Davide Colleoni

Band alignment and chemical bonding at the GaAs/Al2O3 interface: A hybrid functional study

The band alignment at the interface between GaAs and amorphous Al2O3 is studied through the use of hybrid functionals. For the oxide component, a disordered model is generated through density-functional molecular dynamics. The achieved structure shows good agreement with the experimental characterization. The potential line-up across the interface is obtained for two atomistic GaAs/Al2O3 interface models, which differ by the GaAs substrate termination. The calculated valence band offset amounts to 3.9 eV for an interface characterized by the occurrence of Ga–O bonds as dominant chemical bonding, favoring the high-energy side in the range of experimental values (2.6–3.8 eV). The effect of As antisite and As–As dimer defects on the band alignment is shown to be negligible.

Origin of Fermi-level pinning at GaAs surfaces and interfaces

Through first-principles simulation methods, we assign the origin of Fermi-level pinning at GaAs surfaces and interfaces to the bistability between the As-As dimer and two As dangling bonds, which transform into each other upon charge trapping. This defect is shown to be naturally formed both at GaAs surfaces upon oxygen deposition and in the near-interface substoichiometric oxide. Using electron-counting arguments, we infer that the identified defect occurs in opposite charge states. The Fermi-level pinning then results from the amphoteric nature of this defect which drives the Fermi level to its defect level. These results account for the experimental characterization at both GaAs surfaces and interfaces within a unified picture, wherein the role of As antisites is elucidated.

Interfacial Ga-As suboxide: Structural and electronic properties

The structural and electronic properties of Ga-As suboxide representative of the transition region at the GaAs/oxide interface are studied through density functional calculations. Two amorphous models generated by quenches from the melt are taken under consideration. The absence of As–O bonds indicates that the structure is a mixture of GaAs and Ga-oxide, in accordance with photoemission experiments. The band edges of the models are found to be closely aligned to those of GaAs. The simulation of charging and discharging processes leads to the identification of an As-related defect with an energy level at ∼0.7 eV above the GaAs valence band maximum, in good agreement with the experimental density of interface states.

Assignment of Fermi-level pinning and optical transitions to the (AsGa)2-OAs center in oxygen-doped GaAs

The (AsGa)2-OAs defect in oxygen-doped GaAs, consisting of two As antisites neighboring an O center substitutional to As, is addressed through hybrid functional calculations. This defect not only accounts for the nearest neighbor environment of the O atom and the observed charge states but also yields a Fermi-level pinning position and optical transition energies between charge states in excellent agreement with experiment. The present assignment strongly supports the (AsGa)2-OAs center as origin of the Fermi-level pinning in oxygen-doped GaAs.

Diffusion of interstitial oxygen in silicon and germanium: a hybrid functional study.

The minimum-energy paths for the diffusion of an interstitial O atom in silicon and germanium are studied through the nudged-elastic-band method and hybrid functional calculations. The reconsideration of the diffusion of O in silicon primarily serves the purpose of validating the procedure for studying the O diffusion in germanium. Our calculations show that the minimum energy path goes through an asymmetric transition state in both silicon and germanium. The stability of these transition states is found to be enhanced by the generation of unpaired electrons in the highest occupied single-particle states. Calculated energy barriers are 2.54 and 2.14 eV for Si and Ge, in very good agreement with corresponding experimental values of 2.53 and 2.08 eV, respectively.

Nature of electron trap states under inversion at In0.53Ga0.47As/Al2O3 interfaces

In and Ga impurities substitutional to Al in the oxide layer resulting from diffusion out of the substrate are identified as candidates for electron traps under inversion at In0.53Ga0.47As/Al2O3 interfaces. Through density-functional calculations, these defects are found to be thermodynamically stable in amorphous Al2O3 and to be able to capture two electrons in a dangling bond upon breaking bonds with neighboring O atoms. Through a band alignment based on hybrid functional calculations, it is inferred that the corresponding defect levels lie at ∼1 eV above the conduction band minimum of In0.53Ga0.47As, in agreement with measured defect densities. These results support the technological importance of avoiding cation diffusion into the oxide layer.