Band offsets at Ge/GeO2 interfaces: Effect of different interfacial bonding patterns

Using hybrid density functional calculations, we investigate possible origins for the large variation of band offsets measured at Ge/GeO2 interfaces. We consider atomistic model interfaces with both amorphous and crystalline oxides, in which the bond density reduction accounts for the mass densities of the two interface components. To the extent that all the Ge atoms are fourfold coordinated and all the O atoms are twofold coordinated, the band offsets are found to remain constant within 0.1 eV. We then investigate the role of valence alternation pairs consisting of negatively charged Ge dangling bonds and positively charged threefold coordinated O atoms, which have been suggested to occur in sizeable concentrations in substoichiometric germanium oxide. A valence alternation pair is introduced in one of the atomistic models and is found to affect the band alignment by contributing to the interface dipole. The calculated band offsets shift by 0.7 eV for an areal concentration of valence alternation pairs corresponding to one pair per 8 Ge interface atoms. These pairs would act as fixed charges and are not expected to directly contribute to the defect density in the germanium band gap. The occurrence of such bonding patterns offers a possible explanation for the large range of valence band offsets measured at this interface. (C) 2011 Elsevier B.V. All rights reserved.