Origin of Fermi-level pinning at GaAs/oxide interfaces through the hybrid functional study of defects

Gallium arsenide is currently under scrutiny for replacing silicon in microelectronic devices due to its high carrier mobilities. However, the widespread use of this semiconductor is hampered by the intrinsic difficulty of producing high-quality interfaces with oxides. Indeed, proper device operation is generally prevented by a high density of interface defect states which lead to Fermi-level pinning in the band gap. The pinning of the Fermi-level is also found to occur in bulk GaAs and at GaAs surfaces upon submonolayer deposition of oxygen and other metals. Despite intensive efforts which have spanned more than three decades, the defects and the mechanisms underlying this behavior are not fully understood, thereby preventing further progress. Through the hybrid functional study of defects, we assign the origin of Fermi-level pinning at GaAs/oxide interfaces to the bistability between the As-As dimer and two As dangling bonds, which transform into each other upon charge trapping (As-As dimer/DB). Using electron-counting arguments, we infer that the identified defect occurs in opposite charge states conferring to the defect the amphoteric nature responsible for the Fermi-level pinning. In our study, we consider intrinsic defects in bulk GaAs finding a complex defect which undergoes a charge trapping mechanism similar to that of the As-As dimer/DB defect and which accounts for the observed Fermi-level pinning in radiation-damaged GaAs. Moreover, we consider oxygen defects in GaAs. Among the defects considered, we identify the one responsible for the observed Fermi-level pinning in O-doped GaAs. The computed defect properties such as the defect structure, the defect charge states, the charge transition level, and the optical transition energies show excellent agreement with the extensive experimental characterization available for this defect. These results clearly assign the origin of the measured Fermi-level pinning and optical transitions in O-doped GaAs to this O defect. In addition, we note that this O defect also shows a bistable behavior similar to that of the As-As dimer/DB defect. We find confirmation of the general occurrence of the As-As dimer/DB defect in GaAs by considering the GaAs(110) surface and suboxide model structures representing of the interfacial transition layer. Indeed, we find that both these systems show the formation of this defect, thus connecting the GaAs oxidation process to the defect formation. In particular, for the case of the suboxide models we calculate a charge transition level falling in the midgap region of the GaAs band gap in accord with the measured interfacial density of states. Finally, we address the GaAs/Al $_2$ O $_3$ interface. We obtain band offsets in excellent agreement with X-ray photoemission experiments and assess that the prevalent chemical bonding at the interface consists of Ga-O bonds. Then, we study structural and electronic properties of a set of defects occurring on the GaAs side of the interface. We find that the bistable As-As dimer/DB defect can occur either as an isolated defect or within defect complexes. The calculated charge transition level and donor/acceptor character are in agreement with the experimental characterization of the interfacial density of states. This result corroborates the dominant role of the As-As dimer/DB defect in the determination of the Fermi-level pinning at GaAs/oxide interfaces.

Structural and electronic properties of an abrupt 4H-SiC(0001)/SiO2 interface model: Classical molecular dynamics simulations and density functional calculations

Peter Broqvist, Fabien Devynck, Alfredo Pasquarello

Using a density functional approach, we study structural and electronic properties of the 4H(0001)-SiC/SiO2 interface. Through the sequential use of classical and ab initio simulation methods, we generate an abrupt model structure which describes the transition between crystalline SiC and amorphous SiO2 without showing any coordination defect. The first step in our generation procedure consists in identifying suitable interfacial bonding patterns which account for the bond density reduction across the interface. In the second step, the connection to amorphous SiO2 is achieved through classical molecular dynamics. The atomic positions are then relaxed within a generalized gradient approximation of density functional theory. The final model structure shows good structural parameters and an oxide density typical of amorphous SiO2. We investigate the electronic structure of the generated model interface through the local density of states obtained with hybrid density functionals. In our atomically abrupt interface model, the full oxide band gap is recovered at a distance of similar to 5 A from the interface. This extent is in good agreement with estimates derived from internal photoemission measurements and provides support for the abrupt nature of the interface. We obtained band offsets through two different procedures: by evaluating the local density of states and by aligning the band extrema through the local electrostatic potential. Band offsets calculated with various functionals are compared to experimental values. The best agreement is achieved for a hybrid functional in which a screened Coulomb potential is used in the Hartree-Fock exchange term. For this case, the calculated band offsets underestimate the experimental values by similar to 25%, but agree with experiment within a few percent when expressed with respect to the oxide band gap.

2007

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 transition levels of carbon-, oxygen-, and hydrogen-related defects at the SiC/SiO2 interface through hybrid functionals

Audrius Alkauskas, Peter Broqvist, Fabien Devynck, Alfredo Pasquarello

We calculate charge transition levels of various defects at the SiC/SiO2 interface within a scheme based on hybrid density functionals, which accurately reproduce the involved band gaps and band offsets. The defect levels are first evaluated in bulk models of the interface components and then aligned with respect to the interface band diagram through the use of a model interface showing good structural and electronic properties. Interface-specific polarization effects are evaluated separately through classical electrostatics. We considered carbon-related defects involving single atoms and dimers in both the crystalline SiC substrate and the amorphous SiO2 oxide. Our investigation also comprises oxygen-and hydrogen-related defects, including the Si-Si bond (O vacancy), the Si-2-C-O structure, the peroxy linkage, and the hydrogen bridge (Si-H-Si). Among the defects studied, the Si-2-C-O structure represents the best candidate for the high defect density measured near the conduction band of SiC.

2011