Modeling Nitride based Quantum Dots: Multimillion Atom Tight-Binding Simulations

The theoretical calculation of the electronic structure of any constituent materials is the first step towards the interpretation and understanding of the experimental data and reliable device design. This is essentially true for nanoscale devices where the atomistic granularity of the underlying materials and the quantum mechanical nature of charge carriers play critical role in determining the overall device performance. In this work, within a fully atomistic and quantum-mechanical framework, we investigate the electronic structure of wurtzite InN quantum dots (QDs) self-assembled on GaN substrate. The main objectives are three-fold - (1) to explore the nature and the role of crystal atomicity, strain-field, piezoelectric and pyroelectric potentials in determining the energy spectrum and the wavefunctions, (2) to address the redshift in the ground state, the symmetry-lowering and the non-degeneracy in the first excited state, and the strong band-mixing in the overall conduction band electronic states, a group of inter-related phenomena that has been revealed in recent spectroscopic analyses, and (3) to study the size-dependence of the above mentioned phenomena and the electronic states as a whole. We also demonstrate the importance of three-dimensional (3-D) atomistic material representation, and the need for using realistically-extended substrate and cap layers (multimillion atom modeling) in studying the built-in structural and electric fields in these reduced-dimensional QDs. Models used in this study are as follow: (1) VFF Keating model for atomistic strain relaxation;(2) 20-band nearest-neighbor sp3d5s* tight-binding model for the calculation of single-particle energy states; and (3) microscopically determined polarization constants in conjunction with an atomistic 3-D Poisson solver for the calculation of the piezo- and the pyro- electricity