Wrong Bonds Research Articles

Effect of edge threading dislocations on the electronic structure of InN

The open issue of the n-type conductivity and its correlation to threading dislocations (TDs) in InN is addressed through first principles calculations on the electronic properties of a-edge TDs. All possible dislocation core models are considered (4-, 5/7-, and 8-atom cores) and are found to modify the band structure of InN in a distinct manner. In particular, nitrogen and indium low coordinated atoms in the eight-atom core induce states near the valence band maximum and above the conduction band minimum, respectively. The formation of a nitrogen–nitrogen “wrong” bond is observed at the 5/7-atom core resulting in a state inside the band gap. The 4- and 5/7-atom cores induce occupied states resonant in the conduction band due to In–In strain induced interactions and wrong bonds, respectively. These occupied states designate TDs as a source of higher electron concentrations in InN and provide direct evidence that TDs contribute to its inherent n-type conductivity.

Amorphous InSb: Longer bonds yet higher density

Results of x-ray absorption studies of the structure of amorphous InSb are reported. We demonstrate that approximately 1% bond elongation in the amorphous phase (as compared to the crystalline phase) is accompanied by a counterintuitive increase (approximately 5%) in density. We argue that this controversy is due to the formation of wrong bonds in the amorphous phase with both Sb and In atoms effectively preserving their tetrahedral coordination. Our results additionally offer an alternative interpretation of the semiconductor-metal transition observed upon melting of InSb.

Experimental and theoretical investigations on the structure-properties interrelationship of the gadolinium-vanadate-germanate glasses

Glasses in the system xGd₂O₃·(100-x)[GeO₂·V₂O₅] with 0 ≤ x ≤ 20 mol% have been prepared from the melt quenching method. In this paper, we investigated changes in germanium coordination number in gadolinium-vanadate-germanate glasses through molar volume analysis, measurements of densities, investigations of FTIR and UV-VIS spectroscopy, calculations of density functional theory (DFT). Analyzing the structural changes resulted from the IR spectra we found that the gadolinium ions have a pronounced affinity toward [VO₄] structural units which contain non-bridging oxygens necessary for the charge compensation. The introduction of the excess of oxygen yields the formation of [VO₅] structural units. This attains maximum value at 5 mol% Gd₂O₃, in agreement with the density measurements. Further, the addition of the surplus of oxygen implies the transformation of [VO₅] to [VO₄] structural units and the formation of VO₄⁻³ orthovanadate structural units. The UV-VIS spectra show a broad UV absorption band located in the 300-500 nm region. These bands are assumed to originate from the combination of vanadium ions possibly present in the three states of valence. The presence of Ge-Ge wrong bonds attains its maximum values in the samples with x = 5 and 15 mol% Gd₂O₃ (bands centered in the 250-300 nm range). DFT calculations show the massive vibrations of the [VO(n)] structural units coupled with each other via [GeO₆] and [GeO₄] structural units. This leads to the splitting of the bridge modes and a multiplication of the number of these bands.

Transient Absorption in As2Se3 and Ag(Cu)-Doped As2Se3 Glasses Photoinduced at 1.06 µm

Transient photoinduced absorption in As2Se3 and Ag(Cu)-doped As2Se3 glasses was measured after the samples were excited with an intense laser pulse with a pulse width of 10 ns, a peak optical intensity of ∼100 MW/cm2, and a wavelength of 1.064 µm. The decay time of the nonlinear absorption was within 15–21 ms. A photocurrent experiment was also performed under the same illumination conditions, but we observed no photocurrent. Such a slow nonlinearity is presumably attributed not to free-carrier effects, but to photoinduced effects related to wrong bonds in the glasses.

Diffusion in Intermetallic Compounds and Fabrication of Hollow Nanoparticles through Kirkendall Effect

. In intermetallic compounds, random vacancy motion is not possible as it would disrupt the equilibrium ordered arrangement of atoms on lattice sites. In view of this limitation, various atomistic models have been proposed, which allow atom-vacancy exchanges to take place without concomitant long range disordering. For a L12 -type A3B structure, the major element A diffuses faster than the minor element B. The trend is attributed to the different diffusing paths; A atoms can diffuse through site exchanges with a neighbouring vacancy on its own sublattice, while the jump of a B atom to a neighbouring site always creates wrong bonds. For L10-type structures such as γ-TiAl, significant diffusion anisotropy is observed; Ti atoms diffuse on the Ti sublattice, while Al atoms also diffuse on the Ti sublattice. The formation of hollow metal oxide nanoparticles through the oxidation process has been studied by transmission electron microscopy for Cu, Zn, Al, Pb and Ni. The hollow structure is obtained as a result of vacancy aggregation, resulting from the rapid outward diffusion of metal ions through the oxide layer during the oxidation process. This suggests the occurrence of two different diffusion processes in the formation of hollow oxides.

