Curvature Effects on Optical Response of Si nanocrystals in SiO2 having Interface Silicon Suboxides

TL;DR

This study investigates how the curvature of silicon nanocrystals in SiO2 affects their electronic properties, revealing that interface structure and Si-suboxides significantly influence energy gaps and optical responses.

Contribution

The paper introduces a density functional theory approach to quantify curvature effects on Si nanocrystals and proposes a photoluminescence measurement method to assess interface Si-suboxides.

Findings

Si-suboxides reduce energy gaps in spherical nanocrystals

Planar interfaces with Si-suboxides increase energy gaps due to state delocalization

A >0.2 eV blueshift in photoluminescence is predicted for as-grown samples with Si-suboxides

Abstract

Several models of oxygenated and hydrogenated surfaces of Si quantum wells and Si nanocrystals of variable shapes have been constructed in order to assess curvature effects on energy gaps due to the three Si-suboxides. Si-suboxides in partially oxydized models of nanocrystals of spherical shapes (or quantum dots) are shown to reduce energy gaps compared to the hydrogenated nanocrystals, consistent with previous results in the literature. This trend is shown to be reversed when planar interfaces are formed in Si-NC inside SiO2 thin films, as in Si quantum wells. At planar interfaces (or surfaces) the electronic charge density is shown to become extended and to distribute among the Si-suboxides, thus generating along these planar interfaces extended, or delocalized, states. This delocalization of the electronic states then increase the energy gap compared to equivalent hydrogenated interfaces (or surfaces). Determination of geometric effects of curvature on the band gaps are based on density functional theory (DFT) using the real-space numerical approach implemented in the ``Pseudopotential Algorithm for Real-Space Electronic Structure'' (PARSEC) program. A method for measuring interface effects due to Si-suboxides in Si-NC in SiO2 is also suggested: by comparing photoluminescence (PL) between as-grown and annealed Si-NC samples. The DFT calculations suggest that blueshift of more than 0.2 eV of the PL should be observed in as-grown samples having Si-suboxides at their planar interfaces, in comparison to annealed samples above 950 degree Celcius that have fewer or no Si-suboxides once annealed, providing that the bulk of Si-NC remains unaltered.