Photoluminescence of oxygen-deficient defects in germanium oxides: A quantum chemical study

The photoabsorption and photoluminescence (PL) properties of the surface E-' center, -GeX3, and the combined E-'-center-oxygen vacancy, X3Ge-GeX2, defects in substoichiometric germanium oxides have been investigated by high-level ab initio calculations, including complete active space self-consistent field, multireference configuration interaction, and symmetry-adapted cluster configuration interaction methods. Both defects have been shown to give rise to photoabsorption bands between 4 and 6 eV. Geometry relaxation is significant and the Stokes shifts are large for all calculated excited states. A removal of an electron from the Ge-Ge bond leads to its destruction, whereas the creation of an electron hole at lone pairs of O atoms results in elongations of the Ge-O-Ge bonds in the corresponding bridges. Most often, deexcitations of excited electronic states proceed radiationlessly, through crossing points of their potential energy surfaces with those of the lower states. The -GeX3 defect is able to generate several PL bands in the UV (similar to 3 eV) and IR (1.2-1.4 and 0.5-0.6 eV) spectral ranges, whereas the X3Ge-GeX2 defect gives only one red/orange PL band at 2.0-2.1 eV. No intense PL band was found in the blue spectral region of 2.5-2.7 eV, and the two defects are not likely to contribute to the intense blue photoluminescence observed for GeO2 nanowires.