Atomistic modeling of the(a+c)-mixed dislocation core in wurtzite GaN

An atomistic simulation of the threading $(\mathbf{a}+\mathbf{c})$-mixed dislocation core in wurtzite GaN has been carried out. Starting from models generated in the framework of continuum elasticity theory, two core configurations are obtained independently by using an empirical potential and a tight-binding based ab initio method. The most energetically favorable core with a $5∕7$-atoms ring structure is fully coordinated without wrong bonds, whereas the other with a complex double $5∕6$-atoms ring structure contains two rows of dangling bonds. Both core configurations introduce empty states spread over the upper half of the band gap.

Atomic core structure of 90° -bent screw threading dislocations in wurtzite GaN

The atomic and electronic structures of the c threading dislocations with an edge or screw character were compared using a tight binding formalism which takes into account charge transfer. The two dislocations do not exhibit dangling bonds. While the screw dislocation contains only constrained Ga-N bonds, the edge dislocation contains Ga-Ga and N-N wrong bonds. Both dislocations are found to induce shallow and deep gap states.

New core configurations of the c‐edge dislocation in wurtzite GaN

AbstractIn the framework of empirical potentials (Modified Stillinger‐Weber), we modelled and studied the energy and the core structure of the c‐edge basal dislocation in wurtzite GaN. Further to the core configurations already known, we report two new ones without wrong bonds. Our calculations show that the configurations containing wrong bonds are the most energetically favourable in both glide and shuffle sets. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of atomic vacancies and grain boundaries on the mechanical properties of single-crystal GaN nanotubes

We present calculations of the Young modulus of some single-crystal GaN nanotubes with defects by using the empirical Stillinger-Weber potential. Our results indicate that dilute single vacancies and inversion domain grain boundaries (with wrong bonds and without wrong bonds) in single-crystal GaN nanotubes do not significantly affect the Young modulus of the tubes. However, the grain boundary at which the interface is normal to the axis of a nanotube may greatly lower the Young modulus of the single-crystal GaN nanotube.

Photoinduced interdiffusion in nanolayered Se∕As2S3 films: Optical and x-ray photoelectron spectroscopic studies

Photoinduced interdiffusion was observed with above band gap light in nanolayered Se∕As2S3 films. It is discussed in terms of the optical parameters such as band gap, Urbach edge (Ee) [F. Urbach, Phys. Rev. 92, 1324 (1953)], and B1∕2 (Tauc’s parameter) [J. Tauc et al., Phys. Status Solidi 15, 627 (1966)]. Experimental data of B1∕2 and Ee for as-prepared samples do not show clear correlation implied by the Mott-Davis model [N. F. Mott and E. A. Davis, Electronic Process in Non-crystalline Materials (Clarendon, Oxford 1979), p. 287]. It is also shown that the optical parameters can be changed with a change in the Se sublayer thickness. Variations of these optical parameters as a function of modulation period and photoinduced interdiffusion were discussed in terms of the quantum confinement effect and changes in the valence and conduction bands. We proposed a model to explain the mechanism of Se diffusion in As2S3, which suggests that diffusion takes place through the wrong bonds. X-ray photoelectron spectroscopy (XPS) is used to investigate the chemical alternations in the bonding. The proposed model was supported by the XPS data.

What does an $$\left( {\vec a + \vec c} \right)$$ dislocation core look like in wurtzite GaN ?

Here we present results on the first atomic simulation of the threading $$\left( {\vec a + \vec c} \right)$$ –mixed dislocation cores in wurtzite GaN. These calculations are based on a modified Stillinger-Weber potential. For this dislocation two core configurations are shown to be stable, one with a complex double 5/6-atoms rings and the other with a 5/7-atom rings structures. These two cores do not contain wrong bonds.

Ab initio ELNES/XANES spectral calculation of polar and non-polar grain boundaries in β-SiC

A recently developed ab initio method for the ELNES/XANES spectral calculation for solids is applied to two SiC{1 2 2} Σ=9 grain boundary models in β-SiC. Two such models with polar and non-polar interfaces each containing Si–Si and C–C wrong bonds were used. The method takes into account the electron core–hole interaction and includes all dipole transition matrix elements rigorously. The results show considerable differences in the C–K, Si–K and Si–L edges of atoms in the grain boundary region and those in the bulk-like region. The changes are greater for the Si edges than the C edges. In the presence of wrong bonds, the atoms involved can have very significant differences in their ELNES/XANES spectra. These changes are interpreted with separate calculations of atom-resolved partial density of states, the effective charges on each atom, and the bond order values for each pairs of atoms for all atoms in the grain boundary models.

