J. Chen

Core structures of the a-edge dislocation in InN

Stillinger-Weber potential parameters were optimized for InN by fitting to the bulk material properties and point defect energy in order to calculate the core structure of the a-edge dislocation. The initial displacement field is imposed in the perfect crystal according to isotropic linear elasticity theory, and then the system is relaxed to the minimum energy. The different origins of the displacement field generate three cores with four, eight or five/seven atoms. The calculated energies indicate that the four-atom core structure is the most stable for InN.

Formation of a low energy grain boundary in ZnO: The structural unit concept in hexagonal symmetry materials

ZnO thin films prepared by magnetron sputtering on c-plane sapphire may exhibit a columnar growth with an average column diameter depending on the deposition temperature. In this case two epitaxial relationships coexist and adjacent columns are rotated, one from the other, by 90° around the [0001] direction. The long range rotation between domains is in agreement with the theoretical epitaxial relationships, but the local angles oscillate between 27° and 32° due to the formation of interfaces, which settle into low energy configurations with grain boundary dislocations of a Burgers vectors and 5/7 aligned structural units for the 32.2° tilt angle.

Mechanism of formation of the misfit dislocations at the cubic materials interfaces

High-angle annular dark-field scanning transmission electron microscopy and molecular dynamic simulation are applied to study the misfit dislocations at the GaSb/GaAs interface. In the investigated samples, three types of misfit dislocations have been observed: shuffle and glide set Lomer dislocations and 60° dislocation pairs. The dislocation density tensor analysis is next used to quantify the Burgers vector of misfit dislocations and investigate the misfit dislocation formation mechanism. This work demonstrates that, in these hetero-structures, the dominant mechanism underlying the formation of misfit dislocations is the glide and reaction of 60° dislocations. It is shown that the final structure of each misfit dislocation depends on the Burgers vectors of the initial 60° dislocations. Finally, this analysis points out an approach to determine the local rotation at interface due to mixed type dislocations.

The atomic configurations of the a→ threading dislocation in GaN

Abstract The atomic structure of the 1/3 〈1 1 2 0〉 edge dislocation has been simulated with the Stillinger–Weber empirical potential which was previously modified to take into account the homopolar bonds, Ga–Ga and N–N. This dislocation is characterised by a multiple structure based on rings of 4, 8 or 5/7 atoms. This multiplicity is explained by considering the position of the origin of the displacements corresponding to the creation of the dislocation. These displacements are imposed according to the isotropic linear elasticity theory. The choice of the origin is equivalent to consider the nature, differently spaced, of the two {1 0 1 0} prismatic planes. The tips of these two planes form the dislocation cores: 5/7-atom ring for the less spaced planes and 4 or 8-atom ring for the more spaced planes.

Investigation of the anisotropic strain relaxation in GaSb islands on GaP

The strain relaxation at the initial stages of highly mismatched (11.8%) GaSb grown on a GaP substrate following a Ga-rich surface treatment by molecular beam epitaxy has been investigated. High resolution transmission electron microscopy and moire fringe analysis were used to determine the relaxation state in these GaSb islands in the [110] and [1–10] directions. The measurements revealed an anisotropic strain relaxation in these two directions; there is a higher misfit strain relaxation along the [110] direction where the islands are elongated, which is in agreement with a higher density of misfit dislocations. By combining molecular dynamics simulations and TEM results, the anisotropy in the strain relaxation is shown to be related to the asymmetry in the formation of interface misfit dislocations. The P-core glide set 60° dislocations (α type) and the Ga-core shuffle set Lomer dislocations serve as the primary misfit dislocation which contributes to the strain relaxation in the (1–10) interface, and t...

Microstructure analysis in strained-InGaN/GaN multiple quantum wells

The barrier thickness effect on the energy and microstructure properties of InGaN/GaN multiple quantum wells is investigated with Stillinger-Weber potential. The calculation indicates that the energy of a quantum well increases as the GaN barrier thickness rises, and that Ga-N and In-N bonds are shrunk with respect to those of random InGaN alloy. Moreover, a critical value of the barrier thickness exits. If the barrier thickness exceeds the critical value, the bond length of Ga-N in quantum wells reduces as a function of indium concentration. This singular behavior of Ga-N bond is analyzed with a force balance model.

