Effects of atomicity and internal polarization on the electronic and optical properties of GaN/AlN quantum dots: Multimillion-atom coupled VFF MM-sp³ d⁵ s^∗ tight-binding simulations

Single-particle electronic structure and optical transition rates between the HOMO and LUMO states of a self-organized wurtzite GaN/AlN single quantum dot grown along the [0001] axis are calculated within an atomistic 20-band sp³ d⁵ s* tight-binding framework. The GaN/AlN quantum dot used in this computational study is realistically-sized (containing ~9 million atoms) and of truncated pyramid shape having height and base length of 4.5 nm and 23 nm, respectively. These reduced-dimensionality III-N structures are subject to competing effects of size-quantization and long-range internal fields that originate from: a) fundamental crystal atomicity and the interface discontinuity between two dissimilar materials; b) atomistically strained active region; c) strain-induced piezoelectricity; and d) spontaneous polarization (pyroelectricity). The mechano-electrical internal fields in the structure have been modeled using a combination of an atomistic valence force-field molecular mechanics (VFF MM) approach and a three-dimensional Poisson solver, and have found to strongly modulate the intrinsic single-particle electronic and optical properties of the quantum dots. In particular, in contrast to the well-studied InN/GaN systems, the effects of piezoelectric and pyroelectric fields add up (peak pyroelectric potential being larger than the piezoelectric counterpart) and result in a large redshift in the electronic bandgap near the Brillouin zone center (known as quantum confined stark effect), pronounced non-degeneracy in the excited states, strongly suppressed optical transition (increased recombination time), and anisotropic emission spectra.