Inversion domain boundaries in ZnO: First-principles total-energy calculations

We present first-principles total-energy calculations of domain wall energies for inversion domain boundaries and their interaction with point defects in ZnO. We find that the inversion domain boundaries (IDB's) with Zn-Zn and O-O wrong bonds are energetically unstable, and they transform into the so-called ${\mathrm{IDB}}^{*}$ structures. Although the ${\mathrm{IDB}}^{*}'\mathrm{s}$ do not induce electronic states in the bandgap, they are very attractive to electrically active compensating point defects. The segregation of such charged point defects builds up electrical potentials for minority carriers in doped ZnO, which are harmful to optoelectronic applications.

Optical and interface phonons of different kind in BeTe/ZnSe superlattices

AbstractWe have studied Raman scattering for a number of type II novel superlattices BeTe/ZnSe, which have no common cation or anion atoms at the interfaces. Under off‐resonant excitation LO‐phonons in BeTe and ZnSe layers were observed as well as a high‐frequency mode due probably to the “wrong bond” BeSe at the interface. Under excitation resonant with the direct exciton in ZnSe layers we observed ZnSe LO‐phonons of different orders and electrostatic interface modes of the Fuchs–Kliever type. Calculated frequencies of the interface modes are compared with the experimental ones. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural units and low-energy configurations of [0001] tilt grain boundaries in GaN

The potential energy of 〈0001〉 tilt coincidence grain boundaries has been calculated using a Stillinger-Weber potential that was previously adapted to wurtzite (GaN) in order to take into account the Ga--Ga and N--N wrong bonds. The atomic structures of the grain boundaries have been determined for the lowest-energy configuration. They are described in terms of a limited number of structural units corresponding to the cores of the $\frac{1}{3}〈112\ifmmode\bar\else\textasciimacron\fi{}0〉$ edge dislocation. The potential energy curve versus tilt angle shows two energy minima for $\ensuremath{\Sigma}=7$ and 13.

Electronic structure of the wrong-bond states in amorphous germanium sulphides

First-principles calculations of the electronic structures of the wrong bonds were performed for amorphous germanium sulphides in order to explain their compositional dependence. Model cluster calculations of the density of states using a set of geometry similar to the crystalline-GeS 2 coordination can reproduce the peak structure of the experimental valence band photoemission spectra. The bonding and anti-bonding states of the covalent Ge–S bonds form valence and conduction band respectively, and the top of the valence band is occupied with S 3p lone-pair states. The bonding states are modified by S–S bonds and the anti-bonding states are modified by Ge–Ge bonds, mainly through their hybridization with the wrong-bond states between p-orbitals. The lone-pair states do not interact either of them to form a different band, and obscure the modification induced by the S–S wrong bonds. Therefore, we can conclude that the narrowing of the bandgap with increasing Ge content from GeS 2 composition is due to that of the conduction-band bottom with increasing germanium wrong bonds, though the narrowing with increasing S content is moderate due to the presence of the lone-pair states at the valence-band top.

Microstructural and physicochemical study of the buried Fe/AlGaAs(100) interface by transmission electron microscopy and x‐ray emission spectroscopy

AbstractAmong the magnetic metal/semiconductor contacts, the Fe/GaAs system has been widely studied owing to its potential applications in electronic devices. In contrast, there are not many studies concerning the Fe/AlxGa1−xAs contact, and in particular there are no reports concerning the changes induced in the interfacial zone by the presence of Al. In this work, thin polycrystalline iron films were deposited by ion beam sputtering at room temperature on a 300 nm thick Al0.25Ga0.75As layer grown by molecular beam epitaxy onto GaAs(001). X‐ray diffraction analysis showed that the iron films are polycrystalline, and indications of a (002) texture of the film were observed. The fine scale analysis of the interface was achieved by high‐resolution transmission electron microscopy (HRTEM) observations, the results of which are compared with the physicochemical information obtained from electron‐induced x‐ray emission spectroscopy, by analysing the Al 3p valence states at the Fe/AlxGa1−xAs interface. The HRTEM experiments on cross‐section samples indicate that the interfacial zone between iron and AlGaAs is limited to <1.5 nm in thickness. X‐ray emission spectroscopy showed the presence of Al atoms in an FeAl‐like environment at the interface, and the existence of wrong bonds and point defects. The estimated width of the perturbed interface (2.0 ± 0.5 nm) is in agreement with the HRTEM results. Copyright © 2003 John Wiley & Sons, Ltd.

