First Principle Studies of B and P Doped Si Nanocrystals

Ivan Marri,Stefano Ossicini,Elena Degoli

doi:10.1002/pssa.201700414

Abstract

The properties of n‐ and p‐doped silicon nanocrystals obtained through ab initio calculations are reviewed here. The aim is the understanding of the effects induced by substitutional doping on the structural, electronic and optical properties of free‐standing and matrix‐embedded Si nanocrystals. The preferential positioning of the dopants and their effects on the structural properties with respect to the undoped case, as a function of the nanocrystals diameter and termination, are identified through total‐energy considerations. The localization of the acceptor and donor related levels in the band gap of the Si nanocrystals, together with the impurity activation energy, are discussed as a function of the nanocrystals size. The dopant induced differences in the optical properties with respect to the undoped case are presented. Finally, the case of B and P co‐doped nanocrystals is discussed showing that if carriers are perfectly compensated, the Si nanocrystals undergo a minor structural distortion around the impurities inducing a significant decrease of the impurities formation energies with respect to the single doped case. Due to co‐doping, additional peaks are introduced in the absorption spectra, giving rise to a size‐dependent red shift of the absorption spectra.

Highlights

The advent of nanoscience and nanotechnologies and the consequent scaling down of Si structures to nanometer sizes have opened new interesting opportunities
In particular the possibility to play with the size, shape and surface termination of silicon nanocrystals (Si-NCs) togheter with their low toxicity and good biocompatibility has prompted their application in microeletronics [1, 2], photonics [3, 4], non-linear optics [5, 6], photovoltaics [7,8,9,10,11,12], thermoelectrics [13, 14] and biomedicine [15, 16]
Several studies have revealed that doping in Si-NCs is quite different from their bulk counterparts [17,18,19], for example the ionization of the impurities at room temperature may be strongly quenched with respect to the bulk and a shallow impurity level in bulk may become a deep level at the nanoscale

Summary

First principle studies of B and P doped Si nanocrystals

The aim is the understanding of the effect of substitutional doping on the structural, electronic and optical properties of freestanding and matrix-embedded Si nanocrystals. The preferential positioning of the dopants and its effect on the structural properties with respect to the undoped case, as a function of the nanocrystals diameter and termination, are identified through totalenergy considerations. The dopant induced differences in the optical properties with respect to the undoped case are presented. We discuss the case of B and P co-doped nanocrystals showing that, if carriers are perfectly compensated, the Si nanocrystals undergo a minor structural distortion around the impurities inducing a significant decrease of the impurities formation energies with respect to the single doped case. Structure of a co-doped Si85BPH76 nanocrystal, diameter 1.56 nm. B (magenta, dark gray) and P (black) dopants are here located in subsurface positions

Introduction

Methods