Simulation of InAsSb/InGaAs Quantum Dots for Optical Device Applications
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Simulation of InAsSb/InGaAs Quantum Dots for Optical Device Applications Paul von Allmen, Seungwon Lee and Fabiano Oyafuso Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, U.S.A. ABSTRACT Self-assembled InAsSb/InGaAs quantum dots are candidates for optical detectors and emitters in the 2-5 micron band with a wide range of applications for atmospheric chemistry studies. It is known that while the energy band gap of unstrained bulk InAs1xSbx is smallest for x=0.62, the biaxial strain for bulk InAs1-xSbx grown on In0.53Ga0.47As shifts the energy gap to higher energies and the smallest band gap is reached for x=0.51. The aim of the present study is to examine how the electronic confinement in the quantum dots modifies these simple considerations. We have calculated the electronic structure of lens shaped InAs1-xSbx quantum dots with diameter 37 nm and height 4 nm embedded in a In0.53Ga0.47As matrix of thickness 7 nm and lattice matched to an InP buffer. The relaxed atomic positions were determined by minimizing the elastic energy obtained from a valence force field description of the inter-atomic interaction. The electronic structure was calculated with an empirical tight binding approach. For Sb concentrations larger than x=0.5, it is found that the InSb/ In0.53Ga0.47As heterostructure becomes type II leading to no electron confined in the dot. It is also found that the energy gap decreases with increasing Sb content in contradiction with previous experimental results. A possible explanation is a significant variation is quantum dot size with Sb content. INTRODUCTION The energy gap of InAsSb materials lattice matched to InP bridges the 2-5 µm wavelength gap between traditional InGaAsP based optoelectronic devices and quantum cascade lasers. This wavelength region is crucial for the spectroscopy of atmospheric chemical components such as carbon monoxide and methane. InAsSb/InP quantum dots have been grown and characterized by photoluminescence. Emission has been observed up to 2.2 µm [1]. The energy band gap of unstrained bulk InAs1-xSbx is smallest for x=0.62 but biaxial strain for bulk InAs1-xSbx grown on In0.53Ga0.47As shifts the energy gap to higher energies and the smallest band gap is reached for x=0.51, which seems therefore to be the preferred concentration for long wavelength optical devices. The purpose of this work is to examine how the electronic confinement in the quantum dots modifies these simple considerations. The energy levels for the confined electrons and holes are computed using an empirical tight binding model. The relaxed atomic positions in the strained materials are obtained from a valence force field model. THEORY The number of atoms that need to be included in a realistic calculation involving quantum dots can be in the millions and therefore renders first principles approaches to
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computing electronic structure impractical. Alternative approaches such as the envelope function approximation, empirical pseudo-potential and empirical tight bin
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