Quasi-particle calculations on electronic and optical properties of the peroxy linkage and neutral oxygen vacancy defects in amorphous silica

Abstract

Recently, fused silica has been used to prepare the optical windows in the inertial confinement fusion (ICF) equipment. Challenge of application of fused silica is due to the defect-related optical absorption which is considered as the main mechanism of laser-induced damage process. However, due to structural complexity, calculation of the defect-related absorption from the first principles is only limited to small clusters, and a full treatment using the state of art GW and Bathe-Salpeter equation (BSE) method is still lacking.In this work, density functional theory calculations are performed to study the defect structure of the peroxy linkage (POL) and the neutral oxygen vacancy (NOV) defects in amorphous silica. Firstly, well relaxed structure is generated by using a combination of the bond switching Monte Carlo technique and the DFT-based structure optimization. Secondly, the defect structures are generated and studied in both the ground singlet (S0) and the first excited triplet (T1) states. Finally, the electronic and optical properties of the considered structures are studied by applying the self-consistent quasi-particle GW (sc-QPGW) and the BSE methods in Tamm-Dankoff approximation.In the ground state S0, the POL defect is found to be stable and shares a similar local structure to the H2O2 molecule. However, in T1 state, the POL defect breaks into a pair of E' center ( - Si ) and peroxy oxygen radial ( O-O-Si-). For the NOV defect, the optimized Si-Si bond length in the ground state is 2.51 with a variation of 0.1 due to the structural disorder. In comparison to the ground state, the optimized Si-Si bond length in T1 state increases to 3.56 .The scGW/BSE calculation on the defect free structure predicts a quasi particle band gap of 10.1 eV and an optical band gap of 8.0 eV, which are consistent well with the available experimental results. For the POL defect, the scGW/BSE calculation reveals a weak exciton peak at 6.3 eV. Below 6.3 eV, no new exciton peak is found, implying that the experimentally suggested 3.8 eV peak could not be attributed to the POL defect. Calculations of the NOV defect gives a strong and highly polarized optical absorption peak at 7.4 eV which is close to the previous experimental result at 7.6 eV. The structural relaxation induced by NOV also contributes to another absorption peak at 7.8 eV.