熔石英中过氧缺陷及中性氧空位缺陷的几何结构、电子结构和吸收光谱的准粒子计算

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

本文使用密度泛函理论研究了熔石英中peroxy linkage(POL)缺陷和中性氧空位(NOV)缺陷的几何结构,电子结构以及光学性质.采用自洽的准粒子GW计算结合求解Bathe-Salpeter方程的多体理论,研究了缺陷引起的电子结构和光学吸收谱的变化.首先研究了无缺陷非晶结构的电子结构与吸收谱,得到的结果与实验值非常接近.对POL的计算表明,其在基态下的局部结构与过氧化氢分子类似.采用多体理论计算得到的吸收谱表明其最低吸收峰位于6.3 eV处.这一结果不支持实验认为的位于3.8 eV处的吸收峰是由POL缺陷导致的说法.对于NOV缺陷,计算表明其基态的Si—Si键长为2.51?而三重态下的值则为3.56?.相应的GW+BSE计算表明中性氧空位缺陷导致了位于7.4 eV处的吸收峰,与实验测量结果一致.

Recently, fused silica has been used to prepare the optical windows in the inertial confinement fusion （ICF）equip- ment. 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 （So） 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 So, 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 A with a variation of ＋0.1 A due to the structural disorder. In comparison to the ground state, the optimized Si--Si bond length in T1 state increases to 3.56 A. The scGW/BSE calculation on the defect free structure predicts a quasi particle band gap of 10.1 eV and an op- tical 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.