Electronic structure and optical properties of amorphous GeO2 in comparison to amorphous SiO2

Abstract

Amorphous germania (a-GeO2) is an excellent glass former of great industrial and scientific importance. However, in comparison with a-SiO2, its structure and fundamental properties were less well studied. Using a large near-perfect continuous random network (CRN) model with 1296 atoms and no over- or under-coordinated atoms, we have investigated the structural, electronic, and optical properties of a-GeO2 glass. Our results show that the bond length and bond angle distributions in a-GeO2 are larger than in a-SiO2. The gross features of the electronic density of states in a-GeO2 are similar to a-SiO2, but the GeO bonds are weaker than SiO bonds as reflected in the lower calculated total bond order density. The average tetrahedral angle (θ) and bridging angle (φ) are smaller in a-GeO2 than in a-SiO2. The calculated optical absorption spectrum shows two distinctive peaks in excellent agreement with experiment. The calculated refractive index of a-GeO2 (n=1.69) is also in close agreement with the measured value. In contrast to a-SiO2, there is no clear evidence of excitonic peak in a-GeO2. These a-GeO2 and a-SiO2 models could be used as a prototype for other investigations of these glasses or their mixtures containing defects, substitutional impurities and in the form of vitreous nano-particles.