First principles molecular dynamics study of amorphous AlxGa1−xN alloys

We have systematically investigated atomic structures, electronic and dynamical properties of amorphous aluminum–gallium–nitride alloys (a-AlGaN) by performing first principles local basis molecular dynamics simulations. The network topology and defects of the amorphous 216-atom model system have been analyzed with the radial distribution function, the angular distribution function, the ring statistics, and the local coordination. It was found that the models have mixed threefold and fourfold coordinations, and the number of threefold (fourfold) coordinated atoms in alloys decreased (increased) with increasing Al composition. No odd rings are found, indicating that no wrong bonds (homonuclear bonds) appear in the a-AlxGa1−xN alloys. The Ga–N and Al–N bond lengths show a small variation with the Al composition, which is in agreement with recent extended x-ray absorption fine structure experimental measurements. The electronic properties examined by the electronic density of states and local bonding character demonstrate that no mid-band-gap states exist. The band-gap dependence on Al fraction x in a-AlxGa1−xN alloys shows a nearly linear variation with Al composition, and exhibits a small downward bowing behavior. It was also shown that valence band tail states are mostly localized on the threefold coordinated N sites, while the conduction band tail states are mostly localized on the threefold coordinated Ga and Al sites, and the electronic localization tends to become weaker with the addition of Al. We find a mixture of sp3 and sp2 bonds present in the network and their interaction plays a key role in the dynamical properties of a-AlxGa1−xN alloys.

Ab initiostudy of the tensile strength and fracture of coincidence tilt boundaries in cubic SiC: Polar interfaces of the {122}Σ=9boundary

The tensile strength and fracture of polar interfaces of the {122} $\ensuremath{\Sigma}=9$ coincidence tilt grain boundary in cubic SiC have been examined through the behavior of electrons and ions using the ab initio pseudopotential method based on the local density-functional theory. The results are compared with previous results for the nonpolar interface of this boundary [M. Kohyama, Philos. Mag. Lett. 79, 659 (1999)], and the effects of interfacial configurations associated with Si-Si or C-C wrong bonds on the mechanical properties are discussed. In stable configurations of the N-type and P-type polar interfaces, all the interfacial bonds are well reconstructed similarly to the nonpolar interface, and the N-type and P-type interfaces contain C-C and Si-Si wrong bonds, respectively, although the nonpolar interface contains both kinds of wrong bonds. An ab initio tensile test has been applied to the supercell containing both types of polar interfaces, where uniaxial tensile strain normal to the interface is applied in small increments. Only the P-type interface is broken just after the maximum tensile stress of about 48 GPa and the N-type interface is not broken at all. This tensile strength of the P-type interface is larger than that of the nonpolar interface of about 42 GPa, and the N-type interface containing C-C bonds is the strongest. The tensile strength of all the reconstructed interfaces is rather large, and is over 80% of the theoretical and experimental strength of bulk SiC. A critical bond stretching of about 20% is observed for the Si-C bond breaking, similarly to the case of the nonpolar interface. It is shown that the atomic-scale inhomogeneity or singularity associated with the wrong bonds seriously affects the tensile strength and interfacial fracture. For the nonpolar interface, the fracture starts from the back Si-C bond of the C-C bond because of local stress concentration at the atomic scale, and proceeds from bond to bond rather continuously. For the P-type polar interface, on the other hand, there occurs no remarkable local stress concentration except for just before the fracture, because of the highly symmetric configuration. Thus larger tensile strain and stress are introduced before the fracture, and a larger number of interfacial bonds reach the criterion of bond breaking at the same time. This is the reason why the P-type polar interface containing Si-Si bonds is stronger than the nonpolar interface and is broken rather catastrophically. The present results should be applicable to general coincidence tilt grain boundaries in SiC with similar reconstructed configurations.

Antiphase boundaries in GaP layers grown on (001) Si by chemical beam epitaxy

We have investigated the origin of contrast features observed in coalesced GaP islands, deposited by chemical beam epitaxy on (001) Si, by high resolution transmission electron microscopy and conventional dark field electron microscopy. Our results indicate that these features are antiphase boundaries (APBs) lying on {110} planes. Image simulations have been performed to show that APBs can only be seen under specific defocus conditions in high resolution lattice images. The observed contrast is attributed to the presence of Ga–Ga and P–P wrong bonds at APBs. A model is proposed to show that the coalescence of GaP islands on the same Si terrace may not produce APBs, and the formation of such boundaries may require the presence of monoatomic steps, separating the coalescing islands.