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

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The detailed analysis of the structural variations of three GeO2 and SiO2 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 GeO2, while SiO2 preferentially forms quartz. GeO4 tetrahedras of quartz and cristobalite GeO2 phases at the equilibrium volume are more significantly distorted than those of SiO2. Moreover, in the case of quartz GeO2 and cristobalite GeO2, all O--Ge--O bond angles vary when the volume of the GeO2 bulk changes from the equilibrium point, which causes further deformation of tetrahedra. In contrast, the tilt angle formed by Si--O--Si in SiO2 markedly changes. This flexibility of the O--Ge--O bonds reduces the stress at the Ge/GeO2 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].
2011-02-25