First-Principles Study on Structural Properties of GeO2 and SiO2 under Compression and Expansion Pressure

The detailed analysis of the structural variations of three GeO 2 and SiO 2 polymorphs ($\alpha$-quartz, $\alpha$-cristobalite, and rutile) under compression and expansion pressure is reported. First-principles total-energy calculations reveal that the rutile structure is the most stable phase among the phases of GeO 2 , while SiO 2 preferentially forms quartz. GeO 4 tetrahedras of quartz and cristobalite GeO 2 phases at the equilibrium volume are more significantly distorted than those of SiO 2 . Moreover, in the case of quartz GeO 2 and cristobalite GeO 2 , all O--Ge--O bond angles vary when the volume of the GeO 2 bulk changes from the equilibrium point, which causes further deformation of tetrahedra. In contrast, the tilt angle formed by Si--O--Si in SiO 2 markedly changes. This flexibility of the O--Ge--O bonds reduces the stress at the Ge/GeO 2 interface due to the lattice-constant mismatch and results in the low defective interface observed in the experiments [Matsubara et al. : Appl. Phys. Lett. 93 (2008) 032104; Hosoi et al. : Appl. Phys. Lett. 94 (2009) 202112].