically-controlled ones. The model of two-dimensional (2D) islands is adequate for heteroepitaxial InAs/GaAs and CdSe/ZnSe systems at submonolayer depositio
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