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Recently, optical emitters using InGaN nanostructures (quantum dots and nanowires) have attracted much attention for applications in lasers, solid-state lighting, solar cells, consumer displays, as well as diagnostic medicine and biological imaging. Compared to conventional bulk and quantum well (QW) structures, nanostructured devices offer several benefits as follow: 1) Strain in nanostructures is, to a large extent, relaxed and, therefore, threading dislocations can be smaller leading to increased quantum efficiency; 2) The fact that the concentration of strain-induced defects is small in nanostructures allows the use of higher indium content and more design freedom in bandgap engineering in the device, which potentially could lead to full-spectrum LEDs (as well as solar cells); and 3) Nanostructures used in the active region of optical devices provide improved electron confinement (due to strongly peaked energy dependence of density-of-states) and thus higher temperature stability of threshold current and luminescence.
The great majorities of InN/GaN nanostructures crystallize in the thermodynamically stable wurtzite symmetry and are grown along the polar  direction. Since the heteroepitaxy of InN on GaN involves a lattice mismatch of ~11%, these structures generally exhibit atomically inhomogeneous and long-range internal structural and electrostatic fields originating mainly from: (i) the fundamental crystal atomicity and the interface discontinuity between two dissimilar materials, (ii) atomistic strain distribution, (iii) piezoelectricity, and (iv) spontaneous polarization (pyroelectricity). The magnitude of the electrostatic built-in field has been estimated to be on the order of MV/cm. Such fields spatially separate the electrons and holes, which leads to a reduction in the optical transition rate (enhanced radiative lifetimes) and pronounced polarization anisotropy in optical emission/absorption. Therefore, electronic and optical properties of these nanostructures are expected to be strong functions of an intricate interplay between the atomistic structural fields and the quantum mechanical size quantization effects.
The nanoSSL simulator allows one to study the electronic bandstructure and optical properties of wurtzite GaN/InN/GaN disk-in-wire structures. Using the simulator one can: (i) Explore the origin and nature of various built-in fields including crystal atomicity, strain fields, piezoelectric, and pyroelectric potentials; (ii) Quantify the role of these internal fields on the electronic bandstructure in terms of shift in energy levels and split (non-degeneracy) in the excited P states, and (iii) Demonstrate how the atomistically-calculated electronic structures lead to strongly suppressed optical transitions and pronounced growth-plane optical polarization anisotropy in these emerging reduced-dimensionality LEDs.
QuaDS 3-D and NEMO 3-D
This work was supported by National Science Foundation Grant No. 1102192.
Support for the computational resources from the ORAU/ORNL High-Performance Computing Grant 2009 and NSF CRI 0855221 are also acknowledged.
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