Local-fields and disorder effects in free-standing and embedded Si nanocrystallites

TL;DR

This study investigates the electronic and optical properties of silicon nanocrystallites embedded in SiO2 and free-standing, revealing how interface effects, amorphization, and strain influence their behavior and optical response.

Contribution

It provides a detailed ab-initio analysis of how local fields, disorder, and interface characteristics affect silicon nanocrystals' electronic and optical properties.

Findings

Type I band offset in Si/SiO2 heterojunctions.

Amorphization causes a size-dependent redshift in optical gap.

Local field effects significantly alter optical spectra.

Abstract

The case study of a 32-atoms Si nanocrystallite (NC) embedded in a SiO2 matrix, both crystalline and amorphous, or free-standing with different conditions of passivation and strain is analyzed through ab-initio approaches. The Si32/SiO2 heterojunction shows a type I band offset highlighting a separation between the NC plus the interface and the matrix around. The consequence of this separation is the possibility to correctly reproduce the low energy electronic and optical properties of the composed system simply studying the suspended NC plus interface oxygens with the appropriate strain. Moreover, through the definition of an optical absorption threshold we found that, beside the quantum confinement trend, the amorphization introduces an additional redshift that increases with increasing NC size: i.e. the gap tends faster to the bulk limit. Finally, the important changes in the calculated DFT-RPA optical spectra upon inclusion of local fields point towards the need of a proper treatment of the optical response of the interface region.