Effects of spontaneous and piezoelectric polarization fields on the electronic and optical properties in GaN/AlN quantum dots: multimillion-atomsp3d5s*tight-binding simulations

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GaN quantum dots QDs grown on AlN substrates are strong candidates for UV and near-infrared applications. The wurtzite crystal symmetry in these dots induces internal fields arising from the following: i crystal atomicity;ii strained active region; iii piezoelectricity; and iv spontaneous polarization pyroelectricity. Accurate modeling of electronic and optical properties of these QDs must capture the interplay of these atomistic and long-range fields and the size quantization on an equal footing. In this work, single-particle electronic structure and interband optical transition rates of a GaN/AlN QD grown along the c-axis are studied using a coupled molecular mechanics-atomistic 20-band sp3d5s* tight-binding VFF-TB framework. To calculate piezoelectricity, a recently reported model that takes into account both the linear and the nonlinear dependence of polarization on the strain tensors has been employed. The simulated GaN/AlN dot is realistically sized containing ~3 million atoms and of hexagonal disk shape having height and base length of 4.1 and 17nm, respectively. It is found that, in contrast to the well-studied InN/GaN systems, the pyroelectric potential in GaN/AlN dot is larger than the piezoelectric counterpart, and the effects of piezoelectric and pyroelectric fields add up. The internal fields result in a large redshift in the electronic states near the Brillouin zone center known as quantum confined Stark effect, pronounced non-degeneracy in the excited states, strongly suppressed optical transition, and anisotropic emission spectra. Copyright © 2014 John Wiley & Sons, Ltd.