Hybrid carbon nanotube—silica/ polyvinyl alcohol nanocomposites films: preparation and characterisation

Composites of carbon nanotubes (CNT) in polymeric matrices have attracted considerable attention in the research and industrial communities due to their interesting and unique properties. However, the main intrinsic problem of CNT is their insolubility or very poor solubility in either water or organic solvents. Therefore, the dispersion of CNT has become the focus of many researches. In this study, a simple method based on a chemical process was developed in order to improve the dispersion of CNT. Our method consists in grafting carboxylic acid-functionalized carbon nanotubes (CNT-COOH) onto an amino-functionalized silica particles (SiO2-NH2) surface. Prior to assembly, CNT-COOH were prepared by using a mixture of concentrated nitric and sulfuric acids, while SiO2-NH2 were prepared by a silanization with 3-aminopropyltriethoxysilane (APTES). The hybrid carbon nanotube-silica (CNT/SiO2) was dispersed in polyvinyl alcohol (PVA) with 1, 3 and 5xa0wt% (by weight). Techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) measurement have been employed to study the dispersion state in the polymer matrix. The results indicated a good and uniform dispersion of hybrid fillers. Indeed, it was shown that the silica particles play the role of dispersing agents. In addition, a decrease of the resistivity of the composite films was observed as the concentration of CNT-silica fillers in the PVA increases, reaching 3.4u2009×u2009103xa0Ω.m when the hybrid fillers concentration is equal to 5xa0%.

Screw threading dislocations in AlN: Structural and electronic properties of In and O doped material

Density functional theory calculations were performed on undoped AlN screw threading dislocations (TDs) as well as TDs doped by indium and oxygen, prompted by integrated experiments through transmission electron microscopy and spectroscopic techniques demonstrating enhanced In and O concentrations in screw dislocation cores. It is revealed that screw TDs act as conduction pathways to charge carriers, introducing multiple levels in the bandgap due to overstrained, dangling, and “wrong” bonds formed even in the undoped cores. The presence of impurities and especially metallic In elevates the metal-like electronic structure of the distorted material and promotes the conductivity along the dislocation line. Hence screw dislocations in AlN are established as highly prominent conductive nanowires in semiconducting thin films and prospects for novel, highly functional nano-device materials through exploitation of screw TDs are attested.

Energetic, structural and electronic properties of metal vacancies in strained AlN/GaN interfaces

AlN/GaN heterostructures have been studied using density-functional pseudopotential calculations yielding the formation energies of metal vacancies under the influence of local interfacial strains, the associated charge distribution and the energies of vacancy-induced electronic states. Interfaces are built normal to the polar <0 0 0 1> direction of the wurtzite structure by joining two single crystals of AlN and GaN that are a few atomic layers thick; thus, periodic boundary conditions generate two distinct heterophase interfaces. We show that the formation energy of vacancies is a function of their distance from the interfaces: the vacancy-interface interaction is found repulsive or attractive, depending on the type of the interface. When the interaction is attractive, the vacancy formation energy decreases with increasing the associated electric charge, and hence the equilibrium vacancy concentration at the interface is greater. This finding can reveal the well-known morphological differences existing between the two types of investigated interfaces. Moreover, we found that the electric charge is strongly localized around the Ga vacancy, while in the case of Al vacancies is almost uniformly distributed throughout the AlN/GaN heterostructure. Crucially, for the applications of heterostructures, metal vacancies introduce deep states in the calculated bandgap at energy levels from 0.5 to 1 eV above the valence band maximum (VBM). It is, therefore, predicted that vacancies could initiate green luminescence i.e. light emission in the energy range of 2.5 eV stemming from electronic transitions between these extra levels, and the conduction band, or energy levels, due to shallow donors.

The methods to detect vacuum polarization by evanescent modes

We propose the evanescent-mode-sensing methods to probe the quantum electrodynamics (QED) vacuum polarization. From our methods, high-sensitivity can be achieved even though the external field is much smaller than the Schwinger critical field and may be realizable in contemporary experimental conditions. The methods are based on the effect of phase change and time delay of evanescent wave which is transmitted in QED vacuum.