Dangling bond | EPFL Graph Search

In chemistry, a dangling bond is an unsatisfied valence on an immobilized atom. An atom with a dangling bond is also referred to as an immobilized free radical or an immobilized radical, a reference to its structural and chemical similarity to a free radical. When speaking of a dangling bond, one is generally referring to the state described above, containing one electron and thus leading to a neutrally charged atom. There are also dangling bond defects containing two or no electrons. These are negatively and positively charged respectively. Dangling bonds with two electrons have an energy close to the valence band of the material and those with none have an energy that is closer to the conduction band. Properties In order to gain enough electrons to fill their valence shells (see also octet rule), many atoms will form covalent bonds with other atoms. In the simplest case, that of a single bond, two atoms each contribute one unpaired electron, and the res

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

Davide Colleoni

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

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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.

EPFL2015

High-Resolution Photoemission Study of Neutron-Induced Defects in Amorphous Hydrogenated Silicon Devices

Sylvain Dunand, Giulia Rossi

In this paper, by means of high-resolution photoemission, soft X-ray absorption and atomic force microscopy, we investigate, for the first time, the mechanisms of damaging, induced by neutron source, and recovering (after annealing) of p-i-n detector devices based on hydrogenated amorphous silicon (a-Si:H). This investigation will be performed by mean of high-resolution photoemission, soft X-Ray absorption and atomic force microscopy. Due to dangling bonds, the amorphous silicon is a highly defective material. However, by hydrogenation it is possible to reduce the density of the defect by several orders of magnitude, using hydrogenation and this will allow its usage in radiation detector devices. The investigation of the damage induced by exposure to high energy irradiation and its microscopic origin is fundamental since the amount of defects determine the electronic properties of the a-Si:H. The comparison of the spectroscopic results on bare and irradiated samples shows an increased degree of disorder and a strong reduction of the Si-H bonds after irradiation. After annealing we observe a partial recovering of the Si-H bonds, reducing the disorder in the Si (possibly due to the lowering of the radiation-induced dangling bonds). Moreover, effects in the uppermost coating are also observed by spectroscopies.

MDPI2022

Metal Clusters on Surfaces

Michael Hugentobler

The morphology, stability and reactivity of nanometre-sized particles (Au, Pt) is investigated on three different supports. Gold nanoparticles with a diameter comprised between 4 and 6nm are stabilized in nanosized pits of well defined depth in highly oriented pyrolytic graphite (HOPG). These pits are produced by creation of artificial defects, followed by etching under a controlled oxygen atmosphere. At low Au coverage, clusters are found on the edges of the hexagonal pits maximizing the contact to dangling bonds on graphite multisteps. Larger coverage results in Au beads of well defined shape and with a constant bead density per unit length. Most remarkable is the stability of these nanostructures under ambient conditions. Temperatures as high as 650 K do not alter the morphology of the gold clusters. Above 700 K, the gold clusters catalyze the etching of graphite layers when heated under atmospheric conditions. The depth of the etching channel is defined by the depth of the pit and the channel width can be controlled by the cluster size. Hydrogen, oxygen and water are dissociated on the Au clusters at low temperatures (> 400 K). CO and CO2 is produced above 700 K which confirms the etching of the multilayer graphene sheets. The so-called water-gas shift reaction has been observed where oxygen and water dissociate on the Au clusters at low temperatures. Thermal desorption spectroscopy (TDS) measurements on the Pt=YSZ system have been performed and the desorption peaks of CO and O2 have been evidenced. Carbon monoxide desorbs from the surface at a temperature of 535 K and oxygen at 705 K, in good agreement with values reported in literature. Catalytic measurements have confirmed the known strong catalytic activity of Pt. The CO oxidation takes place at temperatures between 450 K and 700 K. The adsorption and desorption temperature of the two educts have been confirmed during the catalysis measurements. Size-selected Aun clusters (n = 5, 7) are deposited from the gas phase at room temperature with well-controlled kinetic energy on rutile TiO2 . Two different surface reconstructions have been used as support: TiO2(110) – (1 × 1) and TiO2(110) – (1 × 2). Systematic studies of morphology, stability and adsorption sites have been performed using scanning tunnelling microscopy (STM). The mean size of the clusters is maintained during deposition at a kinetic energy of 7.1eV per atom, indicating that there is no fragmentation. Au clusters are not stable neither on the TiO2(110) – (1 × 1) nor on the TiO2(110) – (1 × 2) reconstruction during annealing of the substrate to 800 K. A progressive transition into a pronounced 3-d formation is observed. The size distribution of the particles is small and the growth mode (Ostwald ripening) independent of the surface reconstruction. The higher corrugation of the TiO2(110) – (1 × 2) reconstruction results in a pronounced stabilization of the clusters on terraces, in contrast to the TiO2(110) – (1 × 1) reconstruction where 90% of the clusters are found on step edges at elevated temperatures. The catalytic activity of the Au clusters sets in during the 2d - 3d transition. These measurements have to be confirmed and a full understanding of the process should be found.

EPFL2010