Atomistic Simulation of III-Nitride Core-Shell QD Solar Cells

Abstract

Multiscale computational simulations are performed to investigate how electronic structure and optical absorption characteristics of recently reported nanostructured III-nitride core-shell multiple quantum well (MQW) solar cells are governed by an intricate coupling of size-quantization, atomicity, and built-in structural and polarization fields. The core computational framework is divided into four coupled phases: i) geometry construction for the wurtzite lattice in polar and nonpolar crystallographic orientations; ii) structural relaxation and calculation of atomistic strain distributions; iii) obtaining the induced polarization and internal potential distributions; iv) computing the single-particle electronic structure and optical transition probabilities using a 10-band sp3 s*-spin tight-binding framework; and v) obtaining the device terminal characteristics using a TCAD toolkit. Special care was taken in incorporating the nonpolar m-plane crystallographic orientation within the simulator via appropriate lattice vectors, rotational matrices, neighboring atom co-ordinates and sp3-hybridized passivation scheme. From the simulations, it is found that, in contrast to some recent studies, atomistic strain in the m-plane structure induces non-vanishing piezoelectric polarization, which leads to a symmetry-lowering of the structure. Nevertheless, the m-plane structure exhibits a stronger overlap and localization of the wavefunctions, as compared to the c-plane counterpart. Overall, the m-plane structure offers higher spontaneous emission rate and quantum efficiency as well as an improved fill-factor.