Crystal Phase Effects in Si Nanowire Polytypes and Their Homojunctions

Recent experimental investigations have confirmed the possibility to synthesize and exploit polytypism in group IV nanowires. Driven by this promising evidence, we use first-principles methods based on density functional theory and many-body perturbation theory to investigate the electronic and optical properties of hexagonal–diamond and cubic–diamond Si NWs as well as their homojunctions. We show that hexagonal–diamond NWs are characterized by a more pronounced quantum confinement effect than cubic–diamond NWs. Furthermore, they absorb more light in the visible region with respect to cubic–diamond ones and, for most of the studied diameters, they are direct band gap materials. The study of the homojunctions reveals that the diameter has a crucial effect on the band alignment at the interface. In particular, at small diameters the band-offset is type-I whereas at experimentally relevant sizes the offset turns up to be of type-II. These findings highlight intriguing possibilities to modulate electron and hole separations as well as electronic and optical properties by simply modifying the crystal phase and the size of the junction.
We thank L. Vincent for fruitful discussions. We acknowledge financial support by the Ministerio de Economía y Competitividad (MINECO) under grant FEDER-MAT2013-40581-P and the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496 and by the Generalitat de Catalunya under grants no. 2014 SGR 301. M.A. and T.K. acknowledge support from the Nanodesign project "Nanoharvesting" funded by the IDEX Paris-Saclay (ANR-11-IDEX-0003-02). Part of this work was made possible thanks to the HPC resources of IDRIS under the allocation i2015097422 and i2016097531 made available by GENCI (Grand Equipement National de Calcul Intensif) and the computer resources, technical expertise, and assistance provided by the Red Española de Supercomputación. M.P. acknowledges the EC for the RISE Project No. CoExAN GA644076 and CINECA for ISCRA-B Project No.HP10B0LUWN, for computational resources.