Carrier trapping centers in a two-dimensional silica bilayer: Strongly localized shallow gap states and resonances induced by oxygen vacancies

Abstract

Structural and electronic properties of neutral native defects in a two-dimensional ${\mathrm{SiO}}_{2}$ bilayer (2D-${\mathrm{SiO}}_{2}$) are examined using the SIESTA ab initio approach. We identify scissor and rotation modes of oxygen atoms, in the middle of Si-O-Si chains, as low-energy structural excitations responsible for the response of the 2D-${\mathrm{SiO}}_{2}$ lattice to the formation of all native defects in the present study. Furthermore, we find that oxygen native defects (single vacancies and interstitials) should be the most abundant defect species in thermal-equilibrated samples. Regarding native-defect electronic states, single oxygen vacancies and interstitials are found to be amphoteric trapping centers in 2D-${\mathrm{SiO}}_{2}$. Silicon vacancies and interstitials introduce several strongly localized states spanning a large fraction of the gap. Generally, we identify a marked tendency for the appearance of strongly spatially localized defect states, be their energies shallow or deep within the band gap, as well as the emergence of strongly localized resonances in the valence and/or conduction bands. Strong spatial localization of defect states, a hallmark of systems displaying trapping and polaronic effects, is a result of quantum confinement and enhanced Coulombic effects in this 2D system. Carrier trapping and polaron formation are thus expected to be a common feature of defect states and added carriers in 2D-${\mathrm{SiO}}_{2}$.