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.

Electronic and Structural Properties of the Ge/GeO2 Interface through Hybrid Functionals

Jan Felix Binder

The performance of silicon based microelectronic circuits reaches the end of the roadmap. New material systems are required for further improvements in speed and power consumption. Germanium is a possible candidate to substitute silicon for microelectronic devices. Its hole mobility is the highest of all semiconductor materials. Together with its lower band gap it could be an ideal material for energy-saving devices. This thesis is dedicated to first principles studies of the Ge/GeO2 interface through hybrid density functional theory. The substoichiometric region of the interface is of special interest. A wide substoichiometric region is supported by total energy calculations of a set of crystalline model systems. An unexpected structure organization was found through molecular dynamics simulations of substoichiometric GeO. We found that a majority of germanium and oxygen atoms are threefold coordinated, forming valence alternation pairs (VAPs). A detailed energetic analysis located the VAPs in the low-oxygen region of the interface. VAPs show interesting properties : They are prone to charge trapping. The electron trapping level might explain the bad performance of n-type doped devices. Furthermore, VAPs might be at the origin of the difficulties of H passivation at the Ge/GeO2 interface. Since threefold Ge atoms are negatively charged and threefold O atoms are positively charged, VAPs give rise to dipoles. These dipoles may reduce the interface dipole created by the electronegativity difference in the Ge-3O bond. With this mechanism, we can explain the wide range of experimental valence band offsets (VBOs) with the occurrence of different density levels of VAPs at the Ge/GeO2 interface. This suggestion is further confirmed by the determination of the VBO and Ge 3d core-level shift for an atomistic model structure of the Ge/GeO2 interface. Both values are systematically lower than typical experimental values for the Ge/GeO2 interface. Taking the extra dipole into account, our calculated VBOs and XPS shifts are in excellent agreement with experimental values. These results confirm that the structural properties of the Ge/GeO2 interface deviate significantly from its Si counterpart.

EPFL2012

Charge trapping in substoichiometric germanium oxide

Jan Felix Binder, Peter Broqvist, Alfredo Pasquarello

Electron and hole trapping in substoichiometric germanium oxides are investigated through the use of hybrid density functionals. We consider disordered model structures generated by Monte-Carlo bond switching and by ab initio molecular dynamics (MD). The Monte-Carlo model consists of fourfold coordinated Ge atoms and of twofold coordinated O atoms, and does not show trap levels neither for electron nor for holes. At variance, the MD model shows threefold coordinated O and Ge atoms forming valence alternation pairs and is found to present trap states for both carriers. The trapping states correspond to the formation and breaking of Ge-Ge bonds. The associated defect levels are determined within a band diagram of the Ge/GeO2 interface. (C) 2011 Elsevier B.V. All rights reserved.

Elsevier2011

Effect of improved band-gap description in density functional theory on defect energy levels in alpha-quartz

Audrius Alkauskas, Alfredo Pasquarello

We compare defect energy levels of point defects in alpha-quartz using density functionals of different accuracy: a standard generalized-gradient-approximation functional and a hybrid functional. The latter is known to improve the description of band gaps through the incorporation of a fraction of Hartree-Fock exchange. The separations between the defect levels are found to be preserved when going from the standard to the more accurate hybrid functional, indicating that they are already well described with the standard functional. We also find that the defect wave functions obtained with the two functionals are very similar. Consequently, the defect energy levels pertaining to the hybrid functional can be obtained through the perturbational method. (C) 2007 Elsevier B.V. All rights reserved.

2007

Electronic and Structural Properties of the Ge/GeO2 Interface through Hybrid Functionals

Jan Felix Binder

EPFL2012